Basic materials corrosion issues

The use of corrosion tests that appropriately simulate the environment in the proton exchange membrane fuel cell (PEMFC) is discussed. Caution is advised in the interpretation of electrochemical corrosion tests. Metallic bi-polar plates typically achieve corrosion resistance by forming passive oxide films, which must be sufficiently conductive to allow high fuel cell current densities. Austenitic stainless steels are obvious candidate alloys. The key issue with regard to corrosion is that the alloy selected should exhibit passive behavior in its local chemical environment and at the electrochemical potentials imposed upon it by the anode or cathode. Different corrosion tests will yield differing corrosion rates of a given alloy for various reasons: (a), the test measures the corrosion rate at the alloy's natural corrosion potential rather than at an imposed potential; (b), insufficient exposure time was allowed for the alloy to fully develop its passive film; (c), other oxidation or reduction reactions may operate in parallel with the corrosion reactions, which will invalidate the inference of a corrosion rate from a measured net current density. Therefore, out-of-cell corrosion tests should be checked for self-consistency. Corrosion rates are typically slower in the cathode-simulating conditions than in anode conditions. Anode-side corrosion rates are predicted to be higher at low fuel cell power, but cathode-side corrosion rates are likely to be insensitive to power levels. A good, commercial grade stainless steel is unlikely to suffer perforation or collapse of the bi-polar plate within the design lifetime, but other corrosion-related failure modes may become important. Two of these are an increase in contact resistance and the release of corrosion product cations, which can reduce the conductivity of the membrane. A simple model was used to illustrate how the reduction of membrane conductivity was dependent upon corrosion rate and fuel cell power. Keywords: bi-polar plates; corrosion testing; passivation; corrosion potentials; long-term stability; Nafion; stainless steels