Pretreatment for CRCA parts: seven-tank process and checks

The pretreatment CRCA parts receive before powder has to handle a surface that arrives dirty by design. Cold rolled close annealed sheet leaves the mill with a rolling oil film of 1–3 g/m², applied so the coil does not fret in transit and so the press does not gall. On 1 mm sheet, 2 g/m² of oil is a film about 2.2 µm thick: thin enough to look like clean metal and thick enough to ruin a powder film at 190 °C.

That oil is the first thing the seven-tank line has to remove, and it is the stage most often under-controlled because the result is invisible. A part that is degreased badly looks exactly like a part that is degreased well until it reaches the oven.

This guide covers the CRCA route specifically: what the sheet arrives with, what each tank has to achieve, the coating weight and rinse numbers to hold, and the evidence a buyer should ask for with the lot.

What is actually on a CRCA part before pretreatment?

Three things, and only one of them is visible.

The first is drawing oil, 1–3 g/m² at the mill and often more after stamping, because press lubricant is applied on top of it. A drawn or deep-drawn part can carry 3–5 g/m² in the corners and in the flange radius where the oil pools.

The second is surface carbon and smut. Cold rolling leaves a very thin carbon-rich layer, and any annealing without a clean atmosphere leaves it darker in patches. It does not show as scale, but it is not steel oxide either, and a phosphate crystal does not form on it well.

The third is what the fabricating route added: laser oxide on a cut edge, zinc or copper contamination from tooling, talc or chalk from handling, and fingerprints. Fingerprints are the most underrated, because skin oil is a chloride-bearing film and it survives a marginal degrease.

CRCA has one advantage over hot rolled steel: no mill scale. There is nothing to pickle off, so the line is alkaline clean, activate, phosphate, rinse and dry rather than an acid process. Where a CRCA part has rusted in storage, the rust is removed by blasting in the 16 ft × 16 ft chamber to ISO 8501-1 Sa 2½ before it goes anywhere near a tank, because a phosphate cannot convert rust.

What pretreatment CRCA parts need at each stage

Each tank has one controlling measurement. If a shift records those seven numbers, the line is under control; if it records temperatures only, it is not.

Degreasing: the stage that decides everything after it

The degreaser is an alkaline cleaner at 55–60 °C, and the two variables that matter are concentration and dwell. Concentration is held by titration, not by the supplier's scoop, because a tank loses strength to drag-out on every part and gains oil with every part.

A 1,000-litre tank taking 0.1 L/m² of drag-out from a 1 m² part loses 0.1 L per part, and 200 parts removes 20 L of chemistry that has to be replaced. That is the same arithmetic that makes rinse tanks fail: contamination is cumulative and invisible until the part is coated.

The water-break test is the field check. Pull a cleaned part out of the final rinse and watch the film: if it breaks into droplets within 30 seconds, the surface is still hydrophobic and the clean has failed. If it drains as a continuous film, the surface is wettable and the next stage can work.

Three CRCA-specific problems show up at this stage:

• Drawn parts hold oil in the flange radius and in hems. The oil wicks out during dry-off and shows up as a blister at the bend. The fix is orientation on the jig, not a stronger cleaner.
• Stamped parts carry press lubricant on one face only. A part hung with the oiled face down drains well; hung the other way, the oil runs across the clean face.
• Laser-cut edges hold oxide and a thin oil film. The edge is the first place a coating fails, and it is the one place a spray cannot reach on a nested part. The laser-cut edge guide covers what that boundary does to adhesion and coverage.

Stamped parts are the harder case, because press lubricant is applied where the die needs it rather than evenly. A progressive die can leave a heavier film on the strip edge and in the pierced holes than on the face, and the face is what the inspector sees. The degreasing guide for mild steel parts covers the same tank chemistry from the other direction; on CRCA the extra variable is the rolled surface under the oil, which wets differently from a hot rolled or blasted face.

Phosphating: what the coating weight buys you

Zinc phosphate does two jobs. It provides a crystalline surface that powder can mechanically key to, and it is a barrier that slows corrosion from spreading sideways under a damaged film. Both depend on coating weight and crystal structure, not on the colour of the part.

The working range is 1.5–4 g/m². At a crystal density of about 3 g/cm³, 2 g/m² works out to roughly 0.67 µm of layer, which is thin enough that it does not change a dimension and thick enough to cover the grain. Below 1 g/m² the coverage is patchy and the salt spray result collapses. Above 5 g/m² the crystal layer is coarse, brittle and cracks under impact, which is worse than a thin uniform layer.

Crystal size is set by activation. Tank 3 deposits a titanium-based film that gives the phosphate many nucleation sites, and the result is a fine 2–5 µm crystal. Skip the activation and the crystals grow large and sparse, the coating weight reads acceptable on a coupon while the coverage is poor, and the adhesion test parts company with the salt spray result.

Coating weight is measured gravimetrically: weigh a coupon, strip the coating, weigh again, and divide by area. It is a ten-minute test per batch and it is the number an auditor asks for. Judging the phosphate by the grey colour of the part is not a measurement.

Rinse quality, passivation and dry-off

Rinse quality is a conductivity number. Town water at 200–500 µS/cm carries salts that stay on the part after drying, and those salts sit under the powder as a hygroscopic layer. The final rinse is demineralised and held below about 30 µS/cm, with the reading taken at the overflow rather than at the tank inlet.

