Galvanic corrosion of samples consisting of coated aluminium alloy 7075-T6 panels connected to stainless steel 316 fasteners was examined. Two coating systems were evaluated, one containing chromate pretreatment and primer along with a topcoat, and the other a chromate free system. No intentional defects such as scribes were imparted to investigate the attack associated with intrinsic defects in the coatings. The galvanic current between the fasteners and panel was greater for the assembly with chromate-free coatings than for the one with chromated coatings, and for the exposure to a salt spray chamber than for immersion. The attack was much deeper for the chromated system and more localised. The galvanic current increased with time and then was equal to that measured for a sample with an intentional scribe.
The influence of the condition of through-holes on the corrosion of galvanic test assemblies was studied. Holes in coated aluminium alloy 7075-T6 were tested as received, in a worst-case bare condition and with extra protection. Regions away from fastener holes were also tested. Corrosion performance was assessed by electrochemical impedance spectroscopy in immersed conditions. Defects in the coatings at through-holes of as-received panels resulted in degraded corrosion performance, which could be mitigated by the application of additional protection. The presence of a fastener in the hole affected the corrosion performance for all three conditions even without galvanic coupling interactions. Galvanic interactions were studied on panels exposed to ASTM B117 salt fog conditions, where the through-hole condition also had a large effect.
Previous studies have shown how galvanic coupling between stainless steel 316 fasteners and a coated aluminium alloy 7075-T6 test panel could accelerate the coating degradation during exposure to corrosive environments. In this work, impressed current between the fastener and panel was evaluated for use instead of simple galvanic coupling. Two coating systems, including a chromate-containing and a non-chromated coating system, were exposed to ASTM B117 and cyclic salt spray exposure environments. The coatings were scribed prior to exposure to further accelerate the corrosion rate. The nature of attack depended on the magnitude of the applied impressed current and the exposure environment for the chromate-free adhesion promoting surface pre-treatment. However, the nature of the attack did not depend upon the magnitude of applied current for the chromated pre-treatment system. The response of the current during drying and rewetting also varied depending on the coating system.
Previous studies have shown how galvanic coupling susceptibility between stainless steel 316 or titanium alloy fasteners and coated aluminum alloy 7075-T6 depends on the chosen coating system and environmental factors such as relative humidity and chloride concentration. In this study, several machine learning models were developed to predict, analyze, and quantify galvanic corrosion arising between relatively noble fasteners and coated aluminum alloy panels. Different independent factors including pretreatment, primer coating, topcoat, relative humidity, chloride concentration, fastener material, fastener quantity, existence of a defect, type of environment, and time of wetness were evaluated for their effect on galvanic coupling lost volume. Artificial Neural Networks (ANN), Random Forest Regression (RFR), and Multiple Linear Regression (MLR) were used to develop a damage function for galvanic corrosion. ANN, RFR, and MLR models all showed a reasonable fit for lost volume as a function of different inputs.
A galvanostatic test panel design consisting of a coated and scribed 7075‐T6 Al alloy as a working electrode and an uncoated through‐hole 316 stainless‐steel fastener as a counter electrode has been used to accelerate galvanic corrosion and compare various primer coatings and pretreatments and different impressed passed currents in an ASTM G85‐A2 environment. The relative humidity, chloride concentration, and impressed current magnitude were changed to investigate their effects on galvanic corrosion. During the chamber exposure, the electrochemical measurements showed fluctuations in the measured current and potential. Various corrosion morphologies, depths, and volume loss were recorded for panels with different coating systems. Electrochemical measurements, optical profilometry, and photography were used to characterize and measure the corrosion performance of different coating systems. Also, electrochemical impedance spectrometry was used to compare the corrosion resistance and porosity of different coating systems.
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