Let’s Talk About Measuring Wall Thickness

KTA’s Certified Coating Inspector Forum Volume 3, Issue No. 8 – August 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

Most of the KTA Certified Coating Inspector Forums posted to the KTA University site since 2022 have focused on surface preparation and coating application inspection standards, practices, and instruments. However, since we are in the corrosion prevention industry, it is important to discuss how to determine whether metal section loss has occurred (due to corrosion, erosion, or other factors) and to what degree, so that engineering-based decisions can be made regarding structural integrity and the potential need for repair or replacement. This can be accomplished, in part by taking wall thickness measurements, which is typically performed on pipe, pressure vessels, tank shells/bottoms, and other structures where only one side is easily accessible. This issue of the KTA Certified Coating Inspector Forum focuses on measurement of cross-sectional wall thickness using manual, pulse-echo ultrasonic wall thickness gages either with or without coating present.   

Background

Measurement of wall thickness is considered a nondestructive examination/evaluation technique, where a probe with a couplant (a gel used to carry the ultrasonic signal) is placed on a metallic surface and ultrasound is emitted from the probe through the cross-section of the substrate. The velocity of the sound, or “transit time” (after it makes a round trip through the substrate and back to the probe) is halved and displayed in inches or millimeters. This is shown in the formula below, where T=Thickness, V=Velocity, and t= transit time.

T=       Vt / 2

The velocity that sound will move through a material is based on its physical properties. For example, sound will move differently through cast iron/ductile iron, versus carbon steel, versus aluminum. ASTM E797[1] provides guidelines for measuring wall thickness using the pulse-echo method on surfaces below 93°C (200°F), including gage use, verification of accuracy, personnel qualifications, and requirements to consider when developing a procedure. The standard does not indicate a recommended/required frequency of thickness measurements, so that should be agreed-upon prior to obtaining measurements. Some gages offer a scan mode, which allows the operator to scan the surface with the probe versus using a “place and remove” technique. Maintaining couplant between the probe and surface is critical and may be more challenging when using the scan mode.

Instrument Use

ASTM E797 lists three different types of instruments:

• Flaw detectors with A-Scan display/readout;
• Flaw detectors with A-Scan display and direct thickness readout; and
• Direct Thickness Readout

The focus of this article is on the Direct Thickness Readout devices.

While there are several manufacturers of ultrasonic wall thickness gages, and they may function somewhat differently, this column focuses on only one manufacturer (three models for different applications):

• DeFelsko UTG-C, with cabled, integral, and Xtreme probes,
• DeFelsko UTG-M, or multi-echo for taking measurements thru-paint, and
• DeFelsko UTG-CLF for measuring the thickness of cast/ductile iron, cast aluminum, and cast zinc.

A fourth model, the DeFelsko UTG-P is used for measuring the wall thickness of very thin metals and plastics. It is not addressed any further in this column.

The PosiTector UTG C single echo probe uses a dual-element transducer, a focused “V-path”, and V-path compensation to accurately measure the thickness of metals with heavy corrosion or pitting (see Figure 1). The UTG C single echo probe will not ignore the thickness of any coating; for best measurement accuracy, it is necessary to remove any coating present at the point of measurement, even in heavily corroded areas. Simply measuring then deducting the thickness of the coating from the wall thickness measurement is not recommended (see Pitfalls to Avoid later in this column).

The PosiTector UTG M (Multi-echo) probe uses a single element transducer to accurately measure the metal thickness of a new or lightly corroded structure while ignoring the thickness of protective coatings. The ultrasonic beam travels in a straight path to the material’s back wall at 90° relative to the surface. When three consecutive back wall echoes are detected, the probe makes a time-based calculation to eliminate the coating thickness from the gage reading (see Figure 2).

The DeFelsko UTG-CLF probe is used for measuring attenuative materials such as cast/ductile iron, cast aluminum, and cast zinc. A proprietary built-in algorithm distinguishes back wall reflections from grain noise reflections that are characteristic of cast materials. Note that any coating must be removed before using this probe on cast materials.

Verification of Accuracy

Ultrasonic wall thickness gauges should be verified for accuracy prior to each use, in the intended range of use. Note that there is often a small disc mounted directly on the probe attachment (see red circle in the image). This disc is used to “zero” the instrument (using couplant), it is not used for verification of accuracy. Verification of gage accuracy should always be performed in the intended range of use. That is, if ¼” steel is being measured, gage accuracy should be performed using a ¼” step block (described below).

Various “Step Blocks” are available for verifying accuracy (shown below) in both high and low ranges. A couplant must be used when verifying gauge accuracy. In most cases, gauge adjustment is possible (when necessary) to align the readout with the step block thickness. When feasible, greater accuracy is possible when the instrument is standardized on areas of known thickness of the actual material to be measured.

