Chromate on bare steel

TUTORIAL FOR NEWBIES:

Steel can be protected from corrosion with organic coatings like paint or powder coating, or with inorganic coatings including zinc plating (an electro-chemical process) or galvanizing (dipping into molten zinc).

Coatings are sometimes subjected to an accelerated corrosion test, most commonly the neutral salt spray test per ASTM B117 or ISO 9227.

Although the zinc coating protects the steel from rusting ("red rust"), the zinc may also corrode ("white rust"). To deter that white rust, zinc plating is almost always 'chromate conversion coated' and hot dip galvanizing is often 'chromate quenched'.

Until about the year 2000 most chromate conversion coatings used hexavalent chromium because it was more effective than trivalent chromium. But in response to European Reduction of Hazardous Substances pressures, most chromates today are trivalent, and development efforts have made them approximately equal to hexavalent chromates in corrosion resistance.

This chromate conversion coating is done not only on zinc plated surfaces but on cadmium plating, on aluminum, and a few other metals.

Something which is running between the lines on this thread is the question of whether chromate conversion coating can be done on bare steel, and the answer is no, it doesn't convert steel and isn't useful.