Transcript: Coatings, Finishes & Paint

The full text of the narration of all 21 chapters. It was generated automatically from the on-screen captions, so small errors are possible. Back to the video course

0. The Map of Coatings

The part came off the milling machine perfect: every dimension in tolerance, and a clean finish. But if we leave it like this, it won't survive. in a white powder, and a surface that rubs against another surface wears away. Machining gives the part its shape. Finishing and coating keep it that way over time.

In this course we'll go through everything that happens to a part after machining: how each one works, and how to choose the right one. There are several reasons to coat a part, and sometimes one coating covers several of them. The first, and the most common, is protection against corrosion.

The second is wear resistance: a hard coating extends the life of a surface that rubs or wears. The third is appearance: color, shade, gloss, or a matte look. The fourth is electrical properties: some coatings conduct very well, like silver and gold on contacts, and some coatings insulate, like anodizing.

non-stick, solderability, or restoring the size of a worn part. look at a cross-section of the surface and ask one question: what does the coating do to the base material? no material is added at all. Only the surface texture changes.

itself turns into a different compound, through a chemical or electrochemical reaction. phosphating, black oxide, chromate coating, and anodizing work. In a metallic coating, a layer of a different metal is deposited on the part: with electric current, by a chemical reaction, or by dipping in molten metal.

forms a barrier between the metal and the environment. PVD, apply materials that the other methods can't, such as ceramics and carbides. The difference between the families also shows in thickness. A chromate coating on aluminum is very thin, well under one micron.

A PVD coating on a cutting tool is a few microns thick. Electroplating and regular anodizing are usually in the range of 5 to 25 microns. Hard anodizing reaches tens of microns. more, and thermal spray can reach tenths of a millimeter.

Thickness has a direct meaning on the drawing. outside diameter by 50 microns, and reduces a bore by the same amount. On a thread the effect is even bigger: because of the thread angle, the pitch diameter changes by about 4 times the coating thickness.

grows inward into the material, and half grows outward. So you must know whether a dimension on the drawing is before or after coating. A coating protects against corrosion in two main ways. The first is a barrier: the coating separates the metal from moisture and oxygen.

That's how paint, nickel, and chrome work. The drawback is that when a scratch reaches the metal, the protection at that spot is gone. The second is sacrificial protection. scratch, the surrounding zinc corrodes instead of the steel and protects it.

That's how zinc plating and galvanizing work. The same principle also explains galvanic corrosion. presence of moisture, the more active metal corrodes. A classic example is a stainless steel screw in an aluminum part, with no insulation.

the question: which metal are we coating, and what touches it? How do you choose a coating? Ask five questions. aluminum, not steel, and passivation suits stainless steel. indoor space, outdoors, a marine environment, or a chemical environment.

What is the job of the surface? Protection only, wear resistance, electrical contact, or appearance. thickness the drawing can absorb, and which surfaces must stay uncoated. And what do the standard or the customer require? specified by a standard, with type, class, and thickness, not by free choice.

One issue has been changing the field in recent years, and that's regulation. Hexavalent chromium is found in chromate coatings and in some primers. Cadmium was used for many years to protect steel in aerospace. Both materials are toxic and carcinogenic.

Their use is more and more restricted, for example in Europe under REACH, and alternatives are taking their place, such as trivalent chromium coatings and zinc-nickel. We'll cover this in more detail in chapters 5 and 8. What makes a coating expensive? Often, it's not the material itself.

Masking means covering the surfaces and threads that must not be coated. It's manual work, and it can cost more than the coating itself. Racking matters too: every part needs a holding point or an electrical contact, and a part that's hard to hang or to fit into the tank makes the job more expensive.

For a small quantity, you pay a minimum lot charge. A thick coating, or a system of several layers, takes more time. order: thickness, adhesion, and salt spray tests, certificates, and approved processes. The course follows the order a part actually goes through.

finishing, then conversion coatings, metallic coatings, paint, and advanced coatings, and finally testing, standards, and designing a part for coating. which numbers matter, how to design a part for it, what goes wrong, and what drives the cost. Let's begin.

1. Cleaning and Surface Preparation

Most coating failures don't start with the coating itself, but with what was on the surface before it. A coating only bonds to clean metal. underneath, the coating bonds to that layer, not to the part. Then it will peel, blister, or look perfect until the part goes into service.

That's why every coating process starts with cleaning. It's the least impressive step, and it's the one that decides whether everything else holds. several kinds of contamination, and each kind needs a different treatment. rust preventive oil, polishing compound, and fingerprints.

There's solid contamination: chips, dust, and burrs. of the metal itself: rust, oxides, and scale from heat treatment or welding. Soap and water won't remove oxide, and acid won't remove oil. So cleaning is done in stages: first you remove the oils, and only then the oxides.

The most common method is alkaline cleaning: a hot alkaline solution with surfactants. Usually between 50 and 80 degrees Celsius, for a few minutes, by immersion or spray. When there are blind holes, threads, or small parts in bulk, ultrasonics are added. Sound waves at tens of kilohertz create tiny bubbles that collapse on the surface,

and pull dirt out of corners the solution alone can't reach. In electrocleaning, current is passed through the part inside the tank. aggressively, and it's a standard step before electroplating. on oils, but solvent use is restricted today for safety and environmental reasons.

After every step the part is rinsed in water, usually more than once. Any chemical left on the part is dragged into the next tank and contaminates it. Pickling is dipping the part in an acid that dissolves oxides and rust. For steel, hydrochloric or sulfuric acid is usually used, with inhibitors,

so that the acid attacks the oxide more than the metal. On stainless steel the scale is tougher, and a mix of nitric and hydrofluoric acid is used. It's a very hazardous solution that requires a suitable facility. On aluminum, a light alkaline etch is usually done, followed by a desmut:

removing the dark residue layer left on the surface after etching. On cast aluminum, with a lot of silicon, this layer is especially noticeable. And the key rule: acid removes metal. pitting and loss of size, and with tight tolerances you have to account for it.

hydrogen is released on the steel surface, and some of it penetrates the metal. In soft steels, this usually isn't a problem. megapascals, like hardened bolts and springs, hydrogen causes embrittlement. happens hours or days later under load, when the part is already assembled.

So with these steels, pickling is minimized and mechanical cleaning is preferred. usually at around 190 to 220 degrees Celsius, for several hours. And the bake has to start shortly after processing, not days later. We'll cover this in more detail in chapter 7.

The simplest test is the water break test: take the part out of the rinse and look at the water. film that stays unbroken for several tens of seconds. If the water breaks into islands and droplets, there's oil left on the surface. Steel parts prepared for painting have defined cleanliness grades, according to ISO 8501-1.

Sa 1 is light cleaning, Sa 2 is thorough cleaning, common grade for industrial paint, and Sa 3 is white, completely clean metal. St 2 and St 3 are grades for cleaning with hand and power tools. American standards have equivalent grades, and we'll cover them in the next chapter, on blasting.

For especially sensitive coatings there are also laboratory tests, and adhesion tests after coating. solution, and whatever is trapped there doesn't get rinsed out. coating, causing stains, blisters, and corrosion under the coating. So, where possible, design drain and vent holes, and avoid closed pockets and unsealed lap joints.

aluminum, clean each part before assembly, because each material needs different chemistry. cleaning, or a marking that suits the process is chosen in advance. And one more important thing: time. Clean, dry steel starts to rust within hours. So clean parts are coated quickly, and handled only with gloves.

Common problems: oil that wasn't removed, causing peeling and blisters. A residue layer on aluminum, which ruins the conversion coating. Over-pickling, which causes pitting and loss of size. Rust that appears between rinsing and coating, and hydrogen embrittlement in hard parts.

polishing compound, heavy rust, or thick scale, need more steps and more time. Geometry that traps solution needs extra rinses, and sometimes manual cleaning. A hydrogen relief bake adds hours in the oven for every lot. And waste treatment: used chemicals cost money, and that cost goes into the price of every part.

The rule to remember: good cleaning is cheaper than any repair of a failed coating. Next chapter: blasting and mechanical finishing.

2. Blasting and Mechanical Finishing

After chemical cleaning comes mechanical finishing. is changed by force: hit with particles, ground, or rubbed. There are three goals. To remove things chemistry doesn't easily remove, like scale, heavy rust, and old paint.

To create a texture the coating can grip. And to give the part a uniform look: matte, satin, or bright. We'll go through four types: blasting, polishing, brushing, and tumbling. accelerates hard particles and slams them into the surface.

