In his influential talk on water chemistry at the Nordic Barista Cup in 2013, Scott Guglielmino of La Marzocco talked about a strange incidence of boiler corrosion in the city of Cambridge, Massachusetts, in 2007. In that year, the company received seven complaints about boiler failure due to corrosion in Cambridge LM machines. This was more than in the entire United States combined, and Guglielmino was tasked with unravelling the mystery. He discovered that when the Cambridge area experienced icy winter weather, the local authorities applied salt to the roads. Salt lowers the freezing point of water and can help prevent traffic accidents. The salt from the roads was washing into the water catchment areas and making its way into the local water supply.
Salt dissolved in water accelerates the corrosion of metals because when the salt dissolves, it immediately dissociates into sodium cations (Na+) and chloride anions (Cl–). These ions on their own don’t cause corrosion, but they make it much easier for electrons to move through the water, so they greatly increase the rate at which corrosion can occur. The corrosion of a stainless steel boiler occurs in many areas, often around welded areas, where the layer of chromium and nickel that usually prevents the steel from reacting with oxygen may have worn away. In weak points in the boiler, iron can react with oxygen in the water: The iron loses electrons and the oxygen gains electrons; the result is the formation of iron oxide (rust).
The Chemistry of Rust — Anodes, Cathodes, and Electrolytes
A steel stainless boiler rusts by the same principle by which a battery generates electrons to power devices. The only difference is that a battery uses a metal that it intentionally rusts, such as zinc, which donates electrons as it corrodes. A boiler can develop a pit, in a process that is a bit similar to the way you get a hole in your tooth.