Corrosion Chemistry PDF

# Corrosion Chemistry ## Definition Corrosion is the destruction of metal or alloy by chemical or electrochemical reaction with its environment. This implies that, corrosion involves chemical or electrochemical changes onto the metallic surfaces. ## Fundamental Reason of Corrosion Most of metals exist in nature are in combined states such as oxides, carbonates, sulphites, chlorides, silicates etc. These ean be reduced to their pure metallic states from their ores during the extraction process. These extracted pure metals can be regarded as higher energy states than their corresponding ores and thus they have a natural tendency to come back to combined states (Low energy states). Hence, when metal come in contact with environment, its surface begin to decay more or less rapidly. ``` Metal... oxidation Reduction High Energy states metallic compd. OR Dre. +. energy Low energy states. ``` ## Electrochemical Corrosion: (Wet corrosion) If a metal is partly dipped in a salt solution, then a potential difference is developed between the rod surface and the solution. This Potential difference depends upon concentration of solution, nature of metal, and temperature. ``` metal Salt Solution ``` If two such electrodes are electrically connected to complete the circuit, then it forms a galvanic cell, which conduct an electric current. This phenomena of electric current is strongly associated with electrochemical corrosion. Thus, electrochemical corrosion occurs when: * Conducting liquid is in contact with metal * Two dissimilar metals are dipped in a solution. Due to this, separate the anodic and cathodic areas in which current flows through the conducting solution. At anode, oxidation reaction takes place by the loss of electron. Thus there is a tendency at anode to destroy the metal by forming metal ions. Therefore, corrosion always occur at the anodic area. **At anode following reaction takes place** ``` M Metal M + metal ion + ne oxidation. Dissolves in solution to form oxide compd... ``` On the other hand, at cathode, reduction reaction takes place. (Gain of electrons). The dissolved copotitents in the electrolyte gains this electron and form some ions such as OH, Ō etc. ## Rate of Electrochemical corrosion: Rate of electrochemical corrosion depends upon the nature of corrosion product. * If the corrosion product dissolves into the solution or evolves as a gas, then corrosion reaction goes on changing. * If the corrosion product is insoluble compound then it covers the metal surface, therefore decreases the rate of corrosion. ## Mechanism Corrosion of metal in an aqueous solution is an electrochemical phenomenon. In which current flows between anode and cathode areas. The reaction occurring at anode involves dissolution of metal by forming metallic ions with liberation of free electrons. **Thus, at anode following reaction takes place:** ``` M → M + + ne- ``` At cathode, it consumes these electrons either by evolution of hydrogen or by absorption of O2, depending, upon the nature of corrosion process. ``` M + + ne- anode e flow cathode 2H + + 2e → H2↑ acidic solution ``` ### a) Evolution of Hydrogen: The evolution of H2 takes place in the acidic solution like industrial waste, non oxidizing acids (HCI) etc. * Let us consider the iron metal. * At anode, dissolution of iron takes place by forming ferrous iron with the liberation of electrons. ``` Fe → Fe ++ + 2e oxidation R? ``` * These liberated electrons flow through the metal from anode to cathode. * At cathode, H + ions in acidic solation are liberated as hydrogen gas. ``` 2H+ + 2e - → H2↑ ... Reduction R". ``` * **Overall reaction may be given as** ``` Fe + 2H + → Fe ++ + H2↑ ``` * **In this type of corrosion, anodes are very large areas while cathodes are very small areas. Thus, in hydrogen evolution mechanism, the displacement of hydrogen ions from acidic solution by metal ions.** ``` Acidic soution 2H + + 2e - → H2↑ Fe → Fe 2+ + 2e Large (anode) Fe → Fe 2+ + 2e Large (anode) Small IRON cathodic areas ``` * **Generally, all metals above hydrogen in the electrochemical series have a tendency to dissolve in acidic medium with the evolution of hydrogen.