Conversion coating is the process of forming a protective layer by transforming a metal surface through a chemical or electrochemical reaction. The difference from electroplating is that no metal is deposited onto the surface from outside; the surface itself is converted.
Phosphating and chrome-free passivation are common examples.
What is it for?
- Increases corrosion resistance
- Improves paint adhesion to the surface
- Provides surface preparation
Its place within CQI-12
Conversion coating is within the scope of AIAG’s CQI-12 Coating System Assessment.
There is a common mistake here: because conversion coating is the pretreatment for paint, it is assumed not to be assessed separately. It is. It has its own requirements, its own process table and its own control parameters.
The optional Form Builder feature in the 3rd edition of CQI-12 allows customized documents to be prepared that better reflect the actual process of the system being assessed, particularly for the aqueous pretreatment and conversion coating sections.
Typical parameters controlled
Parameters such as bath concentration, temperature, contact time, pH, rinse effectiveness and coating weight are the subject of process control. Monitoring and recording these parameters is the basic expectation of the annual self-assessment.
Related training: CQI-12 coating system assessment training — auditing the pretreatment, conversion coating and curing steps separately is addressed hands-on.