CORROSION CHARACTERISTICS OF TIN-COATED CARBON-STEELS IN ACIDS AND CHLORIDE SOLUTIONS

Electrochemical polarization and corrosion properties of carbon steels having TiN films 0.1-10-mu-m thick formed by reactive sputtering have been examined in 1 kmol.m-3 H2SO4, 1 kmol.m-3 HCl and 3 mass%NaCl, comparing with those of TiN prepared on Ti plates by nitriding. It was found that the corrosion resistance of the TiN-coated steels in short-range immersion in the solutions was improved with increasing thickness of TiN film, d(TiN): In 1 kmol.m-3 H2SO4, an active dissolution current density at 0.1 V (vs. Ag/AgCl-3.33 kmol.m-3 KCl) decreased by a factor of 1000 as d(TiN) increased from 0.1 to 8-mu-m, and the corrosion rate measured after immersion for 84.6 ks decreased correspondingly. In 3 mass%NaCl, a sharp increase in current due to pitting was observed on the anodic polarization curves of the steels having d(TiN) < 6-mu-m. No such increase, however, was observed on the curves of the steel having d(TinN > 8-mu-m. The improvement of corrosion resistance with increasing d(TiN) can be attributed to the decrease in the amount of open pin-holes in TiN films. After prolonged immersion, the steels covered with thicker TiN films also suffered from pitting corrosion because of the penetration of the solutions through microdefects in the films. From the measurement of cathodic polarization curves of TiN and galvanic current densities of carbon steels coupled to TiN, it was found that in an aerated NaCl solution the TiN films on carbon steels acted as cathodic sites and thus galvanic corrosion occurred on the steel substrates. In a deaerated H2SO4 solution, however, no galvanic corrosion was presumed to occur because the hydrogen overvoltage of TiN was fairly large in this solution.