CRCA corrosion and finish compatibility

CRCA corrosion resistance is entirely the coating system. The coil is cold reduced, annealed and skin-passed, with no zinc and no conversion layer, so the finished part is protected only by the film plus whatever the edge geometry allows. That is why two parts made from the same 2 mm sheet, one painted and one not, behave like different materials.

The failure is rarely a red patch in the middle of a panel. It starts at a laser-cut edge, a weld toe or a hole, where the film is thin or absent, and creeps under the coating as a blister. By the time it is visible the steel behind the film has lost section.

This guide sets the exposure classes against the finish systems, gives the pretreatment sequence and the film thickness numbers, and names the failures that no extra coat of powder will fix.

How fast does CRCA corrosion run in each exposure class?

ISO 12944-2 classifies atmospheric corrosivity by the first-year thickness loss of standard steel specimens. C1 is heated indoor space with negligible corrosion. C2 is unheated indoor or rural. C3 is urban and industrial with moderate sulphur dioxide. C4 is industrial or coastal with salt. C5 is severe coastal or industrial, and CX is offshore.

A 1.5 mm panel in C4 loses 50–80 µm of steel in the first year. Uniform loss at 80 µm per year would take 18.75 years to consume the sheet, which sounds comfortable until you remember that corrosion is not uniform. It pits, it concentrates at edges and under deposits, and it accelerates once the coating is breached. The design life of the part is set by the first penetration, not by the average.

The same table explains why "powder coated" is not a specification. A 60 µm film on a C3 part and a 60 µm film on a C4 part are the same coating and different service lives. The class belongs on the drawing next to the finish.

Zinc coating mass is read the same way. A galvanised sheet at 275 g/m² carries 137.5 g/m² on each face, which at 7.14 g/cm³ is 19.3 µm of zinc per side. In C4, where zinc loses 2.1–4.2 µm in the first year, that is 4.6–9.2 years before the coating is consumed at a scratch. Doubling to 600 g/m² gives 42.0 µm per side and 10–20 years, which is why the coating mass belongs on the galvanised-sheet callout and not in a general note.

Which pretreatment does CRCA need before coating?

The pretreatment has one job: turn a contaminated, oxide-covered steel surface into a phosphate layer that the powder can bond to chemically. On CRCA the contamination is mill oil, handling soil and a thin oxide from storage, with no mill scale to remove.

The seven-tank sequence runs alkaline degrease, rinse, acid pickling or derusting, rinse, activation, zinc or iron phosphating, and a final rinse with a passivation stage or deionised water before the dry-off oven. Each tank is a control point. A weak degrease leaves oil that blinds the phosphate; a weak rinse leaves salts that blister the coat; a cold dry-off leaves moisture in the conversion layer. The seven-tank pretreatment process sets out what each stage is controlling, and the mild steel degreasing guide covers the oil and soil the first tank has to remove.

The conversion coating mass is typically 1–3 g/m² for a zinc phosphate on steel. That is a thin layer, measured by weight loss or by X-ray, and it is the reason a coated part passes a cross-cut test and an unpretreated part does not.

How thick does the coating need to be?

Dry film thickness is measured to ISO 2808, either with a magnetic gauge on steel or by microscopic cross-section. The number on the drawing should be a range with a minimum, because a single figure invites a supplier to hit it on the flat and miss it in the recess.

Powder at 70 µm is the working centre for indoor CRCA. The material draw is straightforward: 1 m² × 70 µm = 70 cm³ of film, and at a powder density of 1.5 g/cm³ that is 105 g. At 65% transfer efficiency the gun must deliver 105 ÷ 0.65 = 162 g per square metre, before overspray and booth losses. A part with 2.4 m² of surface therefore consumes about 389 g of powder, which is the number that makes a coating quotation comparable.

Wet paint at two coats of 40 µm has a similar film volume but a lower density, around 1.2 g/cm³, so 80 µm over 1 m² is 96 g of dry film. The solvent that carries it is the difference in the process, not in the protection.

Measurement is part of the specification. A magnetic gauge reads to the top of the surface profile, so on a blasted substrate with a 40–70 µm profile it overstates the film by roughly the profile height unless it is zeroed on an uncoated blasted coupon. On smooth CRCA the reading is direct. Either way the gauge is zeroed on the same substrate, checked against a shim, and read at five points per part, including one edge and one recess.

Thickness alone does not protect an edge. Powder is electrostatically attracted to the face and pulls back from a sharp corner, leaving 30–50% of the face thickness on a laser-cut or sheared edge. A 0.5 mm radius on that edge lets the film wrap, and it does more for edge corrosion than adding 20 µm to the flats.

Which failures are coating failures and which are design failures?

Most coating failures on CRCA are decided before the part reaches the booth.

Blistering is a pocket of moisture or soluble salt under the film. It comes from an incomplete rinse, a contaminated dry-off oven or a part that sat wet in a basket. The blister grows by osmosis and lifts the coating from the steel, and the outdoor enclosure failure guide covers the exposure side of the same defect.

