Different Types of Inner Containers used in water heaters in india ?

There are 3 different type of inner containers

1.   Stainless Steel

These types of Inner containers are used by many brands still – life expectancy of about 5 years depending on hard water TDS of over 500, if TDS is more than 600 the welding joints usually corrode which leads to leakage, stainless steel is a good metal and non-corrosive the only drawback is the TIG Welding between 2 drums, 
Pressure withstanding Capacity upto : 6.5 bar.
Released in 1992 upto 2018

2.   Mild Steel (Enamel or Glass lining or Glass Coating )

Almost all brands add a lining (coating) over the tank like (enamel / glass lining / polymer ( out dated ) / titanium enamel and so on) life expectancy of about 7-9 years depending on hard water. These geysers are used today to cater the needs of high raised buildings (multi storied apartments) with more than 5 floors where the pressure is more than 3.5 Bar to 8.0 bar mild steel as a metal is strong and sturdy but attracts lots of salts making is prone to corrosion at regular intervals, due to the coating on the inner bed of the metal the corrosion rate is reduced up to an extent making the product functional upto a certain period, a metal container is magnetic this enables manufacturers to add a magnesium rod to keep the salts away from the bed of the inner container which prevents the formation of salts over the heating element and prolongs the life of the inner container.
Pressure withstanding Capacity upto : 8.0 to 8.5 Bar
Release Date : 2016 till Date

3.   Copper Containers

A Very Few Brands use the copper container in today's date - the inner tank is excellent and doesn't have any corrosion issues life expectancy of about 5-25 years depending on gauge used copper is a non-corrosive metal and a good conductor of heat, if insulated well the product can save a lot of power by keeping the water for a very long time, copper usually heats up faster than other metals as it is a good conductor of heat, the only disadvantage of this inner container is that it cannot withstand pressure and may fail if used in a high raised building of more than 5 floors or used in pressure pump installed apartments where the water is pumped directly to the apartment, most brands in the 70s till date used only copper as a metal in water heaters but today's manufacturing standards have changed due to rising costs of copper and competition as most manufacturing brands have stepped into enamel mild-steel or glass lined mild-steel.
Pressure withstanding Capacity upto : 6.0 bar.
Released in 1970’s till date