Development of a One-Coat Organic as a Replacement for Conventional Two-Coat Composite Systems on Zinc Nickel

Conventional prepainted organic coatings on zinc nickel substrates consist of a dried-in-place chrome bearing pretreatment layer followed by a thin film solvent-based organic topcoat. This system is generally applied in order to provide additional corrosion protection to the brittle zinc nickel layer subsequent to forming. An improved version of this organic/metallic coating system has been developed which consists of applying a one-coat water-based chrome bearing epoxy to zinc nickel material. In addition to the production efficiencies gained by applying a one-coat prepainted system, enhancements are also realized with regard to corrosion resistance and formability characteristics. It has also been found that a novel curing technique can be utilized in order to allow for higher temperature curing of prepainted materials on bake hardenable substrates without adversely affecting mechanical properties.

One of the first uses of prepainted steel within the automotive industry was ZINCROMETAL™: that is, a chrome primer coated with a zinc-rich epoxy topcoat on the unexposed side of Cold Rolled Steel sheet. During the 1980s, a new product evolved in which an electroplated zinc nickel substrate was coated on one side with a thin film (1μm) two-coat organic system. Initially it was thought that zinc nickel would inherently provide both excellent chloride corrosion resistance, coupled with good welding and formability characteristics. However, zinc nickel itself proved to be less corrosion resistant in field service that was indicated by accelerated laboratory testing. In order to inhibit these corrosion problems, two-coat composite organic systems were developed for the unexposed side of the strip. Until now, these two-coat thin film systems have consisted of a dried-in-place chrome-bearing primer and a solvent-based epoxy topcoat. The organic coating acts as a barrier film, inhibiting catastrophic corrosion from initiating during field service. Without this barrier film, it would be necessary to increase the coating weight of the zinc nickel layer to a level which could adversely affect welding and formability in order to enhance corrosion resistance.

Recently, a significant advance in paint technology has enabled further development of the coated zinc-nickel product. The paint is a chrome bearing water based epoxy resin which can be applied as a one-coat system to one or both sides of the strip. The thickness of this organic coating is approximately 1μm and does not interfere with welding or electrocoating parameters. However, many advantages are achieved with this system. First, it can be applied in one coat, rather than as separate primer and topcoat system, thereby streamlining the painting process. As a water-based epoxy system, it has environmental advantages with respect to disposal and emission regulation. Along with manufacturing advantages, this very thin solid film coating also has the ability to act as a pretreatment, is not moisture sensitive and is compatible with most adhesives and electrocoat paints. The formability of the substrate is often enhanced due to the low coefficient of friction even in the absence of blanking or press lubes. The coating does not appear to age, and shelf life of the painted substrate is limited only by the aging characteristics of the base metal (such as bake hardenable grades). Excellent corrosion resistance can be achieved, surpassing even that of the conventional two-coat varieties.

The substrate peak metal temperature necessary for organic curing and chrome conversion of this material is higher than the 165-177C needed for conventional two-coat systems. However, curing temperatures as low as 193C on bake hardenable (BH) grades have proven to be suitable when utilizing convection ovens. The more normal 218C curing condition can be achieved even on BH steel without the advent of yield point elongation when utilizing very fast induction curing techniques for as little as two seconds.

This paper describes the material properties of this new one-coat organically coated zinc nickel substrate as it relates to the characteristics necessary for successful utilization by the automotive industry. Specifically, chrome dissolution, corrosion resistance, weldability, wet adhesion, formability and paintability will be examined and compared with conventional two-coat systems. In addition, these properties will be analyzed with respect to the peak metal temperature necessary to fully cure this paint system.