Study of the Influence of the Temperature of the Electrolyte During Pulse Anodization on the Properties of the Surface Layers of Technical Aluminum
Authors:   Karpenko Physico-Mechanical Institute of NAS of Ukraine, Hvozdetskii V.M. , Markovych S.I. ,   Central Ukrainian National Technical University, Zadorozhna Kh.R. ,   Karpenko Physico-Mechanical Institute of NAS of Ukraine, Student M.M. ,   Karpenko Physico-Mechanical Institute of NAS of Ukraine
Published: December 2023

luminum alloys are characterized by low abrasive wear resistance, which significantly restricts their wide use in technological environments, especially if they contain abrasive particles. The method of pulse anodizing, which consists in periodically changing the current density, allows to improve the hardness and abrasive wear resistance. However, the influence of temperature on these processes has not been sufficiently studied. The process of pulsed hard anodizing was carried out in a 20% aqueous H2SO4 solution at a current density of 1.4 A/dm2 with a frequency of 100 Hz and a sparability of 75%. The temperature of the electrolyte during the formation of layers was maintained at -5°±1С, 0°С±1, +5°С±1, +10°С±1. The duration of synthesis was 60 minutes. At low temperatures of less than -5С due to the low speed of electrochemical processes, oxide layers with reduced microhardness and thickness are synthesized. At the same time, the phase analysis recorded the synthesis of an oxide layer based on aluminum with the content of two phases: the synthesis of the anodized layer begins with the formation of hydrated aluminum oxide Al2O3•H2O. As the anodizing temperature increases, the rate of electrochemical processes increases, which leads to an increase in the thickness of the anodized layer and its microhardness. At temperatures of 0С and -5С, the anodized layer contains only one phase - Al2O3 H2O, (boehmite) with one water molecule. At anodizing temperatures of +5С and -8С, the anodized layer is formed again in the form of two phases - Al2O3 3H2O (gibbsite) and Al2O3H2O (boehmite) and at a temperature of +10С only Al2O3 3H2O (gibbsite). At a temperature lower than –10°C, the electrolyte turns into a gel-like substance in which electrochemical reactions practically stop, so the synthesis of a pulsed solid anodized layer at a temperature of –5°C was started. However, as a result of the synthesis at T = –5 С and then even higher (up to 0С), the average thickness of the pulsed hard anodized layer increased from 83 μm to 110 μm. With further increase in the temperature of the electrolyte, the thickness decreased and at an electrolyte temperature of +10 С it was 80 μm. Such a change in the thickness of the anodized layer depending on the synthesis temperature was explained by two opposing processes that occur during anodization. As the temperature of the electrolyte increases, the process of synthesis of the layer intensifies first of all, and therefore the thickness of the obtained layer should constantly increase with the increase of the synthesis temperature. However, an increase in the temperature of the electrolyte also intensifies the surface dissolution of the anodized layer, which is aimed at reducing the thickness of the anodized layer. And when the rate of dissolution of the anodized layer begins to exceed the rate of its synthesis, its thickness begins to decrease. Conclusions: 1) The more water and sulfur molecules in the anodized layer, the lower its microhardness and abrasive wear resistance. 2) The minimum wear of the anodized layer, which means the highest wear resistance, was recorded for layers synthesized at an anodization temperature of -8ºС, and the maximum wear and lowest wear resistance at an anodization temperature of -5ºС. 3) High wear resistance of anodized layers synthesized at electrolyte temperatures from -8 to +10ºС under conditions of friction without lubrication is caused by the presence of crystalline water in the anodized layer.