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CHAPTER 5 : CORROSION & NON-FERROUS METAL

5.0 What is Corrosion?

Corrosion is defined as the destruction of a metal by chemical or electrochemical reaction with its surrounding (environment).

Corrosion can occur in a gaseous environment (dry corrosion) or a wet environment (wet corrosion).

Importance of corrosion: 1. Economic – direct or indirect losses 2. Improved safety – failure of critical component 3. Conservation of resource – wastage of metal or energy

Corrosion falls into 2 main categories: 1. General or uniform corrosion 2. Localised corrosion

5.1 General or Uniform Corrosion

The electrochemical reactions occur at the same rate over the entire surface.

This type of attack is mostly found where a metal is in contact with an acid, a humid atmosphere or in a solution.

Example 1:

Zn + HCl ZnCl2 + H2 (g)

Oxidation : anodic Zn Zn+2 + 2e-

Reduction : cathodic 2H+ + 2e- H2

Zn

Zn

HCl

Zn

Zn

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Example 2:

Place the piece of Zn in a solution containing copper sulphate

CuSO4 (blue solution).

Observation: A dark deposit of Cu on Zn and fading of the blue

solution.

Zn + Cu+2 Cu + Zn+2

Oxidation : anodic Zn Zn+2 + 2e-

Reduction: cathodic Cu+2 + 2e- Cu

Conclusion : Any reaction that can be divided into two or more

partial reactions of oxidation and reduction is called

electrochemical.

Prevention : Proper material selection, change the environment,

Cathodic protection.

Zn

Zn

HCl

Zn

Zn

CuSO4

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5.2 Localised Corrosion

There are different types of localised corrosion:

1. Galvanic corrosion

2. Pitting corrosion

3. Crevice corrosion

4. Intergranular corrosion

5. Dealloying

6. Fretting corrosion

7. Cavitation corrosion

8. Erosion corrosion

9. Environmentally induced cracking

i. Hydrogen embrittlement

ii. Stress corrosion cracking (SCC)

iii. Corrosion fatigue

5.2.1 Galvanic Corrosion

Occur when 2 different metals are electrically connected in the

same electrolyte.

The less active (more noble) metal corrodes slower and will be

protected.

The galvanic series will predict which metal will corrode.

The galvanic series is similar to the “emf” but is for alloys in real

environment.

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Experimental corrosion (zinc and cuprum) :

i. A zinc electrode connected to a cuprum electrode and

immerse in an electrolyte such as salt water, acid or

alkaline

ii. The cuprum acts as cathodic and the zinc as anodic

iii. Zinc will be corrode caused by the electrochemical

corrosion

iv. Time to time the zinc will continue to corrode and

became embrittle, fragile and weakening.

Factors affecting the severity galvanic corrosion are :

1. Size of exposed areas of the anodic metal relative to that of

cathodic metal.

i. Smaller cathode relative to anode will cause small

increase in corrosion of anode.

ii. Smaller anode will suffer severe corrosion.

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5.2.2 Crevice Corrosion

Crevice corrosion occurs at shielded areas that contain small

volume of aqueous solution.

Crevice can be a hole, a space between the surface and a poorly

adherent coating.

Principle :

1. Liquid entry but stagnant

2. Corrosion rate of crevice is higher than that on bulk (outside)

3. Crevice corrosion is initiated by changes in local chemistry

within the crevice;

i. Depletion of oxygen in the crevice

ii. Depletion of inhibitor in the crevice

Oxygen concentration can develop when there is a difference in oxygen concentration on a moist surface of a metal that can be oxidized.

Example : 1. a drop of water/ moisture on the surface 2. the oxygen concentration are lesser on the surface 3. the surface that low in oxygen concentration are cathodic 4. the surface that has higher oxygen concentration are anodic 5. because there is anodic and cathodic, the surface below the

water drop are corroded (anodic) 6. the water drop act as electrolyte

Usually occurs at a bad gasket pipe flange, under bolt head and

connections that soaked in liquid.

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5.2.3 Intergranular Corrosion

Is a localised attack along the grain boundaries, or immediately

adjacent to grain boundaries, while the bulk of the grains remain

largely unaffected.

