Was it really a bi-metallic reaction- or simple rust because of the water being trapped and unable to dry off?

as it is in the non-pressurized area of the tube and therefore can't ignite- why not use a grease of some sort? (the big problem being- how to get the grease out of the rest of the tube once you have installed the valve- dragging it down the tube and leaving lubricant all along the way).