Hydrogen embrittlement sensitization

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. 

H. J. Cialone, and J. H. Holbrook, Sensitivity of Steels to Degradation in Gaseous Hydrogen, Hydrogen Embrittlement Prevention and Control, ASTM STP 962, L. Raymond, Ed., American Society for Testing and Materials, Philadelphia, PA, 134-152 (1998). 

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

Corrosion control. Generally corrosion inhibitors, cathodic protection, anodic protection, and coatings are used for this purpose or combination of them. However, cathodic protection is the only method that avoids corrosion completely if the system is not sensitive to hydrogen embrittlement or alkaline medium. Anodic protection is a recent approach when the metal can be passivated in the corrosive solution. In this technique, a current can be applied using a potentiostat, which can set and control the potential at a value greater than the passive potential Ep or below the pitting potential Ep]l for environments containing corrosive species such as chlorides, bromides, etc. 

Unless heavily cold worked, the austenitic stainless steels are resistant to hydrogen stress cracking such as that caused by hydrogen sulfide. They are also resistant to hydrogen embrittlement caused by phenomena other than cathodic charging. If sensitized, austenitic stainless steels can also be susceptible to intergranular corrosion. 

Aluminum alloys, particularly the high-strength compositions, are susceptible to environmental cracking, both in aqueous environments and in air as a function of relative humidity. This susceptibility is particularly sensitive to alloy composition and thermal treatment, which is shown by differences in the dependence of ductility on strain rate. Understanding these differences can contribute to identification of mechanisms of the strain-rate sensitivity. A summary of the influence of strain rate on the ductility of 2000-, 5000-, and 7000-series aluminum alloys in environments represented by 3% NaCl + 0.3% H202 is shown in Fig. 7.84 (Ref 121). The 7000 series shows susceptibility to hydrogen embrittlement at strain rates below 10 5 to 10-6 s 1. Although there is... 

Representative environments for which SCC has been reported in carbon steels are included in Table 7.7. The sensitivity of these steels to changes in composition and environment are illustrated by the effects of potential in Fig. 7.78 to 7.80 and by the slow strain-rate data of Fig. 7.82 and 7.83. These data support the conclusion that environment cracking is related to the susceptibility of the passive films to crack under stress, to the subsequent crack growth due to anodic dissolution and/or hydrogen embrittlement during the period of exposure of the alloy substrate, and to rates of repassivation of the exposed areas. Actual crack-front growth mechanisms are discussed in some detail in a later section. 

The martensitic and precipitation-hardening stainless steels are more susceptible to hydrogen embrittlement than are the austenitic alloys. The susceptibility of these stainless steels is sensitive to microstructure and strength level. Figure 22 shows the resistance to cracking for a martensitic (type 410) stainless steel and some precipitation-hardening stainless steels in an H2S-saturated solution [174]. The low resistance of type 410 stainless steel is typical for most... 

O.M. Alyousif, R. Nishimura, Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling samrated magnesium chloride solutions, Corros. Sci. 50 (2008) 2353-2359. 

Static tests carried out with non-precracked specimens allow us to characterize, for a given environment, the sensitivity of an alloy to stress corrosion cracking and hydrogen embrittlement. TyfpicaUy, the test specimen subjected to a constant load or a constant strain is exposed to a corrosive environment and one measures the time to failure. The test specimens sometimes contain a notch—not to be confused with precracking—that fixes the location where failure will occur. In constant strain tests, the time to failure corresponds to the appearance of the first cracks. This kind of experiment thus indicates the crack initiation time. In constant load tests, on the other hand, the time to failure is the time leading to fracture of the specimen. It corresponds to the sum of the crack initiation and the crack propagation times. 

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