What is Water System Corrosion? The word corrosion is derived from the Latin word “rodere” meaning to gnaw. Corrosion is a natural process involving chemical or electrical degradation of metals in contact with water. The most familiar is the formation of rust when iron is exposed to water. Copper corrosion is less common but perhaps more critical since copper is regulated in drinking water. The rate of corrosion will vary depending on the acidity of the water, its electrical conductivity, oxygen concentration, and temperature. Acidic water with pH values in the range of 6 to 7 is more corrosive to the metals used in plumbing systems than alkaline water. Alkaline water however does not eliminate corrosion if the water also has high electrical conductivity. Total dissolved solids in spite of the beneficial hardness and alkalinity may contribute to the problem.
Factors necessary for Corrosion; For Corrosion to occur there must be the following:
1. An anode where current leaves the metal.
2. A cathode where current enters the metal.
3. An electrolyte such as water to provide the pathway for metal ions.
4. A connection between the electrodes.
Purposes of Corrosion Control:
1. Help protect public health.
2. Provide aesthetically pleasing water.
3. Extend the life of plumbing materials and prevent needless economic drains due to corrosion induced distribution and home plumbing leaks and repairs.
4. Meet Federal and state lead and copper regulations.
Factors that Affect Corrosion: Water Quality parameters such as pH, iron, manganese, hardness, etc. will greatly influence the final product and treatment recommendation. The following water characteristics and their affect on water quality should be part of the overall treatment evaluation.
Alkalinity: As alkalinity increases corrosion decreases.
Calcium: As calcium increases corrosion decreases.
Biofilms and microbial growth: As biological activity increases corrosion increases.
Chlorides, Sulfates and Nitrates: (As part of the TDS), as these increase, corrosion
increases.
Chlorine: As chlorine increases, corrosion increases.
Dissolved Oxygen: As dissolved oxygen increases, corrosion increases.
Iron & Manganese: As Fe-Mn increases, discoloration increases & water quality decreases.
Flow Velocity: Excessive flows can increase corrosion by erosion.
Hardness: As hardness increases, corrosion decreases.
Hydrogen Sulfide (H2S): As H2S increases, corrosion increases.
Phosphate: Decreases corrosion by providing cathodic and anodic inhibition.
pH: As pH increases up to 8.5, corrosion decreases.
Silicates: As silicates increase, corrosion decreases.
Temperature: As temperature increases, corrosion increases.
Total Dissolved Solids (TDS): As TDS increases, corrosion increases
Where Do We Start? What Information is needed? The following chemicals in water are critical to determining the corrosive tendencies of the water.
Laboratory Tests
alkalinity calcium chloride conductivity corrosivity
hardness iron manganese pH magnesium sulfate
hydrogen sulfide total dissolved solids dissolved oxygen
Field Test Parameters (Tests completed in the field for accuracy)
Field temperature = F
Field conductivity = uS
Field pH =
Field dissolved oxygen = mg/L
Visible gas = yes or no
If you don’t have this information on each well I recommend that you take samples and have each parameter analyzed
Corrosion Inhibitors: Phosphates
Orthophosphate inhibitors are used in boiler, cooling and potable water applications, for corrosion, pH and lead control. The orthophosphates can act as anodic corrosion inhibitors in the presence of oxygen (e.g. surface and aerated waters). This type of corrosion control is called passivation. Passivation results initially from chemical precipitation which provides a barrier between the metal and the corroding liquid. In the case of orthophosphates, a thin film which is very insoluble and resistant to dissolution is put down. It is a form of conversion coating. Such coatings result when films capable of preventing the migration of ions from the solid state to the solution state are formed. These thin films retard the flow of electrons through the galvanic cell which is driving the reaction, reducing its effects and controlling corrosion.
Condensed phosphates or polyphosphates, as the names suggest, contain more than one phosphorus atom in the molecule or ion, connected to each other through an oxygen bridge. The chains may contain as few as two phosphorus atoms (pyrophosphates) or an infinite number (polyphosphates). They may be linear or cyclic (metaphosphates). They are prepared from orthophosphates by dehydration and other techniques.Polyphosphates are cathodic corrosion inhibitors. Divalent cations such as calcium (Ca+2) at levels of at least 10mg/L are needed along with the polyphosphates for them to be effective. Positively charged colloidal complexes are formed which migrate to the cathode forming an amorphous polymeric film. This film is self limiting. Analysis of the film shows ferric pyrophosphate and iron/calcium metaphosphate to be the primary constituents. In these applications, the polyphosphate is acting both as a reservoir of potential orthophosphate and as a cathodic corrosion inhibitor. More soluble and less likely to be precipitated in its polymeric form, polyphosphates can be retained longer in the system than orthophosphates. Much of the polyphosphate will eventually revert to the orthophosphate condition, at which time it may react with elements such as calcium, lead, copper and iron, to be precipitated and to form protective conversion coatings.
The property of sequestering exhibited by polyphosphates finds application in boiler, cooling, industrial and potable water treatment for descaling, softening and the elimination of red and black waters. Sequestering offers two major benefits when it is used in water treatment. First, it reduces scale formation by preventing the precipitation of calcium salts and second, it prevents discoloration of the water by inhibiting the precipitation of iron and manganese hydroxides. Polyphosphates reduce scale formation by interfering with the formation of calcium carbonate crystals, thereby reducing the rate of film formation. There is a "threshold effect" in using polyphosphates in this way. Once the threshold is reached, polyphosphates can prevent scaling at calcium carbonate concentrations far above the saturation concentration. One mg/L of polyphosphate can sequester 100 mg/L of hardness. In addition to preventing scale formation polyphosphates can be used to remove scale.
Polyphosphates form colloidal dispersions with metals such as iron, manganese and calcium. Stable negatively charged particles are formed in which the metal is coated with polyphosphate ions. This prevents the particles from coalescing and maintains them in solution. The actual mechanism is not precisely defined at present and it probably varies with each cation. The result is the prevention of scale and of red and black water.
Blended Ortho-Polyphosphates: The properties of the orthophosphates and the polyphosphates are enhanced when blends of the two are used. Since orthophosphates are anodic corrosion inhibitors and polyphosphates are cathodic inhibitors, and since polyphosphates in solution slowly revert to the orthophosphate condition, by formulating blends of these two phosphate forms we can achieve both anodic and cathodic corrosion inhibition, ensure the orthophosphate ion availability over a longer period of time and control calcium, iron, copper and lead deposition, all at the same time. In this way, blended phosphate products can offer better protection than either ortho or polyphosphate can alone. As an added benefit, blends are effective in reducing copper corrosion, which neither ortho or poly types do very well by themselves. Typical maintenance doses range from .4 to 1.2 mg/L.
Typical Approaches to Corrosion Control Projects: Normally pilot scale testing is performed on the water in question. It is done by installing test loops with and without chemical treatment to determine the effectiveness of the treatment chemical and program. These loop pilot tests can be variable and may not reflect actual system results. Additionally, you must also consider the cost of the chemical and continued treatment, the technical support, vender performance and service, and case histories of other utilities using similar products.
Time Required For Results: Studies have shown some results can be seen in as little as two weeks, but the protective film formation needs two to six months of treatment to reach its maximum depth.