Further observations of SCC in alpha-beta brass: Considerations regarding the appearance of crack arrest markings during
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I.
INTRODUCTION
W E have recently described the stress corrosion cracking (SCC) behavior of an alpha-beta Cu-Zn alloy. The material was tested under constant extension rate conditions in distilled water and 1 N Na2SO4 solutions.~ Transgranular and intergranular beta phase cracking were observed, while cracking involving the alpha phase occurred only at the alpha-beta interphase boundaries. The transgranular beta phase fracture surface features strongly resembled those observed in alpha brass SCC studies;, furthermore, the dependence of SCC susceptibility on experimental variables (e.g., solution pH, electrochemical potential, etc.) was also characteristically similar to alpha brass behavior. Analysis of the observations outlined above led us to conclude that alpha and beta brasses stress corrosion crack by similar mechanisms, and that the experimental observations were most consistent with a mechanism involving the chemisorption* of a damaging environmental species at the *A number of investigators have proposed that SCC crack propagation energy is reduced by the adsorption of a damaging environmental species at the crack tip. L4 Depending upon the context of its use, however, the term "adsorption" may have several different meanings. As applied to the discussion of electrocapillarity, adsorption refers to a local concentration increase in the vicinity of the electrode-electrolyte interface. 5 Alternatively, when referring to possible SCC mechanisms, "'adsorption" generally implies chemisorption, ~-" for which some charge redistribution between the substrate and environmental species has occurred. The extent of charge redistribution occurring during a chemisorption event is tyically unspecified: it may presumably include ionic discharge as well as bonding interactions.
crack tip. In the following, we report additional observations of alpha-beta brass SCC behavior which further support the above conclusions.
ameter rod stock of the same Cu-42 wt pct Zn alpha-beta alloy studied previously. Details of the alloy preparation procedure have been given in an earlier paper) the resultant, equiaxed microstructure contains about 76 pct beta phase by volume (as determined by point counting). The specimen gauge sections were electropolished in a mixture of HNO3 and methyl alcohol at 260 K for 1 minute, which produced a smooth, mirror-like surface with a color indistinguishable from that obtained by mechanical polishing. A pair of teflon insulated, 28 gauge wires was soldered onto the ends of the gauge sections (el. Figure 1) and, following flux removal in acetone, the specimens were coated with stop off lacquer such that only the central portion of the gauge section remained exposed. A notch approximately 0.5 mm deep was cut into the center of each gauge length with a small, rectangular cross section file. Mechanical testing was performed in an atmospherically vented, teflon sealed glass environmental cell. The test envi-
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II. EXPERIMENTAL PROCEDURE AND RESULTS H-P 6177B
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