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Corrosion behaviour of Nickel‐aluminium alloys in molten carbonate
Author(s) -
Vossen J. P. T.,
Makkus R. C.,
Janssen A. H. H.,
de Wit J. H. W.
Publication year - 1997
Publication title -
materials and corrosion
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.487
H-Index - 55
eISSN - 1521-4176
pISSN - 0947-5117
DOI - 10.1002/maco.19970480405
Subject(s) - materials science , aluminium , metallurgy , corrosion , oxide , aluminium oxide , nickel , grain boundary , microstructure
The corrosion behaviour of nickel‐aluminium alloys with aluminium contents of 2‐50%, in molten carbonate has been investigated with electrochemical techniques in combination with post‐test analysis of quenched specimens. For the 2 to 10% aluminium alloys a type of aluminium oxide is formed along the grain boundaries of the base metal at potentials of ‐1100 and ‐900 mV. At potentials of ‐700 nmV 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, aluminium and oxygen. The only corrosion product that could he detected by X‐ray diffraction on specimens quenched after polarisation at ‐700 mV or more anodic is NiO; no type of aluminium oxide could be detected, probably due to the small amount of aluminium oxide formed. For the 20% aluminium alloy a type of aluminium oxide is formed along the grain boundaries 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 layer of pure nickel, and an inner 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 ‐700 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 potentials. The only corrosion product formed at all potentials is α‐LiAlO 2 ; the stable form of LiAlO 2 is γ‐LiAlO 2 . α‐LiAlO 2 is probably an intermediate product that is slowly transformed to γ‐LiAlO 2 . The oxide layer provides good protection against further corrosion of the base material. The quasi‐stationary polarisation curves are very similar to those of pure nickel, because no protective oxide scales are formed on the 2 to 20% aluminium alloys during preconditioning at ‐1100 mV.