Introduction:
Corrosion refers to the partial and gradual dissolution of materials used in water treatment and supply systems, pumps, pipes, tanks and valves. In most circumstances, almost all water is corrosive and this may lead to structural failure, water leaks, capacity losses and deterioration of the microbial and chemical water quality (WHO 2011). The pipes used in water distribution are made of different materials such as plastic, metals (galvanized steel, copper, lead and iron), and even concrete. Pipes made from concrete and plastic tend to be corrosion resistant.
In piping systems, however, there are several water quality factors working singly or in combination with others that affect the rate of pipe corrosion and depending on the degree of these factors, newly installed water distribution and piping systems can exhibit signs of corrosive wear in as little as two years after they are installed (Iklimnet.com 2016; WEERC 2012). However, it should be noted that while source water may have corrosive effects on the piping systems, this does not necessarily imply that poor or hazardous water quality is being delivered. In fact, according to Iklimnet.com (2016), water in the North West regions of Canada and the United States has one of the highest water quality ratings in the world yet it still has very corrosive effects on water piping systems. In this case, the corrosion of piping systems is directly related to several water quality parameters such as water pH, amount of oxygen in the water, chemical composition of water, temperature, contact with piping systems, and the pressure and velocity of flowing water. The material used in pipe fabrication also plays a huge role since most of these parameters have no corrosive impact on plastic or concrete pipes, but they do corrode metallic pipes.
Water quality parameters that cause pipe corrosion.
Water pH levels.
The acidity or alkalinity of water samples is measured on the pH scale which ranges from 0 (highest acidity) to 14 (highest alkalinity). The scale’s middle point is 7 and it indicates a point of neutrality. The pH scale is a logarithmic progression which means that a subtle difference of one pH unit is indicative of a ten times change in acidity or alkalinity. In this case, normal rain is slightly acidic with a pH of 5.6 and this is because rain water dissolves some of the atmospheric carbon dioxide. However, highly acidic rain with a 3.6 core on the pH scale has 100 times the level of acidity of normal rain with 5.6 pH.
In copper systems, a pH of more than 8 leads to the formation of an oxide layer which acts a chemical barrier that slows down the effects of corrosion. However, a pH level lower than 8 would dissolve the copper oxide layer thus removing the protective barrier and thus the pipe becomes subjected to corrosion by chemicals in the water (Iklimnet.com 2016). On the same note, if the water is highly alkaline and bacteria such as sulfate reducing bacteria (SRB) becomes established in the pipes, then problems related to MIC (Microbiologically Induced Corrosion) might be experienced. Low pH also causes corrosion in Iron and Lead pipes leading to the formation of both metal salts thus degrading the pipes (Oram 2016; WEERC 2012).
Amount of Oxygen dissolved in the water.
Public water supply systems are considered as open systems since the water in the supply system is constantly replenished with more oxygenated water. The dissolved air in thus water comprises about 22% oxygen while the rest is mostly non-corrosive nitrogen. Oxygen causes degradation of metals via the electrochemical process of oxidation. In this case, the metals are gradually converted to their respective oxides (rust) and they become thinner and weaker. These impurities are deposited within the water lines especially at connection points and corners leading to encrusted buildup. Oxygen content in water under high temperature and pressure is lower but these conditions also catalyze the process of oxidation and this often leads to high rates of corrosion in hot water piping systems compared to cold water lines (Iklimnet.com 2016; Oram 2016).
Chemical and Organic composition of water.
The basic chemical composition of water plays a significant role in determining the corrosive forces acting in pipes systems. For example, medium to high calcium levels help form a protective layer on the pipe which slows down the effects of corrosion due to other factors such as low pH. However, high calcium levels might also lead to encrusted buildups in the pipe systems.
Sulfate ions in piped water have different reactions. According to various short term studies, sulfate ions have been found to be non-reactive with copper while long-term studies indicate that sulfate ions are a causal factor in copper corrosion. SRB (sulfate reducing bacteria) are prevalent in most water mains systems (Seth and Edyvean 2006). If the water contains ammonia, nitrates might be formed leading to the depletion of dissolved oxygen. Oxygen depletion leads to the development of anaerobic regions where the SRB reduce the sulfate ions to hydrogen sulfide. According evidence presented by Jacobs, Reiber and Edwards (1998), sulfides in water can occur naturally or can be produced by the SRB found in the water pipes. It is this work that demonstrated the rather unique impact of sulfides on the corrosion of copper pipes. In this case, it was found that the corrosion rates were higher for water with sulfides compared to water lacking sulfide at pH levels 6.5 and 9.2.
