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HOME > CORROSION > CORROSION IN DRINK WATER SYSTEMS]

Reprinted with with permission from

Wilkes University
Center for Environmental Quality
GeoEnvironmental Sciences and
Engineering Department
Corrosive Waters

Corrosion, Saturation Index, Balanced Water in Drinking Water Systems


SourcesPotential Health HazardsCorrosion IndexCorrosion TestingCorrosion Treatment

  Source and Cause of Corrosion 

Corrosion is a complex series of reactions between the water and metal surfaces and materials in which the water is stored or transported. The corrosion process is an oxidation/reduction reaction that returns refined or processed metal to their more stab le ore state. With respect to the corrosion potential of YOUR drinking water, the primary concerns include the potential presence of TOXIC Metals , such as lead and copper; deterioration and damage to the household plumbing, and aesthetic problem s such as: stained laundry, bitter taste, and greenish-blue stains around basins and drains.

The primary health concern is the potential for the presence of elevated levels of lead and copper in the water. The primary source of lead includes the use of lead pipes, lead lined tanks, and use of 50/50 lead/tin solder. Because of the concern with lead, the EPA banned the use of high lead solders in 1986. The primary source of copper is the leaching of copper from the household piping used to convey the water throughout the home. In some cases, the water is so corrosive that the interior plumbing system needed to be changed and completely replaced with PVC piping. To bad, they did not test the rate and install a neutralize before the piping corroded and caused leaks throughout the home.

Corrosion will occur anywhere a galvanic cell or field can be or has established. To establish the field all that is needed is two dissimilar metals that are connected directly or indirectly by an electrolyte, such as water. This is the same chemical re action that occurs within a battery.

Nearly all metals will corrode to some degree. The rate and extent of the corrosion depends on the degree of dissimilarity of the metals and the physical and chemical characteristics of the media, metal, and environment. In water that is soft, corrosion occurs because of the lack of dissolved cations, such as calcium and magnesium in the water. In scale forming water, a precipitate or coating of calcium or magnesium carbonate forms on the inside of the piping. This coating can inhibit the corrosion of the pipe, because it acts as a barrier, but it can also cause the pipe to clog. Water with high levels of sodium, chloride, or other ions will increase the conductivity of the water and promoting corrosion. Corrosion can also be accelerated by:

  1. low pH;

  2. high flow rate within the piping;

  3. high water temperature; and

  4. presence of suspended solids, such as sand.

If it is necessary to flush or run your coldwater in the morning for a few minutes before you drink because the water has a bitter taste, YOUR Water is probably CORROSIVE. If you see blue-green stains in your basins or this same stain along the joints of your copper piping, YOUR Water is probably CORROSIVE. As corrosive water stands or seats in pipes or tanks it leaches metals from the piping, tanks, well casing or other metal surfaces that water is in contact.


Impact and Potential Health Concerns with Corrosive Water

The cost of corrosion can be expensive. Corrosion can impact you and your families health, aesthetic quality of your water, waste money, and damage your household piping and fixtures. Corrosive water costs you in a number of ways:

  1. decrease the efficiency of hot water heaters and may cause premature failure to the heater;

  2. corrodes and causes premature failure of household plumbing and plumbing fixtures;

  3. imparting a bitter taste to your water because of elevated levels of metals, which causes you in purchase bottled water;

  4. results in the formation of red water or greenish blue stains on drains; and

  5. consumption of water with elevated levels of toxic metals, such as: lead and copper, have been shown to cause both acute and chronic health problems.

Besides the aesthetic concerns, the corrosion process can result in the presence of toxic metals in your drinking water. These metals include: chromium, copper, lead, and zinc. The following are the recommended maximum contaminant levels for regulated public water supplies for the aforementioned metals: chromium (0.05 ppm), copper (1 ppm), lead (0.05 ppm), and zinc (5 ppm). To protect the public the EPA and PADEP requires public water supplies to be non-corrosive and the “Lead and Copper Rule” has set new action levels for lead and copper of 0.015 ppm and 1.3 ppm, respectively. In addition, the EPA has established a recommend maximum contaminant level of 0 ppm for lead, because of the concern with the toxicity of lead in children. If a public water supply is corrosive, the state requires that the water be treated to make the water non-corrosive.

