Know the chemistry.
Know the alloy.
Match them.
Corrosion resistance is chemistry-specific. A material that's excellent in seawater can fail in HCl. Another that handles any acid fails in chloride stress. This guide maps specific environments to specific material solutions.
Most to least noble.
In saltwater, active metals corrode to protect noble metals. Mixing dissimilar metals accelerates corrosion of the less noble one.
| Position | Material | Role in galvanic couple | Typical use |
|---|---|---|---|
| Most noble | Gold, Platinum | Protected cathode | Electronics, jewelry |
| Noble | Titanium | Protected cathode | Marine, chemical service |
| Noble | Nickel alloys (Monel) | Protected | Saltwater, chemical |
| Mid | 316L Stainless | Moderate — depends on couple | General marine/chemical |
| Mid | Copper alloys | Moderate | Heat exchangers |
| Active | Iron/carbon steel | Corrodes in coupled | Structural, dry only |
| Active | Aluminum | Sacrificial anode | Lightweight structural |
| Most active | Zinc, Magnesium | Sacrificial anode | Galvanic protection of steel |
Environment → material.
Monel K-500 or Ti Gr.5
Monel K-500 for strength + corrosion. Ti Gr.5 for lightest weight + corrosion. 316L adequate for low-velocity, short service. Avoid 304 — not rated for seawater.
Inconel 625/718 or duplex 2507
NACE MR0175-qualified. Hardness restriction (typically <22 HRC for wetted surfaces). Avoid standard stainless — fails via sulfide stress cracking.
Hastelloy C-276 or tantalum
Hastelloy C-276 handles most HCl concentrations. For hot conc HCl, tantalum. Avoid stainless — pitted by chlorides. 316L fails quickly in HCl.
Depends on concentration
Dilute: 316L acceptable. Mid-concentration: Hastelloy C-276. Concentrated: 304/316L again (passive oxide forms). Monel for ANY concentration. Alloy 20 for dilute H2SO4.
304/316L stainless
Highly oxidizing — stainless forms passive layer. Used in most stainless manufacturing for passivation. Avoid reducing acids like HCl.
Nickel or nickel alloys
Pure nickel (Nickel 200/201) best for hot concentrated alkali. 316L adequate for dilute. Avoid aluminum (dissolves) and zinc (rapid).
Monel 400
Monel uniquely resistant to HF. Essentially no other common metal works — titanium dissolves, stainless dissolves, nickel dissolves. Monel for all HF service.
Titanium or Hastelloy
Dry Cl2: titanium excellent. Wet Cl2: Hastelloy C-276. Stainless pits rapidly in wet chlorine.
Galvanized or powder coat
General outdoor corrosion. Galvanized steel cheapest, 20+ year service. Powder coat aluminum for premium appearance. Stainless 316L for coastal/industrial.
Different failure modes.
Uniform corrosion
- • Entire surface corrodes at similar rate
- • Predictable — measure by mass loss per year
- • Example: steel in outdoor atmosphere
- • Mitigate: coatings, sacrificial anodes, material upgrade
- • Not usually catastrophic (gradual, detected by inspection)
Pitting corrosion
- • Localized attack forming pits
- • Dangerous — can penetrate thickness while rest of surface looks fine
- • Common in stainless steel + chlorides (seawater)
- • Mitigate: higher-grade stainless (duplex, super-duplex), crevice-free design
- • Check PREN: >40 for seawater service
Crevice corrosion
- • Occurs in tight gaps (under gaskets, threaded joints, weld overlaps)
- • Oxygen-starved crevice becomes anode
- • Common in seawater around stainless fasteners
- • Mitigate: design to eliminate crevices, proper gasket selection
- • Super-austenitic stainless (254 SMO) or titanium resistant
Stress corrosion cracking
- • Cracking from combined stress + specific chemistry
- • 304/316L + chlorides = SCC failure
- • 4340 high-strength steel + H2S = SSC failure (sour service)
- • Mitigate: NACE-qualified materials, stress relief, proper selection
- • Sudden catastrophic failure — particularly dangerous
Pitting Resistance Equivalent Number.
