Seems there is a lot of factors in this % humidity temperature, air flow rust is always happening , my question is what % of humidity do you think you start to see a real increase in the rust forming is 30% or 50% too much? obviously less would be better and less would mean it would take longer to produce the same amount of rust.
Thankfully, I don't think that's true. The % relative humidity may have a miniscule effect but only in closed environments where you can have localized dew points reached because of localized temperature differences. Even in these conditions, the overall humidity is not nearly as important as the temperature (because the rate of corrosion goes up with temperature), whether or not you have localized condensation, whether the dew point has been reached, corrosive elements (eg salt) in the air, and especially when you have disimilar metals in contact with each other. Water and dissimilar metals create a battery effect that is much stronger than wet metal. It helps to remember that all corrosion is the result of a charge differential. No differential - no corrosion. But even the grain structure of a metal can create a battery effect because each grain is slightly different than its neighbour.
All other things being equal, the name of the game is to keep the humidity/temperature below the dew point as opposed to targeting a particular % relative humidity. Mind you, the easiest ways to do that are to keep the overall humidity low and/or to heat the localized air (which raises the temp above the dew point for a given mass of air).
Another way to look at it is to assume a room is full of 100% relative humidity. If you put a light bulb in a cabinet that is inside that air, it will warm the enclosed air in the cabinet above the dew point and prevent corrosion.
Yet another way to look at it is to understand that water in pure vapour form (a gas) does not cause corrosion. On the other hand water in liquid form can cause corrosion by creating a localized battery effect.