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1 S t r o n a 1 Author: Marta Miedźwiedziew METAL CORROSION AND PROTECTION AGAINST CORROSION Introduction The article is intended for high school students having courses in chemistry at both the basic and extended level. The issues presented in this paper can be regarded as an extension of information on the redox reaction or the properties and applications of metals and alloys. The concept of corrosion (Latin corrosio - eating) is the material destruction caused by the influence of the surrounding environment. Corrosion processes can be either chemical or electrochemical reaction, but also the physical processes (melting and other phase transitions, the effect of radiation) or microbiological (damage caused by the action of microorganisms). Corrosion is also called destruction under the influence of friction (fretting corrosion) and hydrogen embrittlement of 1. The effect of atmospheric corrosion [www.fotogaleria.pl] metals (also known as Hydrogen disease, metal hydrogen consumption) - the process of physical or simultaneously destroying the physical and chemical structure of metal by atomic hydrogen which diffuses. Although the corrosion phenomenon also applies to other materials, such as concrete, reinforced concrete, rocks, glass and plastics, it is commonly associated primarily with the destruction of metals. Not accidentally, the corrosion of iron alloys is the biggest practical problem and causes the greatest losses - both direct - associated with costs of the protection of damaged materials and construction, and indirect - associated with exposure to human life and health (corrosion of structural elements of buildings, bridges, transport ) and the threat to the environment (spills of dangerous substances into water and soil). 1

2 S t r o n a 2 Among the corrosive phenomena you can extract different types of corrosion, depending on the nature of the damage (general / uniform, local, pitting corrosion, etc.), the environment (eg atmospheric, water, biochemical corrosion) and the nature of emerging products (rust, creating color raids, creating scale, creating a patina). The basic division of corrosion phenomena, however, is associated with the mechanism of the reaction according to this criterion, and is distinguished by chemical and electrochemical corrosion. Chemical corrosion Chemical corrosion is the direct reaction of metal with the environment without changing the electrical charge. It occurs in an environment of dry gases (mainly with oxygen, but also halogens, hydrogen sulphide and sulfur vapor, bromine, iodine) and liquids non-conductive the electrical current. The most important among the elementary processes of chemical corrosion is oxidation. The oxidation mechanism is composed of molecular oxygen adsorption (the binding of molecules, atoms or ions of the physical interface), then dissociation of the particles on the atoms or ions of oxygen and the chemical reaction with the metal atom, as a result of which an oxide layer is formed on the border of the metal and the environment. x Me + yo 2 => Me x O 2y In the initial period, oxidation is limited by the speed of the reaction. Later - in the rate of diffusion of ions in the formed metal oxide layer (migrating toward the boundary of oxide and atmosphere) and oxygen ions (migrating toward the border of oxide and metal) and electrons (migrating toward the boundary oxide and atmosphere). The rate of diffusion and thereby the rate of corrosion and the extent of damage, depends largely on the temperature, but also the properties of the oxides (or other compounds, if there is another factor in aggressive gas) formed on the metal surface. Precious metals (Pt, Ir, Os, Pd, Ru, Au, Ag) due to their durability do not form oxides under any circumstances and do not corrode. Similarly in nature they are in the unbound 2. Gold nugget [www.wikipadia.pl] 2

3 S t r o n a 3 state [photo 2]. On metals such as W, Mo, Zr, Nb, there is a thin layer of oxide which sublimes rapidly, exposing the surface ready for further oxidation. On the other hand, metals such as Fe, Mg, Cu, Zn get easily oxidized and form a thick, but porous layer of corrosion products so they are not corrosion resistant in most environments mentioned above (however, copper and zinc oxides change into permanent and tight hydroxides, forming a protective layer on the surface of the metal patina in a short time [photo 3]). Finally, the group of metals such as Ti, Ni, Cr and Al create a very thin, but dense and tightly adherent to the substrate layer of the products, thereby protecting the metal surface from further damage. This state of the metal surface is defined as a passive state, and the process of formation of the oxide layer is called passivation. 3. Chinese coin from the first century covered with patina [www.wikipedia.pl] Electrochemical corrosion Electrochemical corrosion - which is the most common type of corrosion - is related to the flow of electrical charges through the phase boundary. It occurs in conductive environments, such as containing electrolytes, water, earth, moist gases. Corrosion processes occur as a result of potential differences on the surface of metal corrosion. In this case corrosion cells are formed, in which the areas with a lower potential become anodes (oxidation of metal occurs on them), and the areas of higher potential become cathodes (the socalled depolarizer reduction, usually of hydrogen and oxygen ions). On the surface during the uniform corrosion of the metal both reactions take place in parallel. The dissolution of metal passing in the form of ions into the electrolyte Me Me n+ + ne releases a stream of electrons which flow in metal to the cathode areas and reduce the electrolyte ions there: H 2 O + ½ O 2 + 2e 2 OH 3

