This article was written by KTA’s Tom Neal & Doug Reardon
Introduction – The controls required for the hot-dip galvanizing process are described in Topic 1 of this series. 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. 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.
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. The definitions are provided in a logical order in which the terms are used, rather than alphabetical. Note that the following is a summary of the information from the definitions. See ASTM A123 for the full text of the definitions.
- Sample —a collection of individual units of product from a single lot…intended to represent that lot for acceptance. If a sample is taken as representing the lot for acceptance, the sample shall be taken at random from the lot without regard to the perceived quality or appearance of any individual unit in the lot being sampled. The sample consists of one or more test articles.
- Test Article —an individual unit of product that is a member of the sample and that is examined for conformance to a part of this specification.
- Specimen —the surface of an individual test article or a portion of a test article, upon which thickness measurements are to be performed, which is a member of a lot, or a member of a sample representing that lot… For a unit of product whose surface area is equal to or less than 160 in.2 [100 000 mm2], the entire surface area of each test article constitutes a specimen. In the case of an article containing more than one material category or steel thickness range … that article will contain more than one specimen, as appropriate…
- Single-specimen article —a unit of product whose surface area is equal to or less than 160 in2 [100 000 mm2] or that is centrifuged or otherwise similarly handled in the galvanizing process to remove excess galvanizing bath metal (free zinc). For thickness testing purposes, the entire surface area of each unit of product constitutes a specimen. In the case of any such article containing more than one material category or steel thickness range … that article will contain more than one specimen…
- Multi-Specimen Article —a unit of product whose surface area is greater than 160 in2 [100 000 mm2]. For thickness testing purposes, articles whose surface area is greater than 160 in2 are subdivided into three continuous local sections, nominally equal in surface area, each of which constitutes a specimen. In the case of any such local section containing more than one material category or steel thickness range… that section will contain more than one specimen…
Determining the Required Galvanizing Coating Thickness
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. Table 1 of ASTM A123 provides the coating grade number (which is also equivalent to the minimum average thickness of the galvanizing in microns) based on the type of item being galvanized (also called the Material Category). The material categories are: Plate, Structural Shape, Strip and Bar. Pipe and Tubing. Wire, and Rebar (see Table 1 of ASTM A123 below).
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.
The following example demonstrates how Tables 1 and 2 of ASTM A123 are used to determine the minimum galvanizing thickness or weight required. If structural shapes greater than 5/8 inches in thickness are being galvanized, Table 1 indicates that the coating grade is 100. Table 2 indicates that the galvanizing thickness of Coating Grade 100 is 100 microns or 3.9 mils. The weight of the galvanizing is 705 grams/square meter or 2.3 ounces/square foot.
Methods for Measuring Galvanizing Weight or Thickness
Coating weight or thickness can be measured by one of four methods. Stripping, Microscopy, Weighing Before and After Galvanizing, or Magnetic Thickness Measurement. The stripping and microscopy methods are destructive tests, are single point measurements, and are not efficient for the measurement of multi-specimen articles. 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. The magnetic thickness measurements are also non-destructive and can be used to take many readings on many articles very quickly. Details on all four methods can be found in ASTM A123, but this article only addresses the use of the magnetic thickness gages.
Measuring Galvanizing Thickness Using Magnetic Gages
When using a magnetic gage, a specialized probe may be needed to measure tight configurations. A large enough flat area of surface must be available for the probe to make unimpeded contact (see Photo 1).
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.
The number of articles to be sampled, the number of thickness measurements to be taken and the acceptance criteria are discussed.
Step 1. Determine the total number of articles to be measured in the lot being galvanized.
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. 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).
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. In other words, the two sizes are sampled separately. And as indicated above, a lot generally consists of articles of the same size and shape.
A single specimen Article size is 160 square inches or less. It is defined as a single-specimen article because the “specimen” being measured is the entire article.
