Hydrogen embrittlement behavior of HSLA line pipe steel under cathodic protection

This paper deals with hydrogen embrittlement (HE) under cathodic protection of steel used or recently proposed for buried pipelines of the oil and gas industry. High strength low alloy steel with different chemical composition, microstructure and ultimate tensile strength ranging between 530 and 860 MPa were considered to compare new steel with traditional grades. Their behavior was assessed through constant load tests, fracture mechanics tests on modified wedge-opening load specimens, slow strain rate tensile tests and interrupted slow strain rate tensile tests in solutions simulating soil moisture and sea water, at room temperature and cathodic potentials between -2 and -0.8 V vs. saturated calomel electrode reference. All the steel was found immune under static load and HE phenomena were observed only during slow strain rate tests at potentials more negative than a critical value, depending on steel susceptibility and strain rate. Quenched and tempered steel with high tempered martensite and very fine precipitation distribution showed better behavior than the rolled steels with banded microstructure, exhibiting the highest HE resistance. The resistance of steel characterized by banded microstructures increased with ultimate tensile strength until ∼700 MPa and then decreased. The increase of ultimate tensile strength occurred by changing the steel microstructure, from hot rolling with coarse ferrite/pearlite to fine ferrite/pearlite microstructure, and very fine acicular ferrite, yields the resistance of steel to HE. A further increase of ultimate tensile strength related to untempered martensite inside the acicular ferrite based microstructure produces a worsening of resistance.