K. Daniel Efird

Coatings tests using standard methods can take from 30 to 90 days for a single test. Efird Corrosion International has developed acoustic emission techniques to detect failure of a coating during testing before the failure can be observed visually. This significantly shortens the time required for a test, provides information on the nature of the substrate/coating/environment interaction, and adds a great deal of flexibility to the design of the test apparatus. Acoustic emission is unique as a test monitoring technique in that it originates in the material, and does not require an external source of energy as do ultrasonic or electrochemical techniques.

Acoustic emission monitoring of a coating in a specific environment allows determination of the failure temperature, and the type of failure occurring. Each coating failure mechanism possesses unique acoustic emission characteristics. Analysis of these characteristics at each temperature reveals what is happening to the coating at that temperature, determines if the coating will fail at that temperature, and evaluates the mode of failure.

The most widely used acoustic emission signal measurement parameters are counts, amplitude, rise time, and the measured area under the rectified signal envelope (MARSE) sometimes referred to as energy counts. The relationship of these parameters is shown in Figure 1.

Figure 1. Commonly measured parameters of an acoustic emission signal.

The acoustic emission stress source with respect to coatings is a result of the penetration of water, gasses, and ions through the coating and their interaction with the coating/substrate interface. This interaction at the interface can result in coating disbondment and blistering, which are acoustic emission sources. A second source of acoustic emissions is the corrosion process on the steel surface under the coating.

Typical Experimental Setups

An experimental setup for acoustic emission monitoring for a cathodic disbondment test of external pipeline coatings is shown in Figure 2. This is a modification of ASTM G 95, "Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings (Attached Cell Method." The test section can be either a coated plate of a section of coated pipe.

Figure 2. Test apparatus for monitoring acoustic emission activity for external pipeline coatings in a cathodic disbondment test.

This method can also be used to test internal tubular coatings or internal tank and vessel coatings according to ASTM. This makes the technique extremely versatile, and allows testing of a wide range of coatings in a wide range of applications. Details of the ASTM C868 Atlas test cell and the internal tubular coating test cell are shown in Figures 3 and 4, respectively. The internal tubular test cell is shown with thermal insulation covering the test sample to simulate downhole conditions. Removal of the insulation allows investigation into the results of a pronounced cold wall effect on the coating performance.

Figure 3. Atlas test cell for monitoring acoustic emission activity as a function of temperature for internal tank and vessel coatings.

Figure 4. Test cell for monitoring acoustic emission activity as a function of temperature for internal tubular coatings. The external insulation is omitted to allow study of the cold wall effect.

Example Results

Acoustic emission monitoring of coatings in an atlas test cell demonstrated that acoustic emission provided an indication of coating failure within 48 hours during environmental exposure at a test temperature and pressure. This permits a test procedure where the test temperature is stepped at 48-hour intervals to determine the failure temperature of a specific coating for a specific set of environmental conditions. Differences in the acoustic emission signature allow evaluation of the coating failure mode, i.e., disbonding, blistering and/or ruptured blisters.

Examples of the results are given in Figures 5 and 6. Acoustic emission tests were conducted on an amine epoxy coating exposed to 3-phase environments (water/oil/gas). The water phase was a buffered NaCl solution (Figure 5) or distilled water (Figure 6). Exposures were for 48 hours at incremental temperatures of 60°C to 100°C.

Figure 5. Acoustic emission activity as a function of temperature for an amine epoxy coated plate with 3% NaCl + 1000 ppm HCO₃ as the aqueous phase.

The coating in the buffered NaCl solution (Figure 5) showed increased acoustic emission activity at 100°C. Post test examination found disbondment between the coating and the steel substrate. With distilled water as the aqueous phase (Figure 6), increased acoustic emission activity began at 70°C, increased at 80°C, and was extremely active at 90°C. Post test examination of the test sample from this experiment found ruptured blisters and extensive disbonding between the coating and the steel substrate. This acoustic emission behavior is typical of that observed in other coatings in similar test environments.

Figure 6. Acoustic emission activity as a function of temperature for an amine epoxy coated plate with distilled water as the aqueous phase.

The Bottom Line

The use of acoustic emission methods developed by Efird Corrosion International to monitor coating performance during testing provides significant advantages over standard test methods: