Macroscopic, thermodynamic, kinetic and microscopic study of nitric acid pickling of Elektron 21 (EV31A) magnesium alloy

The study of pickling of the EL21T6 substrate in a nitric acid bath firstly provided knowledge of the influence of the different experimental parameters (concentration of acid, temperature and stirring of the solution) in relation to the etching rate. This experimental part led to the choice of standard pickling ( T bath = 25 °C, moderate stirring (250 rpm), [HNO 3 ] = 1.20 mol/L, duration of 2 min) so as to obtain constant removal of material, even after several uses of the pickling bath. SEM observations also confirmed that in these operating conditions, pickling concerns both the grains of the matrix and the precipitates of type Mg 3 (Nd, Gd), leading to uniform removal of material from the surface. Working from a number of assumptions, thermodynamic and chemical kinetic studies then allowed a pickling mechanism to be proposed and led to obtaining values for the corresponding kinetic parameters (order of reaction, constant of mean rate, activation energy) to be obtained.