4 S t r o n a 4 or 2 H + + 2e H 2 Of course, even the dissolution of the surface of metal in solution would only be possible in metals with the highest degree of homogeneity. We never have to deal with such course of the corrosion process in real metals and alloys as a result of heterogeneous properties of the surface layer (grain boundaries, defects in the crystal structure, non-metallic inclusions). Corrosion of ferrous alloys in different environments Pure iron, which corrodes relatively slowly, is not used as a construction material. However, alloys of iron (various grades of steel and cast iron) contain addition of carbon such as graphite crystals in the form of cementite Fe3C which in the presence of an electrolyte (water with dissolved gases, and salts) form corrosion cells with iron. In the electrochemical mechanism of corrosion of unalloyed steel, the factor limiting the progress of corrosion in the air is the humidity level; in the water - oxygen transport, and in soil - the transport of oxygen or moisture, depending on current conditions. 4. Effects of electrochemical corrosion [www.odrdzewiacze.com.pl] In clean dry air at ambient temperature, the corrosion of polished alloy steel is very small thanks to creating a thin protective layer composed mainly of Fe2O3 on the surface. In conditions of contact of steel with moisture - almost always present in the atmosphere - there is a process of electrochemical corrosion. The first product of the reaction is ferrous hydroxide, which by further reactions with steam and oxygen forms a series of hydrated iron hydroxide, and then loosely associated with the substrate layer of permeable reddish rust, composed mainly of a mixture of hydrated ferric oxide varieties Fe 2 O 3 H 2 O. Rusting of bare steel in the normal atmosphere is subject to moisture, the degree of air pollution and its temperature. Corrosion in the clean air with a relative humidity less than 100% is almost 4

5 S t r o n a 5 unnoticeable, but increases rapidly even at trace amounts of impurities, especially sulfur dioxide. In coastal areas, corrosion processes can be greatly accelerated by the presence of chlorine in the air. An essential component of the pollution is also dust because properties of hygroscopic dust particles promote retention of moisture on the steel. Natural or industrial water are in most cases good electrolytes and create favorable conditions for the corrosion processes (especially salt water or acid). The most important factors determining the corrosion aggressiveness of the aquatic environment is the nature and quantity of dissolved solids, ph, hardness of water, CO2, and oxygen content, and the presence of organic substances. Corrosion losses can significantly increase both inanimate organic material (np. such as acidifying water peat) and living organisms (bacteria) that stimulate chemical reactions on the surface of the steel dipped in water. Corrosion of ferrous alloys in the soil has by far electrochemical nature. The rate is dependent on several factors, including soil type and uniformity, oxygen, humidity and variations in water levels, the presence of stray currents. Less aggressive are dry limestone and sandy soils and showing high electrical resistance and the most aggressive - salty, heavy clay soils in the area of high groundwater. An important role is played by the bacteria, stimulating chemical reactions (eg, sulfate reduction in a cast), which resulted in fires causing oxygen corrosion of underground structures. The protection against corrosion of metals Corrosion can not be completely prevented in most materials, and therefore, in practice, the aim is not so much to prevent it but to minimize the effects of corrosion damage. Precious metals, occurring in nature in the form of unbound, do not require protection against corrosion, and in case of other materials the protection is reduced to slowing down this process by: 1. Proper selection of suitable materials resistant to foreseeable corrosion risks and avoiding combinations of materials with large difference in electrode potentials, accelerating corrosion processes. 2. Reducing aggressive corrosive environment by removing easy to corrode components (by precipitation or neutralization) or by introducing factors that contribute to passivation (eg adding corrosion inhibitors: chromates, arsenates, selenates to the electrolyte). 5

6 S t r o n a 6 3. The use of non-metallic and metallic coatings that protect metal from access to the environment. Metals which undergo passivation easily (Cr, Ni, Al, stainless steel) or form the protective layer of corrosion products (Cu, Zn, Sn, Cd) are used for metallic coating. They are applied galvanically by immersion in the melt or by plating. For non-metallic coatings include, for example paints, varnishes, enamels, plastics. It is important to precondition surface precisely in order to obtain good adhesion and durability of the applied layer. 4. The use of so-called sacrificial protection where it is not possible to protect the entire surface of the metal shell of the structure. The element exposed to damage (eg ship's hull [photo 5]) is fixed to a block of metal more active 5. Hull sacrificial protection [www.mt.com.pl] than iron (usually the Al-Mg-Zn), which is a compact cell anode. 5. Cathodic protection, consisting of a reduction in electrode potential of metal or alloy (weakening the trend to change to solution) by connecting voltage source of 1-2 V to the negative terminal. This method is used to protect the structure buried in the substrate (eg pipelines ), water tanks, gates, channel cases, industrial design elements. Summary Corrosion is a common phenomenon and concerns different types of materials, but the biggest practical problem, resulting in the greatest loss is the corrosion of ferrous alloys. Corrosion processes may be of a chemical reaction, electrochemical or physical processes. The basic division of corrosion phenomena is related to the mechanism of the reaction, according to this criterion is distinguished by chemical corrosion (involving the direct reaction of metal with the environment without changing the electrical charge) and electrochemical (related to the flow of electric charge through the interface). In most materials it can not possible to completely prevent corrosion, and therefore, in practice, the aim is not so much to prevent it but to minimize the effects of corrosion damage. Precious metals, occurring in nature in the form of unbound, do not require protection against corrosion, and in case 6

7 S t r o n a 7 of other materials it is necessary to slow down this process by careful selection of materials, changing the properties of the environment, use of protective coatings or so called sacrificial protection. Suggested experiment: Krzysztof Orliński, Korozja metali, in: Młody technik [on-line], Bibliografia 1. Z. Bojar, Materiałoznawstwo [script], t.2, WAT, W-wa Korozja niemetali, in: Materiały pomocnicze do ćwiczeń laboratoryjnych [on-line] 3. Krzysztof Orliński, Korozja metali, in: Młody technik [on-line]

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