Galvanic Corrosion

We all know, I hope, that there is a risk of galvanic corrosion when stainless steel and Aluminium are in contact.

But what is the ‘corrosion product’ ?

I have some ‘high quality’ weather instruments which are suffering

Aluminium bodies on an aluminium mount; all fixed with stainless steel fasteners which have corroded-in

My usual favourite dismantling fluid, Plus Gas ‘Formula A’ has proved ineffective … so I fear we may have a layer of Aluminium Oxide, acting as a very powerful ‘retainer’.

Any suggestions, please.

MichaelG.

Heat, even from a heat gun or hair dryer might help. Repeated heating and cooling by spraying on WD40 or Plusgas etc.

The must-have tool for similar situation on aluminium motorbike engine cases with zinc-plated or stainless screws that corrode in place over the years is an impact screwdriver. The old-fashioned type you belt with a hammer. Makes short work of it. But not so good if the structure is made of flimsy thin sheet ally etc.

If you don't have an impact screwdriver, sometimes pounding on the screw head with a hammer and steel drift or punch can shock the threads loose. But you have to be lucky.

Otherwise, you have to drill them out. Left-hand drill bits work best for that as they usually unscrew the shell of the fastener as you go.

Edited By Hopper on 30/04/2022 23:57:43

Edited By Hopper on 30/04/2022 23:59:31

Thanks for the thoughts, Hopper … but I think the suggested methods may be inappropriate for the relatively delicate instruments

**LINK**

http://www.windspeed.co.uk/ws/index.php?option=displaypage&op=page&Itemid=98

**LINK**

http://www.windspeed.co.uk/ws/index.php?option=displaypage&op=page&Itemid=48

**LINK**

http://www.windspeed.co.uk/ws/index.php?option=displaypage&op=page&Itemid=59

… gentle heat & PlusGas is probably the best option.

MichaelG.

You need a Zinc anode attached. This will act as a Sacraficial anode.

Steve.

Yes rather delicate by the looks. Heat and Plusgas definitely more appropriate. Another old trick is to heat a piece of steel bar up cherry red with the propane torch then hold the end of the red hot bar on the screw and let the heat flow into the screw.

Otherwise, careful use of left hand drill bits is usually successful.

Michael

As you surmise the corrosion product is aluminium oxide. The problem arises because the volume of aluminium oxide exceeds the original volume of aluminium which effectively locks the stainless steel into the thread ( or even through holes ).

When assembling, when I remember or when I am not in a hurry, I grease all contact surfaces with teflon grease. Fishing reel grease may be a suitable alternative as it is designed for the same conditions.

I haven't tried the usual copper grease as copper and aluminium are not known to play well together.

Incidentally, for anyone who plays in the marine environment stainless steel and forged steel ( as in the average quality chain or shackle ) do not make good bedfellows. When permanently submerged a steel shackle can corrode out in about a month, depending on size, when attached to a stainless fitting.

A good rule, often not achievable when dealing with fixings into aluminium castings, is to stick with the same metal throughout.

In the case of fixings into aluminium you could try titanium screws which are less reactive.

I have not found anything that will selectively dissolve the aluminium oxide without attacking the surrounding aluminium but would be very interested if anyone has the answer.

Pero

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Thanks Steve … but that’s ‘prevention’ for which it is already too late

MichaelG.

These days you can get non-metallic anti-seize compound from the car parts store, made for use on aluminium cylinders and heads etc with steel fasteners and spark plug threads. Seems to work as well as the old copper slip as far as the anti-seize factor goes. .

Edited By Hopper on 01/05/2022 05:49:20

Horses and barn doors, but this might help at re-assembly time. We used this or something similar on my father's yacht.

LINK

Following up on the Plusgas suggestions (I use it as well) looking at the SDS it is mainly kerosine. Other organic solvents worth a try if you have them to hand. Diesel, a bit stinky but I've seen stuff left to soak in this for a few days getting released eventually. I've also seen toluene and xylene suggested on line. Haven't tried these but you've probably got some around in the form of brake or carb cleaner or paint solvents.

In all cases thinks in days rather than hours for the stuff to do its work.

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Thanks for the confirmation, and the very clear notes, Pero

The assembly was begged from the local Council, when roadside monitoring was abandoned.

I wanted to remove the units for inspection and cleaning before putting them to use; but the damage appears to be already done.

I did manage to release the two screws which clamp the cap to the pole … even this was very difficult and involved using more force than I would be comfortable applying to the instruments.

Your closing lines are very pertinent !

MichaelG.

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P.S. __ I did find this :

https://www.researchgate.net/post/What-is-Aluminium-oxide-soluble-in

.. but haven’t yet worked-out how to introduce sufficient Citric Acid into a tapped hole with a big screw filling it.

Edited By Michael Gilligan on 01/05/2022 06:07:28

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Thanks to you too, Graham … our posts crossed in the æther

Yes, I am familiar with Duralac, and was intending to use it when/if I ever get to the re-assembly stage.

