Abstract
The repeated failures of a wear sleeve in a primary refiner processing hardwood chips for bleached chemithermomechanical pulp (BCTMP) high-yield pulp is costly, and required scheduling for maintenance and replacement. Nickel-chromium thermal sprays can be used to protect wear sleeves in this environment because of their resistance to abrasion and aqueous corrosion. The formation of acetic or sulphuric acids within the refining zone was suspected as a corrodant due to naturally occurring acetyl groups during the pulping process, or from decomposition of residual sulphite impregnation liquor. This report describes a liquor drop test which that simulated vapor phase corrosion and demonstrate the ability of the acids to corrode the wear sleeve alloy. Surface morphologies were analyzed using a scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). Corrosion products were measured using x-ray fluorescence spectroscopy (XRF) and x-ray diffraction (XRD). The tests showed that the nickel phase in the coating is vulnerable to corrosion by both weak acids. This is circumstantial evidence that acid condensation in the refining zone is possible. Based on the test reported here, the two weld overlays tested are promising candidates for wear sleeve service in hardwood high-yield pulping.
