Aluminum surface treatment using three different plasma-assisted dry chemical processes

Aluminum plate surfaces having hydrophilic and corrosion-resistant properties are often required for various mechanical parts or systems. The conventional process for treating aluminum surfaces employs a series of wet processes: grease or organic compounds removal followed by drying, chromate process followed by drying, and finally chemical treatment to achieve hydrophilic property. However, the use of chromium (Cr⁺⁶) in the chromate process creates an environmental problem. A simple and economic dry process was investigated to replace the conventional aluminum surface treatment, i.e., nonthermal plasma followed by the chemical process. Among several nonthermal plasma processes, two types of plasma jets (60-Hz ac plasma and RF plasma) and the pulsed-corona plasma were investigated. The adhesive characteristics between the aluminum surface and the corrosion-resistant chemical were evaluated by the accelerated corrosion test, i.e., copper-accelerated acetic acid spray (CASS) test. RF plasma with more than 30 s of exposure time followed by the epoxy-type corrosion-resistant coating showed a superior performance over the conventional chromate wet process. Pulsed corona process was nearly equal to the conventional process. Therefore, the conventional wet chemical process for aluminum treatment can be replaced by the newly developed plasma-assisted chemical process.