Every particle that hits leaves a small crater, and carries dirt and oxide away with it. Small parts are done in a closed cabinet with gloves. particles by centrifugal force, without compressed air. and 7 bar, and distance from the part, usually tens of centimeters.

Plus the angle, and the type and size of the particle. High pressure and large particles remove more, but also leave a rougher surface. Angular particles, like aluminum oxide, steel grit, and garnet, cut into the surface. They clean fast and create a texture of peaks and valleys: the best texture for paint adhesion.

Round particles, like steel shot and glass beads, don't cut. They peen the surface. They remove very little material and give a uniform satin look. This is the common choice for cosmetic finishing, for example before anodizing. soda, remove paint without damaging the metal underneath.

They're used in aerospace, to strip paint from thin aluminum and from composites. causes a severe lung disease, and in many places its use is restricted or banned. The name sandblasting has stayed in everyday language. The texture blasting leaves is called the surface profile, or anchor profile.

It's measured as the peak-to-valley height, in microns. several tens of microns, and the paint specification sets the exact range. Too shallow a profile, and the paint has nothing to grip. Too deep a profile, and the peaks stick out through the coating, and that's where corrosion starts.

tape pressed onto the surface, or with a stylus gauge. In chapter 1 we saw the cleanliness grades, from Sa 1 to Sa 3. Blasting is how you reach them, and the American equivalents are SSPC-SP7, SP6, SP10, and SP5. is left on the surface, and the profile says what texture was created.

The two are checked separately, and both need to appear in the specification. aluminum with media that was used on carbon steel. after a few days they rust, leaving brown spots on a part that's supposed to be corrosion resistant. That's why there are separate cabinets and media for each family of materials.

The compressed air also has to be clean and dry. the part, and takes us back to the problem from chapter 1. and particles trapped in holes and slots are removed. And coat the part quickly: blasted steel is clean steel, and it starts to rust within hours.

A side note: there's also a process called shot peening. compressive stresses in the surface layer, which greatly improves fatigue life. measurement, and it's different from blasting for cleaning. In polishing, the grit is reduced in steps, from coarse to fine, finishing with polishing compound.

with an Ra of tenths or even hundredths of a micron. Compound residue has to be removed before coating, as we saw in chapter 1. Brushing leaves fine parallel lines in one direction: that's the familiar satin look on stainless steel. It's important to define the direction of the lines on the drawing, because it affects the appearance.

vibratory bowl, together with ceramic or plastic media, and liquid. for hours: burrs disappear, corners round off, and the surface smooths out. It's ideal for small parts in large quantities, because hundreds of parts are processed at once. Blasting distorts thin parts.

and when a thin sheet is blasted on one side only, it bends. For such parts, blast both sides, lower the pressure, or choose a finer particle. Blasting rounds sharp edges and removes a little material. The change is usually small, surfaces it matters, and those surfaces are masked.

stuck in holes, and separate delicate parts so they don't damage each other. or an approved appearance sample, and which surfaces are masked. The word blasting alone, with no details, leaves every decision to the operator. stainless steel and aluminum, and distortion of thin parts.

Rounded corners and damaged dimensions from blasting too long. Uneven appearance between parts, and rust that appears between blasting and coating. And what drives the cost? Masking: manual work for every part. operator time per part. In production runs, a machine is much cheaper.

parts needs inspection, and sometimes parts are rejected. Mirror polishing, with many steps and skilled manual work. And tumbling is the cheapest per part, provided the parts are small and numerous. Next chapter: electropolishing and passivation of stainless steel.

3. Electropolishing and Passivation

Stainless steel is steel with at least 10.5% chromium. forms a very thin oxide layer on the surface, just a few nanometers thick. This layer is called the passive layer. It's transparent and tightly bonded, and when it's scratched, it rebuilds itself.

iron on the surface, particles from tools and work surfaces, and oil residue. In those spots the layer doesn't form properly, and rust spots appear. There are two processes for this: passivation and electropolishing. Passivation is dipping the part in acid, usually nitric acid or citric acid.

The acid dissolves the free iron and contaminants from the surface, leaving a chromium-rich surface. Then a uniform, stable passive layer forms. appearance, and no measurable change in dimensions. scale, it doesn't remove weld heat tint, and it doesn't remove oil.

by cleaning, and scale and weld heat tint by pickling. Nitric acid is the long-established method, and the most common in aerospace. stainless grade, and the time is usually tens of minutes. environment, and the main standards approve it as an alternative.

2700. They define the methods by stainless grade, and the tests. The tests include water immersion or high humidity exposure, checking that no rust appears, which reveals free iron, and isn't suitable for every stainless grade. The austenitic 300 series, like 304 and 316, responds best to passivation.

less chromium and are more sensitive, so the right method must be chosen. The big problem is free-machining stainless, like 303 and 416. sulfide inclusions dissolve in acid, leaving small pits. attacked and turn black. This is called flash attack.

these grades, and sometimes inhibitors are added to the acid. The rule: when ordering passivation, state the exact stainless grade and the standard. Electropolishing is the opposite of electroplating. instead of adding metal, the current dissolves metal from the surface.

The bath is usually a mix of phosphoric acid and sulfuric acid. The secret is in the peaks: a viscous layer of solution forms near the surface. concentrates there, and they dissolve faster than the valleys. microscopic level, and Ra drops, often by about half.

so electropolishing also improves corrosion resistance, and replaces passivation. Electropolishing gives a surface that's easy to clean, with less room for bacteria and residue. piping, medical devices, and vacuum and semiconductor systems. on corners and edges, and the burrs dissolve first. But the corners also get rounded.

It gives a bright, uniform look, and a smooth surface where less builds up. scratches, and it doesn't fix defects. It smooths what's there. A rough surface with a deep scratch comes out bright, with a bright scratch. So the surface is prepared first, by polishing or grinding, as required.

microns up to tens of microns, depending on time and current. With tight dimensions, this has to be accounted for in advance. And the removal isn't uniform: on corners and edges the current is stronger and removes more, and inside holes and internal corners the current is weak and removes less.

tubing, a special cathode is made that goes inside. current, and a mark will remain there. The drawing should state where that's allowed. pickling or grinding before passivation. Electropolishing removes it only if it's thin. surface during passivation, and pitting in free- machining stainless.

Rust spots, when the part wasn't cleaned well enough before passivation. the wrong place, and corners more rounded than allowed. And what drives the cost? Passivation is usually inexpensive, and it's done in batches. Testing and certificates per the standard add a fixed cost to every order.

racking and an electrical connection, and internal surfaces need a special cathode. A specific Ra requirement also means mechanical preparation before the process. And removing weld heat tint is priced as an extra step. Next chapter, conversion coatings: phosphating and black oxide.

4. Phosphating and Black Oxide

In chapter 0 we saw that a conversion coating doesn't apply a layer of foreign material. The surface layer of the part itself turns into a different compound. Today we'll meet two conversion coatings for steel: phosphating and black oxide. Both are thin and inexpensive, and both need a partner: oil, wax, or paint.

On their own, they give very little corrosion protection. The bath is a solution of phosphoric acid, with salts of metals like zinc, manganese, or iron. The acid lightly attacks the steel, and near the surface the acidity drops. Then insoluble phosphate salts precipitate, and grow on the surface as dense crystals.

The result is a gray, matte, porous layer that's firmly anchored to the metal. And the porosity is the advantage: paint grips it like an anchor, and oil soaks into it and stays. activation step that makes the crystals small and dense, the phosphating bath, rinsing, and finally oil or paint.

used mainly as a paint base, on paint lines for sheet metal parts. Zinc phosphate is the most common. As a thin layer, it's an excellent paint base, for example in the automotive industry. And as a heavier layer, with oil, it gives corrosion protection to fasteners and machined parts.

Manganese phosphate gives the thickest and darkest layer, gray to black. It's used for wear: gears, pistons, camshafts, and parts that rub during break-in. The layer holds oil and prevents galling. in grams per square meter, not by thickness. The thickness is usually a few microns.

manganese phosphate, and TT-C-490 for paint pretreatment. alkaline, oxidizing salts, at around 140 degrees Celsius. The iron at the surface turns into magnetite, a black iron oxide. The layer is very thin, about one micron, so it hardly changes dimensions.

