Metallic coatings for carbon steel

by Catherine Howlett | February 1, 2010 9:54 am

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by Robert M. Haddock, CSI
Economic reasons make carbon sheet steel popular for metal roofing, but the material can corrode when exposed to both oxygen and moisture. For this reason, steel typically has a coating, but the protective role differs among products.

Some provide a ‘barrier,’ creating a thin, impermeable film that prevents air and water from reaching the steel substrate. Others are ‘zinc-rich’ and provide ‘sacrificial’ protection—an electrochemical phenomenon protecting the base metal at the expense of the coating metal.

The continuous hot-dip process
Metal coatings are normally applied to the steel coil at the producing mill using a process called ‘continuous hot dipping’. In the first stages of the process, the steel is cleaned, degreased, rinsed, and forced-air dried. It is also ‘pickled’ in an acid bath, and preheated. At this point, the material’s mechanical properties can be affected—if desired—by exacting control of high temperatures, pressure, and cooling.

The coil then passes through a bath of molten metal at temperatures that provide for a metallurgical bond between base steel and coating metal. Ranging between 426 and 593 C (800 and 1100 F), the exact temperature varies with the coating type, since different coatings have different melting temperatures. The metallurgical bond between coating and base steel substrate causes monolithic behavior of the material during fabrication and service.

The coating thickness is controlled in most mills with ‘air knives’—sophisticated pneumatic squeegees that interface with the surface of the coil as it emerges from the bath of molten metal. The material is cooled and the coating solidifies on exit from the bath and entrance to the cooling tower. This process is closely controlled to affect varying surface appearance characteristics. It is during this process that the ‘spangle’—the metal flake appearance in the finish—of zinc-rich coatings is sometimes altered (i.e. minimized). Finally, the material is water-quenched, dried, and recoiled at the end of the line.

Most often just prior to recoiling, a chemical, passivation, or oil treatment (or combination thereof) is applied to extend the material’s shelf life, prevent storage staining, and/or prepare it for the next step of production—whether this means painting or fabrication. The oils help lubricate during the roll-forming process and provide some shelf life. These materials evaporate soon after installation.

The continuous hot-dip process takes place at line speeds of about 244 linear meters (800 linear feet) per minute, translating to as much as 446 m² (4800 sf) per minute, making it a cost-effective method to apply metallic coatings.

Zinc coatings
Perhaps the best-known coating for carbon steel sheet is commercially pure zinc, commonly known as ‘galvanized.’ (It bears mentioning here that galvanized iron [GI], despite being commonly designated on architectural plans, is a product that has been obsolete for decades.)

Common coating application rates for galvanized steel are 0.30, 0.60, and 0.90 oz per sf, designated as G-30, G-60, and G-90, respectively. Long ago, the target application rate for G-90 was 35.4 g (1.25 oz), with 0.90 serving as the minimum requirement. Sophisticated modern application equipment has enabled producers to maintain much more consistent and uniform application thickness, so the target rate of 35.4 g has gone by the wayside. Target application weight now is much closer to the minimum and verified by testing using either a single- or triple-spot sample according to ASTM International procedures.

It is important users understand zinc application coating rates because they have a linear relationship with the roof’s performance and longevity. For example, with other factors being equal, G-30 has a third the life of G-90; consequently, it should not be used for exterior claddings. G-60 is used only in cost-cutting applications and G-90 is the common choice for commercial steel roofing in pre-painted applications.

The total coating thickness of both sides of G-90 is 38.4 µm (1.51 mils). At the target application rate, this means coating thickness on a single side is about 19 µm (0.75 mils). However, due to coating process tolerances, industry standards allow the minimum on one side to be as low as 40 percent of the total, so the thickness on one side could be 15 µm (0.6 mils).

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Due to the slim coating thickness, zinc and zinc-alloy coatings also rely on the galvanic protection at scratches and cut edges. In the presence of an electrolyte (e.g. water), zinc’s active or anodic behavior retards oxidation of the steel substrate. For the same reason, zinc bars are attached to steel-hull ships and often inserted into domestic hot-water tanks—they slow steel’s corrosion.

