Galvanic corrosion in small joints?

[R Smith 1Participant @rsmith1]

Hi all,

 

I’d be grateful for advice on whether galvanic corrosion might be an issue for connecting small parts made from different metals.

 

E.g. 1) if I want to rivet a flat brass bar (0.7mm thick) to an aluminium flat bar(2mm thick) using a 2mm copper rivet, are there likely to be any issues in the long run? Contact area will be about 5 sq mm, so we’re talking a tiny contact spot.

2) Using brass or copper bearings on a titanium axle (bearing width is 4mm, ti axle diameter is 2mm? Are the bearings/axle likely to corrode?

 

Apologies for daft Qs: I just know from wrenching on bikes that when two different metal threads come into contact one is supposed to stick some coppa slip on them, or so I’ve been told.

 

Thank you.

[Martin ConnellyParticipant @martinconnelly55370]

It may all come down to is there an electrolyte in contact with the joint? The reason the USA keep a lot of mothballed aircraft in the dessert is not because it is hot but because it is dry. Without water to dissolve some other chemical and create an electrolyte the corrosion rate is very low. The fastest electrolytic corrosion occurs in the presence of a good electrolyte and warmth, so a warm, salty and humid environment is about the worst place for dis-similar metals in contact. Coatings like paint keep the electrolyte away so help reduce corrosion, coppa slip may be there primarily as a high temperature lubricant that will not gas off or set hard if the parts get hot. It may act as corrosion inhibitor as well but that may not be its primary role.

Where I worked some joints in pipelines had to have insulation kits for the flange joints to keep the metals from making contact such as when joining a stainless steel pipe to a mild steel pipe. This often happened when replacing a pipe in an old pipe run.

Martin C

[DiogenesParticipant @diogenes]

Depends mostly on what kind of use / environment it’s going into – galvanic corrosion requires an electrolyte, so the presence / absence / degree of this strongly influences the outcome.

On a mantelpiece model the assemblies are likely to be fine, but as part of a sea-fishing reel, probably not..

[David George 1Participant @davidgeorge1]

a while ago well perhaps 40 years, I was involved in rope access and anchor points in caves and potholes We were using Steel bolt and aluminium angle brackets which were left in place and in wet or very damp placements, the corrosion was very high and the 8mm high tensile bolts corroded to half their diamiter in as less then six months. We changed the system to resin and stainless steel anchors with no major problems since.

David

[File HandleParticipant @filehandle]

Not long after we moved into our bungalow the ballcock float detached itself from the ball cock. The previous owner must have replaced the brass split pin with a steel one. It was easily fixed, but I doubt that it had been in long, almost ideal conditions for the steel pin to corrode being a sacrificial anode in the situation.
He was also a steam train enthusiast, I use his old workshop and part of his railway track is still in the garden. But I also found out that he was a bit of a bodger, some of his electrical wiring was a deathtrack. One of the spur double sockets had been wired to the ringmain with orange 2 core outdoor flex. OK for the light they obviously plugged into it, but an accident waiting to happen.

[R Smith 1Participant @rsmith1]

Thank you,

It’s a bandonion keyboard action, so no salt water (or any other liquids) involved. It gets quite humid in Scotland though!

The parts will have minimal load on them.

[File HandleParticipant @filehandle]

Just keep it dry it will be fine, water, and especially if it contains dissolved salts, will form a cell as electrons can flow resulting in problems.

[bernard towersParticipant @bernardtowers37738]

The copper slip on fasteners is usually for ease of disassembly and where stainless is concerned to prevent cold welding.

[Kiwi BlokeParticipant @kiwibloke62605]

R Smith. Good question – it’s a worry. But probably, in the vast majority of cases, a needless worry. I have four piano accordions, all bought as restoration projects, so expecting the worst… Only has suffered from significant corrosion: at brass screws into aluminium, at steel screws into wood, and at aluminium to aluminium interfaces. All are between 50 and 70 years old. The corrosion is probably the result of condensation occuring within the box, so at least one has an idea about prevention… The worst corrosion had jammed the slides completely, with masses of oxide formation and pitting of the alloy. You can’t coat these things in corrosion-inhibitors, and the ‘experts’ warn against oiling the mechanism, so it’s perhaps surprising that corrosion isn’t more of a problem.

The Italians built (and still build) accordions with no apparent effort to avoid galvanic corrosion. Perhaps that’s where the Itailan car manufacturers got the idea. In spite of this irresponsible attitude, galvanic corrosion doesn’t seem to be a major problem in well-looked-after squeeze boxes.

Your 3-metal assembly (key rod and pivot?) is standard practice. It works. If you’re still worried, assemble the parts ‘wet’, with Duralac paste – it’s designed to prevent galvanic corrosion, and is used in the aviation industry. Take note of its safety info.

If you haven’t found them already, a limited amount of technical help is available at:

http://www.accordionists.info/ and http://www.forum.melodeon.net/

Both are active fora and run much more slickly than this monstrosity.

