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The Retardation of Crack Propagation for High Strength, Low Alloy Steels in Aqueous Media by Addition of Oxidizing Inhibitors.

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Progress rept. 1 Jul 71-30 May 74,

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Oxidizing inhibitors can be utilized to retard crack growth in high strength, low alloy martinsitic steels in aqueous solution. Artificial crevice cell and crack tip pH measurements have shown that conditions favorable for hydrogen embrittlement of the steel exist in propagating cracks. Localized hydrogen measurements of the fracture surfaces of D6AC specimens which failed under tensile load in aqueous solution show that hydrogen was absorbed into the steel in the vicinity of the crack tip, and that, as proposed by Troiano, hydrogen migrated through the lattice to the region of highest triaxial stress and was trapped in the strained lattice. Hydrazine was effective in reducing the crack growth rate by one order of magnitude for D6AC in aqueous solution. A 2 addition of hydrazine to solution increased K sub Iscc from 12 to 25 ksi square root inch, in both 0.1 M NaCl and nil chloride solutions. Inhibition of crack growth by hydrazine was caused by the formation of nitrite, NO2, which in aqueous solution changed the crack tip conditions of pH and potential so that no hydrogen evolution occurred within the crack, and the passive gamma-Fe2O3 film formed. Localized hydrogen analysis across the fracture surface of specimens which failed in these aqueous hydrazine solutions showed that no hydrogen was absorbed at the propagating crack tip. Thus, failure by hydrogen embrittlement was prevented. Sodium dichromate was also effective in reducing crack growth rates in aqueous solution. For specimens exposed to 0.1 M Na2Cr2O7 solution, the K sub Iscc was raised to 32 ksi square root inch. However, in chloride solution, K sub Iscc was only slightly improved.

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  • Metallurgy and Metallography

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