Zinc coating is preferred by some manufacturers due to its excellent flexibility (i.e. malleability) in fabrication, especially when sharp radius bends are required in the product. Another advantage of using galvanized steel
is its solderability.

Although technically any coating (including zinc) offers barrier protection, zinc is generally referred to as a sacrificial coating because its electrolytic behavior is somewhat unique. By design, the coating goes away over time, sacrificing itself to retard the steel’s corrosion. Its life, then, is directly proportional to its thickness and the elements to which it is exposed. This galvanic activity is a desirable characteristic with respect to the corrosion behavior of steel, especially at surface scratches and cut edges where the base steel is exposed and unprotected by a barrier.

In unpainted applications, galvanized steel has become outdated as it has been replaced by newer technology coatings that significantly outperform it in such instances. Nevertheless, it is still considered an acceptable coating and is preferred by many when a premium organic finish (paint) is used. Although the paint is not impervious to moisture, it retards the galvanic process, prolonging the life of the galvanized substrate. However, due to the hindered galvanic process, corrosion performance at scratches, cut edges, and severe outside-radius bends is somewhat diminished.

Galvanized steel is produced by many mills and is widely available. It is not typically warranted by the producing mills for corrosion performance. Due to galvanic behavior and the natural oxidation process, the zinc diminishes over time. When a substantial volume is gone, the base steel is exposed and the corrosion protection—barrier or sacrificial—is no longer afforded.

This service life varies in different environments. The galvanic process occurs only when an electrolyte is present (i.e. when the surface is wet), so service life is longer in dry climates and at steeper slopes that keep surface moisture well drained. Hence, duration of wetness on the panels’ surface has more to do with service life than rainfall intensity or frequency.

In dry, desert-like climates where roofs seldom dew at night, bare G-90 may perform well for 50 or 60 years. In more humid climates, this is not the case; the roof will reach dewpoint almost every night, it is wet for a third of its lifetime, notwithstanding rainfall.

The aggression of the moisture also affects the life of the galvanized material—drastically reducing it in salt-spray or acid-rain environments. This is because such contaminants make for a much more effective electrolyte, accelerating the galvanic process. Once the coating is depleted, the steel roof need not be replaced, but it is a candidate for a field-applied coating to extend its useful life. However, no known field-applied coating has the same life or performance expectancy as the original metallic one.

Zinc coatings are typified by a broad spangle appearance caused by trace lead or antimony content. The size of the spangle can be controlled or eliminated altogether by the producing mill. In general, minimized spangle is preferred when the material is to be painted.

Spec references for galvanized include:

• Federal Spec QQS-775d;
• ASTM A 924, General Requirements for Steel Sheet Metallic Coated by the Hot-dip Process; and
• ASTM A 653, Standard Specification for Steel Sheet, Zinc-coated (Galvanized) or Zinc-Iron Alloy-coated (Galvannealed) by the Hot-dip Process.

The ASTM reference number is normally followed by steel grade (e.g. “A653 Structural Quality Grade 50”).

The same ASTM spec references are also used for ‘Galvannealed,’ which is a special zinc-iron alloy coating. Other zinc coating treatments, sometimes tailored to specific field-painting applications, are known by various trade names.

Aluminum coating
The application of commercially pure aluminum to steel sheet is a process developed by Armco Steel Inc., and is known by the trade names ‘Aluminized Type I’ or ‘Aluminized Type II.’ The former is typically only used in the automotive industry, while the latter is commonly employed for exterior claddings in coating weight of 18 g (0.65 oz), resulting in a thickness of
62 µm (2.43 mils), total for both sides. Note that although the coating weight is less than zinc (G-90), the resulting thickness is significantly greater, due to the lighter weight of aluminum. It is also available in other coating weights.

