Effect of the Structure of Passive Oxide Films and Surface Temperature on the Rate of Anodic Dissolution of Chromium–Nickel and Titanium Alloys in Electrolytes for Electrochemical Machining: Part 1. Anodic Dissolution of Chromium–Nickel Steel in a Nitrate Solution

Abstract

The anodic dissolution of type Kh18N10 (Cr18Ni10) chromium–nickel steel was performed in a nitrate solution (conductivity of 0.15 S/cm) under pulsed current conditions using pulse durations of 20–100 µs, current densities of 0.01–100 A/cm², and relative pulse durations of 10 to 1 (duty cycle from 10 to 100% (direct current), respectively). Different hydrodynamic conditions were implemented, and the surface temperature was measured. The results obtained are in line with the hypothesis that the process is mediated by the formation of a semiconducting anodic oxide film with point defects that can exhibit different types of conduction. The film is described within point defect model II, and the rate of its electrochemical formation is balanced under steady-state conditions by the rate of its chemical dissolution, which is why the mass decrease per unit charge reaches a limiting value of 0.16–0.18 mg/C (under the pulsed conditions), which corresponds to a current efficiency close to 100% (assuming the highest oxidation state for alloying components of the steel in solution). In going from pulsed current to direct current conditions, the thermokinetic instability of the film is observed, i.e., it forms and then undergoes breakdown due to thermal explosion. Under such circumstances, the current yield of anodic dissolution may not only reach 100%, assuming the lowest degree of oxidation of the alloying components (thermal activation), but exceeds this value as a result of chemical interaction between the film-free surface and the electrolyte.