California Failure Analysis Lab Examines Stainless Steel Corrosion Failure

Stainless steel corrosion failures often result from grain boundary sensitization. Many 300 series stainless steel alloys come in two grades or more. For example 304 stainless steel comes as 304, 304L and 304H. Each grade has a different amount of carbon (C). the 304 grade allows up to .0.08% C; whereas
304 L allows a maximum of 0.03% C. In corrosive environments, this is critical if the piece is welded or heated in the temperature range from 459°C to 850°C. As the carbon content increases in the alloy, the time for grain boundary sensitization decreases. Grain boundary sensitized stainless steels exhibit rapid corrosion  failure at the grain boundary because the chrome has been tied up by the carbon in the grain boundaries.
In this case the California failure analysis expert performed a corrosion failure analysis on a jacketed pipe made from 304 stainless steel which corroded rapidly as a result of grain boundary sensitization. The alloy was not specified correctly. The design drawings should have specified solution treated 304L or 316L plate for the manufacture of the industrial pipe. The plumbing failure expert  contends that if this were done, the plumbing failure would not have occurred. The equipment designer cannot rely on the supplier to pick the proper materials..  

Failure Analysis Expert Examines a Corrosion Failure

Corrosion engineer performed a metallurgy failure analysis to determine the cause of prematurely failing burners in a particular brand of gas grill. The burners in question would fail completely after two or three years of service. The failure analysis of the burner revealed that the grill manufacturer made the subject gas burners from an inferior grade of stainless steel. They used a relatively obscure alloy (J1). This alloy is a variation on 304 stainless steel. The 304 alloy contains 18% chromium (Cr), 8% nickel (Ni) and 2% maximum manganese (Mn), and it allows a maximum of 0.08% carbon (C). The J1 alloy allows the same amount of C but contains only 15% Cr; it also has 4% Ni and 8% Mn. Because the Cr is the active ingredient for the corrosion resistence of stainless steel, the J1 alloy will perform poorly in a corrosive environment. A by-product of burning propane in a gas grill is water. In addition, the burner is hot with other contaminants on it; a corrosion expert would state that this is a corrsive environment. Upper left is a photograph of a burner that failed prematurely. Upper right is an unetched metallugraphic sample of a piece of this burner. One can clearly see that the burner surface shows grain bounbary corrosion. It is believed that the low level of Cr and the presence of water and carbon makes the J1 alloy susceptable to stainless steel corrosion and unsuitable for gas grill burners. This product defect analysis indicates that this is a not a manufacturing defect but design defect.

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.