The use of low-density materials in body panels is increasing as a measure to reduce the weight of vehicles. Honda has developed an aluminum/steel sheet hybrid door—debuting on the 2014 Acura RLX—that is more effective in reducing weight than an all-aluminum door, according to Shoji Kimura of Honda R&D Co., Ltd. Kimura will present details on the hybrid door’s development as part of the “Advances in Lightweight Materials” technical session being held April 16 at the SAE 2013 World Congress in Detroit.
Because aluminum was used in the door skin, bimetallic corrosion at the connection between the aluminum and the steel sheets represented an issue. The difference in the electrical potential of the two metals might promote corrosion at the connection between the aluminum door skin and the steel sheet door panel, in particular at the lower edge of the door, where rainwater and other moisture tends to accumulate.
To address this issue, a watertight structure realized through the use of a high-ductility sealer was employed to help prevent water from infiltrating to the connection between the metals, and steel sheets with a zinc-aluminum-magnesium alloy coating—highly effective in controlling bimetallic corrosion—were employed in the door panels. This produced rust-resistance specifications for the hybrid door able to maintain durability in market use environments.
Corrosion potential of hybrid door
Automotive doors are made up of a skin panel and a door panel, and steel sheets are conventionally used for both panels. By contrast, the hybrid door developed by Honda substitutes aluminum for the skin panel. The door panel and skin panel are joined by means of a hemming structure.
When different metals are in contact in a corrosive environment, the difference in electrical potential between the metals results in the formation of local batteries, and preferential corrosion proceeds in the base metal. Because Honda’s hybrid door uses aluminum for the door skin and steel sheets with a hot-dip galvanized alloy coating (GA) for the door panel, it was necessary to consider bimetallic corrosion between the aluminum and the zinc coating. Zinc displays a more base electrical potential than aluminum in water containing neutral salts, and preferential corrosion would therefore first occur in the zinc-plating layer.
The corrosion of zinc produces OH-, creating an alkaline environment that further accelerates the corrosion of the metal. The zinc in the coating layer would disappear rapidly, and bimetallic corrosion would occur between the aluminum and the steel, with the aluminum undergoing preferential corrosion. As a result, there was a possibility that corrosion would result in a decline in the appeal of the exterior of the hybrid door or cause holes to appear.
Because the existence of water containing chlorides is essential to bimetallic corrosion, attempts to prevent the infiltration of water are the major anti-rust measure.
In the hybrid door, in addition to the displacement produced when the door was open and closed, the difference in the coefficients of linear expansion of the aluminum and the steel sheets would also produce displacement, and there was a possibility that water-tightness could be affected due to the peeling of the sealer. Honda researchers developed a water-tight structure—even against slamming of the door, the heat used in baking the coating, and temperature changes in the market environment—using a high-ductility sealer and coating structure.
Selection of rust-resistant coating
If the interior of the hemming structure was completely shielded from the infiltration of water by the high-ductility sealer, bimetallic corrosion could not occur. However, in bringing a hybrid door to market for the first time, consideration was given to factors such as the occurrence of issues originating in the sealer materials or the production process, or the exposure of the door to unforeseen loads in the market. The use of steel sheets with a high-rust-resistance coating to help ensure sufficient corrosion resistance if water did by some chance infiltrate the door was therefore studied.
The following properties were considered to be demanded of a steel sheet with a high-rust-resistance coating:
• To protect the aluminum, the coating layer on the steel sheets should be formed from a metal that is more base than aluminum
• To extend the life span of the coating layer, the coating should possess self-protective properties able to control corrosion
• The coating should display high corrosion resistance in alkaline environments.
The corrosion potential of steel sheets coated with zinc-aluminum-magnesium alloys (Zn-Al-Mg) is more base than that of aluminum, and these alloys are therefore subject to preferential corrosion when in contact with aluminum. However, they are also known to form a dense basic zinc-chloride protective film on their outermost layer during the initial stage of corrosion and thus display a high level of corrosion resistance. They also display good corrosion resistance in alkaline environments.
Zn-Al-Mg-coated steel sheets were selected, therefore, and their effectiveness in controlling bimetallic corrosion when in contact with aluminum was verified.
Using test specimens, the corrosion resistance of aluminum/GA and aluminum/Zn-Al-Mg combinations was compared using Honda’s basic mode combined corrosion test (CCT).
In the case of the aluminum/GA combination, after 10 cycles the GA coating layer was lost and corrosion of the Al commenced. After 30 cycles, rust holes appeared in the Al. By contrast, in the case of the aluminum/Zn-Al-Mg combination, there was no loss of the coating layer and no corrosion of the aluminum could be observed even after 90 cycles. These results verified the effectiveness of the Zn-Al-Mg-coated steel sheets in controlling bimetallic corrosion.
The tested door had been electrodeposition-coated and a dust sealer had been applied, but structural parts were not fitted.
The researchers found there was no decline in the appeal of the external appearance of the skin panel and door panel, and no corrosion could be observed even in the interior of the hemming structure, indicating the achievement of adequate control of bimetallic corrosion.
Comprehensive durability driving tests conducted to evaluate the multiple durability demands on vehicles also verified the durability of the doors.
This article is based on SAE International technical paper 2013-01-0386 written by Shoji Kimura of Honda R&D Co., Ltd.