Building Materials PDF

138 Chapter 3: Building Materials 3.1. Introduction Construction or building materials are any materials that are used in the construction work. They are of two major categories: natural and synthetic. Natural materials are like aggregates, sand, stones and wood are natural, while cement,...

138 Chapter 3: Building Materials 3.1. Introduction Construction or building materials are any materials that are used in the construction work. They are of two major categories: natural and synthetic. Natural materials are like aggregates, sand, stones and wood are natural, while cement, bricks, steel, concrete, or plastics are synthetic. 3.2. Aggregate Aggregates give the concrete its form and reduce its shrinkage. Aggregates represent about 70 to 80 % of concrete volume. Aggregates are classified according to shape and size. The shape of the aggregates is determined not only by the parent rock but also by the crushing machine used. According to shape, aggregates can be rounded, irregular or partly  139 rounded, angular, flaky, elongated, and flaky and elongated aggregates. According to size, Aggregates are either fine (sand) or coarse (gravel) aggregates 3.3. Bricks Bricks are building materials used to build walls or paving roads. They can be connected using mortar, adhesives or by interlocking them. Bricks can be classified according to quality or the building process. Based on quality there are first, second and third class bricks. First class bricks have standard size, regular shape, uniform yellow or red color, and  140 well burnt. Second class bricks almost have the same characteristics like first class except that the burning temperature is slightly lower than first class. Unlike first and second class, the third class has irregular shape and size, the color is soft and light red and is under burnt. According to building process, Bricks are classified into unburnt bricks, burnt bricks and over burnt bricks. 3.4. Lime Lime is used in construction works as lime mortar. Lime can be hydraulic or non-hydraulic lime. The difference between these two types is that hydraulic lime sets under water but non-hydraulic lime do not set underwater. Quick Lime is a non-hydraulic lime manufactured by burning calcium carbonate  141 containing lime stones. Lime used in construction works should exhibit good plasticity, should be flexible and easily workable, and should harden in short time. 3.5. Cement Cement is a fine powder which sets after a few hours when mixed with water, and then hardens in a few days into a solid, strong material. Cement is mainly used to bind fine sand and coarse aggregates together in concrete. Cement is a hydraulic binder, i.e. it hardens when water is added. Portland cement is made of lime stone (CaCO3) and clay (Kaolin Al2O3SiO2.2H2O) and some iron oxides (Fe2O3). Cements are classified as non-hydraulic or hydraulic, based on the ability of the cement to set in the presence of water. Non-hydraulic cement does not set  142 in the presence of water. Oppositely, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting. Hydraulic cements (e.g., Portland cement) set in the presence of water because of the formation of water- insoluble metal hydrates. 3.5.1.Production of Portland cement Basically, the cement production process involves two main steps; clinker formation step and cement formulation step. Clinker is produced by mixing the cement raw materials at high temperatures, up to 2000 oC, in a rotary kiln. The presence of some ferric oxide in the mixture of the raw materials helps in the formation of the clinker at lower temperatures (around 1300oC). Afterwards, the formed clinker is crushed and grinded in a cement grinding mill. Some additives,  143 such as calcium sulphate or limestone, are grinded in a cement grinding mill too, leading to a fine and homogenous cement powder. The cement is then stored in storage tower before being shipped either in bulk or bagged. Clinker consists of four main phases: 1. C3S: Tri Calcium Silicate (Alite) (3CaO·SiO2) 2. C2S: Di Calcium Silicate (Belite) (2CaO·SiO2) 3. C3A: Tri Calcium Aluminate (celite) (3CaO·Al2O3) 4. C4AF: Tetra Calcium Alumino Ferrite (Brownmillerite) (4CaO·Al2O3·Fe2O3). The silicates are responsible for the cement's mechanical properties, the tricalcium aluminate and Tetra Calcium Alumino Ferrite are essential for the formation of the liquid phase during the burning process of clinker in the kiln.  144 First, the limestone (calcium carbonate) is burned to eliminate its carbon, creating lime (calcium oxide) in what is known as a calcination reaction. This single chemical reaction is a main source of global carbon dioxide emissions. CaCO3 → CaO + CO2 The lime reacts with SiO2 to yield dicalcium silicate and tricalcium silicate. 