著者

竹本 幹男

出版者

Japan Society of Corrosion Engineering

雑誌

防食技術 (ISSN:00109355)

巻号頁・発行日

vol.35, no.2, pp.75-82, 1986-02-15 (Released:2009-10-30)

参考文献数

15

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Utilizing a combined loading Stress Corrosion Cracking (SCC) test apparatus, SCC threshold stress criteria of two material-environment systems where environmentally assisted passive films are formed has been studied. Two systems studied in this report are SCC of carbon steel in boiling nitrate solution (60%Ca(NO3)2+3%NH3NO4) and SCC of 70/30 brass in Mattsson's solution (ammonical solution) at room temperature.The test results obtained are summarized as follows:1) The threshold stress of nitrate SCC of carbon steel could be well explained by the maximum principal stress theory although the elastic fiailure of substrate metal was ruled by the shear strain energy theory. The environmentally assisted passive film of this system is supposed to be thich magnetite, and so brittle that it could suffer brittle fracture at lower stresses than the yield stress of substrate metal. Intergranular cracks might inititate at the fractured passive film along the grainboundaries oriented in perpendicular to the maximum principal tensile stress.2) SCC threshold stress of 70/30 bass in Mattsson's solution was in good agreement with the maximum principal stress theory. The SCC threshold stress is much lower than the elastic failure stress of substrate metal. The tarnished film of this system is supposed to have an elastic modulas higher than that of substrate metal. and fracture stress lower than the yield strength of film-free substrate metal.3) Based on the test results of previous and this reports, it was concluded, that the SCC threshold stress of the systems which developed an environmentally assisted film could be ruled by the maximum principal stress theory, and that the SCC threshold stress of metals which had a selfhealing ability of passive film in atmosphere could be ruled by the strain energy theory which is similar to the elastic failure stress criteria of substrate metal. Whether the threshold stress of APC-type SCC is evaluated by the strain energy theory or maximum principal stress theory seems to depend on the mechanical characteristics of passive film in a given system.