Because of their high corrosion protection capacity and surface quality, Extragal® coated products are recommended for numerous automotive applications, for both exposed and non-exposed parts.
The Extragal® production process, involving a continuous single-step operation after rolling, makes it nearly always the most cost effective pre-coating solution for obtaining the corrosion resistance required in automotive sheet applications.
The crystal structure of Extragal® coatings is not visible to the naked eye. The high surface quality leads to a finished paint appearance meeting the most stringent requirements in the automotive industry for exposed bodywork parts.
Extragal® coatings are relatively ductile, with limited risk of damage in the drawing tools.
Scanning electron micrograph of an Extragal® coating surface
Cross section of an Extragal® coating
Unless otherwise specified, the standard Extragal® coating thicknesses offered (per side, measured at three points) are as follows:
Other coating thicknesses may be considered. Please contact us for more information.
Extragal® coatings are produced by continuous hot dip galvanizing, in which the steel strip is fed through a molten zinc bath. The steel substrate can be selected from most of our cold rolled steels and some of our hot rolled grades.
The Extragal® manufacturing process includes adjustments at all stages of the process, from the steelworks to the skin-pass. It is subject to a rigorous monitoring and inspection policy. As a result of these measures, an exceptional galvanized coating with an optimized surface is obtained. This ensures a very high-quality painted appearance in automotive bodywork parts.
Extragal® coating provides excellent corrosion protection, even in the event of damage (impact, scratches, stone chipping), due to the electrochemical behaviour of the Fe-Zn galvanic couple, in which the zinc acts as a sacrificial anode.
Extragal® has a friction coefficient of approximately 0.10 to 0.20 (depending on the oiling system used), which lends it excellent drawability.
The type and quantity of lubricant and the surface texture are obviously of prime importance during sheet-tool contact; any comparison of coatings must be carried out under identical conditions. Furthermore, the ductility of pure zinc limits the risk of powdering in the drawing tools.
For parts difficult to stamp, NIT surface treatment use is recommended (See surface treatments page).
Extragal® coated products offer a welding range suited to industrial requirements. The welding process, and in particular electrode life (typically 400 spot welds without current adjustment according to ISO standard 18278-2 on a 0.8 mm substrate), can be optimized by fine-tuning electrode composition, geometry and current adjustment frequency as well as welding parameters (current type and intensity, current incrementation, joining pressure, cycle time).
ArcelorMittal specialist teams are available to assist clients in optimizing the welding process.
Extragal® coating has good adhesive bonding behavior, good adhesion to the coating, good adhesion of the coating to the metal and good cohesion of the coating.
The most significant parameters determining bond quality remain the type of adhesive, the joining conditions, the nature of the protective oil, and any chemical treatments that may have been performed.
Extragal® can be phosphated and painted at the user's premises using current trication processes (Zn, Ni, Mn). Alternative "environmentally-friendly" (particularly nickel-free) treatment processes being developed make prior validation necessary for any change in the bodywork surface treatment process.
ArcelorMittal can provide specialized technical assistance regarding these issues.