CORROSION BEHAVIOR OF NICKEL-ALUMINUM ALLOYS IN MOLTEN-CARBONATE

The corrosion behaviour of nickel-aluminium alloys with aluminium cont ents of 2-50% in molten carbonate has been investigated with electroch emical techniques in combination with post-test analysis of quenched s pecimens. For the 2 to 10% aluminium alloys a type of aluminium oxide is formed along the grain boundaries of the base metal at potentials o f -1100 and -900 mV. At potentials of -700 mV and more anodic an outer oxide scale is formed and also aluminium oxide formation takes place along the grain boundaries. The oxide scale contained nickel, aluminiu m and oxygen. The only corrosion product that could be detected by X-r ay diffraction on specimens quenched after polarisation at -700 mV or more anodic is NiO; no type of aluminium oxide could be detected, prob ably due to the small amount of aluminium oxide formed. For the 20% al uminium alloy a type of aluminium oxide is formed along the grain boun daries at -1100 and -900 mV. After longer polarisation times also the aluminium in the interior of the grains is oxidised, which results in a two-layer microstructure: an outer lay er of pure nickel, and an inn er layer that is a honeycomb network of nickel filled with aluminium, oxygen and nickel, while high amounts of aluminium and oxygen could be detected along the grain boundaries. The amount of corrosion product was too small to be detected by X-ray diffraction. At potentials of -7 00 mV and more anodic, an oxide scale of irregular thickness is formed , but no oxide could be observed along the grain boundaries. The only corrosion product detected by X-ray diffraction on specimens quenched after polarisation at potentials in this range is nickel oxide. On the 50% aluminium alloy a continuous oxide layer is formed at all potenti als. The only corrosion product formed at all potentials is alpha-LiAl O2; the stable form of LiAlO2 is gamma-LiAlO2. alpha-LiAlO2 is probabl y an intermediate product that is slowly transformed to gamma-LiAlO2. The oxide layer provides good protection against further corrosion of the base material. The quasi-stationary polarisation curves are very s imilar to those of pure nickel, because no protective oxide scales are formed on the 2 to 20% aluminium alloys during preconditioning at -11 00 mV.