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G30, G40, G60 AND G90: UNDERSTANDING ZINC-BASED COATING WEIGHTS March 7, 2018 | Author: Christal Chambers | Category: N/A DOWNLOAD PDF (328.9KB) Share Embed Donate Report this link

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1 G30, G40, G60 AND G90: UNDERSTANDING ZINC-BASED COATING WEIGHTS EXECUTIVE SUMMARY Zinc-coated, or hot-dipped galvanize...

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G30, G40, G60 AND G90: UNDERSTANDING ZINC-BASED COATING WEIGHTS EXECUTIVE SUMMARY Zinc-coated, or hot-dipped galvanized, steel is a widely used material in numerous applications, thanks to its corrosion-resistance, strength and relatively inexpensive cost. Zinc offers protection to the steel by forming a barrier against the elements, as well as sacrificing itself to prevent steel corrosion. The amount of zinc coating applied over top the core steel for corrosion protection varies depending upon end-use and specification requirements, and can be controlled and measured to industry standards. Zinc coating weights are listed in the U.S. measurement system in ounces per square foot (oz/ft2), and in the metric system as grams per square meter (g/m2). In both systems, the amount of zinc measured is the total average for both sides of the core steel. Common coating weights for galvanized steel are G30, G40, G60 and G90. It is important to note that these coating weights designate a total of 0.30, 0.40, 0.60, and 0.90 oz/ft2, respectively. While these are very common galvanized coating weights used in the construction and appliance industries, as well as other industries, zinc coatings weights can sometimes exceed G300, or 3.00 oz/ft2. A typical use for a coating weight of that caliber could include highway guardrails. The corrosion resistance, and thus, the service life, is proportionate to the amount of zinc coating on the steel. Hence, the service life of a G90 will generally be 3 times that of a G30 and 1.5 times that of a G60 in the same environment. However, the service life of any galvanized steel product is dependent upon the environment in which it is exposed. It is extremely important to be aware of which zinc coating weights are appropriate for specific applications. If the zinc coating weight applied is less than required for its end use, early corrosion or shortened product service life may occur. If the zinc coating weight is more than necessary, there are no negative performance ramifications, but the total project cost could be higher than necessary. INTRODUCTION: Steel is one of the most widely used materials in the world, forming a variety of structural, functional and GALVANIZED STEEL aesthetic products from skyscrapers to HVAC units to high-end art. The relatively low cost, high strength and formability of steel makes it an appealing option for countless applications. Unfortunately, bare steel has a tendency to corrode. According to Galvinfo.com, regarded by many in the industry as the leader in zinc-coated steel sheet information, corrosion is an electrochemical process that, in the case of steel, oxidizes the iron in the steel and causes the sheet to thin over time. The oxygen and steel react, causing rust to form. Rust not only creates an unattractive appearance, but also threatens the stability and integrity of the material, which could ultimately lead to product failure. The International Zinc Association estimates that steel corrosion can cost an industrialized country’s economy up to 4 percent of GDP each year. Not to mention, a corroded product can lead to expensive repairs or replacements for businesses, or to negative reviews for the product manufacturer. Properties and Characteristics Luckily, there are a number of methods employed to protect steel from corrosion. Steel can be inter- of Zinc Coating on Steel nally alloyed (creating stainless steel) or painted, but one of the most effective and economical options is zinc coating, or galvanizing, the steel. Galvanized steel is a proven, economical and effective way to protect bare steel with a surface that is more resilient than paint on its own, although paint can still be applied as a secondary layer of protection over the coated steel. Zinc is a naturally-occurring element present in rock, soil, water, the biosphere, plants, animals and humans. More than half of the 12 million tons of zinc produced annually worldwide is used for galvanizing steel to protect it from corrosion, according to the International Zinc Association. Zinc is beneficial as a steel protectant because it is formable, corrosion resistant and recyclable. Zinc-coated steel, or galvanized steel, is protected from rust in two ways – as a physical barrier and by cathodic protection. As a physical barrier, zinc coating blocks moisture and oxygen from reaching the steel underneath. Zinc’s inherent corrosion resistance is a result of the element’s ability to form dense, adherent corrosion by-products, which leads to a rate of corrosion considerably lower than iron-based materials – up to 10 to 100 times slower, according to the American Galvanizers Association (AGA), a not-for-profit trade association dedicated to serving