As opposed to zinc’s sacrificial nature, aluminum coating offers barrier-type protection. Aluminum oxides are extremely durable and the 0.65-oz coating application, which is designated T2-65, carries a limited 20-year warranty against panel perforation due to normal atmospheric corrosion. (This is a limited material-only warranty underwritten by the mill and normally ‘flows through’ the panel fabrication process, passing on to the end user when specifically requested.) Twenty-year exposure testing of the product has shown it far outlives (and even doubles) the warranted life in most environments.

Although Aluminized does not have the sacrificial protection of zinc, scratch and cut-edge performance is reasonably good. Corrosion seems to progress slowly from such areas because of the durability of the aluminum oxides. Aluminized steel has been used in both painted and unpainted applications, but as the coating is more brittle than zinc, restrictions on sharp bends in fabrication are more stringent. This type of coating is applied by the same hot-dip process, but at slightly higher temperatures.

Having a matte finish without spangle, Aluminized is a good choice for bare applications, or the material can be pre-painted. It generally outperforms most of the other popular coatings in salt or acid environments, although its warranty may exclude a 20-year performance in those situations. Aluminized material has decreased in relative market share in the last two decades because it has not been as actively and aggressively marketed as other coatings have been. However, it is still quite popular within the automotive industry.

Spec references include:

• Federal Spec S-4174 B; and
• ASTM A 463, Standard Specification for Steel Sheet, Aluminum-coated by the Hot-dip Process.

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Aluminum/zinc coatings
Although several aluminum-zinc (AlZn) formulations are used worldwide, the most popular alloy coating domestically is known by its trademarked name, ‘Galvalume.’ This alloy is 55 percent aluminum, 43.4 percent zinc, and about 1.6 percent silicon (by weight). Measured by volume, the coating is approximately 80 percent aluminum.

Developed by Bethlehem Steel, it was made commercially available in the late 1960s. It has since been licensed by BIEC International Inc. (formerly Bethlehem International Engineering Corporation) to 40 or more producers worldwide, several of which are North American companies. It is much more popular here in the United States and in Asia-Pacific (due to strong marketing by the Australian producer) than elsewhere. The product is known by various trade names including Zincalume, Aluzinc, and ZintroAlum.

This AlZn coating blends the barrier protection of aluminum and its oxide durability with the sacrificial properties of zinc, resulting in a synergistic alloy that has superior weathering properties when compared to galvanized, yet maintains the galvanic corrosion protection of zinc at scratches, cut edges, and severe-radius bends. Its cost premium over pure zinc coating is negligible, but performance characteristics are far superior in unpainted applications—hence it has forced bare galvanized into near-obsolescence in commercial applications.

Galvalume is used in various application weights, including 0.50, 0.55, and 0.60 oz. per sf (total both sides). These weights are designated AZ50, AZ55, and AZ60, respectively. The AZ55 coating is the most widely used and is warranted by most domestic producers for 20 years. Its thickness (both sides) is 45 µm (1.76 mils). The warranty is generally an assurance the panel will not perforate (in a ‘normal’ environment) due to corrosion.

Field studies after 25 years of exposure find actual performance of this material far exceeds its warranted performance. In a ‘friendly’ environment, it can be expected to render service life double or even triple that of its conservative warranty. For this reason, some domestic producers are now extending the warranty on AZ55 and painted AZ50 to longer terms.

Galvalume is the leader of coated steel options in unpainted applications. The market trend is also in preference of the AlZn coating as a painted substrate, but galvanized still enjoys significant market share. Due to the fact paint slows down the galvanic process, corrosion performance of Galvalume at scratches and cut edges may not be as good on painted applications as on unpainted ones in the initial years of service Many prefer the galvanized substrate for this reason. However, it has been demonstrated over subsequent years of service, ‘edge-creep’ performance—corrosion that works its way in from a cut edge—is often better with Galvalume than with G-90.

While Galvalume inherits the strengths of both its alloy coating metals, it also takes on their respective weaknesses. Contact with both acids and alkalis should be avoided. Since the coating also tends to retain cosmetic stains such as footprints and handprints, some producers now provide a thin application (about 7.6 µm [0.3 mils]) of acrylic coating to afford temporary stain protection during handling and installation. The coating weathers away after a few years. This option, dubbed ‘Acrylume’ or ‘GalvalumePlus,’ depending on the producer, is used only for unpainted applications and is becoming more popular, as it also has other production and storage-related advantages.

