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Substrates

Author: Joe Parkinson – Technology Manager, Bronx International

There are a number of metal substrates globally available to a potential coil coater. The choice of substrate is usually influenced by the end user’s requirements and available feed.

The feed coils presented to the colour coating line can be:

  • Aluminium
  • Zinc coated steel – galvanised
  • Zinc / Aluminium (45% Zinc / 55% Al with trace Si) coated steel, trade name Galvalume®
  • Zinc / Aluminium (95% Zinc / 5% Al) coated steel – trade name Galfan®
  • Other alloys of Zinc / Aluminium (mainly Magnesium, Nickel or Silicon) coated steel – trade names Supadyma®, ZAM® and Ecogal®
  • Uncoated Steel

Aluminium

Aluminium is the most expensive substrate, but arguably the most long-lasting. It is very good for extreme environmental conditions, for example, close to the sea, exposed to chemicals or high humidity atmosphere. It does not have the strength of steel and hence is limited in its application. It is easily pre-treated in a coil paint line and this results in extremely good paint adhesion and surface appearance. Aluminium coil is used to make sandwich and composite panels, roofing and walling.

Zinc Coated Steel

This is the original technology for the protection of steel. It is also the most economical general purpose product available. The coating is zinc with minor amounts of other elements added to promote the adhesion of zinc to the steel base and, at times to promote a smooth surface. The product can be produced on Non Oxidising Furnace lines, Wet Flux lines and Dry Flux lines. Zinc coated steel is the product of choice when used in wet applications, i.e. in the ground or in heavy snow areas, due to good galvanic protection at cut edges.

Zinc / Aluminium (45% Zinc / 55% Aluminium) Coated Steel – Galvaume®

This is a steel substrate that is coated with 55% aluminium and 45% zinc coating. it is good for most external applications and has a life span to 25 years and beyond, depending on its application. As with aluminium, pre-treatment of this product is very simple and a high level of paint adhesion is obtained. The surface is inherently very smooth. This product is not suitable for applications that are continually exposed to moisture and alkaline conditions, for example, cool room panels or panels in soil. The product is manufactured on Non Oxidising Furnace (NOF) Metal Coating lines. It is generally more expensive than zinc coated as the process equipment is more involved and due to the addition of aluminium. (Aluminium is generally more expensive than zinc). Usually Zinc / Aluminium lines are very large and require high volumes to be competitive, in excess of 120,000 tpa. The technology of Zinc / Aluminium coating is more complex than Zinc, however, it is seen to be a superior quality product as the surface is very smooth, paint adhesion is excellent and overall outside corrosion resistance is very good.

Zinc / Aluminium (95% Zinc / 5% Aluminium) Coated Steel – Galfan®

This is a galvanised steel product that has approximately 5% aluminium in the zinc coating. Usually a license is required to run this product called Galfan®. It is more economical than straight zinc as the coating mass can be reduced to obtain the same performance characteristics. The product is not as widely processed as the Zinc / Aluminium Galvalume® and Zinc. The equipment used to produce Galfan® is basically identical to that for producing zinc coated steel. However, the higher amount of zinc does mean good edge galvanic protection.

Other Alloy Coatings of Zinc and Aluminium

Following the successful development and marketing of the above two Zinc / Aluminium coatings the search for coatings with an even better combination of surface protection and paintability has continued. This has resulted in at least three different alloy combinations of Zinc and Aluminium coatings being developed in Japan. These include:

  • an alloy of 11% Aluminium, 3% Magnesium and 0.2% Silicon
  • an alloy of 6% Aluminium , 3% Magnesium
  • an alloy of 5% Aluminium and minor amounts of Magnesium and Nickel.

As with other Zinc / aluminium (Zn/Al) coatings production equipment similar to the larger NOF galvanising lines is required. This obviously makes capital costs higher but some of the claimed increase in corrosion protection can lead to large reductions (eg up to a half) in the coating weight. This can lead to large raw material costs savings to the steel producer and their customers.

Uncoated

This is uncoated, non-protected, cold rolled steel. It is only suitable for internal applications. it is more economical to purchase than zinc coated steel. Painted Cold Rolled Steel is sometimes used for external applications in some developing countries, but the expected life span is extremely limited.

The painting process of all the different substrates is similar but there will be some differences in the cleaning processes before painting, depending on the substrate type and the protective coatings.

Disclaimer: This information is for general information purposes only and should be viewed as such. For detailed, precise information for your upcoming coil coating line, its best to speak to a Bronx Technologist.

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