Corrosion Chemistry Module 3 PDF

MODULE 3: Corrosion Science and E-waste Management Corrosion Chemistry Corrosion is defined as “the destruction or deterioration of metals or alloys by the surrounding environment through chemical or electrochemical changes. The familiar examples of corrosion are i. Rusting of iron – A reddish...

MODULE 3: Corrosion Science and E-waste Management Corrosion Chemistry Corrosion is defined as “the destruction or deterioration of metals or alloys by the surrounding environment through chemical or electrochemical changes. The familiar examples of corrosion are i. Rusting of iron – A reddish brown scale formation on iron and steel objects. It is due to the formation of hydrated ferric oxide. ii. Green scales formed on copper vessels. It is due to the formation of basic cupric carbonate [CuCO3 + Cu(OH)2]. The metals undergo corrosion due to the oxidation by losing electron and the resulting product is called as rust, which mainly contains oxides, sulphates, carbonates and bicarbonates of the metals. Due to corrosion, metals lose their valuable properties such as conductivity, strength, shining, malleability, ductility etc. Cause for corrosion: Each metal tries to get original state such as ore form (oxides, sulphates, carbonates and bicarbonates of the metals) which are more stable and low energetic. Therefore corrosion is the reverse process of metallurgy. Classification of corrosion Chemical corrosion (Dry corrosion) Chemical corrosion occurs due to the direct chemical reaction between the metal and the gases present in the corrosion environment. This type of corrosion is generally observed in the absence of moisture. Therefore it is also known as dry corrosion. Example: Oxidation of metals or alloys on exposure to oxygen in air. Electrochemical corrosion (Wet corrosion) Electrochemical corrosion involves reactions in aqueous medium and moist-air. The conducting surface of the metal undergoes an electrochemical reaction with the moisture and oxygen present in the atmosphere. This process can be explained on the basis of electrochemical theory of corrosion. Electrochemical theory of corrosion Electrochemical theory of corrosion explains the corrosion on the basis of galvanic cell formation. According to this theory, corrosion of metal takes place due to the formation of anodic and cathodic region on the same metal surface or when two different metals are in contact with each other in the presence of a conducting medium. At anodic region metal undergo oxidation by losing its valance electron and gets corroded. While at cathodic region reduction reaction takes place. Thus cathodic region is unaffected by the cathodic reaction. The electrons liberated at the anodic region migrate to the cathodic region constituting corrosion current. The metal ions liberated at the anode and the anions formed at the cathode diffuse towards each other through the conducting medium and forms a corrosion product. When a part of the metal is dipped in an aqueous solution, the part which is exposed to lower concentration of air (or oxygen) acts as anode, while the part which is exposed to higher concentration of air (or oxygen) acts as cathode. (Because oxygen as greater tendency to absorb electrons) Iron More O2, (Cathode) Less O2, (Anode) Wate Corrosion reaction At anode region metal undergo oxidation liberating electron. M Mn+ + ne- Fe Fe2+ + 2e- Cathodic reaction depends on the nature of the corrosion environment. a) If the surrounding environment is aerated and almost neutral, oxygen and water are reduced - to OH ions. 1 O2  H 2 O  2e  2OH  2 b) If the surrounding environment is de-aerated and almost neutral the cathodic reaction involves the liberation of hydrogen gas and hydroxyl ions. 2H2O + 2e- 2OH- + H2 c) If the surrounding environment is deaerated and acidic the cathodic reaction involves the evolution of hydrogen gas. 2H+ + 2e- H2 Corrosion of iron produces Fe2+ ions and OH- ions at the anodic and cathodic region respectively. These ions diffuse towards each other forming insoluble product Fe(OH)2 (ferrous hydroxide) 2Fe2+ + 4OH¯ 2Fe(OH)2 In an oxidizing environment ferrous hydroxide is oxidized to ferric oxide.  