Effect of Silicon Compounds on Stress Corrosion Cracking of Alloy 600 in Caustic Solutions

Silicon compounds are abundant in both steam generator tube deposits removed from operating plants and in the hide-out return analyses performed during plant operation. Despite their abundance, the effect of these compounds on the susceptibility to intergranular attack (IGA) and stress corrosion cracking (SCC) of alloy 600 (UNS N06600) has not been established clearly in laboratory tests or under operating conditions. The present work studied the effect of different compounds of silicon, such as silica (SiO2) and sodium metasilicate (Na2SiO3), on the susceptibility to IGA/ SCC of alloy 600 in caustic solutions, with and without impurities. Experimental work included measurement of potentiodynamic polarization curves and development of stress corrosion tests with potentiostatic control at 315°C. Results of both tests indicated an inhibiting effect of silicon compounds on stress corrosion of mill-annealed (MA) alloy 600. Polarization curves in solutions with silicon compounds showed a wider range of passivation and a reduction in passive current density and anodic dissolution compared to the curves obtained in sodium hydroxide (NaOH). In corrosion tests performed in solutions containing silicon compounds, cracking appeared at only some of the potentials tested and with much reduced depth. Judging by the results obtained from analyses of the surface deposits, the inhibiting effect appeared to be related to the incorporation of silicon in the oxide layer. Regarding the effect of impurities, these have been observed not to be harmful for the response of alloy 600MA to IGA/SCC, and would appear even to improve this response in solutions containing silicon compounds, probably caused by the formation of insoluble silicates.