Composition
The composition of chromate conversion solutions varies widely depending on the material to be coated and the desired effect. Most solution compositions are proprietary.
The widely used Cronak process for zinc and cadmium consists of 5–10 seconds of immersion at room temperature in a solution of 182 g/l sodium dichromate crystals (Na2Cr2O72H2O) and 6 ml/l concentrated sulfuric acid.
Iridite 14-2, a chromate conversion coating for aluminum, contains chromium(IV) oxide, barium nitrate and sodium silico fluoride.
Chromate coatings are soft and gelatinous when first applied but harden and become hydrophobic as they age. Coating thickness vary from a few nanometers to a few micrometers thick.
Curing can be accelerated by heating up to 70 °C, but higher temperatures will gradually damage the coating over time. Some chromate conversion processes use brief degassing treatments at temperatures of up to 200 °C, to prevent hydrogen embrittlement of the substrate. The embrittlement relief operation isn't performed to eliminate embrittlement from the chromate conversion but the plating substrate that is being chromated. The embrittlement relief is typically specified for electroplated steels > 180 ksi tensile strength and it is normally required prior to conversion coating. It is not the chromate conversion coating but the electrodeposition process that requires embrittlement relief (reference QQ-P-416 or other plating specifications that use chromate conversion)
Read more about this topic: Chromate Conversion Coating
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