Protection Against Corrosion Chemistry Project CORROSION CONTROL AND PROTECTION Thebasicrequirementsofanymetalusedinindustryorinthe homearedurabilityandabilitytogivecontinuedgoodservice. One of the major factors,which makes many metals deteriorate or fail in use is their tendency to corrode. It has been estimated thatfailureofmetalsduetomechanicaldefectsisonly40%as comparedto60%duetocorrosion.Thuscorrosionresultsina bigwasteofnaturalresources,theannuallossesamountingto Rs.1000millionsinindia,asfarbackasthelate9060s.inthe USA, the corresponding in the early 1960s way 9 billion dollars a year. The losses must have undoubtedly mounted to many times by now. Corrosion control and prevention ,therefore , are major aspect of any scheme of metal care.

PROTECTION AGAINST CORROSION ProperProper materialChange of metal Alteration ofUse of inhibators Protective designselectionelectrode potentialenvironment and passivators coatings

Use of pure metalcathodic protectionDeaeration anodic Use of metal alloys galvanic or sacrificial Dehumidificationinhibitor Elimination of tensile protection methodUse of alkalinecathodic Strength Impressed current cathodic neutrilizers inhibitor Protection AnodicProtection Metallic coatingschemical conversion coatingsorganic coatings Hot Dipping Galvanising or zinc coatingPhosphate coating Tinning or tin coating Chromate coating Cementation Sprayed metal coating Electroplating Cladding Vapour Deposition SELECTION OF MATERIAL Thesimplestwayofcontrollingcorrosionistouseamaterialhavingminimumundersizable effectsofcorrosiveenvironment.Ingeneral,somemetalsgivebetterperformanceunder specific environmental conditions. Examples are (1) copper-nickel alloys are resistant to reducing ornon-oxidisingacids(HCL),(2)Leadisveryresistanttodilutesulphuricacidunderstatic condition,(3)aluminiumisresistanttoatmosphericoxidation,(4)tiniscompletelyinertto water,(5)titaniumanditsalloysdonothavetranspassiveregions,(6)ordinarystructuralsheets canbeusedforcontainingconc.Sulphuricacid,butarereadilyattackedbydiluteoraerated acid,(7) chromium containing alloys are useful under oxidisingconditions. CATHODIC AND ANODIC PROTECTION Sincemostofthecorrosionproblemsaregalvanicinnature,bymakinguseofthe electrochemicalpropertiesofreactingsystemdesirablepartsmaybeconvertedintonon deteriorating cathodes through the application of external current i.e.,(1) cathodic ,(2) anodic. (1). CATHODIC PROTECTION When a conductor, buried in soil picked up strays of current from such sources as power cable,telephone cable,tram lines etc ,great risk of stray current corrosion may occur . It is sometimes possible that 15-20% of current may pass through such buried conductor in soil,corrosion will occur at the point where current leaves the conductor and the reaction taking place there will be FeFe2+ + 2 e- In presence of OH-ions,ferrous hydroxide is then produced. Fe2+ + 2 OH-Fe(OH)2 The highelectrical potential with in the conductor will just leak from weakest point in the insulation . Instead of attack being spread over a larger area ,it will be concentrated in one small part . The roles of cathode and anode are mutually complimentary, corrosion can occur only at the anode. Thus ifthemetaltobeprotectedisconvertedintoacathode,corrosioncanbestopped,iscalledcathodic protection. There are two approaches for achieving this cathodic protection. (1). Sacrificial anodicprotection method. (2). Impressed current protection method. (a). SACRIFICIAL ANODIC PROTECTION METHOD. Inthiscase,themetaltobeprotectedismadethecathode,soitcanbesavedfromcorrosionby connecting it to a more corrodable matel,which acts as anode. Zinc and magnesium are being anodic to irons,getcorrodedandironissaved.Aftercertainperiod,theanode(Mg,Zn)isreplacedforcontinuous protection of metal concerned .Since they (Mg,Zn) are sacrificed in process of saving iron,they are called sacrificial anode. (b). IMPRESSED CURRENT PROTECTION METHOD. Inthistechniquecurrenti.e.,asteramofelectronsispassedoverthemetaltobeprotected.Forthis methodtobeeffective,d.c.supplyshouldbemadeavailablethroughouttheperiodforwhichthe structure is to be protected.Thus this method is highly expensive compared to sacrificial anode method. (2). ANODIC PROTECTION It is a new form of corrosion protection ,based on formation of a protective and passive film on metal surfaces by externally impressed anodic currents.many metals such as iron,nickel ,chromium,titanium and their alloys which display active passive transitions,can be passivated by application of suitable and controlled anodic currents which reduce their corrosion rate. Anodic ptotection system for tanks

