Degreasing mild steel parts before coating

One gram per square metre of mill oil is a one-micrometre film. It is invisible on a CRCA panel, it survives a wipe with a rag, and it is enough to hold a powder coat off the steel for the life of the part. Degreasing mild steel parts is not a wash; it is the removal of a measured contamination layer, followed by proof that the layer is gone.

CRCA arrives with mill oil applied by the coil producer, picks up drawing lubricant during pressing, laser oxide at every cut edge and rust wherever the humidity got to it in storage. Each of those has to be removed by a different mechanism, and the order matters: oil blocks the conversion chemistry, rust blocks the oil remover, and scale blocks both.

This page sets out the cleaning cycle for mild steel parts from 0.8 mm to 10 mm, the water-break test that decides whether the cycle worked, and the film build and cure that follow. Galvanised substrates behave differently and are covered separately under degreasing galvanised steel parts.

What is on a mild steel part before it reaches the line?

The soil on a mild steel part is not one substance. Mill oil sits at 0.5–2 g/m², applied by the coil producer as a rust preventive and a forming aid. Drawing lubricant from a press operation is heavier and often contains extreme-pressure additives that resist a mild detergent. Laser oxide and dross sit on cut edges, a few micrometres to tens of micrometres thick depending on assist gas and section. Rust, from light flash rust to laminated scale, sits wherever the sheet was stored badly.

The fifth item is easy to forget: metallic fines and grinding dust. No degreaser removes them, they contaminate the bath, and they show up later as rough patches under the film.

Sort the part before cleaning it. A panel with light oil and a clean edge needs a degrease and a conversion. A panel with rust grade C or D to ISO 8501-1, which means pitting and laminated scale, needs blasting before it sees a chemical bath, because no alkaline cleaner will remove scale and the bath will load up with oxide.

The cleaning cycle that works on CRCA

Alkaline cleaning at pH 11–13, 55–60 °C, for 4–6 minutes is the standard cycle for CRCA, and it works because hot alkali saponifies the oil and lifts the film. It is a different chemistry from the zinc-safe cleaner used on galvanised sheet: mild steel can take a stronger bath, and the stronger bath handles the heavier soils that come with stamping and drawing.

Concentration and temperature drift through a shift, and both drift in the same direction as oil load accumulates. A bath that starts at 4% and ends the shift at 3% with a floating oil layer is cleaning the first hundred parts and greasing the last hundred. Skimming, topping up and dumping on a schedule are what keep a line repeatable, and they are the first thing to check when a batch fails adhesion in one corner of the rack.

Rinsing is the second half of the job. Two rinse stages, with the final rinse running on deionised water, keep dissolved salts from drying on the part and interfering with the conversion coating. A single rinse that is not overflowing carries degreaser into the next tank all day.

Clearing the oil from the inside of lap joints

The oil that causes the worst failures is not on the face. It is inside a lap joint, a hem flange or a boxed section, where the spray cannot reach and the rinse cannot flush. That oil survives the line, sits against bare steel, and bleeds out through the seam when the part reaches 190 °C in the oven.

The symptom is a line of pinholes, bubbles or a brown stain along a seam after cure. It looks like a powder problem and is always a design problem. Three fixes work: specify a drain or vent hole so the joint can be flushed and dried, degrease before welding rather than after, or accept a seam sealer applied before coating where the joint is structural and cannot be opened.

The same logic applies to spot-welded panels. Two sheets spot welded in a grid leave an oil film in the gap between them, and the film migrates out at the weld nuggets. Where the panel is visible, that is a rejection.

Blasting, and when chemistry alone is not enough

The 16 ft × 16 ft shot blast chamber earns its place on rust, scale and weld spatter. Blasting to ISO 8501-1 Sa 2½, near-white metal, removes the oxide and leaves an anchor profile that helps the conversion coating and the powder mechanically as well as chemically.

Blasting is not a substitute for degreasing on thin sheet. A 1.0 mm panel blasted hard will stretch and oil-can, and the profile it leaves can be too coarse for a 60 µm film. Heavy-gauge parts, weldments and anything with rust grade C or D go through the chamber first; the light sheet goes straight to the chemical line.

The order for mixed batches is worth stating in the process sheet. Blast, degrease, rinse, convert, rinse, dry, coat. A part that is blasted and then left on a pallet overnight will have a fresh rust film by morning, and the night shift has undone the work.

Water-break: the 30-second test that decides

The water-break test to ASTM F22 is the simplest acceptance check in the paint shop and the one most often skipped. Flood the panel with clean water and watch the film. A properly cleaned steel surface holds an unbroken water film for at least 30 seconds. A contaminated surface pulls the water into beads and dry patches within a few seconds, wherever the oil remains.

Run it on the parts that matter, not on a sample. The face that faces the customer, the inside of the bend and the area around the weld are the three places where the film breaks first, and they are the places where coating failures start. A part that fails the test goes back through the degrease stage, not forward to the booth.

The test also catches a bath that is still working on the flats but has stopped working in a recess. Water breaks first where the oil is thickest, so the pattern of the break tells you where the spray is shadowed.

