Editorial Feature

An Introduction to Metallic Corrosion

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Corrosion can be defined as the electrochemical reaction of a material with its environment which results in a degradation of the properties of the metal. It is a redox process.

How Does Metallic Corrosion Matter?

The problem of corrosion arises in various environments ranging from urban and marine atmospheres to industrial chemical plant installations. It is a major factor governing the design and operation of various industrial plants and equipment as it reduces their useful life, may cause leakage and contamination, reduce efficiency, and can often result in unscheduled shutdowns or, in some cases, catastrophic failure. It can also harm the environment.

Of course, degradation also occurs with other materials like ceramics and plastics, but the term ‘corrosion’ refers only to metallic degradation.

The control of corrosion presents a considerable challenge to engineers and, in spite of diligent effort, the annual costs of corrosion damage and corrosion-related service failures run into many millions of dollars, estimated at about 4% of the GNP for a typical industrial country. It affects multiple infrastructural facets, such as bridges and buildings, water and waste treatment facilities, oil and gas facilities, and chemical processing plants.

What are the Types of Corrosion?

The process of corrosion may be compared to a galvanic cell. The rates of reaction at the anode and the cathode are influenced by the presence of oxygen or other oxidizing molecules, changes in the rate of flow, reactant concentrations, temperature and pH.

In aqueous environments, corrosion may occur due to the transport of metal ions and their electrochemical reactions. The rate of corrosion is influenced by metallic ion gradients, electrolytic ion gradients, pressure, temperature, and the presence of other particles, whether bacteria, metals or active cells.

There are various types of corrosion. The ASM classification of corrosion is as follows:

  • General corrosion, mostly by uniform thinning, as occurs in
    • Atmospheric corrosion
    • Galvanic corrosion
    • General biological corrosion
    • Stray-current corrosion
    • Molten salt corrosion
    • High temperature corrosion
    • Corrosion in liquid metals
  • Localized corrosion – characterized by uneven depths of penetration at certain sites
    • Crevice corrosion
    • Filiform corrosion
    • Pitting corrosion
    • Localized biological corrosion
  • Metallurgically influenced corrosion – dependent upon the chemistry of the alloy and the heat treatment
    • Intergranular corrosion
    • Dealloying corrosion
  • Mechanically assisted corrosion – with a mechanical component
    • Erosion corrosion
    • Fretting corrosion
    • Cavitation and water drop impingement
    • Corrosion fatigue
  • Environmentally induced cracking – due to the presence of stress
    • Stress corrosion cracking
    • Hydrogen damage
    • Liquid metal embrittlement
    • Solid metal induced embrittlement

Aqueous corrosion may be uniform (general) or non-uniform (local). Uniform corrosion results in general wastage, is reasonably easy to inspect and to prevent, and easy to predict from weight loss experiments or electrochemical data. Local corrosion is much less predictable but its results potentially more serious.  

Passive Oxide Films

All metals except gold form a surface oxide film in air, with the nature of the film depending on the alloy composition and the conditions and temperature of its formation. Films which are strongly adherent and intact protect the underlying substrate against further dry oxidation or wet corrosive attack. However, non-intact oxides such as iron oxide (rust) literally eat away the surface bit by bit.

Oxide films on the surfaces of metals are therefore seen to play a significant part in the mechanism of aqueous corrosion, as illustrated by the rusting of mild steel. Failures so caused are difficult to prevent due to the complex interactions of different corrosion mechanisms, coupled with residual and mechanical stresses in-service. These mechanisms quite often cause non-uniform corrosion that can result in severe local attack leading to failure.

Corrosion Mechanisms

Corrosion can proceed by several different mechanisms:

  • Differential aeration
  • Pitting
  • Galvanic attack
  • Intergranular attack
  • Leaching (selective corrosion)
  • Corrosion and erosion
  • Stress corrosion cracking (SCC)
  • Corrosion fatigue
  • Hydrogen damage

Unlike iron and mild steel, which corrode to form rust flakes that continuously peel off exposing fresh metal to the surroundings, most other metals oxidize readily, but form a thin metal oxide coating that resists further corrosion. Thus aluminum, chromium, magnesium and nickel are useful in creating protective oxides that protect the metal alloy.

Prevention of Corrosion

Corrosion requires three conditions:

  • Electrolyte
  • Exposed metal surface
  • Electron acceptor

Thus it is only necessary to remove one of these in order to prevent corrosion. Some measures are listed below:

Barrier Films

Most commonly the exposed metal surface is treated by applying a barrier of paint or enamel between the metal surface and the environmental moisture.

Electrochemical Methods

A sacrificial anode coating (a coating with another metal that is more readily oxidized than the first) is often used to prevent corrosion. For instance, zinc coating is added to iron alloy steel that rusts easily, to make galvanized iron. The zinc is now corroded by oxidation, reducing the iron which acts as the cathode, preventing corrosion. This is used for buried or submerged structures. The sacrificial anode must be replaced once it is consumed to provide ongoing protection.

Cathodic protection is similar to the above process but involves the use of a more electrochemically active metal. An external DC source is used to supply negative charge to the coating metal. This is commonly seen in underground pipelines and tanks.

Passivation occurs when a metal surface is covered with a thin film of corrosion products, which in turn protects it against further corrosion. An outstanding example is the Statue of Liberty with its aqua blue patina that protects the underlying copper against oxidation.

With anodization, the metal is immersed in a substance under the right conditions to produce uniform nanopores on the oxide film on the metal surface. In these pores oxidation occurs and a hard tough film builds up which is thicker than would be formed by passivation.

Design and Materials Selection

In selecting materials to resist corrosion or taking corrosion protection measures, the total life cycle cost is probably the most significant factor. In this respect, the extra cost of using cheaper materials which have lower corrosion resistance must be calculated in terms of materials inspection, maintenance and earlier replacement.

Materials must be selected on the basis of their ability to resist specific corrosive environments and to withstand the levels of service stresses. The ease of fabricating these materials into the shapes required by the design is also an important consideration.

In the design process potential corrosion problems may be prevented by avoiding:

Shapes with crevices that might cause differential aeration
Contact between incompatible materials which might give rise to galvanic attack
Situations where small anodic sites are in contact with large cathodic areas

Sources and Further Reading

The article was updated on 12th April, 2019.

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