Galvalume-coated steel must be installed to promote free drainage, and is often employed at slopes as low as ¼:12, which is the warranty limitation. Despite its 40-year history and tens of billions of square feet of trouble-free installations at the requisite minimum slope, some specifications and standards mandate ½:12, most notably the National Roofing Contractors Association (NRCA).

Such a requirement is without substantive foundation from a material performance standpoint. The material likes to be able to breathe and air-dry and ¼:12 is deemed (and warranted) by its producers to be adequate for that purpose. Adjacent materials or details that could allow moisture to become trapped against the panel surface should be avoided. Any rooftop appurtenance should be mounted above the drainage plane of the profile. In simple terms: assure drainage of this material and job done.

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Spec references include:

• Federal Spec Army Corps of Engineers Guide Specification (CEGS)-07413;
• Army CEGS-07415;
• Army CEGS-13120;
• Naval Facilities Guide Specification (NFGS)-13121;
• ASTM A 924, General Requirements for Steel Sheet Metallic Coated by the Hot-dip Process; and
• ASTM A 792, Standard Specification for Steel Sheet, 55% Aluminum-Zinc Alloy-coated by the Hot-dip Process.

Other coatings for steel
Other coatings for steel include ‘Galfan,’ which is about 95 percent zinc by volume (almost reciprocal of Galvalume) and terne, which is a solderable tin-lead alloy used over special copper-bearing steel in thin gauges. The latter has been around for more than a century. Its advantages include solderability and a combination of steel’s cost efficiency with the ductility of softer metals.

Both Galfan and terne are only used in painted applications. The former is always pre-painted and the latter is most often post-painted using special paint, although it can also be pre-painted by coil coating. Post-painted terne requires repainting at about six- to eight-year intervals. Newer terne coatings (‘Terne II’ by trade name) are tin-zinc, rather than tin-lead alloys.

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Limitations of coated steel products
Aside from drainage issues, other precautionary measures when using metallic-coated steel are primarily chemical and metallurgical. Contact of these coatings with strong acids should be avoided. Heavy discharge of sulfurous and nitrous oxides from flues and the like shortens coating life adjacent to those areas. When using aluminum or aluminum alloy, strong alkalis are also detrimental to the aluminum. For this reason, use of these products with wet portland cement mortars such as reglet flashings is precluded, unless the metallic coating is first protected with a good, heavy layer of spray- or brush-applied clear coating (e.g. acrylic) to protect it until the mortar cures.

When work adjacent to Galvalume or Aluminized (or aluminum) involves cement mortar, the trades should be sequenced so the masonry installers finish their work prior to placement of metal panels. Cured mortar poses no threat.

There are also some mechanical precautions to be observed. Warranties on Galvalume usually specify a minimum bend radius of ‘2T’ in fabricated shapes—the radius of a bend must be at least double the metal thickness. This is because the material is stretched into tension on the outside of the radius and may develop micro-fractures if such a minimum were not observed. G-90 is a little more flexible and tolerates a tighter radius.

Aluminized (and aluminum sheet) are less ductile and may require even greater bend radii. In most cases, the tooling of roll-forming equipment anticipates these limitations, so there is no need for concern. However, there are exceptions. Sometimes panels or related flashings are brake-formed. Common leaf-brakes often violate the minimum bend restrictions of some coated steel products. The result may be thinning of the coating, micro-fractures, and premature corrosion at tension (outside radius) bends.

Weldability
Contrary to many industry claims, coated steel cannot be welded. Steel can be welded; coated steel cannot. This is because when attempts are made to weld coated steel in a fabrication or manufacturing process, the first step is to completely remove all coating from the area to be welded. Having done that, it is no longer coated steel, but bare steel, and the integrity of the metallic coating cannot be adequately restored.

