CORROSION AND PROTECTIVE COATINGS

CORROSION AND PROTECTIVE COATINGS PROTECTION FROM CORROSION  Proper selection and designing  2.Cathodic and anodic protection  3.Protective coating  (a ) Metal Coating  (b) Inorganic coating  (c) Organic coating  Corrosion Inhibitors Proper selection and designing  Design an equipment by avoiding contact between two dissimilar metals  Maintaining a larger anodic area of the metal  Metals should be close in the galvanic series Cathodic Protection In this method, the corroding metal is forced to behave like a cathode There are two types of cathodic protection a Sacrificial Anodic protection In this method, the metallic structure which is to be protected from corrosion is connected to a more anodic metal by a wire so that the entire corrosion is concentrated on this more active metal The more active metal loses and get corroded and this metal is called sacrificial anode Metals commonly employed as sacrificial anode are Mg, Zn, Al and their alloys Sacrificial Anode  Applications  Important applications of sacrificial anodic method include protection of buried pipe lines, underground cables, marine structures etc b Impressed current cathodic protection In this method, an impressed current is applied in the opposite direction to nullify corrosion current so as to convert the corroding metal from anode to cathode Impressed current can be derived from a direct current source like battery An inert or insoluble electrode like graphite or silica act as anode to complete the circuit The surroundings of anode should be filled with salts and carbon to increased the conductivity Applications  This type of cathodic protection has been applied to water coolers, water tanks, buried oil and water pipes, transmission towers etc This type of protection is employed when Long term protection is needed Large structures are to be protected There is a cheap source of electrical power III.A -Anodisation or Anodised Coatings  Anodised coatings are generally produced on non – ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation process In this process, the base metal is made as anode and the cathode is an inert electrode like graphite The electrolytic bath is usually of H SO , chromic acid, boric acid, phosphoric acid, oxalic acid etc  The base metal to be anodized is suspended from the anode The process is carried out by passing a moderate direct current through the electrolytic bath As the anodized coatings are somewhat thicker than the natural oxide film and they posses improved resistance to corrosion  Anodizing on Al has gained considerable commercial importance Al coated surface require oxidation to convert the metal to its inert oxide Anodising on Al is carried out by an electrolytic process The O evolved at the anode oxides the outer layer of Al to the oxide film, Al2O3 the oxide film initially very thin, grows from the metal surface outwards and increases in thickness as oxidation continues at Al anode The outer part of the oxide film formed is porous and to reduce porosity, the article after electrolysis is kept immersed in a boiling water bath This treatment changes porous alumina into its monohydrate (Al O H O) which occupies more, volume, thereby the pores are sealed Al + 3O Al O + H O Al O Al O H O Anodized coatings may be coloured with organic dyes and inorganic pigments to give decorative effects Applications  Aircraft parts, refrigerators, reflectors, machine parts etc are anodized by this method Al articles used as doors, windows, showcase panels & household utensils are anodized by this method b VITREOUS COATINGS OR CEREMIC PROTECTIVE COATINGS  Ceramic protective coatings can be broadly divided into vitreous enamel coatings and pure ceramic coatings These coatings have the following advantages 1.They posses high refractoriness and inertness 2.They are wear resistant & easily be cleaned 3. They are glossy in appearance 4.They are good thermal & electrical insulators Vitreous enamels are defined as glossy inorganic composition that can adhere to metals by fusion and protect them from corrosion, abrasion, oxidation and high temperature Vitreous enamel coatings consists of a ceramic mixture of refractories and large proportion of fluxes These coatings are usually applied on steel and cast iron equipments The raw materials used for the vitreous coatings are the following Vitreous coatings 1.  Refractories like quartz (SiO ), clay etc 2.  Fluxes like borax (Sodium tetra borate (Sodium Na B O ), cryolite (Na AlF ) alumino fluoride), Soda ash (anhydrous sodium carbonate Na CO ) etc 3.   Opacifiers like TiO , SnO , Al O etc 2 4.   Pigments like metallic oxides organic dyes etc 5.    Floating agents like plastic, clay, gum etc 6.    Electrolytes like MgSO , MgCO , Na Co 3 etc Organic Coatings Polymers and resins (natural or synthetic) usually formulated to be applied as liquids that dry or harden as thin surface films on substrate materials  Advantages: ◦ ◦ ◦ ◦ Wide variety of colors and textures available Capacity to protect the substrate surface Low cost Ease with which they can be applied ©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of Modern Manufacturing 4/e Ingredients in Organic Coatings Binders - give the coating its properties Dyes or pigments - provide color to the coating Solvents - dissolve the polymers and resins and add proper fluidity to the liquid Additives ©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of Modern Manufacturing 4/e IV USE OF CORROSION INHIBITORS Chemicals which are added in small quantities to the corroding medium in order to reduce the corrosion rate are called corrosion inhibitors They reduce corrosion by forming a protective film either at the cathode or anode Thus there are two types of corrosion inhibitors – anodic inhibitors and cathodic inhibitors Anodic inhibitors Chromates (CrO 232), phosphate (PO ) and Tungstates (WO ) of 4 transition metals are used as anodic inhibitors They react with the newly produced metal ions at the anode forming a protective film or barrier there by preventing further corrosion Cathodic inhibitors Cathodic reaction takes place with either evolution of H O depending on the nature of the corroding medium 2 or absorption of 1.  