In order to reduce cosmetic corrosion more stainless steel fasteners are used. However, the additional demand that they be black has caused some difficulties. Electrostatic coatings produce wonderful looking surfaces on the heads while leaving the threaded areas free to perform functionally; however, the cost to coat small trim screws is completely out of the scope of automotive volumes. Powder flow coating and paints clog up recesses, fill in threads on the small parts and cause washers to stick on nut/screw and washer assemblies.
What's left? Many suppliers suggest that black oxide be used. Long used as a black colorant, the finish has had a history of use on steel components. The coating does not increase dimensions because it is a conversion-type finish. This makes it ideal for use with close-tolerance parts such as machine tools, instrument components, camera parts, gages and even firearms. Among its positive properties is the fact that it will not change the surface appearance of the substrate. Luster, roughness, etc. of the finished part will remain the same after black oxiding. Being a conversion coating, it does not peel, chip or rub off. Removal is usually by chemical means, although bead or sandblasting works if a change in surface roughness can be tolerated. Since all steel parts above HRC 32 are subject to embrittlement, baking at 200C poses problems for some finishes. Not so with black oxide since the finish can withstand temperatures as high as 475C before significant color fade or degradation begins. That is, only if the part has not been oiled or waxed.
Black oxide can absorb oils and waxes. This property is used to improve its appearance and corrosion resistance. Tumbling with oil, wax or other compound-impregnated media imparts a glossy shine and luster to the parts. Any non-corrosive and chemically inert material can be used and the process may be via wiping, dipping or tumbling with a variety of media. Sawdust or rags are two popular media that do not damage sensitive and critical surfaces. Finally, the coating is an inexpensive black coating and is easier to apply than most paints and platings.
A negative to the black oxide finish is the fact that the coating is not a corrosion- resistant finish. It is not suitable in any situation for outdoor use, a fact that negates its selection for stainless steel in the first place. In samples of stainless steel parts tested for suitability with black oxide as a coloring agent it was found that the parts exhibited red rust in unacceptable amounts. Series 400 stainless steel parts rusted more rapidly when black oxide finished than did parts left plain-passivated. Series 200 parts rusted faster than did Series 300 parts and showed more undesirable rust product (flake rust vs. red "wash" rust) when oxided than did plain-passivated parts. Series 300 stainless steel, the main material of most threaded fasteners, showed red rust (see photo) on parts with nominal salt spray hours.
An investigation of why this was occurring showed that the nature of the black oxide coating was responsible. The surface layer of the stainless steel had been converted to iron oxide (another term for rust) and ferric sulfide "smut." This smut was black and colored the oxide layer. While post applied oils and waxes reduced the rate at which the finish corroded, the failure was still fairly rapid.
Since numerous manufacturers request black oxide as a stainless steel surface coating for fasteners, the reason why no complaints were evident was researched. What happens in the real world is that the rust that forms on the screws on the outside of vehicles washes off regularly with every rainfall and car wash. When the finish has been completely removed by these means, the surface of the stainless steel screw remains pure stainless steel, which is non-rusting and a dull gray in color. This evidently is acceptable to customers over shiny chromed parts.
When asked for a suggestion for a coating on stainless-steel fasteners be sure to ask what environment it will be serving in. Outside and moderate-to-severe conditions should be serviced by fasteners with one of the high-life organics, probably over an inert base electroplate, or recommend that the part be left in a passivated state. Comments?