The Effects of Permanganate and Other Selected Chemical Inhibitors on Surface and Crack Tip Corrosion

Environmental concern over the use of very effective but highly toxic chromate ionic inhibitors has led to the search for new chemical inhibitors that can protect Al-base precipitation age hardened alloys such as 2024-T351 and AA7075-T6, from corrosion including environmentally assisted cracking. These alloys are commonly used in aerospace applications. Presented in this dissertation are investigations to understand the mechanisms by which alternative ionic solution phase inhibitors function to inhibit corrosion. Extensive comparison to chromate is made. The work presented focuses on both surface corrosion as well as the inhibition of electrochemical processes that can lead to accelerated environmentally assisted crack growth, particularly those that can be explored in simulated film rupture repassivation tests.
The inhibition of corrosion on AA2024-T351 in NaCl solutions, mitigated by either in-situ permanganate ions (MnO4-) or permanganate pretreatment, was examined. Both room temperature pretreatments and solution phase additions were studied as a function of inhibitor concentration. The roles of the inhibitor during anodic and cathodic polarizations were investigated. Inhibition of corrosion at the open circuit corrosion potential (OCP) under conditions where anodic and cathodic reactions are coupled was also examined. The oxidation states of the manganese oxides that contributed to protection 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, as well as an inhibitor of copper replating at OCP.
The inhibition of corrosion on AA2024-T351 in NaCl solutions, mitigated by cerium-permanganate combined inhibitor pretreatments, was examined next. Room temperature pretreatments were studied as a function of total inhibitor concentrations ranging from 10 to 50 mM. Pretreated samples were exposed to 50 mM NaCl solutions. The roles of the inhibitor during anodic and cathodic polarization were also investigated. The Koutecky-Levich correction was applied to experimental cathodic E-i data. Inhibition of open circuit corrosion under conditions where anodic and cathodic reactions are coupled was also examined. The thermodynamics of the Ce- and Mn-water systems was also considered over a range of concentrations. There was no indication of the formation of a combined Mn-Ce molecular species and no reference to such a species was found in the literature. The cerium-permanganate combination was determined to provide nearly the same level of cathodic inhibition seen during the single inhibitor application of permanganate. Cerium alone was inferior to permanganate as a cathodic inhibitor. In addition to strong cathodic inhibition, the combination of Ce and Mn increased inhibition of open circuit corrosion compared to single inhibitor pretreatments as determined by copper redeposition assessment. However, anodic inhibition was not observed with the inhibitors when used alone or when combined.
The scratch depassivation technique was utilized on AA2024-T351, AA7075-T6, and 99.999% aluminum in molybdate (MoO42-) permanganate (MnO4-) and chromate (CrO42-) containing NaCl solutions to examine electrochemical repassivation kinetics over a range of applied potentials and inhibitor concentrations. Environmental fatigue crack propagation is indicated to be inhibited below a critical cyclic loading frequency. The significance of this frequency can be understood in the context of an oxide repassivation time constant. Therefore, repassivation is pertinent to the behavior of crack tips which are depassivated by plastic deformation during mechanical loading under fatigue conditions. Scratch repassivation current vs. time and single frequency impedance measurements were taken in the region of frequency response dominated by surface capacitance. Current decay and capacitance measurements enabled the observation of the effects of inhibitor additions on film formation and oxide growth as protective oxide films, electrically modeled as capacitors, formed on the scratched electrode surface. Impedance measurements overcame the shortcomings of incomplete inventory of anodic oxidation current densities typical of aluminum repassivation at coupled electrode surfaces. Chromate was found to suppress scratched electrode current transients at high potentials on both AA2024-T351 and AA7075-T6. Capacitance measurements suggested the rapid formation and growth of an oxide at enhanced rates when CrO42- was in solution as compared to an uninhibited NaCl solution. Molybdate did not suppress the transient film rupture repassivation current density in AA2024-T351 or AA7075-T6. Impedance measurements in the presence of molybdate do not indicate the enhanced formation and growth of a passive film in the case of molybdate. The observed environmental fatigue crack propagation (EFCP) inhibition due to chromate may be due to enhanced repassivation and subsequently limited hydrogen production and uptake supported by oxide film barrier effects. The observed ECFP reduction with MoO42- is discussed in light of the observed lack of an effect on repassivation kinetics. The buffer capacity of molybdate may affect the local solution chemistry and this chemical effect may also promote EFCP inhibition. Permanganate was found to suppress the transient current density of AA7075 after scratch depassivation but not in the case of AA2024. MnO4- was also found to positively affect interfacial impedance suggesting improved barrier protection for both AA7075 and AA2024. These findings suggest some promise for the inhibition of EFCP with chromate, molybdate, and permanganate.
Single frequency impedance, Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) analysis of abrasion-repassivated Al-Mg-Zn-Cu alloy (AA7075-T6) surfaces give the first direct measurement of enhanced repassivation, inhibitor species incorporation, followed by reduced hydrogen uptake into an Al-Zn-Mg-Cu alloy in a combined experiment. Abrasion was used to simulate film rupture and repassivation. Repassivation was investigated with chromate, molybdate, and permanganate inhibitor additions to aqueous sodium chloride solutions. Chromate (0.1 M) was found to be an effective inhibitor of hydrogen uptake during in-situ surface abrasion in 1.0 M NaCl solution. The evidence presented provides insight into the processes responsible for inhibition of EFCP of AA7075 controlled by hydrogen ingress at depassivated crack tips. Chromate is shown to be capable of enhancing repassivation, forming a compact oxide barrier and retarding hydrogen uptake into the fracture process zones of crack tips. Moreover, these findings provide a new methodology through which corrosion inhibitors may be evaluated for SCC and EFCP inhibitor function.
Ultimately, this thesis extends the scientific understanding of inhibitors and their functions, from surface corrosion inhibition to inhibition of environmentally assisted cracking governed by processes at depassivated crack tips. This research enables better corrosion inhibitor selection based on more informative and pertinent test methods for both surface as well as environmental cracking corrosion phenomena.