Hydrogen Induced Cracking and Stress Corrosion Cracking
Abstract
Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) are two important and dangerous failure forms which can be caused in pipelines and related components by improper operation or material corrosion. HIC forms when the material is exposed to high strength hydrogen which can be absorbed into the surface of the material, making it susceptible to embrittlement and cracking. SCC is hydrolysis-induced cracking caused by mechanical stresses and the presence of corrosive environment and/or contaminants. Although both of these hazardous phenomena are caused by variable high temperatures and other extreme operating conditions, the respective root causes and mechanisms of damage are different. This review article examines the differences between HIC and SCC and provides a synopsis of advances made in the detection, prevention and mitigation of these damaging modes.
Introduction
Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) are two potential failure mechanisms that can occur in steel (and other metals) components subjected to variable high temperatures and other extreme operating conditions. For example, HIC and SCC are common in pipelines and related components and can compromise the integrity of these assets if not controlled through knowledge and effective maintenance regimes. The respective root causes and mechanism of damage in each case are different, and these differences are addressed in this review.
HIC and SCC both tend to occur in the same operating environment, so it is important to understand how to recognize the signs and symptoms of each in order to precisely diagnose and take proactive measures. Inline inspections (ILI) are often used to detect HIC and SCC before they become critical issues. The remainder of this review paper will discuss the differences between HIC and SCC, the potential issues caused by both, and the ILI and other technique used to detect and mitigate them.
HIC
Hydrogen induced cracking (HIC) is a form of cracking that is caused by high strength hydrogen present on the surface of the material, often due to susceptible environments or manufacturing procedures. It occurs when the hydrogen atoms diffuse into the metal, causing a form of embrittlement and cracking. HIC is often accompanied by visible fading of the steel surface, and can be most commonly found in the heel area of pipe bends and the grooves in the welding seam root of a pipe joint.
In pipelines and related components, HIC can be caused by improper material selection, fabrication or welding practices, or even by sudden changes in the operating temperature. Other external sources of high strength hydrogen, such as cathodic protection, may also contribute to HIC in pipelines. It can also be caused by environments with high levels of chlorine and/or acidity from water and/or corrosion inhibitor, generated from the corrosion of the materials.
SCC
Stress corrosion cracking (SCC) is a form of cracking caused by a combination of mechanical stresses and the presence of a corrosive environment and/or contaminants. SCC occurs when the mechanical stresses cause micro-pores to form in the material surface which are then eaten away by the corrosive environment. This can occur in laydowns and bend radius areas, as well as in other strained areas such as welds and connections. SCC can be difficult to detect with conventional inspection techniques and is often not visible on the surface.
The potential causes of SCC include high operating temperatures, high levels of hydrogen sulfide, sulfates, chlorides, and other corrosive elements in the environment, as well as the formation of hydrogen embrittlement at low temperatures. SCC can also be caused by poor design, fabrication and installation of the pipeline components, and by failures at the gasket or flange surfaces. Additionally, SCC can be caused by improper usage, such as operating with an incorrect pressure or flow rate.
Detection and Mitigation
In order to accurately diagnose and mitigate HIC and SCC, it is necessary to use inline inspection (ILI) and other advanced techniques. ILI tools, such as metal loss tools, crack detection tools, and right-of-way tools can detect and measure features, anomalies and potential damage in the pipeline. However, analysis of ILI data cannot replace manual visual inspections and the results should be evaluated by a qualified, trained inspector.
The most important step in minimizing the risk of HIC and SCC is to have a comprehensive maintenance program in place that inspects and tests the pipeline components regularly. The detection of HIC and SCC can be further improved by monitoring the pipeline surface temperature and humidity levels, as these can be key indicators of these damaging forms. In addition, the use of surface coatings, internal liners and corrosion inhibitors can also be used to reduce the risk of HIC and SCC.
Conclusion
In conclusion, HIC and SCC are two important and dangerous failure forms that can be caused in pipelines and related components by improper operation or material corrosion. Although both of these hazardous phenomena are caused by variable high temperatures and other extreme operating conditions, the respective root causes and mechanisms of damage are different. This review article examined the differences between HIC and SCC and provided a synopsis of advances made in the detection, prevention and mitigation of these damaging modes. Inline inspections (ILI) are often used to detect HIC and SCC before they become critical issues, and the most important step in minimizing the risk of HIC and SCC is to have a comprehensive maintenance program in place that inspects and tests the pipeline components regularly.