4Fe(OH)2 + O2 + 2H2O 2[Fe2O3·3H2O] Rust In the presence of limited oxygen, ferrous hydroxide is converted into magnetic oxide of iron (Fe3O4) and is known as black rust. 3Fe(OH)2 + ½ O2 Fe3O4·3H2O Black Rust Note: 1 (i) The cathodic and anodic process occurs at the same rate. (ii) The formation of galvanic cells on the metal surface is due to the following reasons. (a) Contact with other metal (b) Difference in the concentration of air or oxygen. (c) Stress and strain on the material. (d) Precipitation at the grain boundaries. Note: 2 An arrangement of metals in the order of their corrosion resistance in the given environment is referred to as galvanic series. Types of Corrosion Differential metal Corrosion or Galvanic Corrosion: When two dissimilar metals are in contact with each other in a corrosion conducting medium, the metal with lower electrode potential becomes anodic and undergoes corrosion, whereas, the metal with higher electrode potential becomes cathodic and remains unaffected. This kind of corrosion is called as differential metal corrosion. The rate of corrosion depends on the difference in electrode potential. Higher the difference faster is the rate of corrosion Example: When Zinc is in contact with Copper, Zinc metal being placed higher up in the electrochemical series acts as anodic and gets corroded, whereas Copper metal which is placed below Zn in electrochemical series is protected from corrosion. Zn (anode) Cu (Cathode) (Oxidation) (Reduction) Zn Cu  1 Zn Zn 2  2e  O2  H 2 O  2e  2OH  2 This type of corrosion can be observed in the following (i) Steel pipe connected to copper plumbing (ii) Lead antimony solder around copper wires Differential aeration corrosion Differential aeration corrosion occurs when one part of the metal is exposed to lower concentration of air when compared to other part. When a metal is exposed to different concentration of air (O2), part of the metal exposed to lower concentration of O2 will having lower potential becomes anodic and undergoes corrosion. Other part of the metal which is exposed to higher concentration of O2 becomes cathodic and remains unaffected. Thus a differential aeration of metal causes a flow of current called as differential current. Example: When an iron rod is partially immersed in water, the part exposed to atmosphere is more oxygenated and forms cathode and the part immersed in water which is less oxygenated forms anode. There are two types of differential aeration corrosion. i.Pitting corrosion ii.Water line corrosion i. Pitting corrosion Pitting corrosion results when small particle of dust particle or oil drop or moisture (water drop) gets deposited on the surface of the metal. The portion covered is less aerated compared to the large exposed area and thus acts as anode with respect to the surface exposed. Corrosion takes place below the deposit resulting in the formation of pit. Once a pit is formed corrosion rate accelerated due to further decrease in concentration of O2 within the pit. ii. Water line corrosion Water line corrosion can be observed in steel tanks partially filled with water. The area above the water line, which is more oxygenated acts as cathode and is unaffected by corrosion, whereas corrosion takes place along a line just beneath the level of water meniscus because it is exposed to lower oxygen concentration. Mort intense corrosion is observed just below the water line, hence it is called as water line corrosion.  More oxygen, Rust (Cathode) Less Oxygen (Anode) Water Examples for differential aeration corrosion: i. Part of the nail inside the wall being exposed to lower oxygen concentration than the exposed part undergoes corrosion. ii. Window rods inside the frame suffer corrosion but not the exposed parts. iii. Metal surface under dirt, dust, scale or water undergoes corrosion. iv. Paper pins inside the paper get corroded and the exposed part is free from corrosion. v. Partially buried pipe line in soil or water undergoes corrosion below the soil or water. Whereas the exposed part is free from corrosion. Corrosion Control Corrosion of metal is a natural spontaneous process, by which a metal is converted in to more stable compound state. Therefore corrosion control is more realistic than corrosion prevention. The corrosion types are so numerous, the mechanism of corrosion are so different and conditions under which corrosion takes place are so varied that no single method can be used to control all possible corrosion cases. Some of the important methods used in controlling corrosion of metals are, 1. Protective Coating The Protective coating protects the metal from corrosion by acting as a barrier between the metal and the corrosive environment. The principle types of coating applied on the metal surface are, Metal Coating: The process of covering the base metal with a layer of another metal is known as metal coating. By this method the base metal can be protected. Metal coating can be anodic metal