CHAPTER 5 • CORROSION AND NON-FERROUS METAL (06 : 00)

• This topic describes two main categories of

corrosion. It also explains the electrochemical

corrosion phenomena and the differences between

the types of corrosion. This topic also states the

corrosion preventive steps.

SYLLABUS • 5.1 Describe the corrosion process

• 5.1.1 Define corrosion

• 5.1.2 Distinguish two main categories of

corrosion

• a. Dry corrosion

• b. Wet/Electrochemical corrosion

• 5.1.3 Define dry corrosion

• 5.1.4 Explain the basic principle in

electrochemical corrosion (galvanic) and

chemical equations.

• a. Zinc sheet is soaked in hydrochloric acid

(HCl)

• b. Anode and cathode reaction in

electrochemical cell; (iron and copper in

electrolyte)

• 5.1.5 Explain the phenomena in the

electrochemical series: anode metal is

more corrosive to the cathode metal in

electrolyte environment

• 5.1.6 Explain differences between types of

corrosion

• a. Varying stress

• b. Varying oxygen convergence

• c. Grain interface

• d. Crevice

• 5.1.7 Explain the corrosion remedial action

• a. Cathode and anode protection

• b. Material selection

• c. Coatings-metals, organic and non

organic

• d. Design

• 5.2 Describe non-ferrous metals

• 5.2.1 Define term and types of non-ferrous

metals 5.2.2 Explain the characteristics and

application of

• a. Aluminium

• b. Copper

• c. Zinc

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:

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.

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.

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.

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.

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.

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

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

i. it is a coating where higher potential electrode compared to the base metal will be protected.

ii. base metal coating such as cuprum, nickel and chromium as the coating and entering the holes in material.

iii. it cannot protect the base metal if there is holes in the coating because the base metal will become anode

2. Sacrificial coating i. the base metal protected by sacrifice it and the

coated acts as anode ii. 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

iii. organic coating such as paint, tar, oil and varnish

iv. 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

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.

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 b) Silver (Argentum) c) Copper (Cuprum) d) Plumbum/ Lead e) Tin (Stanum) f) Nickel g) Zinc h) Chromium i) Gold (Aurum) j) Molybdenum k) Magnesium l) Cobalt m) Manganese

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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Introduction Metals form about a quarter of the earth crust by weight

One of the earliest material used dated back to pre-historic time

Some of the earliest metals used include: copper, bronze and iron

Stone age Bronze age … ’discovery’ of steel Industrial Revolution in the 18th century

All metals except gold are generally found chemically combined with other elements in the form of oxides and sulphates. Commonly known as ores.

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Pure Metals and Alloys Metal that are not mixed with any other materials are known as pure metals. Metals listed in the Periodic Table are pure metals E.g. Iron (Fe), Copper (Cu) and Zinc (Zn)

Alloys are mixtures of two or more metals formed together with other elements/materials to create new metals with improved properties and characteristics. E.g. Brass (Copper and Zinc), Stainless steel (steel and chromium) Alloy = metal A + metal B + … + other elements

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Ferrous Metals & Non-Ferrous

Metals Ferrous metals are metals that contain iron E.g. Steel (iron and carbon)

Non-ferrous metals are metals that do not contain iron E.g. Zinc (pure metal), Bronze (Copper and tin) (non-ferrous may contain slight traces of iron)

Ferrous Metal = alloy metals that contains iron ( Primary base metal is iron) Non-ferrous Metal = alloy metals that do not contain iron Primary base metal does not contain iron)

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Classification Metals can be divided into 2 groups

Metals

Ferrous Metals Non- Ferrous Metals

Iron Aluminum

Low Carbon Steel Copper

Medium Carbon Steel Brass

High Carbon Steel Bronze

Cast Iron Zinc

Stainless Steel Lead

Tool Steels Tin

Others Others

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Extraction of Iron •Iron is found in iron oxide in the earth. •Three primary iron ores: magnetite, hematite, taconite

•Iron is extracted using blast furnace

•Steps in extraction of iron

Ores is washed, crushed and mixed with limestone and coke

The mixture is fed into the furnace and is then melted

Coke(a product of coal, mainly carbon) is used to convert the iron oxides to iron

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Extraction of Iron Limestone helps to separate the impurities from the metal

The liquid waste is known as slag that floats on the molten iron

They are then tapped off (separated)

The iron produced is only about 90% to 95% pure.

