What aluminum?

[GrahamC]

Recently I had small model airplane engine on which I broke a cylinder hold down bolt (m2.5 socket head cap screw) right off flush with the surface of the crankcase. I use a torque driver so that I don't this sort of thing (or strip threads) so my best guess is it was a below par SHCS. The crankcase is case aluminum.

There was nothing to grab hold of to screw it out. So, I tried the old trick of soaking in a near saturated solution of alum (in a small ceramic bowl on a tea cup warmer). This has always worked in the past and has never done anything to 6061 aluminum but it turned this case aluminum crankcase a dull grey colour (an insert made of 6061 was not changed in the least) kind of like the colour you might expect of oxidized magnesium (perhaps not quite as dark).

It did loosen up the steel SHCS enough I could now turn it with the tip of pin and eventually got it out but it does beg the question - why did the crankcase turn grey? What would be in the aluminum used to make these castings that would react this way to alum? The crankcase has a bronze crankshaft bushing that was untouched.

Also, I had weighed the crankcase before and after - it lost .1 grams in weight from before to after. Some sort of oxidation took place but what and why?

Any ideas what might make this cast aluminum different?

cheers, Graham in Ottawa Canada

 

[bearcar1]

Good day to you Graham, might your casting have just a trace of magnesium in it such as aircraft Ally? I do not know for certain but that is just a rather uneducated guess

Best regards
BC1

 

[rickharris]

Perhaps this helps - from wikipedia:

Aluminium alloys are mixtures of aluminium with other metals (called an alloy), often with copper, zinc, manganese, silicon, or magnesium. They are much lighter and more corrosion resistant than plain carbon steel, but not as corrosion resistant as pure aluminium. Bare aluminium alloy surfaces will keep their apparent shine in a dry environment due to the formation of a clear, protective oxide layer. Galvanic corrosion can be rapid when aluminium alloy is placed in electrical contact with stainless steel, or other metals with a more negative corrosion potential than the aluminium alloy, in a wet environment. Aluminium alloy and stainless steel parts should only be used together in water-containing systems or outdoor installations if provision is made for either electrical or electrolytic isolation between the two metals.

 

[Noitoen]

Also, I had weighed the crankcase before and after - it lost .1 grams in weight from before to after. Some sort of oxidation took place but what and why?

.1 grams is the weight of the screw you've removed.