Is it galvanised steel and not iron? If so I think you will get a reaction between the galv and aluminium which will weaken the aluminium via galvanic corrosion. Ally's a bugger for that, it reacts with so many different metals, some faster than others, and a reaction with iron as it corrodes is guaranteed.
You can avoid it by putting a good insulating layer, a decent hard plastic separator between the two metals, or make sure any contact areas are painted to death with a good Hammerite/Smoothrite type paint to make sure that the two metals can't come in contact with each other.