Stress Corrosion Cracking: My New Baby

I recently started to work on a new project related to stress corrosion cracking or in short SCC. For me new projects equal with a lot of happy moments as I have to overcome a lot of challenges initially (needless to say that I belong to that spectrum in which people strive for challenge). One of the challenges is obviously and naturally about learning stuff and based on evidence learning and Serotonin level in my brain are directly linked making me feel happy.

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To show appreciation for the source of my happiness then, I am sharing here some basics about my project and generally SCC, a serious engineering problem intimidating mainly nuclear power, chemical, oil and gas industries. In my case, my project deals with SCC in gas pipeline so I will go through the basics of SCC in gas pipelines.

Interaction of a reactive environment and a susceptible material with tensile stresses lead to crack initiation and growth and eventually (catastrophic) failure. This phenomenon in which three requirements of environment, material and stress has to be met in order for failure to occur is called stress corrosion cracking (SCC). SCC is different from corrosion as in the latter the element of stress is not required for failure to occur. In fact, SCC can occur at instances where varieties of corrosion protection strategies has been applied on the buried energy pipelines. Gas pipelines are manly made of low carbon steel possessing low cost and suitable mechanical properties though with low corrosion resistance. To mitigate the low corrosion resistance, cathodic protection in conjunction with coating is applied. The aim of cathodic protection is to catholically polarize the surface of pipeline in contact with soil. In other words, the surface of the pipeline is covered with electric current (free electrons) converting active metal surface (anodic sites) to passive or cathodic sites. Coating is also applied to further mitigate corrosion by blocking the contact between soil and the pipe surface. However, interestingly enough, application of coating and cathodic protection although preventing one type can lead to another type of damage known as SCC. To explain the mechanism through which the SCC occurs I need to the categorize SCC phenomenon first.

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Another difference between high-PH SCC and near-neutral SCC is the contribution of temperature. In high-PH SCC, temperature has a role to play and that is why high-PH SCC is normally detected immediately downstream of compressor stations where high temperature is more likely. In contrast, near-neutral SCC can be detected well away from the compressor stations, as it is independent of temperature.Generally, there are two types of SCC damage in the gas pipeline system, so-called high-PH SCC and near-neutral SCC. High-PH SCC is a more classical version of SCC in the energy pipelines discovered from mid-1960s while the latter was not documented until mid-1980s (mainly in Canada). In both cases, localized disbondment of the coating is the source of the damage. However in high-PH SCC, the cathodic current reaches the pipeline inside the disbondment while in near-neutral SCC exactly opposite occurs and cathodic current does not reach inside the disbondment because of the shielding effect of the coating or the soil surrounding the pipe. The area under the disbondment could be filled with solutions from the surrounding environment (which act as a corrosive environment on the surface of steel) and whether or not cathodic current reach the solution under disbondment determine the PH of the corrosive environment. Nevertheless, the reaction between the corrosive environment and pipeline steel generates pits and if these generated pits reach to a critical size and depth can act as stress concentration sites leading to crack initiations. The stress required for SCC to occur can be directly applied (hoop stress) or residual in nature (introduced by cold forming, welding, pipe resistance to soil movement, heat treatment, etc).

There are also differences in the mechanisms through which these two types of SCC develops, but I’m not going through that in this post and I might write a separate post around the mechanism later in the future. But a distinct feature in terms of failure analysis is that in high-PH SCC crack normally propagates inter-granular (along grain boundaries) while in near-neutral SCC crack propagates trans-granular (crossing grains).

 

 

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