Bimetallic corrosion hydrogen evolution

Deaeration has occasionally been used as a means of controlling bimetallic corrosion under conditions of total immersion, and this method of control can be used successfully, if physical conditions permit, provided that the less noble metal is not sufficiently electrochemically active to permit rapid evolution of hydrogen at the more noble metal, as is observed, for instance, in many bimetallic couples involving magnesium anodes. 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

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