Understanding the Difference Between Chromates and Passivates

Sep 3, 2020

Chromates and passivates are terms that we all see quite frequently in the various plating specifications. These terms are often used interchangeably; however, they technically differ. In this Plateco Pointer, we will take a deeper dive into the difference of a chromate versus a passivate coating.

To achieve the desired corrosion resistance outlined in a specification, it is possible to have 3 different deposit layers. The first layer is going to be the zinc coating. With the thickness of the zinc plating the second layer will be deposited: a post-treatment chromate or passivate that is necessary to provide an improved corrosion-resistant finish.

Chromate Verses Passivate

A chromate is a conversion coating in which hexavalent chrome reacts with a zinc finish to convert that finish into a protective coating. Passivates were developed in response to the drive to trivalent passivates.

Passivates may be divided into two categories: thin and thick film. The passivate film thickness ranges from 1 micron or more for thick film passivates to less than half a micron for thin film passivates. Thin film trivalent passivates are specified to provide a thin trivalent chrome coating over the plated zinc finish. Normally, the thin film passivates produce a bluer appearance than the thick film passivates. Thick film passivates are typically heated and contain a higher concentration of metal salts. Because of their thickness, these passivates generally provide more corrosion resistance.