By Bryanna Poczatek and Eric Yeggy
This article aims to address the common misperception that cation-exchange water softening causes corrosion. This is not the case: a properly configured water softener does not make the treated water more corrosive. Here are a few confusing facts that often lead people to this erroneous conclusion. First, naturally soft water is typically corrosive. Therefore, softened water must also be corrosive, right? This is a fallacy because naturally soft water tends to be corrosive due to low pH and low TDS, while cation-exchange softening causes neither of these conditions.
Second, calcium carbonate scale is often cited as a form of corrosion control and since softeners prevent the formation of scale by removing calcium and magnesium, they must therefore cause corrosion, right? Again, this is a fallacy. Despite a lack of scientific evidence supporting the premise that calcium carbonate scale forms a uniform protective layer that protects consumers from lead or copper corrosion, this practice continues to be referenced as a form of corrosion control.
People in Flint, MI began complaining about the water quality shortly after the city began using the Flint River as source water. Ironically, the state responded by blaming the complaints on the high level of hardness in the Flint River water.1 Obviously, the hardness did not protect anyone in Flint from lead corrosion. Below we will examine these misconceptions in more detail and review the science related to softeners and corrosion. But first let’s take a closer look at corrosion.
Impacts of corrosion
Corrosion is a natural process in which a material is degraded by the environment, sometimes to a more stable chemical state through oxidation or reduction reactions. This is a big problem in water distribution systems where metal pipes are continuously being corroded. Corrosion can cause many undesirable effects in plumbing. The physical effects of corrosion include leaks, color in the water and on surfaces, sediment and particulate in the water, taste and odor issues, and physical failure of the pipes.2
Corrosion can also cause dangerous byproducts to be released into the drinking water. Two of the most common corrosion byproducts in plumbing systems are lead and copper; both have serious health implications. Lead can enter the drinking water by being corroded from lead service connections, lead solder used in copper piping or brass/bronze fixtures. Children are the most at risk for the dangers from lead as they absorb 40-50 percent of the lead they ingest, while adults only absorb around 3-10 percent. Pregnant women are also at high risk of lead poisoning. Possible health effects from ingesting lead include low birth weight, developmental delays, lower IQ, damaged hearing, reduced attention span, kidney damage and reproductive damage.2,3
Copper is another common byproduct of corrosion. Copper is essential to human health, however, too little or too much is unhealthy. Copper combines with proteins to produce enzymes that act as catalysts for various body functions. It is important that copper is consumed daily as the body is unable to synthesize copper. Yet when too much is ingested, copper poisoning can occur. Symptoms of copper poisoning include stomach distress, intestinal distress, liver and kidney damage and complications of Wilson’s disease (a genetic disorder that causes copper to accumulate in the liver, brain and other organs).2
Accelerators of corrosion
Corrosion is an inevitable, natural process that occurs in all metals exposed to water with dissolved salts. There are many complex factors that accelerate corrosion; however, common corrosion accelerators include the following.2
- High velocity and/or turbulence
- High temperature
- Low TDS
- Dissimilar metal contact
- Low pH
- Carbon dioxide
- Biofilm accumulation: microbially influenced corrosion
- Chemical agents such as chlorine, chloramines and dissolved oxygen
- An elevated chloride-to-sulfate mass ratio (CSMR)
High velocity and turbulence (lots of turns, or piping that is too narrow for the service flowrate) can accelerate the degradation of the pipe through mechanical erosion and cavitation. Erosion is the physical degradation of the pipe due to fast-moving turbulent water. Cavitation happens when water is moved quickly or suddenly forced in a new direction, leaving low-pressure voids that often appear as bubbles. As these low-pressure bubbles collapse, a shock wave is formed, which can damage piping and fixtures. Heat accelerates almost all chemical reactions and corrosion is no exception. Temperatures of over 70°C (158°F) especially increase the rate of corrosion.
Water that has been completely demineralized (such as RO and DI water) is hungry for ions. This is what makes demineralized water—or water in which all TDS have been removed—corrosive. For this reason, WQA recommends that common metal pipe and fittings (e.g., copper, galvanized, brass, bronze) should not be used downstream of a RO or DI treatment system. Softeners do not remove the TDS and do not cause this type of corrosion. A softener is simply capturing some cations (mostly calcium and magnesium) and releasing other cations in the process (usually sodium or sometimes potassium).
