Module 6 Corrosion PDF

MODULE 6 CORROSION I. INTRODUCTION Corrosion in simple terms occurs in metals because most of the metals are not in their natural state until they return to the ore form which we found them. Almost all metals are found...

MODULE 6 CORROSION I. INTRODUCTION Corrosion in simple terms occurs in metals because most of the metals are not in their natural state until they return to the ore form which we found them. Almost all metals are found in nature in a form of an oxide, sulfide or some other metal compound. Thus, all metals that are derived from ores want to return to its form and corrosion is the reaction process that allows this return to nature to occur. In the case of plastics corrosion occurs because the bonds between the organic molecules making up the plastics can be affected by various environments. Plastic do not rust; deterioration is usually in a form of loss in properties. Example is the aging of the rubber. Plastics and ceramics behave differently than metal because most of them are poor conductors of electricity. Hence, corrosion that occurs in metal are electrochemical in nature which requires flow of electrons. Degradation is the term used for the deterioration of polymers and ceramics. II. CORROSION is the tendency of metals to return to its original state as metal oxides. It is a process in which metals reacts with Oxygen gas (O2) in an oxidation-reduction reaction producing metal oxides. The deterioration of a material or its properties because of reaction with its environment. The Four Components that should be present for corrosion to take place: 1. Anode – is the part/region of the metal where corrosion occurs. It releases positively charged metal ions into the electrolyte , and the electrons are left behind in the metal and flows through it to protect the Cathode. This is where oxidation (loss of electrons) takes place. 2. Cathode – is the part of the metal, into which the electrons flow, and is therefore protected and remains intact. This is where reduction takes place. 3. Electrolyte - is a solution capable of conducting electrical current in the form of ionic flow. - for example: moisture or salt solution 4. An Electrical Path/Salt Bridge – is a connection between the anode and the cathode where electrons flow. 1 Engr.AJ Labayen Emat-Midterm III. TYPES OF CORROSION 1. Uniform corrosion The simplest type of corrosion is uniform attack of all the surfaces exposed to corrodent. It can electrochemical in nature or direct attack. Most plastics and ceramics since they are poor conductor of electrons they corrode by direct attack. Metals will undergo direct attack if there is no oxide or similar film that can promote electrochemical cell action. Example is molybdenum at 760°C in air it will just slowly disappear(sublimes). Similar situations can occur in metals if they are used to handle liquid metals, with metal in molten salts and with metals in corrosive gases. Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance. 2. Pitting corrosion Local corrosion damage characterized by surface cavities/small holes. An insidious form of corrosion because the usefulness of the part may be lost by removal of a relatively small amount of metal. If a single pit perforates the side of a tank or pipe, serviceability will be lost until the pit is repaired. Pitting is usually caused by chemical nature of environment. Proper material selection can minimize the effect of pitting corrosion. Copper alloys such as bronze, brass and monels have lower pitting tendencies in saltwater. 3. Crevice Corrosion Local attack in the crevice between metal-to-metal surfaces or between metal and non-metal surfaces. It usually occurs in poorly gasketed pipe flanges, under bolt heads and attachment immersed in liquids. 4. Galvanic Corrosion Also known as bimetallic corrosion, is an electrochemical process whereby one metal corrodes in preference to another metal that it is in contact with through an electrolyte. Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. One metal (the cathode) is protected, while the other (the anode) is corroded. The rate of attack on the anode is accelerated, compared to the rate when the metal is uncoupled/unconnected. Three Conditions must exist for Galvanic Corrosion to take place: a. Electrochemically Dissimilar metals must be present b. There should be a metal-to-metal contact c. The Metals must be exposed in the same conducting solution/electrolyte The figure illustrates that galvanic corrosion takes place when 2 dissimilar metals placed in contact with each other in the presence of an electrolyte. In the diagram there are two pairs of adjacent metals: (1)Carbon steel bolt & nut – copper plate, and (2) Copper plate-Aluminum plate. In (1) the Carbon steel is the anodic metal, thus it corrodes while the Copper plate remains intact. Meanwhile, in (2) the Aluminum plate is also the anodic metal that rusts while the Copper plate, being more noble, remains intact. 2 Engr.AJ Labayen Emat-Midterm  The galvanic series in seawater lists the common metals in order from the most anodic to most cathodic (noble). The further apart the metals are in this series, the greater the corrosion difference and speed between the two. For example, zinc will corrode in saltwater extremely quickly when in contact with platinum, but 304 stainless steel in contact with 316 stainless steel will have little effect on each other. 