Quite a number of food products, potatoes, vegetables and pharmaceutical products show signs of black discolouration caused by friction against stainless steel components (cold-rolled or brushed sheet in 304L and also 316L), such as vibrating chutes, filling hoppers and conveyor surfaces. There are various causes for this black discolouration.
Which can result in significant product wastage. 
Packo Surface Treatment offers electrochemical treatments that help prevent this black discolouration.