Corrosion & Galvanic Compatibility Guide
A comprehensive corrosion guide: galvanic series, dangerous metal pairs, PREN, prevention methods, corrosion types (pitting, crevice, SCC, intergranular, and more), testing methods (salt spray, ASTM), corrosion in different environments, and key standards.
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Corrosion & Galvanic Compatibility
Galvanic Series · Dangerous Pairs · PREN · Prevention
⚡ Galvanic Series (Seawater)
↑ Cathodic (noble) — protected
| Metal/Alloy | Potential mV |
|---|---|
| Gold, Platinum | +1200 |
| Graphite/Carbon | +300 |
| CFRP (Carbon) | +200 |
| Hastelloy C | +100 |
| Inconel 625 | +50 |
| Monel 400 | +20 |
| SS 316L (passive) | -50 |
| SS 304 (passive) | -80 |
| Copper C110 | -100 |
| Bronze | -150 |
| Brass | -200 |
| Nickel | -250 |
| SS 304 (active) | -300 |
| Titanium Grade 2 | -50 to -300* |
| Silver (Noble!) | -150 |
↓ Anodic (active) — corroding
| Metal/Alloy | Potential mV |
|---|---|
| Lead | -450 |
| Steel A36 | -600 |
| Cast Iron | -650 |
| Aluminum 7075 | -700 |
| Aluminum 6061 | -750 |
| Aluminum 1100 | -780 |
| Cadmium | -800 |
| Zinc | -900 |
| Magnesium AZ31 | -1600 |
| A difference >250mV = galvanic corrosion risk! * Ti = passive (oxide) in normal environments: ~-50mV. Active (no oxide): ~-300mV | |
⚠️ CFRP + Al = ~950mV difference = severe corrosion! A glass-ply barrier is mandatory.
⚠️ SS + Al = ~650mV difference = corrosion. Isolation or coating required.▶ YouTube
⚠️ SS + Al = ~650mV difference = corrosion. Isolation or coating required.▶ YouTube
🚨 Dangerous Pairs — What Not to Combine
| Joint | mV Difference | Risk | Prevention |
|---|---|---|---|
| CFRP + Al | ~950 | Critical! | Glass ply barrier, sealant |
| SS 304 + Al | ~670 | High | Alodine + primer, isolation |
| SS 304 + Mg | ~1500 | Critical! | Never combine! Mg destroyed |
| Copper + Al | ~650 | High | Nylon washer isolation |
| SS + Steel | ~520 | Medium | Zinc plating, paint |
| Al 7075 + Al 6061 | ~50 | Low | Generally acceptable |
| Ti + SS | ~250 | Borderline | Isolation in marine environments |
| Cd + SS | ~550 | Medium | Popular in aerospace intentionally (Cd=anodic) |
📐 The Area Ratio Rule
A critical rule many overlook: it's not only the potential difference that matters — the area ratio matters too!
Small anode + large cathode = disaster — a steel bolt (anodic) in a large stainless plate (cathodic): all the galvanic current concentrates on a small area = fast, catastrophic corrosion of the bolt.
Large anode + small cathode = relatively safe — a steel plate with small stainless bolts: the current spreads over a large area = a slow corrosion rate, usually acceptable.
Small anode + large cathode = disaster — a steel bolt (anodic) in a large stainless plate (cathodic): all the galvanic current concentrates on a small area = fast, catastrophic corrosion of the bolt.
Large anode + small cathode = relatively safe — a steel plate with small stainless bolts: the current spreads over a large area = a slow corrosion rate, usually acceptable.
Rule of thumb: never place an anodic (active) material as a small area next to a cathodic (noble) material as a large area — even if the potential difference is relatively low.
🔢 PREN — Stainless Steel Corrosion Resistance
PREN = %Cr + 3.3×%Mo + 16×%N
| Alloy | PREN | Use |
|---|---|---|
| SS 304 | ~18 | Interior, food |
| SS 316L | ~24 | Light marine, chemical |
| SS 317L | ~30 | Medium marine |
| SS 2205 Duplex | ~35 | Marine, oil/gas |
| SS 2507 Super | >40 | Severe marine |
| 904L | ~36 | Acid |
| Hastelloy C276 | ~70+ | Everything |
PREN>35 = pitting-resistant in marine environments. PREN>40 = Super Duplex = the harshest conditions.▶ YouTube
🌡️ Critical Pitting Temperature (CPT)
The most practical measure for comparing stainless steels: the temperature above which pitting begins in a standard FeCl₃ solution (ASTM G48). The higher, the more resistant.
