The following cases of hydrogen influence on pipeline metal were considered: gaseous hydrogen under internal pressure in notched pipes and electrochemically generated hydrogen on external pipe surface from soil aqueous environment. The burst tests of externally notched pipes under pressure of hydrogen and natural gas (methane) were carried out after the pipe has been exposed to a constant ''holding'' pressure. It has been shown that even for relatively ''soft'' test conditions (holding pressure p ¼ 20 bar and ambient temperature) the gaseous hydrogen is able to penetrate into near surface layers of metal and to change the mechanism of local fracture at notch. The sensitivity to hydrogenating of given steel in deoxygenated, near-neutral pH NS4 solution under soft cathodic polarisation was studied and the assessment local strength at notches in pipeline has been made for this conditions. Here, the relationship between hydrogen concentration and failure loading has been found. The existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was also shown.

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It is now recognised that a complex load history may lead to structural damage that cannot be predicted on the basis of simple uniaxial loading conditions. The application of a single overload can induce sufficient crack growth such that the fatigue limit of a material is compromised. In a similar manner, it is possible to transgress the fatigue limit of a material by applying a number of corrosion cycles which lead to an increase in the size of the defect(s) present in the material beyond that of a 'threshold condition'. Damage accumulation within materials arises owing to the load and environmental history experienced throughout the lifetime of a component or structure. Furthermore, damage owing to load or environment in isolation impacts less on the durability of a structure/component, compared with that when conjoint effects of load and environment are experienced. This paper discusses the contribution that arises from environmental processes, such as corrosion, to final material failure. Synergy owing to corrosion and static/cyclic stress are discussed for both passive (stainless steel) and active (carbon steel) materials.