Corrosion control treatment of drinking water is necessary to prevent deterioration of pipes and fittings, which eventually leads to water loss and pressure loss. It also reduces the leaching of biologically toxic metals such as lead and cadmium. Corrosion control treatment also affects consumers’ perceptions of the water quality by reducing metallic tastes, water color and odors.
Drinking water is inherently corrosive because it contains ions, salts, oxygen and residual disinfectants (chlorine disinfectants are by nature reactive compounds). Treatment chemicals that can reduce corrosion include sodium silicate and phosphates (e.g. zinc orthophosphate, sodium tripolyphosphate and sodium hexametaphosphate), as well as chemicals that impact the water characteristics and change how aggressive the water is toward a given material. These include pH adjusters such as sodium hydroxide or carbon dioxide, and compounds that increase alkalinity such as sodium bicarbonate.
Corrosion control treatment chemicals produce their beneficial effects through a variety of processes.
Silicates, phosphates and various insoluble carbonates (of iron, lead and zinc) form scale on the interior walls of pipes. The scale deposits are composed of many different compounds and are complex in structure, porosity and equilibrium with the water. Which compounds form and deposit depends on water characteristics such as pH, alkalinity and hardness. They are not uniform in their distribution, or their thickness. The scale deposits do not function as a true water barrier such as a paint or coating. Rather these porous deposits retard the electrochemical reactions between the water and the pipe surface by reducing the transport of ions between different sections of pipe surface.
Polyphosphates function less by contributing to scale deposition; rather they are more commonly used to remove undesirable scale. Pitting of the pipe surface frequently occurs in metal pipes. Corrosion compounds will sometimes deposit in and around the pit, and project out of the pipe wall. These porous deposits, called tubercules, do not prevent future corrosion and reduce the flow of water. Tuberculation can be controlled using polyphosphates, which increase the solubility of the corrosion products, moderate the pitting corrosion and can also address aesthetic affects.
Corrosion control treatment chemicals, must be selected and applied based on the water quality parameters of the treated water, as well as the pipe materials that constitute the system. The blend of corrosion control chemicals that works well in a high pH environment with mortar-lined pipe will not perform the same in low alkalinity water and ductile iron pipes. While it can be expensive in both time and money, it is prudent to test new corrosion blends under field conditions on pipe test rigs. For new pipe, or pipe without a protective scale layer, initial doses of the corrosion control chemicals will need to be high to favor scale deposition. After the scale layer has formed, which will take weeks to months, the dose should be lowered to a maintenance dose. Without the maintenance dose, the protective scale will be depleted by the water.
Because corrosion control treatment chemicals are continuously dosed into the drinking water system, it is important to purchase high quality products so that the treatment chemical itself does not contribute contaminants to the drinking water. Evaluation and contaminant testing to NSF/ANSI 60 ensures that these treatment products are safe and contain minimal contaminants. Typical test batteries for phosphates include regular metals, radioactive isotopes and fluoride. Silicates are minimally tested for regulated metals. Products certified to NSF/ ANSI 60 cannot contain regulated metals at more than one-tenth of the EPA maximum contaminant level (MCL)
Products that are certified to NSF/ ANSI 60 can be found on NSF’s official certification listings, which also allow searching by chemical name and by state country or region. It is easy to verify that a specific treatment products is certified by searching by the treatment product’s trade name or manufacturer name. To search for products certified by NSF International to NSF/ ANSI 60, visit info.nsf.org/Certified/PwsChemicals/ .
By Scott Randall, NSF Municipal Water Matters, June 2017 Issue
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