| Alloy | CPT (°C) |
|---|---|
| SS 316L | ~15°C |
| SS 2205 Duplex | ~30°C |
| SS 2507 Super Duplex | ~55°C |
| 254 SMO | ~70°C |
🛡️ Corrosion Prevention Methods
Material Selection
✅ SS 316L instead of 304 for marine environments
✅ Ti-Grade 2 — resistant to everything
✅ Al 5083 for marine environments
✅ PREN>35 for open sea
✅ Ti-Grade 2 — resistant to everything
✅ Al 5083 for marine environments
✅ PREN>35 for open sea
Protective Coatings
✅ Zinc (sacrificial) for steel
✅ Alodine + primer for Al
✅ Passivation for SS
✅ Cadmium for aerospace
✅ HDG for exterior use
✅ Alodine + primer for Al
✅ Passivation for SS
✅ Cadmium for aerospace
✅ HDG for exterior use
Galvanic Isolation
✅ Nylon washer between materials
✅ Glass ply barrier (CFRP+Al)
✅ Faying surface sealant
✅ Anodize + sealant
✅ PR-1422 / PR-1776
▶ YouTube✅ Glass ply barrier (CFRP+Al)
✅ Faying surface sealant
✅ Anodize + sealant
✅ PR-1422 / PR-1776
⚡ Cathodic Protection
Sacrificial Anode
A more anodic metal (Zn, Mg, Al) is sacrificed in place of the protected structure. Common in buried piping, ship hulls, underground fuel tanks. Simple, no electricity needed, but requires periodic replacement.
Impressed Current
An external power source drives current through an inert anode (coated titanium) to protect the structure. Effective for large structures (long pipelines, marine structures) but requires electrical maintenance.
📚 Main Types of Corrosion
Correctly identifying the type of corrosion is the first step in solving the problem — each type requires a completely different prevention strategy.
Uniform Corrosion
MechanismUniform attack across the entire exposed surface
SeverityLow — predictable, calculable
ExampleRust on steel exposed to the atmosphere
The most "friendly" type of corrosion — a uniform rate allows for a corrosion allowance (extra thickness in design).
Pitting Corrosion
MechanismChloride penetration through a local passive layer
SeverityVery high — hard to detect, penetrates deep
ExampleSS 304 in a marine/chlorine environment
Dangerous because the external damage looks small ("pinhole") while a deep cavity can form beneath it — a leading cause of sudden piping failures.
Crevice Corrosion
MechanismAn oxygen-depleted cell in cracks/joints/seals
SeverityHigh — develops quietly beneath the surface
ExampleBeneath O-Ring seals, bolts, bonded layers
Mechanistically similar to pitting but always starts in a geometrically protected area (crack, gap) — good design ("crevice-free design") prevents it from the start.
Stress Corrosion Cracking (SCC)
MechanismCombined tensile stress + a specific corrosive environment
SeverityCritical — sudden and elusive failure
ExampleBrass + ammonia, SS + chloride at high temperature
Among the most dangerous in industry — a crack that grows with no visual warning until sudden brittle fracture. Requires a specific combination of material+stress+environment.
Intergranular Corrosion
MechanismAttack along grain boundaries, usually after welding
SeverityHigh — weakens the material with no external sign
Example"Weld decay" in unstabilized SS 304
Caused by sensitization — chromium carbide precipitation at grain boundaries in the heat-affected zone (HAZ). Solution: "L" grade SS (304L, 316L) with low carbon.
Erosion-Corrosion
MechanismFast flow erodes the protective layer
SeverityMedium-high — depends on flow velocity
ExamplePipe elbows, pumps, valves
A mechanical+chemical combination: the flow physically removes the protective oxide layer before it can regenerate. Solution: larger bend radius, reduced velocity.
Fretting Corrosion
MechanismTiny cyclic movement between two contacting surfaces
SeverityMedium — usually at contact/assembly areas
ExamplePress-fit joints, bearings, bolts under vibration
The tiny (micron-scale) movement repeatedly breaks the oxide layer, creating characteristic reddish-brown oxide powder ("fretting debris").
Hydrogen Embrittlement
MechanismAtomic hydrogen penetrates the crystal lattice, reducing ductility
SeverityCritical — sudden brittle fracture with no warning
ExampleZinc-electroplated bolts with improper plating
A major risk in electroplating of high-strength steels — a bake-out within 4 hours of plating (ASTM B850) is mandatory to remove the hydrogen.