It is occur when different potential between atoms at the grain-

boundaries and create the boundaries of anode and cathode.

It is usually starts from the surface and accelerates internally

causing by bad internal structure.

5.2.4 Stress Corrosion Cracking (SCC)

It is refers to cracking caused by the combined effects of tensile

stress and specific corrosion environment acting on the metal.

Usually occurs in alloys not in pure metals and in certain

environment, examples : copper cracked in ammonia or

aluminium alloy cracked in chloride solubility.

The stress in the materials must has its compressive component

and the presence of both stress and corrosion environment which

causing the cracks to form and spread.

The stress corrosion cracking usually occurs between crystals.

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5.3 Corrosion Control

Cathodic protection is the protection of a metal by connecting it to

a sacrificial anode or by impressing a direct current voltage to

make it a cathode.

Anodic protection is the protection of a metal which forms a

passive film by the application of an externally impressed anodic

current.

Example (steel hulls of ships adjacent to the bronze propellers) :

i. steel is an anode and bronze is a cathode and both are in

sea-water which act as electrolyte

ii. the steel (hulls) will be corroded because of its anodic, so a

more anodic material than steel and bronze is used as

corrosion sacrificial which it is zinc

iii. zinc blocks are fitted to hulls so that the electrochemical

corrosion process will occur only to the zinc

iv. the zinc blocks must be replace time to time because its

worn out of corrosion as shown below

5.4 Material Selection

When selecting materials for engineering design, use materials

that are corrosion resistant for a particular environment and

corrosion handbook or materials data should be consulted to make

sure the proper material is used.

Also, the material positions in electrochemical series need to be

notified.

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There are few combination between metal and good corroded

environment and economical are shown below :

i. stainless steel – nitrite acid

ii. nickel and alloy nickel – caustic

iii. monel – hydrofluoric acid

iv. hastelloi (chlorimet) – hot hydrochloric acid

v. plumbum – liquidify sulphuric acid

vi. aluminium – unpolluted atmosphere exposion

vii. tin – distillation water

viii. titanium – hot oxidation liquid

ix. tantalum – definite resistant

x. steel – sulphuric acid

5.5 Coating

Plastic and oil are non metal material use mainly for coatings.

Metallic coatings which differ from the metal to be protected are

applied as thin coatings to separate the corrosive environment

from the metal. Metal coatings are sometimes applied so that they

can serve as sacrificial anodes which can corrode instead of the

underlying metal.

Metallic coatings :

1. Noble coating

o it is a coating where higher potential electrode compared

to the base metal will be protected

o base metal coating such as cuprum, nickel and chromium

as the coating and entering the holes in material

o it cannot protect the base metal if there is holes in the

coating

o it is because the base metal will become anode

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2. Sacrificial coating

o the base metal protected by sacrifice it and the coated acts

as anode

o organic and inorganic coatings

o the organic and inorganic material are used to protect the

surface from contacting with oxygen or giving the basic

protection by coated with stable material which cannot be

penetrated by humidity/ moisture

o organic coating such as paint, tar, oil and varnish

o inorganic coating is enamel, plastic. Plastic is the main

inorganic materials used as coating by hot dipping and

spraying of corrosion resistant material

5.6 Design

Designing rules :

1. considering corrosion penetration with the need of

mechanical strength when determining the thickness of a

metal used. It is important for piping and tank with liquid

contents

2. welding is better than riveting for contena to reduce crevice

corrosion.

3. use one type of material only for the whole structure to

prevent galvanic corrosion.

4. avoid extra stress and stress concentration in corroded

environment to prevent from crack-stress corrosion. Sharp

edges of component need to be avoided because it can caused

the stress

5. designing simple attachable system or changeable component

if predicted it is easier to break or fail in the service

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5.7 Painting

Paint the surface of metal to avoid corroded material from

contacting the surface.

Paint may be applied by brushing, spraying and dipping.

It may be dried naturally or by stoving.

5.8 Electroplate Metal

Electroplating is the process of using electrical current to reduce

cations of a desired material from a solution and coat a conductive

object with a thin layer of the material such as a metal using

electrolysis.

