Hexavalent chromium [Cr(VI)] has been extensively used in conversion coatings and paints to protect against corrosion of aluminum and iron-based alloys in Department of Defense (DoD) systems. However, chromate-based inhibitor systems are being eliminated as Cr(VI) is toxic and carcinogenic, which is leading to the development of non-chromate alternatives. Most of the non-chromate alternatives are individually not as effective as chromate, but a combination of them may be found to be a suitable replacement. The objective of this project was to gain understanding of the mechanism of selected alternative inhibitors under realistic metallurgical and environmental conditions in order to facilitate the development of non-chromate inhibitor systems that possess effectiveness comparable to that of chromates. With the fundamental understanding of individual compounds, inhibitor systems with combinations of inhibitors whose individual properties are synergistically enhanced were studied.
The project addressed the objective by (1) assessing the anodic and cathodic inhibition functions of non-chromate inhibitors (using standard and multielectrode electrochemical techniques and scratching electrode techniques); (2) determining the adsorption characteristics and concentration of adsorbed species on the alloy surfaces; (3) investigating the interactions between the inhibitors and heterogeneities in the microstrucures (using multielectrode and microelectrochemical techniques); and (4) analyzing the effect of alternating wet and dry conditions and the drying process on inhibition mechanisms.
The inhibition of corrosion on AA2024-T351 in NaCl solutions, mitigated by either in situ permanganate ions (MnO4-) or MnO4- pretreatment, was examined. The anodic and cathodic inhibition effects were studied as a function of inhibitor concentration. Inhibition of open circuit corrosion under conditions where anodic and cathodic reactions are coupled was also examined. The oxygen reduction reaction (ORR) was modeled using the Gueshi and Membrane models after the Koutecky-Levich correction was applied to experimental cathodic data. The oxidation states of the inhibiting manganese oxides were determined using potentiometric electrochemical reduction and in situ Raman spectroscopy. The thermodynamics of the Mn-water system was also considered over a range of concentrations. Permanganate was shown to be both an anodic and cathodic inhibitor, and an inhibitor of copper replating at open circuit potential (OCP).
Vanadate was investigated as a corrosion inhibitor for AA2024 fully immersed in NaCl solutions. Nuclear Magnetic Resonance (NMR) was used to determine the vanadate species in solutions. The dominant species was V4O124- in 50 mM NaCl + 5 to 150 mM NaVO3. Vanadate was found to inhibit the ORR for both AA2024-T4 and high-purity copper. Vanadate anions in solution reduced both charge transfer and mass transport current densities. Less than 1 mM NaVO3 was needed for cathodic inhibition on AA2024-T4. Higher concentrations of NaVO3 were required to raise the average pitting potential. Modeling of the cathodic inhibition mechanisms was pursued using the Gueshi analysis representing an array of active cathodic sites, as well as Langmuir and Temkin adsorption isotherms. The vanadate coverage on copper containing intermetallic particles was found to be dependent on the over-potential. After 24 hour exposures at OCP, anodic inhibition of pitting increased with increasing vanadate concentration in solution for both AA2024-T3 and high-purity aluminum, suggesting that tetrahedrally coordinated vanadate species affect pure aluminum as well as intermetallics in AA2024-T3.
Corrosion inhibitor combinations were studied on AA2024. Vanadate-molybdate combinations acted as cathodic inhibitor, unlike cerium-vanadate. Ce precipitates blocked cathodic sites from vanadate adsorption. This is consistent with modeling of competitive adsorption of anions on copper intermetallic particles. Vanadate-molybdate was as effective as vanadate as anodic inhibitor, and cerium addition to vanadate provided no benefit. Cathodic inhibition of a cerium-permanganate combination was equivalent to that of permanganate and superior to cerium. Cerium-permanganate also increased OCP corrosion inhibition at a level comparable to that of the single inhibitors. No anodic inhibition was found.
Scratch repassivation of AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate and chromate containing NaCl solutions was performed. Chromate suppressed scratched electrode current transients at high potentials. Furthermore the rapid growth of a thick, protective oxide was measured. Molybdate did not suppress transient current but did promote the rapid growth of a passive film.
Cerium, vanadate, and molybdate acted as anodic inhibitor with AA7075, while only cerium and vanadate behaved as cathodic inhibitors. Molybdate was found to behave as an anodic and a cathodic inhibitor for carbon steel 1018. Synergistic behaviors were proven to be occurring partially for Ce/Mo and Ce/V and were especially successful for carbon steel 1018. However, they were not accurately predicted during this project.
Rotating disc arrays were used to study the impact of the density and size of copper intermetallic particles on the cathodic effectiveness of the inhibitor. This was consistent with the comparison between AA2024 and AA7075, based upon the potentiodynamic analysis and scanning electron microscope (SEM) studies. Aluminum/copper multi-electrode array (MEA) was used to simulate copper-rich intermetallic particles in an aluminum-rich matrix. Composition, concentration, and pretreatment were found to have an impact on the effectiveness of the inhibitor. Vanadate was the sole successful inhibitor when testing in NaCl solutions. All inhibitors were successful when pretreated, but better results were obtained at lower concentration, consistent with the Levich analysis performed on AA2024, which concluded that the critical inhibitor concentration needed to suppress ORR is below 0.005 mol/L sodium vanadate. Energy dispersive X-ray spectroscopy (EDS) and Auger electron spectrometry showed that cerium, molybdate, and vanadate are all present on the surface of AA7075. However, only cerium was found to be dependent on the quantity of the inhibitor. Based upon the surface EDS, it appears that the role of concentration observed for the Al/Cu MEA is not due to the amount of inhibitor on the surface but may be due to the species of inhibitors. This would be due to the fact that the pH of the solution is dependent on the concentration of the inhibitor and the inhibitor speciation is directly related to the pH.
Atmospheric tests using the Al/Cu MEA were performed to assess the performance of various inhibitors at various relative humidity. Vanadate and cerium performed successfully only at lower relative humidity (60 and 70%). At higher relative humidity, vanadate and cerium concentration may be too low to perform as cathodic inhibitors.
The results are applicable to a variety of weapons systems and ground support equipment. Cerium, vanadate, and molybdate, which had been successfully applied to AA2024, were shown to be partially transferable to AA7075 and carbon steel 1018. Inhibitor behavior was also found to depend on the type of treatment, e.g., in solution vs. pretreatment. Furthermore, inhibitor combinations have been shown to be beneficial under certain circumstances (e.g., cerium and vanadate efficiently inhibit corrosion under atmospheric corrosion at lower relative humidity).