STP610

    Retardation of Crack Propagation for D6AC High-Strength, Low-Alloy Steel in Aqueous Media by Addition of Oxidizing Inhibitors

    Published: Jan 1976


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    Abstract

    High-strength, low-alloy, martensitic steels are known to be subject to hydrogen embrittlement. This was demonstrated by measurements of crack-tip pH and localized hydrogen content at the fracture surface, and also by observation of electrochemical behavior of simulated “crevices.” The effect of certain oxidizing inhibitors was used to elucidate the mechanisms of stress-corrosion cracking (SCC) and to retard crack growth.

    Hydrazine additions reduced the crack growth rate by an order of magnitude for D6AC in aqueous solutions. A 2 percent addition of hyrazine to solutions either with or without chlorides increased KIscc from 12 (without inhibitor) to 25 ksi∓in. (with inhibitor). The critical flaw size varies as the square of KIscc, so doubling KIscc is the equivalent of increasing the size of a critical flaw fourfold, which in turn makes more likely its detection non-destructively. Localized hydrogen analyses at various locations on the fracture surface of a D6AC specimen pulled to failure after exposure in hydrazine-inhibited solution (with or without chlorides) showed no hydrogen enrichment of the fracture surface (as compared to the bulk specimen remote from the fracture), hence no hydrogen embrittlement.

    While sodium dichromate additions also increased KIscc and reduced the crack-growth rate in chloride-free solutions, they were not effective in 0.1 N sodium chloride solutions.

    Special electrochemical measurements (with and without chlorides) showed that in the presence of the hydrazine inhibitor, the electrode potential of D6AC at the tip of an advancing corrosion fatigue crack was above (more noble than) the potential of the equilibrium hydrogen electrode. By contrast, the electrode potential at the apex of an advancing corrosion fatigue crack in D6AC exposed in dichromate-inhibited, chloride-containing solution was below (more active than) the potential of the equilibrium hydrogen electrode. Thus dichromate additions did not prevent hydrogen entry into D6AC in chloride solutions.

    Keywords:

    stress corrosion, hydrogen embrittlement, crack propagation, inhibitors, fracture properties, alloy steels


    Author Information:

    Parrish, PA
    Metallurgist, U. S. Army Research Office, Research Triangle Park, N. C.

    Chen, CM
    Visiting assistant professor; and, chairman and professor, Materials Science and Engineering, University of Florida, Gainesville, Fla.

    Verink, ED
    Visiting assistant professor; and, chairman and professor, Materials Science and Engineering, University of Florida, Gainesville, Fla.


    Paper ID: STP28679S

    Committee/Subcommittee: G01.06

    DOI: 10.1520/STP28679S


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