corrosion control .pdf

8/14/2019 Corrosion Control .pdf

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CORROSION CONTROL

The rate of corrosion can be controlled by eithermodifying the metal or the environment.

1. Proper Designing

A major factor in the corrosion failure of a component

is a faulty geometrical design.Some important design principles are:

1) Avoid crevices

2) Avoid residual moisture

3) Avoid galvanic corrosion

Galvanic corrosion can be prevented by the followingmethods,

a) Use an electrical insulators

b) Introduce an easily exchangeable corroding places

4) Avoid protruding parts.


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2. Cathodic Protection The principle involved in cathodic

protection is to force the metal to be

protected to behave like a cathode.

Since, there will not be any anodic area onthe metal, corrosion does not occur.

There are two types of cathodic protection.

Sacrificial anodic protection method

Impressed current cathodic protection

method.


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SACRIFICIAL ANODIC PROTECTION METHOD

In this method, the metallic structure to be

protected is made cathode by connecting it with

more active metal (anodic metal).

So that all the corrosion will concentrate onlyon the active metal.

The artificially made anode thus gradually gets

corroded protecting the original metallic

structure. Hence this process is otherwise known as

sacrificial anodic protection.


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Examples of sacrificial anode

This method is used for the protection ofships and boats.

Sheets of zinc and magnesium are hungaround the hull of the ship.

Zinc and magnesium being anodic to ironget corroded.

Since they are sacrificed in the processof saving iron (anode), they are called

sacrificial anodes.


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Protection of underground pipelines and

cables from soil corrosion.

Magnesium rods are inserted in to

domestic water boilers or tanks to preventthe formation of rusty water.

Calcium metal slag's are employed to

minimize engine corrosion.

Applications of Sacrificial Anode


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IMPRESSED CURRENT CATHODIC PROTECTION

In this method, an impressed current is

applied in the opposite direction to nullify

the corrosion current and convert the

corroding metal from anode to cathode.

This can be done by connecting negative

terminal of the battery to the metallic

structure to be protected.

Positive terminal of battery is connected to

an inert anode. inert anode used for this

purpose is graphite or platinised titanium.


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The anode is surrounded by backfill

(containing mixture of gypsum, coke,

sodium sulphate) to improve theelectrical contact between the anode and

the surrounding soil.

APPLICATION OF IMPRESSED CURRENT

PROTECTION

This type of cathodic protection is

applied to open water-box coolers, water

tanks, buried oil and water pipes,

condensers, marine piers, transmission

line towers, etc.,


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Comparison of sacrificial anode and

impressed current cathodic method

Sacrificial anodic

method

No external powersupply is necessary.

This method

requires periodical

replacement ofsacrificial anode.

Investment is low.

Impressed current

method

External powersupply must be

present.

Here anodes are

stable and do notdisintegrate.

Investment is more.


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Soil corrosioneffects are not

taken in to

account.

This is mosteconomical

method especially

when short-term

protection isrequired.

Soil corrosioneffects are taken

in to account.

This method iswell suited for

large structures

and long term

operations.


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Control of corrosion by modifying the

environment

DEAREATION

The presence of increased amount of

oxygen is harmful and increase thecorrosion rate.

Deareation involves removal of dissolved

oxygen by increase of temperature with

mechanical agitation.It also removes dissolved CO2 of water.


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In this method, moisture from the air isremoved by lowering the relative humidity ofthe surrounding air.

This is done by adding silica gel (or) alumina,which adsorbs moisture preferentially on its

surface.

DEHUMIDIFICATION

ALKALINE NEUTRALISATION

The acidic character of the corrosive

environment (due to presence of H2S, HCl, CO2,SO2, etc) can be neutralized by spraying alkalineneutralisers (like NH3, NaOH, lime etc).


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CORROSION INHIBITORS

DEFINITIONA corrosion inhibitors is a substance which when

added in small quantities to the aqueous corrosive

environment effectively decreases the rate of corrosion of the metal.

Inhibitors are classified in to three types,

ANODIC

CATHODIC

VAPOUR PHASE


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Chromates, phosphates, nitrite, nitrate,inhibit the anodic corrosion reaction byforming sparingly soluble compound with anewly produced metal ion (at the anode).

They are absorbed on the metal surfaceforming a protective film or barrier there- by

reducing corrosion rate.

This kind of corrosion rate is not fully

reliable since certain areas left uncovered bythe film can produce severe corrosion.

ANODIC INHIBITORS


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In acidic solution, the main cathodic

reaction is evolution of hydrogen.

