BIMETALLIC CORROSION BASICS

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Bimetallic corrosion, as its name suggests, is corrosion that occurs due to the direct or indi-rect contact between two metals. These metals, called coupling metals, are usually different in property and composition. This type of corrosion can be observed in a number of different environments. Rivets and bolts connecting metal components, an aluminum head on a cast iron block, solder on a copper pipe, steel nails on Aluzinc sheeting, and flanges of connect-ing metal pipes are all examples of everyday situations where bimetallic corrosion may be found.

Although this type of corrosion may sound detrimental at first (and it usually is), there are instances where this naturally occurring process is encouraged and used in a number of beneficial applications.

UNDERSTANDING THE BIMETALLIC CORROSION PROCESSBimetallic corrosion, also known as galvanic corrosion, is corrosion that occurs when two metals, each with different electrode potentials, directly or indirectly touch each other in the presence of an electrolyte. Bimetallic corrosion usually results in the accelerated deg-radation of one metal, while the other metal remains unaffected. In other words, one metal corrodes preferentially, thereby sacrificing itself to galvanically protect the other.

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To understand how bimetallic corrosion works, it is beneficial to first understand the galvan-ic series. When a metal is immersed in an electrolyte, it adopts an electrode potential that represents the amount of energy that is available to drive the oxidation and reduction reac-tions. The electrode potentials of various metals and alloys are calibrated and arranged in a table or list that is referred to as the galvanic series. Metals and alloys positioned higher on the list are referred to as anodic (more electronegative) while those listed lower on the list are cathodic (more electropositive). (Learn more in An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.)

The main component that makes this type of corrosion possible is the difference in elec-trode potentials that exists between two dissimilar metals when they are in contact with an electrolyte. This entire system is known as a bimetallic couple, where one metal acts as the anode and the other acts as the cathode.

The potential difference that is formed between the two dissimilar metals gives rise to a flow of electrons from the electronegative anode to the more electropositive cathode; this flow generates an electric current.

The resulting loss of electrons as they flow from the anode to the cathode triggers an ox-idation reaction at the anode that causes it to corrode, while reduction takes place at the cathode where dissolved oxygen is consumed, as illustrated by the two chemical reactions shown in equations 1 and 2.

Fe → Fe2+ + 2e

O2 + 2H2O + 4e → 4OH

Equation 1 – Typical oxidation of iron (e.g.) at the anode.

Equation 2 – Typical reduction of dissolved oxygen at the cathode.

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Because the potential difference is directly related to the difference in reactivity between the anode and the cathode, the further apart the two metals are in the galvanic series, the greater the potential difference between the metals, and therefore the more severe the rate of corrosion of the anode. In the same way, the closer the metals are in the series, the lower the potential difference, therefore the lower the rate of corrosion. Hence, we can reason that if the two contacting metals were the same then bimetallic corrosion would not occur.

FACTORS THAT AFFECT BIMETALLIC CORROSIONWhile contact between two metal objects is an essential ingredient for this type of corrosion, there are many other contributing factors must be taken into account. (This topic is ad-dressed further in the article Why Do Two Dissimilar Metals Cause Corrosion?) Understand-ing the factors that affect bimetallic corrosion is essential when selecting preventative and mitigative methods. Listed below are some of the factors involved in bimetallic corrosion and how they can affect its rate and severity.

ELECTROLYTE COMPOSITION

Since the electrolyte is the medium responsible for removing oxi-dized metal ions from the anode and providing reduction ions to the cathode, it stands to reason that the quality of the electrolyte will have a major effect on the rate and severity of bimetallic corrosion. Some of the electrolyte properties that influence this corrosion are conductivity, chemical composition and pH.

Increased electrical conductivity in electrolyte solutions often results in more severe corrosion because highly conductive electrolytes are usually accompanied by high concentrations of aggressive ions such as chloride.

The pH of the electrolyte has also been known to affect the rate of bimetallic corrosion as well. Advanced corrosion rates are known to occur as the pH drops below 4 for aluminum and at about 6 for zinc and magnesium.

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ELECTRODE POTENTIAL

As previously mentioned, the relative ranking of the coupling metals in the galvanic series give an indication of the level of galvanic corrosion to expect. Metals that are farther apart in the series result in a greater difference in electrode potential (potential difference) and thus produce a greater induced current in the galvanic circuit.

ELECTRODE EFFICIENCY

Some metals are not as efficient at reducing dissolved oxy-gen as others. Titanium, for example, is not as easily reduced as copper alloys. This makes titanium a very “stubborn” cathode. Therefore, it is possible for a less noble metal to expe-rience a higher rate of corrosion when coupled with a copper alloy as opposed to the more electropositive titanium.

QUALITY OF ELECTRICAL CONNECTION BETWEEN THE METALS

Direct physical contact between the metals is not always necessary to establish a bimetallic/galvanic circuit. The two metals may also be connected indirectly via the use of a conductive material such as a wire or even structur-al steelwork. The quality of this conductor can affect the quality of the electrical connection between the coupling metals and the rate of bimetallic corrosion. Ideally, the conductor should be able to provide sufficient continuity

between the metals to facilitate the flow of electrons. Also, the conductor should possess adequate conductivity so as to not introduce resistivity that can slow electron flow.

ANODE/CATHODE AREA RATIO

Galvanic corrosion is greatly affected by the ratio of the areas of the anode to the cathode. The larger the area of the cathode relative to the anode, the greater the rate of oxygen re-duction and the greater the galvanic current. Increased galvanic current results in the rapid loss in thickness of the anode. The smaller the anode/cathode ratio, the less desirable the situation becomes. For example, steel nails (anode) in a large copper plate (cathode) will experience more aggressive corrosion than copper nails in a large steel plate.

