By Larry Henke
Summary: A common frustration for water conditioning dealers is that of blue/green staining. Although usually caused by corrosion of water pipe and fixtures, its remedies are complex and often elusive. But remedies nonetheless do exist.
Among the more vexing customer complaints is that of blue-green tinted water or blue-green staining on fixtures. Determining the cause, and establishing a cure, can be among the most frustrating challenges for both the water treatment specialist and the customer alike. Part of the problem lies in its complex nature, in the many likely chemical processes and because, in many instances, more than one mechanism is at work and the solution to one may not be a solution to another.
Lacking however, in the water treatment field, has been a systematic and comprehensive approach to the problem and it’s with this in mind that we offer the following discussion.
Although a natural blue hue exists in water, the green, blue or “aqua” staining is most often the result of copper. When copper tainted water evaporates on white tile or fixtures it leaves behind a green/blue stain, copper oxide. Copper-tainted water reacts with soap to form a “copper soap,” a greasy, blue-green residue similar to calcium soap scum, that accumulates near shower stalls and bathtubs. Moreover, copper-laden water can react with shampoo and bath soaps to cause objectionable reactions in hair.
Copper is listed by the U.S. Environmental Protection Agency (USEPA) along with lead as a contaminant in its Lead Copper Rule. Although lead is more harmful, copper is considered a hazard when it exceeds 1.3 milligrams per liter (mg/L) in water. Lead pipe isn’t as common as in the past, but lead-based solders were used in plumbing until the mid-’80s and some brass fixtures have low levels of lead in the alloy that can leach into drinking water under certain conditions. The solvency of lead in water is similar to that of copper in water. Although many of the remedies and analytical techniques are applicable to lead as well, this study will focus on copper. It’s important to note though, that unlike copper, lead will show no visible evidence of its presence and is hence more difficult to detect and eliminate.
Copper is a necessary trace element for human health. Excess levels are excreted from the body, however, when high levels are ingested, liver or kidney damage and/or gastrointestinal disorders may result.
Few locations have copper naturally occurring in groundwater, and those regions customarily have a history of copper-tainted water and, thus, people are aware of its presence. Copper can be introduced in water treatment processes when used as an algaecide. Our focus is on the unnatural introduction of copper into the water, usually the dissolution of copper piping or fixtures into water. This process of copper solvency is called corrosion and involves very sophisticated chemistry.
Corrosion is divided into several categories—any one of which, or a combination of two or more, can contribute to the process. It’s safe to say all piping—metal, plastic and concrete included—undergoes some level of corrosion and, to some degree, all piping processes are affected. Corrosion of iron or copper can be thought of as the metal reacting with the environment to return to its natural form—that of an ore. Among the more common mechanisms of corrosion in water pipe are the following:
Galvanic corrosion—when dissimilar metals come in contact, an electric current can allow the less noble metal to dissolve into the water. By “noble” we mean the cathodic end of a series or list of metals, both pure and in alloy, that react with each other (see Table 1). A full discussion of electrochemical principles on which this list is based is beyond the scope of this discussion. Examples are many but, basically, galvanic corrosion can occur whenever a metal is in contact with another—for example, when steel is in contact with copper, or copper with brass or bronze, or steel with copper, etc.
Another factor in galvanic or electrical cuprosolvency—or copper corrosion—is the conductance of electrical currents through copper piping produced by stray voltages as the result of grounding of appliances, electrical systems such as security systems, cable TV or telephone service. Stray current can also be introduced by conditions as diverse as transient ground current or by a cloud passing overhead, thus making establishment of cause and effect very difficult.
Uniform corrosion—This usually occurs when copper alloy is “pitted” or scored because of chemistry between the water and the copper. Some alloys aren’t as stable in certain waters as others and, thus, some pipe—and even individual sections of pipe—can yield copper into the water. This can be a result of oxidation of the copper piping when exposed to weather or under certain water conditions.
Levels of carbon dioxide, dissolved oxygen, natural organic matter (NOM) or organic acids, ammonia or sulfur all play a role in determining copper solvency. Excess flux used when the pipe is soldered can pool and, through small chemical reactions, produce areas of corrosion. Thus, one joint may contribute copper and another a few feet away will not.
Erosion corrosion—This involves the process where high velocities of water moving in a pipe cause a wearing away of the copper. This can be seen in hot water recirculation systems, or where the water moves too rapidly through the supply pipe. It can also be produced where joints or a sharp bend in copper pipe creates excessive turbulence.
