Well if you have something in low supply, then it will make it difficult for the reaction to go ahead.
So imagine you have steel in a vacuum, there is no oxygen or water for the reaction to occur.
Say you had a partial vacuum then you will have a bit of oxygen but in lower concentrations than if you had the steel in an area which has a lot of oxygen in it at a higher pressure.
If you have more, then the reaction is more probable to occur in quicker time. Here's why:
So if you had a higher concentration of oxygen around the metal's surface, and hence covering a bigger surface area of the metal, then more reactions occur on the surface of the metal, so that the rate of corrosion is increased.
