Let’s Talk About a New Coating/Lining “Tapping Test” Standard

KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 7 – July 2024

William Corbett, COO

AMPP Senior Certified Coating Inspector & Certified Protective Coating Specialist

KTA’s Certified Coating Inspector Forum is designed to provide professional development/continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Introduction

Did I pique your interest with the title of this month’s post? That’s right… in June 2024, ASTM International published a new Standard Practice, D8555 Conducting a Coating/ Lining Detachment Acoustical Sounding (Tapping) Inspection. Staying abreast of new standards and changes to existing standards is a very important role of a certified coating inspector. This column describes the content of a new Standard Practice that has been in use for decades but never formalized into a standard procedure, until now. The standard is available on the ASTM website (www.astm.org).

Origination of the Practice

For many years, engineers and inspectors conducted “sounding” tests on coatings and linings during maintenance outages at power generating facilities to determine potential areas of detachment. The test was considered an alternative to widespread destructive testing and subsequent time-consuming repair of test areas. Despite the advantages of the qualitative “tap test,” a major limitation was the lack of a standard method for conducting it. Without a written standard to govern this or any other testing, the uniformity of the testing process and interpretation of results can vary.  

During the June 2023 meeting of ASTM Committee D33 (Protective Coating and Lining Work for Power Generation Facilities), representatives discussed the value of formalizing and publishing a procedure for conducting acoustical sounding (tapping) of coating/lining systems and indicated it had widespread applicability beyond power generating facilities. ASTM Subcommittee D01.46 (Industrial Protective Coatings) agreed to undertake the project, draft a document, and shepherd it through the consensus process.

What is Acoustical Sounding (Tapping) Inspection?

The idea of acoustical sounding to help determine areas of delamination is not a new concept. Concrete sounding using the drag chain method (see photo) has been widely used by state highway agencies for quickly checking concrete roadway decks for delamination, which can result in potholes and other structural issues. And you have probably used this method yourself. Unless you had a stud finder, you found wall studs by lightly tapping on the drywall with your knuckle and listening for tonal differences.

Coating/lining condition assessments are based, in large part, on visual observation of defects in (or damage to) the coating/lining system. The visual observation process also reveals precursors of defects (e.g., discoloration). These visual inspections are frequently augmented with other tests such as coating thickness measurements and destructive adhesion testing as needed.

Since poor adhesion to the substrate may not be visually evident, acoustical sounding (tap testing) is a non-destructive alternative to widespread, random, destructive testing for quickly identifying areas of potential coating/lining detachment for more intrusive examination. It is particularly useful for linings subjected to immersion service since staining or corrosion products can mask defects, and linings with high cohesive strength can sometimes visually mask disbondment from the underlying substrate.

An acoustical sounding or tapping inspection is conducted by lightly tapping newly installed or previously applied coating/lining systems using a non-sound dampening tool to detect relative differences in reflective sound, which may be an indication of detachment from the substrate. Examples of tools that may be used for a sounding inspection include the handle of a screwdriver, scraper, putty knife, wood chisel (shown), 5-in-1 tool, small drift punch, etc. The tool material cannot dampen the sound. A tool containing a wooden or rubber handle, or a rubber mallet will not work for this type of inspection, and hammers can damage the coating.

With minimal training and proper tool selection, one can perform the test by following the Standard Practice and demarcate areas that produce a hollow/dull/softer sound (relative to a sharp, crisp tap sound) for further investigation by destructive adhesion testing per ASTM D3359[1], D4541[2], D7234[3], or D6677[4].

When performing an acoustical sounding or tapping inspection on a coating or lining, it is important to let the tool do the work. The weight of the tool should be sufficient to perform the inspection; otherwise, damage to the coating/ lining may occur. The tool should be loosely held in the hand. Tethering may be required, and gloves should be used since you will be holding the sharp end of the tool.

While the test is relatively easy to perform, finding areas of potential coating detachment is subject to interpretation of the tonal differences in the produced sound, and just like concrete sounding using dragged chains, areas with high levels of ambient background noise can make it difficult to distinguish differences in sound. It is recommended to have all parties agree on the tool to use, experiment before using it for actual testing, and agree on the force to use and the tonal sound to listen for.

Summary

Certified coating inspectors may be asked to perform coating/lining condition assessments. Adhesion testing to assess the bond of the coating system to the underlying substrate is frequently part of the assessment process; however, these tests are destructive to the coating film and require the areas to be repaired. They can also be time consuming to perform. An acoustical sounding (tapping) inspection, when performed according to ASTM Standard Practice D8555 can be used to quickly distinguish well-adhered coating/linings from potential areas of detachment, which can then be subject to further investigation (including destructive adhesion testing) as required. 

[1] Standard Test Methods for Rating Adhesion by Tape Test, ASTM International (www.astm.org)

[2] Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, ASTM International (www.astm.org)

[3] Standard Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers, ASTM International (www.astm.org)

[4] Standard Test Method for Evaluating Adhesion by Knife, ASTM International (www.astm.org)