Galvanic corrosion insulation materials

Figure 1.42 Insulating material between dissimilar metals, such as copper and aluminum prevents galvanic corrosion...

The new dress-cap substrate material should be either fiber-reinforced plastic (FRP) or Type 316 stainless steel. Being a nonmetallic insulator, FRP would eliminate any possibility of galvanic corrosion. Type 316 stainless steel demonstrates good passivity in urban atmospheres and has been successfully used in contact with copper.1... 

Plastics and rubbers are used as insulating and separating materials between different metallic materials to prevent galvanic corrosion. For this application specific requirements must be fulfilled, such as the necessary mechanical strength and thickness they must be fireproof, and resistant to the environment (including heat-resistant to a specified extent). In addition the following are important ... 

Galvanic corrosion does not occur at all with ordinary organic plastics, because they are insulators. In reinforced plastics the phenomenon is confined to situations where carbon fibre reinforcement is situated in very close proximity to certain metals. This problem can easily be overcome by inserting some insulating material to break the electrical circuit. 

Care must be taken not to allow contact between galvanized steel parts outside a concrete construction (e.g., underneath moist insulation materials) with ungalvanized (black) rebars in the concrete. The chance of corrosion of ungalvanized and galvanized steel parts is summarized as follows ... 

If both an electric and an electrolytic contact are established between different metals, a galvanic cell forms that results in an accelerated corrosion of the metal with the less noble corrosion potential (Chapter 7). With an adequate selection of materials, or by placing an insulating material between the metals, this kind of corrosion can be avoided (Figure 12.4(a)). For small objects, such as electronic components, one can prevent galvanic corrosion by completely immersing a joint of dissimilar metals into a polymer, thus avoiding that the metals come in contact with an electrolyte film that could form in presence of humidity. 

When joining two dissimilar metals together, galvanic corrosion can be prevented by insulating the two materials from each other. For example, when bolting flanges of dissimilar metals together, plastic washers can be used to separate the two metals. 

Materials of construction for the zinc/mercuric oxide cells are limited not only by their ability to survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility with the electrode materials. As far as the external contacts are concerned, these are decided by corrosion resistance, compatibility with the equipment interface with respect to galvanic corrosion, and, to some degree, cosmetic appearance. Metal parts may be homogeneous, plated metal, or clad metal. Insulating parts may be injection-, compression-, or transfer-molded polymers or rubbers. 

Pipes of different materials, such as copper, steel mild steel and galvanized iron are often buried very close to each other in the same trench without any concern for galvanic corrosion. Figure 8.28 shows a method to prevent galvanic corrosion of mild steel pipe which is put in the same trench as close to the copper pipe. The copper pipe is coated and insulated to minimize galvanic corrosion. The mild steel pipe may be protected by a galvanic anode but this is not cost effective. 

One practical way of preventing possible galvanic corrosion is to insulate the two metals in contact from one another as carefully as possible [17]. This is achieved by placing a high ohmic resistance, i.e. an insulating material (Figure B.3.3) such as neoprene or any other suitable polymer, between them. 

Avoid electrical contact between dissimilar metals to prevent galvanic corrosion. If possible, use similar materials throughout the entire structure, or insulate different materials from one another. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

---