Finding suitable replacements for chromates has been a focus of the corrosion inhibition and protection community for decades. The early efforts were largely unsuccessful, prompting the community to take a step back and look closely at the mechanisms of corrosion inhibition provided by chromates. The primary objective of this project was to develop a fundamental understanding of the existing chromate-free inhibitors and inhibitory coating systems with the ultimate goal of providing scientists and engineers developing such coatings information to help them improve their products. This project represented the first large-scale research effort addressing non-chromate inhibitors.
The approach was to address, concurrently, a number of critical, cross-cutting, and underlying issues. These issues formed the basis for individual projects or tasks, which were run simultaneously. They included studies of some leading non-chromate inhibitor technologies—trivalent chromium process (TCP); molybdate, silicate, Zn2+, and praseodymium inhibitors found in commercial chromate-free primers; and additional rare earth metal cations such as Ce3+, or, La3+. The affects of surface treatments on coating adhesion, solubility of modern chromate-free pigments, and the performance of non-chromate coating systems were studied. Finally, novel methods to monitor corrosion of metallic films underneath organic coating systems were evaluated.
TCP coatings were studied on several aluminum (Al) alloys. TCP coating provides both anodic and cathodic protection by physically blocking Al-rich sites (oxidation) and copper (Cu)-rich intermetallics (reduction), resulting in 10 times greater polarization resistance and suppressed anodic and cathodic currents around the open circuit potential. TCP coating can release chromium into solution, which can then form a film on the surface of an uncoated alloy surface exposed in close proximity. The polarization resistance of the uncoated surface near a TCP-coated surface is higher than uncoated controls, which indicates the TCP coating can provide active corrosion inhibition.
The inhibition performance of molybdate, silicate, and praseodymium was studied, and a mechanism for each inhibitor was postulated. Molybdate inhibition is optimal at near-neutral pH. MoO3 imparts anodic inhibition by ennobling the pitting potential. Silicate is a strong anodic inhibitor in highly alkaline conditions, increasing the pitting potential by as much as 1 V. Praseodymium imparts cathodic inhibition at near-neutral pH, lowering the oxygen reduction kinetics by a factor of 10.
A comparative study on the corrosion inhibition caused by rare earth metal cations, Ce3+, Pr3+, La3+, and Zn2+ cations on AA 2024-T3 alloy was performed. Cathodic polarization showed that these inhibitor ions suppress the oxygen reduction reaction (ORR) to varying extents with Zn2+ providing the best inhibition. It was observed that Pr3+ exhibits windows of concentration in which the corrosion rate is minimum, similar to the Ce3+ cation. Scanning electron microscopy studies showed that the mechanism of inhibition of the Pr3+ ion is also similar to that of the Ce3+ ion. Lanthanum is not an effective inhibitor of the alloy as inferred from both electrochemical experiments and microscopic analysis on 2024-T3. Results show that Zn2+ is an effective inhibitor with inhibition arising from the precipitation of mixed Zn hydroxide/Zn hydroxycarbonate surface film.
The blister test was used to study the effect of surface treatments on coating adhesion. Roughness degree and surface topography are important factors for adhesion strength of polyvinyl buterol (PVB) to AA2024-T3. Adhesion strength increases with roughness and surface area due to a larger interaction in the primer/substrate interface. Numerous approaches were tested to sample adhesion strength using the blister test after adhesion degradation of the interface, but no adhesion strength degradation was found. Cleaning and deoxidizing improves adhesion of acetoacetate samples for all conversion coatings except for chromate conversion coating. For epoxy coatings, the addition of the cleaning/deoxidizing step improves the adhesion strength of the chromate conversion coated samples, but no effect is noticeable for titiania-based surface treatment and for TCP treatment.
The solubility of modern chromate-free pigments was found to depend beneficially on reactions both with their environment and with their internal co-pigments, with their transport properties influenced both by ionic current and the fine structure of the primer matrix. All inhibitors studied exhibited significant solubility enhancement with exposure to strong electrolytes such as salt water, with the magnitude of the solubility enhancement governed by the ionic charge of the soluble inhibitor complex. Measurement of inhibitor enrichment on primer surfaces exposed to electric fields indicate both the role of electrophoresis in inhibitor transport and the differential ionic direction of charged inhibitor species to active surface sites. The water and inhibitor transport properties of aerospace primers were found to be regulated by a fine structure characterized by dense polymer layers at the substrate and air interfaces that encapsulate a nanoporous inner layer. The beneficial barrier aspects of this structure are somewhat offset by its tendency to promote water transport within the plane of the primer film.
A range of emergent Cr-free coating systems were characterized to understand the levels of corrosion protection provided to high-strength Al alloy substrates (2024-T3 and 7075-T6) in standardized and laboratory testing and to illuminate the interactions between the various components of the coating system—topcoat, primer, pretreatment—that lead to the levels of corrosion protection observed. Specifically, electrochemical impedance spectroscopy (EIS) and ASTM B117 exposure testing were carried out on coating systems comprising DoD-qualified primers and pretreatment products. The results showed that all Cr-free coating system combinations were inferior to chromate coating system incumbents. The results also allowed a rank ordering of performance. A central finding was that the nature of the pretreatment was critical in overall corrosion protection. The nature of the results enabled development of an empirical model to relate results from short-term EIS measurements to long-term exposure measurements. These results suggest that Cr-free pretreatments do not regularly form the foundations for Cr-free corrosion-resistant coating systems. The idea that a low-chrome system is far superior to a no-chrome coating system is supported in the results.
An approach referred to as Direct Optical Interrogation was used for in-situ monitoring of the corrosion of metallic thin films underneath the organic coating systems. The corrosion damage accumulation under coatings during free corrosion exposures not only depends on the particular aggressive electrolytes but also on the metallization of thin films as well as the local environments. Results show that localized corrosion growth is episodic with short bursts of growth occurring between long periods of passivity. By integrating the electrochemical experiments with the optical image analyzer, the undercoating corrosion damage can be evaluated in novel ways.
Advances in the scientific understanding of these issues will be relevant to non-chromate technologies and should have direct impact on developing new technologies. Furthermore, non-chromate protective system components already are being specified for use on military hardware, and the fundamental understanding of the protective mechanisms is essential for developing maintenance protocols.