The objective for this project was to demonstrate the utility of neutron reflectivity and x-ray reflectivity to elucidate the morphology and protection mechanisms of environmentally benign inhibitors for Al alloys.
The project was in response to SERDP Statement of Need WPSON 08-02 Scientific Understanding of Non-Chromated Corrosion Inhibitors Function. This project was submitted because the technique of neutron reflectivity had never been used to address corrosion or inhibition of aluminum. In fact, the morphology evolution of a conversion coating in a corrosive environment had never been measured by any technique. Success was defined as measurement of the structure and evolution of an inhibitor film and inhibited surface in a corrosive environment by neutron reflectivity.
Tasks were organized around the following six elements and successfully completed:
Every aspect of the approach had to be perfected from deposition of suitable metal films to building of a combined reflectivity and electrochemical cell to observe the evolution of a passive film in situ in a corrosive environment with and without the addition of a soluble inhibitor.
In addition to successful demonstration of required technology, every task produced new observations that could not be made by conventional corrosion methodology.
This project successfully deposited smooth Al, Cu and mixed Cu/Al films on Si wafers and demonstrated that the alloy structure can be manipulated by subsequent annealing. This accomplishment opens up new possibilities to determine the impact of alloying elements on corrosion performance.
The formation and stripping of the native oxide film on Al was observed and it was shown that the native oxide is thin, porous, and partially hydrated. The oxide is easily stripped under anodic conditions, which facilities subsequent deposition of conversion coatings.
By measuring the evolution of bare Al under anodic conditions, it was demonstrated that Al dissolution measured by the corrosion current agrees with that measured by reflectivity. This correspondence means that side reactions do not compromise electrochemical analysis of corrosion processes.
A new conversion-coating deposition protocol was developed based on a simplified, cathodically deposited Trivalent Chromium Process (TCP) that eliminates the ferricyanide and fluoride compounds in commercial formulations.
It was demonstrated that thin, smooth, uniform TCP films suitable for reflectivity measurements can be deposited on aluminum-covered Si wafers. Using a combination of neutron and x-ray methods the composition of the resulting conversion coatings was determined to be Cr2O3∙2.1H2O∙0.85 (ZrO2∙1.6H2O).
The failure mechanism of TCP films in a pitting scenario in the absence of a soluble inhibitor such as Ce(III) was also demonstrated. Below the pitting potential, water exchanges with the film crystal hydrate water but bulk solution does not penetrate the film. At the pitting potential, the protective film swells and bulk water penetrates. The compromised film remains on the metal, creating an isolated environment conducive to pitting.
Using a new step-by-step polarization test, it was shown that addition of a soluble Ce(III) inhibitor leads to anodic healing of an otherwise compromised TCP film. By raising the potential over several hours, the pitting potential can be substantially increased, a process called anodic hardening.
Using neutron reflectivity and chemical analysis, it was shown that during anodic hardening, Ce(III) penetrates the TCP film.
This project achieved 3000-Å, smooth, uniform aluminum layers on Si suitable for anodizing experiments. The kinetics of anodic oxide growth on these films using reflectivity were also observed.
This project discovered a new anodizing protocol based on current control with a voltage limit that affords precise control over the oxide layer thickness.
A small-angle x-ray scattering was used on anodized Al foils to measure the in-plane structure of the porous anodic aluminum oxide. The hexagonal pattern of pores develops within 20 s. The pore size, and interpore distance do not changes after 20 s; the pore depth, however, increases linearly with time until the pores fully penetrate the foil.
Using small-angle x-ray scattering and small angle neutron scattering, a slight alteration of the anodic aluminum oxide structure was observed after cold nickel acetate sealing of anodic aluminum oxide. Hot nickel acetate sealing, however, fills in the pores and covers the air surface of the anodic aluminum oxide.
Neutron reflectivity further reveals the mechanism of cold nickel acetate sealing. At room temperature, nickel acetate deposits at the bottom of the pores. Long-term exposure to the sealing bath leads to penetration of the pore wall and subsequent degradation of the surface on drying. It was concluded that cold nickel acetate sealing cannot replace the hot sealing methods.
The ability to measure evolving surfaces in situ means that the “structure” element of process-structure-performance research strategy is accessible for the first time. When an electrical potential is applied, the method is essentially a powerful accelerated failure test. The data not only reveal failure, but the mechanism of failure.
The new anodizing protocol developed offers improved control over growth and thickness of the anodic aluminum oxide film. The anodizing strategy should be applicable to troublesome metals such as magnesium and iron. Using this approach, it should be possible to reduce the energy consumption required for anodizing.
The anodization study opens new possibilities for improved sealing. Using neutron reflectivity (NR), one can determine the deposit profile and correlate the profile with performance. The observation of sealant penetration of the pore walls opens intriguing possibilities for dramatically improved corrosion performance of aluminum and other metals.