CORROSION BEHAVIOR OF STAINLESS-STEEL AND NICKEL-BASE ALLOYS IN MOLTEN-CARBONATE

The corrosion behavior of five commercially available alloys (AISI 316 L, AISI 310S, Inconel 601, Thermax 4762, and Kanthal A1) in molten car bonate under reducing gas atmospheres was investigated with cyclic vol tammetry and quasi-stationary polarization curve measurements. The rea ctions that proceed on these materials at distinct potentials could be deduced by comparison of the cyclic voltammograms and polarization cu rves with those of pure metals and model alloys. The shape of the pola rization curves of all materials strongly depends on the preceding ele ctrochemical treatment. A polarization curve recorded immediately afte r immersion of a sample resulted in a high anodic current. This implie s that the passivation of the materials is poor. When a specimen was c onditioned at -1060 mV for 10 h before recording the polarization curv e, the anodic current diminished, which indicates passivation. This oc curred for all materials except AISI 316L. A ranking of the corrosion properties was determined from polarization curves of samples that had been conditioned assuming the current densities to be representative. The resistance against corrosion of the alloys increases in the order : AISI 316L < Inconel 601 < AISI 310S < Kanthal A1 = Thermax 4762. Bec ause the current densities were recorded at molten carbonate fuel cell (MCFC) anode potentials, this order may also apply for real MCFC oper ation conditions. The order determined from electrochemical experiment s is in agreement with the results of exposure tests which are more ti me-consuming. This study confirms that the presence of large amounts o f chromium and significant additions of aluminium have a beneficial ef fect on the corrosion resistance of stainless steel and nickel-base al loys.