the needs of specifiers, architects, engineers, contractors, fabricators, and after-fabrication hot-dip galvanizers throughout North America. The by-products, known as zinc patina, serve as an additional barrier protection for the steel. The barrier coat keeps moisture, salt, other liquids and corrosive atmospheres away from the steel. Cathodic protection references zinc’s ability to protect steel by electrochemically sacrificing itself. Zinc is able to do this because it is a less noble metal than steel. Therefore, when steel is exposed through a scratch or defect, the steel itself cannot corrode adjacent to a zinc coating – only the surrounding zinc will corrode. For a metal to be “noble,” it means that the metal is resistant to corrosion and oxidations in environments where moisture is present. The Galvanic Series determines the nobility of metals and displays them according to their resistance to corrosion. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 2 Potential ( VOLT S) Galvanic Series Chart More noble metals are the least susceptible to corrosion. As you can see in the chart, platinum, gold and silver are the most noble metals, and are not vulnerable to corrosive attack. However, steel is less noble, which means oxygen and water causes steel to red rust or oxidize. “When two metals or more are in contact with each other, the less noble will oxidize before the more noble,” says Allen Garrett, Director of Business Development at Majestic Steel, a steel solutions service center headquartered in Cleveland, Ohio. “Moreover, the further apart on the Galvanic Series the metals are, the quicker the less noble metal will oxidize.” Magnesium, zinc, cadmium and aluminum are the metals that are less noble than steel, and therefore will sacrifice themselves for the core steel in cathodic protection. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 3 However, not all of these metals are conducive to high-volume commercial coating for a few different reasons: 1. Magnesium: expensive and explosive if handled incorrectly, making it difficult to work with. 2. Zinc: less expensive than magnesium with a relatively low melting point and is fairly safe. 3. Cadmium: extremely expensive. 4. Aluminum: good for certain steel coating applications, most commonly when the final product is exposed to heat. Therefore, historically zinc and aluminum are the two metals most often used to coat steel to protect it from corrosion. Lest you think things are set in their ways, however, recent developments have started to include small percentages of magnesium into metallic coatings – just one reminder that innovation can occur even after years of standard protocol. Zinc coating thickness alone can be misleading when evaluating zinc applied to steel using different processes, according to the AGA. Therefore, the typical designation used to consistently evaluate the protection value of the zinc is coating weight. The coating weight determines the coating life in conjunction with time, atmospheric conditions and environmental circumstances. Humidity, the presence of water, oxygen, sulfates, temperature, chlorides and nitrates can all dramatically affect the rate of corrosion for zinc coatings, according to Galvinfo.com. How Zinc Coating Weights are Determined for Galvanized Steel Galvanized steel is given designations that indicate the coating weight, specified by the American Society for Testing and Materials, also known as ASTM International. ASTM’s job as a governing body determines the amount of zinc needed on the strip based upon a customer’s request in order to meet end-use requirements. As outlined earlier, common designations include G30, G40, G60, and G90, though zinc coating weights span the gamut and range all the way up to G360. ATSM specifies that the coating bath chemistry for galvanized be no less than 99 percent zinc, and contain between 0.05-0.25 percent aluminum to promote adhesion. The “G” means the coating is galvanic – zinc – while the numbers are in reference to the weight of zinc on the surface of the steel in ounces per square foot, or oz/ft2 units. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 4 Buying Blanket So for G90 steel, when tested with a triple spot test (testing at one edge, the center of the sheet, and or Direct Trade the other edge), one square foot of sheet steel coated with zinc on both sides should have an average minimum coating weight of .9 ounces. Assuming the coating is applied evenly, this means there is .45 ounces on each side of the sheet steel. The table shows the amount of zinc applied by designation in both the U.S. and corresponding equivalent metric measurement systems. This shows the thickness in mils (0.001” of an inch), and also in microns, as well as the metric equivalents. ASTM sets the minimum coating weights for each coating designation, which are determined according to specified testing methods, called a triple spot test and a single-spot test. