A study  about the corrosion phenomena that may occurs in solar system reservoirs
( Copyright IRIS Tecnologie 2005 - Diffusion allowed only by mentioning the owner and the source.)

Preamble

The stainless steel family has a very good resistance to corrosion. This happens thanks to the inert layer on the surface. In the detail, it happens because the chromium component of the alloy, usually around the 12 % of the compound, react with the oxygen in the environment, and the oxidation of this layer protect the beneath metallic alloy.

There are many factors influencing the formation of this protective layer, but these are not important to us in this contest.

The inert layer we are talking about, has an excellent strong resistance to many chemicals aggressions. There are few substances capable of  corrosive aggressions to the stainless steels; among these there are the halogenures* .

One of the worst of this "salts" is the chloride. The chlorides usually acts on the surface layer inducing a perforating corrosions of the related structure. The damage can vary from just some small spots on the surface to deep holes. Corrosions phenomena are more evident, quick and extended, in presence of water or humidity.

This chlorides may be present in several ways, like:

-          contaminated water

-          residuals of regenerating salts used in ionic water demineralizers

-          etc.

Facts

One of the solutions for a better resistance to the chloride could be the use of an higher percentage of chromium in the alloy, or the use  of specials (and expensive) alloys, but the better one is, of course, the elimination of the contamination in the water itself.

I'm convinced that the presence of the chloride in the water is due to the use of  chemical devices for the  demineralization of the water.

The most common type of this device is the ionic water demineralizer. This one, uses resins in order to convert the calcium in the water (calcium carbonate)  in a compound of  sodium salts (sodium carbonate). The water passing through the contained resins lost calcium ions and get  sodium ions. The final results is not a really demineralized water, but just the presence of  a different kind of salts not stucking on the surfaces. This resins lost their chemical capacity in the process and must be periodically regenerated with the use of  sodium chloride (the common cooking salt).    

This sodium carbonated salts are not very aggressive at low temperatures, but they become corrosive when the temperature rises beyond 85°C  (185 °F).

Further more also sodium chloride salts become aggressive towards metals when the temperature increase beyond ambient temperature. These salts should not be actually present in the water, as they are used only in the automated process of the resins regeneration, but it cannot be excluded that traces could still be present in the demineralized water too.  The practical effect on corrosions, in this case, will be raising rapidly when the  contents of Sodium Chloride exceed 150mg/l. 

Using such a demineralized water for the alimentation of a solar water heating panel, where the temperatures can easily exceed 85°C, almost surely leads to perforating corrosions phenomena.  

Solutions

In order to avoid corrosions phenomena it is strongly advisable not to use ionic water demineralizers for the water to be sent to the input of the solar water heating panel.

Substituting this devices with a magnetic or a radio frequency water demineralizer is up to the end user but it is useless. This kind of devices are of course absolutely safe toward the metals, in referring to corrosions phenomena, but the physical change in the molecular status of  the carbonated calcium in the water is lost with the rising in temperature, so these devices becomes absolutely ineffective.

In my knowledge there are not other low cost devices or procedures available.

Using a ionic water demineralizers anyway

The only way to use demineralized water in conjunction with solar heating panels is the following:

The cold water can still be processed with a ionic water demineralizer, but the water supplying the  solar heating panel must be taken BEFORE the  ionic water demineralizer.

When using, as advisable, a thermostatic regulator for reducing and stabilizing the temperature of the water output from the solar heating panel, it is possible to use the water coming from the ionic water demineralizer  for the cold input of the thermostatic regulator ONLY.

In this way the UNTREATED hot water coming from the solar heating panel will be mixed together with  cold THREATED water from the line that will, in this way, reduce at less than the half  the calcium contents in the warm water.

In such a case it will be necessary to take 3 water pipes to the panel:

1-       Cold UNTREATED water (to the panel cold input)

2-       Cold TREATED water   (to the thermostatic regulator cold input)

3-       Warm mixed water (output to the warm water line)

In both the cases, to totally avoid  the calcium carbonated deposits over the surfaces (i.e. the shower cabinet) it will be sufficient just to briefly rinse the wet surfaces with the cold threated water from the tap.