Corrosión

As reported before, conventional group of flat heald, heald C, has used to serve for about two years, while a new group of flat heald, heald X, suffered significant localized corrosion earlier in one or two months. In this study, steel C’ of chemical composition similar to heald C is heat-treated to reproduce the heald C in terms of metallurgical characteristics ( hardness H v ; amount of insoluble carbide ［ MC ］ ; prior austenite grain size d γ ) . The characteristics of heald C ( 484 Hv ; 1.2 ％ ; 12 μ m ) is found to be reproduced by a solution-treatment at 1040 ℃ for 180 s, WQ followed by tempering at 450 ℃ for 20 s, AC. Those of heald X ( 474 Hv ; 2.0 ％ ; 12 μ m ) , lower in hardness and more in insoluble carbide than C, could be reproduced by solution-treatment at 1040 ℃ for around 100 s, shorter time than heald C. The heat treatment condition determined as above is confirmed to corrode the steel C’ in 6.8 vol ％ HNO 3 solution similarly in surface appearance to healds C and X. Their corrosion rates in the same solution vs. soluble Cr content, ［ Cr ］ relationship are nearly equal to the corrosion rates vs. ［ Cr ］ relationship for relevant solution-treated steels. The repassivation potential for crevice corrosion, E R.CREV , in 0.85, 2.8 or 28 mM NaCl solution depends also on ［ Cr ］ , nearly constant between ［ Cr ］ range of 11.5 and 12.8 ％ , and above which becomes more noble with increasing ［ Cr ］ up to 13.4 ％ . Then, ［ Cr ］ values of 11.5 ％ ( heald X ) and 12.5 ％ ( heald C ) make no difference in E R.CREV . Real crevice corrosion for heald X was attributed to more noble spontaneous electrode potential, E SP , of heald X than E R.CREV under increased residual chlorine, as reported before.