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 5 A triple-spot test is used for samples from product more than 18” in width, and it takes the average of three checks per side – one from the middle and one at least 2” from each edge. The total of the average coating weights of each side must be equal to or greater than the specified minimum for that designation. For G90, the total of the per-side average coating weights must be 0.90 oz/ft2. But because the coating will not be exactly equal on both sides, there is also a minimum allowance for the triple spot oneside average. For G90, this is 0.32 oz/ft2. There is also some variation in coating weights across the width of the product, so the test requires that no single spot be too far below the specified minimum. The single-spot minimum considers the total of the weights from each side in one location instead of the average. Again for G90, the single-spot minimum is 0.80 oz/ft2. There are also times when the product is less than 18” wide, and a triple spot test is not possible. In this case, the single-spot test is performed, and the specified minimums for each designation must be met as well. What does this mean in terms of corrosion resistance? Remember that the coating weight determines the coating life – which is the basis for corrosion resistance for steel. Depending on the conditions to which the steel is exposed, the coating life varies. “The corrosion resistance is in linear proportion to the amount of coating on the strip,” says Jeff Reall, Technical Advisor at Majestic Steel. “As you increase the coating weight, you increase the service life of that product.” Generally speaking, if a G30-coated steel product lasts five years until it starts to red rust or corrode, then a G60 will last 10 years and a G90 will last 15 years. In other words, a G60 coating has twice the thickness of a G30 coating, and the life of the product in a given environment is approximately twice as long. Using these assumptions, a G90 coating is nearly 50 percent thicker than G60 and can be expected to perform 50 percent better, according to Galvinfo.com. If you add the top and the bottom, the total zinc thickness on the G90 is about .0015 inches – about half the thickness of a hair. Therefore, a G30 adds .0005 inches – 1/16 the thickness of a hair. Applying Zinc Coating to Steel There are several ways that zinc can be applied to steel, the most common of which is called continuous hot-dipped galvanizing. In this process, coils of flat-rolled steel sheet are unwound and welded end-to-end to produce a continuous strip of steel that will run through the galvanizing line. The steel is cleaned with an alkaline detergent, annealed and then passed through a molten zinc bath. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 6 Air Knives and Coating Line To control the coating to the desired thickness, air knives are used. These air knives are operated by gas pressure and held at designated heights, angles and distances to the strip in order to achieve the appropriate coating weight. Either air or nitrogen can be used. Nitrogen has been proven to limit oxidation and improve surface smoothness, so it is often used in air knives for applications that require high surface quality. The fluid process by which excess zinc is removed is called jet stripping. Once these processes take place, the steel is inspected to make sure it meets requirements. To ensure accuracy, x-ray gauges are used to measure the steel after the coating is complete, providing closed-loop feedback for the automated coating thickness control system. Using Correct Coating Weights in Different Applications When purchasing steel, it is important to specify the correct coating weight on the purchase order to ensure optimal performance based on the environmental conditions to which the steel will be exposed. Other factors to consider when choosing the proper coating weight are the total cost of the materials and the appearance of the final product. If an improperly thin coating weight is used, early corrosion and a shortened service life for the product may occur. On the other hand, using a thicker coating weight than needed could prove to be an expensive mistake. For these reasons, contractors and manufacturers should be fully aware of which coating weight is appropriate for their given application and environment. “Often, people aren’t sure which coating weight they need for their application, and sometimes the higher coating weight isn’t the better one,” says Stephen Wolf, Business Analyst at Majestic Steel. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 7 Theoretical Nominal What is the desired product life given the exposure to corrosion that the steel is going to face? In Weight or Theoretical agricultural applications that will be exposed to Mother Nature and will likely be assaulted by rain, Minimum Weight sun, wind and other environmental factors, the steel should have a higher coating weight. Conversely, something inside the home may be able to pass with a lesser coating weight because it’s not exposed to the elements all day. We’ll discuss anomalies to this example in just a moment. Is appearance a factor for the final product? On basic galvanized product, the general rule of thumb is that the higher the coating weight, the less consistent the surface finish of the coating. That doesn’t mean that corrosion-resistance is impacted in