Investigation of corrosion behavior, hydrogen uptake, and hydrogen embrittlement of armor wires made from carbon steels

Abstract

The effect of cementite morphology on corrosion layer formation, hydrogen uptake, crack initiation, and growth in the presence of hydrogen was investigated in carbon steels which are used to fabricate tensile armor wires in flexible pipelines.

In situ electrochemical micro-cantilever bending (ECCB) test was used to examine the crack initiation and growth path considering hydrogen interaction with the cementite morphology. To realize this, a miniaturized electrochemical three-electrode cell was incorporated into the Triboindenter. The fabricated micro-cantilevers were bent under hydrogen-free and in situ hydrogen-charged conditions. It was shown that in spheroidite microstructure, the crack growth followed a straight path from the notch through the thickness of the micro-cantilever, and under high negative charging potentials (high hydrogen fugacity) the crack growth is independent of grain boundaries or ferrite-cementite interfaces. In the ferrite-pearlite microstructure, competition between the shear mechanism and the interfacial cracking determines the crack growth path.

The linear polarization resistance and weight loss tests were used to examine the effect of cementite morphology on the corrosion layer formation on carbon steels in an aqueous solution containing CO2 and CO2/H2S. After the corrosion test, the corrosion layers that formed on the carbon steel surfaces were characterized using SEM-EDS, FIB, and XPS to reveal the effect of cementite morphology on the layer formation and corresponding corrosion properties. It was revealed that as the cementite phase fraction is increased, the corrosion rate increases in the CO2 environment, while no trend is observed in the CO2/H2S environment. Under CO2 exposure, the corrosion rate of the materials with lamellar ferrite-pearlite microstructures was higher than the materials with a spheroidite microstructure. A mechanism that explains the role of cementite morphology on corrosion layer formation in a CO2 environment is proposed for the studied materials.

Hydrogen thermal desorption and hot extraction analysis were performed on the studied materials after their exposure to three different environments (an aqueous solution with CO2 bubbling, CO2/H2S bubbling, and cathodic charging) to investigate the influence of the charging condition and environment, the cementite morphology, and the corrosion layer on the hydrogen uptake and hydrogen desorption. The hydrogen uptake is substantially higher in CO2/H2S environment compared to CO2 environment for all materials. The hydrogen uptake increase with carbon content in the CO2/H2S environment and under the cathodic charging. The lamellar cementite morphology absorbed higher hydrogen than the material with spheroidite microstructure with almost the same amount of carbon content in the microstructure. The corrosion layer formed on the steels in the CO2 environment strongly affects the hydrogen effusion and the thermal desorption spectroscopy spectrum, while the corrosion layer formed in the CO2/H2S environment does not show this effect.