Stainless steel is an attractive, sleek material for pool accessories and trim, but it has a few limitations. It can become corroded from exposure to chemicals in pool water and time, resulting in a pitted or blackened appearance. Combating the corrosion is a losing battle for most pool owners.
The most likely reason stainless steel turns black in a pool is oxidation, which is caused by chlorine in the pool. The oxidation process, known as rouging, also can occur as a result of exposure to minerals in pool water, which causes orange or red rust stains on stainless steel instead of black stains. Stainless steel blackened due to rouging also may be pitted.
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Black oxidation cannot be removed by wiping or cleaning stainless steel. One option is to polish or grind the stainless steel, but a simpler process is to remove it with a citric acid solution. Citric acid is available in powder form at hardware stores and natural food stores. Stores that sell home canning supplies also may sell citric acid. Dissolving 1/4 cup citric acid in 1 quart water makes the solution. Wipe the mixture on the stainless steel; use a cotton swab or small paintbrush to reach inside crevices. After waiting 10 minutes, scrub the stainless steel, and rinse it with clean water. The process can be repeated if necessary.
Unfortunately, removing the black tarnish from stainless steel is only a temporary solution. As long as the stainless steel is exposed to chlorinated water, it probably will become rouged again. The best bet is either to live with the black stainless steel or replace it with a non-metal material.
One possible, though unlikely, alternative cause of blackened stainless steel is black algae. Algae is a common problem for pools, and black algae is one of the most difficult types of algae to remove. If black algae is the culprit, you'll notice the algae in other areas of the pool, such as its walls and steps, which the black algae may make slimy and slippery. Treating black algae requires scrubbing all pool surfaces and using a metallic algaecide.