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Cracking risk in steel pipelines from external hard zones

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Connect, no.81, November/December 1996

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Relaxed hardness limits for sour service are given for cap regions of pipeline girth welds in BS4515, but are these appropriate when the pipe is cathodically polarised for external corrosion protection?

Ferritic steels are potentially susceptible to in-service cracking because of hydrogen pick-up from a corrosion reaction. Susceptibility to hydrogen-induced cracking increases with material hardness so there is a risk of weld-joint failure as a result of local formation of hardened structures. It is therefore normal industrial practice to specify maximum hardness limits for steels and weldments.

With pipelines, the sour (H2S) environment is within the pipe and hydrogen concentration will be greatest at the inner surface. However, hydrogen diffuses through the pipe but concentrations are lower on the external surface so cracking risk is reduced and higher hardness levels are acceptable. Results from work in this area done at TWI over the last decade have been incorporated into BS4515.

In service, the external pipeline surface may well be subjected to cathodic protection which could restrict hydrogen leakage from the inner surface. This is one reason why the allowable external hardness in BS4515 is well below experimentally-determined values.

A Group Sponsored Project just starting at TWI aims to provide data on the significance of cathodic protection in restricting hardness levels at welds on the outside of linepipe. It aims to confirm that an appropriate level of conservation has been built-in to the standard - neither unsafe nor too restricting.

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