1.0 Introduction
In its broadest sense the term corrosion applied to the destructive alteration of a metal or alloy by chemical reaction with any substance, solid, liquid, or gas. The pattern of attack will be governed largely by the combined influence of several factors relating to the metal or alloy, to the conditions of service, and to the chemical nature of the environment.
- The metal may corrode uniformly over its entire surface, as in the resting of iron in the atmosphere.
- It may suffer only a superficial attack that does not seriously affect the strength of the metal or alloy but does discolour the surface, as in the tarnishing of silver in the atmosphere.
- Corrosion may develop at local areas on an otherwise unattached surface, leading to pitting that may in some instances result in early failure by perforation.
- An alloy may be attacked in such a way as to remove one or more of its constituents, leaving a weakened residue of unattached element with poor metallic properties, as in the dezincification of yellow brass or in the graphitization of cast iron.
- Excessive corrosion may occur in local area where poor design features tend to aggravate the corrosion conditions, as crevices under rivets, bolts and faying surface.
- Use of dissimilar metals sometimes can lead to severe attack of the more active metal where they are joined together.
- High tensile stresses plus specific corrosive conditions may result in cracking of a metal because of a corrosion reaction along critical paths in the metal.
In order to provide a quantitative basis for reporting corrosion rates and intensity of attack, the average weight loss of metal per unit area per unit time or the loss in thickness per unit time is usually recorded. Loss in thickness per unit time is more useful in those instances where the engineer is concerned with life of equipment or structure. Average loss is reported in mils per year (MPY) or inches penetration per year (ipy). Where product contamination is vital, the average corrosion rate is expressed in milligrams of metal corroded per square decimeter per day (mdd).
Corrosion process usually occurs because the metal has a tendency to return to its native state, the form in which it is found in nature. This require that the metal atoms relinquish their valence electrons to the environment and because ions in the process. The newly formed ions may then combine with other ions to from compounds. This, when iron corrodes in water, the iron atoms are converted to ferrous ion upon the loss of two electrons.
Fe Fe2++2e-
The ferrous can then combine with hydroxyl ions to from insoluble ferrous hydroxide.
Fe Fe2++20H Fe(0H)
This compound eventually becomes oxidized through content with oxygen the familiar red iron rust, ferric hydroxide. in the electrons from the iron are accepted by oxygen gas molecules and iron oxides are formed.
1.1 Objectives of the Study
The objective of this study in based on the fault that corrosion which has been a major problem in our modern industries has to be prevented and curbed to the barest minimum by putting it environment into consideration in selecting material a structure or piece equipment.
1.2 Scope of the Study
The main metallic material that is being used in this work is nail (iron). This material will be subjected to different medium (environment) viz air medium salt solution medium, distilled water medium acid solution medium, alkaline solution medium and a lubricating oil medium. The observable change brought about by the environment reaction to the material under survey was studied. Iron of varied coatings will also be used.
1.3 Significance of the Research
A complete understanding of corrosion phenomena in all fact requires intensive and prolonged study. However, a general knowledge of the subject can be gained by the engineer through a review or survey of the various ways in which corrosion manifest itself, the factors that govern corrosion process, and the means available to the engineer for controlling or preventing corrosion.
This work will provide a survey that will serve to guide the material engineer his consideration of corrosion behaviour of metals and alloys virus, it will enable those in the oil and gas sector to install pipelines in an environment that will be favourable.
