Dissimilar-metal corrosion

Two different metals that are connected and immersed in an electrolyte form an electrochemical cell. If a current is allowed to flow, one metal will be consumed and one will remain the same or be increased in some way. These processes lead to dissimilar metal corrosion. In order for dissimilar metal corrosion to occur, it is necessary to have an anode, a cathode, an electrolyte and a connection from anode to cathode. The anode component corrodes whereas the cathode remains unattacked. The tendency for such reactions to take place spontaneously can be judged from the electrochemical series. Three examples follow. 

Copper and iron or steel in juxtaposition can form a cell in which the copper becomes the cathode and the iron the anode. Several reactions are possible. One of these is as follows. 

The Zn + ions react in ordinary conditions to produce ZnO or a zinc oxyhydroxide. These are inert and form insoluble deposits that help to prevent further corrosion. Overall, steel coated with zinc corrodes far more slowly than does bare steel. 

Steel coated with tin was widely used on food cans, or tins , until replaced by aluminium or plastic coatings. Steel coated with tin corrodes faster than steel alone. Unlike the situation with zinc, a scratch in the coating allows an electrochemical cell to form in which the steel (iron) forms the anode and corrodes as follows. 

No protective oxide forms and the corrosion is enhanced compared with uncoated steel. This accounts for the fact that old tin cans on rubbish tips are always badly corroded. 

---

Finally, mechanical joints, e.g. nuts, bolts, rivets etc., are still important joining methods for which attention must be given to compatibility to avoid dissimilar metal corrosion problems and crevice corrosion " . ... 

Dissimilar-metal corrosion can occur even if the two metals are not initially in direct contact. For example, in homes where copper tubing is used for plumbing, there is always a small amount of dissolved Cu2+ in the water. When this water encounters steel piping or a chrome-plated bathroom sink drain, the more-noble copper will plate out on the other metal, producing a new metals-in-contact corrosion cell. In the case of chrome bathroom sink fittings, this leads to the formation of Cr3+ salts which precipitate as greenish stains. 

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. 

Although corrosion engineers are well aware of the mechanism and the detrimental effects of galvanic (dissimilar metal) corrosion, it would seem necessary to make some designers, architects and engineers in other disciplines aware of this very damaging form of corrosion. 

Figure 9.16 Dissimilar metal corrosion of a steel rivet in contact with copper and an electrolyte (a) before and (b) after corrosion...

During dissimilar metal corrosion, the metal that corrodes is ... 

Mansfeld F, Hengstenberg DH, Kenkel JV. Galvanic corrosion of aluminium alloys I. Effect of dissimilar metal. Corrosion, 30, 1974 343-353. 

Galvanic corrosion of a metal occurs as a result of electrical contact with a more noble conductor in a corrosive electrolyte. It is often called "dissimilar metal corrosion. Galvanic corrosion can often be avoided by disconnecting the point of electrical contact. In cases where electrical isolation is not possible, proper materia] selection can help minimize the effect. 

Galvanic corrosion (also called "dissimilar metal corrosion" or wrongly "electrolysis") refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. In a bimetallic couple, the less noble material becomes the anode and tends to corrode at an accelerated rate, compared with the uncoupled condition and the more noble material will act as the cathode in the corrosion cell. 

The surface of a metallic object can be easily reacted in normal atmospheres making it behave quite differently from a piece of unreacted metal, often resulting in a situation resembling dissimilar metal corrosion. A film may be formed which is invisible, actually only a few molecules in thickness, but which may have a potential as much as 0.3 V different from the unfilmed metal. Naturally, such a potential difference is enough to create an active corrosion cell. Steels in soil have a tendency to film with time. "Old" steel, that is, steel which has been in the ground for several years may then become cathodic with respect to "new" steel, even when the two are identical in bulk composition (Fig. 7.37). It is thus strictly a surface phenomenon [21]. 

Metalhc coating may be applied to non-metals to provide a conductive surface. Although some problems (e.g. dissimilar metals corrosion) are thereby minimized or obviated, other corrosion reactions of the metal coating should be considered in the same manner as for plated or solid metals. 

---