KEY POINTS
- Pretreatment cleans the metal and builds a conversion layer that the powder bonds to. Many adhesion and corrosion failures trace back to this stage.
- Industrial lines use 3 to 7 stages. More stages give more control over cleaning, etching and rinsing quality.
- Conversion chemistry can be chrome-based or chrome-free. The choice depends on substrate, certification and local environmental rules.
A powder coating can look perfect when it leaves the oven and still fail months later in the field, peeling at the edges or blistering in coastal air. When that happens, the cause is often not the powder but the surface underneath. Pretreatment is the part of the line that decides long-term adhesion and corrosion resistance.
What pretreatment has to achieve
- Remove contamination: extrusion lubricants, oils, dust and handling residues.
- Remove or control the oxide layer: aluminium carries a natural oxide film that is uneven and contaminated after extrusion and storage, so it is a poor base for the coating. Etching and deoxidising replace it with a uniform, clean surface.
- Build a conversion layer: a thin chemical layer that bonds to the metal and to the powder, and slows corrosion spreading under the film.
- Rinse clean: chemical residues left on the part cause blistering under the coating, so final rinses are often done with demineralised water.
Typical stage sequences
| Line type | Typical sequence |
|---|---|
| 3-stage (basic) | Combined clean and conversion coat (for example iron phosphate on steel) → rinse → final rinse or seal |
| 5-stage | Degrease → rinse → conversion coating → rinse → final rinse |
| 7-stage (architectural aluminium) | Degrease / etch → rinse → rinse → deoxidise → rinse → conversion coating → demineralised water rinse |
These are typical sequences, not fixed recipes. The final design depends on the alloy, the chemistry supplier's process, the certification target and the water quality on site. Ying Group designs the tunnel around the chemistry, not the other way round.
Chrome vs. chrome-free conversion coating
Chromate conversion has a long track record on aluminium and gives strong adhesion and corrosion resistance. However, hexavalent chromium is heavily restricted in many markets because of health and environmental concerns, and it requires careful wastewater treatment.
Chrome-free conversion, typically based on zirconium or titanium chemistry, is now widely used and accepted under major architectural certification schemes. It simplifies wastewater handling and helps factories meet export and environmental requirements. It does need tighter process control: bath concentration, pH, temperature and rinse quality all matter.
All Ying Group pretreatment tunnels can be configured for either chrome or chrome-free chemistry, matched to the substrate and certification requirement.
Aluminium vs. steel
Aluminium profiles generally go through cleaning, etching or deoxidising, and a conversion layer. Steel parts follow a different path: degreasing, then a phosphate or modern thin-film conversion coating. For heavy steel structures heading into C5 corrosive environments, mechanical preparation such as blasting is often combined with chemical pretreatment.
Spray tunnel or dip tanks?
Most continuous conveyor lines use a spray tunnel: parts move through enclosed zones where heated chemicals are sprayed from nozzles, with drip zones between stages to limit carry-over. Dip (immersion) systems reach deep recesses on complex parts. They need a conveyor or hoist that can lower parts into the tanks, often a power & free conveyor with drop sections.
Common pretreatment problems
- Carry-over between stages because drip zones are too short for the line speed.
- Poor rinse water quality, leaving salts on the part that later cause blisters.
- Bath drift: concentration, temperature or pH falling outside the process window when dosing isn't monitored.
- Incomplete drying before the booth, which traps moisture under the coating.
- Under-sized tunnels after the line speed is increased without lengthening the stages.
Good tunnel design addresses each of these: properly sized zones and drip sections, heating matched to the chemistry, PLC-monitored temperatures, and a dry-off oven sized to the line speed.
Frequently asked questions
How many pretreatment stages does a powder coating line need?
Industrial lines typically use 3 to 7 stages. Basic industrial parts may need 3 stages; architectural aluminium for certification and high-corrosion applications usually needs 5 to 7 stages including etching or deoxidising and demineralised water rinses.
Is chrome-free pretreatment as good as chromate?
Modern chrome-free conversion coatings based on zirconium or titanium are widely used and accepted under major architectural certification schemes, provided the bath and rinse quality are well controlled.
Why does powder coating peel off aluminium?
The most common cause is inadequate pretreatment: residual oils, an unstable oxide layer, a weak or missing conversion coating, or chemical residues left after rinsing. Under-cure in the oven is another frequent cause.
Can the same pretreatment line handle aluminium and steel?
It is possible with suitable chemistry and process settings, but the line must be designed for it from the start. Tell the engineers your full substrate mix before the tunnel is specified.