Integrated Mechanical Material Corrosion Consulting, USA
An essential step in materials degradation is the determination of the failure mechanism. For example, stress, metallurgical (microstructure), or environment-assisted cracking, help the determination of the fundamental cause of failure and recommendation on proper measures to prevent failure recurrence. In this context, environment-assisted cracking (EAC), or environmentally induced failure (EIF), is a form of corrosion that produces a brittle fracture in alloys with minimal corrosion. As for example, environmentally assisted cracks in line pipe steels are initiated either as a result of stresses in combination with environmental effects, as in stress corrosion cracking (SCC), or as a result of trapped hydrogen in the steel, as in hydrogen-induced cracking (HIC). To understand better the mechanism of the crack initiation process, key metallurgical and environmental elements that can affect the cracking phenomena were investigated and are reviewed in this paper. The complexity of both cracking phenomena results from the dependence of HIC and SCC on multiple metallurgical, mechanical, and environmental parameters that may all influence both crack initiation and propagation; e.g., composition, microstructure and non-metallic inclusions in the steel, applied stress, water chemistry in the field, and ionic concentrations in the groundwater near the pipe surface to name a few for SCC.
In addition, for HIC phenomena, one can add the concentration of H2S in the fluids transported in the pipe as well as concentration of CO2, pH, etc. Furthermore, it is not well-known why similar steels can have different performance, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is important and needed.
Cracking of line pipe steels is analyzed with particular attention to the crack initiation process. The paper is divided into two parts: The first part covers HIC and the second part covers SCC. This overall terminology includes stress corrosion cracking (SCC), corrosion fatigue cracking (CFC), and hydrogen-induced cracking (HIC). The latter is also referred to as hydrogen embrittlement (HE), or hydrogen-assisted cracking (HAC).