Filiform corrosion appears as thread-like trails under a coating on steel exposed to humid air, usually starting at a scribe or an edge. It needs a defect to start and a humid environment to run, and it is a sign that the pretreatment layer is too thin or too soluble.

Scribe creep is measured after a salt spray test as the distance the rust travels from a deliberate scratch. It is the number that separates a good system from a marginal one, and it depends on adhesion and on the conversion coating, not on colour.

Edge rust starts where the film is thinnest. A deburred edge, a radius and a phosphate layer are the controls; a thicker coat on the face is not.

Galvanic corrosion appears where a stainless fastener or a zinc-plated bracket touches bare CRCA in a wet joint. The steel becomes the anode and loses material at the contact face. An insulating washer or a coated contact face breaks the couple.

Under-cure is a process failure that tests like a material failure. Powder cured below the specified metal temperature stays soft in the film, passes a visual check and fails a solvent rub. A 50 double-rub test with methyl ethyl ketone, or a pencil hardness check, catches it before dispatch. The cure schedule belongs on the process sheet, not in the operator's memory.

Chalking and colour shift are UV effects. Epoxy powders chalk and yellow in sunlight, which is why exterior parts use a polyester resin. A part that fades in two years was usually specified by colour and not by resin chemistry.

Trapped moisture in a box section or a closed hem is a design failure. If condensate can enter and cannot drain, the inside of the part corrodes first and the outside looks perfect until it perforates. A 6 mm drain hole at the low point is cheaper than a coating upgrade.

What finish system should the drawing call for?

For indoor C2–C3 CRCA, iron or zinc phosphate plus 60–90 µm polyester powder is the standard. It gives a hard, colour-stable film with good adhesion, and it is the system the in-house line is built around.

For C4, add a zinc-rich primer at 40–60 µm under the powder or the topcoat, bringing the total to 120–160 µm. The primer supplies sacrificial protection at a scratch and at the edge, which the powder alone cannot do.

For C5, change the substrate to galvanised sheet or stainless rather than building a thicker organic film on CRCA. A duplex system, galvanised plus powder, works when the galvanising is specified by coating mass and the powder is formulated for zinc, but it is a different route with different welding rules.

Bare CRCA is a C1 material. Oiled CRCA is still a C1 material, because mill oil is a temporary handling aid, not a corrosion barrier.

What does Ganesh run for coated CRCA parts?

Coating runs through a seven-tank pretreatment line, a 16 ft × 16 ft shot blast chamber and an 18 ft × 14 ft electrostatic powder booth, with a wet paint booth and a curing oven for specifications that need a primer or a wet topcoat. Dry film thickness and cross-hatch adhesion are measured and recorded on the inspection record, and parts that need a blasted profile for heavy-gauge work go through the shot blast chamber before pretreatment.

The fabrication side is in-house: 12 kW fibre laser with a 40 ft × 16 ft bed, 200 T 5-axis press brake with a 4,450 mm bed, three CLOOS robotic MIG cells and 30+ manual stations. That matters for coating because every upstream operation sets the edge condition the film has to cover. A part laser-cut with a clean edge and deburred before pretreatment coats better than one sheared with a heavy burr and sent straight to the line.

Ganesh has manufactured since 1992 under ISO 9001:2015 and ISO 14001:2015, with five units in Mumbai (Vasai) and Nagpur and an effluent treatment plant for the pretreatment and paint waste.

When CRCA plus a coating is the wrong answer

CRCA plus a coating is wrong when the part is immersed, when it sits in a C5 coastal atmosphere, or when the coating cannot reach the surface that corrodes. A closed box section with no drain will rust from the inside whatever is sprayed on the outside.

It is also wrong when the edge is the critical surface. A punched hole, a laser-cut slot or a sheared flange exposes bare steel on the cut face, and a film that bridges the corner leaves a path for moisture. In those locations a galvanised substrate or a stainless grade is a material decision, not a coating decision.

We do not galvanise in-house, we do not apply zinc flake coatings, and we do not carry out cathodic dip coating. Where the drawing calls for any of those, the route changes and the quotation should name the subcontract step rather than absorb it.

What to put on the drawing

• The exposure class from ISO 12944-2, not the word outdoor.
• Substrate grade and thickness, with the mill certificate requirement.
• Pretreatment type: iron phosphate or zinc phosphate, and the conversion coating mass if it is critical.
• Coating system by layer: primer, powder or wet topcoat, with dry film thickness minimum and maximum for each.
• Colour and gloss, with the colour standard referenced by number.
• Edge condition: deburred, radiused or as-cut, with the radius value.
• Salt spray method to ISO 9227, the hours and the acceptance criterion, including scribe creep.
• Adhesion test to ISO 2409 and the acceptable class, normally G0 or G1.

Send the drawing with the exposure class and the coating specification to request a quote. The materials and grades hub covers the substrate side, the surface treatment and coating hub carries the pretreatment and powder guides, and the paint shop capability page shows the line the part will run through.