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METALLURGICAL CONDITION AND COMPOSITION

Metallurgical composition can also cause changes in the electrode potential of a metal. Quenching, tempering, annealing and other types of heat treatment can all affect the metal-lurgical composition of a metal, which can cause potential differences between two similar metals. Local changes in metal composition may also be brought on by heat-affected zones (HAZ) in welding. For instance, austenitic stainless steel is more cathodic than martensitic metals of the same type.

OBSTRUCTIONS

Uninterrupted cathodic reduction reactions must be sustained at the nobler of the two metals for bimetallic corrosion to occur. In other words, dissolved oxygen from the electro-lyte must be able to be consumed freely at the cathode without interference. The corrosion products produced by some metals can form an effective oxide barrier that can prevent or slow down oxidizers from reaching the metal’s surface, thereby stifling the corrosion pro-cess. Bare metals, therefore, act as better cathodes than metals with oxide barriers because these barriers impede oxygen diffusion to the cathode. In addition, oxide films also tend to add additional resistance to the circuit that can slow down the electrochemical process even further.

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ADVANTAGES OF BIMETALLIC CORROSIONBimetallic corrosion and its associated processes can be very damaging to metallic compo-nents in contact with each other. However, there are a few instances where the processes responsible for this natural phenomenon can be leveraged and used for positive applica-tions. Listed below are some of the ways that bimetallic corrosion can be beneficial.

GENERATING ELECTRICITY IN DRY CELL BATTERIES

Bimetallic/galvanic corrosion is the driving force for producing electricity in galvanic cells such as dry cell batteries. The flow of electricity (the movement of electrons from the anode to the cathode) produces an electric current in the circuit, which gives batteries the ability to produce various voltages. A typical dry cell battery consists of a zinc inner case (anode) and a graphite rod within the case (cathode). To complete the circuit, an electrolyte, usually an aluminum-based paste, bridges the two metals to facilitate the flow of electrons to produce electricity.

CATHODIC/GALVANIC PROTECTION

The fact that one metal (the anode) corrodes preferentially with respect to the other (the cathode) is used to purposely protect specific metal components. For example, zinc-based coatings are commonly applied to structural steel members in a process known as galvaniz-ing. The zinc offers cathodic protection to the steel by undergoing corrosion in the event that the steel substrate becomes exposed. For example, if the steel substrate becomes exposed by a scratch, this will trigger a galvanic cell that will cause the zinc coating around the scratched surface to corrode while the steel substrate remains intact. This process contin-ues until the zinc coating is fully depleted by the corrosion.

For larger structures, where passive galvanic methods are not sufficient, the metal to be protected is connected to another sacrificial anodic metal via an external power source that provides additional current to power the reaction. This is commonly used on long pipelines and offshore oil and gas platforms.

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HOW TO MITIGATE AND PREVENT BIMETALLIC CORROSIONThere are several methods that are commonly used to prevent bimetallic corrosion. Most of these solutions involve disrupting the electrical path between the anode and the cathode, removing dissolved oxygen from the electrolyte or reducing the area ratio between the an-ode and the cathode.

ELECTRICAL INSULATION

Insulation involves the use of non-conductive materials as a barrier between the contact points of the coupling metals. By insulating or separating the direct contact of the coupling metals, the flow of electrons from one metal to another is inhibited. In practical situations, insulation materials may come in the form of rubber or polymer-based washers, gaskets, coatings, etc.

ELECTROLYTE ISOLATION

Electrolyte isolation involves separating one or both of the metals from the medium that facilitates the redox reactions in the galvanic cell. If the anode or cathode is unable to oxi-dize or reduce, then the processes for bimetallic corrosion cannot happen. Water-repellent compounds such as paints, greases and varnishes are just some of the methods used to obscure contact between the electrolyte and the metals. These coatings have traditionally been applied by electroplating, dipping or spraying.

USING METALS WITH SIMILAR ELECTRODE POTENTIALS

Another method of stemming the flow of electrons, and by extension bimetallic corrosion, is to reduce the potential difference between the two metals. This is accomplished by pur-posely selecting contacting metals that are closely matched in the galvanic series (i.e., those having similar electrode potentials). Metals close to each other in the series produce small-er potential differences and, therefore, less galvanic current is available to drive the bime-tallic cell.

CORROSION INHIBITORS

Corrosion inhibitors, usually liquid in nature, can be applied to the electrolyte to dampen the corrosion processes occurring between the two metals. These inhibitors work in a variety of ways, some involving complex chemical reactions. However, one of the most common corrosion inhibitors used to combat bimetallic corrosion involves removing oxygen from the electrolyte. Lack of dissolved oxygen content means little to no reduction can occur at the cathode. Since reactions at the anode and cathode are dependent on each other, the entire galvanic process comes to a halt.

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MAXIMIZING THE ANODE/CATHODE AREA RATIO

Making the area of the anode as large as possible in relation to the cathode can help to sig-nificantly reduce (but not prevent) corrosion of the two metals. This can be achieved through careful consideration and selection of the geometry and physical properties of the contacting metals in the early stages of the design process. (For more on this subject, see Corrosion Control Considerations in the Equipment Design Process.)

CONCLUSIONBimetallic corrosion can cause accelerated corrosion in metals and can be found in a variety of environments across a number of industries. However, this type of corrosion can be eas-ily managed by the application of relatively inexpensive preventative measures. It is, there-fore, important to understand the basic underlying principles behind bimetallic corrosion to select the most effective solution for a given situation.

The same concept applies when using bimetallic corrosion for cathodic protection uses. Proper anode and coating selection is crucial to the success of the chosen protection method.