Oxidation test exposure

The stringent requirements covering oxidation stability are defined by the test method DIN 51352, Part 2, known as the Pneurop Oxidation Test (POT). This test simulates the oxidizing effects of high temperature, intimate exposure to air, and the presence of iron oxide, which acts as catalyst - all factors highly conducive to the chemical breakdown of oil, and the consequent formation of deposits that can lead to fire and explosion. 

The Fujiwara test is used to determine whether a membrane has been oxidized by exposure to halogens. The test measures the presence of halogenated organics in a membrane sample. The test involves a small piece of membrane placed in the bottom of a test tube. One drop of 5N sodium hydroxide solution and 2 drops of a pyridine solution are added to the test tube. The tube is then placed in a water bath at about 90°C, and held there for 30 seconds. A positive test occurs when the pyridine layer in the test tube shows a red or pink color. Note that prolonged heating of the sample will cause the color to fade or turn to brown/yellow.7... 

Specimens with the dimensions of 10 X 1.5 X 1 mm were cut from cast rods and were ground with SiC paper to a 4000 grit surface finish. Before the oxidation tests the samples were cleaned ultrasonically in ethanol. Two kinds of oxidation experiments were carried out isothermal exposure tests at temperatures of 800 to 1000°C for 0.5 h up to 4 h and continuous mass change measurements at 900°C, both in laboratory air. To ensure rapid heating up the exposure tests were started by introducing the samples directly into the hot furnace. The same test conditions were used for the continuous thermogravimetric measurements by raising the hot furnace around the specimen chamber. 

Cyclic oxidation tests were conducted al 800°C in static laboratory air for up to 1500 cycles. Each temperature cycle consisted of exposure for one hour at reaction... 

The oxidation tests in air were cyclic and eventually after cumulative exposures of 1000 h at 800°C for Ii3Al, 775 h at 900°C for Ti Al and 75 h at 1000°C again for Ti Al visual deterioration (bowing, cracking, or spallation) was apparent in the condition of most coatings, which either did or subsequently would have undermined the protection they afforded. In some instances breakdown was observed directly gravimetrical-... 

In an effort to bridge the gap between furnace oxidation studies and arcjet oxidation tests, ManLabs performed a series offumace studies on UHTCs under various ranges oftime, gas velocity, temperature and pressure. " " " Furnace test conditions covered temperatures between 500°C and 2500°C, times from five minutes to four hours at air flow rates in two regimes, low velocity between 0.3 and 3 m/s and high velocity, between 3 and 75 m/s. Cyclic exposures were also performed for most of the candidates. 

Many commercial alloys exhibit what is called a para-linear rate law. Initial scale buildup is parabolic, followed after some time by scale breakaway [if]. A linear weight loss then develops. This linear rate may be related to the initial paraboUc rate, as in the case of scale breakaway and reformation. It may also be independent of the parabolic rate if the reformed oxide is non-protective in nature. The changing of rate laws, over time, is one of the reasons for the desirability of long-term test exposures. 

Metal loss is defined as one half the difference between the original thickness and the remaining thickness after exposure. This thickness extends to just beneath the scale/substrate surface and excludes the thickness of internal attack. This is schematically shown in Fig. 8, as depicted in ASTM G 54, Practice for Simple Static Oxidation Testing. Thus metal loss is defined as ... 

Temperature profiles in continuous isothermal exposure testing (a), discontinuous isothermal exposure testing (b) and thermal cycling oxidation testing (c). 

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