Some Electrochemical Aspects of Corrosion prevention by paints, metallic coatings and inhibition

Abstract

An artificial pit technique was developed to study the galvanic relationship between the substrate and coating metals for a number of systems. The systems studied are steel plated with nickel and chromium, steel coated with sprayed aluminium and steel given dip coating with aluminium and tin-M.S. The study consisted in: a) Measurement of galvanic current and potential flowing between the substrate and the coating metals by using zero resistance ammeter method and VTVM. b) Potential decay when the current is cut off These were done in several corrosive electrolytes. The most significant results obtained are given below: a) In the Fe-Ni-Cr system the presence of nickel undercoat is necessary to eliminate the large galvanic current that flow between iron and chromium unlike the small current flow between nickel and chromium. b) At pores in zinc-aluminium coated steel, steel behaves like hydrogen electrode as long as the pore diameter is small. This has been explained due to the well-known propensity of steels to be charged with hydrogen. c) Sprayed aluminium gives more negative potential than steel in organic acids and can be considered as substitute for tin coating. The results are published. A Comprehensive investigation into the mechanism of protection by primers so as to get insight into the scientific basis for the formulation of new and better primers. The whole range of chromate primers, red lead red oxide primers with different ratios, primers using CNSL and other resins were all taken up for study. The investigation consisted in: a) Analysis of aqueous extracts of pigment vehicle and paints. b) Potential and polarization studies with bare metal in aqueous extract and with painted metal in salt solution. c) Electrical resistance measurements with painted metal. d) Accelerated corrosion rates and water absorption test with painted metal. These investigations lead to the following new results: a) The solubility of pigment in aqueous medium differs with nature of the vehicle employed. b) Extenders can modify the amount of pigment extracted in aqueous solutions. c) The inhibitive character of primers can be attributed to different reasons such as solubility of the pigment, formation of inhibitive soaps, formation of surface complex between the vehicle and metal surface