What is corrosion?
Corrosion is a chemical reaction between chemicals and metals which creates a metal compound, deteriorating the surface of the metal. Corrosion is commonly seen in water pipes as there are a range of chemicals naturally present in water that cause corrosion. Additionally, even ultrapure water is a highly corrosive substance when exposed to atmosphere. Corrosion damage by water is most commonly seen in steel (iron) pipes, but this process can also be seen in aluminum, copper, lead and zinc piping. Surface water, groundwater and seawater can all cause corrosion in metal pipes.
What chemicals cause corrosion?
There are several species in water which will speed up corrosion reactions.
- Lack of hardness – calcium and magnesium precipitate from water and coat water pipes (scale). When these ions are not present, corrosion is more likely
- Salinity – the presence of salt allows metals to lose electrons more easily and speed up the corrosion process
- Dissolved oxygen – the oxidation of metals is faster with higher concentrations of oxygen in the water
- High temperature – corrosion reactions are generally faster at higher temperatures
- Acidity – metals oxidise faster at lower pH (higher acidity)
- Bacteria – there are certain microorganisms that cause corrosion in metals
The more of these species that are present in water, the faster the corrosion process will be. For example, if water is highly saline and at a high temperature, the corrosion reaction will be faster than for highly saline water at low temperature.
Long term effects of corrosion
Corrosion reactions weaken metals, cause surface damage and increase the concentration of metals in water. Long term corrosion can lead to pump failures through rapid pressure release, valves jamming shut or open due to oxidation and pipes cracking and failing due to material weathering. The longer corrosion goes undetected, the more damage that will be done. Corrosion reactions generally occur where they can’t be seen as they happen on the inside of pumps, valves and pipes and corrosion is only noticed when it has caused significant damage.
Measuring the corrosivity of water
Measuring whether water is corrosive or scale forming is done by measuring the Langelier Saturation Index (LSI). The LSI is calculated by measuring the pH, the temperature, the conductivity (or total dissolved solids) and the concentration of calcium bicarbonate (an indicator of the hardness of the water). A negative LSI indicates that the water is corrosive while a positive LSI indicates that the water will form scale. Even a neutral LSI (of 0) can cause slight corrosion. Seawater, which is high in calcium carbonate, has a slightly positive LSI, but due to its high salinity is both corrosive and scale forming.
A full water analysis of the parameters required to calculate the LSI is recommended to determine the likelihood of corrosion. A site water audit which determines the likely presence of dissolved oxygen or bacterial species which can cause corrosion is also recommended if corrosion is a concern.
Methods to prevent corrosion
If water is deemed to be corrosive there are a wide range of methods to prevent corrosion.
- Technologies – there are a wide range of technologies which can reduce the corrosivity of water including Reverse Osmosis, temperature correction, degassing and disinfection
- Materials – the material the water pipes, valves and pumps are made of can prevent corrosion. Plastic materials like PVC or HDPE and metals like titanium do not corrode like steel or copper pipes. Additionally, there are a range of steel alloys like SS316 and 2205 duplex which show enhanced resistance to corrosion.
- Chemicals – there are a wide range of chemicals that can be added to either alter the acidity of water (e.g. lime addition) or inhibit corrosion (e.g. phosphates – phosphonates or sulphites).
Often these technologies will be used in combination to prevent corrosion.
Contact Moerk Water to find out more about preventing corrosion in your water system.