2CaO + SiO2 → 2CaO·SiO2 3CaO + SiO2 → 3CaO·SiO2 The lime also joins with aluminum oxide to produce tricalcium aluminate. 3CaO + Al2O3 → 3CaO·Al2O3 The lime also unifies with aluminum oxide, and ferric oxide to give cement. 4CaO + Al2O3 + Fe2O3 → 4CaO·Al2O3·Fe2O3 (cement)  145 Fig. 2 Cement production steps 3.5.2.Main tests for cement quality: 5. Magnesium oxide percentage: This percentage should not exceed 5%, since its increase means that the quality of the used limestone is poor. Magnesia is rather refractory and does not take part in the cement reactions.  146 6. Powder size ‫النعومة‬ The prepared cement should be very fine, such that at least 98% of it should pass through mesh 200 (200 pinch per linear inch). 7. Setting time: This test is done to test the speed of cement solidification. For this purpose, samples of cement are mixed with standard amounts of water. In this test, the time at which a standard needle loaded with a standard weight cannot penetrate the prepared sample is measured. 8. Date of Manufacturing It is very important to check the manufacturing date because the strength of cement decreases with time. It's better to use cement before 3 months from the date of manufacturing.  147 3.6. Concrete A good quality concrete is basically a heterogeneous mixture of cement, coarse and fine aggregates and water which combines into a hard mass due to chemical action between the cement and water. In concrete production, each component has its specific role. The coarser aggregates (gravel) acts as fillers. The fine aggregates (sand) occupy the holes between the paste and the gravel. The cement with water acts as a binder. 3.6.1. Properties of Fresh Concrete: Concrete should preserve its fresh form the time it mixed until the time it compacted. The properties of the fresh concrete are very critical because it affects its quality after being hardened. Concrete consistency, workability, and settlement and bleeding  148 are important properties that should be taken into account when working with concrete. 1. Concrete consistency Concrete consistency reflects the stiffness or sloppiness or the concrete fluid. For good handling, placing and compacting of the concrete, consistency must be the same for each batch. 2. Concrete workability The workability of a concrete is a measure of how easy a concrete can be placed, compacted and finished without parting of the individual materials. Workability is not the same thing as consistency. Workability is size dependent property; concrete mixes made up with smaller stones are more workable than that with larger stones.  149 3. Settlement and Bleeding Cement and aggregate particles are three times denser than water. Therefore, in concrete mix they have a tendency to to settle and displace mixing water which moves upward the concrete surface. This upward movement of mixing water is known as bleeding; water that splits from the rest of the concrete is called bleed water.  150 CHAPTER (4) Finishing Materials Prepared by Dr. Ahmed A. Younes  151 Chapter 4: Finishing Materials 8.1. Importance of Finishes 8.2. Wall finishes 8.2.1.Plastering 8.2.2.Pointing 8.2.3.Distempering 8.2.4.Painting  152 Chapter 4: Finishing Materials 4.1. Importance of Finishes Surface finishes not only make products look nice, but they also ensure the product performance for long time. Moreover, finishes protect the products from outside damaging reactions, such as corrosion, wear and rust. For example, the paint works as a protective film for the body of your car. If your car is left scratched for long time, this paint off area will likely be corroded and this eventually will damage your car body. Certain types of finishes, such as aerospace coatings, can influence the performance of the product itself. These coatings guard the aircraft from weather resistance and sun destruction, ensuring its safety in the sky.  153 4.2. Wall finishes 4.2.1.Plastering )‫(محارة‬ Plastering is a protective and decorative layer over walls or concrete surface to protect them against the atmospheric effect and give them nice appearance. The plaster is prepared by mixing sand and lime or cement concrete along with water. Plastering can be applied for various purposes. These include increasing the durability of the wall, decorating the structures of the walls, covering the uneven surface and rough walls, preventing water entrance into brick-work, and making a proper base ready for further painting works.  