Electroplating and metal finishing processes include copper

plating, nickel plating, zinc plating, silver plating, tin plating, brass

plating, cadmium and chrome finishes.

Metals plated include brass, copper, bronze, chrome, nickel, and

black nickel, silver and gold.

The process :

i. the metal/ components to be plated are immersed in a

solution called electrolyte

ii. electrolyte allows the passage of an electric current

iii. the parts that require coating, are then placed in the solution

and given a negative charge/ terminal (as cathode)

iv. anodes are connected to the positive terminal

v. upon the passage of an electric current metal ions are

transferred from the electrolyte onto the surface of the

cathode

Electroplating allows for increased corrosion resistance, scratch

resistance, decorative finishes and high temperature protection.

Examples : tin plating and tin alloys for food container and food

contact applications.

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5.9 Oxide Layers

Oxide layers such as zinc oxide and aluminium.

It is higher in density and therefore preventing the oxygen and

water from corrode the metal.

The oxide layers also used as electroplating for metal products.

Example : zinc oxide layers for steel roofs manufacturing.

5.10 Alloys

A metal alloy is a combination of two or more metals or a metal

and a nonmetal.

Alloys are made to improved corrosion resistance.

Steels usually alloyed with chromium and manganese to gain

stainless steel.

5.11 Non-Ferrous Metal

Metals and alloys are commonly divided into these classes :

1. ferrous metals : that contain a large percentage of iron

2. non-ferrous metals : that does not contain iron or only a

relatively small amount of iron

3. a metal alloy : is a combination of two or more metals

or a metal and a nonmetal

Common non-ferrous metals used in engineering are :

a) Aluminium h) Chromium

b) Silver (Argentum) i) Gold (Aurum)

c) Copper (Cuprum) j) Molybdenum

d) Plumbum/ Lead k) Magnesium

e) Tin (Stanum) l) Cobalt

f) Nickel m) Manganese

g) Zinc

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Main properties of non-ferrous metals :

1. low strenght

2. good thermal and electric conductivity

3. free from magnetic field

4. high corrosion resistance

5. easier in manufacturing

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5.11.1 Types of Non-Ferrous Metals, Physical Properties, Mechanic Properties and

the Applications

Types Desription 1. Aluminium, Al

Applications: i. wrapper

ii. light inversion iii. decorative product iv. coating for corrosion resistance

Characteristics:

i. tensile strength 100 N/mm2 ii. compressive strength 100 N/mm2

iii. hardness 40 HB iv. ductility 30% elongation v. melting temperature 660o C

vi. density 2.7 g/cm3 vii. high corrosion resistance in water and atmosphere due to the

formation of a very thin passive film of aluminium oxide on aluminium surface

viii. good in thermal and electrical conductivity ix. good thermal and light inverter

Physical Properties:

i. light weight ii. widely used in casting component

iii. easier formation of oxide causing high in cost for aluminium production from its ore (bauxite) by using electronic method because reduction agent method are not practical

iv. aluminium oxide has its benefit as corrosion protector because its blocking the oxygen from contacting to the metal and avoiding the corrosion from occurred

v. aluminium oxide are hard and wear resistant in properties vi. good electric conductor

Mechanical Properties:

i. good machinability, formability, workability and castability ii. can be rolled to any desired thickness, stamped, drawn,

hammered, forged and extruded to almost any shape and size Types of Aluminium Alloy:

1. Aluminium-Magnesium alloy (Al-Mg) a. 0.14% Cuprum b. 0.5% Silicon c. 0.7% Ferrum d. 0.5% Chromium e. 5.0% Magnesium

strength : 3x than pure aluminium

hardness : 80 HB

good corrosion resistance

applications : hulls (badan kapal) and connected using rivet or TIG weld, automotive gas or oil channel

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2. Aluminium-Manganese alloy (Al-Mn)

containing : 1.5% manganese - to increase the tensile strength 200 N/mm2

good corrosion resistance, formability and weldability

applications : house roof, types of container, bus and lorry bodies, cooking utensils, oil tank and pipe