any way; it’s just that the material can sometimes be less appealing visually, with small blemishes and inconsistencies. For that reason, material with a G30 or G60 coating weight will probably have a more consistent coating than a G90 or higher. One way to obtain a more consistent finish, regardless of coating weight, is to order the material as extra-smooth – also known as skin-passed or temper-passed. When metal-coated steels are ordered extra-smooth, the coated steel is run through a single-stand rolling mill after the coating is applied. The rolls in the mill remove or mask any blemishes in the coating, creating the desired appearance. This process also provides the added benefits of more consistent mechanical properties, better shape, and a resistance to fluting. However, the buyer should know that the surface of the coating would be duller and less light reflective than non-temper passed galvanized. What is the total cost of ownership? Coating weights such as G90 and above are more expensive than G30, G40 and G60. However, we’ve learned that in the same environment, the G90-coated steel will last longer than a lesser coating weight. “There are applications where G30 and G40 are more appropriate, but it is important to consider the total cost of ownership over the expected life of the product,” says Wolf. “It’s like anything in life – you pay a little more for something nicer, assuming it’s going to last you longer.” It’s important to note that there is no guarantee on any coating weight as far as corrosion resistance in any specific environment. To better prepare and understand the different applications, environments can be broken into general categories such as industrial, marine, agricultural and rural. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 8 INDUSTRIAL In an industrial environment, acid, soot and other pollutants come out of exhaust stacks, settle on steel products and attack. According to the AGA, industrial environments are typically the most aggressive in terms of corrosion. Most outdoor applications in city or urban areas can be classified as moderately industrial. MARINE In a marine environment, anywhere within approximately three miles of a coastal situation may fall victim to airborne salt spray or sodium chloride that also can aggressively attack steel. In these circumstances, higher corrosion resistance may be necessary, depending on the actual application. If a long service life is required, then a higher coating weight should be used. AGRICULTURE Agricultural environments expose steel to fertilizers that go airborne, crop dustings and other factors that can attack the product. Steel applications such as feeding bins, grain elevator and animal pens must take into account the chemical and environmental corrosion threats that the product will face. In agricultural applications, galvanized steel is preferable to aluminum materials such as aluminized or Galvalume®, because certain elements inherently present in animal waste attack aluminum very aggressively. Therefore, applications such as animal containment structures or anything that may come in contact with animals or animal waste would be better suited to galvanized steel over aluminum-containing metallic coated steel. RURAL Rural environments are much less aggressive than other environments, due to the relatively low levels of sulfur and other emissions. For this reason, a lighter coating weight can often be used. Each environment creates its own situation, but there are three basic factors that enable corrosion: oxygen, water and the metal itself. If it’s dry, indoors and/or shielded from aggressive elements, metallic coating weights allow for a longer service life. In more humid, outdoor applications, the service life will be shortened. So, as a general rule, the higher the coating weight, the more service life the product will exhibit in each of these settings. Unpainted galvanized steel used in outdoor applications commonly use the G90 coating weight. G90 is also recommended for indoor applications where there may be dampness due to condensation, though a G40 or G60 coating is usually sufficient for indoor uses that remain relatively dry, according to Galvinfo.com. “In an interior environment where it’s environmentally controlled, you don’t necessarily need a significant amount of sacrificial corrosion protection to shield the core steel, so you would typically specify a lower metallic coating weight,” Garrett says. “But in the environments where you have an electrolyte – like near marine environments where there’s salt, or interior pool environments where chlorine is present – those electrolytes can help to accelerate corrosion.” M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 9 For example, the HVAC industry is one of the largest consumers of galvanized steel. For typical HVAC applications, G30, G40 and G60 coating weights can be used in ductwork pipes, elbows and connections. Because of the thinness of the end use product, lighter coating weights are acceptable when the environment is cool and dry. Ideally in the ductwork itself – the main runs of the rectangular part of the duct – G90 coating weight should be