ALTERATION OF ENVIRONMENT Environment is an important factor in determining the severity of galvanic corrosion. Corrosion rates are considerably influenced by the presence of cations and oxygen ,and change in their concentrations would decrese the corrosion currents and would be helpful in controlling corrosion rate. USE OF INHIBITOR Inhibitors are chemicals which render the corrosive media inert or produce passivity on surface of metal. The different types of inhibitors are : 1.inhibitors which are absorbed on surface and form a protective film sealing the surface,and thus restrict the contact of surface of metal with electrolyte. 2. inhibitors which retard the evolution of hydrogen at the cathodic area,this are very effective in acid media. 3. scavengers which react with corrosive agents and remove them from solution e.g. addition of sodium phosphate and sodium nitrate to boiler water. COATINGS Different types of coatings can be applied on metal surfaces to control the corrosion. Three general types of coatings are: 1.METALLIC COATINGS This are applied to protect the parts against structural damage and to preserve their appearance.It can be applied in thin layers by electroplating,spraying and dipping with atomized molten metal.By application of metallic coatings,corrosion is inhibited in three ways: (a). Metal coating is resistant to corrosion whereas base metal is not: for e.g. a sprayed coating of aluminium on steel gives protection from atmospheric oxidation. High strength aluminium alloys are often coated with pure aluminium skin,which provides a corrosion barrier protecting the underlying alloy from stress corrosion. (b). Coating acts as cathode with respect to base metal: In this case coating is resistant to corrosion only if coating is continuous.(c). Coating acts as anode with respect to base metal:In this case ,coating behaves as an anodic to base metal and the corrosion is prevented by sacrificing the coating until appreciable portions of metal coating are removed.This is called cathodic protection e.g. Zinc coated or galvanized steel sheet and fence wire. The steel is protected because the zinc coating is anodic relative to iron and gets sacrificed. If the surface coating gets scratched, the cathode/anode area ratio is the reverse to tin plate situation as the small scratch is cathodic while the large unbroken surface of zinc is anodic.This will cause very small corrosion current leading to corrosion at slow and uniform rate. 2.CERAMIC AND NON METALLIC INORGANIC COATINGS: These are applied toobtain smoothness,durability,corrosive resistance and good appearance.Porcelain coatings of household appliances etc are common examples. An inorganic nonmetallic coating is also used to obtain hard and thick surfaces resistant to the action of various corrosive agents e.g., steel vessels lined with glass are used for handling various chemicals in the industry due to inertness of glass to a large number of chemicals. (3). ORGANIC COATINGS This include organic films such as paints, resins, varnish, rubber etc. Bye use of inorganic fillers ,quality of covering may improvedand cost is lowered .Protection is achieved due to exclusion of moistur,air,and other corrosive media. However,the organic coatings can be used only below B.P of water , as at high temp. coatings deteriorate . for e.g. Tr and asphalt are used to coat buried pipelines to protect them from both bacterial corrosion and corrosion with soil wate PROPER DESIGN Selection of proper design of equipment may be as imp. In reducing corrosion as prooer material selection. Following general design aspects should be taken into consideration: 1.Additional thickness of the metal over required one, reduces corrosion in case of pipes,tanks. 2. Welding of tanks in caes of riveting , generally reduces the possibility of crevice corrosion. 3. Use of galvanically similar material for the entire structure reduces the corrosion problem. 4. In empty tanks, situation for stagnant pools of corrosive liquid should be avoided. 5.Selection of replacablecomponents which are liable to failure due to corrosion solves the corrosion problem to some extent. 6.While designing heating systems,situations for localized hot spot should be avoided. 7.Entertainment of air in liquid system should be avoided.this is particularly imp. In making the deigns of inlets to pumps,tanks. 8.Sharp bends and corners in piping system should be minimized as these cause entrapment of corrosive liquids. SOME SPECIAL METHODS OF CORROSION PROTECTION PLASTIC COATINGS: plastics are applied to metal surfaces in form of claddings, which are made by glueing foils or plates of material.Plastics used are thermoplastics and thermosets. The plastic materials are in general quite resistant towards aq. solutions of non oxidizing acids, salts and alkalies. The thermosets have good resistance also towards organic solvents, in which thermoplastics tend to swell. PHOSPHATING FILM: Phosphating of steel is usually carried out in phosphoric acid sol. containing acid phosphates of zn and mn. the sol. may be applied by dipping or spraying method. As a result of chemical reaction,iron phosphates are formed which are strongly bonded to the metal surface. These phosphates also act as nuclei for crystallization of sparingly soluble mn or zn phosphates.the phosphating process can be accelerated by add.ofoxidizing agents such as nitrates and chlorates to phosphating bath. CHROMATING: It is used as a passivating process after phosphating.chromating is not alone used on steel but can be used on light metals zn& cd.It is cheap,rapid and carried out easily. This