Failure Analysis of Corroding Stainless Steel Probe

Failure analysis expert was asked to determine the cause of deterioration of the tip of a stainless steel (SS) probe used to measure the level of hot tomato juice in a vat. Chemical analysis identified the steel as 304 SS with 0.082% carbon. At best, the carbon limit is on the high end in an alloy that is prone to grain boundary corrosion. Metallographic analysis demonstrated to the corrosion expert that the grain boundaries of the steel were decorated with carbides. The upper left photograph shows the deterioration of the end of the tube. Upper right is a 400X metallograph of a polished and etched sample of the steel. The arrows indicate decorated grain boundaries. Because tomato products have a significant amount of malic acid, 316L stainless steel was reccomended for replacement.