Silver Cad V. Yellow Zinc Hardware

[t6dpilot]

I have read with interest in the past the ongoing debate on silver versus yellow plated hardware and when the change in manufacturing happened. While this doesn't really apply to my cars as they are too late, I thought I would share with you an easy way to get silver plated hardware.

I was recently visiting with one of my VERY talented friends in the aviation restoration world (and who really desires an early 911). He was showing me the museum and restoration shop that he helped found, along with a couple of their restoration projects. Not discounting by any stretch the restoration talents here, but his restoration work on the Mustang and P-40 are over the top. And a lot of this stuff needs to be created from scratch. If you are ever in Cincinnati, OH, do check out Tri-State Warbird Museum and tell Paul and Diane that I sent you.

Anyway, Paul being an absolute stickler for originality on the P-40 restoration needed to come up with a way to have access to silver plated aircraft hardware. They don't make silver AN hardware today - it is all zinc chromate (or is it dichromate?) yellow. He shared with me this easy way to get beautiful silver hardware from yellow plated hardware and I thought I would share it here for you guys. Here is what you do:

1. Purchase Sodium Hydroxide (lye) NaOH from www.chemicalstore.com,
2. Mix some in water and bring to a boil,
3. Drop in your yellow hardware for a few seconds,
4. Voila, beautiful silver plated hardware!

And it doesn't damage the elastic stop nut. His set up is amazingly simple - old pot on a single electric burner. Hope this helps somebody...

[304065]

Interesting. So the yellow stuff is a chromate conversion coating, the sodium hydroxide basically removes it, leaving the zinc plating underneath.

Here's the problem. Fasteners used in critical applications must be baked after plating to prevent the fastener going brittle. While others with much better knowledge of metallurgy can explain far better, basically what happens is that at elevated temperatures, single hydrogen molecules diffuse into the steel, and then recombine in the voids, reducing the ductility of the metal and potentially resulting in cracking.

So after critical fasteners are treated with hydrochloric acid in the plating process, they are baked in an oven for a certain amount of time to release the hydrogen.

Putting them in boiling lye would seem to be the precise opposite of what you would want, particularly for critical fasteners on an airplane. Heat and hydrogen, even in a alkaline bath as opposed to an acid, could potentially lead to hydrogen embrittlement.

I would rather mechanically buff the conversion coating off with some jeweler's rouge on a wheel, at least where the judges would see.

Pop quiz: Name at least five places where a longhood uses 10K or 10.9, or higher property-class fasteners? This is where you would REALLY care.

[Zithlord]

Seat gliders/rails, undercarridge to floor
Brake calipper through bolts
Swing arm bolts in the rear suspension
CV bolts
Flywheel bolts
Quite a few in engine/gearbox

[NorthernThrux]

Evaporust handily takes of the yellow dichromate coating in 10-20 minutes without the heating and potential hydrogen embrittlement. But just to be safe, I'd still use a shop which plates and then processes the parts to the SAE AMS 2759/9 Section 3.3.3.1 standard.

[chris_seven]

Just a few comments on the subject of hydrogen embrittlement and I appreciate that I am being picky:

'Nascent' Hydrogen which is atomic in nature - rather than molecular - is produced in an electrolytic cell due to overvoltage effects.

http://www.fera.org.uk/pdf/Fera%20ac...march%2006.pdf

Hydrogen atoms are small enough to diffuse into a material and interations between hydrogen atoms and imperfections in the crsytal lattice of the material and/or grain boundaries is the most likely cause of the problems.

Although electroplating is a common cause of hydrogen embrittlement any treatments that involve acidic solutions are also a real threat as the reaction of metals and acids generally liberates hydrogen.


Alkaline or neutral solutions are much less effective in this respect as they tend to take up hydrogen rather then liberate hydrogen and in general are much less of a threat and not something I would normally worry about.

There has, however, been some evidence in the last few months that Black Oxide treatments can cause 'caustic embrittlement' and Sikorsky have, for the time being stopped using this treatment but there is no real explanation of the mechanism at the moment.

Most Black Oxide treatments involve boiling in a caustic solution.

I find it difficult to accept that hydrogen is the cause of this embritlement as the amount of hydrogen generated by caustic solutions is very low compared to acidic processes.

The reduction in corrosion protection caused by stripping the chromated and hence passivation form the zinc is likley to be much more damaging and cause the fastener to deteriotrate relatively quickly.

Even if the fastener doesn't corrode the zinc layer will be relatively unprocted and is likely to develop a 'white rust' in a reasonably short period of time and look quite unsightly.