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How are Multi-Specimen Test Articles Inspected for Coating Thickness?


To properly evaluate coating thickness for materials hot-dip galvanized under ASTM A123, a sample of randomly chosen test articles are selected to represent the lot, and the individual test articles from the sample are examined for conformance to the specification requirements. However, within ASTM A123 the terms specimen and test article are used interchangeably for single-specimen test articles, which can lead to confusion regarding inspection requirements and correct subdivision of multi-specimen test articles.

single specimen test article is a test article of surface area equal to or less than 160 in2 [1032 cm2] and comprised of only one material category and steel thickness range as delineated in ASTM A123 Table 1. A multi-specimen test article is comprised of multiple material categories, multiple thickness ranges, or any single material category or thickness range which has a surface area greater than 160 in2 [1032 cm2]. The explanations below can be used to differentiate the coating thickness inspection requirements for multi-specimen test articles.

Sampling & Sub-Division of Multi-Specimen Test Articles

First, randomly select the minimum number of articles from the lot according to ASTM A123 Section 7.3 to become multi-specimen test articles. Then, divide each test article into the different material categories and thickness ranges as delineated in ASTM A123 Table 1, each of which becomes a separate specimen.

Even if articles comprised of different material category and thickness ranges require the same coating grade, such as Grade 75 for both ½ in wall thickness (WT) pipe and ¾ in WT pipe, this does not mean the pipes can be inspected together. The two pipes within the test article are in different thickness categories, and must therefore be separated into two specimens for inspection.

Once the material categories and/or thickness ranges are separated into separate specimens, they must then be evaluated for surface area to determine if a further subdivision is required.  For any specimen of surface area ? 160 in² [1032 cm²], there is no need to further subdivide the specimen. Otherwise, specimens of surface area > 160 in² [1032 cm²] must be further subdivided into three continuous local sections of the equivalent surface area, and each subsection (or third) will constitute a specimen instead. It is incorrect to divide large articles by area first (into three specimens), and then further by material grade and thickness range.

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Example Sub Division of Specimens for Multi-Specimen Test Articles

Acceptance Criteria & Best Practices

To represent the coating thickness of each specimen, average five or more widely dispersed measurement readings over the surface area of each specimen. However, five measurements may not always be sufficient to represent specimens of many pieces or sides. Best practice is to ensure a sufficient number of readings are taken to represent the entire surface of the specimen. Every specimen must have a minimum average coating thickness of one grade below the minimum required per ASTM A123 Table 1, which is based on the order of thickness grade values listed in ASTM A123 Table 2

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Finally, for the entire sample size of test articles, evaluate one material category and/or thickness range at a time to determine if the test articles meet the inspection requirements. When evaluating one material category and/or thickness range, do not consider any others which make up the test article. For any material category and/or thickness range of surface area ? 160 in² [1032 cm²], every specimen in the sample made from that material category and/or thickness range must be averaged, meeting the minimum average coating thickness required per ASTM A123 Table 1.  For any material category and/or thickness range of surface area > 160 in² [1032 cm²], the three specimen measurements for each material category and/or thickness range within each test article must be averaged, meeting the minimum average coating thickness required per ASTM A123 Table 1.

Issues related to insufficient coating thickness can be the result of using an uncalibrated gauge or poor testing methods. This can be prevented by following instructions for calibration, verification, and adjustment from the gauge manufacturer and by following best practices for obtaining coating thickness measurementslisted within ASTM E376, Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Testing Methods. Additionally, utilize only one system of measurement to avoid unit conversions and minimize confusion during an inspection.

New Galvanizing Note

Be on the lookout for an upcoming Galvanizing Note to provide an in-depth explanation of the inspection for single specimen test articles versus the different types of multi-specimen test articles, including four detailed examples for clarification. The Galvanizing Note will be distributed to all TechNotes recipients and will be available for download on the AGA website. For further assistance related to the inspection of multi-specimen test articles, or to be added to the TechNotes distribution list, please contact Alana Hochstein.

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