Free and emulsified water in the diesel as well as metal particles – especially copper, iron, aluminium and tin – accelerate oxidative processes and additive degradation, resulting in the formation of organic acids (carboxylic acids, sulfuric acids). Acids have a highly corrosive effect. The effect is intensified by water. Since water is highly polar, it attracts polar additives (e. g. detergents, dispersants) and impurities. Therefore, as soon as emulsified and free water is present, a sludge-like mass is formed, which sediments at the bottom of the tank and enters the engine when the fuel is used. Clogged in-line filters, valves and injection nozzles are the results.
When using fuel mixtures with biodiesel components, there is a risk that the oxidation stability will be used up after just a few months. The following polymerization reactions lead to the formation of polymer gum, which clogs pumps and injectors. For this reason, operators usually use low-sulfur heating oil with an advanced additive package and high oxidation stability. However, this special fuel is often unavailable for short-term needs, so that fuel is used, containing up to 7% biodiesel.