Stepwise cracking

Hydrogen-induced cracking (HIC) also known as stepwise cracking (SWC). 

Figure 7.83 Stepwise cracking in plate steel exposed to sour gas (Courtesy of Malcolm Hay)...

D. N. Williams. Correlation of the Results of Stepwise Cracking Tests with Service Experience in Sour Gas. Battelle report to NG-18 Line Pipe Research Supervisory Committee of the American Gas Association, October 1983. NG-18 Report 138. 

NACE Standard TM0284, "Evaluation of Pipeline Steels for Resistance to Stepwise Cracking, NACE, Houston, TX, latest revision. 

Fig. 15 Stepwise cracking in type A516-70 pressure vessel steel exposed to NACE TM0284-96 Solution A for 96 h at 25 °C [128. (Reprinted with permission from the copyright holder, NACE International.)...

HIC is also referred to as stepwise cracking. The cracks propagate and interconnect hydrogen blisters to form a step pattern. Similar to sulfide stress corrosion, HIC does not involve substantial metal ranoval but can cause failure of the component through internal crack propagation. 

The most common interpretation of the mechanism of cracking is based on a periodic electrochemical-mechanical process. This suggests that cracking is an alternating sequence of relatively slow anodic dissolution in the crack base and sudden mechanical crack propagation. In some alloys, intermittent cracking has actually been found, but in many other cases, no evidence of stepwise cracking has been produced. 

Hydrogen-induced cracking (H.I.C.) [also known as stepwise cracking (S.W.C.) because of its appearance]... 

This form of attack, also known as stepwise cracking, is typical of carhon steels showing ferritic structures, when in the presence of MnS (Type II) elongated inclusions as a consequence of rolling manufacturing processes. 

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