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Looking for some chemistry expertise/experience here. I bought some ARP head studs, which didn't quite fit. I don't have a thread micrometer, so I can't say exactly how oversized they were, but I wasn't going to get them in without a breaker bar (even then it's a maybe).

So I took a little off with a die and then installed the studs with a little oil. The block is aluminum. My concern is that now I may have a risk of corrosion between the aluminum block and the studs, which have exposed steel since the machining with the die removed the oxidation coating.

My guess is that I should be ok since oil is not very conductive, but I'm not very confident in that guess.

My understanding is that anytime you have dissimilar metals bridged by an electrolyte, that's a battery and the metals will corrode. What about when the dissimilar metals are in direct contact? I oiled the threads, but I'm sure there's some fraction of surface area that's not covered by oil. Is that at a risk to corrode?

So in summary, I have two questions 1) Do I have a corrosion risk between aluminum and steel, when bridged by oil? 2) Do I have a corrosion risk between aluminum and steel, when in direct contact?

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  • No corrosion unless the engine is submerged in salt water ; that will likely cause other problems.
    – blacksmith37
    Sep 9, 2020 at 18:30

3 Answers 3

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A black oxide or "bluing" coating on steel provides little corrosion protection so very little has changed corrosionwise. Parkerizing/phosphate gives a little protection but not enough to make a difference here. And ,as the studs are installed , likely some of the original surface is broken. These surfaces are likely applied to help hold lubricant on the fasteners to reduce the risk of galling of the aluminum. ( Not for corrosion protection).

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  • Why do you say little corrosion protection? My meter shows no conductivity through the black oxide. That seems like excellent protection.
    – user2864293
    Oct 2, 2018 at 17:38
  • It is enough to prevent any rust in storage or inventory; no one wants a brand new fastener with rust.
    – blacksmith37
    Oct 2, 2018 at 22:46
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Electrolysis happens with the passage of electric current.

Steel will corrode in aluminium - removing the protective coating was not a good idea.

Where did you get these new studs? - studs supplied from the manufacturer ie “original” parts rarely have this oversize problem.

If these “new” studs came from a nut & bolt supplier then not only the size may be incorrect, but the grade as well.

I would suggest you source the correct replacement studs from a dealer.

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  • The studs are from ARP. The OEM option was regular head bolts, and I wanted to use studs because I'm increasing the performance of this engine.
    – user2864293
    Oct 1, 2018 at 5:04
  • When you make changes, you should evaluate everything... I would have used a tap and oversized the threads fractionally, but checking the wall thickness and depth. However, the thread diameter change to accomodate a small change in the size of those new studs would not have been a problem (most, but not all, can be oversized to take a helicoil).
    – Solar Mike
    Oct 1, 2018 at 5:09
  • For electrolysis you need an electrolyte. Steel is the more noble of the metals, it is the aluminium that will corrode. See - bssa.org.uk/topics.php?article=89. This is why steel is often zinc plated. The zinc is the least noble, hence sacrificial.
    – HandyHowie
    Oct 1, 2018 at 8:23
  • @HandyHowie water can be sufficient... But you still need a potential difference... And having removed corroded studs, the steel does corrode - just more slowly than the aluminium...
    – Solar Mike
    Oct 1, 2018 at 8:28
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The automobile manufacturers use stainless fasteners in aluminum heads routinely.

You don't have too much worry.

You can use stainless and aluminum together but expect you will have corrosion eventually and prepare for it. But your aluminum or steel will not rust away.

Of course you cannot weld these metals together.

Read around the web, this is well trodden ground.

https://www.practicalmachinist.com/vb/general-archive/will-s-s-bolts-work-aluminum-88508/

https://www.bassresource.com/bass-fishing-forums/topic/20093-what-screws-bolts-for-aluminum-boats/

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  • What manufacturer routinely uses stainless head bolts ? Martensitic or austenitic / magnetic or nonmagnetic ( sort of). Bolts for my car are listed on the net as "steel" ,very unlikely to be stainless.
    – blacksmith37
    Oct 1, 2018 at 20:58
  • I think auten/martensitic are stainless. Anyway what do all the OEMs do when they are running aluminum blocks and steel fasteners? Go to Summitracing, seems like just about all the fasteners there are stainless.
    – geoO
    Oct 1, 2018 at 21:09
  • I believe these particular ones are 8740 Chrome Moly
    – user2864293
    Oct 2, 2018 at 17:44
  • Chrome .nickel , moly; ( 8600 and 8700) these elements mostly provide good heat- treat response ( hardenability).
    – blacksmith37
    Oct 2, 2018 at 22:51

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