Potentiodynamic polarization curves of AA7075 at high scan rates interpreted using the high field model

The automotive industry's push for lightweighting to improve energy efficiency and reduce greenhouse gas emissions has led to the increased use of aluminum alloys (AAs). However, the presence of more noble alloying elements in AAs can create micro-galvanic coupling, resulting in localized corrosion and potential mechanical failure. Potentiodynamic polarization (PDP) experiments are commonly used to investigate corrosion, but the corrosion metrics extracted using Tafel kinetics are sensitive to fitting procedures and experimental parameters. Low scan rates (0.167 mV s⁻¹) are recommended to avoid irreversible surface changes, but these measurements can be affected by the accumulation of corrosion products. Microscale PDP, necessary for mapping a sample's electrochemical properties, is often conducted at high scan rates (>10 mV s⁻¹), leading to deviations from steady-state conditions and inaccurate Tafel parameters. Previous studies have attributed these high scan rate distortions to double-layer capacitance and mass transport, but the role of active dissolution and passivation kinetics has been largely overlooked. This study aims to analyze PDP curves of AA7075 at high scan rates (up to 100 mV s⁻¹) in different electrolytes using numerical simulations incorporating both double-layer capacitance and the high field model to better understand the kinetics of oxide growth on the aluminum alloy during the corrosion process.

The literature highlights inconsistencies in the choice of optimal parameters for potentiodynamic polarization (PDP) experiments, leading to difficulties in interpreting corrosion kinetics. While low scan rates (0.167 mV/s) are often recommended, they can be affected by irreversible surface changes and accumulation of corrosion products. High scan rates used in microscale PDP experiments deviate from steady-state conditions, leading to inaccurate Tafel parameters. Previous work primarily attributed the distortions in PDP curves at high scan rates to the charging current associated with double-layer capacitance and mass transport effects. However, the influence of the metal's active dissolution and passivation kinetics at high scan rates has received limited attention, despite the extensive research on the kinetics of oxide growth on metals. Models like the high field model and the point defect model demonstrate an inherent dependence on scan rates, suggesting their potential importance in understanding high scan rate PDP behavior. Studies on aluminum passivation using these models indicate a potential explanation for the observed changes in PDP at higher scan rates.

AA7075 samples were prepared by light abrasion, rinsing with ethanol and Milli-Q water, and drying. Electrochemical measurements were performed using a three-electrode corrosion cell with a saturated calomel electrode (SCE) as the reference, a platinum mesh as the counter electrode, and a 1 cm² exposed AA7075 sample as the working electrode. The electrolytes used were naturally aerated 0.62 M NaH₂PO₄ (acidic) and 3.5 wt% NaCl (near-neutral). Before each PDP measurement, the sample was left under open circuit potential (OCP) until stabilization. A potential of -0.25 V vs. OCP was applied for 20 s to minimize transient effects. PDP curves were obtained at scan rates of 0.167, 5, 25, and 100 mV s⁻¹ from -0.25 V vs. OCP to 1 V vs. SCE. Five replicates were performed at each scan rate for statistical analysis. Electrochemical Impedance Spectroscopy (EIS) was performed in the frequency range of 10⁵ to 1 Hz at various potentials and sample conditions to determine capacitance. X-ray Photoelectron Spectroscopy (XPS) was used to measure the air-formed oxide film thickness. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was used to measure the dissolution rate of Al₂O₃ in the electrolytes after 24-hour sample immersion. Numerical simulations of PDP curves were performed using COMSOL Multiphysics, incorporating faradaic, capacitive, and high field model components to predict PDP behavior across different scan rates.

Experimental PDP curves showed significant changes in shape and apparent corrosion parameters (*E*_corr^app and *j*_corr^app) with increasing scan rates in both electrolytes. Attempts to model these changes using only capacitive current were unsuccessful, even when considering a wide range of capacitance values. The high field model, however, provided a much better fit to the experimental data. This model considers the oxide film growth and dissolution rate as rate-limiting steps rather than charge transfer. By solving a system of equations describing oxide growth (migration of OH⁻/O²⁻ and Al³⁺ species), dissolution, and considering a Tafel-like expression for the cathodic current, the simulations accurately predicted the trends observed in the experimental PDP curves. The simulation shows that the anodic high field current (*j*_hf) is the dominant factor in explaining the characteristics of PDP emerging at higher scan rates, even when accounting for capacitive current. Parameters from the high field model, such as initial oxide thickness (*d*₀), dissolution rate (*R*_diss), and passivation efficiency (*ε*_p), offer insights into oxide layer properties and behavior. Increased variance in replicate PDP measurements at higher scan rates is attributed to the sensitivity of the high field model to small variations in the initial oxide layer thickness, leading to significant differences in the anodic current. The use of a buffered electrolyte solution helped stabilize the steady-state current (*j*_ss) by preventing significant surface pH changes, further enhancing the model's predictive power. The model also accurately predicts the shift in apparent corrosion potential with increasing scan rate.

The findings demonstrate that the high field model, incorporating anodic oxide growth kinetics, is superior to using only capacitive current considerations in explaining the distortions observed in high scan rate potentiodynamic polarization (PDP) curves of AA7075. The model successfully predicts the changes in PDP curves across different scan rates and electrolytes with a single set of parameters. The model's parameters have physical meanings relevant to the oxide layer, offering valuable complementary information to conventional PDP metrics. This study is crucial for establishing microscale PDP measurements as quantitative techniques. However, there is a need for improving the accuracy of high field model parameter characterization and a deeper understanding of localized corrosion phenomena such as micro-galvanic coupling and local oxide breakdown caused by chloride ions in relation to the high field model.

This research establishes the high field model as a superior framework for interpreting potentiodynamic polarization (PDP) curves of AA7075 at high scan rates. The model successfully explains observed trends better than models based solely on capacitive current. Future work should focus on improving the accuracy of high field model parameter characterization and understanding the effects of localized corrosion events on the model's predictions. This improved understanding will lead to more accurate quantitative analysis of high scan rate PDP measurements, particularly relevant for microscale techniques.

The high field model simulations rely on several fitting parameters, which require accurate characterization for enhanced model validation and predictive accuracy. The model's applicability might be limited in specific electrolytes or AA systems where some assumptions (such as constant properties of the oxide layer or constant surface pH) might be violated. The model does not fully account for localized corrosion events such as pitting corrosion. This limitations are explicitly stated by the authors.