Corrosion protection systems for structural steel

Why the coating system matters

Steel corrodes wherever oxygen and moisture meet bare metal; chlorides from the sea, sulphur compounds from industry and condensation inside unventilated buildings accelerate the process. The coating system is therefore part of the structural design: it decides how long the steel keeps its section before the first major maintenance, how the building looks after ten years and how much the owner will spend on repainting. For an exported steel package the coating also has to survive handling, sea freight in containers or on flat racks, storage on site and erection damage.

World Steel applies coatings in the factory after fabrication, so the client receives steel that is ready to erect with one documented system. This page is the overview and selection guide; the individual systems are described on their own pages: hot-dip galvanizing, epoxy and polyurethane systems, alkyd systems, duplex systems, intumescent fire protection and the coating manufacturers whose systems are applied at World Steel.

Corrosivity categories and durability

ISO 12944-2 classifies atmospheric environments from C1 very low (heated, dry interiors) through C2 low, C3 medium, C4 high and C5 very high to CX extreme (offshore and severe industrial exposure), plus immersion and buried categories Im1–Im4. The category is set by the designer from the site location, distance to the coast, industrial pollution and the micro-climate around each member – the inside of a ventilated warehouse and the eaves of the same building near the sea can fall into different categories. In Australia AS 4312 maps atmospheric corrosivity zones, and AS/NZS 2312.1 and AS/NZS 2312.2 give paint and galvanizing guidance against them.

ISO 12944-1 then defines the required durability as the time to the first major maintenance: low (L) up to 7 years, medium (M) 7–15 years, high (H) 15–25 years and very high (VH) more than 25 years. Durability is a planning figure for the owner, not a guarantee period; warranties are a separate commercial matter between the parties.

Choosing between galvanizing, epoxy, alkyd and duplex

The table below is a first guide only. The final system – number of coats, generic type and nominal dry film thickness – is confirmed against ISO 12944-5 (or AS/NZS 2312.1 / 2312.2) and the coating manufacturer's recommendation for the actual category and durability.

Hot-dip galvanizing suits grating, handrails, ladders and lintels; multi-coat epoxy systems suit large members that need a colour finish; alkyd is the economical answer for dry interiors; duplex combines the strengths of zinc and paint. Fire protection, where required, is added on top of a compatible anti-corrosive primer.

Surface preparation

Most coating failures start at the surface. World Steel removes mill scale and rust by abrasive blast cleaning in the factory to the grade the system needs – typically Sa 2½ to ISO 8501-1 (equivalent to SSPC-SP 10 / NACE No. 2 near-white blast) for zinc-rich and epoxy primers, and Sa 2 or SSPC-SP 6 commercial blast where the specification allows. Welds, edges and imperfections are dressed to the ISO 8501-3 preparation grade (P1–P3) set by the designer, the surface profile is checked with an ISO 8503 comparator, and dust (ISO 8502-3) and soluble salts (ISO 8502-6/-9) are controlled where specified. Australian projects reference AS 1627.4 for abrasive blast cleaning. More background is in the PEB article on surface preparation and painting.

How World Steel specifies, applies and inspects

• Specify: the coating system is taken from the project specification; where the client only gives the environment and durability, World Steel proposes a system based on ISO 12944-5 and the manufacturer's data sheets for approval.
• Apply: blast cleaning and airless spray painting are carried out in the factory to the manufacturer's product data sheets – mixing, induction, temperature, humidity, dew point and overcoating intervals – with stripe coats on edges and welds.
• Inspect: dry film thickness is measured to ISO 19840 / ISO 2808 or SSPC-PA 2, adhesion is checked by pull-off (ISO 4624 / ASTM D4541) or cross-cut (ISO 2409 / ASTM D3359) where specified, and results are recorded in the coating inspection report.
• Protect: coated steel is packed with dunnage and edge protection for export; touch-up paint and repair procedures are supplied for site damage.

Coating work is part of execution under AS/NZS 5131 and EN 1090-2, both of which World Steel is certified to; see also the PEB article on paint and coating systems.