That's why precision parts are blackened: tools, gauges, fasteners, and molds. The look is black, matte or glossy, depending on the part's finish before blackening. And most important: black oxide without oil gives almost no protection. After blackening, the part is dipped in oil or wax, and only the combination protects.

Hot black oxide, at around 140 degrees, is true black oxide: a conversion layer. Mid-temperature black oxide, a little above 100 degrees, is safer to work with. conversion, but a chemical layer of copper and selenium deposited on the surface. It's weaker, and used mainly for touch-up and appearance.

There are black oxide methods for steel, cast iron, and stainless steel. Aluminum has no black oxide. It's made black by dyed anodizing, which we'll meet in chapter 6. And the common standard is MIL-DTL-13924, with classes for steel and for stainless steel. Black oxide: about one micron, for precision parts and appearance, always with oil.

Iron phosphate: the thinnest of the phosphates, as a paint base. Zinc phosphate: a few microns, as a paint base, or for corrosion with oil. And manganese phosphate: the thickest and darkest, for wear and break-in. The rule: a precision part that only needs to be black and lightly protected gets black oxide.

phosphate, and a part that rubs gets manganese phosphate. The black oxide bath is a hot alkaline salt. joints later seeps out as a white powder, and starts corrosion. and zinc, so each part is blackened separately, before assembly.

steel there's also a risk of hydrogen embrittlement. As we Saw in chapter 1, a bake is done after the process. type of oil or wax, the standard and type, and for phosphate, the required coating weight. black oxide, when the bath is too hot or the concentration is wrong.

Uneven color from poor cleaning, and white powder from trapped salts. Rust, when there isn't enough oil or it's washed off. And in phosphate: coarse, uneven crystals, when the activation step is skipped. usually inexpensive, and are done in baskets and barrels, many parts at once.

Large or delicate parts need individual racking, and that adds cost. like salt spray and coating weight, and certificates. not be coated. That's less common, because the layers are thin. chromate, Alodine, and alternatives without hexavalent chromium.

5. Aluminum Conversion Coatings

oxide layer, a few nanometers thick, which protects it to some degree. suffer from corrosion much more than pure aluminum. The natural layer is also uneven, and paint doesn't bond to it well. thin, cheap, fast layer that gives corrosion protection and an excellent paint base.

And unlike anodizing, it keeps the surface electrically conductive. in or sprayed with an acidic solution of chromium compounds. and forms a gel-like layer of chromium and aluminum oxides. about one micron, and it hardly changes dimensions.

Its color is gold, greenish, or iridescent, and in other versions almost clear. The process is short: a few minutes, at room temperature. and Bonderite, and in industry it's also called chem film. In traditional chromate, the layer contains water- soluble hexavalent chromium.

moisture, and restores protection to the exposed spot. aerospace standard for years, and it's also why it's hard to replace. The main standard is MIL-DTL-5541. contains hexavalent chromium, and Type II has no hexavalent chromium.

And if no type is specified, the default in the standard is Type I. The Class tells you the function. Class 1A gives maximum corrosion protection, for a bare part or as a paint base. surfaces, and for enclosures that need electromagnetic shielding.

Both classes must pass 168 hours of salt spray, and Class 3 is also tested for electrical resistance. means: no hexavalent chromium, and electrically conductive. toxic and carcinogenic, and its use is more and more restricted. The main alternative is trivalent chromium, TCP, originally developed by the US Navy.

Today there are such processes approved for Type ll, in both classes. chromate, but they don't have the same self- healing. mainly as paint pretreatment, on industrial production lines. almost clear, so it's hard to see by eye whether the part was coated at all.

Some solutions add a faint tint especially for this. which removes the natural oxide layer and residue, rinsing, the chromate bath, rinsing, and drying. easily, so for about the first day the part is handled carefully.

about 60 degrees Celsius, because high heat dries out the gel and reduces protection. there are pens and brushes of solution for local touch-up. and only the grounding and contact surfaces get chromate. That requires masking, and the areas must be marked on the drawing.

machining after coating exposes bare aluminum, and needs local touch-up. coating, and not to touch the surface with bare hands. On high-copper alloys and castings the result is less uniform, and better preparation is needed. and class, which surfaces must be conductive, and which areas are masked.

off, when the layer is too thick or rinsing is insufficient. Uneven color or stains, from poor deoxidizing, or from differences between alloys. scratches from handling while the layer is still soft. And electrical resistance too high in Class 3.

And what drives the cost? The process itself is cheap and fast, and it's done in batches on racks. Masking for selective coating, like anodizing with chromate areas, adds cost. resistance, the certificates, and manual local touch-ups. Next chapter: regular and hard anodizing, dyeing, and sealing.

6. Anodizing

Anodizing is an electrochemical conversion coating. The part is connected to the positive pole, the anode, and that's where the name comes from. surface into aluminum oxide: a hard ceramic layer that's part of the metal itself. The layer grows out of the part: some of it inward, and some outward.

The result protects against corrosion, resists wear better, insulates electrically, and can be dyed. cells, and each cell has a pore, perpendicular to the surface. there's a thin, dense layer with no pores, called the barrier layer. dye goes into them, but if they aren't sealed, moisture and dirt get in too.

The main standard is MIL-PRF-8625. Type | is chromic acid anodizing: a very thin layer, up to a few microns. aerospace for sensitive parts. There are also versions without chromic acid. Type Il is regular anodizing, in sulfuric acid, and the most common: usually 5 to 25 microns.

It suits protection and appearance, and it's easy to dye. Type Ill is hard anodizing, or hard anodize. close to zero degrees, and at a higher current density. The layer is dense and thick, and the default in the standard is about 50 microns.

And its hardness is comparable to hardened tool steel. And the Class tells you whether the layer is dyed: Class 1 is undyed, and Class 2 is dyed. Only the part that grows outward changes the dimension. outward, and two thirds inward. In Type Ill, it's about half and half.

For example, 50 microns of hard anodizing adds about 25 microns per side. An outside diameter grows by about 50 microns, and a bore shrinks by the same amount. diameter changes by about 4 times the growth per side. In our example that's about 100 microns, and that's enough to stop a bolt from going in.

reduced in advance in machining, and threads are usually masked. Right after anodizing, while the pores are open, the part is dipped in dye. The dye enters the pores and stays inside the layer. Organic dyes give a wide range of colors, but some of them fade in sunlight.

salts is usually used: more durable, but in limited colors, like bronze and black. In Type Ill the layer is naturally dark, so it's dyed only in dark colors. In the last step the pores are closed, by sealing. nickel acetate, and for regular anodizing also dichromate, which gives a yellow-green tint.

Sealing locks in the dye, and greatly improves corrosion protection. unsealed, because sealing slightly reduces the layer's abrasion resistance. grounding and contact surfaces, chromate areas are left, as we saw in the previous chapter. when bent, so anodizing is done after bending, not before.

reduces fatigue strength, and in parts that see cyclic loads this must be taken into account. The 6000 series, like 6061, takes anodizing very well. castings, get a gray, less uniform, and sometimes thinner layer. And in hard anodizing they're also more prone to burning, meaning local damage from high current.

a sharp corner a gap forms in it, and it cracks and flakes. layer, and the standard has a table linking thickness to recommended radius. Inside a narrow, deep hole the layer is thinner, because less current and solution reach it. there will be no coating at the contact point. The drawing states where that's allowed.

into the bath: they burn, and contaminate the bath. They're installed after anodizing. unsealed, masked areas, and whether dimensions apply after coating. dark, damaged spot from locally excessive current. Uneven color between alloys, between batches, or from uneven preparation.

Dye that fades in sunlight or from poor sealing, and powder on the surface from incorrect sealing. because the growth wasn't calculated, and cracks at sharp corners and in parts bent after coating. more than regular: the bath is cold, the time is longer, and cooling takes energy. Masking of threads, grounding surfaces, and precision holes.

contact point, and complex parts need a special rack. Color matching between batches, and between parts in the same assembly. And tests and certificates: thickness, salt spray, and for hard anodizing, abrasion too. thickness, throwing power, and hydrogen embrittlement.

7. Electroplating

negative pole, so it's the cathode. That's the opposite of anodizing and electropolishing. The bath is a solution with ions of the plating metal, for example nickel, zinc, or copper. gain electrons, become metal, and build a layer, atom by atom. metal: it dissolves, and replaces the ions that were used up.

In other processes the anode doesn't dissolve, and the metal is added to the bath as a salt. here we really do deposit a layer of a different metal on the part. current and time. That's Faraday's law: more amps, more minutes, more metal. area, So it's important to know the surface area of the parts in the load.

