Corrosion consists of material erosion caused from a reaction with the surrounding substance. In liquids corrosion is an electrochemical reaction while for gases oxidation is a chemical reaction. Corrosion impact on the heater duration can be considerable.
In order to prevent corrosion, a good selection of the most suitable material is really important, without underestimating the role that other factors, as the power, can play on the heaters duration.
As already written, the main purpose of immersion heaters are hot water systems. Heating elements are subject to corrosion.
Water, when gets in touch with the heater surface, is hotter than in other system parts. Consequently localized corrosion and the setting of scale raise on the heating surfaces. Scattered corrosion can rarely destroy the heater while local corrosion can damage the resistance in a short time. When the corrosive reaction begins it can progress quickly perforating the coat and destroying the heater.
The causes of localized corrosion can be different: water composition, impureness and also factors connected to the use of the system. Unfortunately it is often difficult, when not impossible, to foresee all factors which can cause corrosion and for this reason the practical experience is the best precondition for esteem the danger and the corrosion probability. In a closed system the corrosion danger is sporadic and the reason why is that there is not oxygen in water, which helps corrosion, and also the sedimentation is limited.
Common steel corrosion
Steel is a material which is subject to corrosion: moisture can create a thin coat of rust. In this way it is normal that steel can be used only in limited cases for the heating of liquids. An exception can be closed system with “dead” water, that means without oxygen where the heater can resist well. In the system should not enter fresh air or water because this can cause a rapid corrosion and destruction of the heater.
Stainless steel and nickel alloys corrosion
It seems surprising that also strong steels can be subjected to corrosive events. For this reason the choice of the material according to its purpose is really important. Uniform corrosion rarely causes problems on heating resistances. The reason is that important corrosion events show only in particular environments as in strong acids solutions.
This type of corrosion can be removed through a mixture of suitable substances (e.g. Ti, Nb), and in these cases they are considered stabilised steels, or maintaining the carbon quantity contained in the steel at really low levels, less than 0.03 %.
Also chloride solutions can cause corrosion events and this is why it is important to evaluate its presence on the applications. Prevention of localized corrosion can be improved through the right choice of materials: stainless steel e.g. AISI 316L enriched with molybdenum are better than ordinary stainless steels. Corrosion process depends from so many factors – impurities, liquids currents etc. – and often only the experience can provide a final answer concerning the suitability of a product.