So would a thick layer of Cadmium do the trick? I am not trying to buck received wisdom, I just think the plating these days is super thin!
Antonio wrote:
InconelX […] which may be considered a kind of SS.The amount of iron in these nickel based super alloys is usually way below 10%, just like, say, the iron content of Aluminium Alloy 8019.
LeSving wrote:
Fasteners should, as a general rule, be of the same or lower grade on the galvanic scale.I can’t say where this wisdom stems from, but this seems very wrong. If you join two beams with screws, do you really want the fastener with a very small diameter and a small surface area act as the sacrifical anode?
Archer-181 wrote:
So would a thick layer of Cadmium do the trick? I am not trying to buck received wisdom, I just think the plating these days is super thin!
I don’t think cadmium plating has become much thinner lately – at least aerospace standards remain the same. Also, a coating such as yellow zinc provides more or less the same degree of corrosion prevention as cadmium; the main advantage of cadmium over zinc is its lubricity – it’s just about the only pure metal that’s both galvanically active and slippery. Where this is not critical, zinc plating is fine.
Inkognito wrote:
I can’t say where this wisdom stems from, but this seems very wrong. If you join two beams with screws, do you really want the fastener with a very small diameter and a small surface area act as the sacrifical anode?
In general you don’t wont anything to act as a sacrificial anode, unless you purposely create a sacrificial anode. For structural parts with different materials, the standard is to isolate them with some RTV or similar. It also depends on the corrosion environment. In the deserts of the USA, nothing corrodes. In rainy and coastal Europe, everything corrodes unless proper care is taken. Where this is particularly important is in marine environments; boating, sailing etc, and this is where all experience and knowledge about corrosion and corrosion protection is found.
Three factors need to be in place for galvanic corrosion (in particular) to occur:
Take out any one of those three, and corrosion will not occur. Sometimes this is easy, sometimes it is necessary (but difficult), and sometimes nobody cares because the structure lasts long enough in any case. More often than not, the problem is not so large that anyone really cares all that much. In those cases some sacrificial method is often used to be on the “safe” side. In aviation the typical example is cadmium plated steel bolts. Cadmium is the sacrificial anode. Cadmium is also the metal closest to aluminium on the galvanic scale, so nothing much happens between aluminium and cadmium. If galvanic corrosion occurs, it will in almost all cases be between steel and cadmium, and cadmium will corrode (not the steel, and not the aluminium. Unless you are in marine environments where this simply is not enough).
Another archetypical sacrificial anode, which few people today actually even know is a sacrificial anode, is alclad. 2024 sheet metal is covered in a thin layer of pure aluminium. This pure aluminium is not strong at all, but very corrosion resistant. In addition it is lower on the galvanic scale than 2024. It acts both as an excellent corrosion resistant barrier, and as a sacrificial anode to the 2024. It’s the galvanic protection that is important. It will protect just as good even if scratched and at edges. Usually it is painted over. In those cases it won’t do much good at all, unless someone/something scratches through the surface.
I was thinking more about fastening non structural access panels and such. It will for sure look better with nice SS screws, but corrosion vise it is just about the most stupid thing you can do. Those panels are usually screwed into black steel plate nuts riveted in place. Those are much harder to replace than some screws.
@Archer-181 it has been a while, but last time I checked you could still get used nuts and bolts cad plated for aircraft service, typically for engine overhauls where cad was and is used by OEMs like Lycoming. The military also needs it for overhaul.
Cadmium is much more durable against abrasion than zinc and is spec’d for military aircraft fittings etc for that reason despite AN mil-spec hardware used on GA airframes having just a flash coating of zinc that disappears as fast it does on commercial hardware. Some old time UK vehicles like Norton motorcycles also had cad plating from the factory. For some reason despite much of the old UK automotive stuff being very poor in quality, their industry of the 1980s and earlier always did great plating. The Italians were at the opposite end of the spectrum with their plating so one could surmise that local climate was a factor.
Restorers and hobbyists used to take buckets of nuts and bolts for cad plating when restoring old cars and motorcycles 25 years or more ago, it was quite inexpensive and widely available in the US and elsewhere at that time and lasts almost forever. Nowadays the cost is very high and availability much lower. I wouldn’t use a commercial plater for aircraft hardware anyway as they can’t be trusted to control the bake cycle accurately.
Getting plating done properly has always been russian roulette. I was on motorbikes in the 1970s and 1980s and they have various chrome plated parts. Some were polished chrome and some satin chrome. Most of the platers were useless and the stuff just came off within a year or two.
I would think that with the EU ban on cadmium it will be hard in Europe, but in the US? The engine shop I used (Barrett Precision) used to send buckets of fasteners off for cadmium plating, and it was done well.
LeSving wrote:
In general you don’t wont anything to act as a sacrificial anode
The surface area also plays an important role, a small amount of a stainless steel in a hot dip galvanzied girder construction doesn’t matter much but vice versa it does. Very much. So, no, I wouldn’t want the fastener to be on the lower end of the galvanic scale compared to the main body.
