Metallic Coatings For Corrosion Prevention 5ir2s

METALLIC COATINGS FOR CORROSION PREVENTION Corrosion: =It is termed asdeterioration of the material when it comes in with the environment. =>it happens due to the chemical attack from the surrounding environment and envolves transfer of electrons.

*TYPES OF CORROSION* (1)Generator or uniform corrosion (2)Galvanic corrosion (3)Selective leaching (4)Pitting (5)Crevice corrosion (6)Stress corrosion cracking (7)Intergranular corrosion (8)Erosion corrosion *Methods for RESTRICTING CORROSION* (1)Alter the environment (2)Proper material selection (3)Alter design to optimize geometry (4)Employ cathodic protection (5)Use organic coatings such as paints or powder coatings. (6)Use conversion coatings such as chromates or phosphates (7)Use inorganic coatings such as zinc rich paints or phosphates (8)Use metallic coatings (i)Electroless deposited metals e.g NiCl –mechanically applied zinc as in mechanical plating or Sheoridizing (ii)Electrolytically deposited metals,zinc for functional purpose or decorative Ni/Cr or decorative purposes.

*COROSSION PREVENTION BY COATINGS* (1)Inorganic coatings (2)Metal coatings (3)Organic coatings (i)Inorganic coatings: =This can be applied by chemical acion, with or without electrical assistance. -The treatments change the immediate surface layer of metal into a film of metallic oxide or compound which has better corrosion resistance than the natural oxide film and provides an effective base as key for supplementary protection such as paints. Various techniques are: =  Anodizing

   

Chromate filming Phosphatizing Nitriding ive films

Metallic coating (1)Cladding (2)Flame spraying (3)Electro-deposition (4)Hot-dipping (5)Vapor-deposition (6)Diffusion (7)Chemical conversion (8)Surface modification (9)Ion implantation

(a)Cladding Metal on which coating is done is known as substrate.  Its corrosion resistance is improved by metallurgical binding it with a susceptible care alloy having Good corrosion resistance.  The cladding metal is ive to work-atmosphere and in cases of scratches if the substrate is open to atmosphere acts as “Sacrificial anode”.  The difference in potential is 80 to 100 mV.  Cladding is prevalent at mill stage by manufacturers of sheet, plate or tubing. 

Cladding by pressing, rolling, or extrusion can produce a coating in which the thickness and distribution can be controlled over wide ranges, and the coatings produced are free of porosity.



Coatings of wide range of thickness are produced. They are generally thin in comparison to the substrate. For example, Nickel and Steel sheet are rolled together giving a composite sheet of LINCH Steel, 1/8 inch Ni. Ni, Al, Cu, Ti, Stainless Steel and other metals are cladded over steel. Among principal uses Al cladding in the air-craft industry, Lead, Sheathing for cables etc; In Cladding inexpensive material is coated with expensive material having very thin layer. In cladding, inexpensive metal is coated with expensive metal moving very thin layer. Thus, we get lower cost metal with good corrosion resistance

      

.  In roll bonding, layers of two metals are mated by heavy rolling in a mill after the surfaces have been thoroughly cleaned and treated.  Clad thicknesses of 5 to 10 percent of the base steel thickness are common. Some small areas of unbonded metal will be present. Two metals may also be coextruded through a die. 

Cladding can also be accomplished by other methods such as arc or gas welding in which relatively thick layers of weld metal are deposited either by manual or machine methods on surfaces. Application: The interior of pulp digesters or other pressure vessels requiring an alloy composition to resist the chemical conditions can be constructed in this manner.

Vapour deposition (1) This is accomplished in a high vacuum chamber. The coating metal is vaporized by electrically and vapor deposits on part to be coated. (2) It is very expensive and generally used in coating of high strength parts of ROCKETS AND MISSILES. (3) V.D alters mechanical, optical, electrical, thermal, corrosion resistance of the material. (4) There are two categories of VD process:= a. Physical vapor deposition:-Work piece subjected to plasma bombardment b. Chemical vapor deposition:-Thermal energy heats gases in coating chamber and drives the deposition reaction. The most useful CVD coatings are nickel, tungsten, chromium, and titanium carbide. Titanium carbide is used for coating punching and embossing tools to impart wear resistance.

Flame or thermal spraying -Also known as metallizing. Four methods based on the form of the coating material are generally used: (1) The coating material is in the form of rod which is melted by an oxyacetylene flame and blown onto the surface to be coated. (2) The coating material is in the form of a powder which is heated by an oxyacetylene frame, atomized and blown onto the surface. (3) The coating is in the form of a wire. The wire is fed into the central orifice of a nozzle. It es through an oxyacetylene flame and sprayed to the metal surface.

