Corrosion and Prevention of Corrosion

Corrosion of Metals

Corrosion is the deterioration of materials by chemical interaction with their environment. The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals. The most widely used metal is iron (usually as steel).

Corrosion is a natural phenomenon of conversion of metal into a more stable chemical oxide. All metals do not corrode. Metals placed higher in the reactivity series such as iron, zinc, etc. get corroded very easily and metals placed lower in the reactivity series like gold, platinum and palladium do not corrode. This is because, corrosion involves the oxidation of metals. As we go down the reactivity series tendency to get oxidised is very low (oxidation potentials is very low). Aluminium doesn't corrode unlike other metals even though it is reactive because aluminium is covered by a layer of aluminium oxide already. This layer of aluminium oxide protects it from further corrosion.

Chemistry of Corrosion

Virtually all corrosion reactions are electrochemical in nature; at anodic sites on the surface the iron goes into solution as ferrous ions , this constituting the anodic reaction. As iron atoms undergo oxidation to ions they release electrons whose negative charge would quickly build up in the metal and prevent further anodic reaction, or corrosion. Thus this dissolution will only continue if the electrons released can pass to a site on the metal surface where a cathodic reaction is possible . At a cathodic site the electrons react with some reducible component of the electrolyte and are themselves removed from the metal.

The rates of the anodic and cathodic reactions must be equivalent according to Faraday 's Laws, being determined by the total flow of electrons from anodes to cathodes, which is called the corrosion current (I_coor). Since the corrosion current must also flow through the electrolyte by ionic conduction the conductivity of the electrolyte will influence the way in which corrosion cells operate. The corroding piece of metal is described as a mixed electrode since simultaneous anodic and cathodic reactions are proceeding on its surface . The mixed electrode is a complete electrochemical cell on one metal surface.
The most common and important electrochemical reactions in the corrosion of iron are thus -
Anodic reaction (corrosion) Fe &arr; Fe⁺² + 2e       ---Eq-1
Cathodic reactions (simplified) 2H⁺ + 2e → H₂       ---Eq-2
or H₂O + 1/2 0₂ + 2e → - 2 0H⁻       ---Eq-3

Second reaction (Eq-2) is most common in acids and in the pH range 6.5 - 8.5 the most important reaction is oxygen reduction Eq-3. In this latter case corrosion is usually accompanied by the formation of solid corrosion debris from the reaction between the anodic and cathodic products.
Fe⁺² + 2 0H⁻ → Fe(OH)₂
Pure iron (II) hydroxide is white but the material initially produced by corrosion is normally a greenish colour due to partial oxidation in air. Fe(OH)₂ + H₂O + 1/2 0, → Fe(OH)₃

Further hydration and oxidation reactions can occur and the reddish rust that eventually forms is a complex mixture whose exact constitution will depend on other trace elements, which are present. Because the rust is precipitated as a result of secondary reactions it is porous and absorbent and tends to act as a sort of harmful poultice, which encourages further corrosion.
For other metals or different environments different types of anodic and cathodic reactions may occur. If solid corrosion products are produced directly on the surface as the first result of anodic oxidation these may provide a highly protective surface film, which retards further corrosion, the surface is then said to be passive. An example of such a process would be the production of an oxide film on iron in water, a reaction which is encouraged by oxidizing conditions or elevated temperatures-
Fe + 3H₂0 → Fe₂0₃ +6H⁺ + 6e

Types of Corrosion

Some of the most common types of corrosion are described below-

Uniform Corrosion

This is the most common form of corrosion which usually takes place evenly over large areas of a material's surface.

Erosion Corrosion

Erosion corrosion is defined as the accelerated deterioration of a metal that results from the relative movement between a corrosive liquid and a metal’s surface. As the fluid flows along the surface (usually at high velocities), the metal’s passive oxide layer may be removed or dissolved, leaving the alloy susceptible to damage. During this process, the metal may be removed in the form of dissolved ions or as corrosion products that are mechanically swept from the metal surface due to the force of the flowing fluid.

Pitting Corrosion

One of the most aggressive forms of corrosion, pitting can be hard to predict, detect or characterise. This localised type of corrosion happens when a local anodic or cathodic point forms a corrosion cell with the surrounding surface. This pitt can create a hole or cavity which typically penetrates the material in a vertical direction down from the surface.
Pitting corrosion can be caused by damage or a break in the oxide film or a protective coating and can also be caused through non-uniformities in the structure of the metal. This dangerous form of corrosion can cause a structure to fail despite a relatively low loss of metal.

Crevice Corrosion

This form of corrosion occurs in areas where oxygen is restricted such as under washers or bolt heads. This localised corrosion usually results from a difference in the ion concentration between two areas of metal. The stagnant microenvironment prevents circulation of oxygen, which stops re-passivation and causes a build-up of stagnant solution moving the pH balance away from neutral.
The imbalance between the crevice and the rest of the material contributes to the high rates of corrosion. Crevice corrosion can take place ar lower temperatures than pitting corrosion, but can be minimised by proper joint design.

Intergranular Corrosion

Intergranular corrosion occurs when impuraties are present at the grain boundaries which form during solidification of an alloy. It can also be caused by the enrichment or depletion of an alloying element at the grain boundaries. This type of corrosion occurs along or adjacent to the grains, affecting the mechanical properties of the metal despite the bulk of the material being unaffected.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking refers to the growth of cracks due to a corrosive environment which can lead to the failure of ductile metals when subjected to tensile stress, particularly at high temperatures. This type of corrosion is more common among alloys than with pure metals and is dependant on the specific chemical environment whereby only small concentrations of active chemicals are required for catastrophic cracking.

Galvanic or Bimetallic Corrosion

This form of corrosion occurs when two different metals with physical or electrical contact are immersed in a common electrolyte (such as salt water) or when a metal is exposed to different concentrations of electrolyte. Where two metals are immersed together, known as a galvanic couple the more active metal (the anode) corrodes fast than the more noble metal (the cathode). The galvanic series determines which metals corrode faster, which is useful when using a sacrificial anode to protect a structure from corrosion.

Factors Affecting Corrosion

1. Exposure of the metals to air containing gases like CO₂, SO₂, SO₃ etc.
2. Exposure of metals to moisture especially salt water (which increases the rate of corrosion).
3. Presence of impurities like salt (eg. NaCl).
4. Temperature: An increase in temperature increases corrosion.
5. Nature of the first layer of oxide formed: some oxides like Al₂O₃ forms an insoluble protecting layer that can prevent further corrosion. Others like rust easily crumble and expose the rest of the metal.
6. Presence of acid in the atmosphere: acids can easily accelerate the process of corrosion.

Prevention of Corrosion

Preventing corrosion is of utmost importance in order to avoid huge losses. The majority of the structures that we see and use are made out of metals. This includes bridges, automobiles, machinery, household goods like window grills, doors, railway lines, etc. While this is a concerning issue, several treatments are used to slow or prevent corrosion damage to metallic objects. This is especially done to those materials that are frequently exposed to the weather, saltwater, acids, or other hostile environments. Some of the popular methods to prevent corrosion are given below-

1. Electroplating
2. Galvanization
3.Anodization
4. Passivation
5. Biofilm Coatings
6. Anti-Corrosion Protective Coatings
7. Painting and Greasing
8. Use of Corrosion Inhibitor or drying agents
9. Periodic cleaning of metal surface

 Share