Dear Udit

Alminium/Aluminium alloys are normally  corrosive resistance materials in the surrounding environment.   This is  because the aluminium/aluminium alloy susually  have an inherent protective (shield) film( Al2O3) in the surrounding environment.  So in your case, the  staining of aluminium wire rod was probably formed by other factors ,i.e., oil spill, dirt, animal discharges in the packaging region of the aluminium.    

Aluminium can corrode when it is in a wet or humid environment particularly if the exposure period is long.  The result in often a powder corrosion product that may vary in colour from white through grey to black, the colour usually depends on how fine the corrosion products particles are.