Bonderite and conversion coatings: corrosion protection and adhesion primer
Conversion coatings chemically transform the metal surface into a firmly adhering protective layer. They improve corrosion protection and the adhesion of subsequent paints and adhesives. Brands like Bonderite (Henkel) are exemplary of such pretreatment systems.
In short: What is a conversion layer?
A conversion coating is created by chemically transforming the metal surface into a firmly bonded, non-metallic layer (e.g., phosphating for steel, passivation for aluminum and zinc). It serves as corrosion protection and as a base for paints and adhesives. The appropriate process depends on the base material and subsequent process; the process and product data sheets are binding.
Why pretreatment determines success
Paints and adhesives only adhere as well as the substrate. A bare metal surface carries oils, oxides, and loose particles; it corrodes and offers poor adhesion. Cleaning plus a conversion coating creates a defined, adhesion-promoting, and corrosion-inhibiting base.
The most important procedures
| Proceedings | Suitable for | Purpose |
|---|---|---|
| Zinc/iron phosphating | Steel | Corrosion protection + paint adhesion |
| Passivation | Aluminum, zinc, stainless steel | Corrosion protection + Liability |
| Chromium-free alternatives | aluminum | Corrosion protection without chromium(VI) |
| Pickling/Degreasing (Preliminary Stage) | all metals | Clean, remove oxides |
Guideline values – the specific suitability depends on the product and application and must be checked in the respective technical data sheet.
Typical process flow
- Cleaning/Degreasing: Remove oils, grease, and dirt.
- Rinse: Wash out any remaining cleaning agent.
- Conversion therapy: Build-up layer (dip, injection).
- Rinsing and drying: defined surface for the subsequent process.
- Painting / gluing: within the recommended time window.
Selection: what matters
- Base material (steel, aluminum, zinc, stainless steel),
- Subsequent process (powder coating, wet coating, bonding),
- Corrosion requirements and environment,
- regulatory requirements (e.g., chromium-free),
- Application method (immersion system, syringes, manual).
Chromium-free pretreatment of aluminium
Traditional chromating processes using chromium(VI) are subject to strict regulations. Modern chromium-free conversion processes (e.g., based on titanium/zirconium) offer corrosion protection and paint adhesion without chromium(VI) and are now the standard for many aluminum applications.
Conversion coating vs. pure cleaning
Degreasing alone creates a clean but not permanently protected surface. Only the conversion coating chemically bonds, inhibits undercutting corrosion, and significantly improves the service life of the paint or adhesive bond.
Frequently Asked Questions
Do I need a primer after the conversion coating?
Often, the conversion coating itself provides a good base for adhesion. For demanding bonding applications, an additional primer may be advisable – according to the adhesive manufacturer's instructions.
How long does a conversion-treated surface remain active?
The level of adhesion activity decreases over time. The follow-up process should take place within the timeframe specified in the data sheet.
Is phosphating also suitable for stainless steel?
Passivation is more relevant for stainless steel. Phosphating is primarily used for unalloyed and low-alloy steel.
Sources and technical basis
The information is based on manufacturer and process data sheets (including Henkel/Bonderite) as well as recognized surface engineering principles. Process parameters and suitability are application-dependent and can be found in the respective technical data sheets.
How SILITECH supports
Tell us the material, corrosion requirements and subsequent process – we will recommend a suitable pretreatment system and provide the technical documentation.
