Corrosion of water distribution systems causes billions of dollars of damage every year. Damage caused by corrosion affects the reliability and useful life of residential and commercial buildings, industrial processes and high purity water systems; just to name a few.
The by-products of corrosion, which are commonly described as an off-taste, odor, or appearance, adversely affects the quality of water as it flows through the piping system. This can have some adverse affects on the taste and quality of processed foods and beverages. In addition, it causes undesirable affects on many industrial processes and finished goods.
Some view pipe corrosion as caused by the water “attacking” the pipe. Others experience corrosion as a direct result of using materials of construction that are unsuitable for the water environment. In truth, effective corrosion control begins by making sure that the materials of construction and water quality are compatible with each other. In essence, water quality and materials of construction are two sides of the same coin.
Several factors need to be taken into consideration when designing a water distribution system. This includes federal, state and local codes and regulations that the engineer must meet. Further, every effort needs to be made to keep the costs within budget in the face of rising material and labor costs. Accomplishing this requires a basic understanding of:
Problems corrosion can cause
Forms of corrosion in water distribution systems
Factors that affect pipe corrosion
Selection of materials of construction
Problems Corrosion Can Cause
The by-products of corrosion may be harmful to ones health. For example, the USEPA limits the permissible levels of lead and copper in drinking water. Exposure to high levels of these metals have been linked to stomach and brain disorders. If the lead concentration exceeds 15 parts per billion and/or the copper level exceeds 1.3 ppm in 10% of taps sampled, the utility must take action to mitigate corrosion and reduce the levels of these metals.
Low levels of metals can also adversely affect the taste and quality of manufactured products. Zinc levels greater than 2.5 ppm and iron in excess of 0.3 ppm give a bitter, off-taste to drinking water. These contaminates can also affect the taste of processed food and beverages.
Water supplies that have elevated levels of corrosion by-products are aesthetically unappealing. Iron levels above 0.3 ppm cause rust-red staining of bathroom fixtures. Likewise, black stains are caused by manganese. The by-products of copper corrosion produce a telltale bluish-green stain. These metals will also discolor light colored clothing during laundering.
Pipe corrosion eventually results in leaks and joint failures. This problem has been documented in new copper plumbing systems that experience a rash of pinhole leaks within a few months of installation. Other systems, such as fire protection grids, fail due to the growth of sulfate-reducing bacteria that corrode the pipe as the water sits stagnant under anaerobic (lack of oxygen) conditions. These potential problems go undetected until the pipe fails causing costly water damage to the surrounding area.
The by-products of corrosion often lay down deposits that reduce water flow. This is commonly seen in older black iron and galvanized steel piping systems where iron oxides (rust) and severe tuberculation reduce the inside diameter of the pipe resulting in reduced water flow and a corresponding increase in pump horsepower. These insoluble materials can be transported by the water to points distant from the actual corrosion site where they foul valves, fittings, and heat exchangers.
Forms of Corrosion
Corrosion mechanisms are complex and depend on a number of interrelated variables. These include the type and concentration of dissolved minerals in the water (or lack thereof in the case of high-purity water), the materials of construction, type and concentration of dissolved gases, water temperature and water velocity. These factors combine in complicated ways to produce various forms of corrosion.
General Corrosion: This is characterized as a uniform dissolution of metal over the entire surface area. It is less of a problem than more localized forms of corrosion that promote rapid pipe failure. Acidic or low pH conditions will often result in general corrosive attack.
Galvanic Corrosion: When two dissimilar metals are directly coupled together in a water environment, the less-noble metal electrochemically corrodes in preference to the more-noble metal. Steel fittings that are directly coupled to copper pipe, for example, will corrode since steel is less-noble than copper.
Erosion Corrosion: This is a physical (mechanical) corrosion mechanism whereby protective inhibitor films are broken down by the rapid movement of water over the metal surface. If the water carries suspended solids, the erosion is made worse due to the abrasive nature of the particulate matter.
Pitting Attack: This is a randomly occurring, localized form of corrosion that occurs when dissolved oxygen and dissolved salts initiate pit formation on the metal surface. These rapidly propagate through the pipe wall resulting in pinhole leaks. This form of corrosion is known to affect steel, galvanized steel, copper and stainless steel. Aluminum is also susceptible to pitting attack, which is why it is considered an unacceptable material for piping systems.
Concentration Cell: This form of corrosion is caused by electrochemical attack due to differences in the water environment. This typically occurs under stagnant water conditions that exist in flange crevices, couplings and threaded joints.
Selective Leaching: Many copper-zinc alloys, such as yellow brass and Muntz metal, are prone to attack by selective leaching of the zinc from the brass alloy. Depending on the alloy type, this can occur in both acidic and neutral/alkaline environments. Both uniform and plug-type dezincification can occur.
Factors that Affect Corrosion
The quality of the water to be conveyed by the piping system plays a role in selecting the proper materials of construction. Soft and demineralized water are often viewed as “corrosive” and therefore require pipe materials that are more resistant to corrosion such as stainless steel and plastic. However, hot, deaerated soft water is suitable for distribution in black iron pipe. But water that is high in chlorides and low in oxygen promotes pitting corrosion in stainless steel.
