I should entitle this article "Oops: Additions, Corrections and Comments." Every once in a while I get comments and e-mail pointing out some facet of fastener technology and plating that I did not know about or even where I made an error. Luckily it is not too often that I do not have the full story about one of my topics.
The stainless-steel black-oxide article generated a lot of response and several letters that pointed out that there are non-sulfide-producing products out there working well. Several pointed out that corrosion resistance of more than 200 hrs could be obtained with sealers and other commercially available products. This is true, however, testing more than 17 different current automotive stainless steel fasteners in use by the Big Three showed them all to be black oxide with either oil or wax coatings. These, obviously, do not hold up. The lots were traced back to at least seven suppliers and from what I could gather, they thought that five coaters were involved. It seems that there are other processes to produce black surfaces on stainless steel.
These fine companies use processes that are EPA friendly, quick, fast and economical. They do not produce the ferritic sulfide "smut" but rely on other chemical reactions. While they inform me that corrosion resistance is excellent (more than 200 hrs neutral salt spray), the literature states that a sealer coat is used. As with all blackening operations, cleanability and surface preparation are about 85% of the finished coating. Too little has been emphasized about how clean and contaminate free the parts must be for just about any finish to coat and effectively operate on a fastener. Look for an article on this in the future. As usual I welcome comments at firstname.lastname@example.org or Fax me at 810-9390-0832.
To answer another question about other methods commonly used for blackening. A copper/selenium black salt coating is produced from an acidic bath. Used at ambient temperature, it produces a uniform black color within 1-2 minutes. Copper and selenium are classified as restricted, at the very least and difficult substances to treat and dispose of. Many of the baths used are non-intrusive; they are not released into the environment. They are closed systems with chemicals added to replenish the amounts used. Still, use of potentially toxic chemicals means that something may go wrong, sometime, somehow. Most automotive fastener manufacturers and users are adamant about not doing anything that may reflect back on their corporate image. Many fastener processes and coatings have been turned down for use by the OEMs because they do not wish to be portrayed as backdoor despoilers. (See Products Finishing, December 1998, "What ever Happened to Cadmium?")
Black Magnetite (Fe3O4) can be produced by a caustic soda bath in about 20-30 minutes. Running at almost 300F (exact details and temperatures vary, see chemical manufacturers' literature) the coating is acceptable for most uses. A third method that is not greatly used today is to seal the parts in a retort and heat them to about 800F and then inject steam. This also produces a black Magnetite. In all these cases, a sealer must be used to generate corrosion resistance.
Birchwood Casey informs me that it uses an iron oxalate deposit and blackens fasteners with a stable dye in its patented process. It is EPA approved, run at temperature run and self-contained (chemical being added for replenishment only). They call the process "ColorSafe."
Electrochemical Products informed me that its Ultra-Blak 407 black oxide for stainless steel produces little or no smut and meets all the requirements of its customers.
For more information on sources and methods consult your copy of the Products Finishing Directory and Technology Guide. It's a great information source for everything in the plating industry.
One last comment, to M.M. in Houston, I will be writing an article about the pros and cons of topcoat lubes vs. integral lube chemicals in finishes for fasteners in the coming months. This should answer any further questions that you may have.