Overprotection, cathodic

The activity of absorbed hydrogen is a measure of the damage for a given material. H absorption can be easily investigated in H-permeation experiments [9]. Figure 2-20 shows experimental results for steel/aerated seawater. Significant H absorption occurs only in the range of cathodic overprotection (1/ < -0.8 V). 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

The blister population-potential curves can intersect one another. Thus shortterm experiments at very negative potentials, in the region of cathodic overprotection, give no information on the behavior at potentials in the normal protection range. The susceptibility generally increases strongly with over protection < -0.83 V). 

The voltages AU and rj are defined by Eqs. (24-69) and (24-68a) and have a constant value of about 0.3 V. It is shown in Section 24.4.4 that with overprotection (i.e., by polarization into the range of hydrogen evolution) the cathodically protected range cannot be markedly lengthened. Therefore Eq. (10-5) is basic for the cathodic protection of pipelines. 

Cathodic protection of water power turbines is characterized by wide variations in protection current requirements. This is due to the operating conditions (flow velocity, water level) and in the case of the Werra River, the salt content. For this reason potential-controlled rectifiers must be used. This is also necessary to avoid overprotection and thereby damage to the coating (see Sections 5.2.1.4 and 5.2.1.5 as well as Refs. 4 and 5). Safety measures must be addressed for the reasons stated in Section 20.1.5. Notices were fixed to the turbine and the external access to the box headers which warned of the danger of explosion from hydrogen and included the regulations for the avoidance of accidents (see Ref. 4). 

Cathodic protection, especially if overprotected, through inadequate potential control ... 

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... 

Some measures involving cathodic protection of aluminum using zinc as the sacrificial anode,47 and protection of aluminum ship hulls are found in the literature.37 Overprotection may lead to alkali (cathodic corrosion) attack. Alclad alloy systems are also some form of self-contained cathodic protection systems. 

It is possible with impressed currents to "overprotect". Overprotection may give rise to problems comparable to the corrosion which the system seeks to prevent. Moderate overprotection of steel may not represent too much of a problem except the use of unnecessary current and the consumption of the auxiliary anodes. Higher levels of overprotection however, may give rise to excessive hydrogen at the cathodic surface with consequent deleterious effects. With metals such as aluminium, zinc, lead and tin excess alkali generated at the surface by overprotection may cause damage to the structure. 

Figure 10.9 Diagram indicating the field of potential and pH for reinforcement passive A), in aerated (6) and non-aerated (C) carbonated concrete, subject to pitting (D), under cathodic prevention ( ), under cathodic protection (F), and under overprotection (C) [7]...

In Figure 10.9 zones E and F represent the operational conditions of reinforcement in structures to which cathodic prevention and cathodic protection, respectively, have been applied (Chapter 20). Values at which cathodic protection normally operates are not sufficiently negative to induce hydrogen evolution. But even if it should operate in conditions of overprotection (potential below —900 mV SCE), the same diagram shows that up to potentials of—1100 mV SCE on the protected steel, the situation will nevertheless be less critical than on that unprotected reinforcement where pitting attack occurs. 

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. 

Cathodic protection should be designed not to overprotect the structure to avoid a high rate of hydrogen evolution and, consequendy, HE. Environmental control can be achieved by eliminating the oxidizing agents and critical species by demineralization,... 

In aerated neutral or alkahne solutions, cathodic corrosion reaction is usually the reduction of oxygen. The cathodic corrosion process is controlled by the availability of oxygen, which is related to oxygen diffusion to the cathodic corrosion site. For structures immersed in flowing water, the limiting current of oxygen varies with the flow velocity of the water. This will lead to underprotection or overprotection. 

The cathodic processes in cathodic protection can be limited to the reduction of oxygen and the corresponding alkalinization, but these are not without danger of deterioration. In the case of overprotection, we can have the development of hydrogen the excessive alkalinization of the soil, which tends to remain even in the case of temporary interruption of the current, the damage to the coatings, and their detachment from the metal. 

By measuring the potential of the protected structure, the degree of protection, including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when the protected structure is polarized to the open-circuit anodic potential of the local action cells [10]. 

Thus, from Figure 16.1, at the natural corrosion rate of the metal is Above E the corrosion rate increases. On the other hand, below E the corrosion rate is reduced. At E p, iron will not corrode. Thns, by applying cathodic protection, iron can be fully protected if the potential is lowered to pe- Partial protection is achieved if the potential is pushed to values between E and pe- However, lowering of the potential below leads to overprotection, which is of no benefit in as much as corrosion control is concerned. In addition, a greater expenditure on current than required for fuU protection is needed. 

The question is, how can we show that this has happened In cathodic protection of steel in soil or water it is usual to do this by achieving a potential of -770 mV or -850 mV against a copper/copper sulphate half cell on the surface as the system is switched off (the instant off potential). However, these criteria are not appropriate for steel in atmospherically exposed concrete for a nnmber of theoretical and practical reasons. Two of the practical reasons are the difficulty in accurately measuring an absolute potential over a nnmber of years when reference electrodes calibration may drift, and the fact that if an absolnte minimum (or maximum negative) potential is achieved then some parts of the structure will be overprotected as the corrosion environment varies so rapidly and severely across a high resistance electrolyte like concrete. 

In the United States trials of impressed current cathodic protection have been conducted on pre-tensioned structures (Bennett and Schue, 1998). Many pre-tensioned bridge piles have been cathodically protected in Florida using galvanic anodes. This is unlikely to cause overprotection and hydrogen evolution. 

The rate of attack can be appreciable in either dilute or concentrated alkalies. For this reason, when aluminum is cathodically protected, overprotection must be avoided in order to ensure against damage to the metal by accumulation of alkalies at the cathode surface. Lime, Ca(OH)2, and some of the strongly alkaline organic amines (but not NH4OH) are corrosive. Fresh Portland cement contains lime and is also corrosive hence, aluminum surfaces in contact with wet concrete may evolve hydrogen visibly. The corrosion rate is reduced when the cement sets, but continues if the concrete is kept moist or contains deliquescent salts (e.g., CaCb). 

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