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Spot test to detect quality and thickness of chromate passivation on zinc plated component

2007

We get tons of M.S components zinc plated with yellow passivation layer from 2-3 vendors.
sometimes plated material in stock which was initially tested shows formation of black tiny spots ( initiation of corrosion) after period of 2-3 months.

I need some evaluation method to check quality of pasisvation because presently we check total zinc plating thickness and salt spray test of 2- 3 batches per week. Somewhere I read that solution of diphenyl carbazide is used to see presence of chromate . I not only need to chek presence of chromate but at the same time quality of chromate passivation so as to meet 96 hours of salt spray test.

Zankar
Engineer - Vadodara, Gujarat, India

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2007

Zankar,

It sounds more like a storage problem than a quality problem. If the parts are passing the quality control checks then you need to look for another source for your problem.
Are your parts protected with anything, such as a protective oil, grease-proof paper, VCI or the like? Can the parts have got moisture trapped in with them? Are the parts stored indoors in an environmentally controlled manner (e.g. temerature 5-35C, relative humidity less than 75%)? Are the parts in contact with wooden packaging, this may cause a problem?

Brian Terry
Aerospace - Yeovil, Somerset, UK