But not all the current goes into metal: some of it splits water, and creates hydrogen on the part. and in hard chrome only a small part of it. That's one reason hard chrome is slow and expensive. And that same hydrogen released on the part will come back to us later in this chapter. prefers the shortest path: outside corners, edges, and protrusions.

The plating is thicker there, and inside slots, holes, and inside corners, it's thinner. On a long part it looks like a dog bone: thick at the ends, and thin in the middle. Each bath has a different throwing power: the ability to reach recessed areas. Alkaline baths throw relatively well, and hard chrome throws very poorly.

into slots and holes, and shields that hide edges from the current. Current thieves: pieces of metal near the edges, which draw the excess to themselves. And the right orientation and distance of the part on the rack. significant surfaces, the ones that matter for function or appearance.

On other surfaces the plating can be thinner, and without this definition, the part can't be inspected. Orders of magnitude: decorative chrome is tenths of a micron, over a thicker nickel layer. Zinc is usually a few microns up to tens, and hard chrome, tens to hundreds of microns. It's measured by XRF, with X-rays, which suits most coatings.

on steel, and eddy current suits coatings on a non- magnetic metal. And a microscopic cross-section is a destructive test, but the most accurate. before the bath the part is cleaned, rinsed, and activated in acid, which removes a thin oxide. On materials that are hard to plate, a thin, fast layer is applied first, called a strike.

Stainless steel gets a nickel strike, because its passive layer, from chapter 3, prevents adhesion. Aluminum first gets a zincate treatment, a thin zinc layer, and only then is it plated. And zinc die castings get a copper strike. and peels, sometimes only after heating or bending.

strength steel causes embrittlement, and the fracture appears late, under load. In electroplating the risk is greater: hydrogen forms on the part throughout plating. And coatings like zinc and cadmium seal it in, making it hard for it to get out. A bake is usually required for steels above about 1,000 megapascals, around 31 Rockwell C.

at least several hours, and for harder steels even a full day. And the bake must start within about four hours of the end of plating. machining, a stress relief bake is sometimes also done, before plating. are dedicated tests for hydrogen embrittlement. When in doubt, the product standard governs.

and sharp inside corners get almost none. Radii balance both. Blind holes and narrow slots get little plating, and trap gas and solution. Add a vent hole, or define them as non-significant surfaces. times the plating thickness, so specify a thread with plating allowance, or mask it.

surfaces, masked areas, and allowed contact points. And also the bake requirement, and whether dimensions apply after plating. Common problems: blisters and peeling, from poor adhesion. Burnt plating, dark and rough at the edges, from too high a current density.

contamination, and nodules from particles in the bath. Thin plating in recessed areas, and delayed fracture from hydrogen embrittlement. is more time in the bath, and in hard chrome the cost is especially high. Racking and fixturing, with special racks, auxiliary anodes, and shields.

Masking of threads, holes, and surfaces that must not be plated. Baking, with oven hours and timing tied closely to the end of plating. Tests and certificates: thickness, adhesion, salt spray, and hydrogen embrittlement. And for silver and gold, the price of the metal itself.

Next chapter: zinc, zinc-nickel, and cadmium, for corrosion protection, including in aerospace.

8. Zinc, Zinc-Nickel and Cadmium

sacrificially: it's more active, so it corrodes instead of the steel, even when there's a scratch. coating in the world for bolts, nuts, and small steel parts. It's cheap, and it protects well. The price is that the coating is consumed, and the thicker the layer, the longer it lasts. When the zinc starts to corrode, a white powder appears on it: white rust.

It's not yet rust of the steel, but it's a sign the coating is working, and being consumed. The common standard for zinc on steel is ASTM B633. The Service Condition sets the minimum thickness. SC1, for mild, indoor environments: 5 microns. SC2, for moderate environments: 8 microns.

SC3, for severe environments: 12 microns. And SC4, for very severe environments: 25 microns. and colored passivates based on trivalent chromium. Today, because of regulation, most work is done with trivalent passivates. are not electroplated with zinc at all, because of the risk of hydrogen embrittlement.

added on top: a thin conversion coating, as we saw in chapters 4 and 5. It slows down white rust. added, which further improves protection, and sometimes friction too. white rust appears, meaning until the zinc starts to corrode,

itself is exposed. The specification should define both. Zinc-nickel is an alloy of zinc with nickel, usually 12 to 16 percent nickel. The protection is still sacrificial, but the alloy corrodes much more slowly than regular zinc. Corrosion protection lasts several times longer, and it holds up at higher temperatures too.

system, and in aerospace and defense, as the main replacement for cadmium. The standards are ASTM B841 for general industrial use, and AMS 2417 for aerospace. little hydrogen, Low Hydrogen Embrittlement, for high-strength parts. aerospace fasteners and connectors for many years.

It gives good sacrificial protection, even as a thin layer and in a marine environment. cadmium bolt in an aluminum part causes almost no galvanic corrosion. galling, and a consistent relationship between tightening torque and clamp force. And it's easy to solder to, and it conducts electricity.

Type II with chromate, and Type III with phosphate, as a paint base. 13 microns. Class 2, 8 microns. And Class 3, 5 microns. is toxic and carcinogenic, and its use is very restricted. And in aerospace it's gradually being replaced by zinc-nickel.

protection is moderate, and it's less good with heat. heat, and has reasonable galvanic compatibility with aluminum, but it costs more. aerospace, especially against aluminum and on fasteners, but it's very restricted by regulation. The rule: in general industry, choose zinc with a trivalent passivate.

aluminum, choose zinc-nickel. And cadmium, only when the specification or the customer requires it. diameter changes by about 4 times the coating thickness. bolt plated after manufacture, check that the thread still fits. bake after plating. Zinc and cadmium seal in the hydrogen, so the bake must not be delayed.

temperatures, and near engines and exhausts, zinc-nickel is better. standard, thickness or service condition, the passivate or Type, and a sealer if needed. And also the bake requirement, and the salt spray requirement, for white rust and for red rust. Common problems: early white rust, from a weak or damaged passivate.

areas, and blisters and peeling from poor preparation. embrittlement, and uneven passivate color between batches. cheapest, and bolts are plated in a barrel, thousands at once. Zinc-nickel costs more. Cadmium is especially expensive, and few plants are still licensed to work with it.

Baking, sealing, and long salt spray tests add cost. So does racking large or delicate parts that can't go into a barrel. Next chapter: nickel, hard chrome, silver, and gold.

9. Nickel, Hard Chrome, Silver and Gold

Unlike zinc, nickel, chrome, silver, and gold are more noble than steel. They don't protect sacrificially, but as a barrier. a corrosion site, and sometimes the corrosion there is even faster. So with these coatings, the thickness, density, and integrity of the layer are critical.

corrosion: hardness, wear resistance, conductivity, solderability, and appearance. Nickel is the workhorse of electroplating. Watts nickel is the classic bath: a light gray layer, fairly hard, with good protection. Nickel sulfamate gives a layer with very low internal stress.

It suits thick layers, rebuilding the size of worn parts, and aerospace parts. decorative plating. Its brightness is what gives the thin chrome on top its familiar look. barrier that stops metals from migrating into each other, for example under gold on copper. And two notes: electroplated nickel is magnetic, and some people are allergic to it on skin contact.

The AMS 2460 standard divides chrome into two classes. Class 1 is decorative chrome: a very thin layer, tenths of a micron, over nickel. resistance, and the corrosion protection comes mainly from the nickel underneath. thick layer, usually tens to hundreds of microns, directly on the steel.

It gives very high hardness, wear resistance, and low friction. Hard chrome has a network of micro-cracks. They hold oil and improve lubrication, but moisture can reach the steel through them, so hard chrome's corrosion protection is limited. gear, molds, and for rebuilding the size of worn parts.

at the edges. So it's plated oversize, and then ground to size and finish. Hard chrome lowers the fatigue strength of the steel. So on parts under cyclic loads, shot peening is done before plating, as we saw in chapter 2. low current efficiency, and a lot of hydrogen is produced.

On high-strength steels, baking is mandatory, and within about four hours of the end of plating. which is why alternatives are sought, like thermal spray, which we'll meet in chapter 16. Silver is the best electrical conductor of all metals. It's used on contacts and connectors for high currents, and in RF components and waveguides.

