Fastener finishes

[Jack H.]

Re: Fastener finishes

Zinc is pretty much zinc...silver in color. Now, IF the zinc plated part underwent a secondary plating operation (short bath in chromatic acid), it'll have a yellowish (sometimes 'rainbow' hue) color. But, descriptive text would refer to this as 'dichromate' or 'irradite' and street slang includes 'yellow zinc' or 'yellow cad' since the same final sealing treatment can be done with cad plating too...

[Michael G.]

Re: Fastener finishes

The Ford document referenced below explains the basics regarding zinc and cadmium platings sufficiently to give cursory knowledge of those coatings. The document, however should not be relied upon to understand the zinc phosphate (or zinc phosphate and oil) coatings used for years by GM. GM phosphate coatings are not applied over zinc and/or manganese as stated in that document.

The zinc phosphate coating utilized by GM is applied over bare steel. To coat a part with zinc phosphate, bare steel fasteners are immersed in a a bath of zinc phosphate salt, dissolved in phosphoric acid. Without delving too deep into the chemistry involved, the zinc phosphate salt precipitates onto the surface of the fastener, leaving a grainy medium gray to almost black surface. The color difference is largely due to the size of the grains of salt deposited on the steel: Fine grain is dark gray, large grain is lighter in appearance.

The grains of zinc phosphate do not, by themselves act as much of a barrier to corrosion, they are applied to the surface primarily as a medium to absorb subsequent applications of oil, or in some cases various paints. Those paints and oils would not last long on the surface without the presence of zinc phosphate to hold them there. The addition of those treatments greatly increases corrosion protection and often adds consistent lubricity to the fastener surface thus aiding tightening.

When an early GM drawing refers to "phosphate coating" or "zinc phosphate coating", it almost always refers to some variation of GM-4435M usually code "A", which was zinc phosphate (as described above) with a subsequent application of oil. This finish offered minimal corrosion protection, but was cheap and commonly available. Later versions coated the phosphate with paint and then an oil dip, greatly increasing corrosion performance.

If you have an early fastener that was obviously not shiny plating, it is likely not "black oxide" or any one of ten other names I have heard here, it almost always would have been "phos and oil" as described above.

I hope this helps.

[Chuck S.]

Re: Fastener finishes

Actually all fasteners 1" and less were on page one, 1 -2" or so on the next page, etc.

It is proving problematic to find the thread as the post was by Franz and he is no longer active on this forum...

The fact that he's not active on TDB shouldn't be a problem. As far as I know, his name should still be on the Members List unless it's periodically updated to delete past contributors after a certain time period. In any event...just as you said...Franz Esterreicher (28907) is NOT on the list.

Finding all Franz' past posts was an easy task...he has 31 past posts, and none were related to fasteners. Interestingly, Franz' last thread (I read it) was about deleting past posts, so possibly one or more of Franz' past posts have been deleted by request...if, in fact, that's possible. I think we can safely conclude that if you are correct about it being Franz' post, we can quit looking for the matrices.

I may want to follow up on this...I've made a few posts here I would like to delete.

[Gene M.]

Re: Fastener finishes

To make understanding the plating issue easy, original grade 8 bolts are generally a phosphate or black oxide finish. This is to eliminate the relief of hydrogen embrittlement that electroplating causes. An extra operation to increase cost was not an expense GM was interested in incurring.

Both of these finishes are inferior to zinc electro plating with a black chromate secondary as a corrosion protection. It would take a pretty mean judge to deduct for a black zinc finish in lieu of the original black phosphate or black oxide.

Bead blasting or tumbling them followed by a wire wheel prior to sending them to the platers yields a good looking bolt after re-plating. Just be sure to ask for relief of hydrogen embrittlement from you plater on your graded bolts.

[Michael G.]

Re: Fastener finishes

I don't recommend bead-blasting fasteners. Clean them with a brass wire wheel, then use a steel wheel very lightly to get rid of any residual brass. The brass wheel does not dull the peaks of the threads or remove critical metal from the flanks.

Again, almost ALL gray or black fasteners on early Corvettes were GM4435-M, Code A, Phosphate and Oil. This finish is still cheap and widely available, but it rusts very quickly. If you want your parts to look correct grainy dark gray to black, but not rust for the life of your car, don't use phos and oil, or black zinc plating, go to your plater and ask for GM6047-M. It's other characteristics are similar to phos and oil and it lasts about forever on a minimally driven car. As an additional benefit, treatment for Hydrogen is not necessary, like it is with black zinc.

If you want shiny silver, that will last a while, (and don't want to handle cadmium) go to your plater and ask for 5-8 tenths thick zinc, with clear chromate. Unless you specify a zinc thickness, you get 1-3 tenths, which does nothing but look pretty for a few months indoors. At least the thicker plating will sit indoors for a few months without corroding.