Analysis of different approaches to the study of microelectrochemical heterogeneity of materials
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ANALYSIS OF DIFFERENT APPROACHES TO THE STUDY OF MICROELECTROCHEMICAL HETEROGENEITY OF MATERIALS M. S. Khoma, M. R. Chuchman, H. M. Oliinyk, and E. I. Lychkovs’kyi
UDC 620.193.4
We investigated systems of steel 20 with nickel and zinc coating in a 3% NaCl solution, dis–3
–6
tilled water, and a 45 ⋅ 10 % H2 SO4 + 0.14 H2 O2 + 5 ⋅ 10 % K2 Cr2 O7 solution using capillary probes filled with these environments. After their use, errors in measurements in the volume of the electrolyte are absent. The low conductance of the electrolyte and the dissolution of the metal in the active state are necessary conditions for adequate measurements of local electrode potentials. It is established that the accuracy of local microelectrochemical measurements in an electrolyte drop is higher than that in an electrolyte film.
An important factor that determines corrosion rates of alloys is their structural inhomogeneity. It determines the operating efficiency of galvanic microcells on their surface, which is a prerequisite of local corrosion [1]. For the assessment of electrochemical properties of materials on the microlevel, special methods are used. Among these, the most extensively used methods are based on using a microelectrode in the form of a capillary probe and on the determination of electrochemical properties in a electrolyte film of an electrolyte drop [2 – 4]. We assessed the informativeness of electrochemical investigations with a capillary probe and the influences of the compositions and volume of electrolytes on results of measurements of the local electrode potential. Materials and Testing Technique We investigated steel 20 with a nickel (Ni-PK) and a zinc (Zn-PK) electrodeposited coating deposited from sulfate–chloride electrolyte ( pH 5.0 – 5.2, t = 40°C, i = 200 A / m2 ) and sulfate electrolyte ( pH 4.6 – 5.2, t = 25°C, i = 300 A / m2 ). The investigations were performed in aqueous solutions 45 ⋅ 10– 3 % H2 SO4 + 0.14% H2 O2 + 5 ⋅ 10– 6 % K2 Cr2 O7 (MEKh) [5] with a conductance κ = 0.38 Ω– 1 m– 1, a 3% solution of NaCl ( κ = 4.7 Ω– 1 m– 1 ) and distilled water ( κ = 9 ⋅ 10– 5 Ω– 1 m– 1 ) . The microelectrochemical properties of materials were assessed using the technique of a microelectrode with a capillary probe. Capillaries with an internal diameter of 1 to 2 μm were made of glass tubes in a special automatic unit. They were filled with a working solution of electrolyte and connected with an agar bridge with a reference silver–silver chloride electrode. A capillary together with a microscope were moved over the surface of a microsection with an accuracy of 1 μm with a two-coordinate table. The electrochemical characteristics were measured with a PI-5-1.1 potentiostat in a potentiodynamic regime. The rate of change of potential was 1 mV / sec. Compromise potentials of the systems steel–nickel coating and steel–zinc coating were measured in the volume of electrolyte ( V = 50 ml ) locating specimens connected with a conductor in a cell at a distance of Karpenko Physicomechanical Institute, Ukrainian Academy of Scie
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