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The sulfide stress cracking (SSC) resistance of carbon steels and other alloys is commonly addressed through testing according to NACE TM01771 or NACE TM03162. The Method A of the first standard is focused on tests using uniaxial tensile (UT) while the second standard considers 4-point bend (4PB) type of loads. A common way of qualifying a material according to these standards is the absence of failure of the specimens or SSC crack initiation at the surface of the material after a test duration of 720 hours (1 month). After testing, cross-sectional observations of non-broken specimens often reveal so-called “grooves” that can be significantly different in shape and depth depending on the test method, steel grade or environment considered.
This work was conducted to address the experimental parameters influencing the formation of so-called superficial microgrooves during sulfide stress cracking (SSC) test that render sometimes difficult the decision on material qualification. It gives complementary results to a work published earlier.
Our investigations underline that, in our test conditions and for 2 different steel grades, grooving is not sulfide stress cracking initiation. Grooves can however act as stress concentrators and promote cracking. Calculation using finite element modelling allows to demonstrate that these initiations occur only if some plastic deformation is generated.
The discussion suggests that grooving and cracking are competing processes but that grooving should prevent seeing cracking if the tested material is susceptible.
The alloys used as clad material for this study are members of the so-called “C-family”. It consists of Ni-Cr-Mo alloys, which are known for combining the corrosion resistance of Ni-Cr alloys in oxidizing media with corrosion resistance of Ni-Mo alloys in reducing media. As a result, these materials have proven to be extremely durable in a wide range of highly aggressive media. The development of these materials started in the 1930s with Alloy C. This alloy showed remarkable corrosion resistance in a wide spread of media, low sensitivity for pitting or crevice corrosion and virtual immunity to chloride induced stress corrosion cracking.
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Corrosion of metallic structures is a ubiquitous problem in industries such as power generation, oil and gas, pulp and paper, metals processing etc. which also results in significant financial losses. According to the National Association of Corrosion Engineers (NACE) International report, the global cost of corrosion was ~ 2.5 trillion USD in 2013 - close to 3.4 percent GDP of the entire world. The use of corrosion inhibitors is one of the most effective and economical ways to mitigate corrosion of metal and alloy components. Corrosion inhibitors are substances that are added in small quantities in corrosive media to protect metal and alloy components from corrosion.
Steel rebars in concrete structures are usually protected from corrosion by a thin layer of passive film, which is formed due to the high alkalinity of concrete pore solution.1-2 However, this protective passive film could be damaged by penetration of chloride into concrete structures in marine environments or exposure to the use of de-icing salt for the removal of snow and ice in winter times.3 Penetration of chloride would impair the passive film locally and initiate pitting corrosion.