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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