The iron is then further refined using the basic oxygen furnace and the electric arc furnace to produce steel which is widely used now.

Blast Furnace

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Extraction of Iron

A blast furnace

Blast Furnace Temperatures

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• Ore, coke, and limestone are “charged” in layers into the top of a blast furnace

• Ore is the source of the iron , Coke is the source of

the carbon (coke is derived from coal, by heating in a coking oven)

• Limestone acts as a fluxing slag to remove impurities

like sulphur and silica • 1100-deg. air blown into bottom of furnace, burns

oxygen off the iron oxides, causing temperature in furnace to get above the melting point of iron (approx 3000 degrees)

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• Molten iron sinks to bottom of furnace, where it is

tapped off from furnace and cast into large ingots

called “pigs”…pigs contain high carbon content

(4% or so), plus many impurities, such as sulphur and

silica which wasn’t removed by the limestone.

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Ferrous Metals - Iron and Steel Pure iron is soft and ductile to be of much practical use.

BUT when carbon is added, useful set of alloys are produced. They are known as carbon steel.

The amount of carbon will determine the hardness of the steel. The carbon amount ranges from 0.1% to 4%.

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Types of Steel Steel

•Low carbon steel (mild steel) •Medium carbon steel •High carbon steel (tool steels) •Cast iron

Alloy Steels •Stainless steel •High speed steel

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Low Carbon Steel Also known as mild steel Contain 0.05% -0.32% carbon Tough, ductile and malleable Easily joined and welded Poor resistance to corrosion Often used a general purpose material Nails, screws, car bodies, Structural Steel used in the construction industry

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Medium Carbon Steel Contains 0.35% - 0.5% of carbon Offer more strength and hardness BUT less ductile and malleable Structural steel, rails and garden tools

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High Carbon Steel Also known as ‘tool steel’ Contain 0.55%-1.5% carbon Very hard but offers Higher Strength Less ductile and less malleable Hand tools (chisels, punches) Saw blades

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Cast Iron Contains 2%-4% of carbon Very hard and brittle Strong under compression Suitable for casting [can be pour at a relatively low temperature] Engine block, engineer vices, machine parts

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Cast Iron White: Hard and brittle, good wear resistance Uses: rolling & crunching Equipment Grey: Good compressive & tensile strength, machinability,

and vibration-damping ability Uses: machine bases, crankshafts, furnace doors,

Engine Blocks

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Ductile: High strength and ductility Uses: engine and machine parts Malleable: Heat-treated version of white cast iron

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Stainless Steel Steel alloyed with chromium (18%), nickel (8%), magnesium (8%) Hard and tough Corrosion resistance Comes in different grades Sinks, cooking utensils, surgical instruments

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Stainless Steels Main types: Ferritic chromium: very formable, relatively weak; used in architectural trim, kitchen range hoods, jewelry, decorations, utensils Grades 409, 430, and other 400 Austentitic nickel-chromium: non-magnetic, machinable, weldable, relatively weak; used in architectural products, such as fascias, curtain walls, storefronts, doors & windows, railings; chemical processing, food utensils, kitchen applications. series. Grades 301, 302, 303, 304, 316, and other 300 series.

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Martensitic chromium: High strength, hardness, resistance to abrasion; used in turbine parts, bearings, knives, cutlery and generally Magnetic. Grades 17-4, 410, 416, 420, 440 and other 400 series

Maraging (super alloys): High strength, high Temperature alloy used in structural applications, aircraft components and are generally magnetic. Alloys containing around 18% Nickel.

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High Speed Steel Medium Carbon steel alloyed with Tungsten, chromium, vanadium Very hard Resistant to frictional heat even at high temperature Can only be ground Machine cutting tools (lathe and milling) Drills

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Heat Treatment A process used to alter the properties and characteristics of metals by heating and cooling.