5. Stress corrosion cracking Stress corrosion cracking is different from other corrosion processes in that the material deterioration is due not to significant material removal but to cracking. It is the spontaneous corrosion- induced cracking of a material under static stress either applied or residual. This can occurs in many plastics, aluminum alloys, copper alloys, magnesium alloys, carbon steels, stainless steels, titanium alloys and other metal alloys. High applied or residual stresses increase the tendencies for stress corrosion cracking. Stainless steel is prone to cracking at 50°C-60°C in chloride containing solutions; steels crack in caustic solutions; copper alloys crack in ammonia atmosphere and sometimes in neutral waters. This corrosion happens only when surfaces are under tensile stresses. Hence, the common remedy is to eliminate the stress factor. 6. Fretting Corrosion Material deterioration by oscillatory relative motion of small amplitude between two solid surfaces in corrosive environment. Surfaces subject to fretting corrosion in air often show a red, oxidized appearance like rust. It usually occurs under the bearings, under gears and hubs pressed into shafts and on machine component that experiences vibration. 7. Dealloying Corrosion process whereby one constituent of metal alloy is preferentially removed from the alloy, leaving an altered residual microstructure. A number of alloy system are susceptible to this process, but this is commonly encountered in yellow brass. Dezincification is the removal of zinc from brasses. Selective leaching of aluminum from aluminum bronzes Selective leaching of nickel from cupro nickel alloys Graphitization the dissolution of iron from gray cast iron leaving only graphite 3 Engr.AJ Labayen Emat-Midterm  8. Cavitation The material removal by actional of imploding bubbles. Materials that can resist cavitation have tenacious passive films, high hardness and high strength in which not all metals have these characteristics. Titanium and cobalt based alloys appear to have the best cavitation resistance in wide range of environment. 9. Microbiological Corrosion Corrosion promoted by the presence and growth of living organism such as algae, fungi and bacteria. This corrosion is common in soils and in cooling water systems where there is likelihood of stagnant water. IV. FACTORS CONTRIBUTING TO CORROSION 1. Exposure to moisture Moisture, whether in the form of dew, rain or condensation, is a very significant factor when it comes to atmospheric corrosion. Rain can wash away in the atmosphere that have been deposited in exposed areas, such as in a marine environment, rain also collects in crevices and pockets. Rain can also hasten the corrosion process through constant wetness, especially in areas with galvanized bolts and steel parts or structures. Condensation and dew are unwanted types of moisture when they are not washed away by recurrent rain, which could eliminate or dilute the contamination. Dew films that have become saturated with acid sulfates, sea salt and other acids could produce an aggressive electrolyte environment that promotes the occurrence of corrosion. 2. Chemicals - Presence of acid in the atmosphere: Acids can easily accelerate the process of corrosion. The pH scale ranges from 0 to 14. A pH of 7 is considered to be neutral. Substances with pH of less that 7 are acidic and substances with pH greater than 7 are considered to be basic. Low pH acid waters clearly accelerate corrosion by providing a plentiful supply of hydrogen ions. 3. Temperature - temperature affects atmospheric corrosion. At every 10°C increase in temperature can double corrosion activity. 4. Mechanical stress – stress can not cause the corrosion however, when stress is applied to small cracks it will open and accelerate corrosion process. This will significantly reduce the strength of the metals. 5. Electrochemical reactions - is an electrochemical process because corrosion involves the transfer of electrons between a metal surface and an aqueous electrolyte solution. It results from the overwhelming tendency of metals to react electrochemically with oxygen, water, and other substances in the aqueous environment. V. PREVENTION AND CONTROL 1. Inhibitors Substances that, when added in relatively low concentrations to the environment, decrease its corrosiveness. The specific inhibitor depends both on the alloy and on the corrosive environment. Inhibitors are normally used in closed systems such as automobile radiators and steam boilers. Examples are hydrazine used in boiler feed water and nitrite in freshwater cooling system. 2. Protective coatings Electroplating -electroplated coatings are commonly applied to meet a variety of service requirements, and also for decorative purposes. Service requirements include corrosion-resistance, wear-resistance and contact with chemicals and foodstuffs. The coatings described here are those used chiefly for their corrosion-resistant properties. 