I do have Xylene, but only in small quantity [as a thinner for Canada Balsam] … so I should probably test it on the white residue visible on the screws that I did manage to remove.

MichaelG.

From what I understand, it's perhaps not really a corrosion product, but a result of a process - maybe thats the same thing??

Al in contact with stainless, supplied with a suitable electrolyte ( dirt, air acids, mists, salt, etc) results in the electrons migrating from the Al to the stainless, weakening the Al in the region, and converting Al to Alumium oxide. This in itself is not the problem, as Al oxide is a non-conductor and so the immediate interface corrosion ceases. It does however continue deeper into the Aluminium. And therein lies the problem Al oxide occupies a much greater volume than the pure Al, and so the S/Steel fixing is nicely locked in place, as in a Vise-grip...

There is mention of Copperslip being a no-no, or any copper in contact with Al - But as I know it, is copper not in fact less of a no-no than stainless - But I am not sure what is an 'Active' and a 'Passive' Stainless steel - Or is it that a 'Passive' S/Steel has been passivated in an acid bath?

If I have it correct, the closer in the table two materials are, the less the co-valency problem? Copper is closer than 'passive' stainless...

Does not really help you Michael, but rather interesting.

Stainless / Al must be a problem though, since every 3-5 years I hold in my hand one of my window closers, the aluminium window frame having corroded completely away around the closer's stainless steel fixing screws - 50meters from the Atlantic ocean so lots of salt about.

Have you measured continuity tween screw and body? if none, then the oxide has filled the space completely - then you could perhaps try electrolysis - fill the cavity with salt water and apply a voltage to get current flow for a few minutes and see if the screw loosens - reverse the polarity and repeat?

• Magnesium
• Magnesium Alloys
• Zinc
• Beryllium
• Aluminum Alloys
• Cadmium
• Mild and Carbon Steel, Cast Iron
• Chromium Steel (With Less Than Or Equal To 6% Chromium)
• Active Stainless Steels (302, 310, 316, 410, 430)
• Aluminum Bronze
• Lead-Tin Solder
• Tin
• Active Nickel
• Active Inconel
• Brass
• Bronze
• Copper
• Manganese Bronze
• Silicon Bronze
• Copper-Nickel Alloys
• Lead
• Monel
• Silver Solder
• Passive Nickel
• Passive Inconel
• Passive Stainless Steel (302, 310, 316, 410, 430)
• Silver
• Titanium
• Zirconium
• Gold
• Platinum

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Nice summary, Joe … and thanks for suggesting an electrical continuity test : I will try that after breakfast

Given that I believe the ‘cavity’ is most likely filled with Aluminium Oxide … I do struggle to see how I can now fill it with salt water

Am I being dim ?

MichaelG.

The sacrificial principle is good if it can be applied, but on a point of detail the electrochemical series goes: Iron, Zinc, Aluminium, Magnesium. So Zinc protects Iron but not Aluminium. Magnesium is the boy.

To answer Michael's question, I don't know what the corrosion product is. Aluminium Oxide for sure, but maybe also the Hydroxide, plus products resulting from the disintegration of the alloy which might contain Copper, Zinc, Silicon, Manganese or Tin. An unholy mess!

Aluminium Oxide doesn't react with ordinary chemicals, and the few that would shift it also attack Aluminium. I guess penetrating oil and heat, but predict only limited success. Unless someone knows different!

Dave

A bit of background info. from the horses’ mouths:

**LINK**

https://bssa.org.uk/bssa_articles/bimetallic-galvanic-corrosion/

**LINK**

https://www.ssina.com/education/corrosion/galvanic-corrosion/

MichaelG.

Dismantling - a combination of heat, penetrating fluid, light percussion, and patience tends to work.

To reassemble use the "yellow paste" (Duralac) liberally. You might also consider alternative thread inserts like helicoils or rivetnuts.

(Used) automatic transmission fluid is a superior penetrating fluid.

Source - I own an aluminium boat.

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Thanks for you ‘informed’ advice, Jann

… I still feel a little uncomfortable about the application of heat and/or percussion to these instruments, but as they say:

“needs must, when the devil drives”

MichaelG.

For really stuck fixings on motorcyckes I always ended up with having to drill them out, best with left hand drill bits as previousky mentioned. I know there are more modern anti seize compounds and the galvanic table proximity but I have always used copper grease with no problem disassembling at all even years down the line as have the countless other bike tinkerers.

Michael

Is the any merit in using a mechanical/vibrating engraving tool with a suitably shaped bit to apply a repeated impact to the screw in an attempt to break the bond. It shouldn't damage the sensor but will deliver continuous moderate energy impacts with a short period. It may well be worth a try before more aggressive methods are employed.

In a former life I could have chatted to my colleagues with regard to this type of problem as we deployed ocean buoys with a similar sensor arrangement.

Cheers

Martin

As said, too late to stop the problem now but the 'ignorance' of designers ' to foresee the problem continues.

Entry of moisture /electrolyte down to molecular size is the enemy , permanent contact of oil/grease the friend ..