Passivation is the process of making a material "passive", usually by the deposition of a layer of oxide on its surface, created by the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few nanometres thick.
Your fasteners will be exposed to elements, damp conditions in particular, and any break down of the passivated surface (e.g. through use of tools) will leave the fasteners vulnerable to galvanic corrosion. Although steel and aluminium appear close on the anodic index, there is sufficient potential difference to cause them to fizz in the presence of an electrolyte (water).
In industry in order to raise the potential of steel closer to that of aluminium, the steel is usually cadmium plated and then passivated, cadmium has a near identical anodic index to that of aluminium and thus creates a resistance to galvanic corrosion.
Stainless Steel and aluminium are even further apart (than plain carbon steel) on the anodic index and are therefore an even worse choice, very reactive, and should therefore not be used with aluminium.