[A/S] Re: Fasteners

["nickcrowhurst" <NickandSandra@xxxxxxxxxxx>]

Hello Dick. This came up in my description of chassis corrosion. 
Electrolytic corrosion occurs between dissimilar metals in contact, 
especially when in the presence of an electrolyte such as rain water. 
The ideal with an aluminium skin would be to use all aluminium 
fastenings, but hardened aluminium bolts are like hen's teeth. The 
severity of the corrosion depends on how far apart the two metals are 
on what's called the Galvanic Scale. Suffice it to say that stainless 
steel and aluminium are fairly close, so the corrosion is not as 
severe as with, for example, iron and aluminium. For this reason, 
aluminium masts on sea-going yachts are fastened with stainless 
steel, and , in particular 306 grade stainless. (If you really want 
your hose-clips to stay rust-free, including the screw tightener, it 
should be stamped "all 306"). To further minimize corrosion, the 
stainless fastenings are bedded on a non-conducting paste, gasket, or 
grease. The most common paste is zinc chromate, a yellow thick paste 
which is smeared over the contact surfaces before the fastenings are 
inserted. Your local chandlery or pharmacy may supply it. Failure to 
do this will cause the higher metal in the scale (aluminium, in this 
case), to disintegrate over a period of years. Internal fastenings 
are not so problematic as they should be dry, and free from the 
naturally acid rain or salt water from treated icey roads. I hope 
this helps. Best wishes, from Nick.