Why are bolts of such poor quality in corrosion resistance?

Peter wrote:

But you can’t get really high tensile stainless

Stainless is OK for non structural parts, but common aerospace design wisdom is that for structures in general, and fasteners in particular, Alu alloys and Steel have a better strength-to-weight ratio, with Titanium where high-temp is a must. At least that was common thought until the advent of this stainless thingy:

The history of stainless steel is that during WW1 a metallurgist was trying various additives to iron to increase its strength. He tried chromium but it was no good at all, so he threw it onto his pile of rejects. A month or two later he noticed that while all the others had gone rusty, this one was still as shiny as the day it was made.

No doubt these days it’s possible to make high tensile stainless steel, but it didn’t start that way.

Hot dip galvanized steel is more susceptible to fatigue. Microscopic cracks in the surface act as a notch where a crack will grow from. This is why you don’t see hot dip galvanized bridges, but there is research being conducted at the moment.

Concerning stainless steels, there is a vast amount of different compositions available, all with their own strengths and weaknesses. Bumax apparently even created prestressable stainless steels:

https://www.bumax-fasteners.com/

And if you leave the steels, there are always super alloys

In real life on typical aircraft there are not many external screws with critical strength requirements . Most of them just fix panels to the fuselage and need not be stronger than the alu around their place. So any Chinese stainless screw will do nicely. Any inspector at the annual criticising these replacements has to be a complete muppet and I´d question his competence. Same goes for most screws found inside the fuselage for simple fixtures, no need for high tensile, so again have stainless there. But use MoS2 paste or mild Loctite on threads when doing them up as some nuts may be a “safety” type grabbing the screw thread for friction and stainless can seize there as it is not plated for avoiding seizures . Recently I did the " 5€ Socata" screw job at home , some Torx, some slotted as not enough on stock at home with M 5 thread. That was less than one hour easy action for about 15 of them, for the flatter countersink head type in aviation . Who cares about that mod at all ? Vic

Nice work

I bought a few k of the Socata ones from a “source” (which is now gone) and sold off most of them, but yeah they could be machined up.

I like the collet you are using.

johnh wrote:

The history of stainless steel is that during WW1 a metallurgist was trying various additives to iron to increase its strength. He tried chromium but it was no good at all, so he threw it onto his pile of rejects. A month or two later he noticed that while all the others had gone rusty, this one was still as shiny as the day it was made.

That is an interesting story

Antonio wrote:

that was common thought until the advent of this stainless thingy

Not so, this other one was also SS and I presume its fasteners too

And then of course there was InconelX for hypersonic aircraft, which may be considered a kind of SS.

I recently have had an issue with structural cherry max rivets (hard to remove) having been used for fastening the plastic elevator tips on my Cessna (as opposed to soft aluminum pop rivets)

Even the thin .025 2024 alu clad sheet the plastic is attached to is too soft for these hard rivets and thus labour-intensive for fastener removal without damage. No need for fasteners which are one or two orders of magnitude stronger than the material they are holding…

Another case of 100USD-worth of fasteners where 5USD-worth of pops would have sufficed excelled.

Antonio wrote:

Even the thin .025 2024 alu clad sheet the plastic is attached to is too soft for these hard rivets and thus labour-intensive for fastener removal without damage. No need for fasteners which are one or two orders of magnitude stronger than the material they are holding…
Another case of 100USD-worth of fasteners where 5USD-worth of pops would have sufficed excelled.

These are not only unnecessary, it’s downright wrong to use them. It’s a fundamental engineering principle that the fastener should be less strong than the fastened material. Another situation of the same kind occurred to my colleague a couple of years ago. When working on a small jet (probably a Citation, though I am not sure), they used a stronger adhesive than specified in the maintenance manual. A year or two later they were cursing themselves when taking the same assembly apart. Yet another mistake of this kind is using high-strength threadlocker (such as red Loctite) instead of medium-strength (blue) one – this is a known problem on the Rotax-made engine in the BMW F650 motorcycle: if you get an air bubble in a certain part of the oil system, unscrewing the oil valve to bleed it is a nightmare.

Archer-181 wrote:

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.

This is mostly due to galvanic corrosion. What do you want to corrode? replaceable bolts and screws, or the aircraft structure? Fasteners should, as a general rule, be of the same or lower grade on the galvanic scale.