51315-5559-Failure of a Nickel-Chromium Thermal Spray Due to the Presence of Organic Acid

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Product Number: 51315-5559-SG

ISBN: 5559 2015 CP

Author: Matthew Tunnicliffe

Publication Date: 2015

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The repeated failure of a wear sleeve in a primary refiner processing hardwood chips for BCTMP high-yield pulp was not only costly but also required a regular schedule 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 sulfite impregnation liquor. This report describes a liquor drop test which was designed to simulate 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 relationship between surface corrosion and thermal spray microstructure was studied using cobalt and nickel weld overlays (Stellite 6 and P12) and commercially-pure nickel for comparison. The tests showed that the nickel phase in the coating is vulnerable to corrosion by both weak acetic and sulphuric acids. This is circumstantial evidence that acid condensation in the refining zone is possible. Based on the tests reported here the two weld overlays tested are promising candidates for wear sleeve service in hardwood high-yield pulping. The liquor drop test can be used as a screening test for corrosion resistance.