Effect of electrolyte depletion on the characteristics of the anodic plasma electrolytic nitriding of a VT22 titanium alloy

Abstract

Changes in the saturation capacity of an aqueous electrolyte containing ammonium chloride and ammonia during the nitriding of a VT22 two-phase titanium alloy are studied. Changes in the electrolyte composition are associated with the evaporation of water, ammonia, and ammonium chloride decomposition products and the emission of ions from the boiling solution into the envelope under the action of an electric field. It is found that the thickness of the layer exhibiting high hardness (up to 460 HV), which reaches 20 μm in the case of the treatment in a fresh electrolyte, decreases by 40% in a solution used for 5 h. It is shown that the electrolyte lifetime can be increased due to the vapors and gases from the envelope returning to the electrolyte. The use of an electrolytic cell with the possibility of the vapors and gases leaving the electrolyte during the treatment returning back to it makes it possible to maintain the saturation capacity of the electrolyte within 5 h of operation and ensuring the formation of approximately the same layer as the one formed in a fresh electrolyte. It is found that the electric conductivity of the solution and the gas–vapor envelope increases with electrolyte depletion due to the predominant loss of the dielectric components.