PREN formula: PREN = %Cr + 3.3×%Mo + 16×%N. Higher number = better pitting resistance in chlorides (seawater). Used extensively in oil and gas, marine, chemical service for material selection.
Material PREN values: 304 stainless PREN ≈ 18-19 (not seawater rated). 316L PREN ≈ 24-26 (limited seawater). Duplex 2205 PREN ≈ 35 (general seawater). Super-duplex 2507 PREN ≈ 42 (severe seawater). Super-austenitic 254 SMO PREN ≈ 43. Alloy 625 Inconel PREN ≈ 50+. Hastelloy C-276 PREN ≈ 65.
Minimum PREN for service: Seawater splash zone: PREN ≥ 32. Seawater immersion: PREN ≥ 40. High-chloride hot brine: PREN ≥ 50. Seawater + crevices + elevated temp: PREN ≥ 55. Use PREN to quickly screen materials for chloride service — if material doesn't meet PREN threshold, it will fail in that service.
Beyond PREN: PREN is a rough guide, not absolute. Other factors matter: temperature, chloride concentration, oxygen content, flow conditions, crevices, applied stress. For critical service, review material with corrosion expert and specific service conditions.
FAQ
304 vs 316 stainless — when does it matter?
304 stainless: standard grade. Good for indoor, dry, or mild outdoor. Food processing (non-chloride). 316: add 2% molybdenum for improved chloride and acid resistance. Required for: seawater splash zones (not immersion), food with salt, chloride-containing pharmaceutical, coastal outdoor, pools. Price difference: 316 is 15-25% more expensive. When in doubt, specify 316L — the cost premium is usually justified.
Is titanium corrosion-proof?
Nearly. Titanium forms a stable oxide layer providing excellent corrosion resistance in: saltwater (any concentration, temperature), most acids, most alkalis, chlorine gas. Weaknesses: HF acid (dissolves titanium), reducing environments (reduces protective oxide), very hot concentrated acids. For most corrosion service, titanium is excellent but expensive ($60-80/kg) — specify only when justified.
Galvanic corrosion avoidance?
Rules: (1) Use same material throughout when possible. (2) If mixing materials, isolate electrically with insulating washers, gaskets, coatings. (3) Area rule: make noble material small, active material large (opposite worsens corrosion). (4) Avoid saltwater exposure of galvanic couples. (5) For unavoidable couples, accept faster maintenance schedule. Most galvanic corrosion failures result from overlooking small fastener corrosion in large structures.
Passivation — what and why?
Passivation: chemical treatment of stainless steel to enhance corrosion resistance. Process: immersion in nitric acid or citric acid solution, dissolves free iron from surface, allows protective chromium oxide layer to form. Result: enhanced corrosion resistance, restored after machining. Standard for stainless parts in corrosive service — ASTM A967 specification common. Inexpensive (few dollars per part at modest volume).
NACE compliance details?
NACE MR0175/ISO 15156: standard for materials in H2S oil and gas service. Requirements: specific hardness limits (wetted surfaces typically <22 HRC), material selection restrictions (no high-strength low-alloy steel), specific heat treatment requirements. Hardness is critical — hardness above limit fails via sulfide stress cracking. For any oil and gas service, NACE compliance is mandatory for pressure-containing components.
Plating and coatings for corrosion protection?
Common coatings: (1) Zinc plating — most common steel protection, 10-25 µm, sacrificial anode action. (2) Anodizing aluminum — Type II decorative, Type III hardcoat. (3) Powder coat — 50-150 µm thick organic coating. (4) PVD coatings — thin (2-5 µm) dense ceramic coatings. (5) Hard chrome plating — wear + corrosion. Coating choice depends on environment, cost, aesthetic requirements. For severe service, specify appropriate material rather than relying on coating.
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