INFLUENCE OF THE SOLUTION TEMPERATURE ON THE PASSIVATION BEHAVIOUR OF AN AUSTENITIC STAINLESS STEEL IN POLLUTED PHOSPHORIC ACID

摘要

The wet process used in the phosphoric acid industry generates severe corrosion problems on the equipments made of stainless steels due to the aggressiveness of the acid produced. The aim of this study was to study the corrosion behaviour, as well as the passivity behaviour of a highly alloyed austenitic stainless steel, UNS N08031 (Alloy 31), in a 40 wt.% phosphoric acid polluted with 2 wt.% of sulphuric acid and 0.03 wt.% of chloride ions, at different temperatures. Open Circuit Potential tests and cyclic potentiodynamic polarization curves have been carried out to obtain information about the electrochemical behaviour of Alloy 31. Polarization curves have shown that the material presented a stable passivation range at all the studied conditions and corrosion potentials shifted to more positive values as temperature increases. The loss of passivity of the Alloy 31 shifted towards more active values as temperature increased from 20 °C to 80 °C. The absence of hysteresis loop in polarization curves was attributable to general corrosion. In the studied solution, the presence of phosphate enhances the repassivation of initiated pits by stabilising the pH-value in an activated pit. The results have shown that Open Circuit potential values were located in the passive zone of the potentiodynamic curves, which means that the material passivated spontaneously. Passivation behaviour of Alloy 31 has been investigated by using potentiostatic tests at different potentials. The linear region of the log i vs log t transient was used to obtain the passivation index, n. The value of n was found to decreases as solution temperature increased. The results have shown that the applied potential scarcely affects on the passivation rate of Alloy 31 under the studied conditions. In general, n values are close to unity, which indicates a formation of a compact and highly protected passive film on the alloy surface.