coating or cathodic metal coating. Anodic Coating: Anodic coatings are produced by coating a base metal with more active metals which are anodic to the base metal such as Zn, Al, Mg, etc., The important characteristics of anodic coating are that, even if the coating is ruptured, the base metal does not undergo corrosion. The exposed surface of the base metal is cathodic w.r.t. the coating metal and the coating metal preferentially undergoes corrosion. The protection is ensured as long as the anodic coating metal is still present on the surface. Therefore anodic metal coating is also known as sacrificial coating. Example: Galvanizing is familiar anodic coating and is extensively used to protect iron and steel objects. Galvanization: It is a process of Coating a base metal with zinc metal. It is carried out by hot dipping method. Hot dipping method: It involves the dipping of base metal in molten anodic metal (Zn). The coating metal should melt at a relatively low temperature and the base metal must with standing this temperature without undergoing any changes in its properties. The galvanization process involves the following steps. (a) The metal surface is washed with organic solvent to remove oil or grease present on the surface. (b) Rust and other deposit are removed by washing with dilute H2SO4. (c) The clean and dry sheet is passed through aqueous solution of zinc chloride and ammonium chloride flux and dried. The flux helps the molten metal to adhere (adsorb) on the metal surface. (d) The article is then dipped in a bath of molten Zinc maintained at 425-430oC. (e) The excess Zn on the surface is removed by passing through a pair of hot rollers which wipes out excess of coating and produces a thin coating. Galvanization is used extensively to protect iron from corrosion in the form of roofing sheets, fencing wire, buckets, bolts, nuts, nails, screws, pipes, tubes etc. Galvanized steel are used in construction where high degree of corrosion resistance is required. Galvanized articles are not used for preparing and storing food stuffs since zinc dissolve in dilute acids producing toxic Zinc compound. Surface conversion Coating Inorganic coatings are generally chemical conversion coatings. A surface layer of metal is converted into a compound, by chemical or electrochemical reactions, which forms barrier between the underlying metal surface and the corrosion environment. The chemical conversion coatings are different from other types of coatings in the sense that, they are the integral part of the metal itself. This types of coatings formed on the metal surface by electrolytic method. In addition to the corrosion resistance, also provide increased electrical insulation and enhanced adherence for paints and other similar organic coatings. Anodization Anodization is the process of oxidation of outer layer of metal to its metal oxide by electrolysis. Oxide layer formed over the metal itself acts as protective layer. A protective oxide film generally produced on nonferrous metals like Al, Mg, Cr, Zn, Ni, and their alloys by anodic oxidation process, in which the base metal is made as anode in electrolytic bath of suitable composition (Chromic acid, H2SO4, H3PO4, H2CrO4) and by passing direct current. Lead is generally used as cathode. The anodic oxide film formed on Al in the electrolyte bath tends to be porous and provides good adherence for paints and dyes. The strength and corrosion resistance of the anodized film can be increased by the so called sealing, which involves heating in boiling water or steam or metal salt solution. The treatment changes porous alumina at the surface of coating in to its mono hydrate (Al2O3.H2O), which occupies more volume, thereby pores are sealed. Anode : Aluminium article Cathode : Steel or Copper Temperature : 35OC Electrolyte : 10% sulphuric acid or 5-10% chromic acid or oxalic acid Current density: 10-20 mA/cm2 Electrode reaction At anode : 2Al(s) + 3H2O (l)  Al2O3 + 6H+ + 6e- At cathode : 6H+(aq) + 6e-  3H2(g) Net cell reaction : 2Al(s) + 3H2O (l) Al2O3 (s) +3H2 (g) Anodized articles are used as exterior for roofs, walls, and window frames, soap boxes, Tiffin carriers etc. 2. Cathodic protection Cathodic protection is a method of protecting a metal or alloy from corrosion by converting it completely into cathodic and no part of it is allowed to act as anode. Principle: Corrosion can be prevented by eliminating the anodic sites and converting the entire metal into cathodic area. Cathodic protection can be achieved by sacrificial anodic method. Sacrificial anodic method. In this method