2H+(aq) +2e- H2 (g)

In an acidic solution, the corrosion canbe controlled by slowing down the

diffusion of H+ ions through the cathode.

This can be done by adding organic

inhibitors like amines, pyridine, azoles,etc.

They absorb over the cathodic metal

surface and act as a protective layer.

CATHODIC INHIBITORS


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In a neutral solution, the cathodic reactionis,

H2O + O2 + 2e- 2OH-(aq)

The formation of OH- ions is only due to the

presence of oxygen.By eliminating the oxygen from the medium,the corrosion rate can be reduced.

O2 can be removed by adding some reducingagents like Na2SO3 or by deaeration.

Salts of Zn, Mg, Ni are employed as theyform insoluble metallic hydroxide whichforms impermeable self barriers.


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VAPOUR PHASE INHIBITORS

Vapour phase inhibitors are organicinhibitors which readily sublime and form aprotective layer on the metal surface.

Example : Dicyclohexyl ammonium nitrite,Benzotriazole.

Vapour phase inhibitors are used in the

protection of machineries, sophisticatedequipments, etc. which are sent by ships.

The condensed inhibitor can be easily wipedoff from the metal surface.


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PROTECTIVE COATING

INTRODUCTION

Protective coatings are used to protectthe metals from corrosion.

It acts as a physical barrier between thecoated metal surface and theenvironment.

They impart some special properties suchas hardness, electrical properties andthermal insulating properties to theprotected surface.


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Protectivecoatings

Inorganic coating

Metallic coating Chemical

Conversion

Organic coating

1. Paints2. Varnishes

3. Enamels

4. Ceramic


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Mechanical cleaning To remove loose scale

and rust, using hammer, wire-brushing, grinding

and polishing.

Sandblasting To clean large surface areas in

order to produce enough roughness for good

adherence of protective coating, using sand

with air stream at 25-100 atm.

Solvent Cleaning To remove oil, grease, rust

using organic solvents like alcohol, xylene,

toluene, hydrocarbons followed by cleaning hot

water or steam.

Sample Preparation


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Sample Preparation (contd..)

Alkali Cleaning To remove old paints that are

soluble in alkaline medium using chemicals l ike

NaOH, Na3PO4 etc. After cleaning, the metal is

washed with 1% chromic acid solution.

Acid pickling and etching Base metal is dipped

inside acid solution at a higher tempt for a long

duration. Acids used are HCl, H2

SO4

, H3

PO4

, HNO3

,

under dilute conditions.


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Metallic coatings

Anodic coatingGalvanization:

It is produced by anodic coating metals (Zn, Al,

Cd) on the surface of base metal (Fe) based on

the relative negative electrode potential.

Cathodic coating:

It is produced by cathodic coating metals (Sn,Cr, Ni) on Fe surface based on the relative

positive electrode potential of coat metal.


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Methods of application of

metallic coating

Hot dipping

Metal cladding

Electroplating Cu, Cr, Ni, Au, Ag

Vacuum metalizing

Metal spraying


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Hot Dipping

It is one of the common method of applying

metallic coating on the surface of base

metals.

Hot dipping is a process of coating the base

metal by immersing it in the molten coat

metal.

Examples: Galvanizing and Tinning


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Tinning: In this process tin is coated over mildsteel sheets immersed in molten tin (Sn).

The sheet is subject to acid pickling and passedthrough a bath of molten tin covered with a fluxof ZnCl2.

After coating, the sheet is passed through palmoil to protect from oxidation

Finally the sheet is passed to roller to getuniform thickness.

It is used for the coating of steel, Cu and brasssheets that store food stuffs.


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Metal Cladding

It is the process of sandwitching the basemetal between two thin layers of coating metalby hot-rolling the composite to produce a firmbonding.

The coat metals are usually metals of leastreactivity (Cu, Ni, Ag, Pt, Ti)

The cladding layer should be very thin and itsthickness is only 5% of the total composite

metal. Duraluminium sandwiched between Al sheets

and hot rolled to produce Alkad compositewhich is free from stress corrosion


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ELECTROPLATING

PRINCIPLE

Electroplating is the process in which the coatingmetal is deposited on the base metal by passing adirect current through an electrolytic solutioncontaining the soluble salt of the coating metal.

Electroplating is probably the most important andmost frequently applied industrial method of producing metallic coatings. The metal film

produced is quite uniform with little or no pinholesper unit area.

When the thickness of the deposit increases, thenumber of pinholes decreases.


