What is Galvalume Process(55%Al-Zn)

Q- Galvalume is a coating consisting of zinc, aluminum and silicon that is used to protect a metal (primarily steel) from oxidation. It is similar to galvanizing in that it is a sacrificial metal coating which protects the base metal.

Introduction of Galvalume

55% Al-Zn(Galvalume) Alloy coated product is a flat-rolled steel sheet coated with aluminum-zinc alloy by a continuous hot-dip process.  The nominal alloy composition by weight is 55 % aluminum, 1.6 % silicon, and 43.4% zinc. This product is known by many different trade names throughout the world, viz.; Galvalume® (a registered trademark of BIEC International Inc., and some of its licensed producers, and a trademark of Dofasco Inc. in Canada), -NAL® (a registered trademark of Steelscape Inc.), ZINCALUME® (a registered trademark of BHP Steel (JLA) Pty Ltd.), ZINTRO-ALUMTM (a trademark of Industrias Monterrey S.A.), and GALVALTM (a trademark of Galvak, S.A. de C.V.). Following its introduction in 1972,

It has been well received in the marketplace, especially for metal building roofing and the cumulative worldwide production now exceeds 70 million tons, with an annual production level currently approaching 7 million tons Steel sheet with a 55% Al-Zn(Galvalume) coating has proven to be an excellent product for long-life roofing sheets, especially low slope roofing on industrial buildings. It has been widely applied as bare (unpainted) sheet with the coating being directly exposed to the atmosphere. The product is also used as a substrate for prepainted sheet when a more decorative finish is desired and this use has recently shown significant rates of growth. It offers corrosion resistance and heat reflectivity similar to that of aluminum coated steel, and galvanic protection of bare edges similar to that of zinc coated steel. The coating is applied by means of a continuous hot dip galvanising process. 

Coating

Galvalume sheet

55% Al-Zn(Galvalume) Alloy Coated Steel has been hot-dip coated with a special aluminium alloy that forms a tightly adherent coating on both surfaces of the steel. In the hot dip coating process, a very thin acrylic coating is applied to both sides of the sheet using a sophisticated roll coater. This acrylic coating provides excellent resistance to storage stain & transit corrosion. The formulation contains approximately55 % Al 1.6% silicon and 43.4 % zinc, which allows 55% Al-Zn Alloy Coated Steel to be used in the most demanding roofing and forming applications.

Limitations

It is advised against contact of 55% Al-Zn Alloy Coated Steel sheet with lead, copper, graphite, unprotected steel, uncured concrete or wet, green or pressure –treated wood: exposure of 55% Al-Zn Alloy Coated Steel sheet to water run-down from copper and the use of 55% Al-Zn Alloy Coated Steel sheet in harsh chemical or intensive animal confinement environs.

Corrosion resistance

During the coating process, an aluminium-zinc -iron alloy is formed at the Interface with the steel substrate, whilst an aluminium-zinc-Si   layer is formed at the outer surface. It is this thin but tenacious film of alloy, which protects the steel. The combination of barrier protection provided by the Aluminium network and the galvanic protection of interdendritic zinc provides excellent corrosion resistance 55% Al-Zn Alloy Coated Steel retains its surface luster at temperatures up to 350°C, after which the coating is transformed into a dull gray aluminium-zinc-iron alloy. At this temperature, the alloy still retains its excellent oxidation resistance.

The 55% Al-Zn Alloy Coated Steel range includes products that withstand the corrosive attacks of salt, flue gas, combustion condensates, weather, and atmospheric pollution, oxidation at elevated temperatures. Based on 30-year atmospheric test results, it is estimated that 55% Al-Zn Alloy Coated Steel sheet will outlast G90 galvanized by two to four times in marine, industrial & rural atmospheres. When compared to aluminium coated sheet steel, 55% Al-Zn Alloy Coated Steel sheet has superior corrosion resistance at sheared edges.

Heat reflectivity

55% Al-Zn Alloy Coated Steel reflects approximately 80% of incident light and heat at temperatures up to 350°C compared with Galvanised sheets & clay tiles . This property means that 55% Al-Zn Alloy Coated Steel can reduce the amount of energy required to maintain elevated temperatures relative to other steel products.

