HIGH-TEMPERATURE THERMAL CORROSION TESTING AND PREVENTION

This figure shows several heating cycles on a hanging steel sample. The steel sheet was heated at a rate of 5 K/min in a nitrogen atmosphere with 16% oxygen. Oxidation (mass increase) decreases with each subsequent heating cycle. At the beginning of the test, oxidation of the sheet surface occurs. This can be observed in the early onset and rapid mass increase for the first heating. After a couple of heating cycles, inner oxidation occurs, which is indicated by a slower, diffusion-dependent mass increase.
In this article, we'll examine corrosion products produced by high-temperature exposures and explain how this information can be used in corrosion prevention.

Because high-temperature corrosion tends to produce scale that is multi-layered, line scans and spot maps may be used to illustrate the multi-layers but these techniques are qualitative. They rely on displaying total counts; density of the location can impact the results. Thus, it is also useful to supplement these analyses with quantitative or semi-quantitative data reductions.

The use of TREC-IR tests on a sample can be used to identify the source of the corrosion; this test does not require extensive analysis to interpret the results. A good corrosion control strategy must consider multiple corrosion sources.

The first two factors, temperature and time, are linked; therefore, it is essential to ensure that the tank has a maximum operating temperature of 160F. When the tank is heated above this temperature, the oxidation of the iron catalyst in the thermocouple will be offset by the expansion of the steam which will cause the thermocouple to reach a higher temperature than the tank.

To reduce the effect of this interaction, the maximum operating temperature must be set at 160F; thus, it is necessary to have a thermal management plan in place that reduces the tank temperature when it reaches 160F, and raises the temperature above that level when the thermocouple is activated.

Furthermore, it is also necessary to ensure that there are no thermocouple activations between 160F and 200F; at this point, most corrosion products will have reached the catalyst levels necessary to oxidize the iron, and thermocouple activation will occur.

A strategy must also be in place to monitor the temperature; a failure to do so can result in a tank reaching a temperature which is too high when the thermocouple is activated, which will be wasteful of energy and result in a tank that is not activated when tested, resulting in a false-positive result.

TREA-IR can be used to identify the thermal corrosion process, although this test is more suited to indicate the rate and extent of corrosion than the thermal corrosion product. This test requires only a single sample; therefore, it can be used to monitor tank conditions.

The final test to identify thermal corrosion is the ASTM standard ASTM D-5236. This test is an on-board test; therefore, the test is non-invasive and quick to perform. The test requires only one sample to be taken and requires only a minimum of 4 hours for completion. It is also portable and can be used in an on-site test.

These are quick, non-invasive, non-confrontational tests that can be performed on-site. They are non-invasive to the inspector and the tank, and can be performed by the inspector on a single day's notice.

All of these tests are practical, quick and inexpensive tests that can be performed by the inspector to determine the status of the tank, and their results are evident, visible and recorded in the inspector's notebook. They can also be performed for statistical purposes, indicating the average time to reach the thermal corrosion tipping point, the average temperature during the thermal corrosion process, the percentage of steel, the percentage of aluminum, and the percentage of copper, etc.

In order to obtain the complete picture, and obtain the most accurate data possible, the inspector should collect a total of ten samples for each of the treatments. If the test results indicated any significant differences, the inspector should compare the ten samples, and record the results, including the percentage of corrosion product in each sample. This is a more thorough test, and a more accurate, accurate measure of tank corrosion can be achieved.

In order to obtain an accurate measure of tank corrosion, the inspector should collect a total of 20 samples. If the results indicated any significant differences, the inspector should compare the 20 samples, and record the results, including the percentage of corrosion product in each sample. The tank corrosion data is a more complete, accurate and complete measurement of tank condition that can be obtained when a total of 50 samples are taken. It gives the inspector the complete picture that is true and accurate.

In conclusion, if an inspector takes appropriate precautions, and performs the tests, the result is an accurate and complete tank condition report.

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