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Galvanic Corrosion

We must have seen below images in our surroundings. They are some metallic articles with rust coating on them. Why do metallic things only rusted? Is this good for metal?. 

Rust

All known elements can be classified as metals and non-metals. Metals are electropositive elements with 1, 2 or 3 electrons in their valence shell. They tend to lose electrons and form M+ ions. They are reactive elements which readily react with other compounds. Some common physical properties of metals are malleability, ductility, hard with high melting points etc. Unlike metals, non-metals can exist in all the three possible state of matter; solid, liquid and gases. They are mainly electronegative elements which tend to accept electrons to form anions. That is the reason they form covalent compounds with low melting points. Since there is no ion in covalent bond therefore covalent compounds are bad conductor of electricity and heat. 

So we can say that metals are more reactive towards other substances. They react with atmosphere oxygen to form metal oxides which are alkaline in nature. The formation of metal oxides deteriorates the metal surface and this deterioration is called as corrosion. This deterioration occurs due to reaction of metal with surrounding environment. Corrosion mainly depends on type of metal, environmental conditions, presence of gas and moisture etc. On these bases, corrosion can be classified in different types such as general attack corrosion or uniform attack corrosion is caused by electrochemical reaction of entire exposed metallic surface. It is one of the most common types of corrosion. This type of corrosion is predictable, manageable and also preventable. Another type is localized corrosion which is localized at certain area of the metal structure. It can be in the form of pitting, crevice corrosion and filiform corrosion. 

When a small hole in the metal forms due to de-passivation of a small area, it acts as anode and remaining metal acts as cathode. So we can say that it produces a localized galvanic reaction. Such type of corrosion can be covered and hidden by corrosion-produced compounds. Crevice corrosion also occurs at a specific location and is associated with a stagnant micro-environment. Environmental cracking depends on the environmental conditions that affect the metal such as chemical substances, temperature and stress-related conditions. Flow-assisted corrosion occurs due to removal of protective oxide layer on metal surface. It can occur because of regular flow of wind or water. Inter-granular corrosion is also a chemical or electrochemical attack on the boundaries of a metal surface. De-alloying is also called as selective leaching as it is selective corrosion of a specific element in an alloy.

 

Galvanic Corrosion Definition

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When two metals are located together in a corrosive electrolyte, galvanic corrosion occurs. In such case, both metals act as electrodes; anode and cathode. Metal which acts as anode corrodes and deteriorate fast whereas cathode deteriorates slow. Anodic metal electrode is also called as sacrificial metal in this corrosion. For galvanic corrosion, the metal electrode must be electrochemically different. There must be some electrical connection between both metal electrode and both electrodes must be exposed to an electrolyte. 

Overall galvanic corrosion involves the existence of a galvanic cell in which two metals are submersed in an electrolyte. Such type of corrosion can be observed in marine environments because salt water acts as electrolyte here. If two different metal bodies submersed in close proximity to salt water, it will form an electrolytic cell that can caused galvanic corrosion. It is an electrochemical oxidation-reduction process that occurs with two dissimilar metals are into electrical contact with an electrolyte solution.

Galvanic Corrosion Mechanism

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In galvanic corrosion, two meal electrodes are immersed   into an electrolyte solution. The metal with lower value of electrode potential will oxidize easily and exhibits the anodic reaction. The metal with higher value of electrode potential is nobler so it will show cathodic reaction on its surface. The difference between the electrode potentials of both electrodes determines the galvanic corrosion. It can be represented as given below. 

Galvanic Corrosion

Galvanic corrosion chart. 

Galvanic Corrosion Chart
 
Metals like copper, nickel, tin, lead can corrode by galvanic corrosion in the presence of essential conditions. For example in the combination of Iron-Zinc, the standard electrode potentials Zn is -0.763 V and for iron, it is -0.44 V. So the difference is 0.323 V. Here Zinc has low electrode potential so it dissolves in electrolyte solution and anodic reaction occurs at Zn- electrode;

$Zn \rightarrow Zn^{2+} + 2e^{-}$ 

The electrons move to iron electrode which acts as cathode. The cathodic reaction is;

$2 H^{+} + e^{-} \rightarrow H_{2}$

Another example is Iron-copper combination. Here iron acts as anode due to less value of electrode potential compare to copper. 

$E^{0} _{Fe}$ = -0.44 V
$E^{0} _{Cu}$ = +0.337 V.
$\Delta E^{0}$   = 0.777 V.

Anode reaction: $Fe \rightarrow Fe^{2+} + 2e^{-}$
Cathode reaction: $2H^{+} + e^{-} \rightarrow H_{2}$

Galvanic Corrosion Prevention

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There are many thermodynamic and kinetic conditions of galvanic corrosion that determines the rate of deterioration of metal surface. The greater difference between electrode potential of metal electrodes drives the electric force of corrosion. On the contrary, the contact of boundaries produces the resistance at the boundary of metals. So high contact resistance limits the electrons transfer and reduces the rate of corrosion. The electric resistance of electrolyte solution also decreases the rate of corrosion. Large size of anode and small cathode decreases the corrosion rate. 

Passivation means presence of passive film oxidation products also prevents further corrosion of the metal surface. Passive film can be formed on the surface of Ti, Si, Cr and Al in air, water and dilute acids. Now a day, metal-hulled ships are now fitted with some sacrificial anodes. Here these anodes protect the structural components of the body. They are usually made of those metals which have low electrode potential and can quickly reacts such as Zn and Mg metal.
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