![]() (For more on galvanic corrosion, read: An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion. The electro-potential difference between the metals is a driving force for an accelerated corrosion attack to happen. Galvanic corrosion occurs when two dissimilar metals are in contact with each other while an electrolyte is covering the connecting area. The corrosion process in bolts can cause them to act as sacrificial anodes. In some environments, such bolts may show significant corrosion within weeks of installation, as they are affected in the same way as the sacrificial anodes in a cathodic protection (CP) system.įigure 2. In addition, the bolts and nuts are small relative to the structure to which they are joined, exacerbating the corrosion effect. The flange is high-alloy with a corrosion potential of around 0.5 - a difference of 0.35, well outside acceptable differences. The bolts are of low-alloy steel with a corrosion potential of around 0.85. Typical problems with mixed metals.Ĭorrosion in the low-alloy flange is apparent where coating damage has occurred, but most significant is the level of corrosion in the bolts. The size correlation to the corrosion rate is also shown in Table 1.įor more detailed information about galvanic corrosion and hot-dip galvanizing in contact with dissimilar metals, see Dissimilar Metals in Contact.Figure 1. 2), the area ratio between the cathode and anode is reversed, and although more surface area is affected, the depth of penetration is small the fastener should not fail because of corrosion. ![]() When combining a zinc plate with a stainless steel rivet (Fig. 1), the ratio of the cathode surface area to the anode surface area is large, and the rivet will fail rapidly because of accelerated corrosion. When using a stainless steel plate with a zinc rivet (Fig. This situation is portrayed using a riveted fastener as shown in Figures 1 and 2. ![]() The rate of penetration of corrosion increases as the ratio of the cathode to anode surface area increases as it decreases, the rate of penetration decreases. The existence of two dissimilar metals in direct contact can be no problem whatsoever if there is no electrolytic material present. The formation of a bimetallic couple needs four elements in order to form. If zinc is in contact on the surface with a more cathodic metal and the zinc becomes part of a bimetallic couple and corrodes, then the zinc is not performing its designed function of protecting the base steel. The zinc has been applied to the steel to provide corrosion protection for the underlying base steel. The following diagram shows all of the parts of the bimetallic couple.Īs in all design and fabrication situations, the problem is not as simple as just looking it up on a chart. There must be a return current path which in almost all cases is a direct contact between the two metals.This electrolyte material must be able to conduct ions from one metal to the other and There must be an electrolyte material covering these two metals at the area where they are touching to complete the electrical current path.This metal is the protected partner of the corrosion cell. The other metal must act as a cathode and collect these flowing electrons.One of the metals must act as the anode and generate electrons that can create an electrical current flow.There are four elements necessary for the contact metals to experience corrosion When two metals are in direct contact, there is the potential for the formation of a bimetallic couple. The presence of two dissimilar metals in an assembly is not always a sign of trouble but it could be a problem. ![]() Stainless steel and galvanized materials often are found together in the industry with applications such as galvanized fasteners, stainless steel pressure vessels and roof and siding panels.
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