Initiation of localised corrosion on bearing steel

In some applications roller bearings fail prior to their expected life time. The processes of these failures are not fully understood and there are many different factors which affect the lifetime. One phenomenon which could affect the life time is the corrosion mechanisms of the steel. On defects and inclusions corrosion might be initiated even though the material does not corrode in the matrix. The aim of this project is to study the initial corrosion on martensitic AISI 52100 steel. No similar studies have been published on this kind of steel but some studies on general carbon steel were performed during the 1970s. These studies suggested that manganese sulphur inclusions play a critical roll in the initial corrosion. The present study samples were emerged in deionised water for one to ten minutes. The samples were then studied with XPS, SEM, EDX and FIB-SEM to investigate if and where the corrosion occurred. The XPS analysis showed that similar iron oxides, a few nm thick, mostly covered the samples after 5 min. exposure as well as after handling in air. Using imaging methods no clear signs of preferential corrosion was observed on samples exposed for one or two minutes, but on samples exposed for five and ten minutes. No clear signs of preferential corrosion was observed on samples exposed for one or two minutes, but samples exposed for five and ten minutes did. The analysis of the five minute exposure showed manganese sulphur inclusions which were partially decomposed. The ten minute exposure showed inclusions with thicker oxide films around them, cracks and flaking of the surface could be observed close to several inclusions. It is hypothesized that these phenomena origin from some kind of micro galvanic corrosion and/or sulpur infection of the steel matrix