Sounds like a problem with hard water.
In my area the main problem is a high concentration of lime,
and then it is better to use indirect electric heating.
More or less the warm water tank has a big coil in it,
and the water circulating in that coil is just as warm as needed (let's say 60 degC) and your electric heater is only in contact with this circulating water.
That way your heaters will live in a much nicer environment and will last much longer.
It also seems like those coils is not as sensitive to lime as the direct electric heaters since they work at a lower temperature and the lime does not "leave the water" as much when the temperature is lower.
The downside is that this installation is a little bit more expensive since you need things like a circulation pump, "expansion tank" and a "over pressure valve".
But the positive thing is that the only thing exposed to the hard water is the water tank, so this is the only thing that needs to be changed/cleaned.