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Surface Oxidation of trivalent passivation system

We, a major in Auto Electricals in India. We do zinc and zinc alloy plating for various corrosion requirements and we do stick magnets with epoxy based adhesive and cure them at 150-200 deg.C after plating and passivation. We do find issues with the bonding strength. When we analyse the surface energy levels, we find difference among passivations which shows some improvements after heating.

We like to understand:

1. What is the phenomenon happening here and how the surface energy increases after heat treatment?
2. What is the surface oxidation phenomenon levels of trivalent chrome passivation systems viz. Clear (Blue), iridescent yellow and black?
3. What is relation between surface oxidation and neutral salt spray results?

Can any one helps us in this regard. Thanks in advance.

Narasimhun Parthasarathy
Chemist - Chennai, India
2007

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Hello,
Trivalent chromate passivation on Zinc plated surface is fairly a stable layer and can be heated without the surface undergoing any structural changes unlike the hexavalent type.But the layer is not having self-healing property of Iridescent yellow passivation(Cr+6).Hence any physical damage like scratches may affect the neutral salt-spray results.The amount of Cr+3 on the film will decide the salt-spray test hrs.The amount is in the increasing order for the transparent, thick (with slight iridescence) and black trivalent passivation respectively.I hope this answers your question.

Subramanian Ramajayam
consultant - Bangalore, India
2007