Gas–metal reaction products in the erosion of chromium-plated gun bores

While the general mechanisms in the erosion of gun bore surfaces are known to include coating spallation, gas–metal reactions, and melting, many of the details are issues of considerable uncertainty and debate. The present report addresses one of these issues, the origins of the loss of the protective chromium coating. A survey study was conducted on erosion damage in chromium-plated gun bore surfaces using specimens of three erosion-damaged, chromium-plated, 120-mm M256 gun barrels and two erosion-damaged, 155-mm gun barrels (M199 and XM297). Specimens were examined by optical microscopy, laser scanning confocal microscopy (LSCM), and electron microprobe analyses. New insights regarding the erosion process and the origin of chromium loss were obtained from analyses of the initial damage to the steel at the tips of the fine cracks in the chromium. Products from gas–metal reactions at the gun bore surface are usually difficult to find because of gas wash effects. Observations on unetched specimens revealed that these products remain in place in the relatively protected regions beneath the chromium when chromium crack widths are small. The initial damage from gas–metal reactions is manifested optically as gray layers or gray reaction zones in the steel at the tips of the fine chromium cracks.