CHAPTER ONE INTRODUCTION

1.1 BACKGROUND OF STUDY  1.1.1 CORROSION 

Corrosion is defined as a natural process, which converts refined metal to their more stable oxide. It is the gradual destruction or degradation of materials (usually metals) by chemical reaction with their environment which are most likely inevitable. Corrosion is a natural and costly process of destruction like earthquakes, tornados, floods and vocanic eruptions, with one major difference. Whereas we can be only a silent spectator to the above processes of destruction, corrosion can be prevented or at least be controlled. 

Despite different definitions, it can be observed that corrosion is basically the result of interaction between materials and their environment. Up to the 1960s, the term corrosion was restricted only to the metals and their alloys and it did not incorporate ceramics, polymers, composites and semiconductors in its regime. The term corrosion now encompasses all types of natural and man – made materials including biomaterials and nanomaterials, and it is not confined to metals and alloys alone. The scope of corrosion is consistent with the revolutionary changes in materials development witnessed in recent years.

            FIGURE  1.1: CORROSION ATTACK ON AN OLD SHIP

1.1.1.2 CORROSION AND ITS MECHANISM

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:

                   FIGURE 1..2:   THE ENERGY CYCLE OF IRON INDICATING ITS EXTRACTIVE                                       METALLURGY  

          

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:

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

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

3.      Those areas must be exposed to a common electrolyte.

4.      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 direct current flow.  Mild steel is lost at approximately 20  pounds for each ampere flowing for a year. (Thomas, 1994) .

FIGURE 1.3: 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.

1.1.1.2.1 ELECTROCHEMISTRY OF CORROSION

Corrosion occurs by an electrochemical process. The phenomenon is similar to that which takes place when a carbon-zinc “dry” cell generates a direct current. Basically, an anode (negative electrode), a cathode (positive electrode), an electrolyte (environment), and a circuit connecting the anode and the cathode are required for corrosion to occur (see Figure 1.3).

Dissolution of metal occurs at the anode where the corrosion current enters the electrolyte and flows to the cathode. The general reaction or reactions, if an alloy is involved) that occurs at the anode is the dissolution of metal as ions:

M → Mn+ + en-

Where

M = metal involved n = valence of the corroding metal species e = electrons

FIGURE 1.4: A BASIC CORROSION CELL.

The basic corrosion cell consists of an anode, a cathode, an electrolyte, and a metallic path for electron flow. Note that the corrosion current (z) enters the electrolyte at the anode and flows to the cathode.

Examination of this basic reaction reveals that a loss of electrons, or oxidation, occurs at the anode. Electrons lost at the anode flow through the metallic circuit to the cathode and permit a cathodic reaction (or reactions) to occur. In alkaline and neutral aerated solutions, the predominant cathodic reaction is

O2 + 2H2O + 4e- → 4(OH)                                                                                                   (1.1)

The cathodic reaction that usually occurs in deaerated acids is

2H- + 2e- → H2                                                                                                                     (1.2)

In aerated acids, the cathodic reaction could be

O2 + 4H- + 4e- → 2H2O                                                                                                        (1.3) 

All of these reactions involve a gain of electrons and a reduction process. The number of electrons lost at the anode must equal the number of electrons gained at the cathode. For example, if iron (Fe) was exposed to aerated, corrosive water, the anodic reaction would be

Fe → Fe++ + 2e-                                                                                                                    (1.4)

At the cathode, reduction of oxygen would occur

O2 + 2H2O + 4e- → 4(OH-)                                                                                                   (1.5)

Because there can be no net gain or loss of electrons, two atoms of iron must dissolve to provide the four electrons required at the cathode. Thus, the anodic and cathodic reactions would be

2 Fe → 2Fe++ + 4e-    (anodic)                                                                                               (1.6)

O2 + 2H2O + 4e-→ 4(OH-)    (cathodic)                                                                               (1.7)

These can be summed to give the overall oxidation-reduction reaction

2Fe + O2 + 2H2O → 2Fe++ + 4(OH-)                                                                                    (1.8)

After dissolution, ferrous ions (Fe++) generally oxidize to ferric ions (Fe+++); these will combine with hydroxide ions (OH-) formed at the cathode to give a corrosion product called

rust.

(FeOH or Fe2O3.H2O)                                                                                                           (1.9)

Similarly, zinc corroding in aerated, corrosive water (i.e., Zn → Zn++ + 2e-) will form the corrosion product Zn(OH)2. The important issue to remember is that anodic dissolution of metal occurs electrochemically; the insoluble corrosion products are formed by a secondary chemical reaction.

1.1.1.3 CLASSIFICATION OF CORROSION

Corrosion based on the appearance of the corroded metal can be classified as uniforrm or localized. Corrosion is either uniform i.e  the metal corrodes at the same rate over the entire surface,or it is localized, in which case only small areas are affected.

Classification by appearance, which is particularly useful in failure analysis, is based on identifying forms of corrosion by visual observation with either the naked eye or magnification.The morphology of attack is the basis for classification. Figure 1.5 illustrate some of the most common forms of corrosion.

