Right Combination of Metals, Steels and Alloys to Avoid Bimetallic Corrosion (Galvanic Corrosion)
In the assembly of food process equipment and services, the right combination of steels, alloys or metals must be used to avoid bimetallic corrosion. Bimetallic corrosion occurs between metals, steels, or alloys with considerably different standard reduction potential (Fig. 8.14). A current will flow from the less noble metal to the more noble metal, resulting in the oxidation (corrosion) of the less noble metal. To determine the compatibility of two metals, steels, or alloys, a galvanic corrosion metal compatibility chart (Fig. 8.15) or diagram (Fig. 8.16) can be used. Also galvanic corrosion of welds may occur if the weld metal is less noble than the surrounding materials joined. Higher alloyed filler metal in comparison to the welded materials may reduce the risk for galvanic corrosion. Note that replacement parts could have another chemical composition than the materials of construction used in the existing plant equipment.
FIGURE 8.14 Stainless-steel shell and knives of the roller are combined with roller ends in mild steel. As mild steel is less noble than stainless steel, it will corrode. Work in black steel and stainless steel must always be kept separate. Courtesy of organization Sanitary Design Workshop, © 2016.
FIGURE 8.15 Galvanic corrosion chart for dissimilar metals, steels, and Dalloys. Courtesy of Insertsdirect.com Ltd.
FIGURE 8.16 Galvanic corrosion metal compatibility diagram. Use a rule to line up the two metals, steels or alloys which are being combined. The closer to zero, the lower the risk of galvanic corrosion. (a) Combining stainless steel 316 with zinc (e.g., zinc coating of galvanized steel) will result in severe corrosion of the zinc coating. (b) Combining stainless steel 316 with mild steel (also called carbon steel or black steel) will result in the fast corrosion of mild steel. (c) Aluminum will quickly corrode when combined with copper. (d) Copper slowly corrodes in contact with Monel (contains copper). (e) Copper corrodes when combined with stainless steel 316.