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

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,

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?