A new generation of weldable coating

Abstract

The use of weldable primers for automotive sheeting to improve corrosion performance, reduce system costs and complexity in the paint shop by outsourcing sub processes is on an upward trend in Europe. Outsourcing can, of course, only be accomplished when the Original Equipment Manufacturing (OEM) requirements for high performance coating systems are met. The requirements break down as follows: (i) Enhanced corrosion protection: this is a permanent challenge in the automotive industry and one that precoated sheet can meet. (ii) A lean and clean process: precoated sheet makes it possible to reduce the scope of cavity waxing and seam sealing, depending on the design. (iii) Eco-compatibility: new production processes will only deliver a competitive edge when they can satisfy both economic and ecological goals. One major OEM has been using weldable primers on their vehicles for several years. They have increased the perforation warranty on new vehicles in Europe to 30 years, due in large part to the increased corrosion performance obtained with weldable primers in cavities and flanges. Perforation warrantees from other European OEMs have also seen dramatic increases in the last few years and some of these OEMs are preparing to use or are evaluating pre-primed sheets in order to improve the corrosion performance in cavities and flanges. These coatings also offer other benefits, such as improved forming, lower oil usage and improved adhesive bonding. In the past, the key issues that limited the use of weldable coatings on cold rolled steel were dusting and abrasion during the blanking and stamping processes, and welding issues as well. In the 1980s, zinc coated steel was introduced to the automotive industry, improving the corrosion performance, especially a dramatically improved perforation resistance such that weldable coatings were deemed unnecessary. One niche product was in use at Ford at that time for all bonnets because of its outstanding stone chip resistance, but it was not weldable. Since 1997, new types of weldable primers have been introduced to the automotive industry. One of the driving forces for the use of weldable primers is to increase the corrosion performance of interior cavities, hem flanges and lap joints. Areas that are difficult to clean, pretreat and paint on an assembled part are precoated with a protective chrome free pretreatment and primer while the sheet is in coil form. The flat coil is the ideal substrate to clean and treat effectively and uniformly. When the parts are later assembled, a uniform protective coating is present in all, otherwise difficult to treat, areas. As a result of the uniform corrosion resistant coating being applied prior to stamping and assembly, the costly practice of applying cavity waxes and flange sealers can be eliminated, depending on the design of the car body, without sacrificing corrosion protection. Cavities and flanged joints are particularly susceptible to corrosion, because they cannot be protected by phosphating or e-coating. These interior spaces are inaccessible to the chemicals in downstream processes. Sheets that have been precoated with weldable coatings offer built-in protection in flanged areas. The effectiveness of pre-primed sheets is impressively documented in the magazine Autobild issued on 20 August 2004. It presented the results of a long running corrosion tests. The German technical inspection agency TUV gave the best score for rust precaution measures to the Daimler-Chrysler A-class model. The steel industry currently supplies weldable coated sheets to a major OEM in Germany and the USA and there is a lot of interest from other OEMs worldwide. The biggest hurdle to overcome in achieving a worldwide ‘breakthrough’ of precoated steel in the automotive industry is the additional cost of the weldable coated product itself and the cost of the coating process. However, the demand for continually improved corrosion protection of vehicle body parts remains. To meet these challenges, there is a need for a low cost high performance pretreatment weldable primer system. The new generation of weldable coatings go a long way to meet these challenges. This is based on the new chemistry of the coating formulation. In spite of a reduction of the film thickness by 50%, which in turn reduces the cost of application of the thin organic coating by 50%, this new generation coating process shows a significantly improved corrosion protection. The application of this process includes a short cure time, typically below 30 s and requires a peak metal