Galvanised steel corrosion and finish compatibility

Galvanised steel corrosion runs in two stages, and only the first one is slow. Zinc gives itself up to protect the steel, at a rate set by the atmosphere rather than by the coating thickness alone. When the zinc is gone at a point — a cut edge, a scratch, a weld — the steel underneath corrodes at the rate of bare steel, which is ten to forty times faster.

That is why the useful question is not whether galvanised steel rusts. It does, eventually, everywhere. The useful question is how many micrometres of zinc stand between the steel and the weather, and what the finish on top does to the rate at which those micrometres are spent.

This page works through the zinc consumption rates by exposure category, converts them into years for Z275 sheet and hot-dip coatings, explains why paint and powder fail on zinc when the pretreatment is wrong, and names the four details — cut edge, bend, weld and fastener — where galvanised parts actually fail.

How does galvanised steel corrosion behave with and without paint?

Zinc corrodes at a predictable rate for a given atmosphere, and the rate is published. ISO 9223 gives first-year zinc rates from 0.1–0.7 µm/yr at C2 up to 8.4–25 µm/yr at CX. Divide the coating thickness by the rate and you have the years to first red rust, with the caveat that the rate slows as the zinc corrosion products build a protective layer.

Work it for a Z275 sheet at C3: 19.3 µm per face divided by 0.7–2.1 µm/yr is 9–28 years. At C4 the same coating gives 4.6–9 years. A hot-dip coating on a 6 mm bracket, which typically comes back at 85 µm against the ISO 1461 minimum of 70 µm for that thickness band, gives 40–120 years at C3 and 20–40 years at C4. Those numbers, not the word galvanised, are what belong on the drawing.

Painting changes the rate twice over. The film slows the zinc consumption, and the zinc keeps protecting the steel wherever the film is broken. That combination is the duplex system, and its life is not the arithmetic sum of the two: published synergy factors put it at 1.2–2.5 times the sum, because each layer protects the other's failure mode instead of sharing it.

The bottom row is the one that costs money. A powder coat applied to galvanised sheet without a conversion coat looks complete, passes a film-thickness check, and fails at the zinc boundary in the first two or three wet seasons. The film does not wear out; it detaches.

Why does powder or paint detach from galvanised steel?

Because the coating bonds to a surface that was never converted, and zinc oxide keeps growing under it. Zinc in air forms a thin oxide and hydroxide layer within hours, and that layer is weakly attached to the metal. Powder bonds to it, and the bond is only as strong as the oxide.

Three mechanisms finish the job. Zinc corrosion products are bulkier than the zinc they replace, so they prise the film up from below. Zinc is amphoteric, so any alkaline residue left from a caustic cleaner keeps reacting at the interface. And moisture trapped in the porous zinc layer flashes off in the curing oven, leaving pinholes that give the weather a path to the metal.

The pretreatment sequence that prevents this is specific to zinc, and it is not the same as the one for CRCA:

• Alkaline degrease at pH 9–11.5. Above about pH 12 the cleaner dissolves the coating you are trying to protect; below pH 6 the acid does the same.
• No acid pickle. A hydrochloric or sulphuric pickle that is fine on CRCA strips zinc at a visible rate.
• Condition and convert. A titanium activation followed by a zinc phosphate at 1.5–3 g/m², or a chrome-free passivation where the specification allows it, gives the powder a surface it can bond to chemically.
• Dry off completely before the booth. Residual moisture in the phosphate is the most common cause of pinholes on galvanised work.
• Cure in the powder's window, typically 180–200 °C metal temperature, and verify it with a cure check rather than a timer.

Wet paint follows the same logic with an extra trap: resin chemistry. Alkyd paints react with zinc and form zinc soaps at the interface, and the film lifts in sheets. Epoxy primers are stable on zinc, and a polyurethane topcoat over them holds colour and gloss outdoors. Traditional practice weathered new galvanising for six to twelve months before painting, which let the surface stabilise; a phosphate conversion does the same job in minutes and is the reason a coated part no longer needs to sit outside first.

Which finish system belongs on which galvanised part?

Four systems cover nearly every galvanised part, and the choice follows from the exposure and the appearance requirement rather than from the coating brand.

Bare, for interiors and sheltered externals. Correct at C1 and C2, and for parts inside an enclosure where condensation is the only moisture. The limit is the cut edge: bare galvanised sheet in a wash-down area rusts at the edges first, and no amount of zinc on the face helps a face that has been cut.

Duplex powder, for outdoor enclosures and canopies. 60–80 µm of powder over a zinc phosphate is the standard outdoor industrial system, and it is the one that carries colour and gloss. On a Z275 sheet it is the difference between 5–9 years and 12–25 years at C4.

Duplex wet paint, for large fabricated assemblies. Two coats at 120–140 µm total over hot-dip galvanising, applied after fabrication so that the coating covers the welds and the cut edges. This is the system for a structural frame that cannot be powder coated because of its size or its mass.

