Corrosion Investigation on Conventional and Nanocomposite (Ni-P-Al2O3-TiO2) coated Mild Steel by In-Plant Test in Digester of a Paper Mill

Objectives: To improve paper quality and production, the digesters are now operated at increasingly high temperature and high sulfidity, which increase different type of corrosion attack on the surface of digester. The present study aims to develop an alternative material that can enhance the service life of the digester and reduces the loss generated by corrosion. Methods/Analysis: The nanocomposite coating of Ni-P-Al₂O₃-TiO₂,(MS(NiPATi)) was carried out on mild steel substrate by electroless technique. To carryout coating nanosized Al₂O₃ and TiO₂ particles were dispersed in 1:1 ratio, in Ni-P electroless bath with constant stirring. The plated (MS(NiPATi)_plated) sample was heat treated at 350°C in argon atmosphere (MS(NiPATi)₃₅₀) for improving its adherence to the substrate. In-plant corrosion test was carried, to determine the corrosion attack to the samples in the actual working conditions. To carryout in-plant test, the samples of MS, SS304L, SS2205, MS(NiPATi)_plated and MS(NiPATi)₃₅₀, were exposed for duration of six month in the 5th batch digester of SPML in the actual working conditions. After removal from the digester the exposed samples were analysed by Scanning Electron Microscopy (SEM) and metallurgical microscope. Findings: An overall corrosion view on all the samples results that MS experiences highest corrosion rate while duplex SS2205 least and samples SS304L, MS(NiPATi)_plated, MS(NiPATi)₃₅₀ is nearly the same. The lower part of digester where test rack was fitted is mainly affected due to erosion assisted corrosion. This test, therefore, suggests that in the applied conditions, it would be better to construct the bottom part of the digester either with duplex steel SS2205 or with alternative materials i.e., MS(NiPATi)₃₅₀ to minimize the risk of corrosion.