(4) The coating material in the form of a wire is heated by ing it through the plasma of an electric arc. The resulting molten metal is blown out of the arc by an auxiliary gas stream, as droplets. The process is versatile with low capital investment. In the electrostatic spraying process, the particles released from the spray gun are electrostatically charged and propelled at low velocity by air or revolving spray head. Too much air pressure is to be avoided. This procedure produces a good wrap around without the need to positioning the work piece. The process is suitable for tubular articles. -Resulting coating is made by accumulation of numerous sprayed particles. -Coating materials available for thermal spraying include metals, alloys, plastic and compositThickness of coating is from micro to mm.

*Defects* -Coatings are porous and non-protecting under severe wet condition. -porosity decreases with m.p of metal Zn, Sn and Pb. -The substrate surface should be roughed to obtain mechanical bond. -Sometimes paint coating is applied over sprayed metal to fill voids and provide a better barrier. *Application* -Used in ship-hills, super structure , refrigeration equipment , many fabricated steel products. *Several fabrication of flame spraying*:a. Cold spraying b. Detoration spraying c. Warm spraying d. Wire arc spraying e. Plasma Spraying f. High velocity oxy-fuel spraying.

*Chemical conversion*  -Coatings from chemical conversion are produced by “corroding” the metal surface to form an adherent and protective corrosion product.  -Anodizing consists of anodic oxidation in acid bath to build up an oxide layer.  It gives great improvement in corrosion resistance.  -Oxide coatings are produced on steel by heating in air or by exposing to a hot liquid.  -Another purpose of oxide coatings is to produce uniform surface thus, chances of stress corrosion or crevice corrosion decreases. Other examples are:  Parkerizing ( Phosphatizing in a bath of Phosphoric Acid)

P R O C E S S OF P H O S P H A T I NG Phosphate coatings are generally applied by immersion or spraying. The bath contains generally zinc phosphate in phosphoric acid and an oxidizing agent, such as nitrate. The metallic work-piece es through the following stages. A spray zinc phosphate process for steel and zinc. The following is the process cycle: (1) Alkaline cleaning with an alkali (3-8 g/1) at 82°C . (2) Hot rinsing with water at ITC. (3) Cold rinsing with water at 25°C. (4) Acid pickling by H2S04 (15 wt%) at 60°C. (5) Zinc phosphate bath at 25°C. (6) Neutralization by NaNO2. (7) Lubrication with soap at 66°C

A D V A N T A G E S O F PHOSPHATING

Phosphate coatings have a good resistance to corrosion. They are porous and so make an attractive base for application of paints and organic coatings. The rate of corrosion is substantially decreased when paints are applied on phosphate coatings. They are also used as protection against corrosion in combination with sealing film of oil or grease instead of paint. Phosphate coatings find application in military equipment, cars, refrigerators,washing machines, toys and steel strips for painting.

Chromatizing (exposure to chromic acid and Di-chromates) The coating bath consists of an acidic mixture of phosphate, chromate and fluoride. Fluoride acts as an accelerator. It is applied to aluminum. The mechanism is given below: (a) A1+3HF --> A1F3 + 3H+ (b) Cr03 + 3H++H3P04  CrP04 + 3H20 The coating is characterized by the formation of Al203-2Cr04-8H20.  Corrosion resistance Chromate coatings are mostly applied on aluminum components, particularly on aircraft components and on aluminum objects which are to be coated with paints. They are beneficial in marine environments.  Chromium coatings provide a non-porous bond for all paints.

ELECTROPLATING  Metallic coatings are obtained in a conducting substrate by electro-deposition. The metal which is coated is exposed to a solution container containing a salt of the coating metal in a specially designed tank. The metal which is to be electroplated (work-piece) is made the cathode whereas the anode consists of a rod or sheet of the coating metal.  The cathode is connected to the negative terminal of a DC power source and anode to the positive terminal.  A specified voltage is given to the system depending on what metal is to be electro-deposited.  The anode contains the rod of the metal which is to be electro-deposited, for instance, a copper rod is made the anode if copper is to be electro-deposited. If an anode of inert metal is to be used, a suitable metal salt must be added to the electrolyte.  Pure metals, alloys and mixed metals can be electro-deposited.  Three methods are used - VAT, SELECTIVE PLATING AND ELECTRO-LESS PLATING.

o Vat Plating   

The electro-deposition process is performed in tanks or vats having capacities up to thousands of liters. The work-piece is fixed in a jig and suspended in the electrolyte. Inert anodes which do not dissolve in the electrolyte are suspended a few centimeters from the work-piece. The DC current is supplied by a transformer-rectifier (4-8 V). Thin coatings obtained are mainly used for decorative purposes. Dense coatings are produced by vat plating. The thickness of the coating is proportional to the current density.

