Mechanisms of Hydrogen Damage

8.4.1 Mechanisms of Hydrogen Damage

The mechanism of hydrogen cracking has been explained by development of internal pressure [54] on the assumption that interstitial atomic hydrogen is released as molecular hydrogen at voids or other favored sites under extreme pressure. Such an effect certainly occurs, as is shown by the formation of visible blisters containing hydrogen when ductile metals are cathodically polarized or exposed to certain corrosive media. Under similar conditions, a less ductile metal would crack instead.

168 EFFEC T OF STRESS

Figure 8.12. Delayed fracture times and minimum stress for cracking of 0.4% C steel as a function of hydrogen content. Specimen initially charged cathodically, baked at 150 ° C for varying times to reduce hydrogen content [59] . [Figure 5 from H. Johnson, J. Morlet, and A. Troiano, Hydrogen, crack initiation, and delayed failure in steel, Trans. AIME 212 , 531 (1958).]

An interesting characteristic of hydrogen cracking is a specifi c delay time for appearance of cracks after stress is applied. The delay time is only slightly depen- dent on stress; it decreases with increasing hydrogen concentration in the steel and with increase in hardness or tensile strength [58] . For small concentrations of hydrogen, fracture may occur some days after the stress is applied.

A critical minimum stress exists, below which delayed cracking does not take place in any time. The critical stress decreases with increase in hydrogen concen- tration. These effects are shown in Fig. 8.12 for SAE 4340 steel (0.4% C) charged with hydrogen by cathodic polarization in sulfuric acid, then cadmium - plated to help retain hydrogen, and fi nally subjected to a static stress [59] . The hydrogen concentration was reduced systematically by baking.

Delay in fracture apparently results because of the time required for hydro- gen to diffuse to specifi c areas near a crack nucleus until the concentration reaches a damaging level. These specifi c areas are presumably arrays of imperfec- tion sites produced by plastic deformation of metal just ahead of the crack. Hydrogen atoms preferably occupy such sites because they are then in a lower -

energy state compared to their normal interstitial positions. The crack propagates discontinuously because plastic deformation occurs fi rst, and then hydrogen dif-

HYDROGEN DAMAGE

Figure 8.13 Effect of applied potential on time to failure of 4140 low - alloy steel, R c 46, in boiling 3% NaCl [60] . (Reproduced with permission. Copyright 1975, The Electrochemical Society.)

fuses to imperfection arrays produced by deformation, whereupon the crack propagates one step further. A short notch in a steel surface favors plastic defor- mation at its base, and hence it lowers the critical minimum stress and shortens the delay time. Below − 110 ° C or at high deformation speeds, hydrogen embrittle-

ment and cracking are minimized because diffusion of hydrogen is too slow. It is sometimes assumed that S.C.C. of high - strength steels of hardness >R c 40 (Table 8.1 ) in water or moist air is caused by hydrogen resulting from the reaction of H 2 O with iron. However, the effect of applied potential on failure times shows that, in boiling 3% NaCl, cracking occurs only above (noble to) a critical potential of − 0.40 ± 0.02 V and below a potential of about − 1.1 V (Fig. 8.13 ) with resistance

to cracking between these potentials. Failure in the upper potential region is better interpreted as S.C.C., whereas failure at or below − 1.1 V corresponds to

hydrogen cracking [60] . In support, steels fail by S.C.C. in H 2 O above room tem- perature in shorter times than at room temperature; failure times by hydrogen cracking (cathodic polarization), to the contrary, are longer the higher the tem- perature. Also, cold working of high - strength steels improves resistance to S.C.C. because the critical potential is shifted noble to the corrosion potential, whereas resistance to hydrogen cracking decreases. Accordingly, in practice, it is important that high - strength steel bridge cables be cold - drawn in order to avoid failure by S.C.C. in moist air. In the absence of cold work, they break prematurely despite more - than - adequate test strength, as has happened in the failure of bridges

EFFEC T OF STRESS

in the United States and other countries as well. Furthermore, a surface - decarburized (hence softer surface) high - strength steel does not fail in boiling water or 3% NaCl solution, but is readily hydrogen - cracked when cathodically

polarized. The small amount of hydrogen produced by the H 2 O – Fe reaction has no effect on the hard steel core. The signifi cant effect of adsorbed H 2 O rather than interstitial hydrogen as a cause of cracking of high - strength steels probably also applies to high - strength martensitic and precipitation - hardening stainless steels, Al alloys, Mg alloys,Ti alloys, and β and γ brass, all of which are sensitive to failure in the presence of moisture.