The weld must be protected from corrosion, however, so the fabricator often employs a brush-applied, air-dried paint of sorts (sometimes with zinc or aluminum particulate) for the necessary corrosion protection. This secondary applied coating never has the life or maintenance freedom of the original hot-dip metallic coating. Except in rare cases, such specification is obsolete and a customer disservice. Instead, weldable materials should be specified for such conditions metallurgically compatible with the coated steel roof panels.

Compatibility issues
Zinc and aluminum are both anodic metals and should be isolated from electrolytic contact with more noble or cathodic metals, most notably copper. For the contractor, this means copper flashings should not be used anywhere upstream or in electrolytic contact with the coated steel. Additionally, any mounted equipment involving copper lines that drips condensate or rainwater runoff onto the roof should be avoided at all cost.

Runoff from copper contains copper salts, causing rapid galvanic corrosion of any of these coatings. It is not unusual to see a trail of red rust downslope of a roof-mounted air conditioner after a few years of service. Copper lines should be jacketed with insulation to prevent condensate drippage or rainwater runoff. Alternatively, runoff can be collected in a condensate pan and directed to drains by polyvinyl chloride (PVC) piping, isolating it from the roof panels.

Another common mistake is the use of graphite pencil to mark aluminum-, Aluminized-, or Galvalume-coated steel. Graphite has a severe corrosive effect on aluminum and etches the surface. In the case of coated steel and a wet climate, heavy pencil marks may show red rust in as little as one year. Instead, a felt-tip marker should be used for layout lines and other markings.

A galvanic scale can be used for determining dissimilar metals, and is included in many reference materials. However, the user should be aware this tool does not tell the whole truth. It is a mistake to conclude galvanic corrosion is imminent based on the scale alone. For instance, lead is distant (cathodic) from zinc (anodic) on the scale; but zinc may be soldered with lead alloy without adverse effects. Nickel steel is distant from both zinc and aluminum, but stainless fasteners are not only used, but are also preferred for these metals. Aluminum nails can be used in galvanized steel, but the reciprocal presents a problem.

Metals’ compatibility is too complex to be assessed merely by a quick look at the galvanic scale. Contributing to the complexity is formation of oxide layers that have different characteristics than the base metal. Galvalume producers have always recommended avoiding contact with lead flashings. However, in practice, lead reglet flashing (for instance) rarely affects Galvalume roofing adversely.

The best approach is to ask more questions if metals are found to be distant on the scale. Although coated steel panels are a popular choice for coastal applications, users should be aware of the detrimental effects salt spray has on all these coatings—they simply cannot yield the kind of life mentioned earlier in this type of environment.

Adequate drainage
None of the coatings described in this article tolerate moisture trapped against their surface for prolonged periods. Zinc is markedly less tolerant of this than aluminum, but both benefit from being freely drained and readily air-dried. Warranties typically exclude under-side corrosion. This can result from prolonged exposure of the panels’ underside to wet felts and deck (which is usually the result of leakage).

Topside corrosion can also be induced from the same phenomena where water ponds on the panel or where leaves, pine straw, or other debris retain moisture on the coating’s surface for a time. Occasional inspection and routine cleaning if necessary goes a long way toward preventing such induced coating corrosion. Thanks to its excellent strength-to-weight ratio and good formability and paintability characteristics, coated steel is the most widely used of all metals for roof coverings in the United States. Further, the material is durable enough for engineered, structural applications over open framing. Other factors being equal, coated carbon steel can offer superior wind uplift performance due to their mechanical properties. In many environments the metal can have a service life of four decades or more, and is a cost-compelling choice.

Robert M. Haddock, CSI, is a spokesperson for The Metal Initiative and the president of the Metal Roof Advisory Group. A consultant, technical writer, training curriculum author, inventor, and educator, Haddock is faculty at the National Roofing Contractors Association’s (NRCA’s) Roofing Industry Educational Institute (RIEI), and is also a member of ASTM International, the Construction Specifications Institute (CSI), and the Metal Construction Association (MCA). He can be contacted via e-mail at rmhaddock@earthlink.net[6].

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