[RedDevil]

I bet it boils like a portable electric water boiler does.

Soon after you flip the switch they start making a rumbling noise, caused by tiny bubbles of steam expanding on the heater plate and collapsing when they break loose and rise in the colder water above.
When that gets hotter the bubbles grow bigger and collapse more slowly, making the sound drop in pitch and intensity - until the bubbles reach the surface when it is done.

When you look inside a boiler when the water is still cold you can't even see the bubbles that are making all the noise; they are that small.

Your heater is operating in the same phase as a cold filled electric water heater.
Only a very small amount of coolant gets converted to steam (and back to fluid when it breaks free of the heater core).
Therefore I think degradation is not much of an issue.