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Corrosion Cost and Impact – Australasian Review
Water and wastewater infrastructure are present throughout Australia and New Zealand. The scope of water industries includes water treatment, distribution, plumbing, wastewater treatment, rainfall collection, industrial recycling and desalination.
In New Zealand, the wastewater and stormwater infrastructure include 24,000 km of public wastewater network, more than 3,000 treatment plants and over 17,000 km of stormwater network. In Australia, the total length of sewer pipes is over 110,000 km. While not all local authorities provided data, in 2012 those who did provide information reported a total of 145,481Km of water mains in Australia.There is no easily identified measure of the water infrastructure in New Zealand.
There are a range of products that have historically been used for water and wastewater infrastructure. Common pipe options include concrete, copper, stainless steel and PVC; while fibro has also been used in some situations. Each of these materials has corrosion risks, and the risks will vary based on the environment and the use. Water will incur a different range of corrosion issues to those caused by wastewater.
Causes of Corrosion
The causes of corrosion in the water and wastewater industries vary based on both the material used in the infrastructure and whether that infrastructure is used for water or wastewater.
Stainless steel has been suggested to have a maintenance period of 60 years. Stainless steel corrosion mechanisms in water are identified as crevice corrosion, pitting corrosion, stress corrosion cracking, microbiology-influenced corrosion, and galvanic corrosion. The general corrosion rate in stainless steel infrastructure is less than 2µm per annum. The causal factors in stainless steel corrosion are the chloride levels in the water, the chemicals used in treatment, the presence of any oxidant and the flow rate of the water. In addition, fluoride can be a causal factor when present.Corrosion in concrete structures includes chlorine-induced corrosion, particularly in coastal structures such as wharves and piers, carbonation induced corrosion, and microbial attack in sewers. Concrete is also subject to external corrosion from contact with the soil. Concrete corrosion from sulphuric acid is problematic in sewer systems, where the H2S present in the wastewater reacts to form this acid. There are variations of corrosion from sulphuric acid that appear to be based on the source of the acid. Mineral acid attacks appear to have a different mode of action to that of biogenic acid attacks. As a result, field data relating to corrosion may not necessarily align with laboratory results
Copper is commonly used in Australia and New Zealand for in-home plumbing. It was the material of choice until the early 1990s, when plastic plumbing became more widely accepted. More than 90% of Australian home have copper piping. Copper corrosion is extraordinarily complex. There is an isolated and seemingly random and sporadic nature of corrosion incidents. Further research is required to fully understand the causes. There are four main mechanisms.
- Blue water is a discoloration of water, often resulting from initial use after a period of stagnation. The types of corrosion products include copper hydroxides, with some silicates and sulphates. Copper levels in Blue water exceed the Australian Recommended Drinking Guidelines.
- Pitting Corrosion, resulting in pipe wall failure and leakage.
- Erosion Corrosion, most commonly found in hot water systems. Grooves and gullies form in the pipe, usually in the direction of flow. Erosion can be exacerbated by dissolved gas and particulate matter. The outcome of Erosion corrosion is pipe failure.
- Cuprosolvency, a slow rate of uniform corrosion. This is unlikely to lead to pipe failure; but can result in blue staining of surfaces in contact with the water from these pipes.
Corrosion in sewers involves a range of chemical actions that differ from that of water infrastructure. There are also variations within the sewer infrastructure based on being in contact with the wastewater or the associated gas. A significant proportion of the corrosion from wastewater is due to abrasion or scoring, while low pH (as low as 3) can lead to cement paste degrading to gypsum, with corresponding loss of strength and increasing vulnerability to abrasion and erosion. In the airspace, the gas will attack the pipe with H2S. Manhole covers can be a relative weak point in sewer infrastructure and are at significant risk of corrosion.Fluoride is present in many water supply areas in Australia and New Zealand. Fluorides are corrosive, and the effects can include release of asbestos from fibro mains, absorption of fluoride into the matrix in concrete mains with a subsequent weakening, erosion and deposition downstream, and corrosion of domestic copper pipes, hot water systems, water meters, washing machines, valves, and solder fittings.
Costs of Corrosion
In Australia, the value of sewer infrastructure in 2001 was estimated to be worth $A28 billion. The New Zealand stormwater infrastructure has an estimated replacement value of $NZ8.6 billion, while the wastewater infrastructure has an estimated replacement value of $NZ15.8 billion.
Leaking and burst water mains are problematic in the water industry. The second most common cause of water main failure has been identified as corrosion. Water main failure may result in slow leakage from the network, reducing the capacity of the network to supply water requirements for the community. One example is the small town or Renwick in Marlborough, New Zealand. The town has a small population of 2,100. The main industry in the surrounding districts is viticulture. The water supply is losing 300,000L per day due to leakage from corrosion in the network. The direct cost is the loss of water. The indirect cost is the implementation of watering restrictions throughout the district. Repairs are being implemented by the local council, and will continue for the next 30-40 years, at which stage repairing will become uneconomic.