There the silver acts as a lubricant, and prevents seizing. sulfur compounds, and the black layer spoils the appearance and some contacts. Sometimes it's protected with a thin gold layer, or an anti-tarnish treatment. And a nickel or copper strike is usually applied under the silver, so it bonds well to the part.

Gold doesn't oxidize at all, so it keeps a low, stable contact resistance for years. signals, and on circuits and electronic components, including gold wire bonding. nickel, resists wear, for connectors that are mated and unmated many times. And soft, pure gold is used for soldering and wire bonding.

migrates into the gold over time, and degrades the contact. 45204 in defense. They define purity, hardness, and thickness. micron to a few microns, because every micron of gold costs money. intact, so sometimes several layers are applied, for example copper, nickel, and chrome.

Each layer closes the pores of the one before. With hard chrome, plan a grinding allowance, and define the final dimension after grinding. And on parts under cyclic loads, add shot peening before plating. areas are plated, to save cost. The rest is masked, or plated by selective methods.

requirement, and whether dimensions apply after grinding. Common problems: corrosion through pores in a barrier layer that's too thin. poor preparation, and fatigue failure in a chromed part without shot peening. Tarnished silver, and gold that loses contact, when there's no nickel layer under it.

time in the bath, grinding after plating, baking, and shot peening. For silver and gold: the price of the metal itself, so thickness and area are critical. Selective plating and masking, to save precious metal. And testing: thickness, hardness, adhesion, porosity, and hydrogen embrittlement.

Next chapter: electroless nickel.

10. Electroless Nickel

current decides where the layer is thick and where it's thin. reaction takes place at every point the solution touches. everywhere: on outside corners, inside holes, in slots, and in threads. That's its big advantage. bodies, hydraulics, assemblies with internal passages, and parts with tight tolerances.

The solution contains nickel ions and a reducing agent, usually hypophosphite. electrons to the nickel, and the nickel deposits on the part as metal. Some of the phosphorus enters the layer along with it. part is in the solution. That's why the process is called autocatalytic.

not pure nickel, and the phosphorus content determines almost all of its properties. microns per hour, so it's controlled by time in the bath. harder layer as plated, magnetic and easy to solder, but less corrosion resistant. common: a balanced combination of hardness and protection.

almost amorphous structure, with no grain boundaries. It gives the best corrosion protection, and it's non- magnetic, as long as it isn't heated. phosphorus. For wear, low or medium phosphorus, usually with heat treatment. The common standard is ASTM B733, and in aerospace AMS 2404 is also used.

to 3 percent. Type Ill, 2 to 4. Type IV, 5 to 9. And Type V, 10 and above. Type | has no phosphorus requirement. SC1, 5 microns. SC2, 13. SC3, 25. And SC4, 75 microns. The Class sets the heat treatment after plating: Class 1 none, and Class 2 a hardening treatment.

The other classes are bakes to improve adhesion and to remove hydrogen. means: high phosphorus, at least 25 microns, and no heat treatment. Note: the classes of AMS 2404 are not the same as those of B733. And the words electroless nickel alone, with no standard, define nothing.

As plated, electroless nickel is fairly hard. layer crystallizes, and hard nickel-phosphorus compounds form. and in Class 2 of B733, the requirement is at least 850 HK. The price: the hardened layer becomes more brittle, and its corrosion resistance drops.

And high phosphorus loses its non-magnetic property. So don't harden at high temperature when the part mainly needs corrosion protection. And here too hydrogen is produced, so high- strength steels are baked, as in chapter 7. On steel, electroless nickel bonds well after normal preparation.

electroplating, and for some alloys a bake after plating improves adhesion. On stainless steel, an activation or strike is needed, because of the passive layer. And small particles can be co-deposited in the layer, to create composite coatings. carbides or diamond give extreme wear resistance.

outside diameter grows by twice the thickness, and every bore shrinks by twice the thickness. times the thickness, and long internal bores get the same thickness, which is an advantage. Areas that must not be coated need masking, and in internal bores that's hard. And like any coating, the layer copies the surface underneath. It doesn't hide scratches and defects.

And the drawing should state the standard, Type, thickness or SC, and Class. bake requirement, and whether dimensions apply after plating. preparation, especially on aluminum and stainless steel. Porosity and small pits, from contamination or particles in the bath.

Thickness out of spec, when the deposition rate changed and the time in the bath wasn't corrected. Cracks in a hardened layer, under bending or load, and hydrogen embrittlement in hard steels. electroless nickel bath is consumed, and needs close control and replenishment. the bath, and masking, especially on internal surfaces.

Heat treatment and baking, and tests of thickness, phosphorus, hardness, adhesion, and porosity. saves grinding and machining after plating, so electroless nickel often pays off, despite the price. Next chapter: hot-dip galvanizing.

11. Hot-Dip Galvanizing

In galvanizing, a whole steel part is dipped in a bath of molten zinc, at about 450 degrees Celsius. The zinc in the bath is almost pure: more than 98 percent. the layer is several times thicker, usually tens to more than a hundred microns. And it's metallurgically bonded to the steel.

because the layer is so thick, it lasts for decades outdoors. You see it on steel structures, guardrails, light and power poles, bridges, rebar, and large bolts. The layer isn't uniform in composition: during dipping, zinc and iron diffuse into each other. zinc-iron alloy, with decreasing iron content, and on top of them a layer of almost pure zinc.

scratches and knocks during transport and assembly. The top layer is softer and more ductile, and it absorbs impacts. adhesion is a metallurgical bond, not just a mechanical grip, as with paint. preparation, as we saw in chapter 1: degreasing, rinsing, acid pickling, and rinsing.

and ammonium salts, which cleans off remaining oxide and protects the surface until dipping. reaches bath temperature, and the alloy layers form. drained off, and it's cooled in air or water. Sometimes a passivate is added. electroplating. Thickness is set mainly by the steel itself: its thickness, and its chemical composition.

Silicon and phosphorus in the steel strongly affect the reaction with zinc. layer grows very fast, and comes out thick, gray, dull, and sometimes brittle. This is called the Sandelin effect. 0.04 percent, or between 0.15 and 0.25 percent, and phosphorus below 0.04 percent.

different between the parts. That's not a coating defect, but a result of the composition. Thickness sets the protection life, not the appearance. bolts, hardware, and small parts that are centrifuged. And in Europe and worldwide, ISO 1461.

with no maximum. And the required thickness rises with steel thickness. millimeters the minimum average is 85 microns, and for steel 3 to 6 millimeters thick, 70 microns. proportional to layer thickness: a double layer lasts about twice as long. The most critical part of the design is venting and draining.

Molten zinc has to get into every cavity and out of it, and the air has to escape. A closed cavity with trapped air or moisture is a real explosion hazard in a 450 degree bath. structure must have vent and drain holes, in the locations and sizes recommended by ASTM A385. 1, and large overlaps need venting, or a seal weld all around.

Dipping at 450 degrees creates thermal stresses, and thin, long, or asymmetric parts can distort. symmetric design, help, and ASTM A384 deals with this. steels, are prone to embrittlement after galvanizing. ASTM A143 gives guidance. The layer is too thick for regular threads.

Bolts are galvanized, then spun in a centrifuge to throw off the excess zinc. galvanizing. The internal thread is left without zinc, and the galvanized bolt protects it. won't take on it, so welds are cleaned mechanically before galvanizing. extend the life of the system, or for appearance. That's a duplex system.

the paint will peel. We'll meet this in the paint chapters. Common problems: bare spots, from poor preparation or from slag. A thick, gray, brittle layer from reactive steel, and distortion of thin parts. Drips and excess zinc at corners and in holes, from poor drainage.

And white rust on parts stacked wet, before they had time to dry. priced by weight, and it's relatively cheap per ton, so it pays off very well for structures. A part that doesn't fit the bath in one dip needs a double dip, and that adds cost. So does extra work: vent holes, cleaning welds, and straightening after distortion.

for nuts, and duplex, which adds a whole paint step. Next chapter, we move into the world of paint, starting with the principles.

12. Paint Principles

Paint is an organic layer that protects mainly as a barrier, as we saw in chapter 0. It has four main components. everything together. It sets most of the properties: chemical resistance, UV resistance, and flexibility. some also protect against corrosion, like zinc ina primer.

and spray, and then evaporates. And the additives improve flow, drying, and stability. shrinks: the wet film is thicker than the dry film that remains at the end. film remains. Fast and simple, but a suitable solvent can dissolve the paint again. reacts with oxygen in the air, and the process is slow.

polyurethane, a base and a hardener are mixed, and they react to form a network. This is the most common group in heavy-duty industrial coatings. and we'll meet them in the chapters on industrial paint and powder coating. With two-component paint, there's a limited working time after mixing, the pot life.

