Corrosion of Stellite-6 Overlay Cladding on 304L Stainless Steel in Nitric Acid

Internal gate valve parts corrode in nitric acid (HNO₃) production facilities, which has a significant financial impact. This study uses gas tungsten arc welding in HNO₃ to examine the corrosion of gate valves constructed of 304L stainless steel with a Stellite-6 overlay. Three zones—the Stellite-6 overlay, 304L stainless steel, and the zone at the interface between the Stellite-6 overlay and 304L stainless steel—were represented by samples taken from the valve gate. The findings demonstrated inter-dendritic corrosion and homogenous corrosion attacking the interface zone. The corrosion rates of these three zones were determined with the help of the linear polarization resistance method in 40% HNO₃ at 25 °C. Due to the diluting effect, the interface zone among the three showed the highest corrosion rate. The interface zone’s chromium content was lower than that of the Stellite-6 alloy, which resulted in a lower level of corrosion resistance. Additionally, the zero resistance ammeter method was used to study the galvanic corrosion of the interface zone. With respect to Stellite-6, the interface zone exhibited galvanic corrosion and acted as an anode. On the other hand, its behavior in relation to 304L was discovered to be cathodic. The diluting effect, which led to the development of a chromium concentration gradient between the interface zone and the Stellite-6 bulk alloy, was likewise connected to the galvanic corrosion of the interface zone. Dendrites developed during the Stellite-6 overlay welding process, which produced chemical composition inhomogeneity and accelerated inter-dendritic corrosion. In the inter-dendritic areas, precipitation of chromium carbide was also noticed. These precipitates reduced the dendrites’ peripheries’ corrosion resistance by causing chromium depletion there.