HIGH-SPEED ANODIC DISSOLUTION OF CHROMIUM-NICKEL STEEL IN A NITRATE SOLUTION WITH THERMOKINETIC INSTABILITY OF THE OXIDE FILM

Abstract

It is shown that high-speed anodic dissolution of Kh18Ni10 chromium-nickel steel (Cr18Ni10) in a concentrated nitrate solution occurs through an anode oxide film (AOP), the for-mation of which is carried out as a result of charge transfer, and the dissolution is chemically. The condition of constant film thickness is achieved due to the equality of the rates of these two pro-cesses (electrochemical formation of the film and its chemical dissolution) and is realized under pulsed conditions until the boiling point of the electrolyte is reached at the film-solution interface. When thermokinetic instability of the anodic oxide film occurs at the film-solution interface at surface temperatures exceeding the boiling point of the electrolyte, the interaction of the dissolving surface of the alloy free of the film with the solution (electrolyte) is observed. The dissolution rate in this case may exceed the Faraday rate (anomalous anodic dissolution) as a result of the destruc-tion of the film, for the formation of which the leaked charge was spent. The presence of a barrier film at the metal-film interface is the reason that the change in the surface temperature and disso-lution rate is achieved only by changing the heat transfer conditions of the metal surface.The porous part of the film in contact with the solution is a “heat seal” (limits the heat removal towards the electrolyte), as a result of which a change in the electrolyte flow rate does not affect the surface temperature. Based on the results of calculating heat fluxes, the influence of the temperature of the metal surface in contact with air (the outer part of the surface of the dissolving metal) on the heat transfer coefficient during natural convection and rotation at a speed of 1000 rpm of the elec-trode was estimated, as well as the ratio of the corresponding heat fluxes. An increase in convective heat removal during rotation leads to a decrease in temperature in the anode treatment zone and, as a consequence, to a decrease in the dissolution rate.