Requirement of Good Plaster The surface of a good plaster should be smooth, non- absorbent, not wash by water, and paintable.  154 Moreover, good plaster should be firmly attached to the base surface, not shrink when it dries, fire resistant, and sound insulated.  Plaster Defects and their Solution: If the plaster quality is not good enough it can cause one of the problems listed below. a. Plaster De-bonding Plaster de-bonding means that a plaster is disconnected from the wall. The reasons behind this phenomenon may be a thick plaster layer, insufficient substrate preparation or the use of dusty, oily or dry substrate. To avoid plaster de-bonding, we should remove any dusts or oils from the substrates, prepare the plaster in a good manner and finally add bonding chemical.  155 b. Cracks on Plastered Surface Cracks on plastered surfaces are very common problem that can be observed. Cracks are of different forms and due to different reasons. Crazing cracks are fine cracks like spider web. They happen because of the presence of excess fine content in the sand or due to dry wall on which plaster is applied, when the wall absorbs the water and fines gather on the surface, it leads to crazing. Separation cracks at joints occur at joints of two different materials because of differential thermal movement. Crack with Hollowness occurs due to hollowness in plaster because of extra water in the plaster mix or due to poor workmanship. c. Efflorescence on Plastered Surface When a newly built wall dries out, the soluble salts get out to the surface and appear as whitish  156 crystalline substances. This is called efflorescence. Efflorescence is formed on plasters when soluble salts exist in plaster itself or in one of the building materials such as bricks, sand, cement etc. It badly affects the adhesion of paints with the wall surface. To avoid this problem, all construction materials as well as plaster materials should be free from salt. d. Falling Out of Plaster Falling out of plaster from the plastered walls is of two types, flaking or peeling off. Flaking of plaster means that a small loose mass on the plastered surface is formed due to bad bonding between successive coats of plaster. Peeling off plaster means the formation of a patch in the plastered wall because plaster comes off from the surface. This is also because bond failure between successive coats of plaster. Both forms of falling out of plasters can be  157 prevented by proper material selection, surface preparation and good workmanship. e. Popping of Plaster Popping is the formations of holes that break out of the plaster. It is produced due to the existence of contaminant particles such as burnt lime or other organic constituents in the mix of mortar. Removal of any contaminants from the mortar mix will prevent popping of plaster. 4.2.2.Pointing )‫(ترتشة‬ Pointing is the finishing of mortar joints in brick or stone masonry construction. Pointing is the implementing of joints to a depth of 10 mm to 20 mm and filling it with better quality mortar in desired shape. It is done for cement mortar and lime mortar joints. Pointing finishing is applied to protect the  158 exposed surface from adverse effects due to atmospheric action like rain, sun, wind and snow, or to enhance the appearance. Flush pointing is the most available type of pointing and is generally employed in brick masonry and stone masonry. In flush pointing, mortar is pushed into the gathered joints and joints are made flush with the edge of the stone or brick to provide a uniform appearance. After that, with the help of a trowel and straight edge, edges are precisely trimmed. This type of pointing doesn‘t have a good appearance, but it doesn‘t have any space for dust and water which make it long-lasting. Recessed Pointing is another form of pointing. It has a vertical pointing face and offers a better appearance. A recessed pointing mortar is pushed back inside the wall surface using a proper pointing tool.  159 4.2.3.Distempering (‫(طالء مائي‬ Distemper is a water based paint in which the binding medium consists mainly of either glue or casein. The main ingredients of distemper are chalk, lime, water and some coloring agents if required. They are also known as cement paint; because it can be applied directly on cement walls without any other coating on them. The distempers are offered in powder form or paste form. They are to be mixed with hot water before use. As the water dries, the oil provides a hard washable surface.  Process of Distempering: The application of distemper is carried out in three successive steps (1) surface preparation, (2) prime coating and (3) distemper coating.  