Adhesive bonding of aluminum and steel hybrid-structured vehicles

Significance 

Due to the stringent environmental directives, the current goal in the automobile industry is to reduce both fuel consumption and exhaust emission without compromising on the safety regulations of the vehicles. To this end, more lightweight materials, especially aluminum alloy (AA), have been incorporated in the design of vehicles to reduce weight. However, light alloys cannot be used to replace some vehicle steel components due to their unfavorable mechanical properties. In such cases, advanced high-strength steels of different grades, like dual-phase steels (DPS), have been fabricated in reduced thickness.

DPS generally consists of martensite and ferrite phases and is mainly characterized by low yield ratio. Besides, high work-hardening ratios are commonly used for components requiring good formability, high crashworthiness and high strength. Recently, microalloying has been used to improve the properties of steel. Unlike conventional DPS grades, microalloyed DPS exhibit superior machining characteristics. This explains why microalloyed DPSs are coupled with 7075-T6-AA in some vehicle lap joint assemblies.

Bi-metallic joints are highly susceptible to corrosion under inadequate protection. This may induce electrolyte wicking and trapping in the joint’s crevice, resulting in galvanic interactions between the materials. Both crevice and galvanic corrosion are associated with promoting failure mechanisms that compromise the applications of these materials. In the automotive industry, advanced adhesives have brought out new galvanic corrosion challenges. Whereas different materials can be coupled safely by implementing adequate corrosion protection measures, assessing the level of corrosion protection needed is imperative.

Galvanic corrosion is influenced by numerous factors, including electrolyte conductivity and anode-to-cathode surface-area-ratio. Although galvanic corrosion problems have been extensively studied, there are limited studies on the galvanic behaviors of lap joints incorporating 7xxx-series AA, which are increasingly becoming attractive for automobile applications due to their high strength and low weight. Additionally, the localized and time-dependent nature of these processes is still not clearly understood.

Herein, M.Eng. Vianey Torres, Prof. Rodrigo Mayen‐Mondragon and Prof. Juan Genesca from Universidad Nacional Autónoma de México, UNAM investigated the galvanic corrosion behavior of coupling consisting of a 7075‐T6‐AA and a low‐carbon low‐manganese microalloyed DPS immersed in 3% sodium chloride aqueous solution. Electrochemical impedance spectroscopy and potentiodynamic polarization measurements were performed on individual components to assess the corrosion behaviors of the individual materials. Their work is currently published in the journal, Materials and Corrosion.

The authors reported that for the individual samples, the 7075‐T6‐AA exhibited superior corrosion resistance than the microalloyed 400/315 DPS. However, the corrosion current density of the microalloyed steel was about seven times larger than that of the 7075‐T6‐AA. Thus, the galvanic couple sustained minimal galvanic effect against the expectation when their respective corrosion potentials were compared. Because the negative corrosion potential of the AA was more than that of steel, AA served as the anode of the galvanic couple. This was confirmed by the applied mixed-potential theory and zero-resistance-ammeter measurements that were directly performed on the galvanic couple. The findings further established that using kinetic parameters to predict galvanic effects based on corrosion potentials is more accurate than thermodynamic predictions.

The mixed potential allowed for successful modeling of the galvanic coupling effect. The galvanic current density predicted was close to that determined experimentally via ZRA measurements. In addition, the galvanic effect appeared less severe when the ratio of the galvanic current density to that of aluminum dissolution current was considered. It was important to consider the corrosion current of AA when designing dissimilar joints. Furthermore, it was worth noting that galvanic current densities can be used to provide a rough prediction of the galvanic corrosion strength and to identify the specific measures that might be implemented to avoid further corrosion.

In summary, the galvanic corrosion of 7075-T6-AA/microalloyed DPS bi-metallic couple was assessed. The study provided vital and reliable information about the corrosion performance of 7075‐T6‐AA/microalloyed DPS galvanic couple and is a potential application in adhesive-bonded lap joints. In a statement to Advances in Engineering, Professor. Juan Genesca stated that this information would contribute to the better design of corrosion-free and high-performance lap joints for automobiles.

About the author

Vianey Torres-Mendoza has been an academic technician and professor in the Department of Metallurgical Engineering of the Faculty of Chemistry, FQ, at the National Autonomous University of Mexico, UNAM, since 2006. She is part of the Corrosion Team and collaborates with the GRECCO group, which is part of the Corrosion  Laboratory, LabCorr, of the FQ UNAM.

Currently, Prof. Vianey is in charge of LabCorr, where she focuses on developing methodologies to study the effect of corrosion on bimetallic joints and designs used in the aerospace and automotive industry.

About the author

Dr Juan Genesca is a Professor of electrochemistry and corrosión engineering at the Universidad Nacional Autonoma de Mexico (UNAM). Currently is the General Coordinator of the Unidad de Investigacion y Tecnologia Aplicadas, UNITA UNAM, at the Research and Technology Innovation Park, PIIT, in Monterrey, Nuevo León, Mexico. Juan earned an EngD in Chemical Engineering at the Chemical Institute of Sarria, Barcelona, in 1980.

Juan’s research interest currently focuses on galvanic corrosion and its modeling, and the effect of hydrodynamic conditions on the mechanism and kinetics of CO2/H2S corrosion processes. Juan is the author of 8 books and 110 journal papers. The total citation of his research publications is up to 1556 with the h-index of 21.

Reference

Torres, V., Mayen‐Mondragon, R., & Genesca, J. (2022). Assessment of the galvanic corrosion of bi‐metallic couple 7075‐t6‐aluminum alloy/microalloyed dual‐phase steel. Materials and Corrosion, 73(6), 940–949.

Go To Materials and Corrosion

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