Corona Discharge

Never heard of corona on insulators.

Contaminants on insulators are more likely to cause tracking, the sound is quite distinctive. During damp weather you can hear the crackling sound build up and if it's dark a distinct arc tracking across the surface of the pot can be seen. The whole idea of OH line insulators is rain will wash them clean, damp weather is the enemy.

To be honest I've never come across corona discharge on OH lines I've worked on.

Some power companies wash their insulators with deionized water. They leave the power on and probably ground the tank holding the deionized water. The water is only slightly conductive so there is no high current.

There is also a special silicon grease that some power companies put on their insulators to reduce corona.

On a visit to Drakelow A & C stations as an apprentice I watched guys in the switch yard pressure washing insulators, I jokingly said to the guide wouldn't fancy doing that live…… They were!

Apparently they rely on the air gap between each drop and the low conductivity of the ionised water as you say.

I'd take some bloody convincing to do it!

Actually, it's not a grease, but rather more of a polish or wax. It helps remove surface contaminants, and then is buffed to a smooth shine that helps prevent contaminants from sticking as easily afterward.

In areas that have problems with water spray freezing on insulators (such as near cooling towers or in areas of frequent freezing rain / sleet), I have heard of a company that uses ground up walnut shells power blasted by CO2 to clean off the ice & sleet. It's a relatively good insulating mix, so no problems with flashover. They use it even at fairly high transmission voltages (345kV).

submerge in oil? Not sure the size or scale you are asking about, but the point of breakdown in oil is greater than that of air.