160 a. Surface preparation The surface to be distempered should be carefully rubbed and cleaned. The new plastered surfaces should be kept exposed for a period of two months or so to dry out before distemper is applied on them. If distemper is to be applied on previously distempered surfaces, the old distemper should be detached. b. Priming coating After preparing the surface, the surface is coated with priming coat is allowed dry. c. Distemper coating Normally two layers of distemper are applied. The first layer should be light color and applied with great care. The second coat of distemper is applied after the first coat has dried and become hard.  161 4.2.4.Painting Paints are coatings of fluid materials which are applied as a final finish to surfaces like walls, wood and metal works. Painting is done to protect the surface from the effects of weathering, to prevent wood from decay and metal from corrosion, to provide a decorative finish. Painting process can be applied to new or old wood work, new or old iron and steel surfaces, galvanized iron surface, metals and plastered surfaces. For painting new wood surfaces the following steps should be followed (1) surface preparation, the surface to be painted should be clean, dry and free of dusts or spots, (2) knotting, knots in the wood surface must be killed or covered, (3) priming, applying a prime or a first coat on the wood surface to make the surface smooth, (4) stopping, in this step nail holes and cracks are filled using putty then the entire  162 surface is rubbed with glass paper, (5) under coating, the process by which second and third coats are applied, and (6) finishing, applying the last coat on the wood surface.  163 CHAPTER (5) Dyes and Pigments Prepared by Dr. Ahmed A. Younes  164 Chapter 5: Dyes and Pigments 5.1. Introduction 5.2. Classification of dyes 5.3. Selection of Dyes 5.4. Considerations in Dye Design 5.5. Toxicological considerations 5.6. Dyes versus pigments  165 Chapter 5: Dyes and Pigments 5.1 Introduction If a molecule absorbs light in the visible region (400 nm to 750 nm) corresponding to green colour, then it will appear violet, which is the complementary colour of green. Similarily, if a dye absorbs blue colour, it will appear yellow which is the complementary colour of blue. Thus, the dyes impart colour to fabric by absorbing the complementary colour. The colour of a compound is due to the presence of certain groups containing multiple bonds. These groups which impart colour to a compound are called chromophores. Some examples of chromophores are –NO2 (Nitro), –N = O (nitroso), – N = N – (azo), quinonoid structures, etc.  166 At the same time, there are certain groups which they are not chromophores themselves but they deepen the colour when present with coloured compounds. The groups which deepen the colour of a coloured compound are called auxochromes. Some examples of common auxochromes are : –OH, –NH2, –NHR, –NR2, –Cl, –CO2H, etc. Dyes are organic compounds which are widely used for imparting colour to textiles. They are produced either chemically or from plants. An interesting point  167 about them is that unlike paint, they do not build up on the surface of the fiber but are absorbed into the pores of the material. This becomes possible because of two reasons. (1) The size of the dye molecules is smaller than the size of the pores in the fiber (2) The forces of attraction between the dye and the fiber. 5.2. Classification of Dyes Dyes can be obtained from natural sources such as vegetable matter, mineral or insects or are man-mad from petrochemical feedstock. Amongst natural dyes, indigo is well known for its brilliant blue colour and was obtained by fermenting the leaves of a plant. The red coloured lac dye is extracted from lac, a resinous protective secretion of a tiny insect.  168 A large number of dyes are used for various purposes. These are classified on the basis of their (i) Constitution or (ii) Application i. Classification based on constitution Depending upon the characteristic structural units, the dyes are classified as given in the following table:  169 ii. Classification based on applications. 1. Acid dyes These are soluble in water and are applied under acidic conditions. The acid dyestuff is mostly used for wool and silk and to a less extent nylon and acrylic fibres. The maximum quantity of dye absorbed depends on the amount of H2SO4 present in the bath. Acid dyes are inexpensive dyes. They are fast to light, but they are not fast to washing.  