G'day Andrew
I think it is the sulphate ions which do the damage to the iron. Both
iron and aluminium are trivalent so this could help the reaction.
Corrosion of the iron would be helped by it being an alloy so there
would be galvanic couples on the iron/electrolyte interfacee at which
accelerated erosion would occur. The aluminium ions in the
electrolyte could be deposited as Aluminium Hyroxide leaving iron and
sulphate ions.
That's about the limit of my chemistry knowledge. Some years back I
was involved in Cathodic Protection and have maintained some interest
in corrosion matters since; few people seem to have any grasp of it
or be aware of the issues involved.
Regarding brass: There is a real risk that the zinc would be eroded
from the brass preferentially to the copper in the alloy. You need an
acid ion which would form insoluble and impervious salts with both
copper and zinc; off hand I can't think of one. I think Copper
Chloride is only partially soluble so it is possible that a zinc
chloride solution could be used. It would need to be neutral so
Killed Spirits of Salts may be the answer. A strong solution of
common salt, Sodium chloride, might be worth a try. At least with
brass the galvanic couple between the copper (in the alloy) and the
iron would mean erosion of the iron.
Regarding Yorkshire and Urine: It has been said that Yorkshire men
are full of .... and wind, far be it for me to reinforce that! Urine
was also collected by dyers (a name in my ancestry) because it acts
like alum to fix dyes in cloth)
One good turn deserves another.
Regards,
Ian
--- In 7x12minilathe@..., andrew franks <andyf1108@...>
wrote:
I've been following this thread with interest; it's been ages
since I broke a tap in aluminium, so it's bound to be my turn again
soon. I have been trying to find out what alum actually is. "True
alum" seems to be the double sulphate of aluminium and either
potassium or ammonia *, whereas "papermaker's alum" seems to be just
hydrated aluminum sulphate. Which of these "alums" will attack taps
broken off in aluminium, or will any of them work?
And will the process work for brass, too?
* apparently, there was once a thriving trade in human urine here
in the UK, to supply the Yorkshire alum makers with ammonia, when
they weren't burning seaweed to get potassium!
Andy