** ### b) Absorption of oxygen: * In this process, corrosion of metal occurs in the presence of aqueous neutral solution of electrolyte (NaCl). * Let us consider the iron metal. * On the surface of iron, thin oxide film is obtained. However, if this oxide film develops some cracks, then it forms anode on the surface while remaining part of metal act as cathode. Thus it is clear that, anode areas are very small while cathode areas are very large. ``` Aq. Neutral solution Rusting oxide film. Y2O2 + + H2O 20H Large cathodic area small anodic areas IRON ``` * At anode, iron dissolves as ferrous iron with liberation of free electrons. ``` Fe → Fe 2+ + 2e - oxidation R ``` * The liberated free electrons flow through the metal from anode to cathode and combines with dissolved O2. ``` V2O2 + + 2e - + HOH → 20H Reduction R?! ``` * The Fe 2+ ions at the anode and OH ions at the cathode combines and ferrous hydroxide is precipitated. ``` Fe 2+ +2OH → Fe (OH)2↓ ``` * If enough oxygen is present, then ferrous hydroxide undergoes further oxidation and ferric hydroxide is obtained. ``` 4 Fe(OH)2 + O2 + 2HOH → 4 Fe (OH)3 ``` * If O2 concentration is low, then black anhydrous compound is obtained. ## Types of corrosion: ### 1 Galvanic corrosion: When two dissimilar metals. e.g. Zn and cu are electrically connected and exposed to an electrolyte, the metal higher in electrochemical series undergoes corrosion. This type of corrosion is called as galvanic corrosion. ``` Zn ↔ Zn 2+ + 2e - zn cu ``` In the above example, zn io higher in electrochemical series and thus forms the anode and get dissolved in the solution. Whereas, cu which is lower in electrochemical series and thus acts as cathode. The nature of corrosive environment electrolyte decides the type of cathodic reaction. * In acidic solution, the corrosion occurs by H2 evolution mechanism. While in neutral or alkaline solution, oxygen absorption mechanism occurs. * The electronic carrent flow from zn metal to cu metal. Thus it is clear that, the corrosion occurs at the anodic metal while the cathodic area is protected from attack. * e.g. i) Lead- antimony solder around the cu wire. ii) steel pipe connected to cu plumbing. ### 2) Micro-biological corrosion: Corrosion caused by the metabolic activity of various micro-organisms is called as microbiological corrosion. The micro-organisms are formed in an environment with or without oxygen. And thus they are classified as aerobic and anaerobic microorganisms. * **i) Sulphate reducing bacterias:** __Sporovobrio desud__ * **Responsible for sulphate reducing bacterias are phasicons responsible for aneorobic corrosion of iron and steel. Because they can grow only under aneorobic conditions. They require. In addition to O2, adquate amount of sulphates for nourishment. Other optimum conditions for their growth is temp. which is around 25°C to 30°C. and pH is from 5 to 9.** * **The main corrosion product is black iron sulphide and ferrous hydroxide.** * **ii) Sulphur bacterias:** __Gahiomacillus__ * **Sulphur bacterias are mostly aerobic in nature. Such bacterias oxidise sulphur and produces sulphuric acid, which attacks the metal surface. Sulphur bacterias are grow in acidic conditions.** * **iii) Iron and Manganese micro-organisms:** __These are areobic micro-organisms. They take iron and maganese ions and digest in the presence of O2 and thus insoluble hydrates of iron and MnO2 are formed.__ * **These bacterias can grow in running water at temp. from 5°C to 40°c and pH between 4 to 10.** * **iv) Film forming micro-organisms:** __These includes, bacterias, fungs, algae, diatoms etc.__ * **Film forming micro organizmó are foim microbiological film onto the metal surface. Such films are capable of maintaining concentration gradient of dissolved salts, acid & gases onto the metal surfaces and thus local biological concentration cells are formed and consequently corrosion occurs.