Passivation in the final rinse does two things: it dissolves a trace of the phosphate to leave a sealed, less porous layer, and it provides a temporary corrosion barrier for the interval between the line and the booth. A Cr(III) or Cr-free passivation at pH 4–5 is the common specification; whichever chemistry the plant runs, the pH and the concentration are recorded per shift.

Dry-off is where thin CRCA behaves differently from thick steel. A 0.8 mm panel reaches 90 °C in under two minutes and dries fast. A 3 mm bracket with a folded flange holds water in the fold, and if it enters the booth damp, the powder fizzes on contact and the pinholes are permanent. Hanging the part so every fold drains is a jig decision that costs nothing at the drawing stage and a lot at the rework bench.

Cure, film thickness and the evidence to ask for

Powder cures on metal temperature. A 200 °C oven air reading does not mean a 3 mm bracket reached 190 °C, and an under-cured film passes a thickness check and fails a solvent rub six months later. Cure schedules should state metal temperature and time, and the line should have a recorder or a datalogger on the oven.

The measurements that travel with a lot:

• Dry film thickness at five points per face with a magnetic gauge to ISO 2178. Read the bend radius and the cut edge separately, because powder pulls thin there: a 70 µm face can read 35 µm on a sharp corner.
• Adhesion by cross-hatch to ISO 2409, class 0–1, on a production part per batch. A coupon that was coated separately from the production load proves nothing about the load.
• Salt spray to ISO 9227 on a coupon from the same pretreatment and powder batch, for 240 hours indoors or 480 hours where the part will see weather. Report the creep from the scribe, not just pass or fail.
• Coating weight of the phosphate, gravimetrically, per batch.

What a salt spray comparison actually isolates

Two coupons from the same 1.5 mm CRCA coil, 100 × 150 mm, insulated from each other by one variable. Both are cleaned, phosphated at 2.0 g/m², powder coated at 70 µm in the same pass and cured together. The only difference is the final rinse: one at 25 µS/cm and one at 120 µS/cm.

In neutral salt spray to ISO 9227, at 35 °C with a 5% sodium chloride solution, the coupon with the clean rinse is the one that reaches the 480-hour acceptance point with creep under 2 mm from the scribe. The contaminated coupon fails the same criterion far earlier, usually before 240 hours, because the salt layer under the film is hygroscopic and pulls water along the interface. Same powder, same oven, same thickness.

That is the argument for treating rinse conductivity as a batch record rather than a maintenance item. It is also why the phosphating and conversion coating guide puts crystal structure and rinse quality ahead of coating weight: a 2 g/m² layer over a salty surface is still a salty surface. Where the film has to survive a scratch, the stage-by-stage explanation of the seven tanks is the place to start rather than adding thickness.

For a CRCA part in a sheltered industrial site, 70 µm over 2 g/m² of zinc phosphate is a C3 system in ISO 12944-5 terms, with a medium-durability expectation. A coastal plant room moves to C4, which means 80–120 µm, a blast before pretreatment where rust is present, and a conversation about whether powder alone is the right system.

The Ganesh pretreatment route for CRCA

Parts run through the seven-tank line in-house: alkaline degrease, rinse, activation, zinc phosphating, rinse, passivation and demineralised final rinse, then dry-off before the powder booth. The 16 ft × 16 ft blast chamber handles rusted or scaled CRCA before the line, the 18 ft × 14 ft electrostatic booth takes parts up to the size of a canopy panel, and the curing oven follows on the same flow.

Film thickness is checked with a DFT gauge and adhesion by cross-hatch on production parts, and the line sits inside an ISO 9001:2015 and ISO 14001:2015 system with an effluent treatment plant handling the rinse water. The paint shop page describes the full route from blast to cure, and the coating hub indexes the guides on degreasing, phosphating and the defects that appear two monsoons later.

What to put on a CRCA part drawing

1. Substrate: CRCA to IS 513, grade, thickness and surface condition, so the cleaner and the phosphate can be matched to it.
2. Pretreatment requirement, stated as a process (alkaline clean, zinc phosphate) or by coating weight range, not as "pretreated".
3. Coating system, powder type, colour and dry film thickness range with the faces it applies to.
4. Corrosivity category to ISO 12944-5 and the expected service life, which together justify the thickness.
5. Masking schedule, listing threads, earth points, mating faces and any sliding surface.
6. Adhesion and salt spray acceptance, with the test standard, duration and the pass criterion.
7. Handling and packing, because a coated part stacked wet is a coated part with marks.

The powder coating cost estimator will show how film thickness and part area move the cost before the specification is frozen, which is a cheaper conversation than a re-specification after the first lot.

When this route is the wrong one

An alkaline clean and zinc phosphate line is the right route for CRCA up to about 3 mm in ordinary industrial service. It is the wrong route for three cases.

The first is heavy rust or hot rolled material with mill scale, which needs blasting and often a different conversion coating. A phosphate tank cannot convert what is not steel.

The second is a part that will be welded after coating. Powder burns back 10–15 mm from a weld and the burnt zone is bare steel with a phosphate layer that has been destroyed by heat. Weld first, then coat, or accept a repair coat that will never match the line film.

The third is a specification that calls for a dipped or electrophoretic coating for internal coverage. We run spray powder and wet paint, not dip or e-coat, so a closed box section with no access hole gets line-of-sight coverage only, and the inside of a 60 × 60 mm tube will be bare. Say so on the drawing and design an access hole, rather than discovering it at the audit.

Send the part drawing with the service environment through the RFQ route, and the pretreatment route, the film thickness and the evidence per batch come back with the coating price.