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Even the purest of metals will set up little anodic and cathodic sites within the surface of the material. Steel would tarnish and given time, would pit.
Very quickly, the ion exchange will set up an electrical field that resists future ion exchange, and slows the reaction. In steps saline water, and tends to conduct away the electrical field. The effect is worse if the water is very saline, and/or flowing. This is one of the reasons why salted roads beat hell out of chrome. I believe it's known as "depolarisation".
This electrical field is typically a few millivolts, and so the reaction is not normally very fast. Indeed, it can be slowed radically with the use of a sacrificial anode, that counter-currents the system, tending to hold your expensive bits cathodic (-) with respect to the zinc (+). The zinc does the pitting instead.
Leaking currents are often very damaging as they will arrive with a much more aggressive electrical field that simply swamps your anodes, and they are defeated. Sometimes the voltages measured get beyond single volts.
Saline water encourages the effect as it is far more conductive. The best situation is my beloved fresh water, that makes the propagation of the electrical field much more difficult. In ultra-pure form, the electrical field cannot get from one ship to another. Even the ion exchange within a ship is greatly reduced. The near-absence of chlorine is welcome too. Stainless tends to live longer. Bronzes are too noble to notice anything in fresh water, really.
It freezes though. Yes, it freezes.
Last edited by Rockter; 10-13-2008 at 08:50 AM.