Evaluating the condition of paint and lacquer coatings by a method of measuring the impedance in cyclic tests of samples

Abstract

.Tthe samples were prepared from panels of AMg61 aluminum alloy, a basic ship hull material, and painted by different methods. The edges of the samples (gauge length 4  20  90 ram) were rounded for the purpose of reducing the spread in experimental data since as a rule failure of a paint or lacquer coating starts at sharp edges. Therefore the cross section of the samples in the gauge length tended to approach an elliptical shape. in measuring the impedance the samples were placed in a special cylindrical cell of VT-I titanium alloy filled with a 3% sodium chloride solution. Its housing simultaneously played the role of the auxiliar electrode. The sample was fastened in the end walls of the cell with the use of rubber seals. To decrease the housing of the latter in the area of the water line was coated on the exterior with an oxide film created by thermal oxidation and possessing a hlghelectrical resistance. The impedance measurements were made with an R-568 ac bridge with a frequency of 510 Hz and an ac voltage of not more than 8 mV at room temperature. The ac voltage was provided by a G3-33 pulse generator and measured with a V3-13 voltmeter. The cell with the sample was connected to the bridge with clamps possessing the minimum induction. During development of the method empirical relationships were obtained of the change in resistance and capacitance of the electrochemical cell with the sample to the area of damage of the paint or lacquer coating. To obtain them successive increasing in area damage of a paint coating applied to the previously etched surface of a sample from a panel of AMg61 alloy with one layer of VL02 primer and four layers of KhS720S paint was accomplished. After each new damage inspected for area the resistance and capacitance of the cell with the sample was measured.