Have you ever wondered why detergents for automatic dishwashers contain sodium sulphite? It has nothing to do with cleaning dishes. The sulphite protects the dishwasher! Iron reacts with oxygen to form ferric oxide which is better known as rust. This reaction proceeds more readily at high temperatures, as found in washing machines. Where does the oxygen come from? It is dissolved in water.
The surface of water is in contact with air, so some oxygen can always dissolve. Oxygen also is a byproduct of photosynthesis which of course occurs as aquatic plants grow. The amount of oxygen that dissolves depends on the temperature (less dissolves as the temperature increases), the pressure (less dissolves at higher altitudes) and the amount of other substances already dissolved in the water (freshwater holds more oxygen than salt water). Oxygen in water is a good thing for fish since they can meet their oxygen needs by extracting the gas as water passes through their gills. But it is not so good for metals in washing machines!
Sodium sulfite is an oxygen scavenger. It reacts with oxygen to form sodium sulfate and effectively lowers the dissolved oxygen content, thereby protecting the insides of washing machines from rusting. Such corrosion may be an annoyance at home but it is a huge problem in industrial boilers. Sodium sulphite is commonly used here as well, although in high pressure boilers it presents a problem. It can break down to form sulphur dioxide along with hydrogen sulphide which not only smells like rotten eggs but is itself corrosive. Alternatives are available. Hydrazine, N2H2, is a liquid that can be added to boiler water to virtually eliminate dissolved oxygen. It reacts with oxygen to yield nitrogen gas and water, neither of which present a problem. But hydrazine is a pretty nasty stuff to work with so less toxic compounds are desirable.
An interesting possibility is sodium erythorbate which reacts with oxygen to produce lactic and glycolic acids which themselves can react with oxygen eventually yielding carbon dioxide. You may have encountered sodium erythorbate elsewhere. In hot dogs! It serves double duty here. Sodium erythorbate is a preservative and allows for less nitrite to be used. It is also, as we have seen, an oxygen scavenger and prevents the formation of off-flavors which can be produced as meat components react with oxygen. Incidentally, there is absolutely no truth to the rumor that sodium erythorbate comes from earth worms. This probably resulted from someone noticing a similar pronunciation between “eryth” as “earth” and jumping to the wrong conclusion.