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Trans-granular stress corrosion cracking (S.C.C.) of ferritic steels
Corrosion Science, 1970, Vol. 10, pp. 547 to 548. Persamon Press. Printed in Great Britain
LETTER TO THE EDITOR
TRANS-GRANULAR STRESS CORROSION CRACKING (S.C.C.) OF FERRITIC STEELS
DURING recent years there have been several incidents of severe cracking of steel components carrying and storing reformed or coal gas both on the Continent and in this country, x,2 In all cases there was evidence of the formation of an aqueous environment which would have been acidic because of the CO2 content (10-20%) of the gas. Although the morphology of the cracking was transgranular, unlike that normally found in ferritic steels, it was attributed to s.c.c, rather than hydrogen embrittlement because of the low strength of the steels in which cracking occurred. The unusual form of the cracking and the absence of any causative agent for s.c.c. indicated that this was a new unexplored metal environment stress corrosion system. This type of s.c.c, has now been reproduced in this laboratory using tensile specimens of pipeline steel of composition (0.16C, 0.06Si, 0.83Mn, 0-025S, 0.014P). The specimens were exposed under carefully controlled conditions in pressurized water at 20°C in equilibrium with a COs-CO mixture at 100 psi, the CO partial pressure varying between 4 and 15 psi. Japanese workers have also recently reported reproducing this type of cracking in a CO-rich CO~-CO-H20 environment at a higher pressure and temperature than used here. 3 The results indicate that CO must be present in the environment before cracking can be initiated. Moreover, the greater the concentration of CO the shorter the time to cause cracking and the lower the stress required. The application of potentiostatic control to the specimens indicates that cracking will only occur within a narrow potential range of approximately 200mV. The application of cathodic potential prevents crack initiation and stops cracks growing once they have been initiated, thus indicating that the cracking process is one of anodic stress corrosion rather than hydrogen embrittlement. The necessity for the presence of CO in the environment can be explained in terms of its inhibitive action on the dissolution of Fe in acid solutions, 4 presumably due to its capacity for adsorption on to metallicsurfaces. Thus a general corrosive environment is changed into a passive one enabling highly localized mechanically activated attack to occur upon an area on the surface whilst the remainder of the surface remains relatively inactive. Work is now continuing to define the effect of further environmental, electro547
548
Letter to the Editor
chemical a n d mechanical variables on this new s.c.c, p h e n o m e n a . M e a n w h i l e this letter is published since we feel the w o r k could have relevance to o t h e r i n s t a l l a t i o n s involving exposure o f steel to m o i s t C O s - C O environments.
Gas Council Engineering Research Laboratory, Harvey Combe, Killingworth, Newcastle-upon- Tyne 12.
A. BROWN J. T. HARRISON R. WILKINS
Acknowledgement--The authors are grateful to the Gas Council for permission to publish this work. REFERENCES
1. P. J. RAS, Gas (Netherlands) 4, 81 (1964). 2. W. TAKENS,Gas (Netherlands) 8, 163 (1964). 3. M. KOWAKAand S. NAGATA,Corrosion 24, 427 (1968). 4. G. TRABANELLIet al., Corrosion-Traitement-Protection-Finition 16 (7), 335 (1968).
LETTER TO THE EDITOR
TRANS-GRANULAR STRESS CORROSION CRACKING (S.C.C.) OF FERRITIC STEELS
DURING recent years there have been several incidents of severe cracking of steel components carrying and storing reformed or coal gas both on the Continent and in this country, x,2 In all cases there was evidence of the formation of an aqueous environment which would have been acidic because of the CO2 content (10-20%) of the gas. Although the morphology of the cracking was transgranular, unlike that normally found in ferritic steels, it was attributed to s.c.c, rather than hydrogen embrittlement because of the low strength of the steels in which cracking occurred. The unusual form of the cracking and the absence of any causative agent for s.c.c. indicated that this was a new unexplored metal environment stress corrosion system. This type of s.c.c, has now been reproduced in this laboratory using tensile specimens of pipeline steel of composition (0.16C, 0.06Si, 0.83Mn, 0-025S, 0.014P). The specimens were exposed under carefully controlled conditions in pressurized water at 20°C in equilibrium with a COs-CO mixture at 100 psi, the CO partial pressure varying between 4 and 15 psi. Japanese workers have also recently reported reproducing this type of cracking in a CO-rich CO~-CO-H20 environment at a higher pressure and temperature than used here. 3 The results indicate that CO must be present in the environment before cracking can be initiated. Moreover, the greater the concentration of CO the shorter the time to cause cracking and the lower the stress required. The application of potentiostatic control to the specimens indicates that cracking will only occur within a narrow potential range of approximately 200mV. The application of cathodic potential prevents crack initiation and stops cracks growing once they have been initiated, thus indicating that the cracking process is one of anodic stress corrosion rather than hydrogen embrittlement. The necessity for the presence of CO in the environment can be explained in terms of its inhibitive action on the dissolution of Fe in acid solutions, 4 presumably due to its capacity for adsorption on to metallicsurfaces. Thus a general corrosive environment is changed into a passive one enabling highly localized mechanically activated attack to occur upon an area on the surface whilst the remainder of the surface remains relatively inactive. Work is now continuing to define the effect of further environmental, electro547
548
Letter to the Editor
chemical a n d mechanical variables on this new s.c.c, p h e n o m e n a . M e a n w h i l e this letter is published since we feel the w o r k could have relevance to o t h e r i n s t a l l a t i o n s involving exposure o f steel to m o i s t C O s - C O environments.
Gas Council Engineering Research Laboratory, Harvey Combe, Killingworth, Newcastle-upon- Tyne 12.
A. BROWN J. T. HARRISON R. WILKINS
Acknowledgement--The authors are grateful to the Gas Council for permission to publish this work. REFERENCES
1. P. J. RAS, Gas (Netherlands) 4, 81 (1964). 2. W. TAKENS,Gas (Netherlands) 8, 163 (1964). 3. M. KOWAKAand S. NAGATA,Corrosion 24, 427 (1968). 4. G. TRABANELLIet al., Corrosion-Traitement-Protection-Finition 16 (7), 335 (1968).