Zinc-rich primer, for repaired or site-touched areas. A zinc-dust primer at 60–80% zinc in the dry film restores sacrificial protection at a weld, a scratch or a site cut. It is not a substitute for galvanising on a new part, and it needs its own topcoat outdoors.

The film build ceiling applies to all four. Above about 120 µm, powder on zinc loses impact resistance and cracks at folded edges, and the crack becomes the corrosion site it was meant to prevent.

What happens at cut edges, welds and fasteners?

These four details decide whether a galvanised part reaches its calculated life, and each of them removes zinc.

Laser-cut and sheared edges. The cut face is bare steel, and the laser leaves a zinc-depleted band 0.1–0.3 mm wide at the cut line. Zinc protects bare steel within about 1–2 mm of the edge in dry air, and much less where the edge stays wet. On a panel that sees rain, that edge is the first red rust, and the film creeps back from it.

Bends. A bend below the coating's strain limit crazes the zinc on the outside radius. The craze is not a hole, but it is a path for moisture into the intermetallic layer, and it is where white rust first appears on a formed galvanised part.

Welds. MIG welding burns the zinc back 5–10 mm each side of the bead, which leaves a bare strip on both faces of a lap joint. Grind back before welding, weld, clean, and prime the strip with a zinc-rich primer before the topcoat. A welded galvanised assembly with no paint over the weld will rust along the weld line, not at the edges.

Fasteners and dissimilar metals. A stainless fastener in a galvanised sheet creates a large cathode against a small zinc anode. In a wet joint the zinc around the fastener is consumed first, and the joint loosens as the coating disappears. Use galvanised or zinc-flake fasteners, or isolate the joint with a washer and a sealant.

Storage is the fifth detail and it is not on the drawing. White rust forms when moisture is trapped between stacked sheets, and it appears within days to weeks in a humid store. Stack sheets at an angle, keep them off the floor, and let air move between them; a stack that has gone white needs mechanical removal before it can be coated.

How is coated galvanised work run at Ganesh?

Pretreatment for zinc-coated parts runs on the seven-tank line, with the alkaline stage controlled inside the zinc-safe pH window rather than at the CRCA set point. Powder goes on in an 18 ft × 14 ft electrostatic booth, wet paint in a separate booth with a curing oven, and both routes finish with dry film thickness measurement and cross-hatch adhesion to the drawing's class.

Cutting and forming are the same equipment used for uncoated steel: a 12 kW fibre laser on a 40 ft × 16 ft bed, which cuts zinc-coated sheet with extraction for the zinc fume, a 200 T 5-axis brake with a 4,450 mm bed to 10 mm, and presses from 20 T to 300 T where the part is formed rather than folded. Welding is three CLOOS robotic MIG cells plus 30+ manual stations, with fixtures designed and built in-house.

The company has fabricated since 1992, holds ISO 9001:2015 and ISO 14001:2015, and operates five units in Mumbai (Vasai) and Nagpur. Custom parts run from 0.8 mm to 40 mm in mild steel, CRCA, stainless, aluminium, brass and copper, formed to ±0.5 mm typical, with PPAP and first article inspection available. Customers include Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways.

When a coating on galvanised steel is the wrong answer

Coating galvanised steel is the wrong answer when the zinc is already sufficient and the part will never be seen: an interior bracket at C2 does not need a duplex system, and the pretreatment cost buys nothing. It is also the wrong answer when the exposure is C5 and the section is thin, because neither the zinc nor the paint has the thickness to carry the duty, and 316L is the honest specification.

It is wrong for a part that will be welded after coating, since welding destroys the film and the zinc together, and for a part whose fit depends on a coated surface without a masking instruction.

We do not hot-dip galvanise in-house, so the coating class on the drawing is confirmed against the galvaniser's capability rather than assumed, and the part comes back for inspection or for the paint stage. We do not apply zinc-rich primers as a standalone system for immersion service, and we do not anodise aluminium.

What to put on the drawing

• The substrate: coating designation and mass for coil, or the ISO 1461 thickness class for a fabricated part.
• The exposed faces: which one carries the visible finish and which one may be left bare.
• The exposure category, or the installation description if the category is unknown.
• The pretreatment type, named as a process: alkaline degrease, zinc phosphate, passivation, dry-off.
• Finish layers with dry film thickness ranges, colour and gloss.
• Edge treatment: deburred, sealed, or left bare, and whether cut edges need a stripe coat.
• Weld locations and the re-protection step for each.
• Fastener type and any isolation required at dissimilar-metal joints.
• The adhesion test class to ASTM D3359 and any neutral salt spray duration with separate white-rust and red-rust limits.
• Storage and handling notes for parts that ship before assembly.

Send the drawing with the exposure and the quantity to request a quote. The materials and grades hub carries the grade and coating selection guides, the degreasing guide for galvanised parts covers the first tank in detail, and the seven-tank pretreatment explanation covers what each stage does to a zinc surface. Pretreatment and booth capacity are listed on the paint shop capability page.