Hydrogen embrittlement cracking

Figure 2.1 Schematic comparison of (a) anodic SCC (b) hydrogen embrittlement cracking (FIEC) mechanisms...

There has been much interest in conducting hydrogen embrittlement cracking (HEC) tests in aqueous media which can produce atomic hydrogen on the surface of materials as a result of corrosion or cathodic charging. In most cases, these tests can be conducted at ambient pressure and at... 

Cathodic protection can stifle SCC in some metal systems. However, if cracking is the result of hydrogen embrittlement rather than SCC, the use of cathodic protection can intensify cracking. 

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Hydrogen-induced cracking (HlC) is most commonly encountered in steels but other metals are susceptible, as shown in Table 53.3. The presence of hydrogen atoms in a metal degrades some of its mechanical properties, especially its ductility, leading in some cases to embrittlement. 

Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, NACE, Houston, Texas, (1975)... 

There has been some controversy as to whether s.c.c. occurs by active path corrosion or by hydrogen embrittlement. Lack of space does not permit a full treatment of this subject here. References 14 and 15 are recent reviews on the s.c.c. of high strength steels and deal with the mechanism of cracking (see also Section 8.4). It is appropriate to discuss briefly some of the latest work which appears to provide pertinent information on the cracking mechanism. It should be noted, however, that cracking in all alloy systems may not be by the same mechanism, and that evidence from one alloy system need not constitute valid support for the same cracking mechanism in another. 

Although tests on smooth specimens indicate that cathodic protection of maraging steel is possible, tests on specimens with pre-existing cracks indicate a greater sensitivity to hydrogen embrittlement during cathodic polarisation . The use of cathodic protection on actual structures must therefore be applied with caution, and the application of less negative potentials than are indicated to be feasible in smooth specimen tests is to be recommended if it is assumed that structures contain crack-like defects. 

Anodic oxide film properties depend upon ion concentration in acid chloride and in alkaline solutions films are more compact and crack-free in acid solution . Alloying with more than 47% of nickel gives good resistance to hydrogen embrittlement in potassium hydroxide solution . 

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