3. Duralumin

containing : 4% cuprum (copper)

a strong alloy with tensile strength for 400 N/mm2 after heat treatment

applications : for manufacturing of aircraft body, wire, rivet, metal plate and moulding boxes

4. Aluminium - Silicon alloy (Al-Si)

containing : 10% - 13% silicon

tensile strength : 230 N/mm2

high corrosion resistance against salt water and atmosphere

applications : in casting process for ship block engine and car engine, gear box, shaft and crank box

5. Aluminium - Cuprum alloy (aluminium casting)

containing : 1.5% - 4.0% cuprum (copper)

after heat treatment gaining the tensile strength upto 650 N/mm2

applications : high speed engine block for cars and aircraft

6. Aluminium – Zinc alloy (Al–Zn)

containing : zinc, magnesium, cuprum, small amount of manganese and chromium

high in tensile strength, good corrosion resistance and can be heat treated

applications : widely used for aircraft parts structure where higher strength are needed

7. Aluminium + 0.1% Cuprum + 0.7% Ferrum + 0.1% Manganese

for electric conductor and equipment which does not apply force

applications : construction decorative equipment, metal boxes, bottle caps, cooking utensils

8. Aluminium + 0.1% Cuprum + (1.7-7.5)% Magnesium + Ferrum + Manganese

applications : ship structure, cars, rivet, bar and fences

9. Heat-treatable forged aluminium

applications : for machine manufacturing, houses and aircraft frame structure

used for components that can withstands stress/ force but lighter such as mould equipment for casting, gear box, aircraft structure, cylinder head, piston and for the usage of good corrosion resistance

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2. Copper/Cuprum, Cu

Applications: cables, television and radio equipment, switch, water carrying pipes, soldering material, cooking pot, ship bodies Main Properties:

i. tensile strength 215 N/mm2 ii. compressive strength 300 N/mm2

iii. hardness 80 HB (casting type) 90 HB (cold work) 66 HB (cold work and annealing)

iv. ductility upon elongation 25% (casting type) 55% (annealed) 3% (cold work)

v. melting temperature 1083 oC vi. density 8.9 g/cm3

vii. resistance over atmosphere and water corrosion viii. good electric conductor Mechanical Properties:

i. good electrical conductor – used as conductors in pure state ii. the tensile strength for pure copper about 300Nmm-2 and when

alloyed, increased upto 460 Nmm-2 iii. excellent heat transfer Types of Copper Alloys: 1. Bronzes

is an alloy of copper containing elements other than zinc but cuprum are the main material

properties : 1. higher strength 2. better corrosion resistance 3. antifriction or bearing properties 4. malleable 5. ductile 6. excellent electrical conductor 7. excellent alloying characteristics 8. non-magnetic 9. machinable

3. Brasses Properties:

i. higher strength ii. good thermal and electrical conductivity

iii. good atmospheric corrosion resistance iv. high machinability v. ductility

vi. hardness vii. wear resistance

viii. recyclability Types of Brasses:

1. Brass with 63/37 (basic brass)

suitable for casting process and hot-worked

it is brittle after cold-worked but can be formed by casting, forging, hot rolling and extrusion process

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2. Brass with 70/30 (cartridge brass)

ductile and can be drawn upto 70% of elongation

the tensile strength : 600 N/mm2 (hard) 320 N/mm2 (after cold-worked)

hardness : 60 HB (annealed) 130 HB (in drawing process)

applications : wire, pipe and rod

widely used in drawing operations to produce cartridge cases

3. Brass with 60/40 (muntz metal)

suitable for hot-works such as hot-stamping and hot forging

tensile strength : 450 N/mm2 with low ductility and hard to machined

1% tin added to gain corrosion resistance

4. Zinc

Applications: coating material for steel to prevent corrosion such as for iron chains, house roof, kitchen utensils, water tanks, car batteries, tooth patch material and basic material for paint Main Properties:

i. tensile strength 60 N/mm2 ii. hardness 80 HB (zinc alloy)

iii. melting temperature 420C iv. density 7.1 g/cm3

Physical Properties:

i. basic usage of zinc is for steel coating to prevent corrosion ii. example : galvanic electroplate such as steel coated with zinc

for bolt, screw, fences, pipe, tank iii. pure zinc has the crystallization temperature in room

temperature, so it can annealed itself and cannot be worked (hardened) in room temperature

iv. zinc less pure graded will shows an increasement in hardness and strength in usage

v. applications : as coated for iron and steel (galvanized iron), printer blocks, tube, roof plates

