An Electrochemical Film-Rupture Model for SCC of Mild Steel in Phosphate Environment

摘要

The stress-corrosion cracking of mild steel (0.17% C) in phosphate media (0.1 - 2.0M NaH_2PO_4 at pH = 4 and 20 - 80℃) is studied using slow strain rate and potential sweep techniques as well as SEM, X-ray diffraction and Mossbauer analysis of the surface films. It is shown that the susceptibility of the steel to SCC depends on the phosphate concentration and it decreases markedly with increase of temperature; the highest SCC susceptibility is observed in the region of active to passive transition of the steel (-0.3 - 0.0 V, SCE). It is established that the films formed at potentials in SCC zone are mixed oxide-phosphates composed by Fe_3(PO_4)_2*8H_2O, Fe_3(PO_4)_2*4H_2O, FePCM_4*xH_2O and Fe_3O_4, while the film formed in the passive region (outside SCC zone) is oxide in nature and consists of γ-Fe_2O_3. It is suggested that SCC may be related to the formation of secondary amorphous iron (Ⅲ) phosphates in the surface film. An electrochemical model for crack propagation, based on the film rupture concept in SCC and quantitative electrochemical kinetics considerations, and relating the rate of crack propagation with the electrochemical parameters of both metal dissolution and cathodic reactions at the crack tip and outer metal surface is presented. It is shown that the model could be used for calculating the crack propagation rate and defining electrochemical conditions favorable for SCC.