Degreasing galvanised steel parts before coating

A Z275 galvanised sheet carries about 19 µm of zinc on each face, and a hot caustic cleaner at pH 12 will take a measurable part of it away in a three-minute cycle. That is the whole problem with degreasing galvanised steel parts: the cleaner that strips mill oil off CRCA in one pass is the cleaner that dulls, etches and eventually removes the protection the buyer paid for.

Zinc is amphoteric. It dissolves in acid and it dissolves in strong alkali, so the safe cleaning window sits in the middle, roughly pH 9 to 11.5, at a temperature that lifts oil without attacking metal. Every other decision on a galvanised part — conversion coating, film build, cure — is built on that first step being right.

This page covers the pretreatment and coating route for galvanised sheet and hot-dip parts up to 10 mm, from the seven-tank line through powder and cure, with the checks that decide whether the finish survives a monsoon. The general cleaning sequence is in the pretreatment overview; the part-specific behaviour of zinc is what follows.

What is actually on a galvanised part?

Four things sit between the zinc and the powder, and each needs a different treatment. Mill oil or passivation oil is applied by the coil producer to stop white rust in storage, typically a fraction of a gram per square metre to a couple of grams. Handling oil and fingerprints come from cutting, forming and assembly. Zinc dust and fine oxide sit in the oil film and on the surface. And white rust, which is zinc oxide and zinc hydroxide, grows wherever moisture is trapped between stacked sheets.

The order matters. A cleaner that lifts the oil but leaves the oxide has cleaned nothing, because the conversion coating reacts with oxide rather than with oil. A cleaner that removes both aggressively has usually removed zinc as well. The winning cycle removes the oil gently, removes white rust mechanically before the line, and stops before the zinc layer changes appearance.

Tell the difference between oil film and white rust by eye and by wipe. Oil film is even, slightly darker and wipes away to a lighter grey. White rust is powdery, patchy and white to grey, and wiping leaves it on the cloth. A part with white rust must not go into the degrease tank: the tanks do not remove it, and the oxide load contaminates the bath and every part after it, so white rust is removed mechanically before the line.

What the cleaner does to the zinc layer

The zinc coating mass tells you the margin. Z275 per EN 10346 / IS 277 is 275 g/m² of zinc across both faces, or about 137.5 g/m² per face. At a zinc density of 7.14 g/cm³, that is roughly 19 µm per side. Z120 is about 8 µm per side, and a hot-dip part can carry 85 µm or more. A cleaner that removes 1 µm of zinc per pass has taken 5% of a Z275 sheet and 12% of a Z120 sheet, and it will do it again on the next batch.

Attack shows up as a change in appearance before it shows up as a failure. The surface goes from bright spangled grey to a flat, dull, slightly darker grey, and a white smut appears where the part touched a rack. Powder still goes on, but adhesion drops because the conversion coating has less sound metal to react with and the surface is rougher at the wrong scale.

Temperature compounds the problem. Reaction rate roughly doubles for every 10 K, so a cleaner running at 65 °C instead of 50 °C is three times as aggressive for the same concentration. Most zinc-safe alkaline cleaners specify a band of 45–60 °C, and the band exists for the substrate, not for the detergent.

The cut edge is where the film fails first

Laser cutting and shearing disrupt the zinc at the cut face. The laser vaporises zinc, which boils at 907 °C, well below the melting point of steel, and leaves a narrow band along the edge where the coating is depleted or alloyed with iron. That band is typically 0.1–0.3 mm wide on a laser-cut edge, depending on assist gas and thickness, and it is unprotected steel sitting under a coating film.

Powder covers the edge at a lower thickness than the flat face, because the film thins over a sharp corner by surface tension during cure. The two effects compound: the least protected metal gets the thinnest film. That is why a galvanised cabinet fails at a sheared edge or a hole rim years before the flat panel shows anything.

Three things reduce the risk. Deburr or radius the edge so the film has a surface to wrap around. Increase the film at the edge rather than across the whole part, by spraying the edges first or by specifying a stripe coat where the specification allows. And where the part is heavily cut, blast the edge so the depleted band is removed before coating. The coating failure modes that show up outdoors almost always begin at one of these edges.

Conversion coating: what changes on zinc

Steel and zinc do not take the same conversion coating. On CRCA, an iron phosphate or zirconium film forms quickly and evenly. On galvanised sheet the surface is zinc, sometimes with visible spangle, and the reaction is faster and less even: the grain boundaries and the spangle pattern take the coating at different rates, so a bath that is right for steel can leave a patchy film on zinc.

There are three practical routes. Zinc phosphate gives a heavy crystalline film with good paint adhesion, and it suits hot-dip parts and heavy service. Iron phosphate gives a thin film, usually 0.3–1.0 g/m², and is the common choice for sheet. Zirconium-based conversion gives the thinnest film and works well where the coating is a powder over a clean, lightly etched zinc surface.

Whichever is used, the test is the same: cross-hatch adhesion to ISO 2409 Gt0–Gt1, or ASTM D3359 class 4B–5B, and no film lifting along the cut edge when the tape is pulled. If the coating is patchy, the cross-hatch will show it as a difference between the spangle grains rather than as a clean cut grid.

Film build and cure on a zinc substrate

Galvanised steel takes 60–80 µm of powder comfortably. Below 50 µm the film can be thin enough at edges to show the substrate through, and above about 120 µm the coating becomes brittle: it cracks at folded corners and chips under impact, and once cracked, the crack holds water against the zinc.

