CORROSION AND PROTECTIVE COATINGS

These coating metals usually have higher corrosion resistance than the base metal...  In this method, the coating metal is supplied in the form of tine powder which is converted in to a

CORROSION AND PROTECTIVE COATINGS

 1 Proper selection and designing

 2.Cathodic and anodic protection

 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

1 Proper selection and designing

2. Cathodic Protection

In this method, the corroding metal is forced to behave like a cathode There are two types of cathodic 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

a Sacrificial Anodic protection

Sacrificial Anode

 Applications

 Important applications of sacrificial anodic method include protection of

buried pipe lines, underground cables, marine structures etc

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

b Impressed current cathodic protection

 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

1 Long term protection is needed

2 Large structures are to be protected

3 There is a cheap source of electrical

power

Applications

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 are mostly applied on Iron and steel because these are cheap and commonly used construction materials There are two types of metallic coatings.

Metallic 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

i Anodic coatings

 It is 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

ii Cathodic Coatings

METHODS OF APPLICATION OF METALLIC COATING

 It is used for producing a coating of low melting metal such as Zn, Sn,

Ph, Al etc on relatively higher melting metals such as iron, steel, copper etc This is done by immersing the base metal covered by a layer of molten flux The flux is used to keep the base metal surface clean and also to prevent oxidation of the molten metal Most widely used hot dipping methods are : (i) galvanization and (ii) tinning

 It is the process of coating Zn over iron or steel sheet by immersing it

in molten Zn The procedure involves the following stages

 The iron or steel article is Ist cleaned by pickling with dil H2So4 for 15 – 20 min at 60 – 900C in an acid bath This treatment also removes any oxide layer present on the surface of the metal The article is then washed with water in a washing bath & dried in a drying chamber

a. Galvanization

 It is then dipped in a bath of molten Zn kept at 425 – 4350C The

Surface of the bath is covered with NH4Cl flux to prevent oxide formation

 The article gets coated with a thin layer of Zn It is then passed through a pair of hot rollers to remove excess of Zn and to get uniform thickness for coating Then it is annealed at about 6500C & cooled slowly In the case of Zn coating even if the protecting layer has cracks on it, iron being cathodic does not get corroded

 It is an eg For cathodic coatings It is the process of coating Sn over Fe or steel

articles by immersing it in molten Sn The process consists in Ist treating the iron sheet with dil H2So4 to remove any oxide film After this it is passed through a bath

of ZnCl2 flux which helps the molten Sn to adhere to the metal sheet Next the sheet passes through palm oil which prevents through a pair of hot rollers to remove excess

of Sn & produce uniform thickness for Sn coating.

ii Tinning

 Tinning is widely used for coating steel, Cu and brass sheets which are

used for making containers for storing food studs, oils, kerosene & packing food materials Tinned Cu sheets are used for making cooking utensils & refrigeration equipments

Applications

In this process, a thick homogeneous layer of coating metal is bonded firmly & 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 cladding material over the base metal

b    Welding

c Rolling sheets of cladding material over base metal

 In this process, the coating metal in the molten state is sprayed on the previously cleaned base metal with the help of a sprayer The sprayer coatings are continuous but somewhat porous a sealer – oil is applied

on such a coating to provide a smooth surface However, adhesion strength of metallic spraying is usually lesser that obtained by hot dipping or electroplating It is therefore essential to have a cleaned metal surface Spraying can be applied by the following two techniques

3.Metal spraying

 In this method, the coating metal in the form of 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

i Wire – gun 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.

Powder – metal method

 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 through an electrolyte containing the soluble salt of the coating metal The base metal to be electroplated is made the cathode of the electrolytic cell whereas the anode is either made of the coating metal itself or an inert material of good electrical conductivity like graphic.

4 Electroplating or Electrodeposition

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Electroplating

 For electroplating of Ni, NiSO4 and NiCl2 are used as the electrolyte For electroplating of Cr, chromic acid is used as the electrolyte For Au plating, AuCl3 solution is taken as the electrolyte For Cu plating CuSO4 solution is used as the electrolyte In silver plating, AgNO3 solution is used as the electrolyte.