170 2. Azo dyes These dyes are very important because over 60% of the dyes used are azodyes. The fabric to be dyed is soaked in an alkaline solution of phenol or napthol and is then treated with a solution of diazotized amine. These are used for cotton, silk, polyster and nylon. The colour is not very fast because the interaction is only on the surface. For example, para- red is an azodye. 3. Basic dyes These dyes contain basic groups like (-NH2) group or (-NR2) group therefore these are called basic dyes. These dyes attack the anionic sites present on the fabrics and get attached to them. These are used to dye modified nylons, polyester, wool, cotton, leather,  171 paper, etc. They give good fastness and bright shades to acrylics. Aniline yellow, malachite green and crystal violet are the basic dyes. 4. Direct Dyes Direct dyes are cheap and easy to apply, but of poor fastness quality. These dyes are also known as ‗salt dyes‘ or cotton colours, which dye cotton, other vegetable fibres and viscose rayon. They are readily soluble in water. They generally bleed. To make them fast on fabric add Sodium Bicarbonate for warm colours, and Copper Sulphate for cool colours.  172 5. Disperse dyes The fibres that are most commonly dyed with disperse dyes are cellulose diacetate, cellulose triacetate and polyester fibres. To a lesser extent acrylic and nylon fibres are also dyed with disperse dyes. Polyester fibres being hydrophobic and with significant crystalline content, the assistance of high temperature, high pressure and carriers (which swell the fibre) is taken to achieve satisfactory dyeing. Some common examples of disperse dyes are celliton fast pink B and celliton fast blue B 6. Reactive dyes These dyes attach to the fibre themselves by irreversible chemical reactions. As such their fastness properties are excellent. The fibres most readily coloured with reactive dyes are natural and man- made cellulosic fibres, natural protein fibres and  173 polyamide fibres. With some reactive dyes, the dyeing can be carried out at room temperature. However with most reactive dyes, the dyeing is carried out at high temperatures (upto the boil). Dyes which are drivatives of 2, 4 dichloro –1, 3, 5 – triazine are important examples of fibre reactive dyes. 7. Mordant Dyes These dyes require an additional substance (generally a metal ion) for fixing to the fibre. These are used mainly for dyeing wool. The method involves the precipitation of certain mordant material (binding agent) on the fabrics which then combines with the dye to form an insoluble coloured complex called lake. For acid dyes, metal ions are used as mordants but for basic dyes, tannic acid is used as the mordant. For example, alizarin is a mordant dye. It gives a rose  174 red colour with Al3+ and a blue colour with Ba2+, a brownish red colour with chromium (Cr3+) and a black violet with iron mordant. 8. Vat dyes They are insoluble in water, but they are made soluble by the use of a strong reducing agent, such as Sodium hydrosulphite dissolved in sodium hydroxide. These are the fastest dyes for cotton, linen and rayon. They also may be applied to wool, nylon, polyester etc.  175 5.3. Selection of Dyes To select the proper dye for a fibre, it is necessary to know which dyes have an affinity for the vegetable, animal, or man-made fibres. In general, the dyes used for cotton and linen may be used for viscose rayon, but other fibres having different chemical structures require different dyes. Direct dyes are the easiest to produce, the simplest to apply, and the cheapest in their initial cost as well as in application. They, however, like other dyes have their own limitations. One of these is the degree of colour fastness. Fastness of colour refers to its ability to remain unchanged.  184 mammalian systems. In the case of azo dyes, the enzyme-mediated formation of genotoxic aromatic amines as metabolites must be considered, since it is possible that the intact dye is safe but not all of its metabolites. 5.6. Dyes versus pigments Coloring materials are usually classified as dyes or pigments. Dyes are chemically bound to a substrate such as a textile, whereas pigments require another substance, called a binder, to help them adhere to the substrate. The human body may have served as the first substrate for dyes, but textiles or fabrics have been the most common over the ages. In addition to textile fibers, dyes have been applied to paper, wood, food, cosmetics, fur, and leather.  