This bachelor thesis was made in collaboration with Scania. The objective was to find the cause for defects found in some rear axle welds. It was known axle material was coated with anti-corrosive oil. Oils were examined through ICP-AES, and then compared to the composition found on the surface of the steel samples. Elements found in the oils vastly differed from one another. One of the oils contains large amounts of aluminium while the other contains high levels of calcium. When samples surfaces were analysed using EDS, phases consisting of aluminium and calcium were observed. These results indicate that the wrong anti-corrosive had been used for the axle material which gave substandard welds. The oil used contained elements with a low vaporisation temperature, such as calcium. This causes instabilities in the keyhole, leading to collapse. Collapse of the keyhole facilitates the formation of defects.