Corrosion will happen regardless of whether it's gas or electric, so it's worth doing it for both.Also, my understanding is that since the gas heater's burners are at the bottom, the sediment can act as an insulator, reducing efficiency (although there are varying opinions on this.)Prior to flushing the tank, you should test the TPR valve.

Regarding the screen, recommend any types that are corrosion resistant? 

But chlorine is a corrosive chemical, and some of the components on the roof may not like it so much (Rubber gaskets, galvanized nails)Detergents also act as surfactants.

Bleach is also corrosive and contains dioxins.In your country the OSHA and EPA no longer endorse the use of bleach for mould remediation.Hydrogen peroxide is not the be-all-end-all mould treatment, it is one of the more effective products available to those of us who are not professional remediation services.

Second, while Rob K is correct that there could be cathodic corrosion occurring that usually requires a moist environment which inside a house would indicate other problems.

Of course the pigtails have to be done right too otherwise you have the same corrosion issue at the splice.

Holes in the walls (from pictures and shelves), it appears the bathrooms were never cleaned- there is hard water corrosion, and animal urine on the hardwood floors.

The inspector recommended a water quality test as well as replacement of the main water line.Severe corrosion was noted in the house main electric panel and at the lug connections in the main panel.

A property we are furnishing is on an island surrounded by saltwater so corrosion is a serious concern. 

The damage was done while it was switched temporarily to Flint river corrosive water.