Carburisation and oxidation driven erosion of electroplated chromium in a large-calibre artillery barrel

Chromium electroplating is widely used to enhance erosion resistance and extend service life-time of artillery barrels. In the present study, the mechanism for premature erosion of an electroplated artillery layer was systematically investigated after firing tests by using a combination of electron and ion microscopy. Spallation of chromium layer is closely related with propagation of erosion attack along the chromium-steel interface and the underlying steel matrix upon growth of erosion damage. The pre-existing micro-cracks in electroplated chromium act as the preferential pathways for penetration of erosive propellant gas, facilitating rapid crack propagation. Growth and coalescence of pre-existing micro-cracks further lead to formation of micro-cracks penetrating through the entire thickness of chromium layer, facilitating erosion attack along the chromium-steel interface and the underlying steel matrix upon the presence of erosive propellant gas. Propagation of erosion attack along the interface was initiated by preferential carburisation of chromium layer leading to non-equilibrium M 7 C 3 carbides formation. This further facilitates rapid propagation of erosion attack along the interface and in the adjacent steel matrix via a combination of non-equilibrium carburisation and oxidation processes. The growth of interfacial erosion damage and subsurface erosion pits both contribute to spallation of chromium layer via a decohesion mechanism.