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Why is Abrasive Conductivity Test So Important to Perform - ASTM D4940

Updated: Nov 18, 2021


Corrosion, also known as rust, is a significant problem for any metal. It occurs throughout the world in residential, commercial, and industrial environments. Any time metal is used, corrosion can occur on the metal’s surface.


The corrosion rate depends on several factors that the metal is exposed to, for example, metal in a residential or commercial environment will have a lower corrosion rate than metal in an industrial environment.


Soluble salt, if present within an environment, is a corrosion accelerator. Project specifications often limit soluble salt levels before applying the coating for corrosion protection.


Soluble salt can deposit on a steel surface in many ways. One such way is with abrasives during blast cleaning operations.


The abrasive conductivity test determines the presence of soluble salt, this article will explain everything about the abrasive conductivity test.


Use of Abrasive

Abrasives are widely used during blast cleaning to get clean, rough surfaces for coating applications. The abrasive is made of slag, cooled by air and water.


Slag cooled by seawater or other contaminated or salty water, contains many ionic substances. Ion (salt) contamination from abrasives then transfers to the steel surface. This accelerates the corrosion rate. Failure to remove the salt before coating can cause premature coating failure.


Project coating specifications usually set the upper limit of acceptable abrasive contamination, as well as asks for performing abrasive conductivity tests to ensure compliance.


Abrasive Conductivity Test Frequency

The specification also provides the test frequency; though conductivity tests are performed on the abrasive each time a new batch arrives at the construction site.


Abrasive Conductivity Test Apparatus

The test apparatus includes the following:

  • Conductivity meter

  • Electrode cleaner

  • Calibration solution

  • Pure distilled water

  • Glass beakers (600mL and 100mL)

  • Plastic measuring beaker

  • Funnel

  • Stirring rod

  • Filter paper

  • Timer

Conductivity Meter Accuracy

Before starting the test, the equipment’s accuracy is evaluated using cleaning and calibration solutions. This procedure varies depending on the type of conductivity meter used for the test.


For more information, refer to the instrument’s operating instructions. The following procedures apply to conductivity meters in the image.


Conductivity Meter Calibration Check

  1. Press the ON/OFF button to turn on the conductivity meter, and then press the CAL button until the display shows CAL.

  2. Open the electrode cover and wash the instrument’s electrode with the cleaning agent. Be sure to apply an appropriate amount of calibration solution to the electrode.

  3. When finished cleaning, close the electrode cover. Press and hold the CAL button for more than 2 seconds.

  4. If the instrument accuracy is correct, the reading displayed on the screen will match the specified reading after the time-lapse. If the measured value does not match the solution’s target value, recalibrate/adjust the device per the manufacturer's recommendations.

  5. Clean the electrode with tap water and shake it off to remove moisture. Press the MEA button on the instrument to put it in measurement mode.

Now that we have the instrument’s calibration, let's proceed with the sampling procedure.


SAMPLING PROCEDURE

  • Select a random bag of abrasives for sampling.

  • Measure 300 ml of abrasive and pour into the 600 ml beaker, then add 300 ml of water.

  • Start the timer and stir the solution with the stir bar for one minute.

  • Let the solution sit for 8 minutes, then stir again for 1 minute.

  • Place filter paper in the funnel to get enough solution into the 100 ml beaker.

  • Discard the first 10mL of filtrate.

It is time to check the sample’s conductivity using the conductivity meter. Following the steps below.


Abrasive Conductivity Measurement

  • Turn on the conductivity meter.

  • Rinse the electrode with clean water until the display shows that the water conductivity is equal to or less than 5 µS/cm.

  • Immerse the electrode in the sample in a 100 ml beaker.

  • Mix gently for 2-3 minutes.

  • Read and record the conductivity value of the sample on the screen.

You can now check the specification compliance using the abrasive’s conductivity value you recorded.


SPECIFICATION COMPLIANCE

Use the project’s coating specification to verify that the recorded value is below the upper value set by the specification.


If it does not meet the specifications, use other abrasive bags for additional measurements before taking corrective and preventive measures.


APPLICABLE STANDARDS

Project coating specifications often state the ASTM D4940 needed to carry out the abrasive conductivity test.


CONCLUSION

The conductivity test determines soluble salt or invisible contaminant, presence in abrasives. This test ensures that abrasives will not contaminate surfaces after sandblasting, because the salt contained in steel increases the corrosion rate and makes the coating prematurely deteriorate, making it almost impossible to achieve the ideal service life.


A coating project assigns paint coating inspectors to carry out the abrasive conductivity test. The inspectors get certified by attending paint coating inspection and quality control training to perform such a task.


Videos about protective coating are available on YouTube. You can also find protective coating training courses at the following website: www.iogs.org.


The Institute for Oil and Gas Sector provides the following comprehensive coating inspector courses. Getting into the coating inspection and quality control field you require completion of one of these courses:

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