High-Quality Corrosion Protection for Lightweight Components

Zinc flake coatings provide long-term corrosion protection for vehicle parts and help to reduce the overall weight of the vehicle. At the Automotive Center Südwestfalen investigations have been made into the potential for reducing the thickness of sheet metal and component walls. 

Because of the risk of corrosion, the sheet metal used in vehicles is often thicker and mechanically stronger than is absolutely necessary. A study carried out on behalf of Dörken MKS by the Automotive Center Südwestfalen (ACS) investigated the extent to which sheet metal can be made thinner if high-performance corrosion protection coatings are applied with the aim of reducing the quantity of material used and the weight of the components. 

Simulated tests to reduce sheet metal thickness

The first stage of the study involved evaluating a simplified model of a steering arm, consisting of a steel U-section, to identify how far the wall thickness of the component could be reduced without affecting the functionality of the real component or failing to meet the manufacturer's requirements. Then the finite element method was used to determine the buckling load of a U­section made from steel with the conventional thickness of 2.6 millimetres. 

The simplified steering arm created for the purposes of the study: a U-section with a thickness of 2.6 mm (length 300 mm).; Source: acs - Automotive Center Südwestfalen GmbH

Then the finite element method was used to determine the buckling load of a U­section made from steel with the conventional thickness of 2.6 millimetres. 

Local maximum tension measurements in a genuine steering arm under a tensile load. (Source: acs - Automotive Center Südwestfalen GmbH)

The test indicated that the U­section would buckle under a force of 49.2 kN. As the manufacturer specified a buckling strength of only 35 kN for its steering arms, depending on the vehicle model, it was clear that the steel was much thicker than actually required. Further simulations were run to determine the reduction in the thickness of the U-section that could be achieved with a buckling load of at least 35 kN. These simulations showed that the steel in the U-section could be reduced to a thickness of two millimetres and still provide the required buckling strength. This would allow for a reduction in mass of around 31 percent.

Components with thinner walls and identical performance 

The next stage of the ACS study involved using a practical test set-up to investigate the influence of corrosion on the mechanical properties of the component. A three-point bending test was chosen for this purpose because it is less susceptible to component design differences than buckling load tests. First of all, a simulation was used to determine the minimum bending stiffness. In the model set­up, the simulated U-section with a wall thickness of two millimetres had a bending stiffness of 3.1 kN/mm. 

The minimum metal thickness of 1.95 mm needed to withstand the buckling load was evaluated in different three-point bending tests. (Source: acs - Automotive Center Südwestfalen GmbH)

In the practical tests, two steel U-sections of different thicknesses and with different coatings were evaluated: a 2-millimetre-thick U-section with a high-performance zinc flake basecoat and topcoat from Dörken MKS and a 2.6-millimetre­thick U-section with a phosphate coating and a subsequent electrocoating. After the coatings had been applied, a stone impact was created in the buckling area of the test specimens (in accordance with ISO 20567) and these were then exposed to corrosion in three different ways: the salt spray test (in accordance with ISO 9227), the accelerated corrosion test (ACT 2) and the VDA test (in accordance with VDA 233-102). 

Zinc flake coating protects against red rust 

After 1,000 hours in the salt spray test, no red rust had formed on the test specimen coated with the zinc flake system. During the cyclic exposure and corrosion phase, the results of the accelerated corrosion test and the VDA test were also clear. The 2-millimetre-thick U-section with a zinc flake coating showed no red rust after six cycles, but red rust was visible on the 2.6-millimetre-thick U-section with the phosphate coating and the electrocoating. 

Das mit dem Zinklamellensystem von Dörken MKS beschichtete Blech (Dicke 2 mm) nach der Salznebelsprüh-Prüfung.; Bildquelle: acs - Automotive Center Südwestfalen GmbH

Finally the coated test specimens were subjected to a three-point bending test (based on ISO 14125). The Automotive Center Südwestfalen determined that the bending stiffness of the U-sections with a wall thickness of 2 millimetres (both be­fore and after exposure to the corrosion tests) was between 3.2 and 3.4 kN/mm. The results for the U-sections with a wall thickness of 2.6 millimetres were around 4 kN/mm before exposure and 3.8 to 3.9 kN/mm after exposure. 

Following the various corrosion tests, the component with the zinc flake coating met both the corrosion standards and the mechanical requirements and, in particular, had the stiffness specified in the manufacturer's requirements specifica­tion. The component with walls 0.6 millimetres thicker and a phosphate coating and an electrocoating was able to meet the mechanical requirements because of the greater thickness of the material, but showed clear signs of red rust. 

New options for reducing weight 

In the C3 corrosivity category (as spec­ified in ISO 12944), which relates to an urban and industrial atmosphere, the reduction in the thickness of the component equates to between 25 and 50 micrometres per year. Over a period of ten years, this would amount to 0.5 millimetres. This means that the design of the component using the thicker material takes into consideration the corrosion that occurs during a ten-year life cycle. A comparison of the two components shows that a corrosion protection solution designed specifically for the part can allow the thickness of the sheet metal and also the weight to be reduced significantly, while still fully complying with the car manufacturer's safety requirements. 

Reducing the wall thickness of vehicle components, including the steering arms tested here, opens up a wide range of op­tions for lowering their weight and for introducing new types of lightweight structures. 

The ideal solution to meet car manufacturers' challenging requirements is steel sheet that is designed to withstand the relevant loads combined with a highly resistant zinc flake coating.