Refining of Metals and Corrosion

Refining of Metals

Metals obtained from Section 7 — are still impure. Final step: Refining — making pure metal.

Why Refining?

Fe from Blast Furnace — 4% C + other impurities. Cu (Blister) — 99% pure, but has impurities. Al — Hall-Héroult ~99% pure, needs further purification.

Each application requires specific purity.

Main Methods of Refining

1. Electrolytic Refining — most widely used. 2. Distillation — for volatile metals like Hg, Zn. 3. Liquation — for low-melting-point metals. 4. Zone Refining — for very pure metals.

We will mainly study electrolytic refining.

Electrolytic Refining — Principle

Make impure metal the Anode. Make a thin strip of pure metal the Cathode. Salt solution of the same metal — electrolyte.

Pass electric current.

What happens:

1. Metal from Anode (impure) — into solution as ions.
2. Metal ions from solution — deposited at Cathode in pure form.
3. Impurities — fall below Anode as 'anode mud'.

Finally: pure metal at Cathode; valuable metals like Au, Ag, Pt in anode mud.

Electrolytic Refining of Cu

Setup:

• Thick strip of impure Cu = Anode (+)
• Thin strip of pure Cu = Cathode (-)
• Electrolyte: $CuSO_4$ + dilute $H_2SO_4$.

Reactions:

At Anode: $$Cu(\text{impure}) \rightarrow Cu^{2+} + 2e^-$$

At Cathode: $$Cu^{2+} + 2e^- \rightarrow Cu(\text{pure})$$

Fate of impurities (Au, Ag, Pt, Fe, Zn):

• Au, Ag, Pt — fall below anode as 'anode mud'.
• Fe, Zn — go into solution as ions, but don't deposit at Cathode.

Finally: pure Cu (99.99%) at Cathode.

Al, Zn, Ni — Same Method

For each — that metal's salt as electrolyte.

• Al: $Al_2(SO_4)_3$
• Zn: $ZnSO_4$
• Ni: $NiSO_4$

Anode Mud — An Interesting Yield

Anode mud from Cu refining contains:

• Gold (Au)
• Silver (Ag)
• Platinum (Pt)

This is not 'unwanted waste' — valuable! A 'bonus' product of Cu industry.

Corrosion — Introduction

The biggest enemy of metals: corrosion.

Definition

Slow chemical degradation of metals — by air, water, or other chemicals = Corrosion.

Products: oxides, hydroxides, sulphides, carbonates of metals.

Corrosion of Different Metals

1. Iron — Rust: $Fe_2O_3 \cdot xH_2O$ — hydrated ferric oxide. Brown-red colour. Loose, porous — Fe inside also rusts.

2. Copper — Green Layer (Patina): Cu(OH)₂·CuCO₃ or CuCO₃ — green colour. On ancient copper objects — green layer.

3. Silver — Black Layer (Tarnish): Ag₂S — black sulphide. From $H_2S$ (in air).

4. Aluminium — Silvery Layer: $Al_2O_3$ — thin, dense layer. Protects Al inside (Section 2 reference).

5. Gold — No Corrosion! Au very low reactivity.

Rust — Detailed

Required Elements:

1. Water (moisture).
2. Oxygen (air).

Rust occurs only when both are present. Not from water alone or air alone.

Reaction (simplified):

$$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

Fe₂O₃·xH_2O = rust.

Demonstration — Rust Experiment

Three test tubes:

A: Fe + water (air removed) → no rust. B: Fe + dry air (no water) → no rust. C: Fe + water + air → rust!

Conclusion: Both required.

Mechanism of Rust — Electrochemical

On Fe surface:

• Some areas — Anode (Fe → Fe²⁺ + 2e⁻).
• Some areas — Cathode (O₂ + 2H₂O + 4e⁻ → 4OH⁻).

Fe²⁺ and OH⁻ → Fe(OH)₂ → Fe(OH)₃ → Fe₂O₃·xH₂O.

That is, rust = a 'micro-electric cell'.

Influencing Factors

1. Moisture: more → more rust. 2. Pollution ($SO_2, NO_2$): acid rain → faster. 3. Salt ($NaCl$): more on coasts. 4. Temperature: higher → faster. 5. Surface state: cracks → faster.

Practical Impact

Each year worldwide — lakhs of tonnes of Fe destroyed by rust. India: ~10% of Fe production lost to rust. Economic loss: crores of rupees.

