Usually one assumes that the service life of zinc coatings is directly proportional to their thickness and does not depend on the method of deposition. However, such coatings obtained from various electrolytes differ in their macro-and microstructures, texture, density, glance, roughness, uniformity [1], and corrosion-protective properties. The existing methods of quick tests [2][3][4] are labor-and time-consuming and inaccurate. The method of polarization resistance (Rp) with the use of R5035 and R5126 devices is used for the determination of corrosiveness of media [5,6] and the corrosion resistance of metals, alloys [7], and galvanic coatings [8]. It is highly sensitive and allows one to study the corrosion process without destroying the coating.In the present work, with the use of an R5126 device, we investigate the corrosion behavior of zinc coatings obtained from electrolytes with various chemical composition, with the aim of determining the influence of the technological parameters of their deposition on corrosion resistance. Coatings with thickness from 3 to 24 ~tm were deposited onto cylindrical samples of steel 20 (diameter 6.0mm, length 31.0mm) from electrolytes (see Table 1).We also used electrodes cast from granulated, foundry pig, and electrolytic zinc. Prior to measurements, the electrodes were treated with thin emery paper, degreased, pickled, and washed. The thickness of the coatings and the current efficiency were calculated on the basis of the increase in the mass of the electrodes.To determine the rate of corrosion, we performed several series of experiments. The high sensitivity of the Rp method allows one to make the conditions of corrosion tests close to natural ones. For their realization, we prepared a solution with the following chemical composition (g/liter): 30 Na2SO4, 0.1 K2Cr207, 1.0 A12 (SO4)3" 18H20. The value of pH was measured to be 4.47, and the duration of experiments was in the limits 2.5-3 h. During the experiments, we recorded the time behavior of Rp Note that, in most cases, the weight loss was negligibly small and practically equal to the error of a VLR-200 analytical balance. For this reason, we did not use this method.The corrosion current was calculated on the basis of data on polarization resistance [9]:where K is the proportionality factor and ba and bk are the Tafel constants.The polarization measurements were performed with a PI-50-1.1 potentiostat in the potentiostatic mode with scanning of the potential (0.1 mV/sec) in the range of polarization -150mV < tps t < + 100 mV. We used samples with zinc coatings as working electrodes and a silver chloride electrode in a saturated solution of KC1 as the reference electrode.The corrosion of active electronegative metal Zn in neutral water solutions proceeds with the formation of films formed by the products of corrosion on anodic zones [10] which play the role of protective layers. When the film completely covers the surface of zinc, the rate of corrosion decreases considerably. The film which is formed in water has the com...
