Materials and Corrosion - Coursework Example

Corrosion is the chemical decay of materials which include metals, alloys, polymers and ceramics. It is the disintegration of engineered materials into their constituent components as a result of certain chemical reactions. It can be of various types depending upon the chemical composition of the material, the surrounding environment of the material and it sometimes also depends upon the shape and function of the object of which the corroding material is a part. Very often corrosion results in the formation of oxides and salts and the overall physical properties of the materials affected by corrosion are changed rendering the material useless in most of the cases. The major types of corrosion are discussed in the following paragraphs. Crevice Corrosion Corrosion occurring within or adjacent to crevices, which are commonly left at joints, due to small volumes of stagnant corroding fluids is known as crevice corrosion. The most important feature of this type of corrosion is the contact with stagnant solution which takes place in interstices, near seals and in cervices made for nuts and rivet heads. The metals and alloys used in chemical plants and other industrial applications are covered with passive films of protective coatings in order to prevent contact with high concentrations of Cl- and H+ ions. When such places are left unclean for long durations after long usages of the plant, sand and other substances also get deposited near metals and alloys which prevent the proper application of protective coatings. In such a scenario the crevices and interstices are more likely to come in direct contact with corroding media. The corroding fluids which commonly contains high concentrations of chloride ion is likely to get accumulated in the crevices made for nuts, rivets or any other type of hole. The prolonged contact with such chemical fluids starts the oxidation of the metals. The direct contact with aerated chloride rich media creates a differential aeration cell which destroys the passivity of the material. The oxidized metal starts dissolving into the chemical which is in contact with the metal, in the presence of oxygen and the prolonged contact can corrode the material completely. The best way to prevent crevice corrosion is to prevent crevice corrosion is to prevent crevices. This can be done by using welds in place of bolts and rivets in joints. The design of various components should be done in such a way that there is always a drainage mechanism for the accumulating fluid so that the harmful corroding liquids do not come in contact with the metals for long duration of time. Other effective means of preventing crevice corrosion is the use of corrosion inhibitors and coatings, some of which are discussed below. Inhibitors 1. VpCl-337 which is a vapor phase corrosion inhibitor is also very effective for preventing crevice corrosion. The crevices are fogged by the material and thus large areas of material are coated against corrosion. 2. Ecoline 3690 which is a biodegradable and bio based inhibitor is also very effective for using for crevice corrosion prevention because of its ability to displace moisture and provide protection against aggressive environments. It is very suitable for use in marine environment and high humidity conditions. 3. VCI (Vapor Corrosion Inhibitor) Foam is also a very useful inhibitor for preventing crevice corrosion. It can be used in the form of a foam pad cut into the size of the space where it is to be used. It is very easy to apply as compared to other vapor inhibitors. 4. VCI 2000 is particularly useful in pipes as it can be applied on the crevices on pipes during fabrication. Protective Coatings 1. The most commonly used coating for crevice corrosion is a mixture of PdO and TiO which is applied in the form of a 0.4 pm thick coating. The mixture is found effective in all kinds of conditions where there is a prolonged contact with chemical fluids. 2. Another coating which is present with a brand name of Reactive Gel (RG 2400) is also suitable for prevention of crevice corrosion. The material is applied in the form of a gel which develops a thin glass like surface after reacting with elements in the metal surface. The surface prevents the direct contact of harmful chemicals with the material. Microbial Corrosion The deterioration of materials, particularly metals and alloys, due to the metabolic activity of microorganisms is known as microbial corrosion. It is also called biological corrosion because it is the type of corrosion which involves biological organisms instead of chemical compounds. There are numerous types of bacteria which are responsible for microbial corrosion. These bacteria are very commonly found in environment and include both aerobic and anaerobic bacteria. Most of the cases of microbial corrosion are associated with sulfate reducing bacteria which produce hydrogen sulfide during their metabolic activity and thus cause the corrosion of metal surface due to the presence of hydrogen sulfides. Microbial corrosion occurs in the presence of bacteria which consume nutrients and other elements found in waters and soils. The consumption of nutrients by bacteria results in disturbance of the chemical balance in the region and very often results int the formation of an acidic media which is harmful for metals and other materials such as polymers and ceramics as well. The most common locations in a chemical plant affected by microbial corrosion include stainless steel tanks, pipelines and joints. Proper drainage and regular cleaning can minimize the risk of microbial corrosion (Moncmanova, 2007). Inhibitors 1. Caffeic Acid is a good inhibitor of microbial bacteria which is applied in the form of a film over the metal which is to be protected against microbial corrosion. 