A multi specimen article is greater than 160 sq.in. 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.
Step 2. Sample selection.
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.
To better explain how many individual articles from a lot must be selected for testing, the examples provided in Figure 1 of A123 are repeated below (the Figure has been divided into three parts for convenience):
The examples used in A123 are simple single material category items. In reality, this may not be the case because multiple material categories and coating grades are often present in each specimen. This is especially true for fabricated items as discussed later.
Step 3. Thickness measurement and Acceptance Criteria
Single specimen, single material component. A minimum of 5 spot readings of multiple readings each (generally accepted to be three ) are taken spaced evenly over the specimen (article). The 5 spot readings are averaged. Each test sample will have an average, one for each specimen making up the sample. In the example in A123 above, the three averages are compared to the dry film thickness requirement for compliance and are averaged together to verify compliance to the required thickness. Any of the three  averages may be one coating grade below the requirement provided the grand average is the coating grade requirement. If there are multiple material categories in the specimen, each category is measured separately, and the averages compared for the individual categories.
Multi specimen, single material category. A minimum of 5 spot readings are obtained in each specimen area. The readings in each area are averaged to obtain 3 averages per article. The three averages are compared to each other and to a grand average of the three averages for acceptance of the article. Any of the three  averages for the article may be one coating grade below the requirement providing the grand average for the article meets the coating grade requirement. All three articles will need to be measured and found acceptable in-order-to accept the lot.
Multi specimen, multi component. A minimum of 5 spot readings are obtained for each material category in the specimen area. It is useful to identify each area for clarity.
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. Following is an example of dividing a structure, determining the material categories present and surfaces to be measured, and establishing the acceptance criteria.
Thickness and Acceptance Criteria Example – Example 1 – Pole Structure
The following photos and table are an example of dividing a structure (pole) for measurement and deciding how many surfaces must be measured. The pole in Photo 2 is arbitrarily called left, middle and right. There is no set formula except that the areas must be nominally equal.
Visually inspect each of the three areas to determine the number and type of material categories present. (Note: Not shown in Photo 2 are the end cap and handhold cover plate which would likely be less than 160 sq. in. and thus need to be measured separately, if galvanized. Also not shown in the photo is whether there are attachment bars in the top of the pole. If present, they will also have to be included for measurement). Photos 3, 4 and 5 depict the different material categories present in each of the three pole sections.
Acceptance Criteria – Example 1 Pole Structure
The following table identifies the material categories in the left, middle, and right sections of the pole example together with the requirements for galvanizing thickness.
*Single Specimen Articles. 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.
*Multi-Specimen Articles. 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. Categories are not blended. 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.
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. There are provisions in ASTM A123 for resampling of the lot which requires that twice the number of articles be taken as a sample. 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.
Example 2 – Truss
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. Once fabricated as in this example, the wall thickness cannot be measured to determine the proper coating grade requirement. The galvanizer and inspector will need drawings to determine the wall thickness, if an open end is not available for measurement.
Conclusion – Achieving the proper thickness of hot-dip galvanizing is critical in assuring it achieves its expected performance. 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. 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.
A high-level summary of the selection and measurement process can be summarized as follows:
- Determine whether the articles (items) in the lot are single or multi-specimen articles based on the size of the articles. Assign each article to one of the two categories.
- Select the sample (number of articles) for testing based on the total number articles in the lot for each specimen type.
- For the sample of single-specimen articles, the entire article represents the specimen to be measured. For multi-specimen articles, divide each article into thirds to select the specimens for measurement (3 specimens for each article).
- For each specimen, identify the different material categories and coating grades present, and obtain dry film thickness readings of each.
- Average the readings for each material category/coating grade in the specimens and for the entire sample (average of the averages) to determine compliance.
Written by Tom Neal & Doug Reardon
Other topics in this galvanizing series:
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 5 – Coatings for galvanizing
Topic 6 – Inspection of surface preparation and coating application