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The base metal to be plated is made

cathode of an electrolyte cell, whereas theanode is either made of the coating metalitself or an inert material of good electricalconductivity.

THEORY

If the anode is made of coating metal itselfin the electrolytic cell, during electrolysis,

the concentration of electrolytic bathremains unaltered, since the metal ionsdeposited from the bath on cathode arereplenished continuously by the reaction offree anions with the anode.


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Objectives of electroplating:

(i) To increase the resistance to corrosion and

chemical attack of the plated metal.

(ii) To obtain a polished surface

(iii) To improve hardness and wear resistance

Example: Electroplating of Cu, Au, Ag, Cr, Ni, Sn etc.

Uses of electroplating:

(i) It is often used in electronic industries formaking printed circuit boards, edge connectors,

semiconductor lead-out connection

(ii) It is also used in the manufacture of jewelery,

refrigerator, electric iron etc.


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Electroplating of Cu

For electroplating of Cu on metal surface, Electrolyte: (3-5%)H2SO4 / (15-30%) CuSO4

Anode: Pure Cu metal or Graphite (inert)

Cathode: Metal to be coated

Additive: Boric acid or gelatinIonization reaction of electrolyte is observed,

CuSO4 Cu2+ + SO4

2-

On passing current, Cu2+ + 2e- Cu (at cathode)

SO42- SO4 + 2e

- (at anode)

H2SO4 2H+ + SO4

2-

Due to common ion effect, the ionization rate of Cu2+ is controlled

and the deposition process can also be controlled, with a current

density of 0.5 to 1.5 ampere/dm2.


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Factors affecting electroplating

Surface cleaning for strong adherent Concentration of electrolyte Moderate conc. Is

preferred for uniform coating

Conductivity and stability of electrolyte - Good

Thickness of the deposit for decorative purposethin coating and for corrosion protection multiple

coating.

Current density (current per unit of the base metal)

should be low for uniform controlled deposition Additives: Ensure strong adherence and mirror

smooth coating.

pH of the electrolytic bath between pH 4.0 - 5.5


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Thin-Film Coatings (m)

PVD Coating (Physical Vapor Deposition)

CVD Coating (Chemical Vapor Deposition)

Physical Vapour Deposition, PVD a group of vacuumcoating techniques that are used to deposit thin f ilm

coatings that enhance the properties and performance of

tools and machine components.

PVD coatings are used in a vast array of industries andthousands of applications as diverse as "self-cleaning"

windows, medical implants, cutting tools, decorative

fitt ings and Formula 1 racing parts.


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Thin-film property

category

Typical applications

Optical Reflective/antireflective coatings

Interference filters, Decoration (colour,

luster), Memory discs (CDs),

Waveguides

Electrical Insulation, conduction, Semiconductor

devices, Piezoelectric drivers

Magnetic Memory discs/devices

Chemical Barriers to diffusion or alloying

Protecting against corrosion or

oxidation

Gas/liquid sensors

Mechanical Tribological (wear resistant) coatings

Hardness, Adhesion, Micromechanics

Thermal Barrier layers, Heat sinks

Classifications of thin-films based on their applications


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Deposition techniques


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Deposition by Physical Vapour

Deposition (PVD)

To vacuum pump

PVD Chamber

Suction valve

9/13/2013 6:21 PM 35

Nanotechnology

N2(or)

H2

Heater

Ni Source

Ni film

H2 = 50 psi

N2 = 15 psi

Substrate

2000oC


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PVD is a process to produce a metal vapor that can be

deposited on conductive materials as a thin highly adhered

pure metal or alloy coating.The process is carried out in a vacuum chamber at high

vacuum (10-6 torr).

Single or multi -layer coatings can be applied during the same

process cycle.

Additionally the metal vapor can be reacted with various

gases to deposit Oxides, Nitrides, Carbides or Carbonitrides.

The coating method involves purely physical processessuch as high temperature vacuum evaporation or plasma

sputter bombardment rather than involving a chemical

reaction at the surface to be coated.


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Aluminium Titanium Nitride coated

Titanium Nitride coated punches

Aluminium Chromium

Titanium Nitr ide coated


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Structure of TiAlCN


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Cathodic Arc Deposition: In which an electric arc is usedto vaporize material from a cathode target. Thevaporized material condenses on a substrate, forming athin film.

Evaporative deposition: In which the material to bedeposited is heated to a high vapor pressure byelectrically resist ive heating in " low" vacuum.

Sputter deposition: In which a glow plasma discharge(usually localized around the "target" by a magnet)

bombards the material sputtering away as a vapor.