185 Because pigments do not dissolve in most media, such as water, they require a binder in order to be applied to a substrate. No chemical reaction actually takes place between the pigment and the binder. The binder serves only to incorporate the pigment onto the surface of the substrate, providing another contrast with dyes, which permeate the substrate fully. Normally, binder and pigment are mixed mechanically and then applied together. To serve as paints, pigments are mixed with a carrying agent or solvent as well as a binder. The purpose of the solvent is to keep the mixture fluid until application is complete, after which it evaporates. Chalks and crayons contain pigment and binder but no solvent. The binder in oil paints is linseed oil (from flax), tung oil (from the tung tree), or any of a variety of other (usually) plant products; the solvent is an organic material like turpentine.  186 During the drying process the solvent evaporates, and double bonds between carbons (C=C) in the binder combine with other binder molecules, or polymerize, to form larger and larger molecules (polymers). A large number of pigments are mined or manufactured for the commercial preparation of paints. About 45 year back, white lead [2PbCO3 + Pb (OH)2], Zinc oxide (ZnO) and lithopone (ZnS + BaSO4) were the principal white pigments in use while the coloured pigments consisted of Prussian blue, lead chromates, various iron oxides and a few lake colours.  187 CHAPTER (6) Introduction to Corrosion Prepared by Dr. Ahmed A. Younes  188 Chapter 6: Introduction to Corrosion 6.1. What is corrosion? 6.2. Steps of electrochemical reaction 6.3. Costs of corrosion 6.4. Polarization 6.5. Types of corrosion 6.6. Corrosion Protection and Control  189 Chapter 6: Introduction to Corrosion 6.1. What is corrosion? Corrosion is an irreversible damage of metals by reaction with its surrounding environment. Corrosion of metals typically produces oxide(s) or salt(s) of the original metal.  190 6.2. Steps of electrochemical reaction 1. Mass transfer step: this involves the transfer of ions from the bulk of solution to the electrode surface through the diffusion layer (transfer of Cu2+ from the bulk solution to the cathode surface). 2. Charge transfer step: this involves redox reaction between ions and electrons at the electrode surface. 3. Mass transfer step: this involves the transfer of ions from the electrode surface to the bulk of solution through the diffusion layer (transfer of Zn2+ from the anode surface to the bulk of solution).  191  A redox reaction results when an oxidation and a reduction half-reaction are combined to complete a transfer of electrons as in the following example: Zn(s) + Cu2+(aq) →Zn2+(aq) + Cu(s)  The electrons are not shown because they are neither reactants nor products but have simply been transferred from one species to another (from Zn to Cu2+ in this case). In this redox reaction, the Zn(s) is referred to as the reducing agent because it causes the Cu2+ to be reduced to Cu. The Cu2+ is called the oxidizing agent because it causes the Zn(s) to be oxidized to Zn2+.  A measure of the tendency for a reduction to occur is its reduction potential, E, measured in units of volts. At standard conditions, 25 °C and  192 concentrations of 1.0 M for the aqueous ions, the measured voltage of the reduction half- reaction is defined as the standard reduction potential, E°.  The standard reduction potentials are –0.76 V for zinc and +0.34 V for copper. The more positive (or less negative) the reduction potential, the greater is the tendency for the reduction to occur. So Cu2+ has a greater tendency to be reduced than Zn2+. Furthermore, Zn has a greater tendency to be oxidized than Cu. The values of E° for the oxidation half- reactions are opposite in sign to the reduction potentials: +0.76 V for Zn and –0.34 V for Cu. 193  194 6.3. Costs of corrosion (1) Direct economic losses 1. Replacement of corroded structure 2. Extra cost of using corrosion inhibitors (2) Indirect costs 1. Loss of products by leaks, such as oils, gas or water 2. Loss of production until the corroded part is replaced 3. Loss of efficiency, due to the deposition of corrosion products 4. Product contamination  195 6.4. Polarization Not all oxidation processes result in deterioration of material! Polarization: is decreasing the efficiency of an electrochemical reaction by disturbing its equilibrium conditions (e.g., lowering corrosion rates by adding a corrosion inhibitor to increase the polarization of either the cathode or anode). We have two types of polarization: concentration and activation polarization. Polarization is inversely proportional to the corrosion rate. Thus, increasing polarization decreases corrosion. Passivity: Some metals and alloys under particular environmental condition loss their chemical reactivity and become inert (passive). Passivity is the formation of a thin protective film (1-10 nm thickness) on a metal surface due to its reaction with the surrounding  196 environment under oxidizing conditions. For the formed passive layer to be protective it should be: (1) adherent (2) non-porous, dense and compact)., e.g., the formation of oxide layers of Al2O3, TiO2, Cr2O3. This is why Cr is added to Fe to prepare stainless steel. 