Electrolytic Coating Sheet-Steel Coating Delivers Cathodic Corrosion Protection for Hot Stamping

Author / Editor: Doug Sellers / Jürgen Schreier

Just as galvanizing steel provides long-lasting corrosion protection for cold-formed automotive parts, the GammaProtect® electrolytic coating for steel to be hot-stamped yields an effective antirust finish that withstands the very high temperatures involved in hot stamping. The high-melting-point coating developed by ThyssenKrupp Steel protects against scaling and also prevents cathodic corrosion as actively as the galvanization process does for cold-forming steels. It expands the range of applications for hot stamping to include parts in the “wet” area of the car body.

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The strengths of GammaProtect are apparent especially in direct hot stamping, where steel sheet is heated to around 900°C and then stamped and cooled in a special die. The rapid cooling produces components with strength measurements to 1,650 MPa, thus allowing parts to have thinner walls and lower weight. With a zinc content high enough for cathodic corrosion protection but a composition that ensures a melting point above 870°C, GammaProtect avoids problems associated with zinc liquefaction so that automakers can exploit the advantages of low-cost direct hot forming without having to forgo active corrosion protection.

The coating is sufficiently resistant to tolerate timing variations in production cycles, which ensures process stability. Also, sheet steel coated with GammaProtect can be heated very rapidly, including by such advanced, high-speed techniques as induction and infrared radiation; this enables the holding time in the furnace to be quite short.

GammaProtect has been tested and found suitable for all typical auto industry processing steps. It accommodates part joining by means of adhesive bonding, resistance spot welding and MIG/MAG welding. In addition, the coating is amenable to tailored tempering (image), used to produce hot-stamped parts with customized strength and elongation properties.

ThyssenKrupp Steel AG

Duisburg, Germany