CHAPTER ONE
1.0 INTRODUCTION
Mild steel is one of the major construction materials which is extensively used in chemical and allied industries for the handling of acid, alkali, salt and other solutions. The major problem concerning steel application in industries is or its relatively low corrosion resistance in acidic solution. Metal corrosion is one of the major problems in industry which has attracted a lot of investigations in recent years. Millions of dollars are lost yearly because of corrosion; much of these loss is due to corrosion of iron and steel although other metals may corrode. In addition, corrosion of metal can result in environmental pollution especially in riverine areas where most of pipeline carrying crude oil can easily be corroded thereby causing leakages to the water body which then result in the death of aquatic animals.
Corrosion usually involves two related chemical reactions, oxidation and reduction. In oxidation, the atoms of metal give up electrons to liquid or a gas. In reduction, part of the same metal or an adjoining metal captures electrons from the liquid or gas. Corrosion occurs in the presence of moisture for example when iron is exposed to moisture; it reacts with oxygen to form rust according to the equation:
2Fe(s) + O2(g) + 2H2O(l) 2Fe (OH)2(s) (1.1)
However, in order to mitigate corrosion, the strategy is to isolate the mild steel from corrosive agent. Among the different methods available, the use of inhibitions is usually the best.
1.1 Definition of Corrosion
Corrosion is “the gradual destruction of materials (usually metals) by chemical reaction with its environment” (http://corrosion.Ksc.nasa.gov). Also corrosion is a process through which metals in manufactured states return to their natural oxidation states. This process is a reduction-oxidation reaction in which the metal is being oxidized by its surroundings, often the oxygen in air. This reaction is both spontaneous and electrochemically favoured. Corrosion is essentially the creation of voltaic or galvanic cells, where the metal in question acts as an anode and generally deteriorates or loses functional stability.
Corrosion of material occurs because we use them in environments where they are chemically unstable. Only copper and the precious metals (gold, silver, platinum etc) are found in nature in their metallic state. All others metals including iron-the metal most commonly used-are processed from minerals or ores into metals which are inherently unstable in their environments and have a tendency to revert to their more stable mineral forms. Some metals form protective films (passive films) on their surfaces and these prevent or slow down their corrosion process. Corrosion can be prevented by using metals that form naturally protective passive films, but these alloys are usually expensive, hence other means of corrosion control have been developed.
1.2 Types of Corrosion
There are different types of corrosion; This includes galvanic corrosion, filiform corrosion and crevice corrosion. Others are pitting corrosion, stress corrosion cracking, intergranular corrosion and fretting corrosion.
1.2.1 Galvanic Corrosion
Galvanic corrosion occurs in the presence of an electrolyte such as sea water when dissimilar types of metals join together. Most metals have different electrical potentials. When connected electrically and placed in an electrolyte, the more active metal becomes the anode because it has more negative potential and corrodes faster than if it were alone in the environment. The more noble (less active) metal becomes the cathode because it has more positive potential and corrodes at a slower rate than if it were alone in the environment. Electrical current flows between the metals until their potentials are equal. Galvanic corrosion typically appears in joints where the two dissimilar metals meet.
1.2.2 Filiform Corrosion
Filiform corrosion is a form of concentration cell corrosion on metallic surface coated with a thin organic film. Typically, the surface must be somewhat acidic and the relative humidity in the environment must be 6.5 to 90 percent. This type of corrosion usually begins in a defect within a surface’s protective coating. Filaments of corrosion products begin to grow and cause the protective coating to degrade. The filament take on the appearance of thin threads that lie long branching paths that expand out from the original corrosion site, why the concentration cell breaks its original isotropic symmetry and becomes thread like and spread is unclear.
1.2.3 Crevice Corrosion
Crevice corrosion, also called concentration cell corrosion, forms when a liquid corrosive is trapped in narrow gaps of space between metals, or between nonmetals and metals. Aggressive ions like chlorides must be present in the electrolyte. Once the liquid has settled in the gap, a corrosion reaction begins to take place. The reaction consumes the oxygen in the bottom of the gap, and an anodic area develops adjacent to the oxygen depleted zone. The material on the exterior acts as the cathode.
1.2.4 Pitting Corrosion
Pitting corrosion causes damage by randomly attacking a limited selection of the metal’s surface, leaving behind holes that are larger in depth than width. The “pit” that forms functions as the anode and metal that is left undamaged functions as the cathode. The corrosion process starts with a chemical breakdown in a small spot such as a scratch or a nick, usually occurring unclear a surface coating that has experienced wear or damage.
1.2.5 Stress Corrosion Cracking (SCC)
Stress corrosion cracking (SCC) is a complex form of corrosion that occurs when brittle, dry cracks develop from the combined effects of a tensile stress (A stress caused by bending or stretching a material causing two sides of the material to pull apart) and a corrosive environment.
