Measuring corrosion rate and protector effectiveness of advanced multilayer metallic materials by newly developed methods

Abstract

The paper estimates corrosion resistance of new multilayer metallic materials with internal protector against pitting. Using an electron microscope method, the mechanism of the layers' corrosive destruction has been experimentally substantiated. The authors have suggested chemical and electrochemical methods of accelerated corrosion tests allowing for determining the corrosion destruction rate. The electrochemical method reveals the limiting stage of the process and allows calculating the mass corrosion index and substantiating the choice of protector for the specific corrosive medium. The chemical method allows for quantitative assessment of the internal protector's effectiveness and for defining the multilayer/monometallic material corrosion resistance ratio.

Keywords: Electrochemistry; Materials science.

Fig. 1

Schematic view of four-layer (a) and three-layer (b) metallic materials with internal protector: 1 – layer with high electrochemical potential, exposed mainly to pitting corrosion; 2 – layer with low electrochemical potential (“sacrificial layer” – internal protector); 3 – layer with high electrochemical potential; 4 – layer providing mechanical strength.

Fig. 2

Cross section of natural pitting of a three-layer sample under conditions when the corrosion does not reach the third layer. 1 – external layer, 2 – internal protector, 3 – interlayer boundary, 4 – natural pitting, 5 – lens.

Fig. 3

Mapped section of the sample under conditions when the corrosion reaches the third layer. 1 – external layer, 2 – internal protector, 3 – third layer, 4 and 5 – interlayer boundaries, 6 – lens.

Fig. 4

Section of the first and second layers, for which the distribution of the corrosion products' elemental composition at the spots was defined.

Fig. 5

Schematic view of the assembly for electrochemical study of the corrosion elements and plotting of the corrosion diagrams: 1 – electrodes made of multilayer composite's components, 2 – reference electrodes, 3 – salt bridge, 4 – vessels filled with corrosive medium, 5 – tumbler, 6 – switch, 7, 9 – high resistance millivoltmeters, 8 – calibrated 1Ω resistor, 10 – bank of resistors, 11 – mechanical agitator, 12 – thermometer, 13 – thermostat.

Fig. 6

Corrosion diagram for the galvanic pair 08X18H10T (curve 2) – steel 10 (curve 1) in the iron (III) chloride solution.

Fig. 7

Schematic view (a) and image (b) of a sample for testing: 1 – top plate, 2 – middle plate, 3 – bottom plate, 4 – separator, 5 – glass funnel, 6 – bracket, 7 – bolt, 8 – nut.

Fig. 8

Mass corrosion index of the external (a), middle (b) and bottom (c) layer of the metallic multilayer material at different diameters (d, mm) of an artificial lens, diameter of an artificial pitting of 1 mm and exposure time of 720 h (curve 1) and 2,208 h (curve 2).