Susceptibility to stress corrosion cracking and stress-induced dealloying of leaded and lead-free brasses in simulated drinking water

Duplex α + β’ brasses are widely used in drinking water distribution systems for tube fittings, valves, and ancillaries, because they are cheap, easy to fabricate and exhibit high mechanical strength. However, depending on their composition, they may undergo different corrosion forms, such as dealloying and stress corrosion cracking and may also determine human health concerns due to lead and copper ion release in drinking water. In the case of traditional Dezincification Resistant (DZR) alloys, leaching of arsenic traces may also occur. New stricter regulations introduced in some areas of the world pushed the production of new low lead or lead free DZR alloys. In this research three different brass types, two leaded (CW617N and CW602N) alloys and one lead-free brass (CW724R), were investigated to assess their susceptibility to stress corrosion cracking (SCC) in simulated drinking water (SDW) solution. CW602N is a traditional DZR alloy, while CW724R is a recently developed eco-friendly brass where silicon and phosphorus are introduced in substitution of lead and arsenic to afford machinability and dezincification resistance. The aggressive solution, containing 400 ppm SO42−, 400 ppm Cl− and 50 ppm NO3− as sodium salts, complied with drinking water composition requirements according to Moroccan standard NM 03.7.001. From the quoted alloys, both tensile specimens and C-rings were prepared which were then immersed in SDW and exposed to Slow Strain Rate tests (SSRT) and 120 day constant strain test, respectively. Electrical contacts were ensured on both specimen types and allowed Ecor monitoring and (in the case of C-rings) the recording of electrochemical impedance spectra to investigate the corrosion mechanism. The tests showed that under SSRT conditions both leaded alloys suffered SCC, while CW724R was scarcely susceptible. The C-ring stress-corrosion tests were performed by exposing to SDW either the specimen regions under tensile stress (convex surfaces) or those under compressive stress (concave surfaces) or both region types. Dealloying phenomena occurred and affected regions under tensile or compressive stress, depending on the alloy composition. The nature and depth of the corrosion attack were investigated by optical microscope observations and SEM-EDS analyses. As expected, in the case of leaded brasses dealloying consisted in dezincification of β’ phase, while in CW724R dealloying produced dezincification and selective silicon dissolution, mainly at the expenses of the silicon-rich κ phase. The best behaviour against dealloying attack was obtained in CW602N.