Introduction

Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration 1.

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An estimated cost of the corrosion annually across the world is around $2.5 trillion 2. So this tremendous amount represents a huge financial loss and a lot can be done to reduce it. The cost of corrosion maintenance and prevention is not surprising when we consider that corrosion occurs, with varying degrees of severity, wherever metals and other materials are used.

The environment that in which metals are being used are quiet wide spread and mostly all environments are corrosive to some extent. Corrosion takes place in various places, and in various forms under different circumstances. For example air, moisture, industrial atmosphere like steam, acids, minerals and gases. Generally inorganic materials are more corrosive than that of organic materials for example corrosion in petrochemical industry is more due to sulphur, hydrochloric acid and sulfuric acids than that to the naphtha, oil and gasoline.

There are various types of corrosion, it is easy to classify corrosion by the forms in which it appears. Classification is based on appearance of the corroded surface. Although the same principles are involved in an environmental attack or metal corrosion. The corrosion attack can be uniform, localised, or selective. Depending upon the condition. The corrosion can be identified by visual examination. In an average case naked eye is enough but visual aids can be used to further magnification.

Some of the basic types of corrosion are:

1.     Uniform corrosion or general attack.

2.     Galvanic or Bimetal corrosion.

3.     Crevice corrosion.

4.     Pitting.

5.     Intergranular corrosion.

6.     Selective leaching.

7.     Erosion.

8.     Stress Corrosion.

Report below we discuss the Bimetallic (dissimilar metal) corrosion and uniform or general attack corrosion in terms of their mechanisms and preventative measures.

 

Dissimilar metal corrosion

When two dissimilar metals are in contact with each other in presence of an electrically conductive solution, this gives rise to formation of a galvanic cell and the reactive metal undergoes corrosion 3. When these metals are placed in contact, this potential difference produces electron flow between them. Corrosion of less corrosion resistance metal is usually increased and attack of the more resistance material is decreased, as compared with the behaviour of these metals when they are not in contact. The less resistance metal becomes anodic and the more resistant metal cathodic. Usually the cathode or cathodic metal corrodes very little or not at all in this type. Because the electric currents and dissimilar metals involved, this form of corrosion is called galvanic, or dissimilar metal corrosion.

Main contributors needed to form a dissimilar metal corrosion cells are

1.     Anode:

2.     Cathode:

3.     Electrolyte

4.     Oxidation material

Example of dissimilar metal corrosion

The famous case of dissimilar metal corrosion is the Statue of Liberty which was first designed and constructed in Paris in late 19th century. Which is then transported to United States and erected in New York’s island called Liberty Island. In 1980s a series of inspections was conducted by the National Park Service and by an International team of engineers and architects. There were few problem areas needing attention were identified. The obstacle that most required the restoration of the was galvanic corrosion of the iron armature in contact with the copper skin 4. 

 Galvanic corrosion occurs when dissimilar metals are in electrical contact in the same electrolyte. The difference in electrochemical potential between the dissimilar metals (iron and copper) is the driving force for electrolysis, whereby the armature, forming horizontal anodes at an accelerated rate. The iron armature, forming horizontal and vertical ribs against the copper skin (see image 2.1.1), and the attachment mechanism, whereby copper saddles (which are flush riveted to the copper skin) surround the iron armature, provided a configuration conducive to galvanic corrosion (see image 2.1.2).

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