What type of material is this and how is it corroding?
Though a picture is not really enough information, approximate location at least and any simple tests on the materials shown would help a lot.  

Its structural and ugly so its steel.
Its mostly likely painted steel. I'd say the paint flaked off so it has general uniform corrosion. Why it looks black and not the regular red-brown is probably combination of oxides and sulphides formed as the corrosion products. I don't see a hint that its galvanised steel.
Steel in exterior uses  rarely suffers any corrosion other than  general uniform corrosion.