Electrochemical evaluation of cr-mn austenitic stainless steel in aqueous sulphuric acid and influence of thiocyanate ions

• Published in: Anti-corrosion methods and materials 2020-05, Vol.67 (3), p.281-293
• Main Authors: Bansod, Ankur V, Patil, Awanikumar P, Shukla, Sourabh
• Format: Article
• Language: English
• Subjects: Ammonium; Ammonium compounds; Analytical methods; Anions; Austenitic stainless steels; Calomel electrode; Cathodic polarization; Charge transfer; Copper; Corrosion; Corrosion effects; Corrosion rate; Corrosion resistance; Corrosion tests; Dilution; Electrochemical analysis; Electrochemical impedance spectroscopy; Electrochemistry; Electrode polarization; Electrodes; Frequency ranges; Ions; Manganese; Nickel; Open circuit voltage; Oxalic acid; Oxide coatings; Polarization; Spectroscopy; Stability; Stainless steel; Sulfur; Sulfuric acid; Sulphates; Sulphur; Sulphuric acid; Temperature; Thiocyanates; Water quenching
• ISSN: 0003-5599; 1758-4221
• DOI: 10.1108/ACMM-11-2019-2210

Purpose Low nickel austenitic stainless steel (ASS) has attracted much attention worldwide because of its economical price. This study aims to investigate the effect of different corrosive environments on the corrosion behavior of chrome-manganese (Cr-Mn) ASS. The tests were carried out as a function of H2SO4 concentrations, temperature and addition of ammonium thiocyanate (NH4SCN) (0.01 M). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the corrosion behavior of Cr-Mn ASS. It was observed that with increasing H2SO4 concentration, temperature and with the addition of NH4SCN solution, icorr, icrit and ipassive values increased. EIS data show decreasing charge transfer resistance value with increasing concentration and temperature. Higher corrosion rate with increasing temperature and concentration of H2SO4 is related to the anions (SO42−), which is responsible for reducing the stability of passive films. With the presence of 0.01 M NH4SCN thiocyanate (SCN− anion), there is a higher dilution of the passive film resulting in a higher corrosion rate. Energy-dispersive spectroscopy (EDS) analysis reveals the adsorption of sulfur on the surface in NH4SCN containing a solution. The significant presence of counter ions and the adsorbed sulfur species on the steel surface play a vital role in corrosion behavior. Design/methodology/approach All the experiments were performed on a 3 mm thick sheet of Cr-Mn ASS (202 ASS) in hot rolled condition. The samples were then annealed at 1,050°C for 1 h, followed by water quenching. For microstructural examination, they were electrochemically etched in 10 Wt.% oxalic acid solution at 1 amp for 90 s. A computer-controlled Potentiostat (Biologic VMP-300) was used. After the cell was set up, the working electrode (WE) was electrostatically cleaned at −1 V vs saturated calomel electrode (SCE) for 30 s to remove the air-formed film. Then, WE were allowed to attain stable open circuit potential (OCP) for 1 h, following by the EIS test and potentiodynamic polarization test. The polarization test was started from a cathodic potential (−1.2 V vs SCE) and continued up to an anodic potential (1.6 V vs SCE) a scan rate of 0.1667 mV/s. EIS experiment was conducted on the same instrument by using a sinusoidal AC signal of 10 mV in a frequency range of 1,000,000 to 0.01 Hz at OCP. Findings Potentiodynamic polarization graph shows that with the increase in sulphuric acid concentrati