Page 86 - MetalForming October 2016

Coating Parts
As part of the normal plating process, another chromate layer is added to a part after it is plated to control corrosion of the plated surface, a fact that coaters should be aware of, as this chromate finish often is the source of the adhesion problems they face.
The finish on the plated part can be partially destroyed by a negative chemical reaction between the chro- mate and the phosphate cleaning process used by most powder-coating and paint finishers. As a result, the powder coating, paint or other coating then is being applied over a “loose” surface of partially removed chromate that can lead to adhesion failure. That, in turn, leads to either a scrapped part or to customer dissatisfaction.
Chromate Solution
Gatto Industrial Platers gives metal- part coaters three recommendations to meet this challenge.
1) Ask the platers you work with to
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skip the chromate step in the plating process. Powder coating directly over zinc plating actually produces the opti- mum adhesion result. However, the chromate finish protects the plated part from oxidizing during the period between plating and powder coating. The plater has a couple of options to control oxidation, such as using a bright dip after plating or applying a rust inhibitor.
2) When working with chromate- finished parts, ensure that the chro- mate has fully cured after plating before applying any type of coating. Full cur- ing usually requires 24 hr., and any powder coating or painting performed during the curing period will bring problems.
3) Replace the typically used phos- phate cleaning process with a simple rinse. Parts are relatively clean after plating, and this change avoids the phosphate/chromate reaction and leads to better coating adhesion.
Outgassing Challenge
A metal surface has many micro- scopic pores that can trap plating chemicals.
Later, when powder coaters cure their powder applications—usually at temperatures of approximately 350 F— these residual chemicals turn to gases and release through the surface of the powder, creating pinholes or bubbles in the coated surface that make the finished product unacceptable.
Outgassing Solution
Our experience shows two effective solutions to this problem, one to be addressed by a change in the usual plating process, and the other by a change in the usual powder-coating procedure.
1) Following plating, platers should eliminate the normal chromate-appli- cation step, which can contribute to chemicals remaining trapped, and then bake the finished parts for 1 hr. to
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MetalForming/October 2016
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