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Influence of parameters of the pulsed regime on the formation of nanocrystalline zinc coatings

Keywords:

N.V. Tarasova – Ph.D. (Eng.), Associate Professor, Lipetsk State Technical University


In the paper, the dependence of the zinc crystallite size on the pulse duration (tpulse) and the pause between pulses (tpause), the current density in the pulse (ipulse), and the substrate structure was investigated. The study of SEM-images of steel 08Ю after pulsed cathodic po-larization has shown that a reduction in the grain size is facilitated by a decrease in the pulse duration and an increase in the current density in the pulse, a change in the pause between pulses affects the continuity of the coating and has no clearly pronounced effect on the grain size. It has been experimentally established that the largest grinding of zinc coating crystallites under conditions of pulsed elec-trolysis occurs at tpulse = 1 s, tpause = 1 s and ipulse = 0.07 A/cm2 and reaches a value of d ~ 600 nm. Analysis of the state of the coating deposited on steel 08Ю, 45 and 60, characterized by different structural states, showed that the presence of ferrite/cementite boundaries in the structure of steels 45 and 60 leads to a qualitative change in the appearance of the coating. In this case, the formation of the coat-ing occurs mainly along the interphase boundaries of ferrite/cementite, that is, near the pearlite structural component, which has a more positive electrochemical potential than the ferrite component. Thus, an increase in the electrochemical heterogeneity of the substrate leads to the formation of additional centers of nucleation of the deposited coating. A study of the electrochemical behavior of zinc coat-ings revealed the presence of current maximum in current potentials in the voltage range from -0.62 to -0.40 V, the position of which depends both on the grain size and the thickness of the coating formed, which indicates the possible destruction of the zinc coating in the course of anodic polarization. It should be noted more intense corrosion damage to the coating with a grain size of 4 μm compared with the nanostructured state. It should be noted more intense corrosion damage to the coating with a grain size of 4 μm compared with the nanostructured state. Based on the results of calculations of corrosion rate indicators, a reduction in the grain size to 600 nm leads to a decrease in the depth index of the corrosion rate from 0.15 to 0.07 mm/year. Comparative tests of adhesion of a zinc coating with a grain size of 600 nm to 4 μm and a thickness of 5 μm to a steel substrate showed that in the initial state the zinc coating has good adhe-sion. The results of the evaluation of the adhesion value after the samples were found in distilled water for 72 hours showed an increase in the adhesion of the coating with crystallite size of 600 nm by a factor of 4. Improving the corrosion resistance and adhesion of the zinc coating with a decrease in the crystallite size is largely due to interactions at the metal-coating interface and, above all, by the ratio of the rates of nucleation and grain growth. Measurement of the width of the metal-zinc interface showed that for a sample with d = 2660 nm, the interface width is 1000 nm, for a sample with d = 600 nm – 400 nm. The results are due to an increase in the rate of nucleation and contact of the coating with the substrate as the crystallite size decreases, which is accompanied by an improvement in the performance properties of the material.

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