STP1401

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

    Published: Jan 2000


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    Abstract

    The stress-corrosion cracking of mild steel (0.17% C) in phosphate media (0.1 - 2.0M NaH2PO4 at pH = 4 and 20 - 80 °C) 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 Fe3(PO4)2.8H2O, Fe3(PO4)2.4H2O, FePO4.xH2O and Fe3O4, while the film formed in the passive region (outside SCC zone) is oxide in nature and consists of γ-Fe2O3. 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.

    Keywords:

    Stress-corrosion cracking, electrochemical model, mild steel, phosphate environment, film composition, crack propagation rate, environmental factors


    Author Information:

    Raicheff, R
    Professor, CINVESTAV-Mérida, Mérida, Yuc.

    Maldonado, L
    Professor, CINVESTAV-Mérida, Mérida, Yuc.


    Paper ID: STP10234S

    Committee/Subcommittee: G01.05

    DOI: 10.1520/STP10234S


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