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#7589
efr215
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When I wrote that last bit I just knew that someone would pick up on the seawater thing!

Now I’m no metallurgist but it is my understanding that the villain is chlorides, it is they that promote the stress cracking. The thing is that it is not the seawater per se but the presence of the chlorides that causes the problem and while there is plenty in sea water there is enough elsewhere too, if I read it right as little as 5ppm under the right conditions!

Is this a real problem in this instance? Well possibly, the studs are under stress, (that’s if you remembered to tighten the boogers down!), there is water, there is some heat and there is vibration, not an entirely comforting combination. The really alarming thing is that if all the conditions are just so then the damage can be very rapid indeed, hours rather than weeks it would seem and what is worse the material can appear unaffected, it requires more than a Mk.I eyeball to see the damage.

So would I use stainless? Well on balance probably not, the mechanical properties of high tensile steel, formulated as it is for just this application, has much to recommend it over stainless, the corrosion problem notwithstanding. To minimise rusting I’d certainly consider as fine a finish as possible on the studs, even polishing and then some form of passivation.

The other half of the equation is of course the aluminium head. Is there anything there that can be done to minimise the problems? Well I’m sure there are some fancy and expensive proprietary processes available but if the holes can be cleaned out down to bare metal, sand blasting being my favourite because of the good key that is formed, then the holes can be coated with an epoxy resin. What with the loss of material from the casting by corrosion and by skimping a bit on the stud clearance a sufficiently thick layer to be durable and protective should result. At least no harm will be done to the casting by having a go!