involves an oxidizing treatment of metal surface in chromic acid or chromate both along with accelerating anions.the layers formed contain chromates and chromium oxide. The layers are usually very thin but dense.The adherence of the films formed is very good and the films so formed work as an excellent base for painting . they have little resistance towards abrasion, however , if the layers are damaged at some point,self heating takes place because the diffusion from the surrounding are leads to passivation of exposed surface. GALVANISATION It is a method of applying zinc coatings on metal. Simple and small parts are zinc coated byelectroplating or by dipping in molten metal. Largerobjects may be zinc coated by spraying with molten zinc. The protecting power of electrolytic zinc coatings is often improved by processes such as chromatizinc painting or phosphatizing. In zinc coating of steel by hot dipping known as galvanizing, steel objects are firstly cleaned thoroughly and then dipped into a bath of molten zinc. A fluxing agent is used to dissolve the oxides formed on steel surface between cleaning and galvanizing processes. Thus the flux added facilitates the reaction between iron and zinc.other imp.Methods of zinc coating are :1. Sendmizir 2. Sherardising 3. Spray coating.

TINNING The major application of this process is to provide a hygienic protective coating as well as pleasing appearance to the particles made of steel,copper, brass etc.coatings of tin are widely used in food pacing,food and beverages handling equipments in industry and houses and notably in dairy paint. In general there are three methods of tin plating: 1.HOT DIP COATING PROCESS It involves passage of pickled sheets successively through layers of flux,tin and palm oil. In tin plating, the sheets are not perfluxed but they are introduced in a wet condition and the film of water effectively speeds the fluxing reaction by keeping the flux layer in an embulliant and active condition. The fluxes used are mixtures of ZnCl2, NaCl, NH4Cl, KCl, etc. passing through the tin ,the compound layer is compacted by the lower feed rollers which serve also to carry the sheet onwards where it leaves the tin and enters a deep layer of palm oil called grease pot.in the grease pot, there are a no. of spring loaded pairs of tinned steel rollers which serve to reduce the coating from the thickness entrained to the finished required thickness. 2.WIPING METHOD Here the tin coating is applied by wiping process. In this case tin is applied in stick, molten or powder form, to the prepared and heated surface. The procedure is often employed where particles are large in size, for e.g. cast iron or forged steel marine bearing shells. Such procedure is also useful for tinning the selected portions of the articles.In this process a thin layer of tin is applied to the heated surface of the article. The article is heated gently and evenly by any convenient means. As soon as melting is complete over the whole surface,excess metal and fluxare gently wiped off. Since the first wiper will pick up some quantity of flux, a second light wipe is usually given with a clean wiper kept for this purpose. After the second wipe , a gentle flame may be passed over the surface just to remelt the tin and flatten out wiping marks. 3.ELECTROPLATING PROCESS It is the process of decomposition of coating of metal on the base metal by passing a direct currentthrough a electrolyte solution which contains the soluble salt of the coating metal. The principle of this method is the deposition of coat of the metal over the base metal electrolytically, making use of pure metal as cathode and base metal as anode. These metals are placed in a suitable salt sol. , which serves as an electrolyte. Metals like silver,gold,copper,cadmium etc. are electrodeposited. The electro-deposition depends on several factors such as : 1.Current density 2.The temperature 3.The concentration of electrolyte 4.The time 5.Nature of metal Apart from increasing the resistance of metal to corrosion and chemical attack,electroplating also improves physical appearance,hardness and for decoration purpose. PROCESS: In an electrolytic tank, solution of soluble salt of protective metal is placed. Base metal article is connected to cathode bar and graphite rodor protective metal rod is connected to the anode bar of rectifier and immersed in the electrolytic. Sometimes if needed , the electrolytic tank can be heated by steam or electric heater and can be cooled by water pipes. When electric current is passed through the solution,electrolysis starts and cation of protective metal salt migrate towards base metal cathode and get deposited by capturing electron. ALUMINIZING A combination of aluminium on surface of steel produces a material high in strength as well as resistant to corrosion and oxidation. Aluminium coated steel is called aluminized steel and the process of aluminium coating is called aluminizing. The coating may be applied by several methods: 1.Hot dipping 2.Pack calorizing 3.Spraying 4.Electroplating 5.Vaccum decomposition 6.Roll cladding 7.Powder method