used. In addition, if the application is outdoors, such as a rooftop unit, or in a corrosive-inducing environment, such as an indoor pool, a higher metallic coating weight may be necessary. The Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) drive these specifications. The contractor for each job must follow SMACNA’s standards to ensure proper coating weights for each individual product. How Using The Wrong Coating Can Compromise Your End Use Using the proper zinc coating weight is important for several major reasons. An application or product that has a coating weight that is too low will likely produce a shortened product life span, as well as the potential for unsightly rusting and structural damage. More importantly, safety is a huge factor to consider in regards to load-bearing or architectural applications failing due to improper coating weight specifications. With commercial air ducts, people are breathing in air that comes through the system. If the product corrodes earlier than expected, rust or mold may enter the facility, creating a significant health concern when people breathe it in. Spending a little more on a higher-quality product like G90, or installing antimicrobial-coated steel can help to maintain healthier atmospheres. Load-bearing and structural applications pose obvious threats if there is product failure. Structural applications in highly-trafficked areas may rust or corrode earlier than expected, collapse and cause injury or worse if the incorrect coating weights are used. This type of situation is a huge liability for manufacturers and can lead to what is known as a tear-out. Tim Quinn, Sales Advisor at Majestic Steel, recalls a situation in which a tear-out took place due to improper coating weights being used in an HVAC application. A building inspector at a major Ivy League university performed a micrometer test on the material used in ductwork. The duct had already been hung, with drywall covering it. Once it was determined that the ductwork did not meet code, it had to be pulled out. The money spent on the original product, the time spent to fabricate that product, the time and money to transport the product to the job site and the labor to hang the ductwork, then the cost to tear it out and completely reinstall it ended up wasted because the product wasn’t up to code. Using the wrong coating weight for the project was not only a quality issue, but also posed safety and environmental concerns. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 10 “You can have product failure in the field, short of the product’s intended service life, which creates a big problem there,” Reall says. “For example, government paid-for facilities typically require G90 coating weight minimums for rectangular ducts in HVAC applications. If you get into a situation where somebody finds that there’s an inferior coating weight on the project, it creates quite an embarrassment for everyone involved.” These are just a few illustrations of why it is important to educate oneself on zinc coating weight specifications and how they apply to particular applications. Galvanized steel is a versatile, strong and affordable material that can be used in a myriad of applications. The zinc coating protects the steel from corrosion elements, providing a barrier and sacrificing itself so the core steel may exhibit a longer service life. Here are a few action items we’d recommend taking to ensure the proper coating weights are used for each application: ROADMAP FOR SUCCESS 1. Do your research. To get the most out of your galvanized steel, it’s important to put education before all else. Be aware that multiple coating weights exist, and each one is best suited to certain environments and applications. For more specific details on applications and end use, consult the sound educational outlets we’ve outlined in this technical paper: Galvinfo.com, ASTM International, and SMACNA. 2. Talk to your supplier. Your supplier should know every detail of the steel they provide. From the coating weight to the chemical makeup to common end use applications, the supplier can guide and advise for best practices and usage to ensure optimal results. 3. Consider testing. If there is any doubt or question as to whether or not a product, structure or component is the right coating weight, there are spot tests or lab surveys that can be conducted to verify the material. Once the correct coating weight is determined, testing can confirm that the galvanized steel in use is in fact the appropriate material. Follow these steps to gain an understanding of proper zinc coating weights and specifications, and to help ensure ideal service life and quality of your steel products. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 11 DISCL AIMER THE MATERIAL CONTAINED IN THIS WHITEPAPER IS FOR INFORMATIONAL PURPOSES ONLY AND IS PROVIDED “AS IS” WITH NO WARRANTIES, INCLUDING ANY WARRANTY OF MERCHANTABILITY, TITLE OR FITNESS FOR A PARTICULAR PURPOSE. NO LICENSE TO ANY INTELLECTUAL PROPERTY IS GRANTED HEREIN. M A J E S T I C S T E E L .C O M H O W TO B E A B E T T E R S T E E L B U Y E R 12 View more...

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