Cathodic Hydrogen Absorption

While in (b) fracture occurs at a stress intensity factor of just 28 MPa.ym and is intergranular, in (a) it is transgranular and requires a stress intensity factor of 56 MPa m. 

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E. Protopopoff and P. Marcus, Electrochemistry and Materials Science of Cathodic Hydrogen Absorption and Adsorption, (B. E. Conway and G Jerkiewicz, eds.), PV 94-21 The Electrochemical Society, Pennington, NJ, 1995, pp. 374-386. 

In galvanic coupling, titanium is usually the cathode metal and consequently not attacked. The galvanic potential in flowing seawater in relation to other metals is shown in Table 10. Because titanium is a cathode metal, hydrogen absorption may be of concern, as it occurs with titanium complexed to iron (38). 

All metallic materials can suffer electrolytic corrosion. Fractures caused by cathodic hydrogen only occur when the activity of the absorbed hydrogen and the level of the tensile stress, which can be external or internal, reach a critical value. In general, critical hydrogen absorption is achieved only in the presence of promoters. However, under very severe conditions such as at very low pH or very negative potential, critical hydrogen absorption can occur. Steels with a hardness greater than HV 350 are particularly susceptible. 

With hot-dipped galvanized steel, hydrogen absorption with the formation of blisters can be observed in cathodic protection [38]. 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

Finally, it is necessary to point out that although a particular method of corrosion control may be quite effective for the structure under consideration it can introduce unforeseen corrosion hazards elsewhere. Perhaps the best example is provided by cathodic protection in which stray currents (interaction) result in the corrosion of an adjacent unprotected structure or of steel-reinforcement bars embedded in concrete a further hazard is when the cathodically protected steel is fastened with high-strength steel bolts, since cathodic protection of the tatter could result in hydrogen absorption and hydrogen cracking. 

A measurement of/H can be achieved electrochemically (32). Under galva-nostatic (constant current) cathodic conditions, the fractional efficiency of hydrogen absorption is the ratio of the amount of hydrogen produced electrochemically to the amount absorbed by the metal, as is illustrated in Fig. 25. While this appears to be a very simple direct measurement, it has the normal problems associated with an accelerated electrochemical measurement, including the need to demonstrate that the parameter measured electrochemically will retain the same value under the much slower natural corrosion conditions. 

The parameter g has been determined, from this kind of investigation, to be 5 and constant i.e., —80% of propagation is supported internally by proton reduction (47) under rapid propagation conditions. Since only the internal cathodic reaction supports hydrogen absorption, the rate of hydrogen absorption (/ ha) can be defined by... 

The strongest argument adduced in favour of a hydrated electron as an intermediate in cathodic hydrogen evolution is the statement of Walker that in electrolysing an aqueous solution using a polished silver cathode he succeeded in observing optical absorption in the near-electrode layer, caused by a hydrated electron. However, later it was shown 215,219) reliable proofs of the... 

Note, however, that there are conditions under which inhibitors can give rise to detrimental local corrosion, that is, pitting corrosion. This is the case when the amount of inhibitor is insufficient. Under these conditions, only part of the surface can be covered, thus giving rise to a local element. Corrosive attack is particularly extensive at the uncovered anode areas because of increased corrosion current density and deep cavities penetrating into the material. Similarly, if the inhibitor is too readily reduced at the cathodic areas of the metal surface, increased corrosion can result because compact protective films are not formed. Since there are no universally applicable inhibitors, they must be carefully selected and examined for each specific case. In doing so, inhibition of metal dissolution is not the only point to be considered—there is also hydrogen absorption. 

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