Figure 2 The color evolution of 928 (Left) and 50-1-4Φ (Right) aviation lubrication oil oxidized in high purity dry air without metal sheet (the numbers inset in the square pictures are color numbers measured by petroleum products chromometer)
According to the color numbers (shown as numbers inset in pictures of oil samples in Figure 2) measured by the petroleum products chromometer (model: SYD-0168), the 928 and 50-1-4Φ aviation lubrication oil samples were significantly darkening after oxidized in high purity dry air. As mentioned before, the 928 and 50-1-4Φ aviation lubrication oil samples were treated in same time and temperature to keep every single experiment in nearly identical circumstances. The oxidized 928 oil samples are presented in the left side of Figure 2, while the oxidized 50-1-4Φ oil samples are presented in the right side of Figure 2. Compared with oxidized 50-1-4Φ oil samples, the color of oxidized 928 oil samples is significantly darker, which certainly have higher color numbers (the color numbers are shown as the inset numbers in the square pictures of oil samples in Figure 2). The lowest color number of the oxidized 928 oil sample is 9 which is oxidized at 175 oC for 30 or 60 minutes. However, the color numbers of the oxidized 928 oil samples increase dramatically with increasing oxidation time and temperature. Compared with 928 oil samples, the 50-1-4Φ oil samples have relative higher color stability under elevated oxidation conditions. Considering the color numbers of many 928 oil samples exceed the measure range of the petroleum products chromometer, thus, the results from Figure 2 suggest that 928 aviation lubrication oil has relative unstable color stability under the same oxidation condition of 50-1-4Φ aviation lubrication oil.