Inkognito wrote:
The surface area also plays an important role, a small amount of a stainless steel in a hot dip galvanzied girder construction doesn’t matter much but vice versa it does. So, no, I wouldn’t want the fastener to be on the lower end of the galvanic scale compared to the main body
I have built a whole airplane of auliminium (6061) using using SS rivets. No problems with this whatsoever – In an inland environment. Both of these metals are very corrosion resistant on their own, and together they cause no problems either. In this environment this will last for ages without corroding, even in rain and in lakes. But, take that design to a coastal environment and corrosion starts immediately. The salt in the atmosphere will make the aluminium corrode galvanically while the SS rivets stay like new. What will happen is they will eventually come loose due to aluminium corroding away. Again, what is easiest to replace? a few rivets, or the whole aircraft structure? From a structural integrity point of view, the result of galvanic corrosion is the same.
Boaters have discovered this ages ago, and also found the solution (commercially available). The solution is to dip each rivet in a special compound that will isolate the aluminium form the SS. The only other solution is to use aluminium rivets (lower on the galvanic scale, same as – well aluminium 
). The problem with that (for boaters/sailers) is rivets cannot be made in high corrosion resistant aluminium, like 6061. Not ductile enough, and ordinary alu rivets will corrode immediately in marine environments. Black steel rivets are out of the question from the start due to corrosion. Aluminium with SS rivets is the best solution in marine environments, all things considered. But not unless proper additional care is taken. There is also special “boat alu” (5000 series) that can be welded as well as being super corrosion resistance. This is mainly used on hulls, not high strength parts like for instance a mast, where 6061 is used.
For an aircraft this isn’t of that much consideration, it’s supposed to be in the air after all. Priming and painting will also protect close to 100% (if done right, but this is several dozens of rabbits holes all by itself ) My aircraft is a mix of mirror polish and paint. 6061 will last forever (not shiny ) but it won’t corrode. The only thing that happens to 6061 is the protective layer of oxidation will get thicker. This is seem as the surface gets darker, almost blackish. But, there are these rivets of SS that upsets the continuity of the corrosion protection of the 6061. All my SS rivets are dipped in this special compound. Doing this it will last for ages, no priming/painting needed whatsoever. Without that compound, within days it will corrode, and the whole structure will be destroyed.
Then there are all the other stuff not specifically basic aircraft structure. Engine mounts, landing gear, all sorts of hinges and whatnot. There really is no universal one fit all solution. What works is isolating as much as practically possible every connection where different metals are used. Clearly this isn’t possible everywhere, and in those circumstances you have to make sure that the part that corrodes is easily replaceable. Nuts and bolts are easy to replace. How often they need to be replaced could be anything from every year (or even more often) to never depending on environmental factors.
Carbon composites brings even more weirdness to the table. Carbon is the material highest on the galvanic scale. Everything bolted and screwed onto carbon composites will corrode galvanically, even gold. One of the first things you learn when/if building aircraft is to never ever draw on aluminium with a pencil (graphite) It’s worse than swearing in the church With a carbon structure, there simply is no way you can make fasteners of a higher galvanic grade, because there is no higher grade. Still, aircraft made of carbon composites are the ones least likely to corrode, perhaps due to one single reason, and that is care (isolating) is taken from the very start, because this is such a well known phenomenon. That and the fact that carbon itself don’t corrode, not in the usual sense at least.
The solution is to dip each rivet in a special compound that will isolate the aluminium form the SS.
I am amazed this can be reliable.
My speciality is electronics and I know how to insulate something from a substrate. And it is quite involved if you want it reliable e.g.
Everything bolted and screwed onto carbon composites will corrode galvanically, even gold
How is that solved?
Peter wrote:
How is that solved?
Epoxy and special care using a layer of (inert) non carbon fiber where needed, for instance fiberglass. Some kind of physical isolation must be applied, and/or one must make sure moisture isn’t present?
Peter wrote:
I am amazed this can be reliable.
It will only corrode if all of these are true:
This compound protects by preventing moisture, preventing oxygen and preventing the dis-similar materials from direct contact. If one of those “barriers” breaks, there are two more. Real life experience has shown that it works just fine, but it is a messy process to apply, it’s also time consuming.
All this corrosion stuff is a whole bunch of rabbit holes. The only general thing that can be said is that if no thinking at all is applied, then corrosion will occur. But then again, maybe not, because the environment is a huge factor Inland environment is very different from coastal environment, it’s like night and day, and the geographical distance may only be a handful of kilometers depending on geography. In a desert nothing corrodes, because there is no accumulation of water and the air is dry. And then there is the basic fact that replacing nuts and bolts is simple, cheap and fast. You could also easily go completely overboard. This is typical for military and commercial aviation. They have to stay operational in all kinds of environments, outdoor, for years and years, without additional maintenance restrictions or additions.
Private GA is different. They can easily be protected well enough by simply storing them in a hangar, or even better, a heated hangar. Regarding the engine, just a few watts of heating in storage to get it a few degrees above ambient, and nothing on that engine will corrode, ever (moisture will never accumulate). Washing the plane regularly is one of the best prevention methods. This will get rid of salts and dirt crating ions in the water. Be sure to make no drainage ports are blocked and so on. Fly it regularly to ventilate all parts of the structure. There are a whole bunch of “snake oil” stuff (sprays and stuff) that do actually work very well by preventing accumulation of moisture, and some of them also have corrosion inhibiting pigments of some sort.
