Stress corrosion cracking of alpha-beta brass in distilled water and sodium sulfate solutions
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I.
INTRODUCTION
Most studies of stress corrosion cracking (SCC) in copper alloys have employed alpha brass, and for this material it is well established that cracking may be intergranular, transgranular, or mixed mode, depending upon the alloy composition, the electrochemical environment, and the level of stress. However, in spite of the considerable effort that has been expended in studying SCC of these materials, there is no generally accepted model which describes the mechanism(s) of cracking. Several processes have been postulated to have roles: formation and subsequent rupture of both thick I (visible) and thin 2'3 brittle films, hydrogen embrittlement, 4 local anodic dissolution resulting from passive film rupture at the crack tip, 5'6 and bond strength reduction by adsorption of a damaging species at the crack tip.7 The diversity of the above list and the limited number of observations to date that have unequivocally ruled out a particular mechanism for a given set of conditions 8 indicate the complex nature of SCC in this alloy system. In contrast to the situation for the alpha phase materials, there has been only limited study of SCC in Cu-Zn alloys having sufficient Zn to provide for a beta constituent. It is known, however, that SCC susceptibility of beta-containing alloys is greater than that of the alpha materials; cracking occurs at the corrosion potential in environments as innocuous as dilute salt solutions or distilled water. 9 The beta and alpha-beta alloys also undergo SCC in the ammoniacal and concentrated salt solutions in which alpha phase alloys are susceptible. Since these environments have significantly different electrochemical properties than distilled water, a number of potentially informative comparisons are possible. An early SCC study by Whitaker e t a l . 10 of an alpha-beta alloy in ammonia provides further impetus for renewed interest in the beta-containing materials; the alloy cracked transgranularly, and it was found that the cracks propagated through both phases. Their observations suggest that the cracking mechanism(s) operative in the alpha and beta phases are similar. M.B. HINTZ and L.A. HELDT are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. L.J. NETTLETON, formerly with Michigan Technological University, is now with IBM, Endicott, NY 13760. Manuscript submitted January 24, 1984. METALLURGICALTRANSACTIONS A
The present study examines the response of an alpha-beta alloy to several nonammoniacal environments under a variety of electrochemical conditions.
II.
EXPERIMENTAL PROCEDURE
A 10 kg heat of Cu-42 wt pct Zn (nominal) was induction melted in a high purity graphite crucible and cast into 25 mm diameter, 30 cm long rods in a graphite mold. Asarco/Globe 99.999 pct pure (substitutional impurities) starting materials were used and the melt was covered with high purity graphite during the heating cycle to reduce oxidation and vaporization of the zinc. X-ray fluorescence analysis revealed the actual zinc content of
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