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Dissimilar Metal Corrosion - how do you deal with it


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I've replaced the seatbox ends due to the corrosion caused by the steel bolts onto the sill eating the aluminium over time. these now have rubber washers under the main washer. I have also noticed a fist size hole behind my seat pillar on the base. Just wondering how you guys would repair this? My thought was to cut patch panels out of aluminium sheeting and rivet it on on both sides and then seal it?

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YRM also have an assortment of plastic seals and tapes to isolate parts from another. I recently replaced rusty door bolts by stainless ones, added yrm plastic seals between the hinges and body and applied tef-gel, which should prevent galvanic corrosion and is resistant to marine environments.

Silly expensive stuff, if you do a search here you'll likely find similar products.

 

Joris

 

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I have stainless screws in my motorcycle engines since decades. No problem.

Galvanic corrosion is a problem, that corresponds with thickness of materials and an electrolyte. No electrolyte, no corrosion. Don't bolt aluminium sheet metal with stainless bolts, where salty water can be.

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8 hours ago, Sigi_H said:

I have stainless screws in my motorcycle engines since decades. No problem.

Galvanic corrosion is a problem, that corresponds with thickness of materials and an electrolyte. No electrolyte, no corrosion. Don't bolt aluminium sheet metal with stainless bolts, where salty water can be.

That's just it though - people are throwing stainless bolts around Land Rovers where you're sandwiching steel & aluminium that already naturally rot away, where salty water already gets in.

Stainless bolts tightly holding two bits of nice hot aluminium engine together aren't so bad, but that's a very different application.

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This is exactly why I used the Tef Gel and plastic seals between the components, so I'm not really 'throwing stainless around'. Anyway, if I'm not mistaken most parts concerned are steel (bulkhead, doors, hinges) since it's a 2nd gen Td5 (except for the body at the rear door hinges)

I didn't know about stainless being brittle though, but I don't expect the hinges of the doors to be under thát much stress.

Joris

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SS bolts tend to be brittle, and the galling is a big PITA when it decides to bite as SS is very hard to drill out. Wasn't accusing you specifically of throwing stainless around, just that it's generally to little or no benefit and can be detrimental if used without thought - and let's face it, most owners though process seems to be "Ooooohh, shiiiny!"

Also, BZP is cheap, plentiful, and tough.

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  • 2 weeks later...

Duralac or Tef Gel are good products - but both work in essentially the same way.  They fill the crevises between the metal parts preventing ingress of water (which acts as the electrolite, enabling ion exchange to take place).

If you can keep water out, you won't get electrolytic corrosion.  Duralac & Tef Gel 'creep' into all the crevises which makes them very effective but they are fairly expensive.  Sikaflex is also good - but you have to ensure it gets into all the crevises as it will not creep in itself (messy & time consuming).

For low strain applications, polymer fasteners are worth considering.  There are some pretty good options these days though they can be expensive too.

Of all the metal fasteners, Hot Dip Galvanised steel is the best option (closest together in their Anodic Index).  Most fasteners these days are just electroplated with a very thin layer of zinc.  While these are not bad, proper hot dip are much better.

Copper (in the form of copper-slip) makes the corrosion much worse, even if it makes it easier to remove the nut.  Thick grease (propshaft grease for example) is a better option.

Si

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