Aluminum alloy 7075-T6 is susceptible to localized corrosion when used in aerospace applications. Multi-layered coating systems were applied to aluminum alloys to protect corrosion. Deft primer 02GN084, that was approved to be used in US military aircrafts, is of interest because it has showed promising corrosion protection comparing to an effective chromate system. Praseodymium and CaSO4 were found to be in the Deft primer. Praseodymium was added as a primary corrosion inhibitor while CaSO4 served as an extender material. The role of each component was unknown. CaSO4 might play a role synergistically with praseodymium to inhibit corrosion. Leachability of those inhibitors compared to proven chromate system is also important aspect to fully understand inhibitive mechanism in the real world applications.

The mechanism of inhibition by CaSO4 was investigated. Sometimes, electrochemical analysis is not sensitive enough for detecting marginal inhibitors. Free corrosion exposure can reveal the marginal effect of corrosion inhibitors. Samples of AA7075-T6 were exposed to chloride solutions with varying pH via static immersion exposure with and without CaSO4 as an inhibitor for times ranging up to 30 days. Pitting corrosion damage was characterized by optical profilometery (OP) to find pit depth and pit area. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to characterize the extent of corrosion. Surface analysis was carried out by X-ray photoelectron spectroscopy (XPS). Results showed that sulfate suppressed pitting corrosion under acidic conditions. Calcium did not appear to contribute to inhibition in any of the evaluations carried out. EIS results showed an increase in polarization resistance in the presence of sulfate. Thermodynamic and XPS data suggest that the protective film formed in the presence of CaSO 4 is Al(OH)SO4.

The synergistic and cooperative interaction between of Pr and CaSO 4 are unknown. Optical profilometer was used to characterize pit morphology: maximum pit depth, pit volume, pit area, maximum area-equivalent diameter to determine the effect of inhibition. Polarization and electrochemical impedance spectroscopy were used to substantiate free corrosion exposure results. X-ray photoelectron spectroscopy was used to characterize the surface. Results showed that a single pit parameter cannot represent the inhibition effect. 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.

During field operations, primers store and release inhibitors to protect the substrate materials. Three commercial primers were being investigated in this study; 1) Akzo Nobel 10P20-13 High Solid Primer that contains strontium chromate (SrCrO4), 2) Deft 02GN084 (non-chromate), and 3) Hentzen 16708TEP (non-chromate). Pieces of primers were immersed into different solutions with varied Cl- concentration, temperature, time, and pH. The solutions were then filtered and characterized by inductively coupled plasmas mass spectroscopy (ICP-MS) to analyze elements from the solution. Results have shown that SrCrO4 primer release behavior follows power law kinetics. Solubility limit plays an important role in dictating release kinetics of the chromate primer. The effect of pH and temperature can alter this solubility limit. Chloride seems to promote the release rate because of greater ionic strength. Deft primer releases a large amount of Ca, but does not seem to be time dependent. Hentzen does not seem to release any soluble constituent to any extent.