Protection Hydrogen Embrittlement

Example Calculations of Corrosion Potentials, Corrosion Currents, and Corrosion Rates for Aerated and Deaerated Environments, and the Effects of Galvanic Coupling 

The objective of this example is to illustrate the use of data characterizing the requisite half-cell reactions to estimate corrosion rates. In this 

Corrosion behaviors based on the polarization curves in Fig. 4.28 are analyzed as follows  

It is important to compare the rates of corrosion of A and B in the aerated and then the deaerated solution. In the aerated solution, A is 

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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. 

An interesting field of application is the protection of tantalum against hydrogen embrittlement by electrical connection to platinum metals. The reduction in hydrogen overvoltage and the shift of the free corrosion potential to more positive values apparently leads to a reduced coverage by adsorbed hydrogen and thereby lower absorption [43] (see Sections 2.1 and 2.3.4). 

Acids are substances that increase the hydrogen ion (H ) concentration of the solution they are dissolved in. This, in turn, reduces the pH of the solution, and the corrosion rate increases. Acids may also attack the metal by dissolving the protective film on the metal surface, Presence of acid aggravates the oxygen-influenced attack and also hydrogen sulfide-promoted hydrogen embrittlement [203]. 

It is possible to provide cathodic protection to base plate up to 1 720 MNm yield strength, by coupling to mild steel or possibly to zinc , but zinc and metals more active than zinc tend to induce hydrogen embrittlement. Welds up to 1 380 MNm may be cathodically protected by zinc, but at impressed potentials of — 1-25V (S.C.E.) both 1240 and 1380 MNm welds fail rapidly due to hydrogen embrittlement. Neither mild steel nor zinc couples protect AISI 4340 steel . 

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. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

Cathodic protection in the negative potential zone where reduction of oxygen or water commences, and where the rate of metal oxidation is low. In this case there has to be an inert auxiliary electrode close to the surface to be protected. The protection process consumes current, the quantity depending on solution resistance between the surface to be protected and the anode. This protection can be expensive in terms of energy consumption, and even more if there is hydrogen release and, consequently, hydrogen embrittlement. 

The heart of corrosion science has been identified as electrochemical science coupled with the thermodynamic and kinetic values. Other limbs are oxidation and high-temperature oxidation of metals, protective coatings, passivity, inhibitors, microbial-induced corrosion, corrosion fatigue, hydrogen embrittlement and corrosion-resistant alloys. Having identified the limbs of corrosion science, it is instructive to examine how the various aspects came into existence over a period of time. 

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. 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

Surface films appear to play a major role in the initiation of SCC and may also contribute to hydrogen embrittlement effects. It is assumed that the main role of the surface film is to localize the damage inflicted on the material by the environment. This can be caused by mechanical breakdown of the protective film by slip step or electromechanical breakdown of the passive film.95 SCC may be related to the nature of the surface film. It has been observed that the SCC of C-Steels is related to the presence of magnetite in several low -temperature environments (around 90°C), except... 

A/cm. The choice is between higher protection at the risk of local overprotection (and possible hydrogen embrittlement) and less protection at the risk of corrosion, particularly in regions farther from the anodes. It is also a choice between the high cost of electric power on the one hand and the need for more frequent maintenance due to incomplete protection from corrosion, on the other. 

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