“ASTM G6 “Abrasion Resistance of Pipeline Coatings”
In this test, coatings are subjected to abrasion by a slurry of abrasive in water. Evaluation of wear by slurry abrasion is applicable to coatings applied to the inside of pipe or vessels that carry liquid with entrained solids. The test can be used for comparing different coatings for their resistance to slurry abrasion.
Externally coated tubes are inserted through fittings in the ends of a steel drum, as shown in the Picture 2. The drum, filled with a mixture of water and abrasive, is rolled on a rolling mill at 100 linear ft/minute to effect the slurry abrasive wear. The specified slurry consists of 2/4 mesh alumina in tap water, producing a combined action of light impact and abrasion at the periphery of the drum. The test samples consist of coated steel tubes, 16 inches long by 0.75 inches outside diameter. Up to nine coated tubes can be run in one test, normally three coatings in triplicate. The drum is 12 inches long and 14 inches in diameter. The test is conducted at room temperature (23°C) for 8.3 days.
Regularly, over the duration of the test, the electrical resistance is checked between the drum and the inside of each tube. An abrupt drop in resistance indicates that a breach has occurred somewhere in the coating, either due to chipping or abrasive wear. It is recommended that a reference coating with known wear characteristics be run at the same time as the test coating.
Customized Test Conditions
Often the test conditions of ASTM G6 are not representative of different service conditions. Therefore, the test conditions are customized to be more representative of the field conditions. Among the parameters which can be modified are:
- Faster or slower rotational rate of the drum.
- A milder or more aggressive abrasive medium, eg. silica versus alumina sand.
- A different size of abrasive medium.
- A different liquid medium, eg. oil or a water/oil emulsion.
- Extended run time (eg. 28 days).
- Inclusion of bare or coated concrete rods.
- Single rather than triplicate specimens for coating scoping purposes.
Measurement of Wear
In addition to electrical resistance loss, more detailed measures of wear can be conducted:
- The impedance of each coating can be measured in-situ at intervals during the test. This provides a more quantitative measure of the rate of film loss or physical and chemical deterioration.
- Loss of film thickness, based on measurement of film thickness before and after the test to 0.00005 inches.
- Visual inspection of the coated tubes at intervals during the test, to check for abrasion or chipping.
- Weight loss over the duration of the test. This measurement is only feasible when the weight change for the coating is large relative to the total weight of the coated tube.
ASTM G6. Abrasion Resistance of Pipeline Coatings
Picture 1. Drum on rolling mill with coated tubes in place
Picture 2. View of the interior of the drum with samples and slurry
Picture 3. Tested samples showing varying degrees and mechanisms of slurry abrasion wear