Know the chemistry. Know the alloy. Match them.

In saltwater, active metals corrode to protect noble metals. Mixing dissimilar metals accelerates corrosion of the less noble one.

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.

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.

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: 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 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.

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.