Surface analytical and electrochemical studies of aircraft alloy pretreatments and their influence on corrosion.

摘要

Current research in the areas of aircraft alloy pretreatments and their influence on corrosion have raised fundamental questions regarding the applicability of standard surface preparation and analyzing protocols. Electrochemical techniques along with surface analytical techniques such as Laser Speckle Sensor and X-ray Photoelectron Spectroscopy have demonstrated the ability to detect early pitting corrosion in aluminum alloy AA2024-T3. The ASTM E1078-85 cleaning protocol recommends the use of acetone as an organic solvent. In the presence of ambient light, water vapor and copper; acetone reacts slowly to form acetic acid, which corrodes the copper surface. This reaction is completely inhibited in darkness, suggesting that copper is acting photocatalytically for the reaction between acetone and water. Acetone degreasing of AA2024-T3 following the ASTM E1078-85 protocol shows a similar effect on copper intermetallics. In the presence of a mist of sodium chloride solution and ambient light, the so formed acetic acid with the chloride solution layer supports severe pitting corrosion of the intermetallic particles. Additionally, there is evidence showing the deposition of dissolved metallic copper on the aluminum matrix. The pitting attack in sodium chloride was inhibited in darkness, suggesting that the slow reaction of the surface adsorbed acetone with water was prevented. Corrosion-fatigue studies showed the lowering of the total fatigue life of the alloy in a sodium chloride solution, with prior exposure to acetone.; Widely used surface preparation techniques on AA2024-T3, based on electrochemical and abrasive polishing techniques showed the modification of the alloy surface. Copper enrichment of the surface was observed in all the samples. A surface preparation protocol based on mechanical polishing with copper-free alumina suspensions has been recommended.; X-ray Photoelectron Spectroscopy characterization of hexavalent chromium in the presence of trivalent chromium is relevant to research of chromate conversion coatings inhibition processes and chromate conversion coatings. The photoreduction of hexavalent chromium in chromium trioxide and potassium chromate to tetravalent chromium due to exposure to soft x-rays during XPS analysis was found to be related to hydrocarbons adsorbed on the sample surface during rough pumping with a rotary pump. No photoreduction was observed over a period of 6 hours when the rough pumping was performed with a venturi and sorption pump, which eliminated the traces of hydrocarbons. Cr(IV) was found to be stable under the soft x-ray radiation. In the presence of water vapor Cr(IV) is chemically reduced to Cr(III). The findings bring into question the applicability of current models proposed by Knotek-Feibelman and Cazaux explaining photoreduction of high valent chromium oxides during XPS. A new protocol has been recommended for the studies based on XPS for chromium oxides and chromates.