And between coats there's a recoat window. If you miss it, the next coat doesn't bond well. In industrial coating, a single coat is almost never applied. A system is built, and each coat has a job. for example a zinc-rich primer, which protects sacrificially, like galvanizing. barrier, and lengthens the path of moisture and ions to the steel.

And the topcoat protects against sunlight and the environment, and gives the color and gloss. Why not one thick coat? Thinner coats cure better, and have fewer defects. And a pore in one coat almost never lines up exactly with a pore in the coat below it. A familiar example: epoxy primer, epoxy intermediate, and polyurethane topcoat.

breaks it down and it chalks. Polyurethane resists sunlight, so it goes on top. what remains after the solvent evaporates, and it determines the protection. Dry film thickness equals wet film thickness, times the volume solids percentage. solids, to get 100 microns dry, you apply 200 microns wet.

with a simple comb gauge, so the painter knows right away if it's in range. gauge: magnetic on steel, or eddy current on aluminum, at many points, per a standard. too thick can crack, sag, or not cure fully. So the specification has a minimum, and a maximum too. The main standard for protective paint on steel is ISO 12944.

C1, a heated, clean building interior, and C2, rural, with low pollution. C3, urban, or coastal with low salt, and C4, industrial, or coastal. with high salt, and CX, offshore and extreme conditions. major maintenance: low, up to 7 years, and medium, 7 to 15 years.

High, 15 to 25 years, and very high, more than 25 years. And that's not a warranty period, but an estimate for maintenance planning. harsher the environment and the longer the durability, the thicker the system. chapters 1 and 2 we saw the cleanliness grades and the surface profile.

Paint on an unprepared surface will fail, no matter how good it is. If the steel is colder than the dew point, a thin film of moisture forms on it, sometimes invisible. degrees above the dew point, and within the paint maker's temperature and humidity limits. and at the right time, and respect the drying times and the recoat window.

tape is applied and pulled off, and you check how much paint came off. pulled with a measured force, to see at what force, and in which layer, it fails. Thickness and appearance are also checked, and salt spray or humidity, as required. each coat its type, color, and minimum and maximum dry thickness.

category and required durability, application conditions, and tests. specification is sometimes enough, instead of writing it all out. Common problems: peeling from poor surface preparation, or from a missed recoat window. Blisters from moisture or salts under the paint, and sags and cracks from paint that's too thick.

corners and edges, and chalking and fading of epoxy in sunlight. which in industrial painting is often the largest part of the cost. The number of coats, because every coat is labor, material, and drying time. pays off, because maintenance in the field costs much more.

edges, which get an extra stripe coat by brush, because that's where paint is thin. Next chapter: liquid paint. Conventional spray, airless, electrostatic painting, drying, and baking.

13. Liquid Paint

In spray painting, not all the paint reaches the part. Some of it disperses into the air as mist, and is caught in the spray booth filters. The percentage that stays on the part is called transfer efficiency. is created, and how much solvent is released into the air.

and test method, so we'll compare the methods mainly in relative order. In conventional spray, compressed air in the gun's air cap breaks the paint into very fine droplets. and transfer efficiency is the lowest of all the methods. pressure is limited, by the common definition, to 10 psi, about 0.7 bar.

The cloud is softer, less paint bounces off, and transfer efficiency goes up. In many places, environmental regulations require HVLP or a more efficient method. Both methods are used for fine finishing, metallic colors, small parts, and touch-ups. In airless, no air is used to atomize the paint.

usually hundreds of bar, through a tiny tip, and the paint breaks into droplets because of its speed. in one pass, and the method suits thick, high- solids paints. It's the main method for painting structures, ships, and tanks. The drawback: the finish is less fine, and thin coats are harder to control.

a little air at the cap shapes the fan, and the finish is finer. cause serious injury. Never point it at the body, and never clean the tip by hand. In an electrostatic gun, the droplets get an electric charge, and the part is grounded. around it and reach the back side. This is called wrap-around.

That raises transfer efficiency a lot. rotary bell is used, which spins very fast and atomizes the paint by centrifugal force. Together with an electric charge, it's the most efficient method. grounded, and the rack must be clean of old paint, or the attraction weakens.

electric field doesn't get in. This is called the Faraday cage effect. inside, and there you need adjustment, or manual touch-up spraying. the solvent to evaporate, before the next coat or before the oven. This is called flash-off. Without the wait, solvent trapped under the surface bubbles out, and leaves small holes.

and mild heating, of a few tens of degrees, shortens their drying time. usually between 100 and 200 degrees Celsius, per the paint specification. withstand the temperature, and not every part fits in an oven. itself, not just the oven temperature. A thick part heats slowly.

paint mist away from the operator and the part, and the filters capture it. Clean air also matters for the finish, because dust sticks to wet paint. and caps: threads, grounding surfaces, sealing surfaces, and precision holes. Masking is manual work, and it's often a large part of the time.

is explosion-proof, and the operator wears a suitable respirator. component polyurethane, respiratory protection is especially critical. orange. The cause: poor atomization, or paint that's too thick. Sags form from a coat that's too thick, or a gun held too close.

dry before they reach the part, because the gun is too far, or the solvent is too fast. in the oven, when heating too fast or without a flash-off. contamination on the surface, which repels the paint. This takes us back to chapter 1, to cleaning. And dust and fibers come from an unclean booth, or clogged filters.

moderate efficiency, and suit small parts and quality finishes. areas and structures. Electrostatic and rotary give very high efficiency, for production lines. usually the largest manual task, and paint lost to the filters when transfer efficiency is low. because every coat is another pass through the booth, and baking: oven, energy, and time.

and cosmetic requirements, which need a clean booth, a skilled operator, and inspection. Next chapter: powder coating.

14. Powder Coating

Powder coating uses no solvent at all. The paint comes as a dry, fine powder. Every particle already contains all the ingredients: resin, hardener, pigments, and additives. goes into an oven. There the powder melts, flows, and cures into a continuous film. powder that didn't stick can be collected and reused.

The powder is sprayed with an electrostatic gun, as we Saw in the previous chapter. The particles are charged, the part is grounded, and the powder clings to it even before baking. In a corona gun, a high-voltage electrode charges the powder. It's the most common type. weaker, but the powder gets into inside corners better.

deep inside corners, because of the Faraday cage effect. It's a known limitation of powder coating. is heated, and dipped in powder floating on a stream of air. layer, hundreds of microns, mainly with thermoplastic powders. With powder, whatever doesn't stick is drawn off, and collected in a cyclone and filters.

The collected powder is mixed back in with fresh powder, in small amounts. So material utilization is very high, much higher than with liquid spray painting. has to be cleaned, and sometimes a separate booth is kept for each color. That's why powder coating is especially suited to runs in a single color.

layer, then cures by a chemical reaction, into a network that won't melt again. ten to twenty minutes, and there are also low- temperature powders. moment the part itself reaches temperature, not from when it enters the oven. A thick part heats slowly, so it has to stay in the oven longer.

adhesion and flexibility. And over-cure changes the color, and causes yellowing. The typical thickness is about 60 to 80 microns in one coat. A very thin coat is hard to achieve with powder, so it's less suited to tight dimensions. resistance, but sunlight breaks it down. It's used indoors, or as a primer.

for outdoor use, for gates, railings, and aluminum profiles. Hybrid powder, epoxy-polyester, is intended for indoor use, and yellows less than pure epoxy. And polyurethane gives a smooth, durable finish, for outdoor use. powder primer is applied first, with a topcoat over it. Here too, as in chapter 12, it's a system.

Powder coating only works well on a well- prepared surface. in chapters 4 and 5, and in other cases it's blasted before coating. release gases in the oven. The gases bubble through the powder, and leave small holes. coating, or special powders for outgassing materials.

grounding, ventilation, and safety standards for spray booths are a must. point, so a hanging hole is designed somewhere out of sight. masking must withstand the heat: special tape, and silicone plugs and caps. surfaces need masking, or re-machining after coating.

And electrical grounding surfaces are masked, as with any paint. work, so a radius is recommended, and openings as wide as possible. color, thickness, pretreatment, masked areas, and tests. coat or insufficient flow, and small holes from gases released by the material.

