Effect of the Passive Oxide Films Structure and Surface Temperature on the Rate of Anodic Dissolution of Chromium-Nickel and Titanium Alloys in Electrolytes for Their Electrochemical Machining. Part 2. Anodic Dissolution of Titanium Alloys in Nitrate and Chloride Solutions

Abstract

An experimental study of the anodic dissolution of titanium and its alloys, including pulsed dissolution (up to 100 A/cm2) under control of hydrodynamic conditions and surface temperature in nitrate and chloride solutions, showed that the process is carried out through the electrochemical formation of anodic oxide films (AOF) which can be chemically dissolved. An AOF has a bilayered structure, i.e. two barrier films on the interface with a metal and a solution. Its model is PDM-III (Point Defect Model). At certain conditions, it is possible to achieve a steady state in which the film growth rate is compensated by the rate of its chemical dissolution (observed at pulsed conditions). In this case, over 100% current efficiency based on the ionization of titanium in the oxidation state is achieved. During the conducted experiments, when treated with the direct current, the rate of the electrochemical formation of an AOF exceeded the rate of its chemical dissolution, which led to a decrease in to current efficiency which did not exceed 75%. An increase of the dissolution rate takes place with an increase in the electrolyte flow rate. Upon reaching the thermokinetic instability (TKI) of the AOF (thermal explosion due to a feedback rate of the electrochemical reaction-surface tem-perature-rate of an electrochemical reaction), the interaction of electrolyte components with the surface free of the film takes place, and, as a result, an “anomalous” anodic dissolution is observed with the current efficiency greater than 100%. Regardless the nature of the electrolyte, TKI is achieved at a current density of 1 A/cm2. It is shown that in nitrate solutions with certain parameters of pulsed processing (duty factor 2, dc 50%), the dissolution rate, and, in the case of electrochemical machining, the feed rate of a cathode-tool, can be more than twice higher than the rate of machining with the direct current of the same density.