FIGURE 1.5 :  MACROSCOPIC VERSUS MICROSCOPIC FORMS OF LOCALIZED CORROSION

There should be vivid distinction between macroscopically localized corrosion and microscopic local attack. In the latter case, the amount of metal dissolved is minute (minimal), and considerable damage can occur before the problem becomes visible to the naked eye or can be viewed with the aid of  a low – power magnifying device (Schweitzer, 1998). 

1.1.1.4 CORROSION PREVENTION

Some corrosion prevention methods include material selection, conditioning the corrosive environment, electrochemical control, protective coating and use of corrosion inhibitors. The most common and easiest way of preventing corrosion is through the judicious selection of material once the corrosion environment has been characterized. Standard corrosion references are helpful in this respect. Here, cost may be a significant factor and it is not always economically feasible to employ the material that provides the optimum corrosion resistance; sometimes, either another alloy or some other measure must be used.

Conditioning the corrosive environment if possible may also significantly influence corrosion. Lowering the fluid temperature and/or velocity usually produces a reduction in the rate at which corrosion occurs. Many times increasing or decreasing the concentration of some species in the solution will have a positive effect; for example, the metal may experience passivation. 

CORROSION INHIBITORS:

Corrosion inhibitors are chemicals that react with the metal's surface or the environmental gases causing corrosion, thereby, interrupting the chemical reaction that causes corrosion. Inhibitors can work by adsorbing themselves on the metal's surface and forming a protective film. These chemicals can be applied as a solution or as a protective coating via dispersion techniques.

The inhibitors process of slowing corrosion depends upon:

•       Changing the anodic or cathodic polarization behavior

•       Decreasing the diffusion of ions to the metal's surface

•       Increasing the electrical resistance of the metal's surface

Major end-use industries for corrosion inhibitors are petroleum refining, oil and gas exploration, chemical production and water treatment facilities.

The benefit of corrosion inhibitors is that they can be applied in-situ to metals as a corrective action to counter unexpected corrosion.

1.1.1.5 ECONOMIC IMPORTANCE OF CORROSION

The problems of metallic corrosion is one significant propotion, it has been estimated that appoximatly 5% of an industrial nation’s income is spent on corrosion prevention and its maintenance or preplacement of products lost or contaminated as a result of corrosion reaction. The consequences of corrosion are many and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failure of various kinds of equipment and the need for expensive replacement may occur even though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarized as follows:

•      Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings. For example a leaky domestic radiator can cause expensive damage to carpets and decorations, while corrosive sea water may enter the boilers of a power station if the condenser tubes perforate.

•      Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface.

•      Mechanical damage to valves, pumps, etc., or blockage of pipes by solid corrosion products.

•      Added complexity and expense of equipment which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced.

•      Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localized zones so as to give a crack on the structure, very considerable weakening may result from quite a small amount of metal loss.

•      Hazards or injuries to people arising from structural failure or breakdown (e.g.

bridges, cars, aircraft).

•      Reduced value of goods due to deterioration of appearance.

Corrosion processes are occasionally used to advantage. For example, etching procedures makes use of the selective chemical reactivity of grain boundaries or various micro-structural constituent. Also, the current development in dry cell is as a result of corrosion processes.

1.1.2 ALUMINIUM 

Aluminium always finds very regular and diversified uses in domestic appliances, chemical reactions and storage bottles, vessels and containers, buildings, bridges, packaging foils, automobiles, aircrafts, ships and many others. It is used for variety of applications due to its light weight, very high strength, good thermal and electrical conductivities, good heat and light  reflectivity, its non-rusty nature, non-toxicity and attractive appearance. It is highly electropositive and resistant to corrosion because a hard, tough film of oxide is formed on the surface. The surface film is amphoteric, hence the metal could dissolve readily in both strong acid and alkaline media. Despite these great properties of aluminium, it is not a perfect material for engineering applications in all environments as  they  suffer corrosion caused by chemical interactions with their surroundings (Khandelwal et al., 2010). Aluminium is used in industries like shipping, offshore petroleum exploration, power and coastal industrial plants (for cooling), fire-fighting, oil fuel water injection and desalination plants. 

1.2 PROBLEM STATEMENT

The failure of aluminum equipment and aluminum materials due to acid corrosion in industries is widely reported (Abiola et al, 2012), as such there is a need to minimize this common effect. In virtually all situations, aluminum failure through corrosion can be managed, slowed or even stopped by using the proper techniques.

The most common and easiest way of preventing corrosion is through the judicious selection of material once the corrosion environment has been characterized and by the use of chemical inhibitors is the most practical and cost effective means of controlling corrosion of metals in acid solutions. However, a number of inhibitors of acid corrosion of aluminum are toxic, nonbiodegradable and expensive.

1.3 AIM

The aim of this study is to investigate the inhibitive effect of alanine on corrosion of aluminium in 0.5 M HCl solution using the weight loss technique.

1.4 SCOPE OF STUDY

This study is limited to the study of use of organic inhibitors to reduce the failure of aluminum due to acidic corrosion. This is achieved by determining the inhibition efficiency of alanine by testing different concentration of alanine on aluminum in an acidic solution.

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