CORROSION Corrosion is defined as the involuntary destruction of substances such as metals and mineral building material by surrounding media, which are usually liquid (i. e. corrosive agents). “? Most metals corrode. During corrosion, they change into metallic ions. In some cases, the product of corrosion itself forms a protective coating. “For example, aluminium forms a thin protective oxide layer which is impervious to air and water. In other cases (e. g. iron), however, the coating either flakes off or is pervious to both air and water. So the whole piece of metal can corrode right through. “?
The most common forms of metallic corrosion are caused by electrochemical reactions, wherein two metallic phases (e. g. , iron oxide and iron) react in the presence of electrolytic solution. Another mechanism of metallic corrosion is caused by chemical reaction, which explains how the protective layer of the metal is formed. Rusting is the corrosion of iron which is the most widely used structural metal. Most of it is used in making steel. The wide range of products made from steel includes all types of vehicles, machinery, pipelines, bridges, and reinforcing rods and girders for construction purposes.
Therefore, rusting causes enormous economic problem and is the reason why extensive measures of corrosion protection have had to be developed. Electrochemical corrosion reactions This type of corrosion takes place when two metallic phases with different electrochemical potentials are connected to each other by means of an electric conductor. Chemical corrosion reactions Metals have a tendency to combine with oxygen to form oxides and this is one of the chemical reactions. This tendency is the stronger the less noble the metal.
The layers of oxide on the metal surface which are formed even in dry air may be insoluble and stable against an aqueous medium in contact with them. Therefore, if the oxide layers are dense and adhere well to the metal, they prevent further attack and act as a corrosion prevention layer. An example of this is aluminum oxide. However, iron differs in that, although it does form a surface oxide layer, this layer is loose and enables oxidation to proceed into the depth of the metal. Chemical corrosion also takes place by the action of acids and alkalis on metals.
Hydrochloric acid, for example, reacts with iron, and sodium hydroxide with aluminum (Figure 1). If soluble reaction products are formed, the reaction only ends when either the aggressive medium, or the metal are used up; if salts are formed which are sparingly soluble they can form protective layers. Factors that speed up rusting 1) Presence of electrolytes Acid solutions make rusting go faster. In industrial areas where air is seriously polluted, there are high concentrations of carbon dioxide, sulfur dioxide and nitrogen dioxide. These gases dissolve in rain water to give “acid rain”? which makes iron objects rust faster. Sodium chloride also makes iron rust more quickly. 2) Heat An increase in temperature always increases rate of chemical reactions, including rusting. 3) Humidity “Corrosion starts when the relative humidity of the air exceeds around 65%. Many areas has a higher humidity in winter (80-95%) than in summer (60-80%)”? . In consequence, iron rusts five times faster in winter as it does in summer. 4) Contact with a less reactive metal Consider iron and copper plates joined together and put in water containing dissolved oxygen.
Iron loses electrons more readily than copper. Hence iron forms the anode and copper the cathode of an electrochemical cell. In this case, iron rusts even more quickly than when there was no copper. 5) Other factors Other factors that speed up rusting include the presence of sharply pointed regions in the iron piece, or a high concentration of dissolved oxygen in water. Protection From Rusting Iron is such a useful metal yet it rusts. Rusting is a serious problem. A very sum of many is spent every year to protect iron objects and replace rusted articles.
Several methods can be used to prevent rusting or to slow it down. Applying a Protective Layer Both air and water are necessary for rusting to occur. Any method which can keep out one or both of them from iron will prevent rusting. The most obvious way is to apply a protective layer. 1) Coating with paint, oil or grease Objects that are unlikely to be scratched can be coated with paint (or lacquer, or enamel). For example, bridges, ships and car bodies are painted. Moving parts of a machine are protected by applying oil or grease. 2) Coating with another metal
Iron can be coated with a thin layer of another metal which is resistant to corrosion. Galvanized iron is iron coated with zinc. Some roofs, buckets and dustbins are made from galvanized iron. Tin -plate is iron coated with tin. 3) Using Alloys Of Iron An alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Alloys usually have different properties from those of the component elements. Alloys are sometimes made to stop corrosion of metals and at the same time make it stronger. For example Steel, Carbon steel.
Steel is produced form iron by carefully controlling the amount of carbon present (0 – 1%). To fight against corrosion, steel can be alloyed with other metals such as chromium and nickel to produce stainless steels. Cathodic Protection Rusting is a redox reaction in which iron loses electrons. If iron is connected to a more reactive metal, the other metal will lose electrons in preference, preventing the formation of Fe2+ (aq) ions. Galvanizing (zinc-plating) provides a good example of cathodic protection. “When the zinc coating is undamaged, the iron is covered up and is protected from usting. In case the coating is partly damaged, the iron, though exposed, is still protected. “? Hot dip galvanizing deposits a thick robust layer that may be more than is necessary for the protection of the underlying metal in some applications. This is the case in automobile bodies, where additional rust proofing paint will be applied. Here, a thinner form of galvanizing is applied by electroplating, called “electro galvanization”. The hot-dip process slightly reduces the strength of the base metal, which is a consideration for the manufacture of wire rope and other highly-stressed products.
The protection provided by this process is insufficient for products that will be constantly exposed to corrosive materials such as salt water. For these applications, more expensive stainless steel is preferred. Some nails made today are electro-galvanized. Cathodic protection is also used to prevent corrosion in underground iron pipelines. Bags containing magnesium turnings are connected to the buried pipelines at intervals. (Figure 3. ) The magnesium corrodes instead of the iron. The magnesium should therefore be replaced from time to time. Figure 3.
Protecting underground iron pipelines from corrosion by cathodic protection Most ships are made of iron. To prevent rusting, zinc blocks are attached to the hull of a ship. Zinc will corrode in preference to iron. Electrical Protection Sometimes rusting can be prevented by using electricity. For example, the negative terminal of the car battery is always connected to the car body. This supplies electrons to the iron body, preventing it from losing electrons. In some piers, the steel structures are protected electrically by connecting them to the negative terminal of a d. . source. Anodising:- Anodizing, or anodising in British English, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts Anodizing increases corrosion resistance and wear resistance, Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, niobium, and tantalum. This process is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized; i. e. the iron xide (also known as rust) flakes off, constantly exposing the underlying metal to corrosion. Electroplating:- Electroplating is a plating process that uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material to bestow a desired property (e. g. , abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc. ) to a surface that otherwise lacks that property.
Another application uses electroplating to build up thickness on undersized parts. The anode and cathode in the electroplating cell are both connected to an external supply of direct current a DC battery or, more commonly, a rectifier. The anode is connected to the positive terminal of the supply, and the cathode (article to be plated) is connected to the negative terminal. When the external power supply is switched on, the metal at the anode is oxidized from the zero valence state to form cations with a positive charge.
These cations associate with the anions in the solution. The cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, in an acid solution, copper is oxidized at the anode to Cu2+ by losing two electrons. The Cu2+ associates with the anion SO42- in the solution to form copper sulfate. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons. The result is the effective transfer of copper from the anode source to a plate covering the cathode. The plating is most commonly a single metallic element, not an alloy.
However, some alloys can be electrodeposited, notably brass and solder. Conclusion Corrosion is the gradual deterioration of a metal due to reaction with air, water or other substances in the surroundings. When these substances present, the metal can corrode through the process of electrochemical and chemical corrosion reaction. The most common type of corrosion is rusting which costs several billion dollors a year. To prevent such natural, spontaneous processes, method of protections and constant maintenance work are necessary; only in these ways can a steel structure be adequately