Concentration cell—All copper or steel pipe contains small, localized regions where the alloy isn’t continuous. At these regions, small imperfections in the piping or fixture alloy set up “battery” or “transistor like” cells in the copper tubing where small electric currents dislodge metal ions to the water. In addition, solder, or where excess heat has melted carbon-based material into the piping, can form regions of electrical imbalance.
Microbially Induced Corrosion (MIC)—All water contains microbial life forms. They form biofilms in piping and fittings and can produce aggressive microenvironments that attack copper, releasing it into the water. For example, sulfur-reducing bacteria can form hydrogen sulfides or bisulfides that, under proper conditions, form sulfuric acid that can attack copper, cast iron or other piping materials. Concrete pipe and lining is especially susceptible to this kind of attack.
Other types of corrosion studied and known by scientists and engineers include stress, selective leaching, crevice and inter-granular, but those noted above are the ones most commonly related to water distribution systems.
The role of water
When blue/green stains are confirmed, often the first suspect is the water and the first culprit is often the water softener. The water conditioning dealer is called to “fix the softener” or provide assurances the softener isn’t to blame. Water is of course implicated—after all, it’s a “universal solvent” and a conductor of electricity.
Common water factors related to copper solvency are pH, dissolved oxygen and carbon dioxide. Other chemical parameters include ammonia, natural softness, chlorides, sulfates, organic matter and alkalinity. Physical factors include the water temperature and the velocity of water in the piping. In describing the water, those factors should be included.
Other observations, however, are in order. While water quality can contribute to copper solvency, it alone isn’t to blame. Higher levels of TDS will increase ionic strength and conductivity, and increased conductivity may allow corrosion to occur when otherwise unlikely. Most important however, are the parameters in the water that will react with copper and dissolved oxygen, namely chlorides and sulfates. In addition, higher levels of carbon dioxide will lower the pH, and thus contribute to corrosion.
Naturally soft waters low in alkalinity and calcium, and that may have lower pH values, are known to be corrosive; and it’s this fact that has implicated water softeners through the years, although softeners contribution to the problem has been refuted by USEPA studies. These recent experiments indicate water softeners aren’t a singular cause of corrosion in most circumstances.
Still, the water conditioning dealer can assist the customer in establishing the cause, and then may eliminate the problem. To limit dealer frustration, however, it’s prudent to use a systematic approach to the identification and remediation of blue-green staining. Although this is not strict formula, the method outlined in Figure1 has proven to be useful.
If these efforts don’t solve the problem, an even more thorough analysis can be performed. Such investigations may involve water analysis at several sampling points and an examination of the entire plumbing system. In many cases, sections of pipe are removed for analysis. Assumptions as to the source or cause can be further established to try to correct the problem. This can be rather expensive.
The approach detailed in this article for dealing with blue-green water from copper corrosion isn’t exhaustive—and despite all efforts and attempts—the customer may still have occasional episodes of staining. However, a systematic approach to the problem has a better chance of success, as corrosion factors in domestic waters are complex and thus elude a simple solution.
- American Water Works Association and AWWA Research Foundation, Lead Control Strategies, Denver, Colo., 1990.
- Broo, A. Elfstrom, R. Berghult and T. Hedberg, “Copper corrosion in drinking water distribution systems—the influence of water quality,” Corrosion Science, Vol. 39, No. 6, 1997, pp. 1119-1132.
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- Sorg, T.J., M.R. Schock and D.A. Lytle, “Leaching of Metals from Household Plumbing Materials; Impact of Home Water Softeners,” U.S. Environmental Protection Agency, EPA/600/R-98/044, April 1998.
- Sorg, T.J., M.R. Schock and D.A Lytle, “Ion exchange softening: effects on metal concentrations,” Journal AWWA, August 1999, pp 85-97.
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- U.S. Environmental Protection Agency, “The Effect of Temperature on Corrosion Control,” EPA/815-B-97-004, December 1997.
About the author
Larry Henke has more than 20 years experience in the water treatment industry and is technical director at the Robert B. Hill Co. in St. Louis Park, Minn., near Minneapolis. He’s a graduate of the University of Minnesota and is a member of both the American Water Works Association and the National Ground Water Association. Henke is also a member of WC&P’s Technical Review Committee. He can be reached at (612) 925-1444, (612) 925-1471 (fax) or email: firstname.lastname@example.org