happy to help if I can
From my recollections both Al and Zn can be used to coat steel in processes called aluminising? and galvanising. Both metals are more reactive than iron and so react better in air.. Both form oxide coats which stick, I think the M2O3 structure and MO have similar molecular sizes to the arrays in their metals underneath. They thus stick hard to the metal and thus water/air must penetrate a layer of oxide and another metal before getting to iron.
I suspect technical or
financial reasons might mean Zn preferred. After all Zn is from a blast furnace and whilst not v strong itself makes agood coat as above. Al needs electrolysis and again I recall the extraction costs are v high, I think the obtaining of Al2O3 pure from bauxite ore is as costly as the complete extraction of iron and thus presumably Zinc. Also Al has more worth as it is as a metal esp once oxidised or anodised to thicken the oxide coat. Al however is not v strong so overhead cables at 150000 volts (is it?) are actually Al because CU too dear and yet the Al has a core of steel cable to give strength, so few Al structures around of a big scale...no bridges but plenty of masts etc light and limited strength? Probably some replaced by decent plastics now.
Anyway as I am waffling now hope that's enough to chew on. Look up some key words on a web engine for a decent chemi dept or the Institute of materials
Good wishes as ever
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