Outdoor corrosion rarely has one cause. Environment, water traps, pretreatment, edge coverage, galvanic contact and handling damage interact. Classify atmospheric corrosivity using ISO 12944-2, then specify a complete protection system; “for outdoor use” is not an executable requirement.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
FIVE-STEP FAILURE REVIEW
1. Substrate: carbon steel red-rusts quickly at damage; stainless can fail through iron contamination, chlorides and crevices. 2. Geometry: upward flanges, sealed laps and undrained cavities extend time of wetness. 3. Pretreatment: oil, slag and scale reduce adhesion; sharp edges receive a thinner film. 4. System: powder topcoat is primarily a barrier; zinc or zinc-rich primers can add sacrificial/inhibitive protection. 5. Logistics: packing warm parts, sea-freight condensation or metal straps cutting the coating can start corrosion before installation.
SPECIFICATION METHOD
State at least: environment class, substrate, surface preparation, primer/topcoat, dry-film range, color/gloss, edge and weld treatment, permitted defects, inspection method and repair procedure. ISO 12944 durability is a planning interval to first major maintenance, not a warranty; salt-spray hours do not convert directly to outdoor years.
PROJECT CHECKLIST
Marine distance and salt; industrial pollution; traps/crevices; dissimilar metals; damage risk; drainage; packing; field-repair material.