🧪 Corrosion Testing Methods
Accelerated tests simulate years of environmental exposure in just a few weeks — an essential tool for approving a material/coating before series production.
| Method | Standard | Typical Duration | Simulates |
|---|---|---|---|
| Salt Spray (Fog) Test | ASTM B117 | 24-1000+ hours | Marine/salt environment |
| Cyclic Corrosion Test (CCT) | ASTM D5894 / GMW14872 | Weeks | Cyclic humidity/dry/salt — more realistic |
| Immersion Test | ASTM G31 | Days-weeks | Continuous immersion in a chemical/solution |
| Potentiodynamic Polarization | ASTM G5 / G61 | Hours | Electrical measurement — precise corrosion rate |
| Pitting Resistance (FeCl₃) | ASTM G48 | 72 hours | Pitting/crevice resistance in stainless steel |
| Intergranular Corrosion Test | ASTM A262 | Days | Sensitization susceptibility in SS welds |
| Stress Corrosion Test | ASTM G30 / G36 | Days-weeks | SCC under sustained stress |
| Humidity/Condensation Test | ASTM D2247 | Days | Constant high humidity (100% RH) |
⚠️ The correlation between salt spray and field performance is not linear! "500 hours salt spray" is a relative measure for comparing materials/coatings — not a direct prediction of actual service life in a real environment.
Evaluation Criteria — Visual Rating
Most salt spray standards rate by % of area affected by red rust per ASTM D610, or by time to first appearance of corrosion on a coated surface.
Electrochemical Corrosion Rate
Potentiodynamic polarization gives a numerical value (mm/year) from a Tafel curve — much faster and more precise than a salt spray test, but requires dedicated lab equipment.
🌍 Corrosion in Specific Environments
🌊 Marine/Atmospheric Environment
Chloride (Cl⁻) from salt is the main driver of pitting. Recommended materials: SS 316L (PREN>24), Al 5083/5086, Ti Grade 2. Avoid: standard SS 304, unprotected carbon steel, copper next to aluminum.
🏭 Chemical/Industrial Environment
Completely dependent on the specific chemical — there's no "one solution fits all". Concentrated sulfuric acid = carbon steel is actually resistant (a protective sulfate layer forms); diluted = destructive to steel. A specific chemical compatibility check (corrosion chart) is mandatory.
🔥 High-Temperature Oxidation
Above ~500°C the mechanism is completely different from wet corrosion — direct oxidation of the metal in air. Requires high-Cr/Ni alloys (SS 310, Inconel 600/601) that form a stable protective oxide layer at heat.
🏗️ Soil/Underground
Corrosion depends on soil resistivity and stray currents. Buried piping is usually protected by a combination of: external coating (coal tar/FBE) + cathodic protection (sacrificial anode or impressed current).
Cryogenic
At low temperatures the corrosion rate itself drops significantly, but brittle fracture of the material itself must be checked — not every corrosion-resistant material is also suitable for cryogenic temperatures.
💧 Desalinated/DI Water
Mineral-free water (DI/RO) is actually more aggressive than seemingly "aggressive" chemicals — the lack of ions causes the water to "pull" ions from the metal itself. Requires SS 316L or plastic (PVDF/PTFE) in water purification systems.
📋 Key Corrosion Standards
| Standard | Topic |
|---|---|
| ASTM B117 | Salt Spray (Fog) Testing — the most common test |
| ASTM G31 | Laboratory Immersion Corrosion Testing |
| ASTM G48 | Pitting and Crevice Corrosion Resistance (FeCl₃) |
| ASTM G5 / G61 | Potentiodynamic Polarization Measurements |
| ASTM A262 | Intergranular Attack Susceptibility (Stainless) |
| ASTM G30 / G36 / G38 | Stress Corrosion Cracking Testing |
| NACE MR0175 / ISO 15156 | Materials for sour oil/gas environments (Sour Service) |
| ASTM B850 | Post-Coating Baking to prevent hydrogen embrittlement |
| ISO 9223/9224 | Atmospheric corrosivity classification (C1-CX) |
| ASTM D1654 / D610 | Corrosion rating evaluation on coated surfaces |
💡 NACE MR0175 is critical in the oil/gas industry — it determines which materials are permitted in H₂S environments (sour service) to prevent sulfide stress cracking, a subtype of SCC.