High temperature Behavior:

Bare 55% Al-Zn Alloy Coated Steel sheet can be used at temperatures up to 3000C without discoloration and up to 6500C without heavy oxidation and scaling. Prolonged exposure to temperatures above 3000C can result in changes to the base metal characteristics of conventional 55% Al-Zn Alloy Coated Steel sheet.

Yield advantage of Glvalume

Aluminum makes up 55% of the Alloy Coated Steel coating by weight, but comprises 80% by volume. The result is a coating that weighs less while delivering more square feet per ton than galvanized steel.

Resistance to standing water:

Cyclic immersion tests by Bethlehem Steel confirmed that 55%Al-Zn coated sheet endures ponding—standing water—several times longer than either galvanized G90 or aluminum-coated Type 2 sheet before the onset of rust.  This quality makes the alloy coated steel an ideal material for low-slope roofs.

Salt Spray Corrosion Resistance:

In salt spray tests conducted with bare cut edges exposed, the corrosion resistance is typically three to four times that of G90 galvanized.

Coating Microstructure: 

The coating makeup is a duplex microstructure that results form aluminium –rich phase solidifying first, as the coating cools, forming a network dendrites. Interdendritic spaces are filled by the zinc-rich phase. The intermetallic layer is an Al-Fe-Zn-Si alloy that metallurgically bonds the coating to the steel substrate & further aids in corrosion resistance. The micro-structure of the 55% Al zinc coating contains light Al rich phase and the dark Zn rich phase which contains 22% Al.

The microstructure distribution:

 80% by volume – Al rich phase

20% by volume – Zn rich inter-dendritic phase

The inter-metallic layer formed between the substrate and the coating overlay is comprised of two layers.

  • Outer layer adjacent to the overlay containing: 18% Fe : 60% Al: 22% Si
  • Inner layer adjacent to the steel base : 20% Fe, 70% Al ,8%Si,<2% Zn

The thickness of the alloy layer ranges from 1µ – 3.25µm and generally maintained at @1 – 1.5 µm.

Thickness of the alloy layer depends upon

  • Bath temperature
  • Bath Si content
  • Strip entry temperature
  • Residence time in bath
  • Cooling rate

Appearance:

55% Al-Zn Alloy Coated Steel has smooth surface that is aesthetically pleasing and makes for a superior surface for pre-painting and post-painting. Uniform visual appearance of unpainted 55% Al-Zn Alloy Coated Steel sheet cannot be guaranteed. Normally occurring variations in surface appearance typical of all hot-dip products will still be present and will not be masked by the thin, clear acrylic film. If uniform visual appearance is critical, then prepainted 55% Al-Zn(Galvalume) Alloy Coated Steel sheet should be ordered.

Applications

The 55% Al-Zn(galvalume) Alloy Coated Steel are used widely in applications both indoors and outdoors.

Architecture:

Architectural panels, pre-engineered metal buildings, roofing (both conventional and standing seam), sidings, ventilating ductwork, rain gutters, downspouts, solar collector panels, culvert pipes (corrugated steel pipes), etc.

Storage & Maintenance:

55% Al-Zn Alloy Coated Steel® is supplied with a chemical surface passivation or oiled to temporarily limit any risk of black rust formation. During transport and storage, all necessary precautions must be taken to keep the material dry and to prevent the formation of condensation. Improved protection can be obtained by the application of chemical coating.

To preserve the surface, handling should only be carried out using clean, dry gloves. Sliding of the sheets over rough surfaces or each other should be avoided. As with galvanized or painted steel products, bundles of 55% Al-Zn Alloy Coated Steel sheets or products made from steel in all finishes must be kept dry in transit. After transit, material should then be covered and stored off the ground, at a slight angle, to prevent water or condensation from being trapped between adjacent sheet surfaces. If the bundles become wet, sheets should be separated, wiped with a clean cloth without delay and then placed so that air circulation completes the drying process. These procedures are recommended to avoid possible deterioration of the coating, which could result in non-uniform appearance.

