The effect of sodium chromate on the cathodic current availability pertinent to the micro-galvanic-induced corrosion of high-strength AA7XXX alloys under simulated thin electrolyte films representative of atmospheric conditions was investigated utilizing a combination of electrochemical and surface characterization techniques. The rotating disk electrode technique provided a means to simulate the effects of water layer thickness to differentiate thin film conditions from full immersion conditions, and enabled the study of the mass-transport-limited oxygen reduction reaction (ORR) on AA7XXX alloys as a function of chromate concentration. The ORR current density decreased by up to two orders of magnitude upon addition of 10 mM chromate, however, the degree of inhibition was observed to depend on the Cu content of the alloy. Chromate was reduced irreversibly to form a Cr3+-rich film on the alloy surface that blocked cathodic sites and hindered ORR. This film was confirmed by X-ray photoelectron spectroscopic characterization of the chemistry and thickness of the chromate-induced layer formed on the specimens after exposure to chromate. The layer was approximately 13 nm in thickness and consisted of mixed Cr3+/Cr6+ oxides with some metallic Cr. Studies on a Pt electrode demonstrated the intrinsic ability of chromate as an effective inhibitor for ORR.
The scanning vibrating electrode technique (SVET) was utilized to experimentally validate the applicability of finite element modeling (FEM) in simulating macro-galvanic-induced corrosion of AA7050 coupled to SS316, in environments representative of the boldly exposed surface of an actual fastener couple. The FEM boundary conditions were modified from the SVET environments in which the AA7050-SS316 couple sample was initially exposed, in order to better represent the steady-state corroding surface of the localized corrosion-prone AA7050. Better agreements between the SVET-derived data and the model in the case of macro-galvanic coupling behavior were achieved for near-neutral conditions, compared to acidic conditions. The current density at the electrode/electrolyte interface was determined with the validated model. In addition, the percent difference between the measured current density at the SVET probe height and that at the electrode surface was observed to scale with the magnitude of current density at the electrode surface, with the largest discrepancy seen at the galvanic couple interface. Plausible reasons for the deviation of the model predictions from the SVET-derived data are discussed.
The performance of chromate in protecting AA7050-T7451 coupled to 316SS in simulated fastener environments, including those representative of the boldly exposed surfaces and downhole conditions, was investigated utilizing a number of electrochemical and surface characterization techniques. The influence of pH and Al3+ on the galvanic coupling behavior and damage evolution on AA7050 as a function of chromate concentration were assessed. The degree of chromate inhibition was observed to decrease as pH decreased, owing to chromate speciation and reduced capacity to suppress the hydrogen evolution reaction (HER) compared to the oxygen reduction reaction (ORR). The addition of 0.1 M Al3+ significantly increased HER kinetics and produced a large buffer effect which overwhelmed the ability of chromate to slow damage propagation on AA7050. Assessment of cathodes indicated that Cu was more important than 316SS in driving damage initiation, but less active than 316SS in supporting high-rate damage propagation in simulated crevice environments. The implications of this study for actual bimetallic systems are discussed.
Dissimilar metal assemblies are frequently encountered in complex structures, such as in aerospace applications, and pose a major challenge in terms of galvanic-induced localized corrosion. One example of such an assembly is that of high-strength aluminum alloy (AA) components joined with stainless steel (SS) fasteners. Defects in the corrosion protection coating systems at these joint locations facilitate the wicking of aggressive electrolyte species into the confined fastener crevices, creating a hidden macro-galvanic corrosion cell which can aggravate attack and degrade the fatigue life of the localized corrosion-prone base AA component.[1-2] In this situation, inhibitor release from the coating system, and transport into the fastener crevice, would be crucial in mitigating the localized attack. Previous studies on inhibitor activity on AA-SS galvanic couples have mainly focused on the boldly exposed surface conditions and associated chemistries due to the difficulties encountered in reconstructing and/or simulating complex fastener geometries. As a result, there is limited understanding of the physical, electrochemical, metallurgical, and geometric factors that govern both the location and mode of corrosion damage in a fastener crevice, let alone the rationalization of how an inhibitor, such as chromate, can influence these factors. Recent downhole analyses of an AA7050-316SS fastener crevice utilizing X-ray tomography and coupled multi-electrode arrays (CMEAs) showed that although there was a significant decrease in fissure density with no (near) surface fissures on addition of chromate, galvanic attack on AA7050 was still occurring at the bottom of the fastener hole.[3] This result raised questions surrounding the possible effects of crevice depth on solution chemistry in the fastener crevice which may have impacted the speciation and/or activity of chromate in solution.The present work will build on the work of Rafla and Scully[3] to advance current knowledge of the role of chromate on the interaction of the various factors that govern damage in AA plate-SS fastener systems. This work will utilize a combination of electrochemical techniques – rotating disk electrode (RDE), zero resistance ammetery (ZRA), potentiodynamic polarization experiments – and surface characterization techniques - optical microscopy and scanning electron microscopy (SEM) - to assess the performance of chromate on AA7050-316SS galvanic couples in elevated chloride environments with the addition of Al3+ simulating deep fastener crevice environments. The results will reveal whether or not chromate suppresses HER kinetics on 316SS that may be occurring deep inside a fastener crevice and/or if there is a critical [Cl-]/[CrO4 2-] above which chromate does not stifle damage propagation on AA coupled to SS in aggressive chloride environments. References V. N. Rafla et al., Corr. 74, 2018, p. 5-23R. S. Marshall et al., Corr. 75, 2019, p. 1461-1473V. N. Rafla and J. R. Scully, Corr. 75, 2019, p. 587-603 Acknowledgement This work is support...
