Pitting Resistance of Diffusion-Alloyed Carbon Steels

Abstract

Pitting corrosion is a dangerous type of corrosion fracture of metals observed, as a rule, in solutions containing oxidizers (e.g, oxygen) and, at the same time, activating chlorine ions and ions of other halogens. In this case, bounded regions of the metal suffer fracture. However, since the surface damaged by pitting plays the role of an anode, the corrosion process runs at a high rate due to the contact with the remaining part of the surface playing the role of a practically nonpolarized cathode. This leads to the formation of deep point defects and pits. The principal electrochemical condition required for the development of pitting is the shift of potential to a value more positive than a certain critical value called the potential of pitting formation EPT . According to the generalized experimental data, pitting corrosion is typical of the metals and alloys readily susceptible to passivation, i.e., of iron, stainless steels, aluminum and its alloys, titanium, etc. [1–5]. The resistance of metals and alloys to this type of corrosion is determined, on one hand, by the nature, composition, structure, and state of the surface and, on the other hand, by the solution, its composition (i.e., aggressive ions), and temperature. In the present work, to study the pitting resistance of carbide layers obtained on 35, 45, and U7A steels after treatment according to the technology of diffusion carbide surface alloying (DCSA), the specimens were subjected to electrochemical and long-term corrosion tests in chloride-containing solutions.