Galvanic Corrosion of Aluminum Alloy Coupled with Ultra High-Strength Steel Under Thin Chloride Solution Layers
Authors: Tran Nam Van , Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: February 2020

Aluminum alloy (Al-alloy) has been employed to replace steel in many automobile sectors for reducing the weight and improvement of fuel consumption, because Al-alloy has excellent physic properties such as high strength and low density. Al-alloys are often coupled with other materials in rivet or welding-plate assemblies. Thus, these joining locations are inherently susceptible to galvanic corrosion under the presence of an electrolyte solution. Galvanic corrosion between Al-alloy and other materials can often lead to failure mechanism in automotive applications. While the dangers of galvanic corrosion are recognized in bulk solution, little work has done to investigate their galvanic corrosion under the limit of electrolyte solution such as droplets or thin layers. These cases are found popularly in atmospheric environments. Where airborne salts deposit on the surface of automotive assemblies, then deliquescing to form droplets or thin electrolyte layers. This research is carried out to investigate the galvanic corrosion between Al-alloy and ultra high-strength steel under thin electrolyte layers.
An Al-alloy and 1180 MPa ultra high-strength steel (HSS) sheets were cut to dimensions of 10×10×1 mm3, then embedded in parallel with a gap of 0.5 mm in epoxy resin. These Al-alloy/ HSS couple cells was mechanically polished to #2000 with SiC papers, degreased with ethanol, and rinsed with distilled water before each experiment. The cell was placed horizontally into a Temperature-Humidity controllable chamber. A 2M NaCl solution layer with different thicknesses of 100, 50, 25 µm was placed on the cell to inspect their galvanic corrosion behavior.
Anodic and cathodic polarization curves for Al-alloy and HSS covered with 2M NaCl solution layers were performed. The open-circuit potential (OCP) of Al-alloy were −0.80 - −0.79 V vs SSE under solution layers with the thickness in range of 25 - 100 µm. A passive current range was observed. However, the effect of solution layer thickness on the passive current was uncertain. The lowest breakdown potential of the passive layer was −0.68 V under the 100 µm-solution layer. Regarding to the cathodic curves for HSS, the OCPs were in range of −0.63 - −0.58 V. The cathodic polarization curves under solution layer with various thickness are similar to that obtained in bulk solution. A distinct limiting current region caused by the oxygen reduction was observed in the potential range −0.68 to −0.96 V, followed by a rapid current increase due to hydrogen evolution. The limiting current density was inversely proportional with the thickness of solution layer.
The galvanic current density (i
g) between Al-alloy and HSS for 72h of coupling under 2M NaCl solution layer with various thicknesses was recognized. i
g increased as decreased the thickness of solution layer. i
g’s were ~38; 32 and 12.5 µA·cm-2 under thickness of 25; 50 and 100 µm, respectively. This higher galvanic current under thinner solution layer is contributed by the dominant of the oxygen diffusion stage. The galvanic potentials were ca. −0.72 V under regardless of the different-thickness solution layers. After 72h of corrosion, localized corrosion was severe on Al-alloy surface. The aluminum hydroxide was precipitated to not only cover the Al-alloy surface (except for the local corroded regions), but also accumulated as a fence between two electrodes. This caused the distinct change of pH value at each electrode. Bubbles were observed on Al-alloy corresponding to the hydrogen revolution process. Besides, amount of red corrosion product found on HSS indicating that HSS was still corroded during coupling.
The EIS measurements were performed after 24, 48 and 71h of corrosion. In these cases, the transmission line model (TML) is useful to simulate the electrochemical behavior of Al-alloy/ HSS couple. The results showed the TML type frequency dependence emerged in the high-frequency range (105 to 103 Hz), which could not be observed in the bulk solution. The phase shift goes further than −50 oC on a plot of θ vs log(f) indicating the accurate corrosion rate can be obtained from the impedance value at low-frequency range. At low-frequency region below 0.5 Hz, a diffusion impedance can be observed. It is attributed to the effect of the diffusion stage of metallic ions inside the local corroded regions.
These results indicated that galvanic corrosion of Al-alloy coupled with HSS is severe under thin solution layers than that under bulk solution. The EIS method is useful to evaluate the galvanic corrosion of Al-alloy under thin solution layers.

Acknowledgments

This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The authors thank to Nihon Parkerizing Co., Ltd for supporting the specimens