the base metal to be protected from corrosion is brought in contact with more anodic metals like Zn, Mg, Al, etc., (i.e. metal having lower standard electrode potential compared to the base metal). These metals are called as sacrificial metals. The sacrificial metals acts as anode and hence it undergo corrosion, while the base metal acts as cathode and so it is protected. Anode is periodically replaced by fresh block of sacrificial metals. This type of cathodic protection is used to protect buried pipe line, underground cables, water tanks, boilers, ships etc., from corrosion. Example: To protect Cu from corrosion, it is brought in contact with Zn. Zn having lower standard electrode potential acts anode and get corroded, while Cu having higher standard electrode potential acts as cathode and get protected from corrosion. e- Zinc stripe (Anode) metal to be protected (Cathode) Corrosion Penetration Rate (CPR) The speed at which any metal in a specific environment deteriotes due to a chemical reaction. It gives the amount of corrosion loss per year in thickness and the speed at which corrosion spreads to the inner portions of a material. It is expressed interms of mils penetration per year (mpy) or millimeter per year (mmpy). Multiple data are needed in estimating the corrosion rate of any given metal including, (i) Weight lost (the decrease in weight of the metal during the period of reference), (ii) The density of the metal, (iii) The total surface area present initially and (iv) Length of time taken. Weight loss method It is commonly used method for the measurement of uniform corrosion. It involves the exposure of a weighed piece of test metal or alloy to a specific environment for a precise time. This is accompanied by thorough cleaning to remove the corrosion products and then determining the weight of the lost metal as a result of corrosion. The corrosion rate is best described in terms of the thickness or weight loss where the surface of the metal disintegrates uniformly across the area that has been exposed. The corrosion penetration rate (CPR) iscalculated by using the following mathematical relation: CPR=KW/ ρAt Where, K=constant, K=534, if CPR is in mils per year (1 mil= 0.001 inch) or K= 87.6 if CPR is in millimeter per year (1 inch= 2.54 cm or 25.4 mm) W= total weight lost after exposure time ‘t’ in mg t= timetaken for the loss of specimen A= the surface area of the exposed specimen cm2 for mm/ year ρ= the specimen density in g/ cm3 E-waste Management Waste management (or waste disposal) is the activities and actions required to manage waste from its inception to its final disposal. This includes the collection, transport, treatment and disposal of waste, together with monitoring and regulation of the waste management process. Waste can be solid, liquid, or gaseous and each type has different methods of disposal and management. Waste management deals with all types of waste, including industrial, biological, and household. Waste management is intended to reduce adverse effects of waste on human health, the environment or aesthetics. A large portion of waste management practices deal with solid waste, electronic-waste and biomedical waste which are the bulk of the waste that are created by household, industrial, medical, agricultural and commercial activity. 1. Electronic Waste Electronic waste or e-waste describes discarded electrical or electronic devices. E-waste or electronic waste is created when an electronic product is discarded after the end of its useful life. The rapid expansion of technology means that a very large amount of e-waste is created every minute. Sources Sources of e-waste includes  IT equipment such as computers, laptops, networking devices, cables, power adapters;  Household appliances like televisions, telephones, mobile phones, calculators, fridge, air conditioners, washing machines, microwaves, gaming consoles; electrical and electronic devices such as tube lights, bulbs, LED lights, remote controls, electronic toys, treadmill;  Medical devices such as monitoring and control equipment, ultrasound testing machines, X- ray, stabilizers etc.  Electronic waste is the fastest growing segment in the waste generated across the globe.  Increasing mass production increases the amount of E-Waste leading to higher levels of pollution and hazardous effects on the planet. Types The types of e-waste are listed down below:  Major Appliances  Small Appliances  Computer and Telecommunications Appliances  Lighting Devices  Electrical and electronic tools  Electronic Toys  Medical devices  Monitoring Devices  Vending Machines Effects of e-waste on environment and human health Effect on environment: The effects of e-waste on the environment can be devastating. Even though the long-term effects of e-waste are still unknown, it certainly has some negative impact on soil, water, and air quality. These are all necessary parts of a healthy planet. 