W. Adam Sigler and Jim Bauder
Montana State University Extension Water Quality Program
Department of Land Resources and Environmental Sciences
Corrosivity
What is Corrosivity?
Corrosivity is a measure of how aggressive
water is at corroding pipes and fixtures.
Corrosive water can mobilize lead and copper from pipes into drinking water and can
eventually cause leaks in plumbing.
Corrosive potential of
water is increased by:
→ pH (lower than 6.5 or higher than 8.5)
→ water flow rate (faster flow)
→ water temperature (higher temp)
→ dissolved gases (more dissolved gas)
→ conductivity (higher conductivity)
→ dissolved solids (high dissolved solids)
→ certain bacteria (more bacteria)
→ suspended solids (more sediment)
→ chlorine (more chlorine)
Determining Corrosivity
One common index of corrosivity is the Langelier Index (LI). The LI is calculated using pH, temperature, total dissolved
solids, alkalinity, and total hardness. The LI is a measure of the balance between pH and calcium carbonate (CaCO3). As
the LI value becomes more negative, the water is increasingly under-saturated with CaCO3 and therefore has increased
corrosion potential. As
Langelier Index
Description
General Recommendation
the LI value becomes
more positive, the water
-4
Severe Corrosion
Treatment Recommended/Consider Lead/Copper Test
is increasingly
-3
Moderate Corrosion
Treatment Recommended/Consider Lead/Copper Test
over-saturated with
CaCO3. Over-saturation
-2
Moderate Corrosion
Treatment May Be Needed/Consider Lead/Copper Test
results in CaCO3
-1
Mild Corrosion
Treatment May Be Needed/Consider Lead/Copper Test
precipitation which can
-0.5
None-Mild Corrosion
Probably No Treatment
coat and protect pipes
from corrosion but can
0
Near Balanced
No Treatment
cause scaling in pipes,
0.5
Some Faint Coating
Probably No Treatment
hot water heaters, and
fixtures. While not a
1
Mild Scale Coating
Treatment May Be Needed
perfect analytical tool,
2
Mild to Moderate Coating
Treatment May Be Needed
the LI serves as a useful
guide for assessing
3
Moderate Scale Forming
Treatment Advisable
corrosive ability of well
4
Severe Scale Forming
Treatment Advisable
water.
Controlling Corrosion
Adapted from Wilkes University Center For Environmental Quality;
Corrosion, Saturation Index, Balanced Water in Drinking Water Systems
Corrosiveness may be increased by installing water softeners, aeration devices, increasing hot water
temperatures, chlorinating water or improper matching of metal pipes. Corrosion control options include
pretreatment systems, installation of non-conductive unions, reducing hot water temperature, and replacing
metal piping with CPVC. Pretreatment systems include neutralizing tank filters and caustic liquid treatment.
These systems change the pH, hardness, and/or alkalinity to achieve a less corrosive water chemistry.
Additional Resources:
Corrosion in Drinking Water Systems; Wilkes University Center for Environmental Quality
http://www.water-research.net/corrosion.htm
Lead and Copper Fact Sheet; MSU Extension Water Quality
http://waterquality.montana.edu/docs/homeowners.shtml (listed under “Drinking Water”)
Household Drinking Water Protection and Treatment; MSU Extension Service
http://waterquality.montana.edu/docs/homeowners.shtml (listed under “Drinking Water”)
Northern Plains and Mountains Regional Water Program– Drinking Water Initiative
http://www.region8water.org
W. Adam Sigler and Jim Bauder
Montana State University Extension Water Quality Program
Department of Land Resources and Environmental Sciences
Hardness
What is Hard Water?
Water is considered hard when it has a
Classifying Your Water
relatively high concentration of calcium
Hardness as mg/L CaCO3 Hardness in grains per gallon Classification of Water
and magnesium. Hard water received
this name because it requires more soap
0-60
0 - 3.5
Soft
for a good lather, making the water
61-120
3.6 - 7.0
Moderately Hard
“hard” to clean with. In addition to
making washing more difficult, hard
121-300
7.1 - 17.5
Hard
water can cause spotting on glasses,
over 300
over 17.5
Very Hard
deposits in hot water heaters, and scaling
Hardness can be reported in milligrams per liter (mg/L),
on sinks and fixtures. This can lead to
parts per million (ppm) which is equivalent to mg/L,
reduced water pressure and shorter hot
or grains per gallon (1 grain = 17.1 mg/L)
water heater life. Benefits of hard water
include reduced risk of pipe corrosion and, within limits, a
better taste. There is also some evidence that harder water
could reduce risk of cardiovascular disease.
Treating Hard Water
Water softening is the most common treatment for hard water. It is
possible to install a water softener on a washing machine or
dishwasher and some new dishwashers come with a water
softener. To treat the water for the entire house a water softening
system can be installed. Water softeners exchange calcium and
magnesium with another ion which does not contribute to
hardness. Traditionally sodium has been used in water softeners
but potassium is also available.
Water softening does not reduce total dissolved solids, it
simply exchanges the calcium and magnesium for sodium or
potassium. In some cases, people choose to soften the main
household water supply, but bypass the softener with a separate
drinking water tap. This allows people to receive the positive
benefits of drinking hard water but avoid the negative effects of
hard water on hot water heaters, washing machines, and household
plumbing. Another option to avoid drinking the additional sodium
from sodium softening is to install a reverse osmosis filter at the
drinking water tap. Reverse osmosis can remove sodium and
alleviate health concerns associated with high sodium intake.
Note about water softeners
In areas with clayey soils, the sodium discharged in the recharging process can increase the risk of septic drainfield failure.
Softening with potassium is a possible alternative. Softening with sodium can also increase the corrosive nature of water by
reducing concentrations of protective calcium and magnesium and increasing concentrations of highly conductive sodium.
Discharge of salt during softener recharge may also disrupt the solids settling process within the septic tank. Consider routing
recharge waste water away directly to the leach field.
Additional Resources:
Hard Water Calcium and Magnesium; Wilkes University Center for Environmental Quality
http://www.water-research.net/hardness.htm
Household Drinking Water Protection and Treatment; MSU Extension Service
http://waterquality.montana.edu/docs/homeowners.shtml (listed under “Drinking Water”)
Northern Plains and Mountains Regional Water Program– Drinking Water Initiative
http://www.region8water.org/