Application of Electrochemical Impedance Method to Studying Corrosion of Porous Powder Steels

Electrochemical impedance spectroscopy (10⁴–10^–3 Hz) and weighing methods are applied to studying the corrosion behavior of powder austenitic 316L stainless steel with a porosity = 15, 20, and 38% and compact steel of the same composition in a 3% HCl + 5% NaCl solution and oil pool water (pH 6.0; total mineralization 184.8 g/l, preferentially chlorides of Na, Ca, and Mg). Hodographs and phase angles of the electrode impedance are analyzed for high ac frequencies. The procedure of corrosion rate determination for powder steel from impedance data is described. The corrosion rate of powder steel calculated to the geometric surface area of a sample is shown to increase in acid and neutral chloride solutions with an increase in the porosity of samples from 15 to 38%, and, at 20%, with the exposure of samples to the electrolytes (from 2 to 120 h). Advantages of the impedance method over the weighing method are demonstrated for determination of the corrosion rate of porous powder steels. Based on impedance measurements, the specific surface areas of powder steel samples are assessed.