Galvanic corrosion of zinc

It is obvious from the data that galvanic corrosion of zinc in rural atmosphere can be five times the rate in corresponding atmospheric corrosion and three times that in marine atmospheres. Mild steel appears to be the most efficient cathode among the metals studied. More detailed discussions are given in the literature.92... 

Cu -F Zn Cu -F Zn " " which causes additional galvanic corrosion of zinc. 

R.M. Souto, Y. Gonzalez-Garcia, A.C. Bastos, A.M. Simoes, Investigating corrosion processes in the micrometric range a SVET study of the galvanic corrosion of zinc coupled with iron, Corros. Sci. 49 (2007) 4568-4580. 

By itself, this reaction does not pose a great danger because only small amounts of Cu are available for reaction. However, due to the displacement reaction nuclei of metallic copper form at the pipe walls and cause galvanic corrosion of zinc and steel in their immediate vicinity. As a consequence, one observes the formation of pits that eventually lead to perforation of the pipe walls. 

V, and Tpe= -0.44 V. If the zinc and the iron electrodes connected by an electrical conductor are dipped into standard electrolytes in this cell, then iron would serve as the cathode (Fe2+ + 2 e —> Fe) and zinc as the anode (Zn —> Zn2+ + 2 e ). The result would be a tendency for zinc to dissolve in the electrolyte, and this process is known as the galvanic corrosion of a less noble metal (zinc) in comparison with the more noble metal iron in this system. The reversible emf of the corrosion cell would be... 

The zinc, cadmium or tin plating reduces the galvanic corrosion of magnesium, as seen in Figure 4.6. The compatibility of various fasteners shown in the figure leads to the order of effectiveness tin > cadmium > zinc. 

As noted earlier pitting corrosion of zinc is usually not encountered, with the exception of corrosion in nonuniform soils, distilled water containing vertical samples of zinc panels, and galvanized steel in hot water tanks. In hot water tanks containing soft water,... 

Another form of surface coating is provided by galvanizing, the coating of an iron object with zinc. Because the latter s standard potential is —0.76 V, which is more negative than that of the iron couple, the corrosion of zinc is thermodynamically favored and the iron survives (the zinc survives because it is protected by a hydrated oxide layer). 

Hydrogen embrittlement of steel is not strictly related to corrosion of zinc, but it is briefly mentioned here because in hot dip galvanized steel applications it is often confused by users with corrosive attack. 

He studied the results of six sets of exposure investigations to relate the corrosion of zinc and galvanized steel to concentration of sulfur dioxide. All six investigations were different, and the data were evaluated differently, and thus no direct comparison of the published results were possible. However,... 

Spence and Haynie (1990) combine the dry and wet deposition terms derived for the corrosion of zinc surfaces to propose the following linear damage function for predicting the long-term corrosion (p,m) of galvanized steel surfaces ... 

The corrosion of zinc in urban atmospheres (Table 2.4 and Fig. 2.2) is historically about 2-6 p,m/year this corresponds to a life for zinc roofing sheet of more than 50 years, and 12-50 years for the galvanized coating on... 

Reinhart (1976) has reported on the use of zinc alloys and zinc wire ropes that were exposed in the Pacific Ocean at depths of 720-2070 m for periods varying from 123 to 1064 days. The zinc alloy composition was 99.9% zinc, 0.9% lead, and 0.1% iron. The wire ropes were galvanized steel cables of various types. The data obtained from the study are given in Tables 3.23 and 3.24. From the data shown in Table 3.23, the corrosion rate of zinc in Pacific Ocean seawater is seen to decrease with the duration of exposure, except for zinc at the 2400 ft depth, at which the corrosion rate increased with increasing time of exposure. Also, the corrosion of zinc was greater at depth than at the surface. In addition, the report indicated that the corrosion of zinc was not uniformly influenced by changes in the concentration of oxygen in seawater between the limits of 0.4-5.75 mL/L. 

Simm (1984) has studied the conjoint action of carbonation and chloride ions on the corrosion of zinc in mortar. Highly concentrated, high chloride mortars at 100% RH and 25°C can corrode zinc at 100 pm/year, but high carbonation or high chloride on its own causes a loss of zinc of only 5 pm/year. With neither chloride nor carbonation, no corrosion occurred. Sergi et al. (1985) have electrochemically looked at zinc in solutions pH 9.0-14.0 in relation to the use of galvanized steel in concrete. 

Dreulle, N., and Dreulle, P. (1973). Die Beziehung zwischen Umweitverschmutzung und Korrosion von Zink und feuerverzinktem Stahl (Relationship between environmental pollution and the corrosion of zinc and galvanized steel). Metall, 27(6), 626-628 (in German). 

Gilbert, P. T. (1953). The effect of impurities in the metal on the rate of corrosion of zinc and galvanized coatings in the atmosphere. J. Appl. Chem. 3, 174—181. 

Ruckert, J., Neubauer, F., and Zietelmann, C. (1983). Influence of Bituminous Roofing Materials on the Corrosion of Zinc and Galvanized Steel Gutters and Pipes. Gemeinschaftsausschuss Verzinken eV, Dusseldorf, Report 91 (in German), 11 pp. 

Galvanic corrosion is an accelerated corrosion of a metal due to formation of a corrosion cell with a metal or non-metallic conductor that exhibits a higher corrosion potential. For example, if a water pipe made of zinc-coated steel (galvanized steel) is connected to a brass fixture and caution is not taken to electrically isolate the two metals, a corrosion cell is established (Figure 7.5). To simplify the situation, we have replaced, in Figure 7.5, the zinc-coated steel by pure zinc and the brass by copper. The cathodic reaction is the reduction of dissolved oxygen, which takes place on both metals. The corrosion cell formed between the zinc and the copper leads to an accelerated corrosion of zinc near the joint. 

The protection mechanism is similar to that of metallic zinc coatings zinc is less noble than steel and protects the substrate by forming a galvanic corrosion cell, in which zinc is the anode. This mechanism is particularly active in presence of defects in the coating that provide an electrolyte path to the substrate surface. In addition, the atmospheric corrosion of zinc yields voluminous solid corrosion products (oxides, carbonates, etc.) that are capable of blocking pores or small defects in the coating, thereby reinforcing its barrier effect. 

Goodwin, F. E The Effect of Environmental Acidification on Atmospheric Corrosion of Zinc," 16th International Galvanizing Conference, European General Galvanizers Association, London, 1991, pp. GHl/1-12. 

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