Additional detail:
Conversion coatings involve a change or interaction between the substrate and coating material, as opposed to plating and other processes in which a material is applied over the substrate.
Phosphating - conversion coating that relies on a reaction of phosphoric acid to integrate the coating material into the surface of the substrate. Electrical current can be used to accelerate the process, but this is not typically done. Numerous materials can be applied in this manner - zinc, magnesium, iron. Iron phosphating is also known as Parkerizing. See the attached file for a thorough technical description.
Phosphate & oil - phosphating followed by an oil sealer. Magnesium is typical, but this can be done with zinc as well.
Zinc plating - zinc that has been applied through an electrochemical process.
Zinc chromate - a compound, not a coating. Is typically applied as a pigment in primer or paint.
Dichromate - conversion process used as a sealer over a metallic plating, typically zinc or cadmium. Also referred to as "yellow zinc" or "yellow cad", depending on which plating it is applied over. Yellow is common, but clear dichromating can be performed as well. Note that it can also be used over zinc phosphate, galvanizing and directly on zinc alloys such as pot metal (carburetors). Its primary purpose on zinc is to prevent "white rust". The dichromate is applied in an acid bath with dichromate salts. Older dichromate solutions used hexavalent chromium, which is toxic and highly controlled nowadays. Trivalent chromium is now used instead. See US Patent 2,035,380 for the origins of this process.
Galvanizing - zinc is applied by spray or by dipping in a bath of molten zinc.
Black oxide - a conversion coating in which iron on the part surface is converted to a black, oxidized layer. Corrosion protection is mild, be can be enhanced with an oil follow-up. Not the same nor as protective as phosphate & oil.
QPQ nitrocarburizing - a conversion coating that leaves a nice, deep, even black surface with improved corrosion resistance and lubricity. Essentially it's a black oxide, but with a different process. Not used on automotive parts to my knowledge (not vintage ones anyway), but one item in my "bag of tricks" for certain tooling.
