CHAPTER ONE

1.0 INTRODUCTION

Corrosion is a serious problem in this modern age of technological advancement. This accounts for a lot of economic losses and irreversible structural damage. The cost of corrosion failures annually for any nation is difficult to estimate per annum, but it has been stated that the wastage of material resources by corrosion ranks third after war and disease (Olugbenga et al.2011). Efforts have been made to restrain the destructive effects of corrosion using several preventive measures (Loto et al. 1989, Popoola et al.2011 and Davis et al. 2001). The effects of corrosion in our daily lives can be direct by affecting 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 (Denny et al. 1996).  The corrosion of steel reinforcing bars in concrete usually proceeds out of sight and suddenly results in failure of a section of bridges or buildings.

Virtually all metals will corrode to some extent; the fossil–fuel boilers and fossil-fuel fired power generators equipment experience corrosion problems in such component as steam generator and water walls surrounding the furnace (Natarajanf & Sivan, 2003). Perhaps most dangerous of all is corrosion that occurs in major industrial plants, such as electrical power plants or chemical However, the consequences of corrosion are economic and could lead to:

•      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 heattransfer 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.

Corrosion affects most of the industrial sector and may cost billions of dollars each year for prevention and replacement maintenance. Thus, the modern world has made investigations to overcome this problem by conducting enrichment studies of corrosion inhibitors. Corrosion inhibitors will reduce the rate of either anodic oxidation or cathodic reduction or both. This will give us anodic, cathodic or a mixed type of inhibition. In an attempt to find corrosion inhibitors that are environmentally safe and readily available, there has been a growing trend in the use of biological substrate such as leaves or plant extracts as corrosion inhibitors for metals in acid cleaning processes.

As a result of increasing awareness on environmentally friendly practices for sustainable development, the demand for non-toxic inhibitors to replace toxic ones has increased tremendously. Thus, in recent years, several plant extracts have been investigated for the inhibition of acid corrosion of metals. This is because plants contain naturally synthesized chemical compounds that are biodegradable, environmentally acceptable, inexpensive, readily available and renewable source of materials.

Corrosion is not only dangerous, but also costly, with annual damages in the billions of dollars! If this is difficult to believe, consider some of the direct and indirect effects of corrosion which contribute to these costs:

Not only that the economic costs are frightening, there is also potential loss of life and damage to the environment problems, which can have widespread effects upon modern industrial businesses. It is essential, therefore, for operators of industrial process plants to have a program for controlling corrosion.

1.1 Literature Review

Corrosion may be defined as a destructive phenomenon, chemical or electrochemical, which can attack any metal or alloy through reaction by the surrounding environment and in extreme cases may cause structural failure. The corrosion occurs because of the natural tendency for most metals to return to their natural state (reverse of metallurgy); e.g., iron in the presence of moist air will revert to its natural state, iron oxide.

Corrosion could be basically carried by water intrusion and some environmental factors. Water intrusion is the principal cause of corrosion problems encountered in the field use of equipment. Water can enter an enclosure by free entry, capillary action, or condensation. With these three modes of water entry acting and with the subsequent confinement of water, it is almost certain that any enclosure will be susceptible to water intrusion. At normal atmospheric temperatures the moisture in the air is enough to start corrosive action. Oxygen is essential for corrosion to occur in water at ambient temperatures. Other factors that affect the tendency of a metal to corrode are acidity or alkalinity of the conductive medium (pH factor), stability of the corrosion products, biological organisms (particularly anaerobic bacteria), Variation in composition of the corrosive medium and temperature.