1.2.6 Intergranular Corrosion
The microstructure of metals and alloys consists of a granular composition. Grains are small crystals whose surfaces join the surfaces of other grains to form grain boundaries. Grain boundaries separate the grains. Intergranular corrosion, also called intercrystalline corrosion, occurs on or adjacent to the grain boundaries of a metal.
1.2.7 Fretting Corrosion
Fretting corrosion, sometimes referred to as wear oxidation, friction oxidation, chafing and brinelling, is a type of erosion-corrosion. It occurs as a result of the combined effect of corrosion and fretting of a metal surface. The corrosion process involves removal of material from the surface where contact occurs and motion between the surfaces is restricted to small vibrations. Fretting corrosion appears as local surface dislocations and deep pits.
1.3 Factors influencing the corrosion of metals
The factors which influence the corrosion of metals can be divided into two classes, namely (i) Nature of metal (ii) Nature of corrosive environment
1.3.1 Nature of Metal
1.3.1.1 Purity of Metal: The more impure the metal, the more is the rate of corrosion. This is because impurities act as minute electrochemical cells.
1.3.1.2 Physical State of Metal: Physical state of metal like grain size, stress affects the corrosion rate; stressed areas undergo more corrosion.
1.3.1.3 Position in Galvanic Series: The more the oxidation potential (that is, if metal is higher in the galvanic series), it is more anodic and hence its rate of corrosion is high.
1.3.1.4 Nature of Oxide Layer: Corrosion rate depends upon the nature of oxide layer whether it is stable, unstable, porous etc.
1.3.1.5 Passive Nature of Metal: Some metals like titanium, aluminium, chromium show more corrosion resistance than expected from their position in the electrochemical series. These are called passive metals. This is due to the formation of thin protective layer on metal surface.
1.3.1.6 Nature of Corrosive Products: If the corrosion products are soluble or volatile in nature, then corrosion increases.
1.3.1.7 Relative Area of Anode and Cathodic: The smaller the anodic area, the more is the corrosion rate because oxidation of anode occurs at a faster rate.
1.3.2 Nature of Corroding Environment
1.3.2.1 Temperature: The rate of corrosion generally increases with increase in temperature.
1.3.2.2 Moisture: The rate of corrosion of a metal increases with the amount of moisture in the environment.
1.3.2.3 pH Value: The rate of corrosion of a metal increases with decreased in pH of the medium.
1.3.2.4 Nature of Electrolyte: Presence of salts in the electrolyte increases the rate of corrosion.
1.3.2.5 Presence of Impurities in Atmosphere: Impurities increase the rate of corrosion. Corrosion is more in industrial areas and sea. This is because of presence of gases like CO2, H2S, SO2.
1.4 Methods of Preventing Corrosion
Corrosion prevention can take a number of forms, depending on the circumstances of the metal being corroded.
Corrosion prevention techniques can be generally classified into six groups, namely, environmental modifications, metal selection and surface conditions, cathodic protection, corrosion inhibitors, coating and plating
1.1.1 Environmental Modification
Corrosion is caused by chemical interactions between metal and gases in the surrounding environment. By removing the metal from or changing the type of environment; metal deterioration can be immediately reduced.
This may be as simple as limiting contact with rain or sea water by storing metal materials indoors, or could be in the form of direct manipulation of the environment affecting the metal.
1.4.2 Metal Selection and Surface Conditions:
No metal is immune to corrosion in all environments, but through monitoring and understanding the environment conditions that are the cause of corrosion, changes to the type of metal being used can also lead to significant reductions in corrosion. Monitoring of surface conditions is also critical in protecting against metal deterioration from corrosion.
1.4.3 Cathodic Protection
Cathodic protection works by converting unwanted anodic (active) sites on a metal’s surface to cathodic (passive) sites through the application of an opposing current. This opposing current supplies free electrons and forces local anodes to be polarized to the potential of the local cathodes.
Cathodic protection can take two forms. The first is the introduction of galvanic anodes. This method, known as a sacrificial system, uses metal anodes, introduced to the electrolytic environment, to sacrifice themselves (corrode) in order to protect the cathode. While the metal needing protection can vary. A second method of cathodic protection is referred to as impressed current protection, which is often used to protect buried pipelines and ship hulls, requires an alternative source of direct electrical current to be supplied to the electrolyte. The negative terminal of the current source is connected to the metal, while the positive terminal is attached to an auxiliary anode, which is added to complete the electrical circuit.
1.4.4 Corrosion Inhibitors
Corrosion inhibitors are chemicals that react with the metal’s surface or the environment gases 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: (i) Changing the anodic or cathodic polarization behaviour (ii) Decreasing the diffusion of ions to the metal’s surface and (iii) 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.
