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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 stable 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 problems 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. Too bad, they did not test the rate and install a neutralizer 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 reaction 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:
- low pH;
- high flow rate within the piping;
- high water temperature;
- 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 sits in pipes or tanks it leaches metals from the piping, tanks, well casing or other metal surfaces that it contacts.
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.
Treatment of Corrosion
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.
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 Calcite (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.
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.
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.
In 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 hot water 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.
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