TiggerToo
Well-Known Member
What compound forms when aluminium is in contact with stainless steel? And what acid will dissolve it?
Aluminium salt
- Thread starter TiggerToo
- Start date
Fr J Hackett
Well-Known Member
Under normal circumstances there should be no galvanic corrosion between the two but in an environment where chlorine is present i.e. salt water then corrosion will commence and quite rapidly which is why in a marine environment the two need to be isolated from each other.
The product is probably Aluminium Chloride or Hydroxide and should be water soluble. If the deposit is not soluble then it will be Aluminium Oxide which is soluble in Hydrochloric acid.
alant
Well-Known Member
Well, it's either an aluminate, or a compound like aluminium chromate perhaps.
vyv_cox
Well-Known Member
Photo shows a masthead bracket made from an aluminium alloy and a stainless antenna fitting. An insoluble salt had formed between them over many years, hydrochloric acid removed it in a very short time, photo shows it fizzing nicely. The aluminium bracket was not attacked. However, aluminium-magnesium alloys, normally specified for marine use, are considerably more resistant to hydrochloric acid than pure aluminium or aluminium-copper alloys. So test carefully before trying it.
Fr J Hackett
Well-Known Member
Photo shows a masthead bracket made from an aluminium alloy and a stainless antenna fitting. An insoluble salt had formed between them over many years, hydrochloric acid removed it in a very short time, photo shows it fizzing nicely. The aluminium bracket was not attacked. However, aluminium-magnesium alloys, normally specified for marine use, are considerably more resistant to hydrochloric acid than pure aluminium or aluminium-copper alloys. So test carefully before trying it.
Can't imagine an aluminium salt effervescing, the oxide would certainly be dissolved by HCl
Seajet
...
' Duralac ' paste ( used liberally in the aircraft industry ) is a jolly good isolator between differing metals, for example when pop riveting monel rivets and stainless bits to an alloy mast - a tube of it may seem pricey but it will probably last the average yottie several lifetimes !
KAL
Well-Known Member
I got all excited when I saw the thread title. Thought it might be about the 'salt' that seems to form on some of my aluminium portholes then falls onto the mattresses below.
I figure it's some kind of aluminium salt, but I can't seem to stop it forming at all.
Maybe someone like Vyv might have a solution for me, as long as it's not HCl (aq)...
I figure it's some kind of aluminium salt, but I can't seem to stop it forming at all.
Maybe someone like Vyv might have a solution for me, as long as it's not HCl (aq)...
Fr J Hackett
Well-Known Member
Are there any screws or bolts through the aluminium? because plain aluminium or one of it's alloys should be quite inert as it is protected by a very thin layer of oxide that reforms instantaneously as soon it's removed. It is only when there is another metal and something like saltwater present that you start galvanic corrosion.
vyv_cox
Well-Known Member
As with all alloys that rely on the formation of an oxide film for corrosion resistance, which for us mostly means stainless steels and aluminium alloys, all is well until the film is breached locally. This results in pitting, a condition that can spread rapidly, resulting in destruction of the surface. In stainless steels the addition of 2% molybdenum, converting 304 to 316, reduces but does not eliminate pitting. In aluminium alloys, assuming the correct one has been chosen initially, anodising is the preferred method. Unfortunately this does not last for ever and any small defects such as cracks and scratches can form pits.
I would not try any strong chemicals but you might find that mild caustic solutions, e.g. sodium carbonate, will be helpful. When I first rode motorbikes in the 1960s there were regular articles in the magazines called 'go boil your head' or similar, about cleaning cylinder heads by boiling in sodium carbonate solutions. It may be that mild abrasives, e.g. Scotchbrite pads, will also be needed. A few coats of lacquer should help to prevent further corrosion.