** ### 5) Pitting Corrosion: Pitting corrosion occurs by the formation of pinboles, pits and cavities in the metal. Because of the formation of these pinholes, which penetrate deep into the metal. Thus pitting is very destructive nature. ``` more oxygenated cathode. Y2O2 + + H2O + 2e - → 2OH more oxygenated cathode. anode is attacked Fe → Fe 2+ + 2e - IRON Fe 2+ + 2OH → Fe (OH)2 (corrosion plaiact) ``` ## Factors affecting the rate of corrosion: The rate and extent of corrosion depends upon the following factors: 1. Nature of the metal * a) Position in galvanic series: * When two metals or alloys are electrically connected and exposed to an electrolyte, then metal which is higher in electrochemical series suffers from corrosion. * The rate of corrosion depends upon relative difference in their electroebemical positions. * Greater is the difference, faster is the rate of corrosion. * b) Relative areas of anodic and cathodic parts: * When two dissimilar metals are in contact, the corrosion of the anodic part is directly proportional to the ratios of areas of anodic and cathodic parts. * The rate of corrosion is more rapid, if the anodic area is smaller, because the current density at the smaller apodic area is much higher. Thus rapid destruction of anodic part metal takes place. * c) Purity of metal : * Impurities in a metal, causes beterogenity and form small electrochemical cell with rest of the metal. * Thus anodic part gets corroded. * e.g. Zn metal containing impurity such as Pb or fe, undergoes corrosion around the impurity. * This is due to formation of local electrochemical cells. * The rate and extent of corrosion increases with the increasing exposure time and percentage of the impurities. * d) Solubility of corrosion product: * In electrochemical corrosion, if the corrosion product is soluble in the corroding medium, the rate of corrosion increases. * On the other hand, if the corrosion product is insoluble then it covers the metal surface and thus the rate of corrosion decreases. 2. Nature of the corroding Environment: * a) Temperature: * As temp. increases, with increase of temperature, the rate to of corrosion as well as diffusion increases. * But the rate of solubility of oxygen decreases. * Thus, corrosion generally increases with rise in enviromental temperature. * b) Presence of impurities in atmosphere: * Atmospheric air contains some corrosive gases such as CO2, H2S, SO2 and fumes of HCI, H2SO4 etc. * In the presence of these gases, acidity of the liquid occurs adjacent to the metal surfaces. which increases it's electrical conductivity and thus increases the corrosion current flowing in the thereby, local electrochemical cells on the exposed metal surface. * c) Presence of suspended particles in atmosphere: * Atmospheric corrosion is influenced by the presence of solid particles in air. * If these particles are chemically active in nature. then they absorb moisture and act as strong electrolytes and thus increases the rate of corrosion. * if these particles are chemically inactive in nature, then they absorbs both sulphur gases and moisture and slowly thet enhances the rate of corrosion. * d) Effect of pH: * Generally, acidic media are more corrosive than alkaline and neutral media. The corrosion of iron in oxygen tree water.is Glow. However, the corresponding corrosion rate in the presence of oxygen is much higher. * Zn buffers minimum corrosion at pH=11, while tin corrodes rapidly at pH greater than 8.5. ## Methods to minimise the rate of corrosion: Only under ideal service conditions corrosion can be stopped completely. However ideal condition means, uniform composition without heterogenity and absolute uniform environment are impossible to-altain. Thus, it is possible only to minimise the rate of corrosion considerably. Since types of corrosion are so numerous and the conditions under which corrosion occurs are so different that diverse methods are used to control corrosion. The choice of the method depends upon the environmental conditions to which metal is exposed. Some of corrosion control methods are described as following: ### 1) Proper Designing: * A proper selection of metallic material for any particular environment and proper design is the best way of controlling corrosion. * The design of the material should be such that, corrosion even if it occurs, which is uniform does not result in intense and localised corrosion. **Important design principles are:** * Avoid the contacts of dissimilar metals in the presence of a corroding solution. If this principle is not taken into conoideration, then corrosion oscars on the more active metal while less active metal remains protected. * If two dissimilar metals in contact, but they should be as close as possible to each other in the electrochemical series. * Whenever, the direct joining of disimilar metal is unvoidable, then insulating titting may be applied between them to avoid metal-metal electrical contact. * The angdic metal should not be painted or coated, when