High temperature oxidation is an important part of materials life testing, which can be used to study the oxidation resistance of materials in a simulated surrounding atmosphere at high temperature. This type of testing is useful for predicting the life of many engineering materials, including metals and alloys, in service. The test result can provide basic data for prediction of service life under realistic conditions, and some programs can be used to predict oxidation-related corrosion on the basis of the test data.
High temperature oxidation testing is generally conducted between 500 and 1200 °C, with 1100 °C being the most common temperature used. These tests are designed to assess the ability of materials to withstand oxidation in air and the results are typically expressed as the number of hours to Failure (Time to Failure, e.g. TTF). In order to ensure the test results are meaningful and reproducible, certain strict procedures have to be followed, such as controlling the atmospheric condition, temperature and air velocity rate or V-ratio, selecting suitable sample shape and size, preparing the samples to remove any surface contaminants and so on. The oxidation layer is carefully monitored at different temperatures and times, and at the end of the test, the samples are examined to determine the extent of Oxidation, Cracking, and Spalling.
The most important step prior to the oxidation test is preparing a high temperature and protective atmosphere. Generally, a furnace or muffle furnace is used to heat a chamber where the test sample and a protective atmosphere can be contained. The protective atmosphere during the oxidation test should minimize the oxidation rate and, therefore, minimize the amount of oxidation that eventually occurs. Commonly used atmospheres include an inert gas, such as Nitrogen, as well as an oxidizing gas, such as Oxygen, which can be controlled to give a defined level of oxidation.
Another important step is cleaning the sample surface. Contaminations on the sample surface will accelerate oxidation and may also give inaccurate results. Therefore, clean samples are required prior to the test. Remove any external oxidations or contaminants, such as oil and grease, from the sample surface by clean with a solvent, followed by a hot aqueous chemical cleaner, such as sulphuric acid, or chemical etching. Electric induction heating is also possible for cleaning, but the selection of an appropriate chemical process depends on the materials being tested.
In addition to the preparation steps, the oxidation test conditions, including the temperature and duration, must be carefully controlled and monitored. Generally, the oxidation test is conducted by increasing the temperature in stages, until the desired test temperature, usually from 500-1200°C, depending on the material, is reached. Each stage should be at least twice as long as the previous one, and it should be long enough to allow the predicted oxidation rate to be attained.Furthermore, the air velocity around the sample must also be controlled, with a V-ratio of 10-15 cm/min, to ensure that the temperature of the sample is uniform and that there is sufficient oxygen in the atmosphere. The atmospheric pressure should also be monitored, as changes in pressure can also affect the quality of the test results.
At the end of test, the sample needs to be observed and evaluated. Generally, the sample surface is examined by optical microscope and the amount and type of oxidation can be determined. The samples can also be sampled for analysis by Scanning Electron Microscopy (SEM). The test results are then compared against predicted results to check their accuracy.
In conclusion, high-temperature oxidation tests are used to assess the oxidation resistance of materials in a simulated environment at high temperatures. Carefully controlled test conditions, such preparation steps and evaluation of results will ensure meaningful and reproducible test results.