One of the primary determining factors in predicting the tendency of pipe to corrode is the concentration of dissolved gases in the water. Dissolved oxygen plays a critical role in the corrosion of most metals. Iron and steel corrosion rates increase with an increase in the concentration of dissolved oxygen. Oxygen-pitting is a localized form of corrosion that causes damage to black iron and galvanized steel piping systems. However, oxygen is necessary to establish and maintain a passive, protective corrosion barrier on stainless steel and aluminum.
Carbon dioxide gas forms carbonic acid when dissolved in water. This lowers the pH and thereby promotes general corrosion of piping systems.
Hydrogen sulfide gas is generated as a by-product of sulfate-reducing bacteria that thrive in oxygen-free environments. This gas is very corrosive as it depresses the pH resulting in a localized, acidic environment that attacks iron and concrete pipe. The chemical reaction produces iron sulfide, a black deposit that builds up under stagnant water conditions. This is commonly seen in fire protection systems and is the main reason for periodic hydrant flushing. It is also very common in sewer lines.
Water temperature plays a role in determining the rate of corrosion. A general rule of thumb is that the corrosion rate of iron doubles with every 50 F increase in temperature. All things being equal, hot water is more corrosive than cold water.
Water velocity gives rise to a physical form of corrosion. Flow in excess of 4 feet per second promotes erosion of softer metals like copper, for example. Piping systems must be designed to flow at a rate that is compatible with the water quality and materials of construction. Stagnant water conditions promote bacteria growth and create an environment for micro-biologically influenced corrosion (MIC).
pH, while related to the other corrosion factors, is not the sole determining cause of corrosion. pH measures the relative acidity of water. A low pH below 7 is acidic whereas a high pH above 7 is basic or alkaline. Iron and steel corrosion rates are stable within the pH range of 4.5 to 8.5. Tuberculation (build up of iron corrosion by-products) generally occurs within pH 7.5 to 8.5. For galvanized pipe, the protective zinc coating is stable within pH 6.5 to 12. However, zinc corrodes rapidly at pH values less than 6.5.
Many municipal water treatment plants are required to adjust the water quality to minimize corrosion in the distribution system. In general, adding sodium bicarbonate (baking soda) to increase the alkalinity and pH helps to passivate metal pipe surfaces to reduce corrosion. Phosphate and/or silica is also used to form a protective inhibitor film on metal surfaces. It is difficult to predict the effectiveness of chemical additives as corrosion control agents, however, without routine sampling and testing to measure lead, copper, iron and zinc levels.
Selection of the Materials of Construction
Corrosion control begins with the selection of the materials of construction to be used in the water distribution system. As the saying goes, the engineer must “design with corrosion in mind.” Of course, consideration must be given to cost since materials that are more corrosion resistant are often more costly to purchase, fabricate and install.
Prior to World War II, black iron pipe was commonly used in water systems. Galvanized steel pipe was later substituted since the zinc coating afforded a protective, sacrificial barrier that inhibited corrosion of the underlying steel and thereby prolonged the useful life of the piping system.
More recently, millions of miles of copper tubing and pipe have been installed for water service. Copper is very durable and is suited for hot and cold water service. However, waters low in hardness and high is dissolved gases (oxygen and carbon dioxide) have been linked to a rash of pin-hole leaks. However, the mechanism for this type of copper corrosion failure is poorly understood and is just now being identified. Because copper is a soft metal, it is not suitable for use in high flow applications where the velocity is greater than 4 feet per second.
Lead pipe has been used for many years to make service connections from municipal water supply lines to residential plumbing systems. Lead can also be found in the solder used to make joint connections in copper piping. As water flows through the metal pipe, lead can be leached into the drinking water. As a result, lead solder is no longer used for making copper joints. In many cases, lead pipe must be removed from service connections and replaced with an alternative that is less prone to corrosion, to reduce the lead concentrations in drinking water.
When engineers seek a corrosion resistant material to convey high purity water they generally turn to stainless steel. However, stainless steel is not immune to corrosion. As the name states, it just “stains less.” Chlorides promote pitting of stainless steel. Stainless steel also suffers from rouge discoloration on the metal surface. Stainless is more difficult to weld and tends to corrode in the heat affected zone.
Metal pipe such as black iron, lead and copper have been the mainstay of water distribution systems for many years. More recently, plastic pipe is finding applications as an alternative to metal pipe. The use of plastic pipe and tubing may be limited by local plumbing codes, however, so it is best to identify any restrictions on its use ahead of time.
PVC (polyvinyl chloride) pipe is available in the familiar Schedule 40 (white) grade and the less common Schedule 80 (gray). Sch40 PVC is rated up to 140 psi and is commonly used for drain and sewer lines. Sch80, being thicker walled, is rated up to 200 psi and is frequently used in higher pressure, higher purity water applications. PVC is suitable for use in non-pressure applications between 0 and 150 F.
CPVC (chlorinated polyvinyl chloride) pipe is rated for up to 100 psi service and 180 F. The main advantage of CPVC over PVC is that it is more ductile and has greater flexure and crush resistance.
PEX (crosslinked polyethylene) tubing is used for hydronic heating loops and general water service. It is suitable for water temperatures between freezing and 200 F.
HDPE (high density polyethylene) is gaining popularity for use in water distribution systems because it is corrosion-free with a service life of 50 to 100 years. Like the other types of plastic pipe, it does not corrode, tuberculate or support bacteria growth like metal pipe.
HDPE has additional advantages in that the joints are butt fused using a “fusion welder,” which yields a strong heat-fused joint that offers flexibility and fatigue resistance.