William Fahrenholtz

Effect of Phase on the Electrochemical and Morphological Properties of Praseodymium-Based Coatings

Effect of Phase on the Electrochemical and Morphological Properties of Praseodymium-based Coatings B.L. Treu, W.R. Pinc, W.G. Fahrenholtz, M.J. O’Keefe, E.L. Morris, and R.A. Albers Missouri University of Science and Technology 101 Straumanis Hall Materials Research Center 401 W. 16 Street Rolla MO 65409 Deft Inc. 7451 Von Karman Ave. Irvine, CA 92614 Currently, chromate conversion coatings (CrCCs) and chromated primers are used to provide corrosion protection for high strength aluminum alloys (i.e. Al 2024-T3 and Al 7075-T6) used in aircraft construction. Although chromates are highly effective corrosion inhibitors, these compounds are toxic and carcinogenic [1]. A variety of non-chromate corrosion inhibitors have been developed as environmentally friendly alternatives to chromates [2]. Among the possible alternatives to chromates, rare-earth (RE) compounds have proven to be effective corrosion inhibitors when the proper RE phase is incorporated into the appropriate type of coating. Previous research showed that praseodymium oxide (PrOx )compounds are effective corrosion inhibitors in epoxy-polyamide primers when deposited onto high strength aluminum alloys with CrCCs [3]. These primers have been commercialized by Deft, Inc. and have been qualified to DoD requirements [4]. The mechanism by which PrOx compounds inhibit corrosion of high strength aluminum alloys when introduced into an epoxy-polyamide primer is unknown. However, it is understood that the phase of the rare-earth present in the coating play an important role in the ability of the coating to be able to afford optimal corrosion protection. Therefore, electrochemical evaluations, scanning electrochemical microscopy with electron diffraction spectroscopy (SEM-EDS), and x-ray diffraction (XRD) characterization of epoxy-based primers containing either hexagonal Pr2O3 or cubic Pr6O11 inhibitors on chromate conversion coated Al 2024-T3 will be conducted. Also, the response of coated panels prior to and following ASTM B117 salt spray exposure will be studied. Test panels were prepared by spraying a uniform layer of a PrOx containing primer onto aluminum alloy 2024-T3 panels that had been previously treated to form a chromate conversion coating. Electrochemical testing was conducted using precisely machined circular holes designed to simulate defects in the coatings that expose the underlying metal. These surfaces provided a more controlled environment for characterization than typical machine or hand scribes that are used for salt spray corrosion testing as described in the military specifications. Analysis of primers containing PrOx inhibitors showed that the Pr migrated to damaged areas of the coating and formed Pr-rich precipitates following salt spray exposure. The presence of Pr-rich precipitates was also confirmed by in the scribes of panels following salt spray exposure whereas none were found in scribes prior to salt spray testing. This analysis provided direct evidence of dissolution, transport, and re-precipitation of inhibitor species in response to a corrosive environment. SEM-EDS characterization has shown Pr-rich species precipitate from both Pr2O3 (Figure 1A) and Pr6O11 (Figure 1B) containing coatings in a dense or localized pattern and localized clusters are separated by ~ 50 μm or more in the scribe in Pr2O3 containing coatings. When PrOx is introduced into the coating as either Pr2O3 or Pr6O11, a mixed Pr-hydroxycarbonate, identified by XRD, evolves on top of the coating during the course of salt spray exposure. The phase of the Pr-rich precipitates in the scribe have not yet been determined, but it is likely they are a mixed Pr-hydroxycarbonate as well. XRD patterns collected for Pr2O3 containing coatings identify Pr(OH)3 as the phase present in the asdeposited coating as well as in coatings following salt spray exposure. Previous studies have shown that Pr2O3 readily hydrates to form Pr(OH)3 [5]. XRD patterns collected for Pr6O11 containing coatings identify Pr6O11, PrO2, and Pr(OH)3 as the phases present in the asdeposited and post-salt spray coatings. While Pr6O11 does not hydrate as readily as Pr2O3, an increase in temperature will increase the rate at which disproportionation proceeds where Pr6O11→ PrO2 + Pr(OH)3 [5]. Significant progress has been made toward understanding the changes that occur in Pr-based coatings when they are exposed to corrosive environments. There is evidence that Pr-species are mobile and migrate from the primer matrix during salt spray exposure. It is still unclear why Pr-rich precipitates form in discrete areas of the scribe, especially in areas that appear to be prone to salting. The primary goal of this study is to determine the how the phase of the PrOx compounds employed as corrosion inhibitors in epoxy-polyamide primers affects the electrochemical and morphological properties of Prbased coatings.