1.4.5 Coating
Paints and other organic coatings are used to protect metals from the degradative effect of environmental gases. Coatings are grouped by the type of polymer employed: common organic coatings include:- (i) Two-part urethane coating, (ii) Both acrylic and epoxy polymer radiation curable coatings, (iii) High-solid coatings, (iv) Powder coatings, (v) Water soluble coatings and (vi) Vinyl, acrylic or styrene polymer combination latex coatings.
1.4.6 Plating
Metallic coatings or plating can be applied to inhibit corrosion as well as provide aesthetic, decorative finishes.
Plating can be divided into groups. This includes electroplating, mechanical plating, electroless and hot dipping.
1.4.6.1 Electroplating
A thin lager of metal is deposited on the substrate metal (generally steel) in an electrolytic bath.
1.4.6.2 Mechanical Plating
Metal powder can be cold welded to a substrate metal by tumbling the part, along with the powder and glass beads, in treated aqueous solutions.
1.4.6.3 Electroless
A coating metal, such as cobalt or nickel, is deposited on the substrate metal using a chemical reaction in this non-electric plating method.
1.4.6.4 Hot Dipping
When immersed in a molten bath of the protective, coating metal a thin layer adheres to the substrate metal.
1.5 Corrosion Inhibitors
A corrosion inhibitor is a substance when added in a small concentration to an environment reduces the corrosion rate of a metal exposed to that environment.
Types of Corrosion Inhibitors
1.5.1 Anodic Inhibitors
Anodic inhibitors usually act by forming a protective oxide film on the surface of the metal causing a large anodic shift of the corrosion potential. This shift forces the metallic surface into the passivation region. They are also sometimes referred to as passivators. Chromates, nitrates, tangstate, molybdates are some examples of anodic inhibitors.
1.5.2 Cathodic Inhibitors
Cathodic inhibitors act by either slowing the cathodic reaction itself or selectively precipitating on cathodic areas to limit the diffusion of reducing species to the surface. The rates of the cathodic reactions can be reduced by the use of cathodic poisons.
1.5.3 Mixed Inhibitors
Mixed inhibitors work by reducing both the cathodic and anodic reactions. They are typically film forming compounds that cause the formation of precipitates on the surface blocking both anodic and cathodic sites indirectly. Hard water that is high in calcium and magnesium is less corrosive than sift water because of the tendency of the sacts in the hard water to precipitate on the surface, of the metal forming a protective film.
1.5.4 Volatile Corrosion Inhibitors
Volatile corrosion inhibitors (VCI), also called vapor phase inhibitors (VPI), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine are transported with steam to prevent corrosion in the condenser tubes by neutralizing acidic carbondioxide or by shifting surface pH towards less acidic and corrosive values.
1.6 Maesobotrya barteri plant
Maesobotrya barteri var. sparsiflora (English name bush cherry; Efik/Ibibio name: Nnyanyated) is a variety of flowering plant belonging to the family phyllanthaceae, or by some author’s classification Euphorbiaceae Sensu Lato, native to Cote d’ivoire. Its fruits are edible.
1.6.1 Medical uses of Maesobotrya barteri
Root cuts of Maesobotrya barteri are infused in gin for treating arthritis (Uzodima, 2013).
The mashed leaves are applied as a wound dressing in Ghana and a leaf-decoction is given in Ivory Coast to dispel giddiness. Also it is used to cure malaria (www.academica.edu/4214204/A.Dictionary).
1.6.2 Phytochemical Analyses of Maesobotrya barteri
The phytochemical analysis of Maesobotrya barteri leaf extract by Akan (2014) showed it to contain the following parameters, contained in table 1.1.
Parameter Inference
Tannins +
Phlobotannins –
Saponins +
Anthraquinone –
Cardiac glycoside ++
Deoxy sugar ++
Terpenes ++
Alkaloids –
Flavonoids –
++ = relatively more abundant; + = relatively less abundant; – = not detected.
Table 1.1 Phytochemical composition of ethanol leaf extract of Maesobotrya barteri
1.7 Aim and Objectives of the Study
The aim of this research was to inhibit the corrosion of mild steel in H2SO4 solution by Maesobotrya barteri extract using weight loss method.
The objectives of the study shall be achieved by:
- determining the effect of extract concentration and temperature on the corrosion rate of mild steel in H2SO4 solution;
- calculating the inhibition efficiency and corrosion rate of the Maesobotrya barteri extract on the corrosion of mild steel in H2SO4 solution,
iii. evaluating the thermodynamic parameters of the corrosion process; and
- assessing the adsorption mode of the extracts unto the mild steel surface and determining the adsorption isotherm obeyed.
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