Corrosion rate, anode

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

In all cases of localized corrosion, tlie ratio of the catliodic to tlie anodic area plays a major role in tlie localized dissolution rate. A large catliodic area provides high catliodic currents and, due to electroneutrality requirements, tlie small anodic area must provide a high anodic current. Hence, tlie local current density, i.e., local corrosion rate, becomes higher witli a larger catliode/anode-ratio. 

Wrought lead—calcium—tin anodes have replaced many cast lead—calcium anodes (14). Superior mechanical properties, uniform grain stmcture, low corrosion rates, and lack of casting defects result in increased life for wrought lead—calcium—tin anodes compared to other lead alloy anodes. 

Table 4 shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. 

The second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

Asahi also reports an undivided cell process employing a lead alloy cathode, a nickel—steel anode, and an electrolyte composed of an emulsion of 20 wt % of an oil phase and 80 wt % of an aqueous phase (125). The aqueous phase is 10 wt % K HPO, 3 wt % K B O, and 2 wt % (C2H (C4H )2N)2HP04. The oil phase is about 28 wt % acrylonitrile and 50 wt % adiponitrile. The balance of the oil phase consists of by-products and water. The cell operates at a current density of 20 A/dm at 50°C. Circulated across the cathode surface at a superficial velocity of 1.5 m/s is the electrolyte. A 91% selectivity to adiponitrile is claimed at a current efficiency of 90%. The respective anode and cathode corrosion rates are about mg/(Ah). Asahi s improved EHD process is reported to have been commercialized in 1987. 

Galvanic Corrosion Galvanic corrosion is the corrosion rate above normal that is associated with the flow of current to a less active metal (cathode) in contact with a more active metal (anode) in the same environment. Tables 28-1 7 and 28-li show the galvanic series of various metals. It should be used with caution, since exceptions to... 

The corrosion rate is controlled mainly hy cathodic reaction rates. Cathodic Reactions 5.2 and 5.3 are usually much slower than anodic Reaction 5.1. The slower reaction controls the corrosion rate. If water pH is depressed. Reaction 5.3 is favored, speeding attack. If oxygen concentration is high. Reaction 5.2 is aided, also increasing wastage hy a process called depolarization. Depolarization is simply hydrogen-ion removal from solution near the cathode. 

In any galvanic couple, the corrosion rate of the active material (anode) will t3rpically increase, and the corrosion rate of the noble material (cathode) will typically decrease or cease altogether. 

Most galvanic corrosion processes are sensitive to the relatively exposed areas of the noble (cathode) and active (anode) metals. The corrosion rate of the active metal is proportional to the area of exposed noble metal divided by the area of exposed active metal. A favorable area ratio (large anode, small cathode) can permit the coupling of dissimilar metals. An unfavorable area ratio (large cathode, small anode) of the same two metals in the same environment can be costly. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

In this type of corrosion, metal ions arising as a result of the process in Eq. (2-21) migrate into the medium. Solid corrosion products formed in subsequent reactions have little effect on the corrosion rate. The anodic partial current-density-potential curve is a constant straight line (see Fig. 2.4). 

Two areas of passivity are located in Fig. 2-2 where Fe has a very low corrosion rate. In contrast to cathodically protected metals in groups I and II, the corrosion rate of anodically passivated metals in groups III and IV cannot in principle be zero. In most cases the systems belong to group IV where intensified weight loss corrosion or local corrosion occurs when U > U" There are only a few metals belonging to group III e.g., Ti, Zr [44] and A1 in neutral waters free of halides. 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

The rate of self-corrosion of zinc anodes is relatively low. In fresh cold water, it amounts to about 0.02 g m h , corresponding to a corrosion rate of 25 /rm a. In cold seawater, the value is about 50% higher [10]. These figures refer to stagnant water. In flowing water the corrosion rates are significantly greater. Zinc is not practically suited for use in warm waters because of its tendency to passivate. 

However, if part of the reinforcing steel is aerated, a cell is formed as in Section 2.2.4.2. With a high surface area ratio SJS and with well-aerated cathodes, very high corrosion rates can occur at anodic regions. 

Oxygen dissolved in aqueous solutions, even in very low concentrations, is a leading cause of corrosion problems (i.e., pitting) in drilling. Its presence also accelerates the corrosion rate of other corrodents such as hydrogen sulfide and carbon dioxide. Oxygen plays a dual role both as a cathodic depolarizer and an anodic polarizer or passivator. Within a certain range of concentration the... 

The aggressive ions either break down the protective films or prevent their formation and, in effect, increase the corrosion rate. In presence of chloride and sulfate ions the corrosion attack is more localized and, as a result, causes deep pitting. Inhibitive ions, on the other hand, tend to limit the attack and decrease the corrosion rate by forming protective films. The film is similiar to adherent carbonate-containing rust, which polarizes the anodic areas. When aggressive and inhibitive ions are present together, the aggressive ions, if present in sufficient quantities, interfere with the deposition of the protective layers. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

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