Evolution of H2 in acid medium 2H+ + 2e-  H (g) + Evolution of H can be prevented by slowing down the diffusion of H ions to + the cathode or by increasing H over voltage Diffusion of H ions can be prevented by adding organic inhibitors such as amines, urea, thiourea etc These are adsorbed at the surface as a film Arsenic oxide or antimony oxide is added to increase the H over voltage These oxides form adherent film of metallic arsenic or antimony at the cathodic areas II Absorption of O2 in metal or alkaline medium H O + ½ O + 2e-  OH 2 The formation of OH ions can be prevented either by removing O or by decreasing the diffusion of O in to the cathode O 2 from the medium is removed either by adding reducing agents like Na SO , N H etc or by mechanical dearation Na SO + O  N H  + O 2 Na SO N + 2H O 2 Salts of Zn, Mg or Ni are added to the corroding medium to reduce the diffusion of O towards cathode These salts react with OH ions at the cathode forming insoluble hydroxides which are adsorbed at the cathode II Absorption of O2 in metal or alkaline medium H O + ½ O + 2e-  OH 2 The formation of OH ions can be prevented either by removing O or by decreasing the diffusion of O in to the cathode O 2 from the medium is removed either by adding reducing agents like Na SO , N H etc or by mechanical dearation Na SO + O  N H  + O 2 Na SO N + 2H O 2 Salts of Zn, Mg or Ni are added to the corroding medium to reduce the diffusion of O towards cathode These salts react with OH ions at the cathode forming insoluble hydroxides which are adsorbed at the cathode [...]...3. Protective coatings An important method for protecting a metal from corrosion is to apply a protective coating The protective coatings may be of metal, inorganic or organic The coated surface isolates the metal from the corroding medium The coating applied must be chemically inert towards the environment Metallic Coatings  Metallic coatings are mostly applied on Iron and steel because... ceramics and other inorganic compounds, even some polymers  Substrates: metals, glass, and plastics  Very versatile coating technology ◦ Applicable to an almost unlimited combination of coatings and substrate materials ©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of Modern Manufacturing 4/e Applications of PVD  Decorative coatings on plastic and metal parts such as trophies, toys, pens and. .. Iron and steel because these are cheap and commonly used construction materials There are two types of metallic coatings i Anodic coatings  The base metal which is to be protected is coated with a more anodic metal for eg Coatings of Zn, Al and Cd steel are anodic because their electrode potentials are lower than that of the base metal ie Fe ii.   Cathodic Coatings  It is obtained by coating a... permanently to the base metal on one or both the sides This method cnhanceds corrosion resistance The choice of cladding material depends on the corrosion resistance required for any particular environment Nearly all existing corrosion resisting metals like Ni, Cu, Al, Ag, Pt and alloys like stainless steel, Ni alloys, Cu alloys can be used as cladding materials Cladding can be done by different means a.   Fusing... obtained by coating a more inert metal having higher electrode potential Than the base metal Eg Coating of Sn, Cr, Ni on Fe surface The coating should be continuous and free from pores and cracks These coating metals usually have higher corrosion resistance than the base metal METHODS OF APPLICATION OF METALLIC COATING 1 Hot Dipping  It is used for producing a coating of low melting metal such as... coatings on plastic and metal parts such as trophies, toys, pens and pencils, watchcases, and interior trim in automobiles  Antireflection coatings of magnesium fluoride (MgF2) onto optical lenses  Depositing metal to form electrical connections in integrated circuits  Coating titanium nitride (TiN) onto cutting tools and plastic injection molds for wear resistance ©2010 John Wiley & Sons, Inc M P Groover,... evaporation, one form of PVD, showing vacuum chamber and other process components ©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of Modern Manufacturing 4/e 7.Chemical Vapor Deposition (CVD) Involves interaction between a mixture of gases and the surface of a heated substrate, causing chemical decomposition of some of the gas constituents and formation of a solid film on the substrate  Reactions... thin wire is melted by an oxy – acetylene flame and vaporized by a blast of compressed air The coating metal adheres to the base metal Al is coated on aircraft steel parts using this techniques Powder – metal method  In this method, the coating metal is supplied in the form of tine powder which is converted in to a cloud of molten globules by a blower and are adsorbed on the base metal surface 4 Electroplating... converted in to a cloud of molten globules by a blower and are adsorbed on the base metal surface 4 Electroplating or Electrodeposition  it is probably the most important and most frequently applied industrial method of producing metallic coatings Electroplating is carried out by a process called electrolysis Thus in this process, the coating metal is deposited on the base metal by passing direct current... agent  Metals that can be plated: nickel, copper, and gold  Notable application: copper for plating through‑holes of printed circuit boards ©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of Modern Manufacturing 4/e 6.Physical Vapor Deposition (PVD) Family of processes in which a material is converted to its vapor phase in a vacuum chamber and condensed onto substrate surface as a very thin ... paints, enamels and other protective coatings The most commonly used surface conversion coatings are chromate coatings, phosphate coatings and chemical oxide coatings 1.Chromate Coatings  There... method b VITREOUS COATINGS OR CEREMIC PROTECTIVE COATINGS  Ceramic protective coatings can be broadly divided into vitreous enamel coatings and pure ceramic coatings These coatings have the... further classified in to chemical conversion coatings and vitreous coatings a CHEMICAL CONVERSATION COATINGS OR SURFACE CONVERSATION COATINGS These coatings are produced on the surface of a metal