The use of dissimilar metals in the plumbing can lead to galvanic corrosion. Direct contact between dissimilar metals will initiate the formation of a galvanic cell, which is created whenever two different types of metal are conductively connected to each other, and then immersed in an electrolyte such as water. Within the galvanic cell, the more anodic metal will be corroded and an electrical current is created because of the process. A common example of this is when galvanized pipe is directly connected to copper pipe. It is also worth noting that while using a brass fitting to bridge the gap between galvanized and copper pipe will not accelerate this galvanic activity, it also will not completely prevent it.4,5,6 Dielectric couplings can be used to mitigate this direct contact between the dissimilar metals. A dielectric coupling contains an insulating material that physically separates contact between the two metals.
The deposition of scale can also create mini-galvanic cells. Scale does not exist as a homogeneous layer. Microscopic examination of scale deposits has revealed that as scale accumulates, different types of metals can be incorporated into the scale. These different types of metals that are present in the scale are conductively connected and act as galvanic cells in miniature.
Low pH also increases the corrosiveness of water. pH is the measure of hydrogen ion activity in a solution and is used to express the intensity of the acidity of a solution. Solutions with a low pH (< 7) are acids; solutions with a high pH (> 7) are bases. Acids are compounds that release hydrogen ions, which oxidize the metals in pipes, accelerating corrosion. Carbon dioxide will potentially contribute to the formation of carbonic acid, thereby making the water more corrosive. The presence of high levels of carbon dioxide tends to correlate with specific geologic conditions.
Another common accelerator of corrosion is biofilm accumulation, which causes microbially influenced corrosion (MIC), a form of localized corrosion. Bacteria attach to a surface of a pipe, colonize and grow, forming a biofilm. The biofilm is nutrient-dense and resistant to disinfectants such as chlorine. MIC is a complex phenomenon and biofilms influence corrosion in a multitude of ways. Theories on the mechanisms involved include increasing electron transfer, destroying the surface protective film layers, changing the redox potential of pipe surfaces and creating acidic corrosive substances.7
Certain chemicals (such as chlorine, chloramine and dissolved oxygen) can also make water more corrosive. For example, the presence of oxidizing agents such as dissolved oxygen can cause metals to lose electrons and lead to corrosion.
Another common cause of corrosion is removal of sulfate, and/or addition of chloride, referred to as an elevated CSMR. Increasing the CSMR will accelerate corrosion in the presence of materials that contain lead. A July 2007 paper published in the AWWA Journal, “Chloride-to-sulfate mass ratio and lead leaching to water,” summarized this effect as follows: “While sulfates inhibit corrosion by forming passive protective film layers and reducing galvanic currents between dissimilar metals, chlorides prevent the formation of such passive layers and stimulate galvanic current.”8 If the source water contains natural levels of chloride and treatment is installed to remove sulfate, this will push the CSMR up and potentially accelerate corrosion. Another way that the CSMR could be elevated is by improper configuration of a softener with inadequate rinse time. If salt remains in the softener bed after regeneration, due to inadequate rinse time, the chloride in that salt will drive up the CSMR and potentially accelerate corrosion.
The potential risk caused by the CSMR can be evaluated in this way. If there are materials in the premise plumbing that contain lead (such as galvanized pipe, brass/bronze fittings, faucets or lead solder) and the CSMR is at or above 0.2, there is a significant risk of corrosion and lead exposure for the customer. If there are materials in the premise plumbing that contain lead and the CSMR is > 0.5 with an alkalinity < 50 mg/L as CaCO3, there is a serious risk of corrosion and lead exposure.9
Common misperceptions about softened water and corrosion
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 water and 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 softening does not lower the pH (might slightly raise it, making water less corrosive) and does not lower the TDS.10
Another common misperception is that calcium-carbonate scale is an effective form of corrosion control. There is a lack of scientific evidence supporting this claim, however. Scale does not form in uniform homogeneous layers that would protect pipe from corrosion. Scale can be porous or soft and is highly irregular. Corrosion can still occur under conditions that are favorable to the formation of hard-water scale or even when hard-water scale is already present.