1.2 Mechanism of Corrosion

In nature, metals are not found in Free State due to their reactivity. Metals are generally in high energy state because some energy is added during their manufacturing process from the ores. Low energy – state ores are more stable than the high energy – state metals. As a result of this uphill thermodynamic struggle, the metals have a strong driving force to release energy and  go back to their original form. Hence the metals revert to their parent state or ore under a suitable corrosive environment. The electrochemical process involved in corrosion by nature is opposite to the extractive metallurgy involved in manufacturing of the metals. Therefore, corrosion is sometimes considered as the reverse process of extractive metallurgy as can be seen below:

The energy cycle of iron indicating its extractive metallurgy in reverse (Kahhaleh et al.1994)

According to electrochemistry, the corrosion reaction can be considered as taking place by two simultaneous reactions:

The oxidation of a metal at an anode (a corroded end releasing electrons) and the reduction of a substance at a cathode (a protected end receiving electrons). In order for the reaction to occur, the following conditions must exist:

•          Two areas on the structure must differ in electrical potential.

•          Those areas called anodes and cathodes must be electrically interconnected.

•          Those areas must be exposed to a common electrolyte.

•          An electric path through the metal or between metals be available to permit electron flow. When these conditions exist, a corrosion cell is formed in which the cathode remains passive while the anode deteriorates by corrosion. As a result of this process, electric current flows through the interconnection between cathode and anode. The cathode area is protected from corrosion damage at the expense of the metal, which is consumed at the anode. The amount of metal lost is directly proportional to the flow of direct current.  Mild steel is lost at approximately 20  pounds for each ampere flowing for a year. (Thomas, 1994).

Figure 1.1:   The Component of an Electrochemical Corrosion Cell

At the anode, metals are oxidized and the electrons are liberated from the metal to form positive     metal ions. The liberated electrons dissolve into the electrolyte, and deposition is formed on the cathodic metal. Anode corrodes while the cathode remains intact.

1.3 Forms of corrosion damage

1.3.1 Uniform or thinning corrosion

In this form of corrosion attack, the entire surface of the metal is corroded, and the metal thickness reduced by a uniform amount. This would occur with a homogenous metal when no difference in potential existed between any points on the surface.

1.3.2 Fretting corrosion 

Fretting corrosion occurs when two or more parts rub against each other. The rubbing action removes the corrosion products and exposes new metal to the electrolyte.

1.3.3 Pitting corrosion

This is the most common type of attack that occurs with heterogeneous metals such as steels and other alloys. It is a localized attack, where the rate of corrosion is greater at some areas than at others. This is caused by differences in potential between different points on the metal surface

1.3.4 Galvanic corrosion

Galvanic corrosion occurs where two different metals or alloys come in contact. The severity of galvanic corrosion depends upon the difference in potential between the two metals, and the relative size of the cathode and anode areas

1.3.5 Intergranular corrosion 

Corrosion occurs at the grain boundaries due to a difference in potential between the anodic grain boundaries and the cathodic grains. “Sensitized” stainless steels, where carbides have been precipitated in the grain boundaries during improper heat treatment or in the heat-affected zone of a weld, are particularly susceptible to intergranular corrosion.

1.3.6 Erosion corrosion 

Erosion is the removal of metal by the movement of fluids against the surface. The combination of erosion and corrosion can provide a severe rate of corrosion.

1.3.7 Crevice corrosion 

Crevice corrosion occurs when there is a difference in ion, or oxygen, concentration between the metal and its surroundings. Oxygen starvation in an electrolyte at the bottom of a sharp V-section will set up an anodic site in the metal that then corrodes rapidly.

1.4 Methods of Corrosion Protection 

1.4.1 Application of Protective Coatings 

Metallic structures can be protected from corrosion in many ways. A common method involves the application of protective coatings made from paints, plastics or films of noble metals on the structure itself (e.g., the coating on tin cans). These coatings form an impervious barrier between the metal and the oxidant but are only effective when the coating completely covers the structure.

Flaws in the coating have been found to produce accelerated corrosion of the metal.

1.4.2 Cathodic Protection 

Cathodic protection using an impressed current derived from an external power supply is a related form of protection in which the metal is forced to be the cathode in an electrochemical cell. For example, most cars now use the negative terminal on their batteries as the ground. Besides being a convenient way to carry electricity, this process shifts the electrical potential of the chassis of the car, thereby reducing (somewhat) its tendency to rust.