ASTM G76-18 Standard Test Method for Solid Particle Impact Erosion Tests Using Gas Jets

The ASTM G76-18 Standard Test Method for Erosion Testing by Impact of Solid Particles Using Gas Jets, developed by the American Society for Testing and Materials (ASTM) , describes a test method for determining material loss by impact erosion of solid particles carried by gas using jet nozzle type erosion equipment.

This test method can be used in the laboratory to measure solid particle erosion of different materials and as a screening test to rank the solid particle erosion rates of materials in simulated service environments. Actual erosion service involves a wide range of particle sizes, velocities, angles of attack, environments, and similar conditions. Therefore, any single laboratory test may not be sufficient to evaluate the expected service performance. This test method includes a well-characterized procedure for measuring solid particle impact erosion for which interlaboratory test results are available.

For the purposes of this standard, the following terms and definitions apply:

• Erosion refers to the gradual loss of original material due to mechanical interaction between a solid surface and a liquid, a multicomponent liquid, or impinging liquid or solid particles.
• Impaction refers to a process that results in a continuous train of impacts between liquid or solid particles and a solid surface.
• The erosion value is the value obtained by dividing the volume loss of the sample material by the total mass of abrasive particles striking the sample.
• The normalized erosion rate is the value obtained by dividing the erosion value of the sample material by the erosion value of the reference material.

This test method uses a repeated impact erosion approach that involves a small nozzle delivering a gas stream containing abrasive particles and impinging on the surface of a test specimen. A standard set of test conditions is described. However, deviations from some of the standard conditions are permitted if described in detail. This allows laboratory-scale erosion measurements under a range of conditions. This standard describes test methods for preparing specimens, performing erosion exposure, and reporting results.

The importance of this test method in any overall measurement program for evaluating the wear behavior of materials depends on many factors related to the conditions of service applications. Users of this test method should determine the degree to which the results obtained correlate with results from field performance or with results obtained using other test systems and methods. This test method can be used to rank the wear resistance of materials under specified test conditions.

The apparatus used in these tests can abrade material from a test specimen under well-controlled exposure conditions. A schematic drawing of the outlet nozzle and particle-gas feed system is shown in the figure provided in the standard. Deviations from this design are permissible, but adequate system characterization and control of critical parameters are required. Deviations in nozzle design and dimensions should be documented. The nozzle length-to-diameter ratio should be 25:1 or greater to obtain an acceptable particle velocity distribution in the flow. The recommended nozzle consists of a 1,5-mm-long tube with an approximately 50-mm inner diameter manufactured from an abrasion-resistant material. Nozzle wear should be monitored during service and should not exceed a 10 percent increase over the initial diameter.

Required features of the instrument should include means for controlling and adjusting particle impact velocity, particle flux, and sample position and orientation relative to the impinging stream (impact angle).

Various means can be provided to introduce particles into the gas stream, including a vibration-controlled hopper or screw feed system. The system must provide a uniform particle feed and be adjustable to accommodate the desired particle flow values.

A method of measuring particle velocity shall be available for use with the abrasive equipment. Examples of acceptable methods include high-speed photography, dual spinning disks, and laser velocimetry. Particle velocity shall be measured at the location to be covered by the sample and under test conditions.

This test method can be used on a variety of sample sizes and configurations. A suitable sample configuration is a rectangular strip approximately 10 mm x 30 mm x 2 mm thick. Larger samples and other shapes may be used if necessary, but must be documented. The abrasive material used should be uniform in essential properties such as particle size, moisture, chemical composition, etc. Sampling of the material to obtain representative test samples should be in accordance with acceptable statistical practice. ASTM E122 applies.

This test method defines the following standard conditions.

• The nozzle tube must have an inner diameter of 50 mm plus/minus 1,5 mm by a minimum length of 0,075 mm.
• The test gas shall be nominally dry air. The test report shall state the amount of water contained in the test gas, at what pressure, and how the measurement was performed. Interlaboratory testing has used cylinder-type compressed air with a water content defined as minus 50 degrees dew point. A similar level of dryness is recommended regardless of which gas source is used in the test.
• Abrasive particles should be nominal 50-µm, equivalent to those used in interlaboratory test series. Abrasive should be used only once. Typical size distribution (determined by sedimentation): 20 percent 83 to 100 µm, 42 percent 57 to 50 µm, 48 percent coarser than 50 µm.
•  Abrasive particle velocity measured at sample location 30 plus/minus 2 m s⁻¹ At this speed, the gas flow rate is approximately 0,13 L/s and the system pressure is approximately 140 kPa, but the pressure depends on the specific system design.
• To achieve steady-state conditions, the test duration should be 600 seconds. Longer times are permissible as long as the final wear crater is not deeper than 1 mm.
• The angle between the nozzle axis and the sample surface should be 90 degrees plus/minus 2 degrees.
• The test temperature should be normal ambient temperature (typically between 18 degrees and 28 degrees).
• The particle feed rate should be 0,033 plus/minus 0,008 g/s. This is approximately 2 mg mm on the sample surface under standard conditions.⁻²· s⁻¹This corresponds to a particle flux of . Particle flux determination requires measuring the eroded area on the sample and is subject to significant error. The measured width and depth profile of an erosion crater produced using the specified conditions is shown in the figure and shows a typical erosion width/depth relationship.
• The distance from the sample surface to the nozzle tip should be 10 mm plus/minus 1 mm.

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