2. Chlorogenic acid is also very commonly used as an inhibitor for microbial bacteria as it stops the growth of bacteria, however acts as a passive medium when in contact with metal surface. 3. Oil soluble corrosion inhibitors like Morpholine which is non degradable by bacteria and stops the activity of bacteria is also effective for using in pipelines. 4. 8-hydroxy-N′-(2-(quinolin-8-yloxy)acetyl)quinoline-5-sulfonohydrazide or HQS is also a very effective inhibitor and its performance is considerably enhanced in the presence of biocides. Protective Coatings 1. Zinc rich epoxy primer present in the market with the name of sigmazinc is very effective for protection against microbial corrosion due to its fast drying ability and long term protection against bacteria. 2. Epoxy tiecoat is also useful due to its strong adhesive properties. It covers the surface of metal completely developing a sealed surface which protects it from the action of harmful chemicals originating from bacterial activity. Epoxy tiecoats also prevents the growth of bacteria. Corrosion Fatigue The mechanical degradation a material under the joint action of corrosive environment and cyclic loading is called corrosion fatigue. The material suffering from corrosion fatigue fails at lower values of load than the actual strength of the material against cyclic loading. The materials fail due to the development of cracks due to the simultaneous action of corrosion and cyclic stresses. The fatigue strength of materials reduces considerably in an aggressive environment and thus it is necessary to take preventive measures to stop corrosion of materials subjected to cyclic loading. Corrosion fatigue can be prevented by controlling the vibrations occurring in a chemical plant. Certain inhibitors and coatings can also be applied for preventing and delaying the corrosion fatigue cracks from originating (Ahmed, 2006). Inhibitors 1. Sodium Molybdate is found to be very effective for inhibiting corrosion fatigue. 2. Organofunctional silanes are also very useful for using as corrosion inhibitors against corrosion fatigue in aluminum alloys. 3. Vapor Corrosion Inhibitors can be used for protection against corrosion fatigue in steel and alloys of titanium. 4. Other organic inhibitors which are application specific can be formed by the combination of amines and carboxylic acids. Protective Coatings 1. High solid zinc epoxy primer can prolong the initiation time of fatigue corrosion cracks. 2. Zinc phosphate alkyd primers are also useful where aesthetics of the material to be coated are also to be considered. Fretting Corrosion The corrosion occurring at the rubbing contact of two metal surfaces is known as fretting corrosion. The phenomenon is commonly found in places where the materials are subjected to compressive stresses in addition to the occurrence of small vibrations. The best way to prevent fretting corrosion is through lubrication or by the use of hard materials considerably resistant against abrasion. The abrasion of the top surfaces makes it easy for the corroding media to penetrate the material and thus the corrosion rate is considerably enhanced. Sealant materials, protective coatings and inhibitors can be used to absorb vibrations and exclude oxygen and moisture for preventing corrosion. Inhibitors 1. Passive lubricants which serve the purpose of lubrication as well as inhibition can be the best available inhibitors. 2. Organic alkyd inhibitors are suitable for use when fretting corrosion occurs in the mechanical parts of electronic equipments. 3. Molybdenum disulfide is also very effective for inhibiting fretting corrosion in steel. 4. Aerosol lubricants with inhibition capabilities can also be used in parts subjected to high abrasive action. Protective Coatings 1. Solvent free epoxy flow coatings can be used for minimizing corrosion and provide lubrication at the same time. 2. Coal tar epoxy coating is another medium which can provide a lubricated finish while protecting the materials against corrosion. Cathodic Prevention The material to be protected against corrosion can be made the cathode of an electrolytic cell; such a process for corrosion prevention is known as cathodic prevention. During the process the anodic dissolution of metallic structure is reduced through the reduction of electrical potential energy difference between the anodic and cathodic sites in the corroding media. Sacrificial anode cathodic protection is one of the various ways in which cathodic protection is carried out. The alloys of commonly used metals are made anodes; the selection is done on the basis of electrochemical series. The nature of corroding media which acts as the electrolyte also dictates the metal or alloy used as anode. During the electrolytic process, the alloy from anode gets deposited on the corroding cathode and in this way it is protected. Another way of doing cathodic protection is by the use of impressed current cathodic protection technique in which the anode is inert. The basic mechanism for cathodic prevention is same for different types of corrosion however the selection of anodes and the electrolytes is also dictated by the type of corrosion against which cathodic prevention is being employed (McCafferty, 2010). References Ahmed, Z. (2006) Principles of Corrosion Engineering and Corrosion Control, Butterworth-Heinemann. Buchheit, R. (2004), Corrosion and Protection of Light Metal Alloys, The Electrochemical Society. McCafferty, E. (2010) Introduction to Corrosion Science, Springer. Moncmanova, A. (2007) Environmental Deterioration of Materials, WIT Press. Sastri, V. (1998) Corrosion Inhibitors: principles and applications, Wiley. Read More