Cooperative and synergistic corrosion inhibition of AA 7075-T6 by praseodymium and CaSO4

In a primer coating system used in aerospace applications to protect aluminum alloy substrate, praseodymium is added as corrosion inhibitors while CaSO4 is primarily added as filler materials. The interaction of Pr and CaSO4 is unknown. The goal of this study is to characterize any cooperative or synergistic inhibition between these two. Cooperative inhibition can be defined when one inhibitor enhances inhibiting effect of the other that already has inhibiting ability. Synergistic inhibition can be defined when one inhibitor activates the inhibiting effect of the other that originally does not inhibit. Optical profilometry, electrochemical techniques and X-ray photoelectron spectroscopy were used to characterize corrosion results. The results showed that several pit parameters will affirm the inhibition effect. Electrochemical results cannot always detect modest corrosion inhibitors. Cooperative inhibition was detected in pH 5 while synergistic inhibition was observed in pH 8. Synergistic inhibition occurs because SO42- helps with gelation of Pr to passivate the surface.