1. Impact on the soil First, e-waste can have a damaging effect on the soil of a region. As e-waste breaks down, it releases toxic heavy metals. Such heavy metals include lead, arsenic, and cadmium. When these toxins leach into the soil, they influence the plants and trees that are crowing from this soil. Thus, these toxins can enter the human food supply, which can lead to birth defects as well as a number of other health complications. 2. Impact on the water E-waste that is improperly disposed of by residents or businesses also leads to toxins entering groundwater. This groundwater is what underlies many surface streams, ponds, and lakes. Many animals rely on these channels of water for nourishment. Thus, these toxins can make these animals sick and cause imbalances in the planetary ecosystem. E-waste can also impact humans that rely on this water. Toxins like lead, barium, mercury, and lithium are also considered carcinogenic. 3. Impact on the air When e-waste is disposed of at the landfill, it’s usually burned by incinerators on site. This process can release hydrocarbons in the atmosphere, which pollutes the air that many animals and humans rely on. Furthermore, these hydrocarbons can contribute to the greenhouse gas effect, which many scientists think is a leading contributor to global warming. In some parts of the world, desperate people sift through landfills in order to salvage e-waste for money. Yet, some of these people burn unwanted parts like wires in order to extract copper, which can lead to air pollution as well. Effect on human health: Electronic waste contains toxic components that are dangerous to human health, such as mercury, lead, cadmium, polybrominated flame retardants, barium and lithium. The negative health effects of these toxins on humans include brain, heart, liver, kidney and skeletal system damage. It can also considerably affect the nervous and reproductive systems of the human body, leading to disease and birth defects. Improper disposal of e-waste is unbelievably dangerous to the global environment, which is why it is so important to spread awareness on this growing problem and the threatening aftermath. To avoid these toxic effects of e-waste, it is crucial to properly e-cycle, so that items can be recycled, refurbished, resold, or reused. The growing stream of e-waste will only worsen if not educated on the correct measures of disposal. Pollutants Health Effects Source of Constituents Lead  Cause to damage the central and Available in solder in peripheral neural system, blood printed circuit boards, systems and kidney  It effects badly on child brain glass panels, and gaskets development, damage to the circulatory in computer monitors. system and kidney. Cadmium  Irreversible toxic effects on human Available in chip health. resistors and  It accumulates in kidney and liver. semiconductors.  Damage neural  Mercury  Cause chronic damage to the brain. Available in relays and  Cause respiratory and skin disorders switches, and printed due to Bio-accumulation in fishes. circuit boards. Chromium  It causes bronchitis. Available in galvanized steel plates and decorator or hardener for steel housing. Plastic and  While burning produces dioxin Available in Cabling and PVC that causes reproductive and developmental problems. computer body. Brominated  It disrupts endocrine system functions Available in electronic Flame equipment and circuit retardants Boards. Barium  It cause muscle weakness and Present in front panel of phosphorus damage to heart, liver, and spleen. CRTs. and heavy metals. copper  It causes stomach cramps, nausea, liver Present in copper wires, damage, or Wilsons disease. printed Circuit board Tracks. Nickel  Causes allergy to the skin results Present in nickel- dermatitis while allergy to the lung cadmium rechargeable results in asthma. batteries. Lithium  It can pass in to breast milk and may Present in Lithium-ion harm a nursing baby. battery Beryllium  It is Carcinogenic (lung cancer). Present in Motherboards.  