Chlorides are a contributing factor to increased conductivity of water and since electrical conductivity is directly related to corrosion, it is generally implied the Chlorides are a causal factor to metal pipe corrosion. Chloride ions also form soluble salts with most metals but the corrosion potential does not necessarily increase as Chloride ion concentrations increase. Recent studies by Lytle and Schock (2008) showed that pitting corrosion of copper pipes occurred at high pH of 9 with Chloride ions present. At the degradation points, the copper metal was only in contact with Chloride anions which led to the conclusion that the presence of Chloride had an impact on corrosion.
Chlorine residue has been attributed to copper pipe corrosion since it is an electron acceptor and thus water treatment plants began switching to Chloramines as final water disinfectants. Chloramines have lower potential of forming DBPs disinfection by-products and have higher residual stability compared to Chlorine (Zaklikowski 2006). However, the same Chloramines act as sources of ammonia since they decompose to Chlorides and ammonia as the water continues to age in the piping systems. Ammonia can also be found in groundwater and can be carried through to water distribution systems if the water is not chlorinated at the breakpoints (Zhang and Edwards 2009). The existence of ammonia in distribution systems leads to its conversion into nitrate and nitrite by nitrifying bacteria found in the water mains in a process known as nitrification (Rahman 2008). The process is carried out by nitrite oxidizing bacteria (NOBs) and ammonia oxidizing bacteria (AOBs). First, the AOBs oxidize the ammonia into nitrite (Liu et al. 2005) then the nitrite is oxidized to nitrate by NOBs. In both cases, hydrogen ions are produced and these contribute to low pH (increased acidity) of water especially if the alkaline concentrations were initially low (Rahman 2008).
Galvanic Corrosion due to Dissimilar Metals.
Galvanic corrosion (electrolysis) occurs when different metals come into contact with one another. In this case, when two conflicting metals come into contact, one of the metals gives up electrons and over time, the metal that gives up the electrons corrodes at the contact point. These cases mostly occur when copper and galvanized pipes are interconnected, when copper pipes come into contact with steel pipe hangers and studs. However, this corrosion only occurs at the immediate contact area. Dielectric fittings are usually used to contain the problem but they do not repair the already damaged pipe (CDC 2016; Iklimnet.com 2016).
Other water quality parameters that cause pipe corrosion.
Other water quality parameters that could lead to water pipe corrosion include higher temperature and water velocity. High water temperature leads to faster oxidation hence faster corrosion while the converse is also true. Water velocity issues are often found in closed loop pipe systems where water is pumped at high pressure for distribution. Mostly, erosion of pipes occurs at turbulence areas and this turbulence is often caused by sudden directional changes in flow (turns and elbows), excessive water pressure, or due to flow obstacles such as excess older and burrs. In some case, erosion may also occur when water moves from pipes with large diameters to those with smaller diameters at high pressure (Iklimnet.com 2016; WSSC 2016).
Water treatment to control corrosion:
Most piping systems in distribution networks mainly comprise iron piping and this has resulted in one most costly and complicated problems faced by water distribution networks as pertains to water quality and pipe corrosion. If the different piping materials were ranked by corrosion rates, iron is the easiest material to corrode followed by lead, then copper and galvanized steel pipes. However, lead pipes have become sort of obsolete and only exist in old establishments that have not yet overhauled their distribution networks. This is mainly due to the fact that Lead is now considered toxic and thus using it as a water conduit and in cases where it will be corroded will eventually cause lead poisoning. Iron pipe corrosion mainly occurs due to oxidation which leads to the formation of iron oxide (rust). The rust has several disadvantages in that a significant amount of pipe mass is lost leading to leakages. In other cases, the accumulated rust becomes encrusted leading to head loss and reduced load capacity. Finally, iron pipe corrosion destroys the aesthetic quality of water with consumers reporting cases of ‘red water’, and also causes drinking water to have a metallic taste. Copper is the most commonly used plumbing material for home installations and is also less reactive than iron which makes it last longer even when exposed to low pH and other corrosion inducing factors described (McNeill and Edwards 2000).
Water quality and pipe corrosion: Flint River Water Case Study.
A classic example of water quality and pipe corrosion issues is the Flint River water which is highly corrosive to lead and iron pipes especially since these materials are widely used in Flint City, Michigan. The Flint River Water has high Chloride content and has no corrosion inhibitor added. This led to the Flint water crisis where consumers received toxic water with poor quality due to lead and iron corrosion which led to lead poisoning and poor water aesthetic quality respectively. The problem began in 2015 and is still to be mitigated.
References:
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