Copper-zinc alloys stress-corrosion cracking

Standard Test Methods for Use ofMattsson s Solution of pH 7.2 to Evaluate the Stress Corrosion Cracking Susceptibility of Copper—Zinc Alloys, ASTM G 37-85, American Society for Testing and Materials, Philadelphia, Pa., 1992. 

Alloys containing only a few per cent of zinc may fail if the stresses are high and the environment sufficiently corrosive. Most types of brass, besides the plain copper/zinc alloys, appear to be susceptible to stress corrosion. An extensive investigation of the effect of additions to 70/30 brass was carried out by Wilson, Edmunds, Anderson and Peirce , who found that about 1% Si was markedly beneficial. Other additions were beneficial under some circumstances and none of the 36 additions tested accelerated stress-corrosion cracking. Further results are given in later papers ... 

Stress-corrosion cracking of copper-zinc alloys can occur in environments other than ammoniacal solutions (Ref 114, 147, 151, 152). Included are nitrogen-bearing compounds such as amines and aniline, as well as sulfates, nitrates, nitrites, acetates, formates, and tartrates. These environments can produce tarnish films of Cu20 similar to the films formed in ammoniacal solutions. Both the rate of formation and... 

K. Sieradzki, J.S. Kim, A.T. Cole, R.C. Newman, The relationship between dealloying and transgranular stress-corrosion cracking of copper-zinc and copper-aluminum alloys, J. Electrochem. Soc. 134 (1987) 1635-1639. 

Figure 20.3. Trends of dezincification, stress-corrosion cracking, and impingement attack with increasing zinc content in copper-zinc alloys (brasses).

Copper-zinc alloys (brass) are extensively used in heat exchangers. They are susceptible to SCC when in contact with ammonia or other amino-compounds such as urea. The phenomenon was first observed in the British colonies when brass cartridges were stored for prolonged times near horse stables. It has since also been known as season cracking. Stress corrosion cracks in brasses are most often, but not always, intergranular. 

ASTM G 37 (Practice for Use of Mattsson s Solution of pH 7.2 to Evaluate the Stress Corrosion Cracking Susceptibility of Copper-Zinc AUoys) is an accelerated stress corrosion cracking test environment for brasses (copper-zinc alloys). The use of this test environment is not recommended for other copper alloys since the results may be erroneous, providing completely misleading rankings. This is particularly true of alloys containing aluminum or nickel as deliberate alloying additions. 

Pure copper is immime to stress corrosion cracking (SCC). However, alloys of copper containing more than 15% zinc are particularly subject to this type of corrosion. 

Thompson and Tracy carried out tests in a moist ammoniacal atmosphere on stressed binary copper alloys containing zinc, phosphorus, arsenic, antimony, silicon, nickel or aluminium. All these elements gave alloys susceptible to stress corrosion. In the case of zinc the breaking time decreased steadily with increase of zinc content, but with most of the other elements there was a minimum in the curve of content of alloying elements against breaking time. In tests carried out at almost 70MN/m these minima occurred with about 0-2% P, 0-2% As, 1% Si, 5% Ni and 1% Al. In most cases cracks were intercrystalline. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

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