GB/T 38804-2020Test method for oxidation of metallic materials exposed in high-temperature steam (English PDF)

5.2 Test section's temperature

The temperature of the test section reflects both the temperature of the steam and the specimen. The temperature of the test section is controlled by a temperature control device, and thermocouples are used for real-time temperature monitoring. The selection and use of thermocouples shall comply with the provisions of JJG 141, JJF 1637 or JJG 368. The temperature deviation of the test section shall comply with the following provisions:

a) it shall be less than ±3 °C when below 650 °C;

b) it shall be less than ±4 °C when at 650 °C ~ 800 °C;

c) it shall be less than ±5 °C when above 800 °C.

5.3 Reaction chamber's pressure

During the test, a pressure gauge or pressure sensor is used to monitor the pressure of the reaction chamber. The selection and use of pressure gauges and pressure sensors shall comply with the provisions of GB/T 1227. The pressure deviation of the reaction chamber shall be minimized. When the test pressure value is greater than 10 MPa, the pressure deviation of the reaction chamber shall be less than ±2 %.

5.4 Test steam

The test steam is prepared from deionized water or distilled water, and the test water shall comply with the provisions of Class III electronic water in GB/T 11446.1-2013. The test water shall be deoxygenated before the steam is produced, and the oxygen content in the steam or test water shall be indicated in the test report. If other gases are added to the steam, the final gas content ratio of the steam shall be indicated in the test report.

5.5 Flowmeter

During the test, the steam flow rate or water flow rate shall be monitored. The selection and use of flowmeters shall comply with the provisions of JJG 2063.

6.1 Shape and size

The storage environment of specimens shall be kept dry.

7.1 Preparing the specimen

After machining, the specimen is subjected to surface treatment according to 6.3. Measure size and mass and record the initial value. Avoid touching the specimen directly with your hands during measurement.

7.2 Placing the specimen

7.2.1 When placing the specimen, it should be suspended so that the surface of the largest area of the specimen is parallel to the direction of steam flow. The diameter of the specimen hanging hole shall be 1 mm, and if it is larger than 1 mm, it shall be indicated in the report. There shall be enough space between the suspended specimens to prevent the specimens from touching each other.

When hanging the specimen, the contact between the hanging wire and the specimen shall be reduced, and nickel-chromium alloy is recommended as the hanging wire.

7.2.2 When hanging specimens cannot be used due to equipment layout and other problems, the specimen can be placed flat in the tubular specimen holder.

When laying the specimen flat, the contact area between the specimen and the specimen holder shall be minimized. Each specimen shall be placed separately and shall not be in contact with each other. The surface of the largest area of the specimen shall be parallel to the direction of steam flow. The material of the specimen holder should be corundum.

7.3 Deoxygenating the reaction chamber

After the specimen is placed in the reaction chamber, close the reaction chamber to ensure that air cannot enter the reaction chamber, and use inert gas argon or nitrogen to purge the reaction chamber so that the oxygen content (volume fraction) in the reaction chamber is less than 0.0003 %.

7.4 Heating up the reaction chamber

The reaction chamber begins to heat up after deoxygenation, and the temperature rise rate shall not be too fast to cause the oxide scale to exfoliate.

The recommended temperature rise rate is 55 °C/h ~ 220 °C/h. When the difference between the temperature of the test section and the preset test temperature is less than 50 °C, steam is filled. When the temperature of the test section reaches the preset temperature and stabilizes, it is regarded as the starting time of the test timing. If the test needs to be carried out under high in square centimeters (cm2); a1 - the average mass gain per unit area of the reference specimen, in milligrams per square centimeter (mg/cm2);

B - the total area of the non-machined surface of the specimen before the test, in square centimeters (cm2).

8.3 Oxide scale thickness

The measurement of oxide scale thickness requires cross-sectional observation of the specimen. The oxide scale shall be protected by electroplating, inlay, etc. before preparation of the cross-section specimen. The cross-section of the oxide scale is prepared by wire cutting or sanding, and the cross-section shall be perpendicular to the surface of the specimen. When measuring the thickness, the cross-sectional characteristics of the oxide scale shall be considered, and the oxide scale thickness at the edge position or exfoliating position shall not be recorded as the general thickness. During recording, it shall observe a cross-section with a length of at least 10 mm, and record including but not limited to the maximum thickness, minimum thickness, and average thickness above 5 data points. If there is delamination in the specimen, the inner layer, outer layer, internal oxidation zone and total thickness of the oxide scale (inner oxide scale + outer oxide scale + internal oxidation zone) shall be recorded separately.

8.4 Observation of oxide scale morphology

8.4.1 The surface morphology of oxide scale should be observed and recorded using a scanning electron microscope. See Figure A.1 for the surface morphology of typical oxide scale.

8.4.2 The cross-sectional morphology of oxide scale should be observed and recorded using an optical metallographic microscope or a scanning electron microscope with backscatter mode. See Figure A.2 for the cross-sectional morphology of typical oxide scale.