Investigation of the effect of chrome-free pretreatment prior to cataphoresis coating on the corrosion performance of AA7075 and AA5754 aluminium alloys

In the traditional cathophresis process, paint adhesion to metal substrates is achieved by phosphating. However, it is not possible to use a phosphating process when aluminium substrates are used. The main reason for this is that the surface of the aluminium material used as a substrate deteriorates when kept in the phosphate bath for a prolonged period.Silicon in aluminium alloys is the main reason phosphate cannot stick to the surface. The amount of silicon changes depending on the type of aluminium alloy, and higher silicon levels make phosphating less effective. AA5754 and AA7075 aluminium alloys, widely used in the automotive industry, were used in this study. In order to improve adhesion and corrosion resistance in the cost-effective and widely used cathophresis process in the automotive sector, a chrome-free pretreatment using the commercial product EcoAL 20A solution was applied instead of the phosphating pretreatment. In the experimental work, the etching time (5 minutes) and temperature (25°C) were kept constant. Chrome-free pretreatments were applied to these two types of aluminium at different concentrations of EcoAL 20A (0.75%, 1% and 1.25%) and immersion times (45 seconds, 60 seconds and 75 seconds) prior to the cathophresis. The coated samples were then subjected to tests such as dry adhesion, water resistance, humidity resistance, cycle testing, CASS testing and stone chip testing. Based on the results of these tests, the optimum chrome-free concentration was determined to be 1.25% and the optimum pre-treatment time was determined to be 75 seconds for both aluminium series. In this study, a new process was developed by incorporating a chrome free step in place of the phosphating process.