The new steel bolts fitted to my PA28 (and lots of GA) have some sort of coating, zinc/bright zinc plating. It never seems to be a very thick layer.
Sooner or later they start to corrode again. I was wondering is there some sort of coating one could put on an additional layer on – I am thinking silver plating by electrolysis.
Even if it cost £ 5 per bolt for a nice thick layer of silver before the paint, it would be cheaper in the long run. Any ideas welcome!
This is a very big subject and very complicated.
Use CÍC like COR-BAN 22 or LPS3 on interior surfaces, paint on exterior.
Your AMM will list approved CICs for your aircraft
I agree it’s shameful to see rust on a bolt. I use mainly stainless steel on non-structural parts (torx screws) but all my nuts and bolts are cadmium plated, so don’t rust.
Are the bolt you are using plated?
Also agree on the anti-rust spray like ACF-50 or XCP or CorrosionX.
This is a very big subject and very complicated.
Some detail might be interesting. What I do know is that the “wrong” plating can make the material brittle, and there are possible electrolytic repercussions.
Historically, aviation fasteners were cadmium plated which is a great material but has been banned by Brussels (except for military, AFAIK; same for lead and ~200 other substances) and the US manufacturers are trying to find alternatives for export. Actually aviation may be exempted too…
ACF-50 is the way to go. Great stuff.
Peter wrote:
What I do know is that the “wrong” plating can make the material brittle, and there are possible electrolytic repercussions.
The cause is not really the wrong kind of plating but rather a failure to follow the proper procedure: the phenomenon is called hydrogen embrittlement, it’s due to saturation of the base metal with hydrogen that is formed on the cathode, but can be easily reversed by baking the parts at around 190-200°C for several hours. Electrolytic repercussions are numerous, but it’s certainly better to let the small fasteners corrode if it helps keep structural parts intact.
We recently had some exhaust work done on our TB20 and I understand the ‘standard’ replacement bolts were in the region of £100 each! Crazy prices. Is this normal!?
I think they are a bit more now – an old thread.
It is normal for airframe parts. Socata are somewhat worse because they stopped making the TBs 20 years ago, and are having to gradually replace their “old man in a wooden shed” supply chain with buying parts from bigger companies, and these don’t want to make small batches of anything.
We recently had some exhaust work done on our TB20 and I understand the ‘standard’ replacement bolts were in the region of £100 each! Crazy prices. Is this normal!?
It is only normal if the manufacture uses nonstandard bolts. These can typically per the MM specification be sourced only from the aircraft or engine manufacturer and are produced in limited volume as @Peter describes. It is therefore better for the owner if the manufacturer originally specified widely available bolts in the MM, available from multiple competing sources other than the aircraft manufacturer.
My plane has some nonstandard structural bolts, typically metric in my case and if they are disassembled and reassembled, it is done with care and an elevated level of concern that they remain intact 
If the hardware is truly non structural, and especially if it is not specified in the MM, one can typically use commercial hardware or AN hardware as/if applicable.
The problem is that a European maintenance company will not normally allow a deviation from the MM, and if you point them at a regulation permitting it, especially one which has a revenue reducing effect, they are likely to suggest you take your plane elsewhere 
With European GA builders the main issue is metric fasteners, which don’t benefit from a system like the US AN/MS numbering. As usual, “Threads possibly related to this one” has some discussions
Back to the topic, why is it a problem plating steel (high tensile usually) so it actually works? My guess is that it simply is a challenge, which is why if I want anything to last I buy stainless steel – usually chinese parts from Ebay. In the 1970s and 1980s I was on motorbikes and basically all the steel would be chrome plated, and it always started to corrode within a few years. And chrome is not an ideal coating; it is brittle and chips off. Cadmium is much better.
In GA, a big issue is electrolytic action, although I believe stainless is OK on aluminium. But you can’t get really high tensile stainless.