6.5. Types of corrosion Corrosions are of two main types, general and localized corrosion 1. General (Uniform) Corrosion: This type of corrosion involves a uniform rate of metal damaging over the entire surface area that is exposed to the corrosive environment. For example, dissolution of Zn and Al metal in acids or iron rusting; steel exposed to environment. Uniform  197 corrosion can be caused by atmospheric attack or galvanic corrosion. a. Atmospheric corrosion It takes place due to the direct contact between a metal and its surrounding atmosphere. It involves formation of metal oxide layer, e.g., rusting of iron. The corrosion rate is influenced by several factors, such as type of metal, type of atmosphere (dust,  198 humidity, pollutant. Water vapor, gases, etc.), temperature (the corrosion rate increases with increasing temp.), time of exposure (the extent of corrosion increases with increased exposure time), and metal surface conditions (more roughness, more corrosion). b. Galvanic Corrosion It is a form of corrosion that happens when two dissimilar metals are physically connected in the presence of an electrolyte. The factors affecting galvanic corrosion are potential difference between anode and cathode, distance between anode and cathode and cathode/anode (C/A) area ratio.  199 2. Localized corrosion Localized corrosion takes place within localized crevices or holes within the metal surface. There are two main forms, pitting and erosion corrosions. a. Pitting Corrosion This form is extremely localized corrosion that leads to the creation of small holes in the metal surfaces. This type of corrosion is highly destructive. The rate of pitting corrosion increases when there is lack of oxygen around a small area; this area becomes anodic while the area with excess of oxygen becomes cathodic. Moreover, stagnant solutions accelerate the pitting corrosion rate. In order to prevent or minimize  201 6.6. Corrosion Protection and Control Recognizing the symptoms and mechanism of a corrosion problem is an important preliminary step on the road to finding a convenient solution. There are basically five methods of corrosion control: 1. Material Selection 2. Design modifications to the system or component 3. Changing the Environment 4. Changing the Metal Potential 5. Use of protective coatings (Zn, Sn, paints, plastic, … etc.)  202 1. Corrosion Control by Material selection (selection of proper material depends on the environment) synthetic), plastics, ceramics, carbon and graphite, and wood] 2. Corrosion Control by Proper Design: Corrosion can be minimized or eliminated by proper design, installation, and operation of systems. 3. Corrosion Control by Changing the Environment Removal of O2 from the system Control of pH (H+ ions) Addition of masking/complexing agents to remove the aggressive ions Addition of corrosion inhibitors to decrease the rate of oxidation and/or reduction reactions  203 Reactants of the cathodic reaction accompanying the corrosion process 4. Corrosion Control by Changing the Metal Potential.  Anodic Protection shifts metal potential into a range where passivation occurs. This is done by applying an external DC current to the anode (prior to its use) to form a protective passive film.  Cathodic Protections shifts the metal potential to a more cathodic value (to negative direction). This is done by two different methods. (a) Cathodic protection with impressed current (b) Cathodic protection with sacrificial anodes The CP is widely used to protect (1) steel tanks and pipelines of water and fuel, (2) ships, (3) chemical  204 processing equipment (4) offshore oil platforms and onshore oil well casings.  Cathodic Protection Methods (a) Sacrificial Anode Method Current is supplied by connecting the object to a more reactive (easily corroded) metal as a sacrificial anode. i.e. formation of galvanic cell in which the sacrificial anode is consumed (sacrificed). (b) Impressed Current Method Here, an external current is applied from a DC source (e.g. a battery) and auxiliary anode. i.e. an electrolytic cell is formed where corrodible or inert auxiliary anode can be used.