|
Page
2
|
|
|
Drinking water corrosion control standard
|
Aesthetics and Customer perception
The metals of most interest from an aesthetic standpoint are copper, iron, and zinc.
- Copper in drinking water causes a metallic taste, blue-green particles and discoloration of the water, blue green staining of porcelain fixtures, staining of swimming pools and hot tubs, and green tinted hair when certain shampoos are used. The primary sources of copper are copper used in service lines and consumer plumbing and brass fixtures and faucets.
- Iron in drinking water discolors water, forms precipitates, causes rusty water, causes red water staining of laundry and fixtures, imparts metallic taste to water, and promotes growth or iron bacteria and sulfate-reducing bacteria. Rusty of red water is one of the most common customer complaints received by water utilities. The sources of iron in drinking water related to corrosion are unlined cast iron piping, unlined steel piping, and galvanized piping and fittings in plumbing systems that have lost their zinc coating. Iron can also be naturally present in many source waters.
- Zinc causes bitter or astringent tastes in drinking water at concentrations of 4 to 5 mg/l, High concentrations give water a milky appearance at room temperature and a greasy appearance when boiling. Zinc is present in waters largely due to corrosion of galvanized metal in the present plumbing or small distribution mains.
Premature Piping Deterioration and Economic Concerns
Internal corrosion can have significant adverse economic impacts on the water transmission and distribution systems and consumer plumbing. Typically, consumer plumbing is most seriously affected by internal corrosion because the piping is normally unlined and its diameter small. Such piping will have a tendency to lead to, or develop, tubercles that reduce both flows and pressure.
Environmental Concerns
Corrosion of water distribution piping and subsequent leaching of metals raises environmental concerns associated with the presence of these metals. The metals enter the wastewater collection system and either pass through the treatment plant and into receiving waters or accumulate in the sludge and end up in a landfill, or other locations, depending on the disposal method. Metals corroded from water piping are the largest source of contaminants in waste water.
Common Types of Corrosion
When minerals are dissolved in water, the resultant ions provide the necessary conductivity to permit corrosion currents to flow, with negatively charged anions moving to the anode and positively charged cations moving to the cathode. Their accumulation at the respective electrodes is limited by other reactions that take place at these points. For corrosion of any type to occur, the presence of all the components of an electrochemical cell is required. If any component is absent, a corrosion cell does not exist and corrosion will not occur.
- Uniform corrosion is simply the presence of a cathodic reaction involving the overall metal piping surface.
- Galvanic corrosion is where to dissimilar metals come in contact with one another setting up a disparate electrochemical reaction.
- Erosion corrosion occurs when high velocity fluid flow and/or flow of abrasive materials prevents formation of protective film, allowing fresh material to be continually exposed to the corrosive environment. Fretting and cavitations are each special forms of erosion corrosion.
- Area effects are also important, especially in galvanic and localize corrosion. Consider differences between cells with a very large anode area compared to a cathode and one in which the cathode is significantly larger that the anode. Since metal is corroded at the anode only, the rate of corrosion will be proportional to the rate at which the anodic reaction proceeds, however, there must be corresponding cathodic reactions. The cathodic reaction therefore controls the rate of the overall reaction.
- Exfoliation is characterized by the flaky blistering appearance on the surface and is common in aluminum alloys.
- Selective leaching involves the removal of one element in an alloy.
- Intergranular corrosion is a localize attack at the grain boundaries which proceeds much as pitting corrosion but along grain lines primarily due to the small differences in metallurgical properties.
- Stress corrosion cracking can occur when a metal is both stressed and exposed to relatively mild corrosive conditions, causing the metal to fail at applied stresses well below predicted mechanical failure.
- Hydrogen induced cracking is similar to stress corrosion cracking and occurs under stresses which would not ordinarily lead to failure but which are made worse by corrosion processes.
- Microbiologically induced corrosion is the result of the growth of bacteria and other biological organisms in a system, causing or worsening corrosive conditions.
Monitoring Corrosion Rates
The best method of monitoring corrosion rates is by a corrosion meter. This is an electrical resistance instrument with probes measuring the change of resistance of the probe element as it is exposed to corrosive conditions. Such equipment is manufactured by Metal Samples, 152 Metal Samples Road, Munford, Alabama 36268, telephone 205-358-4202, fax 205-358-4515. Their portable unit is called their MS1500 and the microprocessor controlled unit is called their MS2500.
By being able to instantaneously monitor corrosion rates, determination of corrosion control additives can be quantified. The ability of a corrosion control additive to provide corrosion control is therefore dependent, as we have expressed above, on its ability to reduce or minimize any or all of the components necessary for an electrochemical cell to exist. The level of reduction or minimization is quantified by a measurement with the corrosion meter.
A functional corrosion control additive should provide a minimum of not less that 50% reduction in corrosion rates compared to the same water without the presence of the corrsosion control additive. A good range is 50% to 75% reduction as it is impossible to reach 100% due to the many reactions and minerals involved in forming the electrochemical cell.
After initial installation of the probes and corrosion meter, base line readings are taken on the water supply without the corrosion control additive. The corrosion control additive is introduced and readings are taken again after 48 hours of system acclamation. The numbers are compared:
Initial reading minus final reading after corrosion additive
Initial reading | x 100% = Y |
|---|
|
|---|
| 100% minus Y = per cent reduction |
|---|
|
|
|
Aqua
Smart Inc. 4445
Commerce Drive SW, Suite A4, Atlanta, Georgia 30336 USA
1-800-AQUASMART
|
|
