Sub-surface Electrochemical Effects on the Spontaneous Deposition of Cerium Conversion Coatings on Aluminum Alloys

The combination of CeCl3 and H2O2 in cerium-based conversion coating (CeCC) solutions causes the dissolution of the aluminum alloy substrate, resulting in formation of voids a few microns below the surface during deposition. Localized, excessive dissolution of aluminum occurs near intermetallic particles over approximately 10% of the substrate surface. Alternate deposition methods were explored whereby either H2O2 or chloride content was reduced to minimize dissolution. an alternative oxidant, NaClO4, added to offset the impact of lower H2O2 content. CeCCs were deposited without sub-surface voids, but the resulting coatings had poor corrosion protection compared to CeCCs deposited with H2O2 alone. Coatings prepared with alternate Ce3+ salts using different ratios of CeCl3 and Ce(NO3)3, along with higher concentrations of H2O2, were thicker and more uniform than coatings with lower H2O2 content. However, CeCCs deposited from solutions containing H2O2 and CeCl3 had the highest overall electrochemical impedance and exhibited the best corrosion protection

Characterization of Localized Surface States of Al 7075-T6 during Deposition of Cerium-Based Conversion Coatings

The combination of chloride ions and H2O2 in solutions used to deposit cerium-based conversion coatings led to localized dissolution of aluminum alloy 7075-T6 substrates. Potentiodynamic scans indicated that exposure of the alloy to a solution containing 0.3 M chloride ions and 1 M H2O2 led to active dissolution. This process resulted in selective etching of the aluminum alloy substrate that produced a nonuniform surface and voids that penetrated a few micrometers into the alloy. When H2O2 was replaced by an alternative oxidizing agent, NaClO4 , cerium-based conversion coatings were deposited without substrate dissolution. Chloride ions and H2O2 selectively etch aluminum alloy 7075-T6 due to electrochemical reactions that take place. The reactions readily dissolved the native oxide that was present, allowing for the aluminum substrate along with embedded intermetallic particles to be exposed to the electrolyte, which propagated localized dissolution

Oxidation Behavior of Ultra-High Temperature Ceramics Using Different High-Temperature Test Facilities

The development of advanced thermal protection system (TPS) materials for use in next generation aerospace vehicles is needed. Ultra-high temperature ceramics (UHTCs) are considered viable candidates for hypersonic vehicle TPS materials due to their high melting temperature and good thermal conductivity. However, the need for new TPS materials has also prompted the need for the development of relevant test methods that simulate flight environments. Therefore, the focus of this talk will be to investigate the effect of high temperature test facility environments on the oxidation behavior of UHTCs. Three testing methods will be used to assess UHTCs at high temperatures (up to 2000 °C) and heat flux up to 200 Wcm-2. The first is an oxyacetylene torch set up according to ASTM E285-80 with oxidizing flame control and maximum achievable temperatures in excess of 2000 °C. The other two are high temperature static oxidation furnaces such as a thermal gravimetric analyzer and a box furnace capable of operating up to 1650 °C. The former is capable of in situ detection of weight loss and weight gain due to oxidation of the material under controlled high temperature gas mixtures thus allowing us to measure oxidation rates. In this study, ZrB2-SiC composites were processed using spark plasma sintering and were evaluated for oxidation behavior using our high temperature test facilities. The test facilities will be discussed in detail and correlated with preliminary materials evaluation results. We will also discuss collaborative testing efforts (plasma, solar furnace, and hyperthermal oxygen atoms) and on going development work at UA (dissociated air mixtures) in order to further develop an understanding for the effect of test environment on oxidation behavior of UHTCs

Characterization of Transport Processes in a Praseodymium-containing Coating

The electrochemical response of Pr-containing epoxy-polyamide primers on aluminum alloy 2024-T3 substrates was investigated. the effect of electrolyte pHs on corrosion behavior was studied before and after salt spray exposure. X-ray diffraction analysis confirmed that the praseodymium phase that was added to the primer converted into Pr(OH)3 in the as-deposited coating and remained present in the primer up to 3000 hours of salt spray exposure. Examination of panels prior to salt spray testing indicated that Pr was not detected in the scribes; however, following salt spray testing, Pr-rich species were found in localized clusters in scribes. Electrochemical tests were performed on test panels with machined defects using electrolyte pHs ranging from 5 to 8. the largest change in passivation range occurred at pH=8, and was two times higher than the change observed for bare Al 2024-T3. Corrosion protection of Pr-based primers is due to the ability of Pr-rich species to dissolve from coatings during exposure to corrosive environment and is influenced by pH