What is it that commonly impacts the lives of human beings around the entire world? Among the list of the most important issues is the problem of water pipe corrosion and its complex interrelated causative factors. When the piping material comes in contact with water, invariably there is degradation due to corrosion. The different constituents of treated drinking water, the variety of materials chosen by designers for the water piping systems, the environmental factors such as soil conditions and the variety of techniques being adopted for corrosion prevention all add up to create a very complex problem.
The water quality parameters that basically influence the electrochemical deterioration of the world's piping systems include water chemistry and pH value. The water's alkalinity, temperature and dissolved oxygen and other gases influence the onset of corrosion. These water parameters can affect scale formation, the longevity of protective coatings and the deterioration rate of the piping material.
Corrosion in the Presence of Water
Corrosion is an electrochemical oxidation reaction that reconverts the metal to its original ore (metallic oxide) state in which it remains stable. Almost all metals will corrode in the presence of oxygen and moisture. Dissimilar metals connected directly or through an electrolyte form a galvanic cell that initiate the corrosion reaction. (Find out more about dissimilar metals and how to avoid galvanic corrosion in An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.)
The Impact of Water Chemistry
Water chemistry impacts the onset as well as the acceleration of corrosion. The presence of salts such as sodium chloride accelerates the corrosion rate and its severity. Calcium carbonate (CaCO3) present in the water can cause scale formation (a film of calcium carbonate) on the pipe's inner surface. The scale may provide a protective barrier thus minimizing corrosion, but invariably also causes the pipe to clog and obstruct the water flow.
If the water is alkaline (i.e., high pH), microbiologically induced corrosion (MIC) can start due to the presence of microorganisms. (Think you might have MIC? Find out for sure by reading Testing For Microbiologically Influenced Corrosion in Pipelines.)
A high dissolved oxygen content can cause the corrosion rate to accelerate. Researchers found that reducing the dissolved oxygen reduces the corrosion rate in ductile iron, galvanized steel and carbon steel pipes significantly.
The process of internal corrosion releases metallic ions from the pipe to the water stream on a continuous basis. Pipe plumbing fixtures used in water systems can contain lead as well as copper, hence, the corrosion of these systems can release lead and copper through a leaching process. In this context, the United States Environmental Protection Agency (EPA) has stipulated that drinking water should be monitored for copper and lead content, and corrective measures must be taken wherever the level exceeds the prescribed limits.
According to the US CDC, the effects of internal corrosion also include discoloration, odor and taste concerns, health issues and economic matters. Induced stray currents, the soil condition surrounding metallic pipes, water chemistry and the presence of dissimilar metals in pipes and fixtures can influence the corrosion rate.
The corrosivity of domestic drinking water can be indirectly controlled by continuous measurement and adjustment of the pH value and the levels of silicates, phosphates, calcium and magnesium, thus simultaneously controlling water hardness. While hard waters minimize the chances of metals leaching from plumbing, they also contribute to a peculiar taste.
The corrosion rate of the pipe's internal surfaces is also influenced by calcium carbonate, water temperature and residual chlorine. Dissolved oxygen acts as an acceptor of electrons in the oxidation of metallic water pipes. In short, the quality of potable water is affected by corrosion of the plumbing constituents.
High flow rates too can cause erosion corrosion or in some cases cavitation corrosion near the zone of flow restrictions. The presence of gritty contaminants in the water can accelerate erosion corrosion. (Learn more about erosion corrosion in Erosion Corrosion: Coatings and Other Preventive Measures.) Similarly, corrosion is aggravated by higher temperatures.
Economic Aspects of Water Pipe Corrosion Prevention
Apart from causing the leakage of a precious resource (water), corrosion can damage valuable assets such as fixtures and piping, cause interruption of the regular water supply, and pose a health hazard due to contaminated water. In some municipalities, water supplied through corroded pipes have been found to contain unacceptable level of hazardous metals and salts.
A study titled "Water Main Break Rates in USA and Canada: A Comprehensive Study", conducted by researchers at Utah State University, found that the average water loss due to leaks on account of main breakage is around 10% in North America. In some cases water leakage is as high as 20% to 30%.