Numerous studies have demonstrated that the Cu-rich intermetallic particles (IMPs) are the major facilitators of localized corrosion of high strength Al alloys in chloride-containing environments.[1-4] In the context of cathodic activity, these Cu-rich IMPs catalyze the fast oxygen reduction reaction (ORR) rates required to sustain anodic dissolution of the peripheral Al matrix and/or preferential dealloying of active elemental constituent(s) of the Cu-rich IMPs. The latter process results in Cu replating on the surface which increases the cathodic surface area. The situation is exacerbated when these Al alloys are coupled with more noble fasteners such as stainless steel (SS) as found in aircraft structures; macro-galvanic interactions between the Al plates and SS fasteners lead to substantial damage, including within the fastener holes.[5-8] The SS fastener is speculated to increase the driving force for Cu replating, and the replated Cu in turn sustains fast cathodic reaction kinetics.[4] Traditionally, soluble chromates have been used in conversion coatings and as pigments in protective coatings to mitigate localized corrosion of Al alloys.[9-10] Chromate cathodically reduces to form an insoluble Cr3+-rich film on the alloy surface that blocks cathodic sites and hinders the ORR as well as the dealloying of the S-phase (Al2CuMg).[10-13] While chromate is well known to be carcinogenic, and efforts are underway to replace it with an environmentally-friendly inhibitor, efforts to date on potential alternatives have proven generally unsuccessful.[14] Consequently, chromate remains the choice inhibitor for Al alloy applications particularly in the aerospace industry. As such, it remains of utmost academic and industrial interest to continue to gain more in-depth understanding of chromate inhibition mechanism(s), especially as it pertains to the corrosion behavior of complex structures with micro- and macro-galvanic couples with the intent to enhance the knowledge required to facilitate the establishment of non-toxic chromate replacement systems. In the present work, the rotating disk electrode (RDE) technique is used for a comparative study of the inhibitive effect of sodium chromate on the ORR on polished and pre-corroded AA7050 electrodes. The results will serve as boundary conditions for the computational modeling of galvanic corrosion inhibition of AA7050 coupled to 316SS in a plate-fastener arrangement. The RDE is used to simulate thin electrolyte films with varying diffusion boundary layer thicknesses as rotation rate is increased. In the inhibitor-free chloride solution, a Pt working electrode is used to determine boundary layer thickness as a function of rotation rate, eliminating the effects of a surface oxide film. Initial results on polished electrodes show that at optimal chromate concentrations where the cathodic current density is significantly reduced on AA7050 and 316SS, the ORR kinetics on pure Cu is still substantial. This implies that higher inhibitor concentrations may be required to suppress the ORR on corroded AA7050 replated with Cu. AA7050 pre-corrosion is simulated by employing two approaches: 1) free corrosion at the open circuit potential in inhibitor-free solution and 2) electroplating Cu onto AA7050. The results are compared to the ORR kinetics on pure Cu. Furthermore, the scanning vibrating electrode technique (SVET) will be carried out on a typical AA7050/316SS galvanic couple to map local current density distributions as a function of chromate concentration. Surface characterization will be performed using optical microscopy and scanning electron microscopy (SEM) and surface film analyses will be conducted utilizing x-ray photoelectron spectroscopy (XPS). References [1] Q. Meng, G.S. Frankel, J. Elect. Soc. 151, 2004, p. B271-B283 [2] G.O. Ilevbare, J.R. Scully, Corr. 57, 2001, p. 134-152 [3] Y. Zhu, K. Sun, G.S. Frankel, J. Elect. Soc. 165, 2018, p. C807-C820 [4] V.N. Rafla et al., Corr. 74, 2018, p. 5-23 [5] R.S. Marshall, R.G. Kelly, A. Goff, C. Sprinkle, Corr. 75, 2019, p. 1461-1473 [6] R.S. Marshall et al., Corr. 75, 2019, p. 1461-1473 [7] C. Liu, V.N. Rafla, J.R. Scully, R.G. Kelly, CORROSION 2015, paper no. 5579 (Houston, TX: NACE International, 2015) [8] C. Liu, J. Srinivasan and R.G. Kelly, J. Electrochem. Soc. 164, 2017, p. 845-855 [9] G.S. Frankel, R.L. McCreery, Elect. Soc. Interface, 2001, p. 34-38 [10] M.W. Kendig and R.G. Buchheit, Corr. 59, 2003, p. 379-400 [11] V.N. Rafla and J.R. Scully, Corr. 75, 2019, p. 587-603 [12] G.O. Ilevbare and J.R. Scully, J. Elect. Soc. 148, 2001, p. B196-B207 [13] R. Gupta, B. Hinton and N. Birbilis, Corr. Sci. 82, 2014, p. 197-207 [14] O. Gharbi et al., npj Mat. Deg. 2, 2018, p. 12-19
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.