An alternative to chromates for corrosion protection for aluminum alloys
Aluminum alloys, such as Al 6061-T6 and Al 7075-T6, are widely used in industry due to their high strength to weight ratio and good mechanical properties. The corrosion of these alloys, however, is an expensive and critical problem since the alloys are susceptible to pitting and crevice corrosion in marine environments. The most significant environmental factor, which contributes to the corrosion of these alloys, is the chloride ion found in marine environments or water condensed from humid air contaminated with soluble chloride salts. Traditionally, chromate based conversion coatings have been used for many years for the protection of aluminum alloys. Chromates are efficient corrosion inhibitors for aluminum and its alloys in near neutral marine environments containing aggressive anions such as chlorides. Although the hexavalent chromium ion, Cr6+, may be a superior corrosion inhibitor and used in numerous industrial systems, it is environmentally unsafe. Over the past several years, federal agencies, such as the Environmental Protection Agency (EPA) and the Department of Defense (DoD), have increasingly limited the use of chromium containing compounds due to their toxic and carcinogenic effects. In addition, there is a direct economic challenge associated with costs for environmental compliance along with increased liability for claims of exposure in the workplace with the continued use of chromates. Therefore, there is a need to identify new corrosion inhibitors for aluminum alloys. As an alternate conversion coating, a new titanate conversion coating was researched and developed for the Al 2024-T3 alloy and was shown to be effective. The objective of this research was to determine if the coating process could be applied to Al 6061-T6 and Al 7075-T6. The coating process involves immersion of the alloy in a titante solution bath, which produces a passive film. The corrosion resistance of coated samples has been evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic electrochemical techniques. Electrochemical testing and energy dispersive X-ray analysis indicated that the titanate ion would retard corrosion in a similar manner to the chromate ion if fluoride ions (F-) were not present on the surface. A study was also conducted to determine if Al 6061-T6 and Al 7075-T6 were easily susceptible to crevice corrosion in a marine environment. The study yielded important results regarding protection of the alloy against crevice corrosion by the titanate ion. Corrosion was only seen on samples not exposed to the titanate ion. A conclusion may be made that titanate coatings appear to be viable alternatives to chromate coatings but further investigation will be required in order to determine an optimum conversion coating bath, which will produce impedance magnitudes comparable to those measured for the Al 2024-T3 alloy.
Chemical engineering|Materials science
"An alternative to chromates for corrosion protection for aluminum alloys"
Dissertations and Master's Theses (Campus Access).