Research studies on softeners and corrosion
Many research studies have investigated the effects of cation-exchange softened water on corrosion. One such study, “Leaching of Metals from Household Plumbing Materials: Impact of Home Water Softeners,” was conducted by the US EPA National Risk Management Research Lab. Their objectives were to evaluate metal leaching from metallic pipes and faucets and determine any changes of the critical chemical characteristics of the water passing through the water softener that would accelerate the rate of corrosion. Two different water qualities were used in the study: lime-softened finished tap water with a hardness of 160 mg/L and groundwater with a hardness of 300 mg/L. The researchers concluded there is no evidence that ion-exchange softened water systematically produced higher metal levels than the non-softened waters under otherwise identical conditions.10
Another study done by the British Standards Institute for the UK Water Treatment Association examined the corrosivity of natural hard water versus ion-exchange softened water against a range of metals, including aluminum, mild steel, copper, brass and stainless steel. Two identical model central water-heating systems were installed and removed after one, three and six months for determination of corrosion rates, using visual inspection and chemical analysis. This study concluded there was no significant difference in the corrosion rates for brass, copper, mild steel and stainless steel between the two rigs. There was initially a higher rate of corrosion of aluminum from ion-exchange softened water, but this decreased continually over time.11
A third study done at the METALogic research institute in Belgium, “In situ corrosion investigation on the effect of hard and softened water to domestic copper and galvanized steel drinking water systems,” investigated the long-term effects of ion-exchange softened water on corrosion. Four representative domestic drinking-water installations were used: two for the behavior of copper corrosion and two for the behavior of galvanized-steel corrosion. The corrosion rate was measured after six and 12 months. Analysis also included measurement of the pH, redox potential, conductivity and hardness. These researchers did not find any evidence that corrosion in copper or galvanized steel pipe is more severe when in contact with softened water compared to hard water.12
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.
1. “State says Flint River water meets all standards but more than twice the hardness of lake water,” Michigan Live, January 17, 2015. https://www.mlive.com/news/flint/index.ssf/2014/05/state_says_flint_river_water_m.html.
2. Reyneke, G. (2017, May 17). “Key Concepts in Corrosion Chemistry and Treatment.” Lecture presented at the 2017 WQA Convention & Exposition in Orange County Convention Center, Orlando, FL.
3. CDC. (2007). “ToxGuide for Lead.” Retrieved from https://www.atsdr.cdc.gov/toxguides/toxguide-13.pdf.
4. Edwards M., Clark B., Cartier C., St. Clair J. Triantafyllidou S., Prevost M. (2013). “Effect of connection type on galvanic corrosion between lead and copper pipes,” Journal AWWA.
5. St Clair, J., Cartier, C., Triantafyllidou, S., Clark, B. and Edwards, M. (2016). “Long-Term Behavior of Simulated Partial Lead Service Line Replacements.” Environmental Engineering Science, 33(1), 53-64.
6. Welter G., Giammar D., Wang Y., Cantor A. (2013). “Galvanic Corrosion Following Partial Lead Service Line Replacement.” Water Research Foundation.
7. Sand, W. (1997). “Microbial Mechanisms of Deterioration of Inorganic Substrates – A General Mechanistic Overview.” International Biodeterioration and Biodegredation, 40(2): 183-190.
8. Edwards, M. and Triantafyllidou S. (2007). “Chloride-to-sulfate mass ratio and lead leaching to water,” Journal AWWA.
9. Nguyen, C. K., Stone, K., Clark, B., Edwards, M., Gagnon, G. and Knowles, A. (2010). “Impact of Chloride: Sulfate Mass Ratio (CSMR) Changes on Lead Leaching in Potable Water.” Water Research Foundation, 4088.
10. Sorg, T., Schock, M. and Lytle, D. (1998). “Leaching of Metals from Household Plumbing Materials: Impact of Home Water Softeners.” United States Environmental Protection Agency.
11. Munn, P. (2012). “Results from testing corrosivity of hard and softened water in model central heating systems at BSI, Loughborough.” Midland Corrosion Services Ltd.
12. Verdonckt, C. and Nijs, C. (2007). “In situ corrosion investigation on the effect of hard and softened water to domestic copper and galvanized steel drinking water systems.” METALogic.
Bryanna Poczatek is the Technical Affairs Coordinator at the Water Quality Association. She joined the association in 2016 as an Associate Project Leader in the Product Certification Department. In early 2017, Poczatek transferred to the Technical Affairs department and works alongside Director Eric Yeggy on various technical projects. Poczatek represents the industry as a voting member of the Water Equipment & Policy Research Center Industry Advisory Board, funded through the National Science Foundation (WEP IUCRC). She also participates in numerous industry committees and task forces through NSF International and other organizations. Prior to joining WQA, Poczatek worked as a Project Coordinator for Syngenta Crop Protection LLC and as a Research Assistant in the Biochemistry, Biophysics and Molecular Biology Department at Iowa State University. She holds a Bachelor’s Degree in biology from Iowa State University.
About the organization
WQA is a not-for-profit international trade association representing the residential, commercial and industrial water treatment industries. More information is available on corrosion accelerators through WQA’s educational materials. Core and Premier members can also contact the WQA Technical Affairs department and request the latest research on these corrosion accelerators. The association’s position paper on this topic can be obtained by contacting the authors in the WQA Technical Affairs department.