Prevention of Corrosion

Many ways to prevent rust — in modern industry.

1. Painting

Layer of paint on Fe surface. Protection from both oxygen and moisture.

Advantages:

• Cheap.
• Easy.
• Available in various colours.

Disadvantages:

• If paint scratches off — rust below.
• Needs periodic replacement.

Use: cars, windows, garden iron items.

2. Oiling/Greasing

Thin layer of oil.

Advantages:

• Immediate.
• Simple.
• Easy to apply.

Disadvantages:

• Dries up in some time.
• Needs reapplication.

Use: machine parts, locks, tools.

3. Galvanisation

Coating Fe surface with Zn.

Zn — more reactive than Fe. If layer scratches off — Zn corrodes first (not Fe!).

This is 'cathodic protection'.

Method: Dip Fe in molten Zn — Zn layer formed.

Use:

• Taps
• Pipes
• Buckets
• Roofs
• Fences

'Galvanised iron' (GI) — common market name.

4. Chromium Plating

Cr layer by electrolytic method.

Advantages:

• Shiny.
• Durable.
• Corrosion-resistant.

Use: taps, car parts.

5. Electroplating

Coating one metal with another by electrolytic method.

Common Examples:

• Ni on Fe
• Sn on Fe
• Cr on Fe

6. Anodising

Forming thicker Al₂O₃ layer on Al. (Refer Section 2.)

Only Al — no foreign substance.

7. Use of Alloys

Alloys are better than pure metal. Example: Stainless steel — Fe + Cr + Ni.

A layer of Cr — protects Fe.

Alloys — What and Why?

Definition: A homogeneous mixture of two or more metals — or one metal + one non-metal.

Major Examples:

Alloy	Composition	Use
Stainless steel	Fe + Cr + Ni	Utensils, knives
Brass	Cu + Zn	Decoration, utensils
Bronze	Cu + Sn	Statues, medals
Solder	Pb + Sn	Welding
Duralumin	Al + Cu + Mg + Mn	Aircraft
Magnalium	Al + Mg	Light fixtures

Advantages of Alloys:

1. Increased hardness.
2. Corrosion resistance.
3. Lower melting point (in some).
4. More useful.

Example: pure Au — soft. 22-carat gold (Au + Cu) — durable.

Electroplating — Detailed

A famous refining-protection method.

Principle

By electrolytic method — coating one metal with a thin, uniform layer of another metal.

Basic Setup

Example: Ag plating on Fe object

• Cathode (-): Fe object (the one to be coated).
• Anode (+): Ag strip.
• Electrolyte: $AgNO_3$ solution.

Reactions

At Anode: $$Ag \rightarrow Ag^+ + e^-$$

Ag into solution.

At Cathode: $$Ag^+ + e^- \rightarrow Ag(s)$$

Ag — deposited on Fe object.

That is, the concentration of electrolyte remains constant.

Commercial Uses

1. Gold plating:

• Gold on cheap jewellery.
• Looks expensive, but light weight.

2. Silver plating:

• On dining utensils.
• On decorative items.

3. Cr plating:

• Taps, bathroom fittings.
• Car parts (chrome trim).

4. Ni plating:

• On coins (in some countries).
• Electrical switches.

5. Tin plating:

• Inside food cans (Fe + Sn = 'tin can').
• Sn — non-reactive with food.

Advantages of Electroplating

1. Thin, uniform layer.
2. Precise control.
3. For various metals.
4. On complex shapes too.

Practical Example — Tin Can

Fe + Sn coating = food can.

Why Sn?

• No reaction with food.
• Easy plating.
• Cheap.

If Sn layer scratches → Fe rusts. Hence open tin cans rust quickly.

Comparison — Galvanisation vs Plating

Property	Galvanisation	Electroplating
Principle	Physical	Electrochemical
Metal	Zn	Cr, Ag, Au, Sn
Purpose	Protection	Protection + Decoration
Example	GI tap	Cr-plated faucet
If Fe scratches	Zn first corrodes	Fe rusts under layer

An Interesting Fact

'Galvanisation' — named after Luigi Galvani. He studied 'bioelectricity' in 1791. Same name — 'galvanometer' and 'galvanisation'.

Comparison of Anti-Corrosion Methods

A Summary Table

Method	Principle	Advantages	Limitations
Paint	Physical barrier	Cheap	Bad on scratches
Oil	Barrier	Immediate	Short term
Galvanisation (Zn)	Cathodic protection	Durable	Expensive
Cr plating	Electroplating	Shiny	Complex
Anodising	Oxide layer	Special	Only for Al
Alloy	Composition	Permanent	Initial cost

Which Method When?