Three stages of heat treatment

1. Heat the metal to the correct temperature 2. Keep it at that temperature for a the required length of time (soaking) 3. Cool it in the correct way to give the desired properties

Cold working induce stress in metal lead to work hardening prevent further work from taking place

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Heat Treatment

Types of heat treatment: Annealing Normalizing Hardening Tempering Case hardening

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Annealing Annealing is the process whereby heat is introduced to mobilise the atoms and relieve internal stress

After annealing, it allows the metal to be further shaped

It involves the re-crystallization of the distorted structure

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Normalizing This process is only confined to steel.

It is used to refine the grain due to work hardening

It involves the heating of the steel to just above Its upper critical point.

Phase diagram of Iron-Carbon

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Hardening Hardening is the process of increasing the hardness of steel by adding a high amount of carbon

The degree of hardness depends on the amount of carbon present in steel and the form in which it is trapped during quenching.

Once hardened, the steel is resistant to wear but is brittle and easily broken under load.

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Tempering Tempering is the process to reduce hardness and brittleness slightly of a hardened steel workpiece.

It helps to produce a more elastic and tough steel capable of retaining the cutting edge after tempering

Prior to tempering, the steel must be cleaned to brightness with emery cloth so that oxide colour is visible when reheated

Tempering temperature 1/α hardness Tempering temperature α toughness

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Tempering

Guidelines for tempering Tempering of cold chisel

230 C = 446 F

300 C = 572 F

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Case Hardening Case hardening is a process used with mild steel to give a hard skin

The metal is heated to cherry red and is dipped in Carbon powder. It is then repeated 2-3 more times before Quenching the metal in water to harden the skin.

This allows the surface of mild steel to be able to subject to wear but the soft core is able to subject to Sudden shock e.g. the tool holders

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Case Hardening - Carburizing Carburizing involves placing the mild steel in box packed with charcoal granules, heated to 950 º C (1742 oF) and allowing the mild steel to soak for several hours.

It achieves the same purpose of case hardening

Carbon Steels Used for

Construciton

• Those steels in which the residual elements (carbon,

manganese, sulphur, silicon, etc.) are controlled,

but in which no alloying elements are added to

achieve special properties.

A36 Carbon Structural Steel

• For years, the workhorse all-purpose steel for nearly

all structural “shapes” (beams, channels, angles,

etc.), as well as plates and bars, has been:

Wide Flanged Beams “W” shapes • Recently (last few years), A36 has been displaced

as the steel of choice for the major “shape”

subcategory called wide-flange beams, or “W”

shapes. The replacement steel is a high-strength,

low-alloy steel, known as A992 (see below). For the

other non-wide-flange beam structural shapes, A36

remains the predominant steel.

Structural pipe and square tubing • Pipe: A53 Pipe, Steel, Black and Hot-Dipped, Zinc-

Coated Welded and Seamless.

• Tubing: A500 Cold-Formed Welded and Seamless

Structural Tubing in Rounds and Shapes.

• A501 Hot-Formed Welded and Seamless

Carbon Steel Structural Tubing.

High-Strength, Low-Alloy Steels • High-Strength, Low-Alloy Steels:

• A group of steels with chemical compositions specially developed to impart better mechanical properties and greater resistance to atmospheric corrosion than are obtainable from conventional carbon structural steels. Several particular steels used often in construction, and their ASTM specifications, are:

• A572: High-Strength, Low-Alloy Columbium-Vanadium Steels of Structural Quality.

• A618: Hot-Formed Welded and Seamless High-Strength, Low-Alloy Structural Tubing

• A913: High-Strength, Low-Alloy Steel Shapes of Structural Quality,

• Produced by Quenching and Self-Tempering Process

• A992: Steel for Structural Shapes for Use in Building Framing

This is the steel which has substantially replaced A36 steel for

Wide-flange structural shapes.

Corrosion – Resistant Steels • A242: High-Strength, Low-Alloy Structural Steel.

• A588: High-Strength, Low-Alloy Structural Steel with 50 ksi Minimum Yield Point.

• A847: Cold-Formed Welded and Seamless High-Strength, Low-Alloy Structural Tubing with Improved Atmospheric Corrosion Resistance.