The ability of austenitic stainless steel (Alloy-28) to withstand corrosion when exposed to a combination of industrial phosphoric acid and an oxidizing agent

Phosphoric acid is an interesting industrial chemical compound used for several applications. However, preparation of phosphoric acid with a concentration of 51–54% is usually accompanied by the formation of new organic impurities, which require the intervention of oxidizing agents to be eliminated. The purification process is performed through advanced special equipment, the materials of which must be highly resistant to degradation. In particular, Alloy-28 is used as the main material in the installation for concentration process. This research aims to determine the optimum concentration of HNO₃ that guarantees the highest purification level by eliminating the maximum amount of organic impurities and to study the corrosion resistance of Alloy-28 in phosphoric acid media (52%P₂O₅) containing oxidizing agent. Measurements of optical density (OD) combined with organic impurities concentration (C_org) revealed that mixing PA mixed with 8 kg of HNO₃ per tone of P₂O₅ is the optimum formulation. In fact, when the optimum quantity of HNO₃ was added to PA, the OD and C_org decreased from 0.83 and 450 ppm without oxidant to 0.2 and 240 ppm, respectively. Static and dynamic immersion tests were performed in a reactor. Results revealed that when 8 kg/t P₂O₅ of nitric acid was added to PA, corrosion rate showed negative values reflecting an increase in the weight of Alloy-28 samples due to the formation of a passive layer. Electrochemical measurements confirmed that Alloy-28 is a passivable metal after adding the optimum amount of the oxidant, indicating that Alloy-28 possesses high corrosion resistance and strong passivation ability.