Formability

The 55% Al-Zn(Galvalume) Alloy Coated Steel sheet exhibits excellent formability. It can be formed about as readily as continuously annealed galvanized sheet steel. Lock forming and roll forming are readily accomplished. A lubricant should be applied and chromium-plated rolls used to minimize coating metal pick-up on roll-forming dies. Solvent-based lubricants, which do not leave residues, are suggested.

 Weldability

55% Al-Zn Alloy Coated Steel sheet is readily weldable with conventional resistance and arc welding processes. Conditions for resistance welding are similar to those used on galvanized steel. Spot welding electrodes should be redressed as required to maintain nugget size. RWMA Class 2 or dispersion-strengthened copper alloy electrodes are suggested. 55% Al-Zn Alloy Coated Steel sheet can be arc welded with the shielded metal-arc and gas metal-arc processes. The lower zinc content of the coating of 55% Al-Zn Alloy Coated Steel sheet results in considerably less fuming during arc welding, providing reduced fume hazards to welders. Nevertheless, adequate ventilation is required to remove zinc oxide fumes. The decreased fuming also results in less porosity, giving better quality of weld.

Corrosion Resistance of Welds:

Spot & seam welding may remove the coating from 55% Al-Zn Alloy Coated Steel sheet exposing the base steel. This areas may be too large to be galvanically protected by the adjacent coating & should be covered with metal sprayed zinc, aluminium-zinc, zinc rich paint or organic coatings.

Soldering

Although 55% Al-Zn Alloy Coated Steel sheet can be soldered, the presence of thin film of aluminium oxide on the surface makes soldering more difficult than when soldering galvanized products. When soldering is necessary, the techniques & fluxes used to solder aluminium should be used. 

Fastening

From a mechanical standpoint, any style of fastener suitable or use with sheet metal can be used to join 55% Al-Zn Alloy Coated Steel sheet to itself or other parts. Corrosion characteristics of the fastener material should be carefully considered from two standpoints. First, the fastener should be equally as corrosion resistant as the 55% Al-Zn Alloy Coated Steel coating to ensure long life of fabricated product. Second, the fastener material should be compatible with the coating; that is, it should be selected to avoid accelerated corrosion caused by intimate contact between certain types of dissimilar metals.

Suitable fasteners made of nylon, other plastics or stainless steels, and carbon steel fasteners with thick zinc alloy heads. Fasteners made of lead or copper must never be used because they seriously reduce the corrosion resistance of the 55% Al-Zn(Galvalume) Alloy Coated Steel material.

Sealants

Sealants are often used between overlapping areas of steel     sheet to form watertight joints & to achieve superior environmental durability. Natural-cure silicone rubber sealants are recommended. These materials need no primer. They are flexible and non-corrosive as well as resistant to heat, cold, water and ultraviolet rays. In selecting silicone rubber sealants, avoid those materials that contain acetic acid or amines, which can promote corrosion during the early stages of curing.

Other sealants like butyl rubber and styrene butadiene rubber also may be used successfully.

Paintability

55% Al-Zn Alloy Coated Steel can be painted after degreasing and surface treatment when delivered oiled. 55% Al-Zn(Galvalume) Alloy Coated Steel coated with thin ,           organic coating can be painted directly without any prior surface treatment. 55% Al-Zn Alloy Coated Steel has smooth, consistent surface that is suitable for painting ,

INSTALLATION :

To preserve the surface apperance of 55% Al-Zn Alloy Coated Steel sheet,only    clean,dry gloves should be used during handling.Care should also be exersied to prevent the sheets from sliding over rough surfaces or each other.55% Al-Zn Alloy Coated Steel sheet steel joints can effectively closed using appropriate sealants such as neutral-curing silicone rubber.

WARRANTY

55% Al-Zn Alloy Coated Steel sheet is conditionally warranted against rupture , structural failure or perforation due to corrosion for a period upto 20 years.

Summary

  • 55% Al-Zn Alloy Coated Steel is 55%Al-Zn alloy coated strip manufactured by the hot dip process.

One most peculier features is excellent corrosion resistance as compared to galvanized or Aluminium coated strips. It is this property which makes 55% Al-Zn Alloy Coated Steel most suitable product for roofing system.Other features include good heat reflectivity, weldability and paintability comairable to that of galvanized coatings

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