Letter 3092

Need advise on plating thickness measurement 

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Can you please let me know which is the most preferred method for measuring plating thickness for Zinc-Iron electroplated over cold rolled steel rivets. ASTM B 487 (Microscopic) or B 568 (X-Ray) or B 499 (Magnetic) or B 504 (Coulometric)?. What are the advantages and disadvantages of above methods. I heard that the Iron in Iron-Zinc platings renders it difficult to get repeatable readings using the Magnetic method (like a Fisher plating thickness gage) is this true?. Thanks in advance for your answers.

Ganesh Raj
automotive components - Wylie, Texas