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Effect of CP on the Occurrence of SCC in X80 and X100 Pipe Steels in a Near-Neutral pH Environment

In this work, stress corrosion cracking (SCC) of X80 and X100 pipe steels under various cathodic protection (CP) levels in near-neutral pH environment was investigated. The results showed that X100 tended to form longer SCC cracks compared to X80 steel.

Product Number: 51317--9147-SG
ISBN: 9147 2017 CP
Author: Li Yan
Publication Date: 2017
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Cathodic protection (CP) in conjunction with coating is generally recognized as the most effective mitigation method for pipeline external corrosion and stress corrosion cracking (SCC). However if not designed properly for a given pipeline system the applied CP can cause detrimental effects instead such as coating disbondment leading to corrosion and SCC hydrogen induced cracking (HIC) or hydrogen embrittlement in general. It has been recognized that modern high-strength pipeline steels can be more susceptible to hydrogen embrittlement or related environment assisted cracking such as near-neutral pH SCC compared to their low-strength steels counterparts. However the role of CP in the prevention or facilitation of such cracking is still not clear. The objective of this study is to examine the effect of CP on the initiation and propagation of SCC in two different high strength pipe steels namely X80 and X100.Proof rings with constant loads were employed to investigate the occurrence of SCC in X80 and X100 pipe steels under various CP conditions. The results showed that the effect of CP level on SCC in X80 and X100 steels were markedly different. At CP levels of -750 and -776 mV X80 steel was less susceptible to SCC than X100 steel i.e. X80 steel developed lower crack density and smaller crack lengths. With increasing CP level from -750 to -776 mV (SCE) the crack density and the maximum and the mean crack lengths in X80 steel decreased while those in X100 steel increased. No SCC cracks were observed in both steels when CP level was increased to -1126 mV (SCE). Another critical finding of this study was that CP may not be effective in retarding the propagation of existing SCC cracks. This could be attributed to the significant IR drop associated with narrow SCC cracks preventing CP current to penetrate.

Key words: SCC, CP, constant stress, crack initiation, crack propagation, high strength steel

Cathodic protection (CP) in conjunction with coating is generally recognized as the most effective mitigation method for pipeline external corrosion and stress corrosion cracking (SCC). However if not designed properly for a given pipeline system the applied CP can cause detrimental effects instead such as coating disbondment leading to corrosion and SCC hydrogen induced cracking (HIC) or hydrogen embrittlement in general. It has been recognized that modern high-strength pipeline steels can be more susceptible to hydrogen embrittlement or related environment assisted cracking such as near-neutral pH SCC compared to their low-strength steels counterparts. However the role of CP in the prevention or facilitation of such cracking is still not clear. The objective of this study is to examine the effect of CP on the initiation and propagation of SCC in two different high strength pipe steels namely X80 and X100.Proof rings with constant loads were employed to investigate the occurrence of SCC in X80 and X100 pipe steels under various CP conditions. The results showed that the effect of CP level on SCC in X80 and X100 steels were markedly different. At CP levels of -750 and -776 mV X80 steel was less susceptible to SCC than X100 steel i.e. X80 steel developed lower crack density and smaller crack lengths. With increasing CP level from -750 to -776 mV (SCE) the crack density and the maximum and the mean crack lengths in X80 steel decreased while those in X100 steel increased. No SCC cracks were observed in both steels when CP level was increased to -1126 mV (SCE). Another critical finding of this study was that CP may not be effective in retarding the propagation of existing SCC cracks. This could be attributed to the significant IR drop associated with narrow SCC cracks preventing CP current to penetrate.

Key words: SCC, CP, constant stress, crack initiation, crack propagation, high strength steel

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