Chromates / Passivates

 

Passivating is part of the electroplating process and for zinc plating this has traditionally been performed using chromate based solutions. The passivate, or more correctly termed ‘chromate’ solution, is used after the plating process to provide a sealing layer over the zinc layer. Passivating the zinc extends the corrosion resistance of zinc plated parts significantly and can also be used to add a particular colour or appearance. Typical colours are clear, yellow, black and olive green.

Trivalent Chromates are a more recent development in passivation solution technology containing Cr(3), following the demand from the automotive industry for all materials to be ‘hexavalent chrome free’. One of the reasons you’ll never see chrome plated bumper bars and fittings on your car anymore, as the solution contained Cr(6) as its primary element. Trivalent chromate solutions are also classed as thick or thin film. Thick film trivalents were developed to give comparable corrosion resistance to the hexavalent chromates used in zinc passivation.

Hexavalent Chromates are the traditional passivation solution containing Cr(6) as popularized by chrome electroplating providing excellent corrosion protection in thin film form and even better as a thick film. Thin film versions also give excellent colour and appearance without appearing iridescent or streaky. Unfortunately hexavalent chromates are not as popular as once before due to environmental and chronic health concerns. Advances in trivalent chromates has seen comparable corrosion resistance and appearance requirements being met, but not without significant increases in raw material and maintenance costs.

Moving forward all new passivates are  trivalent chromates to satisfy RoHS (Restriction of Hazardous Substances Directive) and REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) directives.

 

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