Erosion and corrosion properties of WC coatings and duplex stainless steel in sand-containing synthetic sea water

Abstract

Two different thermally sprayed coatings have been investigated with respect to erosion and corrosion resistance and compared with a duplex stainless steel. The coatings consist of WC particles in a metal binder, one with a pure Co binder and one with a binder consisting of mainly Ni, Cr, Co and Fe. The materials have been exposed to flowing synthetic sea water containing sand. The test equipment consists of an inner rotating cylinder and an outer stationary cylinder. In between there is a 4 mm gap through which synthetic sea water flows. The cylinder walls are divided into ring-shaped specimens. Less sand attacks the inner cylinder than the outer owing to centrifugal forces. Weight loss rates were determined by weighing the specimens. In addition, corrosion rates were determined by electrochemical measurements. By cathodic protection of the specimens the rate of erosion was found. In that way, the relative importance of the corrosion and erosion rates has been studied. The results show that the corrosion properties of the coatings strongly affect the material loss rate. Coatings with a highly corroding matrix, such as Co, suffer from a strong synergistic effect where the corrosion gives enhanced erosion. This effect can be more than three times as high as the corrosion itself. It is also seen that the weight loss rates vary with exposure time, being larger from the start. This is an important issue to consider when testing these materials. Modelling of the flow conditions has been performed to determine sand particle energy and the angle of attack on the surface.