Soft water causes water heaters to corrode?

Those of us practicing the art on the commercial and industrial side of the fence are familiar with the Langlier Stability Index (LSI) and Ryznar Saturation Index (RSI) which are helpful in evaluating the potential of scaling or corrosion in waters that have NOT been softened by Ion Exchange. Unfortunately, some people misunderstand those calculations and think that they apply equally to ion-exchange softened waters; they do not.

Softening by ion exchange replaces “hard” Calcium and Magnesium ions with sodium/potassium ions; each of which are part of a carbonate compound.

Neither hard water or ion-exchange softened water is necessarily corrosive in and of itself, the potential for corrosion depends on so much more than just whether the water is “hard” or “soft”.

The sacrificial anode in a water heater is designed to protect the shell and other components of the device by reacting with potentially corrosive conditions. This causes degradation of the anode, which is why we call it “sacrificial”.

In situations where the anode is degrading, one should evaluate WHY the corrosive condition is occurring, and then establish HOW to resolve it.

When we discuss anodic protection of residential water heaters, there are four things that contribute to accelerated corrosion:

     1. Low TDS water

     2. Low pH water

     3. High TDS water

     4. High pH water

Low TDS water implies water with fewer ionic contaminants (Generally with a TDS <50 mg/L), so it is naturally more “aggressive” and will often seek balance by dissolving metals from the plumbing system that it is routed through. That’s why we don’t route RO filtered or Deionized water through copper pipe or regular water heaters. “Naturally Soft” water falls into this category, another place where some people get confused.

Low pH water with a pH <6.5 of course is what we know as “Acidic” water, and it also seeks balance by attacking all minerals and metals that it comes into contact with.

High TDS water refers to water with >500 mg/L (ppm) TDS. This water is loaded with electrically conductive impurities, which then of course will contribute to galvanic corrosion. The higher the TDS, the greater the potential for corrosion to occur.

High pH water refers to water with a pH >8.5 and can also contribute to corrosion. Corrosion potential increases exponentially as the pH rises above 9

So, how does this relate to a salt-based water softener, and why do some people think that softeners cause sacrificial anodes to degrade?

A salt-based softener will not lower TDS or pH, so we’ll rule those out of this particular discussion.

A salt-based softener will certainly raise TDS and possibly pH in certain cases. It is quite common to see TDS increase by approximately 10% with Ion-exchange softened water, so if the hard water coming into the home has a TDS of 180, then it would be normal to see the softened water with a TDS of around 200 ppm. This increase in TDS is not because there are more dissolved solids in the water, but because the sodium ions are more electrically conductive.

On hardness levels <25 gpg, one will rarely see a TDS >500 ppm, so the additional conductivity added by softening is rarely ever an issue in most areas.

When raw water contains hardness >25 gpg, the additional conductivity can indeed be a complicating factor, so we have to bear that in mind as a potential contributor to corrosive conditions.

None of this guarantees that water softeners cause water heaters to fail though, so where does this “old-plumber’s tale” come from?…Another anecdotal factor to bear in mind is that “in the old days” when softeners lacked effective electronic controls, it wouldn’t be uncommon that a softener was returned to service after regeneration without being properly rinsed. This would of course induce a very high conductivity solution into the plumbing and create a corrosive condition within the water heater. Lots of water heaters were replaced, and the softeners were rightfully blamed, which inferred incorrect blame on ion exchange softening technology.

How does this all translate into water heater longevity? – Well, the first thing is that hard water can significantly reduce the longevity and energy efficiency of a water heater through scale formation, and softening the water will protect against it; BUT if that water is very hard, is highly alkaline, or has a low pH it can be corrosive after softening. As with all things, we must know and understand the details, for the devil is in the detail.

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