Galvanic or Two-Metal Corrosion

A potential (emf) difference usually exists between two dissimilar metals when they are immersed in a corrosive or conductive solution. If these metals are placed in contact (or electrically connected), this potential difference produces an electron flow between them. Corrosion of the less corrosion-resistant metal is usually increased and attack of the more resistant material is decreased, as compared with the behavior of these metals when they are not in contact. The less-resistant metal becomes anodic and the more-resistant metal cathodic. Usually the cathode or cathode metal corrodes very little or not at all in this type of couple. Because of the electric currents and dissimilar metals involved, this form of corrosion is called galvanic or two-metal corrosion. 

Galvanic or two-metal corrosion can influence erosion-corrosion when dissimilar metals are in contact in a flowing system. 

The dry cell battery is a typical example of galvanic corrosion, or two metal corrosion as it is otherwise called. When two dissimilar metals are immersed in a conductive or corrosive medium, there is always the potential for a change in them. Once these metals are connected this difference induces electron flow between them. The less corrosion resistant metal is attacked more than the more resistant metal. This is an electrochemical process. In the case of a dry cell battery, the carbon electrode acts as the cathode (the more resistant materials) and zinc as the corroding anode. The natural phenomenon of corrosion is used in this case for producing electricity. 

Galvanic corrosion is the enhanced corrosion of one metal by contact with a more noble metal. The two metals require only being in electrical contact with each other and exposing to the same electrolyte environment. By virtue of the potential difference that exists between the two metals, a current flows between them, as in the case of copper and zinc in a Daniell cell. This current dissolves the more reactive metal (zinc in this case), simultaneously reducing the corrosion rate of the less reactive metal. This principle is exploited in the cathodic protection (Section 53.7.2) of steel structures by the sacrificial loss of aluminum or zinc anodes. 

Formation of a Galvanic Cell. When a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte, a galvanic cell is created. The electromotive force and current of the galvanic cell depend on the properties of the electrolyte and polarization characteristics of anodic and cathodic reactions. The term galvanic corrosion has been employed to identify the corrosion caused by the contact between two metals or conductors with different potentials. It is also called dissimilar metallic corrosion or bimetallic corrosion where metal is the conductor material. 

Metallic coatings are either more noble than the substrate, as in chromium plate on mild steel, or are base metals which corrode more easily than the substrate. In the former case, the underlying metal is protected by a continuous impervious film of the noble metal which is itself resistant to attack. This method is adequate provided the surface coating contains no holes or flaws and remains intact. Penetration of this layer by the corrosive agent leads to galvanic corrosion at the interface of the two metals. If the object is coated with a more base metal, then protection is by both the physical barrier of the metal film and by cathodic protection at any subsequent defects (Fig. 19). 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

The aqueous corrosion of metals is due to an electrochemical cell being formed between two different metals in electrical contact (galvanic corrosion) or two... 

When two dissimilar metals are immersed in an electrolyte they usually develop different potentials in accordance with the theory already presented. If the metals are in contact the potential difference provides the driving force for corrosion. Severe corrosion often occurs as a result of the contact between two metals. In shell and tube heat exchangers where the tubes are fabricated from a corrosion resistant alloy, and the shell is made from mild steel for instance to reduce the capital cost, corrosion is very likely unless adequate protection is made. The less resistant of the two metals is caused to corrode, or to corrode more rapidly, while the resistant metal or alloy corrodes much less or may be even completely protected. The basis for galvanic corrosion is illustrated on Fig. 10.6. Metal A has a lower electrode potential than metal B. Ions migrate in the conducting solution while electrons flow across the junction of the two metals, as a result metal A is corroded at C. 

Galvanic corrosion occurs between two metals with dissimilar electrochemical potentials. In this form of corrosion, one of the metals is more electrochemically active and corrodes, while the second metal is protected by the corroding metal. The metals can even be of the same material if the electtochemical potential of one of the materials has been charged because of stresses or differential aeration. Previous studies have indicated that most hull corrosion is galvanic in nature (22). 

Choice of an appropiate surface treatment and a suitable primer are important because adhesion to the substrates presents difficulties [11.37]. Primers based on modified alkyd resins or two-pack epoxy-resins for derusted ferrous metals mainly contain zinc phosphate and zinc OKide as corrosion protection pigments. Nonferrous metals are first washed with an ammoniacal wetting agent before applying the primer that contains a binder based on synthetic resins (e.g., PVC copolymers, chlorinated rubber) which ensure good adhesion to the substrate. The same primer must be used on zinc or galvanized surfaces because the use of alkyd resins causes embrittlement [11.38] The primed surfaces are largely topcoated with alkyd resin systems. 

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