Ion plating: In which the material is heated to a highvapor pressure and a plasma is established to ionize theevaporating species. These species physically implantinto the substrate producing strong coating bond.

Different types of PVD coating


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Sputtering

Coating can be done for both conductive (dc) and non-conductive (RF sputtering) materials

DC sputtering the workpiece and substance to be

coated are connected to high voltage dc power supply.

Vacuum chamber is fil led with controlled amount ofargon gas to establish a pressure of 10-4 torr.

The supplied direct current energizes the chamber

creating a plasma between the workpiece and the

material to be coated.

The argon atoms get ionized and accelerated to bombard

on the workpiece resulting in the sputtering of atoms,

which are transported and coated on the substance


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The parts to be coated are first cleaned. The cleaningprocess varies depending on the level of quality from the

electroplater, substrate material and geometry.

The parts are loaded into the vacuum chamber on custom

fixtures designed to optimize the chamber load size andensure coating uniformity.

The vacuum chamber is evacuated to 10-6 torr (high vacuum)

to remove any contaminants in the system.

The vacuum chamber is backfi lled with an inert gas argon

and ionized, result ing in a glow discharge (plasma). This is

the gas cleaning stage and prepares the parts for the ini tial

metal deposition.

PVD Process


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A high current, low voltage arc is initiated on the target

(solid material used for deposition). The metal is

evaporated and instantaneously ionized.

These metal ions are accelerated at high energies into

the vacuum through an inert gas or reactive gas and

subsequently deposited on the part.

The basic properties of the metal being evaporated

(target) remain unchanged during the metal deposition

cycle.

Changing the volume of gas and type of gas during the

reactive deposition cycle changes the nature of the

coating.


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Zirconium nitride (ZrN) is a hard, yellow-gold colored

coating with exceptional wear and corrosionresistance, used in plumbing and door hardware

industry.

Introducing measured amounts of nitrogen into thechamber during the zirconium deposition cycle

produces zirconium nitride.

Chromium nitride is produced in much the same way.

Simply by adding an additional gas such as acetylene(C2H2), you can create chromium carbonitride. This is a

gray to black color.


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TiN 2900 HV Gold

ZrN 2800 HV Gold TiAlN 2600 HV Brown

TiCN 4000 HV Silver

CrN 2500 HV Silver

DLC 1000 to 5000 HV - Black

Coating processes are performed at 300 0C or upto 2800 0C.

The higher temperature processes usually produce optimum

coating properties but sometimes results in softening ofsubstrates especially steel.

Thickness usually in the range of 1 to 2 m.


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CVD involves the formation of a solid f ilm on a surface of a heated

substrate by means of a chemical reaction in a gas or in the vapor

phase.The complex molecule in the vapor state impinges on the hot

substrate, decomposes and forms a thin film. These reactions are

promoted by resistance, RF or infrared radiation heating.

Example: Monds Process

Ni(CO)4 Ni + 4CO150

C

CVD process

Requirements for a CVD process

1. Vacumised chamber (10-3 mbar) connected to a rotary pump

2. Complex chamber with heating facility

3. Vapour transport SS tubes ( quarter inch)

4. Substrate holder inside the chamber with heater (flat heater

900oC)

5. High Temperature valve to control the flow rate of complex vapour


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To vacuum pump

CVD Chamber

Suction valve

9/13/2013 6:21 PM 46Nanotechnology

Pre

Vap

N2(or)

H2400oC

Heater

Substrate

Ni film

H2 = 50 mm Hg

N2 = 15 mm Hg

Deposition by Chemical Vapour

Deposition (CVD)

Solidcomplex

HTV


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Step 1: The vapor source of the film-forming material

may be a solid, liquid, vapor or gas. Solid materials,having sufficient vapor pressures, need to be vaporized

to transport them at moderate temperature to the

deposition zone where the substrate is placed, and this

is normally achieved by heating.

Step 2: Another issue is the uniformity of arrival rate of

vapor sources by transport to the hot substrates. This

uniformity in transporting the vapor source varies with

the transport medium used to transport the source to thedestination (i.e. either by high vacuum or fluid (gaseous

fluid).

CVD Process steps


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Step 3: Deposition is the third step in a process of

developing thin-films, in which the actual growth of film

over the surface occurs. Deposition pattern is

determined by the source, transport of vapor and

conditions of deposition zone.

Step 4: Once the deposition is over, the next step is the

analysis of coated thin-films by various techniques.

Analysis of thin-films can be thought of as the final stageprocess of monitoring but it is important in all steps of

thin-film deposition.

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