Chromate Free Corrosion Inhibition Of Aluminum Alloys

Furthermore, synthetic hydrotalcite anion exchange clay pigments were synthesized with vanadates and other possible inhibitor anions. Hydrotalcites allow the use of inhibitors that are too soluble for direct use in organic coatings without leading to coating blistering. A number of hydrotalcite pigments were synthesized and compared to a SrCrO4 standard using electrochemical impedance spectroscopy and salt spray exposure of scribed organically coated panels. Typically, vanadate pigmented PVB coatings were observed to have total impedance within an order of magnitude of SrCrO4 (2 x 107 ohms·cm2). Further, all vanadate coatings were observed to provide some scribe protection during 750 hours of salt spray exposure, however, these coatings also had a tendency to blister. Release from vanadate hydrotalcites was characterized using neutron activation analysis. Interestingly, vanadate hydrotalcite pigments that released relatively small total concentrations of vanadium resulted in the best performance. Low concentrations of vanadium may promote the formation of tetrahedrally coordinated species which were shown to act as inhibitors earlier in this study.

Abstract: The pursuit to find a chromate-alternative has led to the development of several chromate-free aerospace primers and coating systems that offer good protection. However, fundamental understanding of the functionality of the chromate-free pigments that are embedded within these coating systems is lacking. The objective of this study was to understand the fundamental mechanism of corrosion inhibition of aluminum alloy 2024-T3 by molybdate (MoO42-), silicate (SiO32-), and praseodymium (Pr3+) with the goal of developing the kind of understanding that was accomplished for chromate. Furthermore, since most inhibiting conversion coatings and pigments act by releasing soluble species into the local environment, it was of interest to understand the mechanism of inhibition in aqueous 0.1 M NaCl solution.

Aerospace aluminum alloys such as Al alloy 2024-T3 and 7075-T6 are subject to localized corrosion due the existence of intermetallics containing Cu, Mg or Zn. Current protection measurement employs substantial use of chromate and high VOC organics, both of which are identified as environment and health hazards. The approach of this study is to utilize a combination of organofunctional silanes and a compatible inhibitor integrated into high-performance waterborne resins. First, an extensive pigment screening has been done to find replacements for chromates using the testing methodology for fast corrosion inhibition evaluation and pigment. Zinc phosphate and calcium zinc phosphomolybdate were found to have the best overall performance on Al alloys. Some new corrosion inhibitors were synthesized by chemical methods or modified by plasma polymerization for use in the coatings. Low-VOC, chromate-free primers (superprimer) were developed using these pigments with silane and acrylic-epoxy resins. The developed superprimer demonstrated good corrosion inhibition on aluminum substrates. The functions of inhibitor and silane in the coating were investigated. Both silane and inhibitor are critical for the performance of the superprimer. Silane was found to improve the adhesion of the coating to the substrate and also facilitate corrosion prevention. Addition of zinc phosphate to the coating improved the resistance of a scratched area against corrosion. The microstructure of the acrylic-epoxy superprimer coating was studied. SEM/EDAX revealed that the superprimer has a self-assembled stratified double-layer structure which accounts for the strong anti-corrosion performance of the zinc phosphate pigment. Zinc phosphate leaches out from the coating to actively protect the scratched area. The leaching of pigment was confirmed in the ICP-MS analysis and the leaching rate was measured. Coating-metal interface and the scribe of coated panels subjected to corrosion test was studied. ToF-SIMS studies confirmed the presence of silane at the interface and the hydrolysis of the silane. The abundant presence of silane was believed to improve the adhesion and also facilitate the corrosion prevention. The protection mechanism of the acrylic-epoxy superprimer was proposed. The self-assembled double-layer structure of the acrylic-epoxy superprimer consist of a less-penetrable hydrophobic layer (epoxy-dominated) on the top and a hydrophilic layer (acrylic-dominated) accommodating the inhibitors underneath. This unique structure of the acrylic-epoxy accounts for the good protection of the coating. Furthermore, the inhibition mechanism of zinc phosphate was explored and compared to those which have been reported. Based on the protection mechanism of the superprimer, electrodeposition was explored in order to achieve a more organized coating with a better engineered metal/coating interface. The electrodeposited coatings were found to have higher barrier property and anticorrosion performance.

Inorganic polycrystalline hydrotalcite, Li2[Al2(OH)6]2·CO3·3H2O, coatings can be formed on aluminum and aluminum alloys by exposure to alkaline lithium carbonate solutions. This process is conducted using methods similar to traditional chromate conversion coating procedures, but does not use or produce toxic chemicals. The coating provides anodic protection and delays the onset of pitting during anodic polarization. Cathodic reactions are also inhibited which may also contribute to corrosion protection. Recent studies have shown that corrosion resistance can be increased by sealing hydrotalcite coated surfaces to transition metal salt solutions including Ce(NO3)3, KMnO4 and Na2MoO4. Results from these studies are also reported.

"These proceedings represent contributions to the Symposium on the Corrosion and Protection of Light Metals, held at the 204th Meeting of the Electrochemical Society, October 12th through 17th, 2003, in Orlando, Florida."--P. iii.

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 CaSO4 is Al(OH)SO4.