Galvanic corrosion factors affecting

Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Some of the other factors affecting galvanic corrosion are area ratios, distance between electrically connected materials, and geometric shapes. Galvanic corrosion of the anodic metal takes the form of general or localized corrosion, depending on the configuration of the couple, the nature of the protective films formed and the nature of the metals. 

Haynie, F. H., Spence, J. W., and Upham, J. B. (1976 and 1978). Effects of air pollutants on weathering steel and galvanized steel A chamber study. Atmospheric Factors Affecting the Corrosion cf Engineering Metals. ASTM STP 646, Proc. Golden Anniversary Symposium. ASTM, Philadelphia, pp. 30-47. Also report EPA-600/3-76-015 (1976), 85 pp. 

Baboian, R., Final Report on the ASTM Study Atmospheric Galvanic Corrosion of Magnesium Coupled to Other Metals, Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646, S. Cobum, Ed., 1978, ASTM International, West Conshohocken, PA, pp. 17-29. 

Galvanic corrosion and the factors affecting it have been discussed in Chapter 20. However, a few precautionary comments are in order for aluminum, since it is anodic to most common materials of construction, with the exception of magnesium and zinc. In the presence of a good electrolyte, as little as 15 mV difference in corrosion potential of the two metals can have an effect, and if the difference is 30 mV or greater the anodic material will definitely corrode sacrificially to protect the contacting cathodic metal. A recently revised report on galvanic corrosion, with emphasis on automotive applications is available [73]. 

Protection of galvanized surfaces from atmospheric corrosion is due to the formation of films of basic salts, primarily carbonate. The most widely accepted formula is Zn.j(0H)(,(C03) . The galvanized surface of zinc is rapidly attacked when environmental conditions lead to the formation of soluble films. One of the most important factors affecting the corrosion of zinc in the atmosphere is the duration and frequency of moisture contact. 

It is important to realize that galvanic corrosion effects can be manifested not only on the macroscopic level but also within the microstructure of a material. Certain phases or precipitates will undergo anodic dissolution under microgalvanic effects. Because the principle of galvanic corrosion is widely known, it is remarkable that it still features prominently in numerous corrosion failures. Figure 7.17 illustrates the main factors affecting the formation of a galvanic cell [14]. 

The following factors significantly affect the magnitude of galvanic corrosion ... 

Component design is also a factor in galvanic corrosion as the current circuit geometry affect the magnitude of galvanic corrosion and the polarization process. Any obstacle to polarization would accelerate galvanic corrosion. 

There are in addition several other factors that accelerate corrosion and must betaken into account these include crevices, galvanic coupling, tensile stress, aeration, presence of impurities, surface finish, etc. If these were also taken into consideration then several million experiments would have to be performed to compile such data. There are many instances where two or more chemicals exert a marked synergistic action such that low dissolution rates obtained in either environment become much greater in the presence of both. Further, the corrosiveness of a chemical will be affected by the presence of certain impurities, which may act as either accelerators or inhibitors. To take all these factors into account would add to an already impossible task and as Evans has remarked, There are not enough trained investigators in the world to obtain the empirical information to cover all combinations of conditions likely to arise . Unfortunately corrosion science has not yet reached the stage where prediction, based on a few well established laws, allows selection of materials to be made without recourse to a vast amount of data. 

The extent of galvanic effects will be influenced by, in addition to the usual factors that affect corrosion of a single metal, the potential relationships of the metals involved, their polarisation characteristics, the relative areas of anode and cathode, and the internal and external resistances in the galvanic circuit (see Section 1.7). 

Sulfur oxides and other corrosive species are brought to react with the zinc surface in two ways dry deposition and wet deposition. Sulfur dioxide has been observed to deposit on a dry surface of galvanized steel panels until a monolayer of SO2 formed (Maato, 1982). In either case, the sulfur dioxide that deposits on the surface of the zinc forms sulfurous or other strong acids, which react with the film of zinc oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of sulfur dioxide to sulfur-based acids may be catalyzed by nitrogen compounds in the air—usually referred to collectively as NQt compounds—and it is believed that this factor may affect corrosion rates in practice. The acids partially destroy the Film of corrosion products, which will then re-form from the underlying metal, so causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Above about 85% RH, corrosion rates increase further—probably as a result of the formation of basic zinc sulfates. 

The second category of factors that affect corrosion includes temperature, pressure, metallurgy, redox potential, biological effects, velocity, and galvanic effects [4,5]. In general, corrosion rates increase with increasing temperature or pressure. Both temperature and pressure affect reaction rates and gas solubility. Temperature will also affect the formation of protective scales. 

One of the key factors in any corrosion situation is the environment. The definition and characteristics of this variable can be quite complex. One can use thermodynamics, e.g., Pourbaix or -pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided the chemical makeup of the environment is known. But for practical situations, it is important to realize that the environment is a variable that can change with time and conditions. It is also important to realize that the environment that actually affects a metal corresponds to the microenvironmental conditions that this metal really sees, i.e., the local environment at the surface of the metal. It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that investigates only the nominal environmental condition without consideration of local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction. 

Other factors that affect the rate and level of corrosion are heterogeneities in the concrete and the steel, pH of the concrete pore water, carbonation of the Portland cement paste, cracks in the concrete, stray currents and galvanic effects due to contact between dissimilar metals. 

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