Qualifications of Inspector Personnel

While there are no specific qualification requirements to perform wall thickness measurements according to ASTM E797, Section 6 of the standard states that inspection personnel performing wall thickness measurements may need to be qualified to nationally/internationally recognized NDT standards such as ANSI/ASNT CP-189, SNT-TC-1A, NAS-410, or ISO 9712 when required by the project specifications or other contractual agreement. It is prudent to determine whether there are contractual requirements for qualification prior to conducting wall thickness measurements. If an owner doesn’t believe ASNT (or other) certification is necessary, a project-specific qualification test may be developed so that an operator can demonstrate the necessary skills (i.e., instrument operation, data interpretation, troubleshooting, reporting, etc.) prior to obtaining actual measurements on a project.

Pitfalls to Avoid

Here are six pitfalls to avoid when obtaining wall thickness data using a direct thickness readout device:

1. Verify the instrument is set up to read the thickness of the type of metal being inspected before taking measurements. Measuring the thickness of cast/ductile iron with an instrument set to read on carbon steel or aluminum will produce incorrect values.
2. Verify the correct probe is used (i.e., single echo, multi-echo, attenuative) based on the application before taking measurements.
3. Before taking measurements, verify the instrument is measuring correctly by using a Step Block of a thickness that represents the anticipated thickness. Taking measurements with an instrument that hasn’t been verified for accuracy may produce invalid data.
4. Verify sufficient couplant is on the probe/surface to carry the ultrasound.
5. Determine the frequency of measurements in advance. ASTM E797 does not address frequency.
6. When using a single echo probe, be sure to remove the coating; otherwise, its thickness will be included in the wall thickness measurement. While it may seem acceptable to simply measure the coating thickness with a dry film thickness gage/probe first, then subsequently deduct the thickness of the coating from the wall thickness measurement, sound travels through coatings at a different speed than through metal, so there is an opportunity for significant error using this approach. Coating thickness gages use magnetic induction or eddy current to measure thickness on metallic substrates, while wall thickness gages employ ultrasound.

Application Examples

Here are a few examples of how wall thickness measurement science is applied. Wall thickness is often monitored anytime it is considered critical for continuous, safe operation of a structure or structural component.

Cargo tank trucks

Cargo tank trucks transporting hazardous materials or waste require routine inspection according to the Pipeline and Hazardous Materials Safety Administration (PHMSA), Department of Transportation (DOT) Specification 49 CFR 180.407. This inspection is performed by qualified inspectors to ensure safe operation and maintenance of cargo tanks, including those that are used to transport hazardous materials, by identifying any evidence of wear, malfunction, or corrosion. Thickness testing must be performed every two years for unlined tanks transporting corrosive materials, when a tank has not been in hazardous materials transportation service for a period of one year or greater, or when there are visual signs of damage. This inspection requires the use of a wall thickness gage operated according to the manufacturer’s instructions. The thickness gauge used for the test must be capable of accurately measuring steel, steel alloy, and aluminum wall thickness to within ±0.002” (±0.06 mm). Wall thickness measurements must meet the “In-Service Minimum Thickness” requirements as specified in 49 CFR 180.407 to remain in service. Inspectors must record the minimum thickness measurements as specified in 49 CFR 180.417(b).

Pipelines

Pipelines carrying oil or natural gas, as well as fuel storage tanks are frequently checked for wall thickness to verify minimum wall thickness and detect early signs of corrosion/erosion prior to an integrity breach, which may be catastrophic to human life and the environment.

Tank Walls/Bottoms

Tank Walls/Bottoms in oil, asphalt, and even water storage tanks can be inspected to verify adequate section, to prevent environmental pollution or other issues.

Tank Cars

Rail tank cars are similar to cargo tank trucks and may be used to transport hazardous materials or waste, thereby requiring routine measurement of wall thickness.

Structural Steel Plate

New structural plate received with pitting or corrosion product can be assessed for wall thickness to determine whether it is suitable for acceptance/use as determined by referencing ASTM A6[2].

Summary

While it is common for a certified coating inspector to measure and record coating thickness, it is also possible that they may be required to measure and record wall thickness to determine potential corrosion and section loss. This can be done using ultrasound devices, and requires removing the coating prior to obtaining measurements, or using instruments that are designed to measure through the paint. Like coating thickness gages, wall thickness gauges must be verified for accuracy prior to use and inspector qualification according to various NDT standards may be required by contract.

[1] Standard Practice for Measuring Thickness by Manual Ultrasonic Pulse-Echo Contact Method, ASTM International (www.astm.org)

[2] Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling