A large share of complaints about blistering, flaking and early rusting on painted metal products originates not from the paint itself but from the surface preparation carried out before painting. Even the best paint cannot adhere permanently to an oily, rusty or non-activated surface.
The basic steps of pre-treatment
1. Degreasing
Oils and dirt picked up during forming and machining create a separating layer between paint and metal. Alkaline degreasers (BiyoDeoil) remove this contamination in immersion and spray lines. Whether the surface is clean can be checked with a simple water-break test: on a clean surface water forms a continuous film, while on an oily surface it breaks into droplets.
2. Pickling and rust removal
Mill scale and rust on hot-rolled steel are removed with inhibited acid baths (BiyoPickle). The inhibitor limits excessive attack on the base metal and the risk of hydrogen embrittlement while the acid dissolves rust and scale.
3. Conversion coating
Phosphating forms a micro-crystalline layer on the metal surface to which paint can adhere, and provides additional protection against under-film corrosion.
- Iron phosphate (BiyoPhos Fe): An economical solution for indoor products that can combine cleaning and coating in a single step.
- Zinc phosphate (BiyoPhos Zn): For outdoor and automotive applications requiring higher corrosion resistance.
4. Rinsing and drying
Adequate rinsing after each chemical step prevents one bath from contaminating the next. The quality of the final rinse water directly affects the risk of blistering after painting.
Passivation of stainless steel
Stainless steel can partly lose its “stainless” character due to free iron picked up during processing. Passivation (BiyoPass) removes this contamination and strengthens the natural protective oxide layer. ASTM A967 is a widely referenced standard for passivation treatments.
How is quality measured?
- Coating weight: Adequacy of the phosphate layer,
- Cross-cut test: Paint adhesion,
- Salt spray test: Corrosion resistance.
Maintenance and refurbishment
For the maintenance of existing rusty surfaces, rust removers and rust converters (BiyoRust Off) prepare a suitable base for a new coat of paint.
Frequently asked questions
How often should I check the phosphating bath?
Depending on production volume, baths are usually checked by titration every shift or at least once a day. Keeping acidity, temperature and concentration within the target range is essential for consistent coating quality.
Should I choose iron phosphate or zinc phosphate?
For products used indoors that do not require high corrosion resistance, iron phosphate is an economical solution. For outdoor, automotive and long-warranty products, zinc phosphate is more suitable. The decision should be based on the customer’s salt spray requirements.
Why is rinse water quality so important?
Salts left on the surface after the final rinse can attract moisture under the paint and cause blistering. It is therefore recommended to use low-conductivity water for the final rinse and to measure conductivity regularly.
Conclusion
The pre-treatment line is the invisible but most critical part of the paint line. Regular bath analysis and the right product choice are the most effective ways to extend warranty periods and reduce return costs.
Related products: Metal Surface Treatment Chemicals
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