Corrosion on rail axles can be a serious problem. Historically it has led to accidents that have resulted in extensive damage and fatalities, by causing crack initiation and propagation causing catastrophic component failure. To mitigate the risk, current UK maintenance standards set strict limits on permissible damage. These limits have resulted in a safe network, in terms of axle corrosion, with no failures in 20 years. However, anecdotal evidence suggested that this had been achieved through the unnecessary scrapping of many axles for minor corrosion damage. Over scrapping has a cost to industry financially, logistically and environmentally.
The scope of the problem was first established through consultation of industry data. Once it was shown that there was merit to the anecdotal reports, an axle survey was undertaken at a UK overhaul depot, the largest identified in literature. A new three dimensional survey technique was developed, improving on previous two dimensional surveys, and analysed using bespoke software based on image processing techniques to identify and separate corrosion pits. Pits identified through the survey were then analysed, using fracture mechanics techniques, to assess the risk of crack initiation they represented. It was found that no pits came close to current limits, in terms of depth, and presented a extremely small risk of crack initiation. This indicated that current limits and procedures were resulting in axles being scrapped with low levels of corrosion damage, suggesting improvements could be made.
To explore how corrosion damage changed over time, a series of novel experiments were performed. These experiments sought to replicate the rail axle environment during UK operations through the application of a representative corrosive medium to axle samples. The results of these experiments allowed an estimation to be made of the change in corrosion severity on rail axles under different starting conditions. The predicted changes were applied to the pits identified during the axle survey to assess the potential risk of changes to current corrosion maintenance practises.
By analysing the experimental results it was shown that removing the protective passivity layer to inspect axles resulted in significantly increased rates corrosion damage, compared to the case where the passivity layer was left in place. In some cases, this meant that correction of axles was required when it would not have been if no inspection were carried out.
Based on the results of the analysis, it was recommended that an extension to inspection intervals be considered. This would reduce the scrapping of axles, without substantially increasing the risk of axle failure. This was due to the low level of damage observed on in service axles and the significantly slowed rate of corrosion if the passivity layer was left in tact.
