The Mechanism Of Pitting Corrosion And Rare-Earth Surface Modification On Aluminum Alloy

The evolution of corrosion pits on aluminum alloy immersed in chloride solution occurs in three distinct stages: nucleation, metastable growth and stable growth. This paper describes the initiation of corrosion pits and stimulates the pitting growth on aluminum alloy immersed in chloride solution. The rate of growth of stimulated individual corrosion pits is found to be controlled by transfer rate of the dissolving hydrogen which is produced on the electrode interface.Because current environmental legislations moving towards total exclusion of Cr^6- and because of tightening regulatory pressure to reduce the hazardous waste of chromium, many attempts are being made to develop non-toxic alternative methods of corrosion protection. Another object of present research has focused on rare-earth (RE) surface modification with emphasis on elimination of hazardous chemicals used in corrosion protection of aluminum alloy.Ex situ TEM was used in conjunction with EDS to monitor evolution near the preselected inclusions during initial stages of localized dissolution on the AA 6061 immersed in 0.1M NaCl solution. On the alloy TEM foil, Mg₂Si phase particles were observed to preferentially dissolve after 3 minutes immersion. Nevertheless, different dissolution behaviors were observed to form in the vicinities of the different structure Fe-rich precipitates; the trenches were observed nearby the anorthic phase particle, but no obvious priority dissolution was detected in the vicinity of hexagonal phase particles. The preferential sites of alloy dissolution were found to depend on both the component and structure of the intermetallics. The shift of electrochemical potential was found to relate the dissolution of the heterogeneous phases.The importance of pitting corrosion was investigated under potentiostatic dissolution conditions with pencil electrodes contained in inert supports. The artificial cavities created simulated localized corrosion conditions. Current-time behavior at voltages in excess of the critical pitting potential (-300mV vs. SCE) was examined for pure aluminum specimens in concentrated chloride solutions. The effect of changing the concentration of the dissolving metal cations within the artificial cavity was studied by altering the composition of the bulk solution. Solutions of NaCl ranging from 0.01 to 4M were used. Mass transfer models were developed for the observed transient and quasi steady-state periods of dissolution.Ex situ TEM technique was also used to monitor deposition behavior of cerium oxide near the pre-selected inclusions on the AA 5083 immersed in 0.1 M cerous ethanol solution. On the 5083 aluminium alloy TEM foil, inclusions were observed to preferentially depositing sites after 30 minutes solution treatment. Nevertheless, different deposition rate behaviors were observed to produce in the vicinities of precipitates; and the cerium oxides were observed in open cavities area near Si-rich inclusions, but no cerium oxide was detected on the substrates of aluminum. The preferential sites of cerium oxide dipositon were found to depend on both the corrosion potential the heterogeneous phases; also on the oxidizer, and pH of the solution.In the research devoted to the elimination of toxic materials in methods of surface modification for corrosion protection, deposition cerium oxide by cathode electrophoresis processes have been developed to improve the pitting resistance of aluminum alloys without the use of chromates. Impedance data collected in 0.1 M NaCl remained capacitive for 31 days, which is indicative of the lack of localized corrosion. Apparently local cathodes are eliminated during the surface RE modification process, thereby reducing the driving force for pitting.