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Corrosion Behaviour of a Highly Alloyed Austenitic Alloy UB6 in Contaminated Phosphoric Acid

DOI: 10.1155/2013/363826

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The influence of temperature (20–80°C) on the electrochemical behaviour of passive films anodically formed on UB6 stainless steel in phosphoric acid solution (5.5?M H3PO4) has been examined by using potentiodynamic curves, electrochemical impedance spectroscopy, and Mott-Schottky analysis. UB6 stainless steel in contaminated phosphoric acid is characterised by high interfacial impedance, thereby, illustrating its high corrosion resistance. The obtained results show that the films behave as n-type and p-type semiconductors in the potential range above and below the flat band potential, respectively. This behaviour is assumed to be the consequence of the semiconducting properties of the iron oxide and chromium oxide regions which compose the passive film. 1. Introduction In the phosphoric acid industry, the main stages of the wet process phosphoric acid (WPA) manufacture involve the attack of phosphate ore by concentrated sulphuric acid (98%), filtration of the pulp, and concentration of acid [1]. Phosphoric acid in pure state is not very corrosive compared to nitric or sulphuric acids, but this process generates severe corrosion problems of the equipments made of stainless steels due to the presence of impurities such as chlorides, fluorides, and sulphides [1–4]. Depending on the nature of phosphates and the type of phosphoric acid manufacturing process used, the equipments (reactors, agitators, pumps, drain, etc.) are subjected to slower or faster deterioration [1]. The choice of materials used in this industrial process plays an important role since they must have both good chemical and mechanical resistance. These two characteristics are not always easy to obtain and a tradeoff between these properties must be reached [5]. In this sense, austenitic stainless steels are a good choice for phosphoric media. In this study, a highly alloyed austenitic stainless steel (UB6) has been used. Stainless steels proved their good corrosion resistance in acid solutions. It was shown in the literature that chloride and fluorides ions accelerated the anodic process by altering passivity and activating the material dissolution rate [6–14]. UB6 stainless steel is used extensively in phosphoric acid industry, because of its good corrosion resistance; passive films formed on its surface have been the subject of some investigations [15]. The major disadvantage of these new alloys is their high cost compared with conventional stainless steels, due to the higher percentage of the alloying elements such as Cr, Ni, and Mo, as well as the complexity of the fabrication process

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