Mechanical Properties:

i. brittle in normal temperature and can be forge at the temperature 100°C - 150ºC

ii. at 200°C, it become more brittle and can be form in powder iii. corrosion resistance

5. Lead

Plumbum, Pb Applications: water pipe, cable coating, coating for chemical container, weights and counter-balances, shielded against x-ray and radiation-rays in nuclear plant and as alloy in tin as solder metal Main Properties:

i. tensile strength 18 N/mm2 ii. hardness 4 - 8 HB

iii. melting temperature 327C iv. density 11 g/cm3

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Physical Properties: i. a heavy metal

ii. high in density iii. low melting point iv. good resistance to corrosion v. soft

vi. malleable vii. possesses low strength

viii. good lubricating properties ix. high absorbing power for radiations, such as x-rays x. good heat and electric conductor

6. Bearing Material Mechanical properties :

i. high compressive strength so it does not squeeze out under heavy loads

ii. high hardness and wear resistance to provide a longer life iii. good thermal conductivity to prevent the bearing metal from

becoming overheated iv. resistance to corrosion v. able to retain oil

vi. antifriction quality

Types of Bearing Materials : 1. White metal alloy

divided into the tin-base and the lead-base alloys 2. Phosphor bronzes

used for heavy loads at low speeds 3. Poly-tetra-fluoro-ethane (P.T.F.E) or Teflon

it is a non-metallic material and it is a thermoplastic material

used under light loads and at low speeds

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Metal Properties Applications

Aluminium Lightest of the commonly used

High electrical and thermal conductivity

Soft, ductile and low tensile strength

The base of many engineering alloys

Lightweight electrical conductors

Copper Soft, ductile and low tensile strength

Much easier to joint by soldering and brazing

Corrosion resistant

The base of brass and bronze alloys

It is used extensively for electrical conductors and heat exchangers such as motor car radiators

Lead Soft, ductile and very low tensile strength

High corrosion resistance

Electric cable sheaths

The base of „solder‟ alloys

The grids for „accumulator‟ plates

Lining chemical plant

Added to other metals to make them „free-cutting‟ Silver Soft, ductile and very low

tensile strength

Highest electrical conductivity of any metal

Widely used in electrical and electronic engineering for switch and relay contacts

Tin Resists corrosion Coats sheet mild steel to give „tin plate‟

Used in soft solders

One of the bases of „white metal‟ bearings

An alloying element in bronzes Zinc Soft, ductile and low tensile

strength

Corrosion resistance

Used extensively to coat sheet steel to give „galvanized iron‟

The base of die-casting alloys

An alloying element in brass Chromium Resists corrosion

Raises strength but lowers ductility of steels

Improves heat-treatment properties

Used as an alloying element in high-strength and corrosion resistant steels

Used for electroplating

Cobalt Improves wear-resistance and „hot hardness‟ of high-speed steels

Used as an alloying element in „super‟ high-speed steels and in permanent-magnet alloys

Manganese High affinity for oxygen and sulphur

Soft and ductile

Used to de-oxidize steels and to offset the ill-effects of the impurity sulphur

Larger amounts improve wear resistance Molybdenum A heavy, heat-resistant metal

that alloys readily with other metals

Used as an alloying elements in high-strength nickel-chrome steels to improve mechanical and heat-treatment properties

It reduces mass effect and temper-brittleness Nickel A strong, tough, corrosion

resistant metal widely used as an alloying element

Used as an alloying element to improve the strength and mechanical properties of steel

Tends to unstabilize the carbon during heat treatment, and chromium has to be added to counteract this effect in medium and high-carbon steels

Used for electroplating