4 Engr.AJ Labayen Emat-Midterm  a. Cadmium plating- Cadmium has excellent corrosion-resistance, especially in marine applications as a coating on iron and steel. Its main use is with military and sea going equipment and in steel components on aircraft. For industrial use it is now being replaced by zinc coatings, which are cheaper. Cadmium is poisonous and must not be used on articles or equipment for use in contact with food or drinking water. It should not be applied to articles which may be subjected to temperatures in excess of 250C, as poisonous fumes are then given off. b. Chromium plating- Bright chromium plate is the most widely used electroplated coating. It combines resistance to corrosion, wear and heat, use in contact with food stuffs, and decorative qualities. c. Nickel plating- Nickel deposits are most widely used as a base for chromium in producing bright chromium plate. Apart from its decorative appeal, nickel is used as a hard wear-and corrosion- resistant coating on steel and on copper, zinc and aluminum alloys. d. Tin plating - Tin plating is employed as a protective coating on both ferrous and nonferrous metals, particularly where the product will come in contact with foodstuffs. It is also used for electrical components, for protection against corrosion and as an aid to soldering. e. Zinc coating-The best-known zinc coating is galvanizing, which is a hot-dip process almost exclusively used on industrial, constructional and domestic goods which require a thick coating. Zinc is unsuitable for electroplating articles likely to come in contact with food stuffs, since it dissolves in dilute acids and alkalis. Paints - wide range of paints is available for industrial use, and their primary function is to give added corrosion-resistance to the metal surface as well as providing a decorative appearance. To reduce the viscosity so that the paint can be easily applied, solvents or thinners are added. These solvents and thinners evaporate during drying and do not form part of the paint film. To provide the maximum protection, a paint system should ideally consist of three types of paint: Primer– applied to the clean surface and designed to adhere strongly and prevent corrosion; Undercoat– applied as an intermediate film to provide a basis for the correct final colour and either matt or low-gloss to provide a good ‘key’for the finishing paint; finishing paint– the choice of this paint will depend on the degree of protection and the decorative effect required. a. Chlorinated-rubber paints: Chlorinated rubber is produced by the chemical action of chlorine on raw rubber. Has outstanding resistance to chemical attack and to penetration by water. It is used in breweries, chemical plants, bridges, cranes and ships. b. Acrylic paints-Paints based on acrylic resins are widely used, particularly for motor vehicle applications. Their advantages are speed of drying to give tough films with excellent durability and color retention. c. Epoxy resins: Epoxy-resin-based paints are used where a high degree of chemical resistance is required. 5 Engr.AJ Labayen Emat-Midterm 3. Cathodic Protection Sacrificial anodes are among several forms of cathode protection. Sacrificial anodes are highly active metals that are used to prevent a less active material surface from corroding. Sacrificial Anodes are created from a metal alloy with a more negative electrochemical potential than the other metal it will be used to protect. The sacrificial anode will be consumed in place of the metal it is protecting, which is why it is referred to as a "sacrificial" anode. Usually, pure active metals such as magnesium, zinc and aluminum are used as a material for sacrificial anodes. Galvanic corrosion is the most common type of corrosion common to pipes connected to equipment that are made of different metal. Some piping systems may have sacrificial pipe sections to concentrate the galvanic corrosion in one place. They provide corrosion protection to the remainder of the system. Sacrificial pipe sections should be routinely inspected, and renewed when required. For the pipelines used in marine industry it is preferred to use zinc and aluminum metals as sacrificial pipe. Sacrificial anodes are also used in heat exchanger such as condensers and seawater strainer. Magnesium, zinc, and aluminum are the most common metal used. Among the three metals, aluminum is the safest material to be used for all applications. Meanwhile, magnesium is not used in the cargo- ballast tank of oil carriers owing to the spark hazard. 4. Material Selection The most common and easiest way of preventing corrosion is through the selection of materials once the corrosion environment has been characterized. However cost may be a significant factor. It is not always economically feasible to employ the material that provides the optimum corrosion resistance; sometimes, either another alloy and/or some other measure must be used. 6 Engr.AJ Labayen Emat-Midterm VI. CORROSIVE ENVIRONMENTS Corrosive environments include the atmosphere, aqueous solutions, soils, acids, bases, inorganic solvents, molten salts, liquid metals, and, last but not least, the human body. On a tonnage basis, atmospheric corrosion accounts for the greatest losses. Moisture containing dissolved oxygen is the primary corrosive agent, but other substances, including sulfur compounds and sodium chloride, may also contribute. This is especially true of marine atmospheres, which are highly corrosive because of the presence of sodium chloride. Dilute sulfuric acid solutions (acid rain) in industrial environments can also cause corrosion problems. Metals commonly used for atmospheric applications include alloys of aluminum and copper, and galvanized steel. Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen, as well as other minerals several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper–nickel alloys, and nickel–chromium– molybdenum alloys are highly corrosion resistant in seawater. Soils have a wide range of compositions and susceptibilities to corrosion. Com positional variables include moisture, oxygen, salt content, alkalinity, and acidity, as well as the presence of various forms of bacteria. Cast iron and plain carbon steels, both with and without protective surface coatings, are found most economical for underground structures. 7 Engr.AJ Labayen Emat-Midterm