It is not very common that plated finishes are top coated with organic finishes (like electrocoat, powder coat or liquid paint). But certainly, the dual coat system is used in several industries. Top coating any type of “inorganic” plating over steel substrates with an “organic” polymeric finish can be challenging for adhesion and aesthetics. Their compatibility depends on the chemistry of the two finishes from the suppliers.
Plated finishes are deposited over the course of several consecutive baths with subsequent rinse stages between and sealants after. Plating is primarily used as a standalone finish. It can be very difficult to distinguish from appearance alone that a final rinse/sealant is dirty, contaminated or just not compatible with an organic finish. For the most part, final rinsing does not matter in the plating application because the aesthetic properties of plating can be more important than the corrosion requirements. However, upon the application of an organic coating on top of the plating, cleanliness, contamination, or incompatibility of a final rinse/sealant will create problems. Those issues arise either when the two different finishes react or during the heat cycle to cure the organic finish.
Conventional plating these days typically does not provide significant corrosion protection as they had in the past. Several protective materials were identified as significant health hazards. Those were replaced with less harmful yet less effective materials. The old saying “they don’t make them like they used to” comes to fruition here.
Giering Metal Finishing Inc. is well versed in organic and inorganic finish combinations. We exemplify almost 70 years of coating experience to help our customers prevent issues like these and combat potential rejections.
Photos above are of parts that were punched and formed from cold rolled steel sheets. They have only been zinc plated, not yet top coated with electrocoat. Significant signs of contaminant film on top of plating are seen from a dirty rinse/chromate sealant. These residual films will cause defects in the top coated organic finish.
After top coating in electrocoat, the imperfections caused by contamination in the plated finish telegraph thru the top coated finish can seen above.