in contact with a dissimilar cathodic metal, because any break or crack in coating lead to rapid destruction. ### 2) Cathodic protection: The main principle involved in this method is to force the metal to be protected to behave like a cathode. Thereby corrosion does not occurs. There are two types of cathodic protections: * **a) Sacrificial anodic Protection method:** * In this method, the metallic structure to be protected is connected by a wire to a more anodic active metal, so that all the corrosion is concentrated at this more active metal. * The more active metal itself gets corroded slowly, while the Parent metallic structure (which becomes cathode) is protected. * The more active metal so used is called as "Sacrificial anode". * The corroded sacrificial anode can be replaced by a fresh anode when consumed completely. ``` soil M9 -insulated ca wire I underground iron pipetine (cathode) Zn or Mg оск. ``` * **a) Iron pipetine protected by connecting to my block.** * **b) Industrial Hot water tank Protected by suspended zn or Mg blocks/rods.** * **The sacrificial anodes generally used are Zn, Mg, Al and their alloys. This method can be used for the protection of iron pipelines, underground cables, industrial hot water tank etc.** * **b) Impressed current cathodic protection:** * In this method, an impressed current is passing in opposite direction to nullity the corrosion current and convert the corroding metal from anode to cathode. * Generally, D.C. source is wed to pass impressed current with on insoluble anodes like graphite, stainless steel, platinum etc. ``` soil graphitodet 17D.C.source. Insulated copper wire. Lunderground pipeline (act as cathode) ``` * **A sufficient direct carrent is applied to an insoluble anode immersed in the corroding medium, and connected to the metallic structure which is to be protected.** * **This type of cathodic protection can be wed in water-box coolers, water tanks, pipeline, condensers etc.** ## Dry corrosion or Direct chemical corrosion This type of corrosion occurs mainly through the direct chemical action of atmospheric gases like O2, CO2, H2S, Cl2, Br2, SO2 etc. with metal or alloy surfaces. The extent of corrosion depends on the: * chemical affinity between metal and corrosive environment. * The ability of metal to form a protective film. Thus, the surface of metals are directly attacked by the atmospheric gases and gets coated with corresponding compounds like oxides, sulphides, chlorides carbonates etc. such type of corrosion is called as direct chemical corrosion or Dry corrosion **Generally, direct chemical corrosion occurs due to oxygen.** It has been found that, oxygen present in the medium directly attacked the metal surface at low or high temperatures in the absence of moisture. It is represented by this equation ``` Exposed surface 2M + O2 → 2MO Porous metal oxide fillm Metal +02 (of air) Metal Further → attack thro Pores/cracks. continues. ``` **When oxidation starts, a thin oxide film is formed on the surface of the metal** The nature of oxide film plays an important role in oxidation process. **This film decides the prevention or continuation of corrosion.** The oxide film so produced can be classified into three categories: * **a) stable Film:** * 1 Alkali and alkaline earth metals on oxidation produce oxide film of smaller volume than the respective metals from which they were formed. This result in the formation of porous film through which oxygen can diffuse to bring the further attack of the metal. * 2) On other hand, heavy metals like Al, Cr, Pb, cu etc. form oxide film of greater volume than the metal from which they were formed. This non-porous stable film prevent the diffusion of oxygen and hence the rate of further attack decreases. * **b) unstable oxide film:** * When oxide film formed is unstable, then it decomposes back into the metal and oxygen. ``` 2M0 → 2M + O2 ``` Therefore, oxidation process or corrosion is not possible in case of noble metals like Ag, Au, Ptele. * **c) volatile oxide Film:** * When the oxide film formed is volatile, it rapourises as soon as it is formed. Therefore, the underlying metal surface is exposed for further attack of oxygen and thus causes continuous and excessive corrosion. * e.g. Molybdenum oxide (MoO3) is volatile.

Corrosion Chemistry PDF

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