Poor coverage in inside corners, under-cure or over-cure, and peeling from poor pretreatment. which is manual work, and color changes, which require cleaning the recovery system. The oven, in energy and time, and thick parts need a longer time. coated at all, and field touch-ups are hard, because powder needs an oven.

with almost no solvent and high material utilization, but it needs an oven, and fixed colors. touch-ups, and gives thin coats, but it contains solvent, and has more waste. Next chapter: electrocoat, or e-coat.

15. Electrocoat (e-coat)

water-based paint, and direct current is passed through it. It works like electroplating, but with paint instead of metal. particles are positively charged, are drawn to it, and settle on the surface. The voltage is usually a few hundred volts, and it sets the film thickness.

layer, and it's also common on metal parts for agriculture, appliances, and metal furniture. The secret is that the film being built is an electrical insulator. drops, and the current moves on to areas not yet coated. corners, and internal surfaces that spray can't reach.

electroplating in chapter 7. But here the film limits itself, so the thickness is very uniform. doesn't get in the way like that, because the part is fully immersed in the solution. and a conversion coating, usually zinc phosphate, as in chapter 4. E-coat is very sensitive to the surface, and without good pretreatment, the protection suffers.

In the bath itself, the part stays for a few minutes, under voltage. filtered from the bath itself, by a fine filtration called ultrafiltration. paint is used: transfer efficiency is above 95 percent. Celsius part temperature, for about 15 to 20 minutes.

much thinner than powder, but uniform and complete, even inside. benchmark for corrosion protection, but like any epoxy, sunlight breaks it down. layer, with a surfacer and topcoat over it. On internal surfaces, it stands alone. light colors. It's used when the e-coat is the final layer, for example on black exterior parts.

cathodic is much more common, because it protects better against corrosion. uniform thickness, and very high material utilization. The paint is water-based, with little solvent, and the line is fully automatic, at a high rate. not always enough for a harsh environment or for appearance.

and the whole bath is one color, so the color can't be changed easily. coat suits high-volume production, not individual parts. And here too, baking requires a part that can withstand the heat. galvanizing in chapter 11, drain and vent holes are needed.

without paint, and paint and rinse water don't drain out. rack, and there's no paint at that point, so it's placed somewhere unimportant. Very tight joints and very narrow slots get little paint, or none at all. the whole part is immersed. Usually plugs are used, or the area is machined after coating.

thickness, pretreatment, whether a topcoat goes over it, allowed contact points, and tests. Common problems: unpainted areas in cavities, from an air pocket or missing drain holes. A film that's too thin, from low voltage or an unbalanced bath. bath or rinse, and small holes from gases, or from oil that wasn't cleaned off.

Weak adhesion and corrosion resistance, from poor pretreatment. And runs of unrinsed paint, at holes and folds, from poor drainage. line, which only pays off at high volumes, and the pretreatment, which is another long row of tanks. keeping the paint quality up, the baking, and a topcoat, if needed.

coat is among the lowest there is, thanks to the high utilization and automation. coat gives a thin, uniform film that reaches everywhere, in mass production. Next chapter, we start on advanced coatings, with thermal spray.

16. Thermal Spray

gas stream throws it at high speed onto the surface. these splats, one on top of another, build the coating. decomposing can be sprayed: metals, alloys, ceramics, and carbides in a metal binder. several millimeters, so thermal spray is also used to restore the size of worn parts.

The big difference from welding: the material melts in the gun, not on the part. temperature, usually up to about 150 degrees Celsius. So there's no distortion, no change to the part's heat treatment, and no heat-affected zone. But adhesion is mainly mechanical: the splats grip the roughness of the surface.

then blasting to create a profile, as we saw in chapter 2. too, so it's cooled and the temperature is controlled. It's simple and cheap, but the coating is more porous, and adhesion is weaker. between them melts them, and compressed air throws the droplets.

method is very common for zinc and aluminum, for corrosion protection of structures and ships. arc becomes a very hot plasma, which melts even ceramics with a very high melting point. That's how ceramic coatings and thermal barriers are made, for example in turbines. combustion accelerates the particles to supersonic speed.

coating is very dense, with porosity below one percent, and strong adhesion. chrome: very hard layers, with very high wear resistance. example on landing gear, hydraulic pistons, and shafts. No hexavalent chromium bath, no hydrogen from the process, and usually better wear resistance.

grinding, usually with a diamond wheel, because the layer is so hard. The coating is built from splats, not from crystals that grew out of the part. much porosity there is, how many oxides, and how well the splats bonded to each other. denser coating, and slow processes, like flame, give more porosity.

through them to the part, so the coating is sometimes sealed with a sealer. And adhesion has a limit: under a strong point load, or impacts, the coating can spall. cavities are hard to coat, and sometimes it's impossible. surface, usually tens of centimeters, and whatever doesn't fit that distance doesn't get coated.

At sharp corners and edges the coating is weak and tends to spall, so a radius or chamfer is used. And HVOF and plasma need a soundproof booth, dust extraction, and robotic gun motion. is machined first, to the coating depth plus a grinding allowance. At the ends of the recess a taper is made, not a square shoulder, so the coating won't spall.

shields, and whatever isn't coated must be protected from both blasting and spraying. is defined, and the coater adds whatever is needed on top. for example HVOF with WC-Co, final thickness and coated area, finish, and hardness. and tests: adhesion, porosity, hardness, and thickness.

Common problems: spalling, from poor surface preparation or sharp corners. Cracks from stresses in too thick a coating, or from accumulated heat. unsuitable process, and corrosion under the coating, through connected pores, without sealing. And material: carbide and ceramic powders are expensive.

process qualification and test pieces in every batch. But restoring an expensive worn part often costs much less than a new one. Next chapter: PVD and CVD, very thin coatings, built atom by atom.

17. PVD and CVD

or molecule by molecule, and the material reaches the part as a vapor. microns: much thinner than any coating we've seen so far. titanium nitride are several times harder than hardened steel. The main uses are cutting tools, molds, punches, and wearing parts, and also decorative coating.

turned into vapor in a vacuum chamber, for example titanium or chromium. This is done with an electric arc, or by ion bombardment, called sputtering. fed into the chamber, for example nitrogen, and a ceramic layer forms, like titanium nitride. about 200 to 500, depending on the coating, and there are also lower-temperature processes.

rotate on a fixture, so all sides get coated. Deep holes and narrow cavities get less. long-established and very common, for tools and general use. withstand higher temperatures, and suit high- speed and dry cutting. friction, and common on plastic injection molds and forming tools.

TiCN is harder than TiN, for cutting and forming. temperature, for wearing parts in engines and assemblies. evaporated. Gases are fed into a reactor, react chemically on the hot part, and leave a layer on it. The temperature is high: usually around 900 to 1,050 degrees Celsius.

sight problem, and adhesion is very strong, thanks to the temperature. And the layer can be thicker, usually a few to more than ten microns. re-hardening after coating, and there's a risk of distortion. carbide, mostly on cutting inserts, which can withstand the temperature.

And there's also PACVD, a plasma version, at a lower temperature. hardened tool steels, molds, and tools that are recoated. reaches everywhere, with strong adhesion. Suited mainly to cemented carbide inserts. tempering temperature gets PVD. Cemented carbide inserts for heavy cutting usually get CVD.

surface stays rough, so molds and wearing parts are polished before coating. the chamber there's thorough cleaning, and inside the chamber there's also ion cleaning. cutting tools the edge is slightly honed before coating. support the coating. A very hard layer on a soft base cracks, like an eggshell.

dimensions don't need to change, and it's only accounted for on very tight fits. And PVD-coated tools can be stripped, reground, and recoated. example TiAIN or a specific type of DLC, and often the coater's trade name. coat and to mask, for example no coating on threads or mating surfaces.

Preparation: polishing and finish before coating, and edge honing on tools. The base material and its heat treatment, so the coater can confirm the temperature is suitable. indentation test that looks at the cracks around the indent, and hardness. insufficient cleaning or too sharp an edge, and coating cracking, from too soft a base.

Softening or distortion of the part, from a process temperature too high for the material. uncoated areas, because of line of sight, or unsuitable rotation and fixturing. hours of vacuum, heating, and cooling, and the price depends on how many parts fit in a batch. Fixtures and masking for each part shape, preparation and polishing, and edge honing.

