"Is there a recommended test to determine the vapor-phase corrosion-inhibiting properties of lubricants?"
The standard test method for measuring vapor-phase corrosion inhibition is ASTM D-5534. In this test, a steel specimen is attached to the top of an ASTM D-3603 test apparatus that contains the fluid to be tested at 60 degrees C (140 degrees F). The specimen is then exposed to water and oil vapors for a period of six hours. At the end of the test, the specimen is examined for evidence of corrosion. If corrosion is present, the lubricant is scored as a fail. Obviously, if there is no sign of corrosion, the lubricant is given a pass.
While it is important to have a lubricant that can protect against water ingression and the eventual humidity created by the water and hot machine oil, what is more important is taking a proactive approach to reduce water ingression in the first place. There are many viable solutions, which tend to be application dependent, including desiccant breathers, headspace purging, expansion chambers, etc. One of the easiest, cheapest and most fundamental ways to reduce ingression is to simply seal the system as best you can. This would include not leaving tank hatches open, not relying on OEM breathers to stop moisture and not running water across the top of a sump to cool it.
If the water does enter the system, you must be able to recognize its presence, analyze its state and concentration, and remove it as quickly as possible. Water can exist in oil in three states or phases. The first state, known as dissolved water, is characterized by individual water molecules dispersed throughout the oil. Most industrial oils such as hydraulic fluids, turbine oils, etc., can hold as much as 200 to 600 parts per million of water (0.02 to 0.06 percent) in the dissolved state, depending on the oil's temperature and age, with aged oils capable of holding three to four times more water in the dissolved state than new oil.
Once the amount of water has exceeded the maximum level for it to remain dissolved, the oil is saturated. At this point, the water is suspended in the oil in microscopic droplets known as an emulsion. In a lubricating oil, this condition is often referred to as haze, with the oil said to be cloudy or hazy.
The addition of more water to an emulsified oil/water mixture will lead to a separation of the two phases, producing a layer of free water as well as free and/or emulsified oil. For mineral oils and polyalphaolefin (PAO) synthetics with a specific gravity of less than 1.0, this free water layer is found on the bottom of tanks and sumps.
Recognizing the states and analyzing the concentration can help you make the decision on how to best remove the water, but as stated earlier, stopping the ingression in the first place should be the ultimate goal.