Development of Representative Tests to Quantify the Susceptibility to Stress Corrosion Cracking of α,β’-Brass Used for Gas Transfer Devices

Stress corrosion cracking (SCC) is believed to be the principal cause of the cracks observed in brass components used in gas distribution network. In this framework, it is of interest to develop accelerated tests to determine the SCC susceptibility of the brass used in the gas transfer valves. To be representative and reliable, a reference test has to reproduce most of the damages observed on network components after break-up during operation. The present study aimed at developing SCC equipment and experimental protocol leading to a relevant evaluation of the SCC susceptibility of α,β’-brass.

The specimens tested, corresponding to α,β’-brass CuZn40Pb2 (CW617N), were machined from stamping rods used to manufacture gas transfer valves. The accelerated tests were performed in NaNO₃ solution selected with reference to research on α-brass (1). The first step of the study corresponded to develop a SCC cell and an adequate experimental protocol, which enable to apply, control and measure specific conditions (load, potential, environment…) leading to various SCC damages; the second step was to compare the SCC damage developed after the accelerated tests to that observed on network components after break-up during operation. The goal was to determine the best parameters for the accelerated tests leading to a SCC damage similar to that observed in service.

Results correspond to the design of the SCC cell developed and the methodology used to validate the applied conditions, and the measurements performed through SCC device. Furthermore, SCC damage developed after the tests was studied on the basis of crack characterisation, quantification of the dissolution process and chemical analyses of the corroded samples. Discussion focuses on the validity and the predictive nature of the test, linked to environment and load levels expected in normal condition of use of brass network components (2) (3). The influence of the manufacturing of brass network components on the SCC susceptibility (modifications in microstructure, work hardening and residual stresses) will be discussed and, in this way, prospects will be presented.

1. S.A. Fernandez and M.G. Alvarez, Corros. Sci., 53, 82 (2011).

2. C. Mapelli, A. Gruttaduaria and M. Bellogini, Eng. Fail. Anal., 17, 431 (2010).

3. C. Mapelli, D. Mombelli, S. Barella and A. Gruttadauria, Eng. Fail. Anal., 27, 141 (2013).