Optimization of coating process parameters and surface characterization for vanadium-based conversion coating on 2024 aluminum alloy

摘要

This paper aims at designing a simple corrosion resistant vanadium based conversion coating for high strength 2024 aluminum alloy. First, to achieve highest anti-corrosion performance of vanadium conversion coating, process parameters such as vanadium salt concentration (5, 7.5, 10 g/l), solution pH (3, 3.5, 4, 5, 6, 7), solution temperature (15, 25, 35, 45 degrees C) and immersion time (10, 15, 20, 25, 30 min) using different electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and direct current polarization (DC) in 3.5 wt. % sodium chloride solution were optimized. Then, the surface characterization of optimized vanadium conversion coating was performed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), water contact angle measurement and atomic force microscopy (AFM). Results showed that AA2024 substrate which was treated in chemical conversion solution containing 10 g/l of sodium vanadate salt, pH of 4, room temperature and immersion time of 25 min had the highest anti-corrosion performance. Based on the results of electrochemical tests, corrosion current density and polarization resistance for untreated substrate were 2.28 mu A/cm(2) and 26k Omega. cm(2), respectively. These values for optimized vanadium treated substrate were 0.13 mu A/cm(2) and 147k Omega. cm(2), respectively, which displayed significant improvement in anti-corrosion resistance of bare substrate. SEM images showed the presence of a fine and compact conversion coating on the surface of vanadium treated substrate. Results of EDS demonstrated the presence of approximately 3 wt.% of vanadium element on the surface of vanadium conversion coating. By coating process under optimum conditions water contact angle on the surface of vanadium treated substrate was increased compared to untreated substrate. According to AFM images, by applying vanadium conversion coating surface roughness of the treated substrate was reduced.