Hydrogen embrittlement cathodic protection

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. 

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. 

Let us begin with two common observations involving separated anodes and cathodes. The cathodic protection level obtained on metallic surfaces is often noted to vary with position. The metal is usually less well protected as the distance of the metal surface from the sacrificial or impressed current anode increases. Alternatively, the structure may be overprotected at positions close to the anode, leading to potentially embrittling hydrogen production. Similarly, it is well known that it is more difficult to plate metals electrolytically or throw current into corners or recesses, while exposed edges may receive a thicker plating deposit. The main explanation for this behavior is that the aqueous solution... 

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. 

Trials. The effectiveness of chloride extraction depends on characteristics of individual structures, such as the concrete composition, the actual chloride-penetration profile and the depth of cover. So, it may be useful to carry out a trial on an area (about 1 to 10 m ), which must be representative of the structure to be treated and should last at least 4 to 8 weeks. The results of such a trial in terms of the chloride profile before, during and after chloride extraction gives an indication of the duration required and can be used to show that chloride-extraction treatment of the particular structure will be effective under field conditions. Trials are most certainly recommended if prestressed structures are to be treated with chloride extraction. Careful monitoring of the potential of the prestressing steel should be carried out to establish the risk of hydrogen embrittlement. As a safe criterion, the potential should not become more negative than -900 mV SCE, as apphes for cathodic protection [13]. 

The criteria for cathodic protection are not free from criticism. It is beheved that all the listed criteria are deficient to some extent and therefore qualitative in practical appKcation. However, one should be optimistic that any level of cathodic polarization is beneficial, and a broad range of ca-thodically applied potentials will yield adequate protection. As a result, the use of any criterion listed in Table 4 [24] will produce adequate cathodic protection if applied judiciously. The amount of cathodic protection should be sufficient to reduce the corrosion rate to an acceptable range. Caution should be exercised to avoid overprotection. Overprotection results in the premature consumption of sacrificial anodes or excessive amounts of impressed current demands. Moreover, the application of too much cathodic protection can result in damage to the structure to be protected as a result of hydrogen embrittlement. 

Prestressed concrete pipelines occasionally require cathodic protection. Protection must be done carefully to avoid damage to the prestressing wire from hydrogen embrittlement or SCC [75]. 

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