Does Soft Water Cause Corrosion? September 8, 2022 11:00

Does Soft Water Cause Corrosion?

does soft water cause corrosion
What is Soft water?

Do you hear people refer to water as soft water or hard water? Our world is made up of two different types of water: soft water and hard water. Soft Water refers to water that has low concentration of calcium and magnesium minerals and low PH.  Hard water refers to water rich in calcium and magnesium minerals and has high PH. Soft water is water that comes from rainfall. It’s safe to drink but how it impacts our lives and our pipes is a different story.

Scientists classify water based upon the concentration of dissolved minerals. Technically, hard water is defined as having a high concentration of multivalent positive ions. These ions such as Ca2+ and Mg2+ are normally collected by water, as it flows through the ground. Soft water has a lower concentration of calcium and magnesium ions.

Why we want soft water? This type of water can lather with soap, cleaning scum and dirt off easier than hard water. Household cleaning is performed more efficiently, leaving clothes and hair softer and cleaner. Thus, little detergent or soap is used, saving you money and prolonging the life of your dishwasher and washing machine.

What is soft water corrosion?

Copper pitting is a localized type of corrosion that leads to the thinning of the pipe wall in the area. Copper pitting comes in several types that depend on the temperature and pH of water flowing through the pipe. Type 1 pitting occurs when cold water with a high sulfate to chloride ratio flows through the pipe. Type 2 pitting occurs when hot water with pH below 7.2 flows through the pipe. Type 3 pitting occurs when soft water with pH below 8.0 flows through the pipe.


One of the factors that causes people to erroneously conclude that cation-exchange softened water is more corrosive than hard water is the erroneous assumption that naturally, soft water is similar to cation-exchange softened water. Naturally soft waterand softened water are different in many ways. Naturally soft water is very corrosive. It is commonly found in surface waters of the Pacific northwest, New England and the southeastern US. It is corrosive because it has a low pH and low TDS. Cation-exchange water softeners do not lower the pH (might slightly raise it, making water less corrosive) and does not lower the TDS.

Corrosion is a complex phenomenon and there are many factors that accelerate corrosion in the drinking-water distribution system and premise plumbing. For years, there has been confusion surrounding cation-exchange softened water and corrosion, making it important to distinguish between naturally soft water and cation-exchange softened water. Naturally soft water is corrosive as it has a low pH and low TDS; however, cation-exchange water softening does not contribute to any factors that accelerate corrosion. Also, there is a lack of scientific evidence supporting the claim that hard-water scale is an effective form of corrosion control. Thus, considering all these factors, as well as results from research studies, WQA’s position is that a properly configured cation-exchange water softener does not make water more corrosive.

Do water softeners ruin copper pipes?

does soft water ruin copper pipes

Corrosion by water is not purely down to the "softness", but a whole range of other properties, such as pH, temperature, sulphate and chloride content, etc. The hardness can either be permanent or temporary, depending on what causes it. Usually hardness is caused by the presence of calcium carbonate or bicarbonate in the water. These presences will affect the pH of the water and hence the potential corrosiveness of it. Furthermore, the corrosive nature of the water will be related to the material your pipes are made from. In some cases iron pipes are better than copper, in others the reverse is true; when lead pipes have been used, they are more susceptible to corrosion at low pH and soft water, but there are very few lead pipes around nowadays. The best corrosion resistant pipework is plastic pipes. 

Ion exchange softened water is not generally corrosive. Why do I say “generally”? – In certain conditions, if the conductivity of the water is already high, such as water with 30 gpg (grains per gallon) of hardness or higher, the softening process will contribute enough sodium/potassium to the water that is becomes very conductive which can indeed accelerate galvanic corrosion reactions.

So what does this mean? – Softened water is safe, softened water is generally good for the environment, softened water is good for your home, softened water is good for your business, and softened water is very good for your pocketbook.

How does this impact our pipes? This type of water can affect how long your metal pipes will last and how fast the pipes will corrode. Water that is soft will not create mineral build up within the pipes but it can create pinhole leaks as soft water is lacking in minerals making it aggressive towards unlined metal (copper) pipes. The reason is as this type of water is lacking minerals, it will pull minerals such as copper as it travels through the pipe systems. Eventually, the corrosion will perforate the pipe walls, creating pinhole leaks.



Hard water scale used to only be considered an aesthetic issue, but in today’s climate of energy efficiency and environmental sensitivity, one would be foolish to overlook all the benefits of softened water.

Softened water can:

  • Reduce the energy costs to heat water
  • Reduce the amount of soap/detergent required to clean clothing
  • Reduce or eliminate the use of dishwasher rinse-aids
  • Reduce or eliminate the use of harsh chemical cleansers
  • Minimize bacterial growth in washing machines
  • Prolong the working life of faucets and fixtures
  • Prolong the working life of appliances like dishwashers, steam irons, humidifiers, washing machines and ice makers
  • Prolong the working life of water heaters and boilers
  • Lower the overall carbon footprint of a home or business


Detecting Corrosive Water Problems

In some cases, it may be obvious that you have corrosive water based upon symptoms exhibited by your water. You may notice the telltale bluish-green stains in your sinks, metallic taste of the first water out of the tap, or small leaks in your metal plumbing components. In addition to these symptoms, water testing can be used to measure the level of corrosivity of your water and to determine if metals like copper and lead are above recommended or health-related standards.

Symptoms of Corrosive Water

  • Bluish-green stains in sinks
  • Metallic taste to water, especially first water in the morning
  • Small leaks in plumbing system

Many characteristics of water determine its corrosivity including pH, calcium concentration, hardness, dissolved solids content and temperature. Water that is soft and acidic (pH < 7.0) tends to be more corrosive but the generally accepted measures of water corrosivity are the stability or saturation indices. These indices use chemical characteristics of the water, such as hardness and pH, to estimate the corrosiveness of the water. The Langelier Saturation Index (LSI) is the most common and the index for which there is a secondary drinking water standard. Negative values of the LSI indicate corrosive water while positive results indicate non-corrosive water. Another common index is the Ryzner Stability Index or RSI. A RSI greater than about 6.5 indicates water that is probably corrosive with higher values being increasingly corrosive.