Corrosion prevention

When nitrogen trichloride is given off from the water in a pool it reacts with the atmospheric humidity to form hydrochloric acid. This then condenses onto cold surfaces and attacks them. Handrails in pools are often constructed from high quality stainless steel. This choice is based on its corrosion resistance. Stainless steel wires and fastenings are also used to support flumes and other structures within the pool building. Stainless steel in contact with hydrochloric acid is not stainless. It is damaged in two ways. One is superficial. An unsightly film of rust forms and, where accessible, can be removed with abrasive cleaners. The other is unseen and extremely dangerous. The hydrochloric acid penetrates the crystalline structure of the stainless steel and forms microscopic cracks that can only be detected using sophisticated engineering techniques. The cracks migrate through the metal and weaken it to such an extent that it eventually breaks. There have been many examples of this type of failure - it is strongly suspected as the cause of the recent collapse of a leisure pool roof in Moscow that killed 28 people.

Other swimming pool systems are also affected by the acidic atmosphere. In order to reduce heating costs the air is often partially recirculated and passed through dehumidifers or heat exchangers. The high acid content often corrodes these units leading to premature failure.

As part of the development of the Poolcure system Total Pool Chemicals commissioned the University of Manchester Institute of Science and Technology (UMIST) to carry out analyses of the air in two pools. One pool was using Poolcure with sodium hypochlorite and the other hypochlorite alone. The results were dramatic. The acid content of the air in the non-Poolcure pool was 250 times higher than that in the Poolcure pool. (Pages 18 and 19 Poolcure Manual -PDF)

UMIST also inspected both buildings for evidence of atmospheric metallic corrosion. They found severe damage in the non-Poolcure pool, even though it had only been in operation for 4 months. On the other hand, the Poolcure pool had minimal levels of atmospheric metallic corrosion.

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