Main breaks occur mainly due to corrosion and cracking (circular and longitudinal cracks) of metallic water pipes. Failure rates were found to be highest in cast iron and ductile iron pipes, and lowest in polyvinyl chloride (PVC) pipes. The risk of corrosion increased with the presence of corrosive soils because most water supply pipes are buried underground. The pipe break rates had a direct correlation with the corrosion index of the soil in the case of ductile iron and cast iron pipes. Most water supply utilities face a high risk of water pipe corrosion because the soil corrosion index of the territory is so high. Only a small number of utilities face moderate risk of water pipe corrosion. (The article An Introduction to Soil Corrosion is a good resource for this topic.)
Wherever the soil corrosion index is high the designers now prefer PVC as the piping material because they face zero risk of corrosion. The initial purchase cost of PVC pipes is high, but the lower life-cycle costs inclusive of low water loss costs should justify the choice. However, nonmetallic pipes also bring the risk of leaching their own set of undesirable traces of pipe material constituents and contaminants.
Coatings for Corrosion Prevention of Water Pipes
In the US and Canada, the components used by water utilities for water and distribution systems as well as water treatment systems must meet the requirements of NSF/ANSI 61 (National Sanitation Foundation - International Public Health and safety Organization) standards.
American Water Works Association (AWWA) standards prepared by its steel pipe committee deal with the requirements for testing the coatings and linings applied internally for protecting the surfaces of potable water pipes from corrosion. For each coating/lining material used for pipes, a separate testing standard has been established. Coating materials covered by the standards include polyurea, cement-mortar, fusion bonded epoxy (FBE), coal tar enamel or bitumen, liquid epoxy and polyurethane lining/coating, as well as other materials.
Polyurea coatings have superior corrosion resistance and chemical resistance. They are used to coat water pipes and potable water tanks. They have the advantage of fast curing, superior adhesion to ferrous metals, high elongation and abrasion resistance. The disadvantages include their high cost and rigorous surface preparation requirements. Though they are expensive to buy, the durability and lower maintenance cost provides justification for an higher initial cost. (For further information on polyurea coatings, please see 8 Things to Know About Polyurea Coatings.)
An inexpensive cement mortar provides a stable protective film on the ferrous substrate of water pipes. Discontinuities in the lining do not affect the protection and the risk of microbiologically induced corrosion is minimal. However this lining material is not suitable for water with heavy chlorination or systems with a water flow velocity exceeding 1200 feet per minute (20 feet/second).
Asbestos cement piping used in the past resulted in traces of asbestos in drinking water. Likewise, cement mortar coatings could result in the leaching of trace contaminants.
Fusion Bond Epoxy (FBE)
Fusion bonded epoxies (FBEs) belong to thermoset type of heat-cured polymers with very good adhesion to the steel substrate. They also have good abrasion and corrosion resistance. They are called fusion bond epoxies because of the nature of cross-linking and also because of the non-conventional application method used. The powder form of FBE is applied on cleaned and heated steel pipe surfaces at 450°F (232°C). The curing time is around 3 to 5 minutes. Stringent surface preparation is a necessary requirement for this coating.
Bitumen or Coal Tar Enamel (CTE) Coating
However, there are several disadvantages, such as:
- The risk of CTE causing leaching of trace amounts of contaminants into potable water
- Personnel being exposed to undesirable and potentially unsafe working conditions during application
- The stringent requirements of surface preparation
These factors have made CTE unpopular for the internal surfaces of potable water pipes.
- Excellent adhesion to clean ferrous substrates
- High mechanical strength
- Lower risk of cavitation corrosion and erosion corrosion
Designers can prescribe a lower coating thickness due to its favorable mechanical properties, which result in improved economy and efficiency of the water system.
The disadvantages of polyurethane coatings include the requirement for expert application and expensive equipment, as well as a very rigorous surface cleanliness.
Public utility water systems deteriorate and often fail due to corrosion of pipe surfaces. Factors impacting the internal corrosion of metallic pipelines include the use of dissimilar metals, water chemistry, dissolved oxygen levels and water temperature. (Related reading: The Role of Corrosion in the Flint Water Crisis.)
Water losses due to corrosion-related main breaks are estimated to be very high. The internal surfaces of metallic water pipes are often fortified with layers of anti-corrosion coatings. To address the health concerns, the chosen coating material for potable water infrastructure must meet the requirements of NSF/ANSI 61 standards.