Trivalent Chromates 101

To coat metal parts, chromates are used to enhance the life of the part. Typically, chromates are used on zinc plated parts whether it be alkaline non-cyanide or chloride applications. These trivalent chromate conversion coatings offer prolonged corrosion protection to finished parts.

In the past, hexavalent chromates were predominantly used throughout the metal finishing industry. Today, to meet RoHS compliances and environmental safety standards, the metal finishing industry
has moved in the direction of trivalent chromates. Trivalent conversion coatings have shown to produce a scratch resistant coating that more tolerable to heat than hexavalent coatings. The latest technology in trivalent chromates has shown acceptable corrosion resistance in salt spray (ASTM) passing over 120 hours to white rust. A trivalent chromate coating does not slow down the corrosion independently, but serves as a protection barrier for the zinc deposit which does protect the ferrous substrate. Although the coating is typically very thin, the chemical film formed holds the ability to heal small scrathes and imperfections through recoating. Thusly, they have been termed “self-healing”.

When trivalent chromates are used in process, they are the last chemical step in the finishing unless the use of a chemical seal or paint is required. To form a functional conversion coating, the finished parts need to be allowed to dry at ambient air temperature for at least 24 hours. The curing process of the coating can be expedited by heated dryers or other mechanical means.