It might be an electrolytic process. Robert - you use filtered water but I don't think that is the the same as ph neutral steam distilled water - filtering usually only removes particulate matter, depending on the filter of course - I don't know what you are using.
A good electrolyte is water, which contains oxygen, and either a salty or acidic concentration. If you use distilled water (ie - get rid of one variable right from the start), then the only things affecting the ph value of the water is what it comes into contact with - the stone, the container, the razor and the tape, for instance. Of these, the stone must have the biggest impact. So - your stone is either particularly acidic or alkaline (salty).
As japanese sword polishers advocate the use of bicarb then the stones they use must have an acidic nature. But, as the stones are all 'individual' the pH value must differ from stone to stone. I guess we can say there is a range from high to low acidity, and this will leach out of the stone into the water (electrolyte) to increase its potential for causing rust.
What you are describing Robert sounds like exposure to an acid-rich electrolyte - acids remove metal and promote pitting, whereas salts just convert the oxidised metal that is there. If your water is simply filtered then it may be acidic (below pH 7) and add to the problem. Water with a low pH is known to be corrosive and to carry metal ions, including iron.
A rough estimate can be made by assessing whether the water is soft (acidic) or hard (alkaline). Hard water leaves soap scum, a film on the surface of hot drinks, furring of kettles.
I have a couple or three japanese natural stones and some pH testing papers (much more sensitive than the old litmus papers) somewhere - if I can find them I'll see what the reading from stone to stone is.
But this is just theory! It could equally well be due to some obscure arrangement of the celestial bodies... it didn't say "
on no account hone any razors today" in your horoscope, did it?
Regards,
Neil