(B) Selective Plating  In this process, electro-deposition can be made on the desired localized areas without the need for masking and without immersion of components.  The anode is mounted in an insulated handle and covered by an absorbent pad soaked in the electrolyte.  The work is connected to the negative side of a DC power source and circuit is completed by the of absorbent pad with the workpiece.  The process makes masking unnecessary.  The deposition rate is higher than vat plating and this process enjoys the benefit of portability.

Electro-Less nickel (EN) plating Electro-Less nickel (EN) plating is a chemical reduction process that depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy.  Electro-less plating processes are widely used in industry to meet the end-use functional requirements and are only rarely used for decorative purposes.  In the EN plating process, the driving force for the reduction of nickel metal ions and their deposition is supplied by a reducing agent in solution. In contrast to electrolytic nickel coating, electro less nickel coating is deposited without any current as the name indicates.  The metal is formed by the reduction of nickel ions in solution by a reducing agent. Sodium hypophosphite is used as a reducing agent. This plating power is thus relatively constant at all points of the surface of the component, provided the agitation is sufficient to ensure a uniform concentration of metal ions and reducing agents.  The electro-less deposit are therefore very uniform in thickness all over the part’s shape and size. This offers distinct advantages when plating irregularly shaped objects, holes, recesses, internal surfaces, valves, threaded parts, and so forth.

 In a true electro-less plating process, reduction of metal ions occurs only on the surface of a catalytic substrate in with the plating solution. Once the catalytic substrate is covered by the deposited metal, the plating continues because the deposited metal is also catalytic. The following is the mechanism of reduction: (a) Oxidation of Hypophosphite to Orthophosphate in the presence of a catalyst.

-

(H2P02) +H2O Heat

Catalyst

+

H + (HP03)

--

+ 2H (abs)

[H2 is absorbed on the catalyst surface and (H2P02) -Is transformed to (HP03) ]

-

++

(b) Reduction of Ni by adsorbed hydrogen + Ni2+ + 2Ha b s o r b e d Ni+2H (c) Some hydrogen is used to reduce the amount Of (H2P02)

--

--

to form H20, OH

(H2P02) +H (abs)

--

and P. --

H2OOH + P --

(d) Oxidation of hypophosphite to (HPO3) and Release of gaseous hydrogen. --+ (H2P02) +H2O H +HPO3 + H2 Complexing agents are added to prevent the oxidation of reduced nickel and to control the pH. Inhibitors are also added to prevent the decomposition of the solutions in the bath.

Electro-less plated coating is more smooth and continuous. P R O P E R T I ES  The corrosion resistance of electro less nickel is superior to that of electrodeposited nickel.  The coating may contain 6-12% phosphorus which increases the resistance to corrosion.  A uniform coating thickness is obtained by electro less nickel.  The adhesion of electro less nickel coating to the substrate is excellent.  The coatings have high strength, limited ductility and a high modulus of elasticity.  Nickel coating is not resistant to caustic solutions.  Nickel deposits have unique magnetic properties, except deposits containing more than 8 percent phosphorus are essentially nonmagnetic.  In Ni-P coatings, phosphorus is present as supersaturated solution in fine microcrystalline solid solution, bordering on amorphous or liquid-like (glass-like) metastable structure, and this phosphorus is responsible for the non ferromagnetic behavior of as-plated Ni-P deposits with more than 8 percent phosphorus.

 A second generation of EN plating has been developed by Co-depositing micrometer-sized particles of silicon carbide with the nickel, thereby creating an extremely wear- and corrosionresistant coating. In this composite coating, the nickel alloy matrix provides corrosion resistance while the silicon carbide particles add wear resistance.