There are no regulations in Pennsylvania or many other states that require private water wells or individual water wells or springs to be tested or treated for corrosivity or even toxic metal, pathogenic organisms, or organic chemicals, IT is up to YOU to monitor the Safety of your own private water well.


Corrositivity Index -Langelier Saturation Index

The Langelier Saturation Index is a means of evaluating water quality data to determine if the water has a tendency to be corrosive or scale forming. In order to use this index, the following laboratory analysis is needed: pH, conductivity, total dissolved solids, alkalinity, and total hardness.

In manipulating the data, the actual pH of the water is compared to the theoretical pH (pHs) based on the chemical analysis. The Saturation Index =

SI = pH - pHs

The Saturation Index is typically either negative or positive and rarely 0. A Saturation Index of zero indicates that the water is “balanced” and is neither scale forming or corrosive. A negative SI suggest that the water is corrosive. A corrosive water can react with the household plumbing and metal fixtures resulting in the deterioration of the pipes and increased metal content of the water. This reaction could result in aesthetic problems, such as bitter water and stains around basins/sinks, and in many cases elevated levels of toxic metals. A positive SI indicates that water may be scale forming. The scale, typically a carbonate residue, could clog or reduce the flow in pipes, cause buildup on hot water heaters, impart an alkali taste to the water, reduce the efficiency of the water heaters, and cause other aesthetic problems. Table 1 presents a typical range of SI that may be encountered in a drinking water and a description of the nature of the water and general recommendations regarding treatment.

Table 1.
SI Values and Recommended Treatment

Saturation Index

Description

General Recommendation

- 5

Severe Corrosion

Treatment Recommended

- 4

Severe Corrosion

Treatment Recommended

- 3

Moderate
Corrosion

Treatment Recommended

- 2

Moderate Corrosion

Treatment May Be Needed

-1

Mild Corrosion

Treatment May Be Needed

-0.5

None- Mild Corrosion

Probably No 
Treatment

0

Near Balanced

No Treatment

0.5

Some Faint Coating

Probably No 
Treatment

1

Mild Scale Coating

Treatment May Be Needed

2

Mild to Moderate Coatings

Treatment May Be Needed

3

Moderate Scale 
Forming

Treatment Advisable

4

Severe Scale Forming

Treatment Advisable


Water Testing for Corrosion

To determine the corrosion potential for the water, the “Langelier Saturation Index” can be used. To calculate the saturation it is necessary to determine the alkalinity, pH, calcium hardness (or total hardness), conductivity and total dissolved solids content of the water. The saturation index is then determined based on a particular water temperature, typically 25 C.

In addition, it is recommend to have the water checked for evidence of testing the water for lead and copper. This is conducted by determining the lead and copper content of the water after the water has been left in the piping overnight. The first draw is collected and then a second sample is collected after the line has been flushed, typically three to five minutes. The first draw sample is the first one liter of water collected from a cold water tap which has been shut off for at least six hours. This is the sampling procedure EPA is requiring community water systems to use to determine compliance with the new action levels. Samples are then analyzed by atomic absorption spectrophotometry for lead and copper.

It is strongly recommended that a homeowner or new homeowner have the corrosivity of the water tested at least once every few years. Corrosive or Aggressive water could result in aesthetic problems, increased levels of toxic metals, and deterioration of household plumbing and fixtures. In one case that the University was involved, the homeowner’s water was so corrosive that the interior household plumbing was changed twice prior to the University becoming involved and evaluating the wells water quality. The homeowner spent nearly $ 9000.00 to replace and repair plumbing, fix damage interior and only after this expense and 15 year of living in the home the water was finally tested. The testing indicated that the Saturation Index was -4.25, which indicates that the water is very corrosive. Following the testing, it was determined that a neutralizing filter was needed. The total cost to install the filtration system was only $ 850.00.

If the homeowner would have gone through the expense of a $ 50.00 to $ 75.00 water analysis prior to purchasing the home, it is possible that the cost of installing the system could have been included in the real estate transaction and it would have prevented the expense and frustration of having to change the household plumbing twice within 15 years.


