On many aircraft there are some steel parts used where corrosion occurs and the galvanic reaction between dissimilar metals causes problems.
Can anybody see any issues with having selected parts hot dipped galvanised before painting? I will give one example, on a PA28 there is a steel step which bolts onto the fuselage and I often see these rusting on older Aircraft. I also see corrosion on the fuselage where the step bolts on.
If one looks at the Galvanic Table below, it seems there are two advantages
1. When the paint breaks down, there is less of a galvanic voltage difference between Aluminium and Zinc than the original steel which must be a good thing.
2. The zinc plating is least noble so after the paint breaks down, the zinc will get “eaten” away before the aluminium which must be a good thing?
Issues I can see would be
(a) The heat when the steel part goes into the hot dip tank might cause some sort of hardening/ change the strength of the steel (in which case it could be electroplated rather than hot dipped galvanising) – I believe hot dip gets a thicker layer of zinc on the component compared to electroplating.
(b) The galvanising layer might cover up cracking in the part but I don’t really believe this is an issue as the part will be inspected before and then painted after the process – the only issue is some sort of high build primer might be needed to make the part smooth again.
Moving on to critical parts, I am replacing my rear wing spar attachment plates as there was some corrosion. It was caught in time and the aluminium surface below is now in perfect condition after the corrosion polished out. . New steel parts have been ordered from Piper and I am told that these new plates have a a Zinc Chromate layer. With new paint and the correct assembly process the job should be good for another 39 years so that’s good enough for me!
However, why don’t Piper hot dip galvanise these parts?
I don’t know enough to say about the electrolytic aspects but hot dip galvanizing is indeed highly effective.@pilot_dar may know more.
One issue is that it is hard to get paint to stick to zinc. I have used various paints intended for that purpose but none of them seem to work; they just chip off. However I have not yet tried epoxy primer. And it much depends on whether the component flexes; any flexing tends to break the paint bond quite well.
The step is hardly a structural part. The worst that can happen is that you might break your ankle if it snaps off while you are jumping on it 
The footsteps on the Socata TB (I believe both the fixed ones, and the retracting ones on the TB20/21 GT) are also steel. I think the retracting ones have a brass or bronze bush in them. Whether this reduces the corrosion issue I don’t know. You could do something similar i.e. insulate the steps from the airframe.
That’s an interesting table and it explains why the magnesium which I have in my workshop (I sometimes machine things out of magnesium) corrodes more or less as you are watching it
Piper was famous for using galvanized steel for firewalls instead of stainless steel. In other areas I think as Peter suggests the issue is paint adhesion. It’s certainly been an issue on cars where galvanized body panels are used.
Socata also use galvanised steel for the firewall. Quite thin – less than 1mm AFAICT. They don’t bother painting it. On the cockpit side you can’t really see it except in the footwells, and on the engine side they stick the horrible foam all over it (which one has to scrape off and replace after a few years).
Peter wrote:
One issue is that it is hard to get paint to stick to zinc.
Correct me if I am wrong, but a zinc coating can be treated with chromate primer to convert its top part to zinc chromate, which should hold paint nicely.
why don’t Piper hot dip galvanise these parts?
1. Galvanising adds cost.
2. Galvanising adds weight – about 400 g/m2 on thin steel parts, according to EN ISO 1461.
3. The directors of Piper wish to feed their families, so 39 years and counting is not an unreasonable life for a cheap steel part.
4. It could be argued that aircraft technicians also need to eat. They can earn money by removing, treating and repainting rusty parts.
I don’t think there’s any FAA or EASA fatwa against having steel parts bright zinc- or cadmium-electroplated before painting – even supposing that the Feds would ever know. A few thin coats from a rattle can of ZG-90 is almost as effective.
