Form of Galvanic Corrosion

The appearance of galvanic corrosion is very characteristic. It is not dispersed like pitting corrosion, but highly localised in the contact zone with the other metal. 

In memory of Luigi Galvani, an Italian physician who in 1786 discovered the electrical effect of the coupling of two dissimilar metals. 

The attack of aluminium is regular and progresses in depth as craters that are more or less rounded. The zone affected by galvanic corrosion often has a shinier aspect than 

All aluminium alloys undergo identical galvanic corrosion. In addition, intercrystaUine and exfoliation corrosion may occur in those alloys of the 2000, 5000, 6000 and 7000 series that are susceptible to these forms of corrosion. 

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The most serious form of galvanic corrosion occurs in cooling systems that contain both copper and steel alloys. It results when dissolved copper plates onto a steel surface and induces rapid galvanic attack of the steel. The amount of dissolved copper required to produce this effect is small and the increased corrosion is difficult to inhibit once it occurs. A copper corrosion inhibitor is needed to prevent copper dissolution. 

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

The real electrical potential of various metals and their alloys may, under practical boiler operating conditions, be considerably different from their standard potential under ideal conditions. Thus, a reversal of potential may take place in the boiler plant system, with unexpected forms of galvanic corrosion occurring. 

Several items of pre-boiler equipment (and post-boiler equipment) may suffer from selective forms of galvanic corrosion. One fairly common... 

Cathodic protection can be viewed as a form of galvanic corrosion, put to good use. In this case an active metal (most often zinc, but under special circumstances magnesium or aluminum) is employed as a sacrificial anode. It is attached to the steel structure being protected in one or several locations and does not constitute part of the... 

When two dissimilar metals are electrically connected together, such that electrons can flow from one metal to the other, it is probable that the anodic, metal dissolution reaction will concentrate on one metal, while the cathodic reaction occurs on both. This accelerates the corrosion of the anodic component. The acceleration will be particularly severe if the area of the cathodic metal is much larger than that of the anodic metal (Fig. 1). While the anodic metal will corrode more, the cathodic metal will normally corrode less, and one form of galvanic corrosion provides a method of corrosion control (know as cathodic protection), in which the anodic, corroding metal is provided deliberately in order to restrict the corrosion of the cathodic metal. 

Technical metals that contain impurities of other metab Examples are technical metals such as zinc and tin, which contain silver or iron as impurities. The standard electrode potentials of silver and iron are more electropositive than those of zinc or tin, causing zinc and tin to corrode through a form of galvanic corrosion, while the impurities remain intact. Because of their more positive electrode potentials, silver and iron are cathodicaUy protected by zinc or tin. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

The corrodent is a liquid metal in this form of stress corrosion cracking. Mercury at ambient temperature and metals including zinc (from galvanized steel-work) and copper (from electric cables) when melted during welding or in a fire cause rapid failure of certain metals. 

A detailed discussion of galvanic corrosion between dissimilar metals in contact in a corrosive environment has been given in Section 1.7, but in the case of coating discontinuities the effect of the anode/cathode area relationship and the nature of any corrosion products formed at small discontinuities may modify any choice made on strict considerations of general galvanic corrosion theory based on the potentials of the coating and substrate in the environment under consideration. 

The increase in cathodic kinetics due to the action of biofilms on passive alloy surfaces can also increase the propagation rate of galvanic corrosion. Potentiodynamic polarization studies show that cathodic kinetics are increased during biofilm formation on passive alloy surfaces. Tests on crevice corrosion samples of passive alloys S30400 and S31600 revealed that crevice initiation times were reduced when natural marine biofilms were allowed to form on the exposed external cathode surface. (Dexter)5... 

Massive electrochemical attack known as galvanic corrosion [58,59] is the most severe form of copper corrosion. It can completely remove the copper from the structures (Figs. 17.25 and 17.26). It can occur when the wafers are exposed to a corrosive electrolyte for an extended period. It can also occur if the slurry does not contain enough or effective corrosion inhibitor. The source of such a galvanic potential on the patterned copper surface may be due to the fact that some copper structures connected to transistors have a different electrical potential than the rest of the wafer surface. Another possible cause of this type of galvanic potential is related to the barrier material induced metal metal battery effect. Most copper CMP slurries have been developed for Cu structures with Ta or TaN as a barrier material. In some cases, other metals may also be used in addition to the barrier metal. For example, a metal hard mask could contribute to the galvanic corrosion effects. It is also possible that some types of copper are more susceptible to corrosion that others. The grain... 

CORROSION, OXYGEN DEFICIENCY - A form of crevice corrosion in which galvanic corrosion proceeds because oxygen is prevented from diffusing into the crevice. 

Ohmic potential drops also play a role in galvanic corrosion and other forms of localized corrosion, in which the anodes and cathodes are spatially separated. Ionic current must then flow some distance through the electrolyte. Two galvanically coupled electrodes will not reach the exact same potential as a result of the ohmic potential drop that will occur along the current path through the electrolyte. 

Severe corrosion may occur in active A1 or Mg alloys in neutral solutions of heavy metal salts (salts of Cu, Fe, or Ni). This type of corrosion occurs when the heavy metal salts plate out to form active cathodes on the anodic magnesium surface. This type of galvanic corrosion can lead to localized pitting corrosion. 

The forms of internal corrosion in water systems are uniform corrosion, galvanic corrosion, localized corrosion, concentration cell corrosion, MIC, and erosion- corrosion. 

Nonferrous metals (e.g., aluminum and zinc) are more corrosion resistant than iron and steel. They corrode in a different way and at a much slower rate. The corrosion products of aluminium and zinc are white salts which form on the surface. Aluminum and zinc are used to provide sacrifical protection to iron and steel in the form of galvanizing and metal sprays. 

Localized corrosion is either macroscopic or microscopic. The major forms of macroscopic corrosion in addition to uniform are galvanic, erosion, crevice, pitting, and exfoliation. Microscopic corrosion includes intergranular corrosion, selective leaching, and stress-corrosion cracking. 

The steel under the paint is frequently a form of galvanize for corrosion resistance. The metal is chemically treated to help adhesion and corrosion resistance. 

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