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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.
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.
Pulsed Eddy Current (PEC) technology is a widely accepted inspection method now covered by several industry standards such as ISO(1) 20669, API(2) RP 583, and the new ASME(3) Section V (BPVC for Boiler and Pressure Vessel Code), article 21. PEC is a versatile inspection technology which provides an average remaining wall thickness through insulation and coatings. The technique can also be used to safely assess the minimum remaining ligament under corrosion scabs or blisters without surface preparation. PEC is resilient to liftoff variations and provides volumetric measurements of remaining material. It is capable of both detecting and assessing general corrosion on the outer surface of the pipes such as scabs and blisters, and detecting erosion or Flow Accelerated Corrosion (FAC) on the inner surface.
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Calcium carbonate precipitation in the presence of pollutants is a carrier of importance for their transport to the sediments and for their subsequent release depending on the local microenvironment conditions. Zinc is often present in industrial waters (heat exchangers and boilers) mainly for corrosion protection. Depending on the alkalinity and calcium concentration of waters used in water intensive processes, calcium carbonate fouling is common. Moreover, among other metals, is present in natural waters together with iron and copper, where the concentration of Zn(II) may reach at levels of several ppm.
Pipeline under, solids deposition deposit corrosion (UDC) is a localized corrosion phenomenon that develops beneath or around solid deposits, which settle at the bottom of low flow/intermittent flow pipelines. These deposits are complex mixtures of water, organic, inorganic, and biological materials, and their composition can vary significantly depending on the properties of the product being transported and the operating conditions in the pipeline.