The inhalation of fumes and dust causes chronic beryllium disease. Methods of disposal Reusing Reuse of end-of-life electronic equipment has first priority on the management of electronic waste because the usable lifespan of equipment is extended to a secondary market, resulting in a reduced volume of waste stream encompassing treatment. Scientific landfilling A scientific landfill is termed so because of its scientific design during construction. One of the biggest problems of ordinary landfills is the seeping of pollutants from e-waste into underlying soil and water, contaminating both. Scientific landfills eliminate the risk of e-waste seeping underground as the base layer is constructed of 90 meters of clay, thus arresting any seepage or leakage within the landfill. On top of the base layer, a drainage layer made of soil, measuring 15 meters in length and a vegetative layer of 45 centimeters to minimize soil erosion. Recycling When the e-waste items arrive at the recycling plants, the first step involves sorting all the items manually. After sorting by hand, the e-waste items dismantled and categorized into core materials and components. The dismantled items are then separated into various categories into parts that can be re-used or still continue the recycling processes. Then components are either sold as raw materials or re-used for fresh manufacture. Advantages of recycling 1. Reduces Landfill Sites The more we recycle, the less waste we generate, which applies to all kinds of waste. Reduced landfill sites allow for more land to be utilised for more meaningful purposes such as agriculture and housing development. 2. Preserves Natural Resources As highlighted earlier, recycling e-waste enables valuable materials to be recovered and reused to manufacture new products, which saves energy, reduces our carbon footprint, and preserves Earth’s finite natural resources. 3. Prevents Toxic Chemicals from Polluting the Ecosystem Electronic components contain a number of toxic chemical substances such as nickel, cadmium, lithium, mercury, and lead, which pose health and environmental hazards.Buried in landfills, these toxic chemicals leach into our soil, waterways, and ecosystem, contaminating agriculture, livestock, and sea life, ultimately ending up in our food and causing long-term damage to our health and the environment. 4. Creates New Business & Employment Opportunities The current challenges in e-waste collection and recycling present an immense opportunity for innovative research & development and creative business models for sustainable solutions.A prime example is the world’s first e-waste microfactory by UNSW’s Professor Veena Sahajwalla. Designed as a modular system that can be installed in a space as small as 50 square metres, the microfactory crushes the e-waste, with a robot removing its useful components to be reheated in a small furnace. Its innovative and portable design makes it easy and cheap to transport to the waste site, making e-waste recycling much more cost-effective and accessible. 5. Promotes Mindful Consumerism Making e-waste recycling a regular practice is a useful reminder of the impact our consumer decisions and behaviour have on our environment. Rather than contributing to the wasteful and irresponsible throw-away culture, it’s time for us to embrace mindful consumerism by thinking before buying and repairing/recycling before discarding. Extraction of copper and gold from e-waste  A hydrometallurgical route is adapted for the technically feasible recycling of copper and gold from waste printed circuit boards (WPCBs).  This process comprises the liberation of the metallic fractions from downsized WPCBs, a two-stage acid leaching process to provide a bulk separation of copper and gold from the other metals present, and subsequent purification of the copper and gold-containing solutions by solvent extraction using highly selective phenolic oxime and amide extractants, respectively.  Complete dissolution of the base metals is achieved using 3 M nitric acid at 30 °C and the selective separation of copper from this leach liquor was achieved by solvent extraction using ACORGA M5640 (5-nonyl salicylic acid) dissolved in kerosene.  The residues from base-metal leaching were treated with a mixture of 3 M sulfuric acid and 3 M sodium bromide at 70 °C, resulting in greater than 95% gold dissolution.  The selective separation of gold from this precious metal leachate was achieved by solvent extraction using 0.1 M tertiary amide extractant dissolved in toluene.  This process delivers complete copper and gold recycling from WPCBs under relatively benign laboratory conditions and represents a proof of concept for liberating valuable and critical metals back into active service from end-of-life electronic devices.