H2S - Only corrosive in presence of water?

I am studying landfill gas corrosion on discarded metals and their effect on the environment - Land fill gas is predominantly Methane & Carbon Dioxide but it contains a trace element of H2S, which when in the presence of water produces sulphuric acid (Which is obviously corrosive to some metals) but is the trace gas alone corrosive, or does it only become corrosive when mixed with water to produce acid?