223 Oxidation of MoS 2 at ambient temperature is more likely when both water and oxygen are present. In most  oxidation scenarios related to storage, both H 2O and O 2 are present to some degree. There are two  reactions that have been proposed to occur:  

2 MoS 2 + 4 H 2O + O 2 → 2 MoO 3 + 4 H 2S (1)  2 MoS 2 + 4 H 2O + 9 O 2 → 2 MoO 3 + 4 H 2SO 4 (2) 

In one study, H 2S was detected during wear tests of dry MoS 2 powders in a humid atmosphere.13 In another  study, no detectable sulfates at the surface region of sputter -deposited MoS 2 coatings were found after  consid erable oxidation of the surface in humid air.14 The observation of H 2S production and the lack of  sulfates in humid atmospheres indicate that Eq. (1) was active in these cases.    In another study, oxidation in a humid atmosphere was shown to produce sulfate ion and MoO 3 in an  approximately 2:1 ratio, and acid was detected after washing the reaction products in water.15 This suggests  that Eq. (2) was occurring. However, no analysis of evolved gases was conducted, so that Eq. (1) might also have been operative, with some production of H 2S gas.    The presence of H 2SO 4 is of concern because it can corrode the surfaces of metals such as aluminum and  steel. In fact, the production of the acid H 2SO 4 has caused the establishment of pH limits of aque ous  extracts of MoS 2 powders in several specifications.16   The presence of H 2O appears to accelerate the oxidation of MoS 2 in air. The study in Ref. 15 revealed that  the rate of oxidation of MoS 2 (as measured by the rate of MoO 3 production) was two to three times higher  in an air atmosphere that was saturated with H 2O compared to a dry air atmosphere. (This experiment was  conducted at temperatures of 85°C to 100°C at 1 atm pressure, so that negligi ble condensation of the H 2O  from the gas phase would be expected.)   Increased humidity has also been shown to degrade tribological properties. Ref. 14 demonstrated  decreasing wear life of coatings treated  with increasingly humid atmospheres. R ef. 17 showed that the  friction coefficient of burnished MoS 2 coatings increased as the humidity increased.    A study was  previously  conducted at The Aerospace Corporation to better quantify the effects of oxygen  and humidity exposure – as well as storage time – on the oxidation of MoS 2.18 The study used polycrystalline  MoS 2 powders , which are used i n fabricating burnished and bonded MoS 2 coatings, and also composites  like bearing retainers and slip- ring b rushes . The average particle size was 0.7 µ m. After washing the  powders in aqueous ammonium hydroxide (NH 4OH) to remove residual MoO 3, samples were placed into  one of three storage environments: air with 4% RH, air with 33% RH, and air with 53% RH. Samples  were  placed in the bottom of small beakers, forming pile s of MoS 2 powder ~1 mm thick. Oxidation of MoS 2  samples was analyzed after 1– 3 years of storage by dissolving any MoO 3 present in aqueous NH 4OH and  measuring the resultant Mo content in solution using atomic absorption (AA) spectroscopy.    Analysis gave the fractional amount of MoS 2 that was oxidized to MoO 3. These values are shown in Figure  2 for each storage atmosphere and storage time. The results for 4% and 33% RH are the same within the  error limits, but the amount of oxidation for 53% RH is considerably greater than that for 0% or 33% RH . In  addition, the amount of oxidation continues to increase for 53% RH after one year of storage, while the corresponding values for 4% and 33% RH do not.    The similarity of the 4% and 33% curves, and the rapid increase in oxidation between 33% and 53% R H,  could indicate a two- step oxidation mechanism: the first step proceeds to completion without requiring  substantial H 2O in the storage ambient, while the second step only proceeds at an apprecia ble rate for  humidities greater than some threshold RH value between 33% and 53% .