Corrosion polarization, influence

Geometry of components and their design also influence galvanic corrosion. As current does not flow around the corners, the geometry of the circuit affects the degree of galvanic corrosion. Polarization may be affected by a break in the continuity of the current. 

J.T. Waber and B. Fogan, Mathematical Studies on Galvanic Corrosion ly Influence of Electrolyte Thickness on The Potential and Current Distributions of Coplanar Electrodes using Polarization Parameters, Journal of Electrochemical Society, vol. 103, pp. 64-72, 1956. 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

It should be clearly pointed out that with anodic interference according to the data in Fig. 2-6 in Section 2.2.4.1, the corrosivity of the electrolyte for the particular material has no influence on the current exit corrosion. On the other hand, the conductivity of the electrolyte has an effect according to Eqs. (24-102) and (20-4). Chemical parameters have a further influence that determines the formation of surface films and the polarization resistance. 

Use of carefully selected surfactants in well treatment fluids is a way to accomplish this. Rock wettability can be altered by adsorption of polar materials such as surfactants and corrosion inhibitors, or by the deposition of polar crude oil components (173). Pressure appears to have little influence on rock wettability (174). The two techniques used to study wettability, contact and and relative permeability measurements, show qualitative agreement (175-177). Deposition of polar asphaltenes can be particularly significant in carbon dioxide enhanced oil recovery. 

The quality of the ethanol has a strong influence on its corrosive effects. Three categories of problems have been identified general corrosion (caused by ionic impurities, mainly chloride ions and acetic acid), dry corrosion due to ethanol polarity, and wet corrosion caused by azeotropic water. Corrosion inhibitors should thus be incorporated in ethanol-diesel blends. 

Figure 2 Influence of IR voltage error on galvanic corrosion. Increasing IR voltage error decreases the extent of anodic polarization of the anode and cathodic polarization of the cathode.

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

It is important to study the potential distribution inside the crevice and its relation to the polarization curve in order to obtain a better understanding of the mechanism by which the crevice corrosion occurs. Factors that influence both the potential distribution and the polarization curve include the metal environment reaction products and the crevice geometry as well as the well-known promoters of crevice corrosion acidification and chloride ion build-up within the crevice. When excess oxygen entered the crevice via flushing air-saturated solution through it, the corrosion rate decreased which was consistent with the proposed need to keep the cathodic reaction outside the crevice, separate from the anodic reaction, for a stable crevice corrosion to occur [151]. 

Fig. 4.9 Influence of relative positions and shapes of anodic and cathodic polarization curves on the corrosion current, lcorr. (a) Anodic diffusion control, (b) Cathodic diffusion control, (c) Anodic and cathodic diffusion control. Ecorr and lcorr refer to corrosion under diffusion control. (Ecorr) and (lcorr) refer to corrosion without diffusion control.

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

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