Development of a Composite Coating for Pre-Coated Automotive Sheet Metal 862027
The development of a new corrosion-resistant coating composite for automotive sheet metal is described. The three-layer coating combines the galvanic properties of a thin electroplating with the passivating and barrier protection of an inorganic intermediate layer and an organic outer layer. The combinedeffect of the composite is to act as three lines of defense against corrosion. The organic top layer provides barrier protection, while the inorganic intermediate layer provides passivation to slow down corrosive reactions once the barrier is breached. The plating layer provides galvanic action as the last line of defense, being called into play only after the initial two coatings are exhausted. The zinc coating then corrodes galvanically to protect the underlying steel.
Corrosion tests indicate that this combined coating is more effective than the individual coatings alone. This composite thus allows a relatively thin zinc electroplating to be used, and welding and forming problems are minimized. Incorporation of internal lubricants in the topcoat allows deliberate control of the lubricity of the coating to permit maximum latitude in forming operations. Metallic pigments in both the inorganic and organic layers form electrically conductive pathways necessary for welding. The baked-on organic surface is compatible with adhesives, thus allowing further freedom in manufacturing.