Alternatively, they may result in catastrophic rupturing of the water main, resulting in property damage from flooding, costs for users of affected infrastructure (such as roads) being delayed and diverted in transit, and in costs associated with remediation and repair. A burst water main in Adelaide’s north-eastern suburb caused flooding in 40 residents. While it may have been significant, the failure that caused the flooding was one of many. SA Water has identified a failure rate of 20-27 failures/100Km/year between 2006 and 2015. This contrasts with other water providers across Australia, where Unity Water in Queensland has the lowest rate (<5), while Yarra Valley Water in Victoria has a reported rate of nearly 40 failures per 100Km per year. While corrosion is only the second most common cause of water main failures, the rate of failures provides some indication of the cost to the community of the corrosion-related failures. The estimated less of water due to these failures has been measured at between 15 415L/connection/day.
During a pipeline failure event, there are intangible costs that can have a significant effect upon the wider community, including disruptions due to flooding, road closures and loss of trade. The cost of corrosion of water and in Australia has been estimated at $A91 million per annum. On a pro-rata basis, this would suggest an annual cost in New Zealand to be in the range of $NZ17 million.
Ratliff notes that based on the age of water pipes, replacement of those installed from the late 1800s to the 1950s is now imminent, and the replacement of pipes installed since the 1950s will require ongoing activity for the rest of the 21st century. In the USA, more than 1,000,000 miles of pipe are nearing the end of their economic life and will require the investment of over $US1 trillion in the next two decades. On a pro-rata basis, the Australian mains infrastructure is around one tenth of the US infrastructure, while the New Zealand water mains are likely to be one fifth of the Australian mains systems. Using the same replacement costs, this means the investment in Australia is likely to be $A125 billion in the next two decades, while the cost for New Zealand is likely to be $25 billion.
Microbial induced corrosion of reinforced concrete sewer pipe is currently considered one of the most serious and costly problems. The Water Industry Network (USA) conducted a survey in 2000, which estimated annual rehabilitation costs to be $13.75 billion per year. A study in Germany in 2007 reported the cost for the repair of corrosion damaged sewer pipe in Germany is estimated to be over $50 billion. In Australia in 2001, the annual cost due to the failure of water/wastewater pipeline alone in Australia was estimated to cost $250 million.
Optimisation programs are being established by local governments to address the cost implications of maintenance and renewal. The Hastings District Council has implemented a two-stage approach for managing their sewer infrastructure. The first stage involves the determination of condition-based residual life and programming for rehabilitation or further monitoring. This is followed by stage two, targeted at optimizing the prioritization of short term (<5 years) repairs using risk matrix scoring. The funding requirements have been evened out across the infrastructure, including managing the risk of an age- based replacement where the renewal of large portions of the trunk sewers would coincide within a small 10-20-year window period.
Corrie (2015) noted that risk-based inspection plans are increasingly accepted in organisations that understand the importance of extending the life of high-value assets for as long as possible. These plans provide an indication of the downstream dollar cost of deferred maintenance. They also compare the cost effectiveness of a range of protective options and offering optimal maintenance plans for each.
An associated factor in management of corrosion is managing of the core infrastructure in areas where load is increasing, often to the point of maximising capacity. In such instances, corrosion may not be the limiting factor in a cost/ deferred cost decision. Rather, the need for increased capacity may result in remediation plans being deferred while augmented infrastructure is put in place.
An ideal maintenance management plan for large and complex infrastructure should be designed to avoid a huge outlay every few years, with a potential complete plant shutdown. A management plan could be to divide the infrastructure into “blocks” or sections and a rotating maintenance plan is developed. By doing this, there is ongoing maintenance and frequency of inspection is managed by the risk factors found in each area, as well as by the ease of access to each section. As a result, areas that are more prone to corrosion can be inspected and maintained more regularly than those presenting a lower risk.
Within sewers, management includes corrosion prevention, where forced air movement and venting are used to minimise hydrogen sulphide, and bacterial control is provided using chlorine or other sterilant. While this is possible to manage on a location basis, it is difficult to do on a large scale or in inaccessible areas. Another form of prevention is the use of materials that are not affected by acid attack such as PVC or HDPE pipes, or use of impervious liners for precast concrete pipes . Introduction of chemicals that react with H2S can reduce the rate of production of sulphuric acid. Magnesium hydroxide was identified as a product that is capable of such a reduction, with a resultant decrease in the rate of corrosion.
As with Oil and Gas infrastructure, water and wastewater infrastructure need to be managed based on a range of environments35. Factors that need to be managed include abrasion from strong, turbulent flows (particularly if the flow contains abrasive particles), concrete shrinkage and associated steel reinforcement corrosion, and the presence of chemicals that may lead to biogenic sulphur attack.
Implementing New Technologies
New technologies are providing tools to assist in identification of corrosion risks, and corresponding management of the infrastructure. The SeweX model is an example, where a simulation tool for predicting hydrogen sulphide and methane production in sewers, as well as other water quality parameters.
The CSIRO has been collaborating with water utilities to model sewer corrosion by focusing on data-driven approaches. The work intends to help water utilities to reduce the uncertainty of corrosion factors such as the H2S concentration and pipe condition over time. The overall outcome will ideally be a significant cost saving.
The Bondi Ocean Outfall Sewer, (BOOS) was commissioned in Sydney, Australia in 201135. Corrosion prevention measures included:
- Increased reinforcement cover, providing improved crack control
- Special concrete designs, including admixtures delivering higher durability, lower shrinkage, better compaction and reduced porosity
- Enclosed settlement tanks where the build-up of bacteria (aerobic and anaerobic) can be managed by the tanks being taken off-line
- Protective coating materials