For CVD on steel, re-hardening after coating, and special and multilayer coatings cost more too. But a coated tool often lasts several times longer, so the coating usually pays off very well. Next chapter: special coatings, like PTFE, dry film lubricants, and more.

18. Special Coatings

corrosion, hard coatings, and coatings for appearance. friction and galling where you can't lubricate, materials that stick to the surface, and corrosion protection for high-strength bolts, without the risk of hydrogen embrittlement. known by the trade name Teflon, dry film lubricants, and zinc flake coatings.

Teflon: a polymer in which a carbon chain is completely wrapped in fluorine atoms. Its friction is very low, almost nothing sticks to it, and it resists almost every chemical. It works over a wide temperature range, up to about 260 degrees Celsius in continuous service. The drawback: PTFE is soft, and wears easily under load and rubbing.

temperature, close to 400 degrees, to fuse into a film. lower temperature, and resist wear better. They're known, for example, by the trade name Xylan. and on fasteners that need to be assembled and disassembled without galling. PTFE can be combined into a hard coating, giving both hardness and low friction.

are pores. They're filled with PTFE, giving a hard, durable surface with low friction. And in electroless nickel, as we saw in chapter 10, PTFE particles can be dispersed in the layer. The hard coating carries the load, and the PTFE provides the lubrication. A dry film lubricant is a thin layer of solid lubricant, in a resin binder that holds it on the surface.

slide over each other, like a deck of cards, and it carries high loads. Graphite withstands high temperatures, and PTFE gives low friction and prevents sticking. The film is very thin, usually about 5 to 13 microns, so it hardly changes dimensions. allowed: in vacuum, in space, in dusty places, and at extreme temperatures.

rubbing wear, fretting. In aerospace, the well- known standard is AS52772. surface that provides grip, like light blasting or phosphating, as in chapter 4. the data sheet. There are also air-drying types, for field repair. On internal surfaces, like internal threads and bores, it's hard to spray a uniform film.

longer, and a hard base extends the life of the film. and standard, thickness, areas to coat, preparation, and baking temperature and time. and aluminum in a binder, and is applied without electric current. The zinc protects sacrificially, as in chapter 8, and the flakes also form a barrier.

It's applied by dip-spin, dipping and spinning ina basket, or by spraying, and then baked in an oven. source of hydrogen embrittlement, and it suits high-strength bolts. and salt spray results are especially good, usually better than electroplated zinc of similar thickness. The standard for fasteners is ISO 10683, and well- known trade names are Geomet and Delta-Protekt.

PTFE solves sticking, friction, and chemicals, but it's soft, and needs a high bake temperature. without oil, but it wears over time, and it's hard to apply evenly inside. And zinc flake solves corrosion on bolts without hydrogen, but its thickness affects the thread. thread that's multiplied by about 4. So plan clearance, or define a maximum thickness.

the part, and any coating already on it, can withstand the temperature. cleaning or preparation, and a film that wore quickly, from too high a load or a soft base. and incorrect baking, which leaves a soft film or damages the part. And missing coverage in internal threads and bores.

masking when only certain areas are coated, and baking. testing of thickness, friction, adhesion, and salt spray. would otherwise require changing the whole design. sure the coating was really done the way we asked.

19. Testing and Standards

A good coating and a bad coating often look the same. enough, whether the layer adheres, or whether hydrogen got into the steel. So every requirement on the drawing needs a test method, and an acceptance criterion. measured, at how many points, and what passes, turn it into a requirement you can verify.

adhere? How well does it protect? And did it harm the part? coating on steel, like zinc, paint, or chrome. It's fast, cheap, and common, per ASTM B499. anodizing or paint on aluminum. For anodizing, the standard is ASTM B244. together, like gold over nickel. The standard is ASTM B568.

thickness, and many measurement points, because thickness varies across the part. polished, and the thickness is measured directly under a microscope. and defects. It's the reference method, but the part is destroyed. The standard is ASTM B487. thickness is calculated from the time, per ASTM B504.

often used: pieces coated together with the batch, which are the ones cut. at the edge, burnishing, heat and quench, and tape. In all of them, you check that the coating doesn't peel or blister. much paint came off, per ISO 2409 or ASTM D3359.

and in which layer it fails, per ISO 4624 or ASTM D4541. And for thermal spray, there's a pull test on a defined specimen, ASTM C633. The most common test is neutral salt spray, per ASTM B117 or ISO 9227. The panels sit in a salt fog, at a constant temperature, for a set number of hours.

what counts as failure: white rust, red rust, blisters, or peeling. notes that salt spray results alone rarely correlate with service life in the field. It's a test for comparison and quality control, not a prediction of how many years the coating will last. humidity, and sometimes UV radiation too, are closer to field conditions.

coating itself is measured, Vickers or Knoop, at a low load, on a cross-section, per ASTM E384. That's how hard chrome, heat-treated electroless nickel, and hard anodizing are checked. For hydrogen embrittlement, ASTM F519 tests the process itself. along with the parts, and are then held under a sustained load, usually for 200 hours.

that plate high-strength steels run this test regularly. chemical tests that show where pores reach the base metal. happens when one fails. ASTM B602 gives guidance. requirements, and often attaches measurement results and a documented bake.

And on a new part, a first article is inspected in depth before a production run is approved. and class, thickness and measurement method, and significant surfaces for inspection. acceptance criteria, hydrogen testing and baking if relevant, and certificates. method, like a good coating, or corrosion resistant. That can't be verified.

recess, and an uncalibrated gauge, or one meant for a different base material. and destructive tests on an expensive part, instead of witness samples. hydrogen test, and destructive tests that sacrifice parts. plus first article inspection and process qualification.

But testing is much cheaper than a failure in the field. selection. How to choose the right coating, and specify it correctly on the drawing.

20. Design and Selection

The most common mistake is choosing a coating out of habit: we always put zinc here. The right way starts with the question: what does the part need? outdoors, near the sea, or in contact with chemicals? Does it wear, rub, or gall? Does appearance matter? And what temperature does it work at?

Each answer narrows the list, and only then do you choose a coating. On ordinary steel, almost everything works: zinc, galvanizing, paint, and nickel. hydrogen embrittlement, as we saw in chapters 7 and 8. sometimes a hydrogen-free coating is better, like zinc flake, or thermal spray.

On stainless steel, passivation is usually enough, and if a coating is needed on top, it needs a strike. anodizing, or paint, and electroplating directly on it is almost impossible without zincate. material can soften in CVD, galvanizing, or powder coating. Blasting, thermal spray, and PVD need line of sight, and don't reach deep holes.

everywhere, so they're better suited to complex shapes. oven, and sometimes only liquid paint or thermal spray is left. uniform coating, like electroless nickel, black oxide, or PVD, or plan an allowance and grinding. suit an automated one, like e-coat, powder, or barrel plating.

restricted, so where possible, choose the alternatives. moisture between them, the more active metal corrodes. We saw this in chapter 0. aluminum part gets a suitable coating, like cadmium or zinc-nickel. And the parts can be isolated with paint, a gasket, or a compound.

conductive coating, like chromate Class 3, and on the drawing they're marked separately. And coat before assembly. Inserts and pins are installed after coating. passivation per ASTM A967, and sometimes electropolishing. per MIL-PRF-8625, and in the grounding areas, chromate per MIL-DTL-5541, Class 3.

A high-strength automotive bolt: zinc flake per ISO 10683, with no hydrogen. A hydraulic piston rod: HVOF WC-Co, with grinding, as a replacement for hard chrome. A cemented carbide insert: CVD. ISO 1461, with paint per ISO 12944 on top, ina duplex system.

A good coating callout on the drawing answers all of the coating shop's questions. per MIL-PRF-8625, and type and class, for example Type Ill, Class 1. measured. And areas: what's coated, what's masked, and where contact points are allowed. and whether dimensions are before or after coating.

And tests and certificates, as we saw in chapter 19. understand what you wanted, the drawing isn't complete. Send the drawing to the coating shop as early as possible, while it's still in design. coating will come out thin, which drain holes are missing, and which masking will cost a lot.

corner, a drain hole, or moving the grounding surface. On a new part, a first article is approved first, and only then a production run. over time give a consistent result. Changing suppliers requires requalification. coatings, metallic coatings, paint, advanced coatings, and testing and design.

material, the shape, the environment, and the coating are chosen together. that didn't account for the process, or an unclear requirement on the drawing. Thank you for watching the Coatings, Finishes and Paint course, from METAL HUB.