CORROSION
Is a natural process. Just like water flows to the lowest level, all natural processes tend toward the lowest possible energy states. Thus, for example, iron and steel have a natural tendency to combine with other chemical elements to return to their lowest energy states. In order to return to lower energy states, iron and steel frequently combine with oxygen and water, both of which are present in most natural environments, to form hydrated iron oxides (rust), similar in chemical composition to the original iron ore.
} Fig. 1 The corrosion cycle of steel
} 2Fe + 2H2O + O2à 2FE(OH)2
REQUIREMENTS FOR CORROSION.
}A metal capable of undergoing an anodic reaction. All metal can.
}An environment containing a corrodant or cathodic reactant capable of oxidizing the metal to a chemically combined state.
For steel, suitable corrodants are dissolved oxygen; hydrogen ions e.g. mineral acid HCL.
}An environment that is electrolytically conducting.
}A metal/environment interface that allows the transfer of electric charge
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FORMS OF CORROSION.
Corrosion occurs in several widely differing forms. Classification is usually based on one of three factors:
} Nature of the corrodent: Corrosion can be classified as “wet” or “dry.” A liquid or moisture is necessary for the former, and dry corrosion usually involves reaction with high-temperature gases.
} Mechanism of corrosion: This involves either electrochemical or direct chemical reactions.
}Appearance of the corroded metal: Corrosion is either uniform and the metal corrodes at the same rate over the entire surface, or it is localized, in which case only small areas are affected.
} Completeness requires further distinction between macroscopically localized corrosion and microscopic local attack. In the latter case, the amount of metal dissolved is minute, and considerable damage can occur before the problem becomes visible to the naked eye.
} Macroscopic forms of corrosion affect greater areas of corroded metal and are generally observable with the naked eye or can be viewed with the aid of a low-power magnifying device.
} Galvanic corrosion: It occurs close to the junction of two dissimilar metals in conducting solution containing cathodic reactant. Natural potential difference between the two metals drives the corrosion as one metal becomes the anode (corrodes) and the other becomes the cathode (protected).
} GALVANIC SERIES IN AN AGGRESSIVE ENVIRONMENT (SEA WATER).
} NOBLE PLATINIUM GOLD GRAPHITE TITANIUM STAINLESS STEEL (PASSIVE) NICKEL (PASSIVE) SILVER SOLDER
BRONZE COPPER BRASS TIN LEAD CAST IRON CARBON STEEL CADMIUM ALUMINIUM ZINC MAGNESIUM
Dealloying corrosion: This is the selective removal by corrosion of one element from the alloy,e.g. zinc from brasses (dezincification).
Crevice corrosion: Occurs in flooded crevices present at a metal surface exposed to an aerated aqueous environment.
}It occurs most commonly in chloride rich environments.
}It’s associated with passivatable metals.
}It takes time to initiate.
}It’s difficult to stop once started.
}It occurs in crevices formed between two similar or dissimilar metals.
} Pitting corrosion: It occurs on normally passive metals. Local failure of passive film results in restricted areas of intensely localized corrosion often surrounded by unattacked metal.
Intergranular corrosion: Occurs when reactive material forms at grain boundaries of metal, and the parent metal becomes depleted of essential constituent.
Stress corrosion cracking: Occurs on simultaneous exposure to specific environment and a tensile stress.
Erosion corrosion: Occurs when high velocity fluid disrupts passive films or prevents their formation.
The Effects of Corrosion
The effects of corrosion in our daily lives are both direct, in that corrosion affects the useful service lives of our possessions, and indirect, in that producers and suppliers of goods and services incur corrosion costs, which they pass on to consumers. At home, corrosion is readily recognized on automobile body panels, charcoal grills, outdoor furniture, and metal tools. Preventative maintenance such as painting protects such items from corrosion.
A principal reason to replace automobile radiator coolant every 12 to 18 months is to replenish the corrosion inhibitor that controls corrosion of the cooling system. Corrosion protection is built into all major household appliances such as water heaters, furnaces, ranges, washers, and dryers.
Of far more serious consequence is how corrosion affects our lives during travel from home to work or school. The corrosion of steel reinforcing bar (rebar) in concrete can proceed out of sight and suddenly (or seemingly so) result in failure of a section of highway, the collapse of electrical towers, and damage to buildings, parking structures, and bridges, etc., resulting in significant repair costs and endangering public safety.
Perhaps most dangerous of all is corrosion that occurs in major industrial plants, such as electrical power plants or chemical processing plants. Plant shutdowns can and do occur as a result of corrosion. This is just one of its many direct and indirect consequences. Some consequences are economic, and cause the following:
} Replacement of corroded equipment
} Overdesign to allow for corrosion
} Preventive maintenance, for example, painting
} Shutdown of equipment due to corrosion failure
} Contamination of a product
} Loss of efficiency—such as when overdesign and corrosion products decrease the heat-transfer rate in heat exchangers
} Loss of valuable product, for example, from a container that has corroded through inability to use otherwise desirable materials
} Damage of equipment adjacent to that in which corrosion failure occurs.
} Safety, for example, sudden failure can cause fire, explosion, release of toxic product, and construction collapse.
} Health, for example, pollution due to escaping product from corroded equipment or due to a corrosion product itself.
CONTROLLING CORROSION.
Design: Corrosion can be designed out
} Eliminate water or water retention.
} Avoid crevices
} Avoid stagnant areas
} Taking care when using dissimilar metals
Materials: More corrosion resistant materials may be used. Some include-
Stainless steels: It often possess very good resistance to uniform corrosion but may be susceptible to localized corrosion.
Phosphor bronzes: It has good resistance to flowing seawater.
Coating: Separates metal from corrosive environment.
} Widely practised for external surfaces.
} More difficult to apply for internal surfaces.
} No coating is wholly impervious. Coatings degrade with time.
Chemical treatment:
}Add chemicals to remove or neutralize cathodic reactants.
}pH control
}oxygen scavengers
Electrochemical protection: Polarizing the metal by the application of an external current, changes its potential (E).
}Metal made passive
}Very widely used in soil and sea water.
CONCLUSION
Corrosion of metals costs the Nigerian economy several billions of naira per year at current prices. Approximately one-third of these costs could be reduced by broader application of corrosion-resistant materials and the application of best corrosion control.
REFERENCES
Corrosion Vols 1 and 2 Metal Environment Reactions, Edited by L. L. Shrier Pub Butterworth-Heinemann LtdCorrosion engineering Mars Guy Fontana McGraw-Hill, 1986
Handbook of corrosion engineering By Pierre R. Roberge
Corrosion engineering: principles and practice Pierre R. Roberge McGraw-Hill Prof Med/Tech, 2008
The Corrosion Handbook, Uhlig, H. H. (Ed.), Wiley, New York and Chapman and Hall, London (1948)