Conversion coating on steel: phosphate or zirconium

Steel takes a conversion coating more readily than zinc, and the two common routes are iron phosphate and zirconium. Iron phosphate gives a crystalline film of 0.3–1.0 g/m² that grips powder mechanically and chemically, and it tolerates a slightly imperfect clean better than zirconium. Zirconium gives a thinner, more uniform film with fewer stages and less sludge, and it works well on a very clean surface.

Neither will form through oil. That is why the water-break test precedes the conversion stage: the chemistry is the most sensitive step on the line and the most expensive to get wrong, because a patchy conversion coating is invisible until the cross-hatch test.

Adhesion is checked by cross-hatch to ISO 2409 or ASTM D3359 and classed 0 to 1 for acceptance. On a phosphate film, a good result shows clean cut squares with no lifting at the intersections. Lifting along the cut line, rather than across the grid, points at the conversion coating rather than the powder. The phosphating guide covers the bath chemistry in more detail.

Film build, cure and the edge

Mild steel takes 60–80 µm of polyester powder comfortably. The number is not arbitrary: below 50 µm the film thins over sheared edges and shows the substrate, and above 120 µm it becomes brittle enough to crack at a fold and chip under impact. Once a crack reaches steel, the corrosion cell that forms under the film spreads sideways faster than the same damage would spread on an uncoated panel.

Cure is a metal-temperature question, not an oven-setting question. Polyester powder needs 180–200 °C at the substrate for 10–15 minutes. A heavy bracket on the same rack as a 0.8 mm panel reaches temperature later, and the panel can be overbaked while the bracket is still undercured. Rack parts of similar mass together, and record the oven profile for the batch.

Thickness is measured with a magnetic dry film thickness (DFT) gauge. On mild steel the reading is direct, which makes it a reliable check, but take the reading on a flat area away from edges and welds; a gauge near a bend reads high because of the geometry, not the film.

What salt spray does and does not tell you

ISO 9227 neutral salt spray is an accelerated comparison, not a service-life prediction. A CRCA part with a good iron phosphate conversion and 60–80 µm of polyester typically runs into the hundreds of hours before red rust appears at a scribe, and the exact number depends on film build, edge condition and conversion quality. Two parts from the same drawing can differ by a factor of two on edge quality alone.

Write the requirement as a method with acceptance limits: the standard, the duration, whether the panel is scribed, how far corrosion may creep from the scribe, and whether blistering is permitted. That turns a laboratory number into a purchase specification, and it is the only form in which salt spray belongs on a drawing.

What Ganesh runs for mild steel parts

Parts are cut on a 12 kW fibre laser with a 40 ft × 16 ft bed or blanked on presses from 20 T to 300 T, formed on the 200 T 5-axis brake, welded on the CLOOS cells or the manual stations, then blasted where the surface needs it in the 16 ft × 16 ft chamber and cleaned on the seven-tank line. Powder goes on in the 18 ft × 14 ft booth, cures in the oven, and DFT and cross-hatch adhesion are recorded for the batch.

The plant holds ISO 9001:2015 and ISO 14001:2015, and the effluent treatment plant covers the chemical side of the line. Genset canopies for Mahindra Powerol and tractor components for Mahindra run through this route; the paint shop capability page lists the full sequence, and the surface treatment hub collects the other substrate guides.

What the finish specification has to settle

• Material grade and thickness, plus the surface condition on arrival — oiled, pickled or rusty.
• Whether the part is laser cut, sheared or blanked, because the edge condition differs.
• Cleaning restriction if the customer specifies a process, or a note that the part must be water-break free before conversion.
• Blast requirement, with the ISO 8501-1 preparation grade where scale or rust is present.
• Conversion coating type, if the customer's specification names one.
• Powder type, colour, DFT range and which faces are coated.
• Cure limit, where the assembly includes a component that cannot take 200 °C.
• Adhesion method and acceptance class.
• Salt-spray method, duration and acceptance limits.
• Masked areas: threads, earth studs, mating faces and any conductive contact.
• Packing, because a film damaged in transit fails before it reaches assembly.

The RFQ checklist covers the commercial fields, and the powder coating cost estimator shows how masking, coats and batch size move the coating price.

When this is not the right route

Powder over a phosphate conversion is the right finish for mild steel parts in general industrial service, including canopies, brackets, frames and enclosures. It is not the right answer everywhere.

For a part that will be buried, submerged or washed with chemicals, a liquid system with a specified primer and an ISO 12944 corrosivity category is the correct specification, and powder alone will not meet it. For a part with a galvanised or plated finish already applied, the pretreatment chemistry changes and the part belongs on the galvanised route. For a part that needs electrical conductivity at the joint, powder is the wrong finish at that interface and the area must be masked or left plated. And for sections above 10 mm, bending leaves the press brake and the part becomes a weldment before it becomes a coating job.

Send the material, the surface condition on arrival and the corrosion requirement with the drawing, and the quotation will state the cleaning and conversion route it assumes.