During operation, wind turbines are subjected to a wide range of varying temperatures, wind speeds and loads. These operating conditions, in combination with potential detrimental influences from solid contamination and tribochemical attack due to oil chemistry and water contamination, could result in bearing damage (fig. 1) , which would considerably shorten bearing service life.
One of the requiremets to prevent these failures is superior surface performance that favours the running-in process, is resistant to corrosion and is inert against unfavourable reactions from aggressive lubricant chemistry. Through SKF’s long involvement with the wind energy industry, the company has identified black oxidation as one of many solutions to improve operational reliability.
Black oxidation process
Black oxide is a surface treatment that is formed by a chemical reaction at the surface layer of the bearing steel and is produced when parts are immersed in an alkaline aqueous salt solution operating at a temperature between approximately 130 and 150 °C. The reaction between the iron of the ferrous alloy and the reagents produces an oxide layer on the outer layer of the bearing components, consisting of a well-defined blend of FeO, Fe2O3 and resulting Fe3O4. The result is a dark-black surface layer of approximately 1–2 μm in thickness. The total process consists of about 15 different immersion steps, in many of which it is possible to vary chemical contents, concentrations, temperatures, immersion times and fluid behaviour within the tanks.