Also, the helpful Charlie Marshall, http://www.cgmmusical.co.uk, can’t be a million miles from you.

Good luck with your bandoneon!

 

[Nigel Graham 2Participant @nigelgraham2]

One couple to avoid is stainless-steel and aluminium-alloy, though it does vary a bit by the specific alloys.

The steel eats the aluminium for breakfast; a point worth bearing in mind when building miniature railways with aluminium rails, especially ground-level where earth and decomposing leaf-litter can be a problem; or in salty coastal areas.

Galling between stainless-steel fasteners is a bit less likely if the screw and nut is of different grade.

…

Re David George’s observation…

I know!

The original belays of that type used ‘Rawl’ “self-drilling” tubular-steel expansion sleeves for the M8 X 12mm set-screws commonly holding the aluminium-alloy belay hanger-plates themselves. Local cavers would install the sleeves, which stayed permanently in the limestone. When visiting the cave we’d fit the hangers and ropes on the way down but remove them on exit, leaving the sleeves there. Many are still Down Below, now unused, quietly rusting away.

So the dissimilar-metals problem was not very common except on kit left underground for a long time.

Nevertheless I was never quite sure which came first: worn-out threads or corroded sleeves.

“Self-drilling”: the sleeve ends had sharp serrations so turning them into simple percussion-drills when screwed to a holder which one struck with a hammer while rotating it by hand.

I gather the Rawlplug company was horrified to discover we were trusting our lives to shelf-bracket fasteners! Well, two screws at a time to halve the load on each. In theory.

.

In later years I helped operate a laboratory test-tank filled with fresh water kept sweet with swimming-pool additives and filters. Although the chemicals, including calcium-hypochlorite disinfectant and copper-sulphate algicide, were at the same very weak concentrations as in a pool so safe for us, it was electrolytic enough for test-pieces made from aluminium alloy and stainless-steel screws to emerge after a few days with white measles.

“But it’s anodised!” their ‘owners’ plaintively complained.

I had previously worked for a metals-finishing company, so knew the problem; but could never get through to them that the decorative black finish on their prototypes was porous!

[Kiwi Bloke]

Oh, good grief! Proof reading failure. Correction, inserted ‘one’, in bold, below…

On Kiwi Bloke Said:

R Smith. Good question – it’s a worry. But probably, in the vast majority of cases, a needless worry. I have four piano accordions, all bought as restoration projects, so expecting the worst… Only one has suffered from significant corrosion:…

Just to add unnecessary detail, in the corroded box, the reeds had minimal light surface rusting and the bellows were fine. These are expected victims of damp storage, so the rampant alloy corrosion seems a bit strange – although condensation still seems a reasonable explanation (cold box taken into warm, humid room, etc.). Perhaps the box lived near the seaside. A steel reed, rivetted to an aluminium or zinc reed plate, might be thought to be an ideal candidate for galvanic corrosion, but that’s how they are, and I haven’t heard of this being a problem. Strange things, squeeze boxes…

[R Smith 1Participant @rsmith1]

@Kiwi Bloke

Thanks. Since I’m designing my own everything, I do have some choice in the materials and how things are joined, so thought it was worth asking… A lot of choices come down to ease of making, assembling and disassembling for servicing. Perhaps, I’ll use brass axles instead of Ti (I just had some good ti ones kicking about). Italians use steel axles and brass bushings in accordion keyboards, but the problems seem to always come from the exposed parts of the steel axle rusting and jamming during disassembly.

I agree – the real killers in accordions are steel-wood touch points and just regular rust on steel.

 

…Don’t oil the mechs… I worked on a top of the line Excelsior that had the bass mechs oiled for some reason. Probably by a cheeky repairman during a service rather than the factory, as I’m not aware of factories ever using oil. A couple decades down the line the amount of gunk that got stuck in the oil was quite impressive.

[Kiwi BlokeParticipant @kiwibloke62605]

You might consider plastic (eg teflon, acetal, molybdenum disulphide-loaded materials, etc.) bushes to bear on Ti axles. But, if you’re designing without the constraint of convention, what about flexures instead of pivots for the keyboard?

My experience of keyboard axle corrosion has been where there was steel-timber contact, not where it was exposed. The now oversize and abrasive axle enlarges the key pivot holes when it is (with difficulty) removed, necessitating the holes being peened smaller. Stupid design.

I thought someone had gone berserk with an oil can on one accordion, but it turned out that the treble coupler-slide mechanism, penetrating the base plate, had been filled with grease, presumably to make it ‘air-tight’. Oil, separated from the grease, was leaking out. I think this grease-filling may have been done at the factory, because I’ve seen two of the same make with this ‘feature’.

[R Smith 1Participant @rsmith1]

I don’t want to go too far from the convention on my first scratch build, and I’ve already strayed quite well from the path.

Steel-timber is the worst, indeed!

[Mark RandParticipant @markrand96270]

Avoid the oak family, use teak, preferably or maybe padauk and the corrosion problem shouldn’t be as great. more so if you can use anything to protect any exposed steel (nickel plating?)

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