Abstract

A nickel-based alloy knife gate valve exposed to hot potash brine failed in less than two years in service. Failure was realized when the valve was unable to stop the flow. Visual examination of the valve internal body parts revealed heavy scaling and corrosion damage, although the center body part and the two knife gates were intact. Corrosion scale samples were subjected to chemical analysis by X-ray diffraction (XRD) augmented by energy dispersive X-ray spectroscopy (EDS). The XRD technique was unable to identify approximately 85% of the components in the scale due to lack of crystallinity. The crystalline portion of the scale was found to contain a mixture of molybdenum, chromium, nickel and tungsten oxides, and trace amounts of tungsten carbide and nickel sulphate. EDS analysis revealed high concentrations of oxygen, molybdenum and tungsten. Metallographic examination of the corroded surface revealed preferential attack of the inter-dendritic secondary phases in the cast microstructure. The results of the examinations show, the higher carbon content in the cast alloy had consumed a portion of the corrosion resisting alloying elements by forming carbide precipitates. This resulted in the depletion of corrosion resisting alloying elements in localized areas. These depleted localized areas were therefore susceptible to corrosion attack upon exposure to corrosive process fluids.

Introduction

Potash is mined from deep underground deposits left by ancient inland seas or extracted from saltwater bodies. The typical composition of potash is 40% potassium chloride (KCl), 55% sodium chloride (NaCl) and 5% clay. About 95% of potash is used for fertilizer in agriculture; the remaining 5% is used in commercial and industrial products such as soap, water softeners, de-icers, drilling muds etc.

Corrosion is one of the major problems in the potash production industry which can affect the stable and continuous operation of the plant. Metallic materials are susceptible to corrosion due to the high concentration of dissolved chlorides in the potash brine. Corrosion resistant nickel-based alloys are often used for process piping; fittings; valves and equipment to prolong their service life.

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