Metallic plating process driven entirely by chemical reactions ‑ no electric current

is supplied

 Deposition onto a part surface occurs in an aqueous solution containing ions

of the desired plating metal

◦ Workpart surface acts as a catalyst for the reaction in the presence of reducing 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

5.Electroless Plating

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 film

 Coating materials: metals, alloys, 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

6.Physical Vapor Deposition (PVD)

 Decorative 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, Fundamentals of

Applications of PVD

 All physical vapor deposition processes consist of the following steps:

1. Synthesis of coating vapor

2. Vapor transport to substrate

3. Condensation of vapors onto substrate surface

 These steps are generally carried out in a vacuum chamber, so evacuation of

the chamber must precede PVD process

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Processing Steps in PVD

 Setup for vacuum

evaporation, one form of

PVD, showing vacuum

chamber and other

process components

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Physical Vapor Deposition

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 occur in enclosed reaction chamber

 Reaction product nucleates and grows on substrate surface to form the coating

 Most CVD reactions require heat

 Variety of coating and substrate materials

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

7.Chemical Vapor Deposition (CVD)

 Typical reactor

used in CVD

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Chemical Vapor Deposition

 Industrial metallurgical processes

◦ Mond process to reduce nickel from its ore

 Coated carbide tools

 Solar cells

 Refractory metals on jet engine turbine blades

 Integrated circuit fabrication

 Other applications for resistance to wear, corrosion, erosion, and thermal shock

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Applications of CVD

Embedding atoms of one (or more) foreign element(s) into a substrate surface using a high ‑energy beam of ionized particles

 Results in alteration of the chemistry and physical properties of layers near the substrate surface

 Produces a much thinner altered layer and different concentration profile than diffusion

 Alternative to diffusion when the latter is not feasible due to high temperatures required

8.Ion Implantation

 Advantages

◦ Low temperature processing

◦ Good control and reproducibility of impurity penetration depth

◦ Solubility limits can be exceeded without precipitation of excess atoms

 Applications

◦ Modifying metal surfaces to improve properties

◦ Fabrication of semiconductor devices

©2010 John Wiley & Sons, Inc M P Groover, Fundamentals of

Advantages and Applications of Ion Implantation

a CHEMICAL CONVERSATION COATINGS OR SURFACE

CONVERSATION COATINGS

These coatings are produced on the surface of a metal or alloy by chemical or electrochemical reaction The metal is immersed in a solution of suitable chemical which reacts with the metal surface producing and adherent coating These coatings protect the base metal from corrosion Moreover many of these coatings are particularly useful to serve as excellent bases for the application of paints, enamels and other protective coatings The most commonly used surface conversion coatings are chromate coatings, phosphate coatings and chemical oxide coatings

 There are produced by the immersion of the article in a bath of acidic

potassium chromate followed by immersion in a bath of neutral chromate solution The surface film consisting of a mixture of trivals and hexavalent

Cr is formed Chromate coatings possess more corrosion resistance and can also be used as a base for paints These are applied on Zu, Cd, Mg and Al

1 Chromate Coatings

 These are produced by the chemical reaction of base metal with aqueous solution of

phosphoric acid and a phosphate of Fe, Mn or Zn The reaction results in the formation of a surface film consisting of phosphate of a surface film consisting of phosphates of the metal These coatings are usually applied by immersing or spraying or brushing These coating do not give complete corrosion resistance but can serve as base for painting These are applied on metals like Fe, Zn, Cd, Al and Sn.

II.Phosphate coating

These types of coatings are formed on the surface of metals like Fe, Al, Mg etc

by treating the base metal with alkaline oxidizing agents like potassium permanganate This treatment increases the thickness of the original oxide film

on the metal, there by increasing the corrosion resistance Oxide coatings form a good base for paints These oxide coatings have got only poor corrosion resistance However, for better protection the thickness of the oxide film can be increased 100 to 1000 times by electrolytic oxidation or anodisation.

 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 H2SO4, 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.

III.A -Anodisation or Anodised Coatings

 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.