DIFFUSION COATING or PACK CEMENTATION In this method, the surface of the metal to be coated is modified by diffusing into it at a high temperature ,a metal or an element, which would provide the required resistance when combined with the parent metal. Such coatings are called 'diffusion coatings'. Thus, a surface product entirely different from the metal is produced. Diffusion coatings can be applied to a range of metals and alloys, such as Nickel, Titanium and Molybdenum, but the widest use is on ferrous metals. Examples are Zinc diffusion coatings and Aluminum diffusion coatings.  Powders of carbonaceous materials, Aluminides, Chromium salts, Borides, or Silicides are packed around the object in a container.  Cementation consists of tumbling the work in a mixture of metal powder and a flux at elevated temperatures, allowing the metal to diffuse into the base metal i.e. the mass is taken to high temperature for sufficient time for the carbon, aluminum, chromium, or boron to diffuse into the metal surface. Aluminum and Zinc coatings can be prepared in this way.  Diffusion coatings of Chromium, nickel, titanium, aluminum, and so on, can also be prepared by immersing metal parts, under an inert atmosphere, in a bath of molten calcium containing some of the coating metal in solution.  Coatings are also sometimes produced by gas - phase reaction.  For example, CrCl2, when volatilized and ed over steel at about 1000 ° C (1800 ° F), results in formation of a chromium – iron alloy surface containing up to 30% Cr in accord with the reaction: 1.5 CrCl2 + Fe ----> FeCl3 + 1.5Cr (alloyed with Fe) Similar surface alloys of silicon – iron containing up to 19% Si can be prepared by reaction of iron with SiCl 4 at 800 – 900 ° C (1475 – 1650 ° F). 

P R O P E R T I ES  A hard, abrasion resistant surface of some 20 to 50 µm in thickness can be thus produced.  These coatings are used to impart abrasion and/or high-temperature resistance to gas turbine blades and vanes, among other similar uses.  Aluminum coatings are frequently used on heat exchanger surfaces subject to attack by sulfur gases (H2S, SO2, and SO3) as it is highly corrosion resistant.  Alonising also provides strength to the metal. Carbon Steels, Low Alloy Steels and Stainless Steels are Alonized. The Sherardizing process involves diffusion of a zinc coating into steel by tumbling steel parts with zinc dust at high temperatures. Similarly, Chromizing is a process to diffuse Chromium.

Ion implantation Ion implantation is a process of producing thin surface alloy coatings by bombarding the metal with ions in vacuum.  Ion implantation can be used for any element that can be vaporized and ionized in a vacuum chamber.  Since material is added to the surface, rather than onto the surface, there is no significant dimensional change or problems with adhesion. Advantages:  Implantation is used to alter surface properties, such as hardness, friction, wear resistance, conductance, optical properties, corrosion resistance, and catalysis.  The process is easily controlled, offers high reliability and reproducibility, requires no post treatment, and generates minimal waste. Limitation:



If exposed to high temperatures, however, implanted ions may diffuse away from the surface due to limited depth of penetration and penetration does not always withstand severe abrasive wear.

Commercial availability is limited by general unfamiliarity with the technology, scarcity of equipment, lack of quality control and assurance, and competition with other surface modification techniques. Areas of research includes ion implantation of ceramic materials for high temperature internal combustion engines, glass to reduce infrared radiation transmission and reduce corrosion, as well as automotive parts (piston rings, cylinder liners) to reduce wear.  Such coatings of, for example, Ti, B, Cr, or Y have specialized applications for wear and high temperature oxidation resistance. APPLICATIONS  Glassed Steel or Glass Lined Steel used in Industry, Water tanks, Food Plants, Brewery etc; Smooth Surface is an advantage when ease of cleaning is a requirement of sticky material.  Concrete lined pipes or vessel.

SURFACE MODIFICATION  Surface treatments involving directed energy beams are receiving increasing attention.  Laser Treatment improves resistance to uniform corrosion in HCl and Citrate solution and has no effect on Pitting potential.  Lasers are used to impregnate metals with hard particles for improving wear resistance.

HOT DIPPING Hot-Dip coatings are applied to metals by immersing them in a molten metal bath of Low Melting Point.  It is the oldest example of coating of the metal.  Chief examples are Zn ,Sn , Pb and Al.  “GALVANISING” is the most popular example.  Coated parts are heat treated to form an alloy bond between coating and substrate creating a barrier i.e. part of steel itself.  Thickness of coating thicker than electroplates because very thin dip are difficult to produce.  Galvanized steel is used in chemical process, pulp and papers, automotive and transportation industry.  “Zn” which forms a coating over the substrate forms a ive layer of “ZnO2” thus protecting steel in dry condition.  In case of exposure of metals due to drilling or other activities.  Zn acts as Anode while steel acts as cathode. Thus, dissolution of Zn takes place as it acts as Sacrificial anode. Severity of Galvanic Cell depends on: 1. Duration of wetness 2. Conductivity of electrolyte 3. Ratio of areas of metal in