Corrosion Treatment

Corrosion control is a complex science, requiring considerable knowledge of corrosion chemistry and of the system being evaluated. Corrosive water can be managed by installing pretreatment systems, installation of non-conductive unions, reducing hotwater temperature, and replacing copper piping with PVC. The pretreatment process treats the corrosivity of the water by changing the Saturation Index through an increase or decrease in the pH, hardness, and/or alkalinity. The resultant Saturation Index is typically more positive and preferably the SI is between -0.5 to +0.5.

In addition to changing the SI, one approach includes establishing a thin film of calcium or magnesium carbonate on the inside of the piping which acts as a physiochemical barrier. The pretreatment systems typically used in application for homeowners or small private water supplies includes either a neutralizing tank filter or caustic liquid treatment feed system. The neutralizing filter is more commonly used. The neutralizing filter uses crushed limestone, magnesia or some other mixture and as the water passes through the filter, the filter neutralizes the excess “acid” and results in the partial dissolution of the media. Therefore, the neutralizing filter actually increases the hardness of the water and raises the pH. The following are the chemical equations for the filter reactions.

H2CO3 + CaCO3 ---------> Ca(HCO)3
2 (H2CO3) + MgO ---------> Mg(HCO3)2 + H20

The main drawbacks of the neutralizing filter include:

  1. Increase water hardness to more than 120 mg as calcium carbonate or > 7 grains per gallon , i.e., Hardwater.

  2. The use of finely ground limestone may result in the reduction of water pressure and some fine media may enter the system.

  3. The system requires weekly backwashing. This is typically automated, but it does put a strain on any on-lot disposal systems.

  4. Neutralizing filters can be used where the raw water pH is 6.0 or greater. A limestone media will raise the pH to only about 6.9 to 7.0. If a higher pH is needed, a magnesia filter media should be used. GET YOUR WATER TESTED before installing a system.

The caustic feed system offers more options and is more flexible than the neutralizing filter, but requires additional safety precautions; more expertise to install, setup and operate; and possibly more extensive testing prior to and following installation. The system can treat waters with a lower pH without adding hardness to the water. Typically a sodium based solution is used as the caustic source, so the sodium concentration of the water will be increased. Therefore, households that have individuals on a low sodium diet need to make the doctors aware of the treatment system.

The waters with a pH of 4.0 to 6.8 a soda ash (sodium carbonate) is typically used. The soda ash is usually feed into the system at a rate to produce a resultant pH of approximately 7.0. When the raw water pH is less than 4.0, a caustic soda (sodium hydroxide) solution is used. Note: Solutions of sodium hydroxide are extremely aggressive and used only be handled by trained individuals.

In general, treatment of groundwater can be accomplished by adding some hardness or alkalinity, or both, and raising the pH to slightly increase the scale-forming tendency,thus creating a film or barrier to the corrosion. Installation of dielectric (non-conductive) unions between dissimilar metals such as copper pipe and steel hot water heaters can limit corrosion by breaking the galvanic circuit.

One of the more effective methods of controlling corrosion and leaching of toxic metals into the water is preventive, such as using dielectric couplings, installing PVC piping, and stainless steel equipment If copper plumbing is used, non-lead solder such as 95/5 tin/antimony solder should be used.

It is important to keep in mind that the corrosiveness of the water can be increased by the installation of water softeners, aeration devices, increasing hotwater temperatures, chlorinating the water and improper matching of metal pipes. Some water treatment equipment such as softeners and aeration systems can aggravate corrosion. Softeners remove the protective calcium and magnesium, and introduce highly conductive sodium into the water. Aeration devices for iron, sulfur or odor removal add oxygen, which is extremely corrosive in water. Higher water temperatures and suspended materials accelerate the rate of corrosion by increasing the reaction rates or causing physical damage to the pipes. Chlorine is a strong powerful oxidant which can promote corrosion.

Over the next few years, all regulated community water systems will be required to monitor their levels of lead and copper, with samples being taken from cold water taps in consumers' homes. Water systems with levels of lead or copper in excess of the action levels will be required to implement corrosion control procedures. Therefore, it is up to YOU to check and monitor the safety and potability of YOUR own private water supply.

For information on treatment options, please visit the Wilkes University links page.


 
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