Assessment of Chromate Effects on the Galvanic-Coupling-Induced Localized Corrosion of AA7050-T7451

Dissimilar metal assemblies are frequently encountered in complex structures such as in aerospace applications and pose a major challenge involving galvanic-induced corrosion. One example of such assemblies is that of precipitation-strengthened aluminum alloy (AA) components and stainless steel (SS) fasteners. These precipitation-strengthened AA components are inherently susceptible to localized corrosion due to micro-galvanic interactions that develop between the Al matrix and the constituent (or intermetallic) particles that form as a result of the strengthening process. In addition, the strengthening process renders these AAs unsuitable for traditional welding, and as an alternative, they are commonly joined with 316SS fasteners. Although the structures are coated to mitigate galvanic-induced localized corrosion of the AA component, defects in these coating systems are inevitable during the operational and maintenance life cycles of the AA-based structures. In natural corrosive environments, including the thin electrolyte films present under atmospheric conditions, this situation would often create a macro-galvanic cell in the vicinity of the fastener joint, including within the confined fastener crevice, between the localized corrosion-susceptible base AA component – the anode in the galvanic cell – and the 316SS fastener – the cathode in the galvanic cell – in which the AA component may be polarized above critical potential thresholds (e.g., pitting and repassivation potentials), resulting in greater localized corrosion than if there was no external 316SS cathode. These aggravated attack sites, particularly the hidden deep fissures within the fastener crevice, can serve as locations for crack initiation with the potential for accelerated crack propagation rates, which would be detrimental to the service and fatigue life of the AA-based structure. This situation has warranted an increasing body of research with regards to the macro-galvanic-induced corrosion of these AAs with the intent to develop an understanding of the physical, electrochemical, and metallurgical factors that govern both the location and mode of corrosion damage in a fastener geometry, including the rationalization of how an inhibitor, such as chromate, can influence these factors. Although extensive work has been carried out on chromate effects on the micro-galvanic corrosion behavior of base precipitation-strengthened AAs, information is generally lacking on the impact of chromate on the damage distribution and morphology pertinent to complex geometries such as plate-fastener configuration and how the unique fastener crevice environments affect the activity of chromate compared to the boldly exposed surface conditions under atmospheric conditions. Although the dangers of chromate have been widely recognized, with a mandate to phase out chromate-based inhibitor systems, a comprehensive understanding of its role in the galvanic-induced corrosion phenomena is crucial for the optimization of testing frameworks and design of environmentally-friendly inhibitor technologies needed to replace toxic chromate-based corrosion mitigation systems. This work aims to improve the mechanistic understanding of the galvanic-induced localized corrosion of AA7050-T7451 coupled to 316SS in simulated atmospheric environments from the perspectives of damage distribution dependence on cathodic activity and inhibitor action on the main cathodic contributors. This understanding is accomplished by means of a combination of experimental and modeling techniques through three main areas: 1) characterization of the baseline cathodic behavior of AA7XXX in an attempt to establish the factors that affect the degree of suppression of cathodic reactions on this class of AAs, 2) assessment of the current capacities of the individual cathodic contributors to drive anodic dissolution of AA7050 in simulated atmospheric environments, 3) correlation of the findings from areas 1 and 2 to those attained on actual AA7050-316SS galvanic couples under various environmental conditions pertinent to atmospheric exposure.

The rotating disk electrode (RDE) 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 10-2 M chromate, however, the degree of inhibition was observed to depend on the Cu content of the alloy. Chromate was reduced irreversibly to form a Cr(III)-rich film on the alloy surface that blocked cathodic sites and hindered the 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 Cr(III)/Cr(VI) oxides with some metallic Cr.

The RDE technique was extended to investigate the impact of chromate on the cathodic behavior of 316SS and high-purity Cu in dilute and elevated chloride environments. The objective was to determine which electrode would be the more significant cathode that would drive galvanic corrosion of AA7050 as a function of environment, and to assess the role of chromate in modifying their cathodic activity to mitigate the attack. In 0.6 M NaCl, the ORR kinetics on 316SS was suppressed by more than two orders of magnitude upon addition of 10-4 M chromate, and high-purity Cu produced the largest cathodic currents with and without chromate in solution. In 5 M NaCl, chromate was assessed to be less effective on all the studied cathodes, with high-purity Cu remaining the most significant cathode. Upon introduction of Al ions to 5 M Cl- in order to simulate pre-corrosion of AA7050 leading to the release of Al3+ into a concentrated chloride environment, cathodic reactions were accelerated on the electrodes regardless of the presence of chromate. In this situation, 316SS became the dominant cathode compared to high-purity Cu.

The efficacy of chromate in protecting AA7050 coupled to 316SS in simulated external and fastener crevice environments was investigated utilizing a number of electrochemical and surface characterization techniques. The influence of pH and Al ions 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 ORR. The addition of Al ions 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.

An existing steady-state finite element method (FEM) modeling framework based on the Laplace equation was adapted to predict the macro-galvanic current distributions on an AA7050-316SS couple, in environments representative of the boldly exposed surface of an actual fastener couple. Boundary conditions had to be modified from that of the bulk experimental environment in order to achieve better agreement between the total currents calculated with the model and those measured experimentally with the scanning vibrating electrode technique (SVET) at a certain height above the electrode surface. Once validated, the model was used to predict the current densities at the electrode/electrolyte interface and better interpret the results obtained with SVET, even though it was incapable of capturing localized events that occurred experimentally such as pitting and precipitation of corrosion products.

Lastly, the galvanic current interactions on an AA7050-316SS couple in chromate-containing NaCl environments under thick films and conditions of wet-dry cycling were investigated utilizing the coupled micro-electrode array (CMEA) approach. The CMEA approach provided a means to analyze the in-situ electrochemical kinetics as a function of spatial location and time. In Inhibitor-free environments, the total net anodic charge increased with increasing conductivity and aggressiveness of the environment, with AA7050 electrodes supplying greater than 50% of the total net cathodic charge in the more aggressive environments. Under thick films, chromate was less effective at suppressing cathodic kinetics on the 316SS and AA7050 net cathodes as chloride concentration increased. Under wet-dry cycling conditions, the effectiveness of chromate was diminished when compared to thick film conditions, due to the alternation in equilibrium chloride concentration as electrolyte thickness changed upon onset of drying and wetting. Furthermore, chromate exhibited a diminished ability to suppress cathodic currents on the AA7050 net cathodes in comparison to the 316SS electrodes.

This dissertation provided clarifications and further insights into the mechanistic understanding of the galvanic-coupling-induced corrosion phenomena as regards precipitation-strengthened AAs, including the shortcomings of chromate as regards protection of these AAs when exposed to conditions pertinent to aerospace service environments. This work highlighted the importance of Cu-rich intermetallic particles and replated Cu when considering the driving force of cathodes in sustaining anodic dissolution in typical AA macro-galvanic systems exposed to atmospheric conditions. From the technological perspective, it is intended that the knowledge acquired through this work will aid inform the principal attributes that should be exploited in the testing and development of potential chromate-free inhibitor alternatives.