Surface treatment and corrosion protection under AS/NZS 5131

Planning: coating quality levels PC1 and PC2

Section 9 covers surface preparation and the corrosion protection system – painting, hot-dip galvanizing and related treatments. Unlike most of AS/NZS 5131, the level of documentation here is driven by the service environment and the surface preparation method, not by the construction category (Clause 9.2.1). The standard defines two coating quality levels:

In practice almost every exported structural steel package with a blasted multi-coat system is PC2. The atmospheric corrosivity category itself is classified using AS 4312, and the system is selected from AS/NZS 2312.1 (paint) or AS/NZS 2312.2 (hot-dip galvanizing). Clause 9.2.3 lists what the construction specification must state – coated areas, system and colours, treatment of bolted connections and concrete interfaces, surface cleanliness, profile, soluble salt limits, DFT, cure, inspection level and records. A work method statement is required for each stage (Clause 9.2.2), and Clause 9.2.4 requires member identification to remain traceable through any subcontracted blasting, painting or galvanizing.

Surface preparation and treatment grades

Before blasting, weld spatter, slag and flux are removed, salts and dirt are washed off and oil is solvent-cleaned to AS 1627.1 (Clause 9.3.2). Unless the specification says otherwise, exterior steel is blasted to Sa 2½ (ISO 8501-1) and interior steel receives at least St 2 power-tool cleaning or an Sa 1 sweep. Clause 9.4 then sets the blasting controls: clean, dry angular abrasive; oil- and water-free air; no blasting when the steel is within 3°C of dew point, humidity exceeds 85% or lighting is under 200 lx; a profile of 40–75 µm measured to AS 3894.5; and priming within 4 hours. Mechanical cleaning (Clause 9.5) is a fallback where blasting is impracticable, and wire brushing is not allowed.

Clause 9.8.4 adds treatment grades for the steel itself, aligned with ISO 8501-3 preparation grades:

Thermally cut edges (flame, plasma, laser) must be ground before coating because the hardened cut zone resists profile and adhesion (Clause 9.8.5). Enclosed spaces, lapped surfaces and inaccessible areas must be specified and protected before assembly (Clauses 9.6–9.7). Steel in contact with concrete carries the same system as adjacent steel, and embedded painted members are protected for the first 50 mm (Clause 9.3.7).

Paint application

Clause 9.9 reads like a paint-shop audit list. Painting is done by competent personnel (for example ACA-certified or PCCP-registered applicators) using paint complying with the AS/NZS 3750 series or equivalents in Table 9.9, kept within shelf life and with batch numbers recorded. Key controls include:

• No application when steel is within 3°C of dew point, air is below 5°C, humidity exceeds 85% (unless the product allows it), or the surface is wet or in strong sun (Clause 9.9.10).
• Stripe coating by brush on welds, edges, bolts and corners before each spray coat (Clauses 9.9.11, 9.9.13); alternate coats of the same product tinted for contrast (Clause 9.9.8).
• Masking: TF faying surfaces and bearing areas under bolt heads, machined surfaces, and a 75 mm band at site weld locations (Clause 9.9.9; see the bolting chapter).
• DFT checked on every coat with a calibrated gauge to AS 3894.3; wet film gauges used during application (Clause 9.9.15).
• Fasteners hot-dip galvanized to AS/NZS 1214 rather than painted, unless specified otherwise (Clause 9.9.18).

Painting is supervised by a competent person, such as an ACA or NACE Level 1 coatings inspector (Clause 9.9.20). Damage repairs follow the paint manufacturer and Clause 9.12, with primer and finish coats overlapping sound coating by 25–50 mm after site welding. More on systems is in corrosion protection systems.

Hot-dip galvanizing

Galvanizing is carried out to AS/NZS 4680, with threaded fasteners to AS/NZS 1214 (Clause 9.10). Most of the work happens at fabrication design stage:

• Venting and draining: every hollow section needs at least one vent and one drain hole, each sized to at least 25% of the largest internal diagonal and no smaller than the wall thickness – several smaller holes may make up that area, but none under 10 mm (Clause 9.6.3). Closed sections are never sealed before galvanizing.
• Weld porosity is ground out and re-welded before dipping; distortion-prone fabrications may need stress relieving (Clause 9.10.4).
• Mill certificates and traceability records are available before galvanizing (Clause 9.10.3), and identification survives the bath, for example with heat-resistant tags (Clause 9.2.4).
• Coating thickness, adherence and repairs follow AS/NZS 4680; wet storage stain made at the plant is removed before dispatch (Clauses 9.10.7–9.10.10).

Where galvanized steel is to be painted (a duplex system), the zinc is degreased and sweep-blasted removing no more than about 10 µm, then painted within 6 hours (Clause 9.11). See hot-dip galvanizing and duplex coating systems. Cathodic protection must meet AS 2832 (Clause 9.13), and where stainless steel is welded to carbon steel the coating continues at least 50 mm onto the stainless (Clause 9.14).

Inspection of coatings (Clauses 13.8–13.10)

For PC2 work, two ITPs are required: one for surface treatment (Clause 13.8.1 – cleanliness, profile, treatment grade, contact surfaces, concrete interfaces) and one for paint coatings (Clause 13.9.1 – materials, storage, masking, conditions, priming, application, DFT, continuity). Each ITP lists calibrated inspection equipment, sample report forms, how nonconforming areas are marked, who inspects and when. The paint ITP must also name the supervisor and their qualifications.

• Inspectors are competent persons – typically ACA or NACE certified coatings inspectors or equivalent international qualifications (Clauses 13.8.2, 13.9.2).
• A pre-painting condition report records the steel condition, blast grade and treatment grade before any paint is applied (Clause 13.8.4).
• A daily inspection report per work area records paint batches, system and quantities, for example in the AS 3894.12 format (Clause 13.9.4).
• The prime coat is a hold: no further coats until it is inspected and defects are corrected (Clause 13.9.5).
• Galvanizing is inspected by the approach in AS/NZS 4680 nominated in advance; variation in hue alone is not grounds for rejection (Clause 13.10.1).

For an offshore fabricator these records are the evidence trail: in English, traceable to piece marks, and available to the client's inspector. Refer to the current edition of AS/NZS 5131 for the full requirements.