Design and Materials Selection
Selection is a system decision
Materials selection starts with the service environment and required life. Candidates must compare corrosion resistance, mechanical properties, fabrication behavior, availability, cost, and inspection needs. A material that resists uniform attack may still fail by stress-corrosion cracking, pitting, or galvanic effects. Use iso-corrosion diagrams and service history cautiously, qualify welding procedures, and account for heat-affected zones.
Design details that control corrosion
Design for drainage, avoidance of crevices, and access for inspection and cleaning. Prevent water traps, dead legs, and gaps under supports. Separate dissimilar metals or use compatible transition joints and insulating kits. Account for coating damage: a coated cathode with a small bare anode is a severe galvanic geometry. Specify corrosion allowance, cathodic protection, coatings, and inhibitors as part of the design, not as afterthoughts. Check that cathodic protection does not overprotect high-strength steels or disbond coatings.
Traps
A common error is choosing the most corrosion-resistant alloy while ignoring fabrication cracks, cost, or galvanic coupling to less noble components. Another is treating stainless steel as universally resistant; chloride pitting and stress-corrosion cracking are environment-specific. Do not confuse corrosion allowance with a guarantee of life, and do not assume a coating eliminates galvanic corrosion when damaged.
Worked example
Required corrosion allowance = corrosion rate × design life × safety factor. For 0.15 mm/y, 20 years, and a 1.5 factor: 0.15 × 20 × 1.5 = 4.5 mm. Add this to the minimum structural thickness, then confirm with inspection and remaining-life assessment. Rule of thumb: design for drainability and avoid small anodes; if a detail cannot be inspected, it should not be relied upon for integrity.