Exchanging a badly rusted wishbone

[mikec]

Thanks for posting this with so many details. Did you use a new screw for the front bush? Did you put in any anti-corrosion agent to prevent corrosion between console and the screw in the future?

 

[Joga]

Thanks for posting this with so many details. Did you use a new screw for the front bush? Did you put in any anti-corrosion agent to prevent corrosion between console and the screw in the future?

Yes, new genuine screws was purchased and mounted. Cant quite remember if I used any corrosion inhibitors, but I would like to think that I added some into the thread cavity from the other end after mounting the long screw.

 

[mikec]

I just remember that you put heli-coils in there, which are probably some sort of steel.
Was there really no way to save the threads, or did you just not try very hard? I haven't looked under my car at this yet, but were there any threads protruding from the back that could have been cleaned up, or maybe even cut off, before unscrewing?

 

[Joga]

I just remember that you put heli-coils in there, which are probably some sort of steel.
Was there really no way to save the threads, or did you just not try very hard? I haven't looked under my car at this yet, but were there any threads protruding from the back that could have been cleaned up, or maybe even cut off, before unscrewing?

No screw threads are protruding, on the contrary actually.
Regarding helicoil; this screw is to be torqued rather much so I wanted strong threads.

 

[mikec]

No screw threads are protruding, on the contrary actually.

... and they still don't want to come out? I thought someone wrote, somewhere around here, that the corroded threads of the screw get inside the bracket and ruin the threads that way, but if they don't even stick out, how can they do that? If they get stripped a different way, then can they get so corroded that no penetrating oil will help, because it can't repair what's not there?

I'm not an expert on metals, so I don't know, but is corroded aluminium/steel (i.e. in this case, the bracket/steel threads) really all that strong? If not, then the bracket threads were hopefully designed not to corrode totally, as to not lose tensile strength of the assembly, and well, may be salvageable? Wishful thinking? Hmm, corrosion probably expands the volume of whatever metal, creating a heck of a tight 'fit' in there.

Kind of going in circles here, trying to figure out how they get stripped...

Looking at it more closely, does the section of Al thread closest to screw corrode to the screw, bond to it more strongly than its surrouding 'brethren' Al , and get ripped apart from its 'brethren' on rotational movement of the screw? Again, don't know exactly what goes on in there, so hypothesising.

From what I remember in high school, oxidation?, and material 'lowest on the scale' gets sacrificed?

Just wondering if there really is a way to loosen the interface between the very corroded bracket/screw, or if its corroded, you're screwed and there's nothing you can do.

Maybe that's a topic for some engineering/metallurgy forum though.

 

[Guest 4123]

Corrosion in this context is caused by the galvanic reaction between steel and aluminium, the aluminium corroding much more rapidly in the presence of water. Coating the threads with grease might help prevent this reaction to some degree. Unfortunately using stainless Helicoils exacerbates this problem - don't expect such a Helicoiled thread to release easily in the future!

RAB

 

[mikec]

Does the corroded aluminium 'stick' to the steel bolt more than the aluminium metal it came from?

Unfortunately using stainless Helicoils exacerbates this problem - don't expect such a Helicoiled thread to release easily in the future!

Really! How come? Chrome and nickel content?

 

[Guest 4123]

Just compare the different anodic indices of the different materials from here:

Galvanic corrosion - Wikipedia

en.wikipedia.org

The larger the difference, the greater the corrosion under similar conditions. The material with the most negative index will corrode and the bigger the difference, the greater the corrosion.

One reason why you shouldn't put sterling silver and stainless tableware in a dishwasher at the same time!

Corrosion can be prevented or minimised by plating of the components - that's why it's important to use new bolts for such applications.

RAB