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Beyond The Pool: Corrosion Problems Involved in Swimming Pools

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Many swimming pools were built as part of leisure facilities or schools. These typically comprised of a cast in situ reinforced concrete base and walls with cast in situ promenade decks around the pool. These structures are now often in severe distress due to corrosion of the reinforcement in the walls and promenade decks.

All swimming pools experience galvanic corrosion if they use chlorine sanitizer. Chlorine is salt based, so by adding chlorine to a pool you are actually adding salt. A saltwater pool, however, has approximately ten times the salt level of a traditional chlorine pool. If a saltwater pool has ten times as much salt as a traditional chlorine pool, it means that the rate of galvanic corrosion also increases ten times.

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Corrosion prevention, often referred to as "cathodic corrosion prevention," is a massive field that touches an incredibly broad spectrum of industries, including maritime shipping, manufacturing, petrochemical, water distribution, food processing, major construction and even dairy farming.

The use of cathodic protection to stop this degradation of the reinforcement has been developed to be the standard repair method. The use of anodes in the pool water has become the most popular system of cathodic protection of swimming pools. The anodes are installed in boxes, recessed into the pool side walls of the swimming pool. The economic benefits of this form of repair are so impressive that it is now being used worldwide.  

In this installation, the swimming pool walls and base are protected using anodes in the pool water. The support columns and other concrete parts which also suffer from reinforcement corrosion are protected by internal anodes placed in holes drilled in the concrete. The cathodic protection system is computer-controlled. This gives an accurate continuous control of the output current to each part of the structure based on real-time readings from reference electrodes. This gives a better and more even protection from corrosion, increases the life of the anodes and switches off the anodes in the water when bathers are in the pool. The computer control system has a modem and a telephone connection allowing remote monitoring and control of the system.

Every swimming pool, and especially every saltwater pool, should have a sacrificial anode installed. The addition of this simple and low-cost device will dramatically reduce the damage a pool experiences as a result of galvanic corrosion. While you may still experience localized anodization and oxidation of metals in a pool, especially in situations where two different metals are in direct contact, a sacrificial anode is the bare minimum level of protection that every pool needs. It is absolutely silly to not have one of these — plus they can easily be adapted to any existing system.

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Conventional Steel Corrosion and Durability Design

Conventional corrosion is an electrochemical redox reaction, thus when steel is in contact with an electrolyte and oxygen, then steel mass will be lost, this is more pronounce in sea water. Corrosion, compared to time is generally a linear process and is uniformly spread over the exposed area.

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Table 1. Recommended value for the loss of thickness (mm) due to corrosion for piles and sheet piles in fresh water or in sea water

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On the basis of this table the common method utilised in accounting for corrosion is to utilise a sacrificial thickness by increasing the thickness of the pile by at least 4mm.

However, for construction in the Arabian gulf this method may not be the optimal solution due to the climatic and seawater conditions. The gulf coastline experiences some of the most extreme weather conditions with summer temperature reaching up to mid to high forties, with the salinity of the Gulf generally being highly variable with some sections near the coast reaching a concentration of 10 % (Fookes et al). In general, the salinity of the Gulf, at 4 %, is also higher than the open ocean, at 3 %.

The sacrificial thickness specification for a pile in sea water in zone of high attack is 3.75 mm, which means that a corrosion rate of 0.075 mm/year is adopted. However, according to research presented in CIRIA C634 that is the minimum rate of corrosion reported. The average corrosion rates reported range from 0.08 to 0.2 mm/side/year. For the harsh aggressive environment of the Arabian Gulf compounded with high and variable salinity of sea water, with the high temperatures a higher corrosion rate in design is recommended for optimal durability. The highest corrosion rates range from 0.17 to 0.34 mm/side/year. For a worst-case scenario, the highest corrosion rate will see a loss of 17 mm of steel, and if a sacrificial thickness of 4 mm is utilised, it will only protect the integrity of the member for 12 years.

Table 2. Corrosion Rates found in Literature

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