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Extragal® double-sided pure zinc galvanized steels

Applications

Because of their high corrosion protection capacity and surface quality, Extragal® coated products are recommended for numerous automotive applications, for both visible and non-visible 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.

Technical characteristics

Surface appearance
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 severest requirements of the automotive industry for visible bodywork parts.

Hardness
Extragal® coatings are relatively ductile, with limited risk of damage in the drawing tools.

Morphology

Scanning electron micrograph of an Extragal® coating surface

Cross section of an Extragal® coating

Coating thickness

Unless otherwise specified, the standard Extragal® coating thicknesses offered (per side, measured at three points) are as follows:

Similar standard

Minimum

(double-sided)

(g/m²)

Minimum
(per side)
µm          g/m²

Maximum
(per side)
µm          g/m²

Z100

 100

 7.0           50

  9.0           65

Z140

 140

 10.0          70

 12.0          85

Z200

 200

 14.0         100

 17.0         120

Z225

 225

 15.5        112.5

 18.5        132.5

Other coating thicknesses may be considered. Please consult us.

Coating process

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 almost any 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 rigorous control and inspection. 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.

Typical layout of a galvanising line

Recommendations for use and secondary processing

Corrosion
Extragal® coating provides excellent corrosion protection, even in the event of damage (impact, scratches, gravel impingement), due to the electro-chemical behavior of the Fe-Zn galvanic couple, in which the zinc acts as a sacrificial anode.

Drawing
Industrial experience shows that the drawing performance of Extragal® products is superior to that of other coating systems.

Extragal® has a friction coefficient of approximately 0.10 to 0.14 (with standard oiling), 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.

Welding

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.

Adhesive bonding
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.

Surface treatment
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 are being developed; however any change in the bodywork surface treatment process must undergo prior validation.

Recommendations:

  • Alkali degreasing to remove any organic residues and oxides present on the surface,
  • F- ions should be present in the bath to neutralize any Al3+ ions that may reduce its activity.

ArcelorMittal can provide specialized technical assistance regarding these issues.

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