- Select the system from ISO 12944 exposure and maintenance targets.
- Durability bands plan first major maintenance; they are not warranty periods.
- Drainage, edge rounding, crevices and metal isolation drive real life.
- Salt spray is a controlled comparison, not outdoor-year conversion.
Values in this guide are practical starting points for RFQ, DFM and prototype validation. They do not replace load calculations, regulations, material certificates, qualified procedures or approved production drawings.
Classify the Actual Micro-environment
ISO 12944-2 classifies atmospheric corrosivity as C1, C2, C3, C4, C5 and CX. A sheltered urban exterior may resemble C3; humid industrial or moderate salinity C4; high-humidity industrial or high-salinity coast C5; extreme marine exposure CX. Confirm from local deposition, time of wetness and field data. A horizontal water trap and a ventilated vertical face on the same product may have different micro-exposure.
Durability Is a Maintenance-planning Term, Not a Warranty
ISO 12944-1 durability bands are L up to 7 years, M 7–15, H 15–25 and VH over 25 years. They are planning intervals to first major maintenance, not warranty periods. Specify corrosivity, durability, surface preparation, primer/intermediate/topcoat and total dry film thickness together.
Select a Route by Exposure and Damage Risk
Low-to-moderate exposure may use zinc conversion pretreatment plus durable polyester powder. Higher-risk steel may use blast cleaning with zinc-rich primer, intermediate and weatherable topcoat, or post-fabrication hot-dip galvanizing. Where color and scratch tolerance matter, evaluate a duplex galvanized-plus-paint/powder system. Aluminum uses suitable conversion plus weatherable powder/liquid coating. Stainless grade is selected for chlorides and cleaning, with iron contamination and crevices controlled.
Geometry Often Controls Coating Life
Drain every horizontal pocket by slope or hole. Avoid back-to-back capillary gaps, sealed overlaps and deep uncoatable crevices. Round sharp laser or sheared edges because film is thinner at sharp corners. Select weld continuity for sealing and fatigue, and do not place drains where mud accumulates. Isolate dissimilar metals with washers, barriers and sealant.
Match Tests to Failure Modes
Measure film by magnetic/eddy-current methods; adhesion by cross-cut or pull-off where appropriate; weatherability by UV/moisture exposure; cyclic corrosion may better represent some outdoor mechanisms than continuous salt spray. ISO 9227 is useful for comparative quality control but 500 or 1,000 hours cannot be converted directly to outdoor years. Scribed results depend on scribe method, substrate and edge sealing, so specimen preparation and acceptance must be defined.
Design for Maintenance and Shipping Damage
Define repair system, overlap, cleaning and reinspection for scratches, cut edges and field welds. Use soft separators, securement and condensation control; prevent steel bands from touching finished surfaces. Inspect after installation and then at intervals based on exposure and damage. Early touch-up is far cheaper than repair after underfilm corrosion spreads.
REFERENCES
ISO 12944-1/-2/-5 (environment, durability and coating systems), ISO 1461:2022 (hot-dip galvanizing), ISO 9227 (salt spray), ISO 2409 (cross-cut testing), and ISO 2360/ISO 2178 (coating thickness).