Cure is not a constraint for zinc. Polyester powders cross-link at a metal temperature of 180–200 °C for 10–15 minutes, and zinc melts at 419 °C, so the substrate is nowhere near its limit. What does matter is evenness. A part with heavy sections and thin panels heats at different rates, and the thin panel can be overbaked while the heavy bracket is still undercured. Overbaked powder loses adhesion and gloss; undercured powder stays soft and fails a solvent rub.

Thickness measurement needs care on a thin galvanised sheet. A magnetic DFT gauge reads the distance to the steel, and the zinc layer sits inside that reading, so a 19 µm zinc layer adds to the displayed film. Calibrate on an uncoated sample of the same galvanised sheet, not on a bare steel plate. This is the most common reason a galvanised part appears to have 20 µm more powder than it does.

Rinsing and drying decide the last 10%

Between the conversion stage and the booth there are two rinse stages, and they exist to stop chemistry from drying on the part. Carry-over from the degrease tank into the conversion tank changes the pH of a bath that is balanced for zinc, and carry-over from the conversion tank onto the part leaves a salt film that blisters under powder.

Watch the final rinse rather than the cleaner. A rinse that is not overflowing builds up dissolved salts through the shift, and the last parts of the batch dry with visible water marks. Water spots are not cosmetic: each one is a patch where the conversion coating did not form properly, and the powder adheres to a salt crystal rather than to metal.

Drying matters just as much. Galvanised parts that leave the line damp will form white rust in the fifteen minutes before they reach the booth, especially in a humid plant. A dry-off oven at 110–130 °C removes the film without over-drying the surface, and parts should reach the booth warm or at least fully dry. Stacking wet galvanised parts on a trolley is the fastest way to produce a batch that fails adhesion on one face.

What salt-spray hours actually prove

Neutral salt spray to ISO 9227 runs at 35 °C with a 5% sodium chloride solution at pH 6.5–7.2, and it accelerates corrosion without reproducing any real environment. It is a comparison tool, and it is only meaningful when the acceptance criterion is stated with it.

For a galvanised part, two numbers matter: time to white rust and time to red rust, measured on a scribed and an unscribed area separately. A Z275 substrate with a conversion coating and 60–80 µm of polyester powder might reach several hundred hours before red rust at a scribe, while an identical part with a poor degrease might fail in a fraction of that with blistering along the edge. The hours alone do not tell you which happened; the photographs and the failure mode do.

Write the requirement as method, duration, scribe condition and acceptance. "500 hours NSS to ISO 9227, no red rust beyond 2 mm from the scribe and no blistering" is a testable specification. "Salt spray tested" is not.

The galvanised part route at Ganesh

Galvanised sheet and hot-dip parts go through the seven-tank line rather than a single combined wash: degrease, conversion and the rinses that follow each have their own bath, so the degrease stage can be run to a zinc-safe condition without compromising the stages that come after it. The acidic stage in the standard sequence — the descale or acid activation tank — would dissolve zinc, so on galvanised work it is bypassed or run zinc-safe: dilute, short dwell and pH-controlled. The oxide removal it would otherwise do is replaced by mechanical white-rust removal before the line. Heavy white rust is removed in the 16 ft × 16 ft shot blast chamber before the line, and the parts are coated in the 18 ft × 14 ft powder booth and cured in the oven.

Film thickness is measured with a DFT gauge calibrated on the same substrate, and adhesion is checked by cross-hatch. Sheet up to 20 mm is laser cut on the 12 kW fibre machine, and bending runs to 10 mm on the 200 T brake, which covers most galvanised canopy, enclosure and bracket work. The paint shop and the full pretreatment sequence are described under pretreatment and powder coating, and the rest of the substrate-specific guidance is indexed under surface treatment and powder coating.

What to put on the drawing

• Substrate and zinc coating mass, Z120 or Z275 or hot-dip thickness in µm.
• Whether the part is cut from galvanised sheet or galvanised after fabrication, because the two have different edges.
• Cleaner restriction if the customer specifies one, or a note that the pretreatment must be zinc-safe.
• Conversion coating type if the customer's specification names one.
• Powder type, colour and DFT range, with edge coverage called out.
• Cure schedule or the customer's oven profile limit.
• Adhesion method and acceptance classification.
• Salt-spray method, duration, scribe condition and acceptance limits.
• Masked areas: threads, earth studs and mating faces.
• Handling and packing, because white rust can form between dispatch and assembly.

The powder coating cost estimator is a quick way to see how masking, coats and batch size change the coating price before the drawing is frozen.

When this is not the right route

Powder over galvanised steel is a good system for outdoor cabinets, canopy panels and brackets in moderate environments. It is the wrong choice in four situations.

Where the part is a heavy hot-dip weldment with 85 µm or more of zinc and visible drainage spikes, the surface has to be dressed before coating, and the dressing cost often exceeds the coating. Where the service environment is coastal or chemical, the answer is usually a duplex system with a specified primer, not a single powder coat. Where the part needs a conductive path, powder on zinc is the wrong finish at the interface and should be masked or replaced with a plated finish. And where the part is above 10 mm, bending moves off the press brake and the assembly is usually better designed as a weldment.

Send the zinc specification, the edge condition and the salt-spray requirement with the drawing, and the quotation will state the pretreatment route it assumes.