1. At home:

• Taps (GI)
• Doors (paint)
• Food utensils (stainless steel)

2. In industry:

• Pipes (galvanised)
• Cars (paint + Cr plating)
• Railway tracks (special alloy)

3. Special:

• Aircraft (Al + anodising)
• Ships (special alloy + paint)

Cathodic Protection — An Interesting Method

Make Fe a 'cathode' — and 'sacrifice' another metal.

Principle:

• A more reactive metal (Mg, Zn) — connected to Fe.
• On corrosion — that metal first (sacrificial).
• Fe protected.

Examples:

• Hulls of ships: Mg/Zn strips on Fe.
• Underground pipes: Mg-block attached.
• Aqueous storage tanks: Zn-anode.

This is 'sacrificial protection'.

Mg/Zn keep getting consumed — replaced periodically.

Stainless Steel — A 'Miracle'

Composition: Fe + Cr (>10%) + Ni + others

Why doesn't this rust?

Because of Cr — a very thin layer of $Cr_2O_3$ on the surface. This layer — dense, pore-free. Protects Fe inside.

Even if scratched — Cr forms $Cr_2O_3$ again (Cr in air automatically).

That is, 'self-healing' protection.

Alloys — In Detail

Stainless Steel

• 18% Cr + 8% Ni + 74% Fe.
• Kitchen utensils, knives.
• India: Tata, Jindal — major.

Brass

• Cu + Zn.
• Decoration, utensils, musical instruments.
• Yellow colour.

Bronze

• Cu + Sn.
• Statues, medals, bells.
• Brown colour.

Solder

• Pb + Sn (50:50).
• Electrical welding.
• Low melting point (~180°C).

Amalgam

• Hg + another metal.
• Dentistry (Hg + Ag + Sn).
• In gold extraction.

Properties of Alloys

Compared to pure metal:

• Hardness ↑
• Corrosion resistance ↑
• Melting point usually ↓
• Electrical conductivity usually ↓

An Interesting Comparison

Pure Au: very soft — jewellery cannot be made. 22-carat Au (Au + Cu): hard — jewellery can be made. 18-carat Au: even harder. 'White Gold' (Au + Pd/Ni): white colour, durable.

That is — alloy makes 'gold' more useful.

[Board Important] Remember 4-5 examples of alloys.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Electrolytic Refining (Cu example)

• Anode: impure Cu (thick strip).
• Cathode: pure Cu (thin strip).
• Electrolyte: CuSO₄ + H₂SO₄.
• At Anode: $Cu \rightarrow Cu^{2+} + 2e^-$.
• At Cathode: $Cu^{2+} + 2e^- \rightarrow Cu$.
• Impurities: Anode mud (Au, Ag, Pt) + solution (Fe, Zn).

2. Corrosion Products

Metal	Corrosion Product	Colour
Fe	Fe₂O₃·xH₂O (rust)	Brown-red
Cu	Cu(OH)₂·CuCO₃	Green
Ag	Ag₂S	Black
Al	Al₂O₃ (layer)	Silvery

3. Required for Rust

Both: Water (moisture) + Oxygen (air). Not from one alone.

Basic Reaction: $$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

4. Anti-Corrosion Methods

1. Paint (cheap).
2. Oil/Grease (immediate).
3. Galvanisation (Zn coating — Cathodic protection).
4. Cr plating (shiny).
5. Electroplating (thin layer).
6. Anodising (on Al).
7. Alloy (stainless steel).

5. Famous Alloys

Alloy	Composition
Stainless steel	Fe + Cr + Ni
Brass	Cu + Zn
Bronze	Cu + Sn
Solder	Pb + Sn
Duralumin	Al + Cu + Mg + Mn
Amalgam	Hg + others

6. Galvanisation (Special)

• Layer of Zn on Fe.
• Zn corrodes first (cathodic protection).
• Taps, pipes, buckets.

7. Electroplating Example (Ag on Fe)

• Cathode: Fe object.
• Anode: Ag strip.
• Electrolyte: AgNO₃.

8. Board's 'Golden' Questions

1. Electrolytic refining — diagram.
2. Mechanism of rust and prevention.
3. Method of galvanisation.
4. 5 famous alloys names and uses.
5. What is anode mud? How valuable?

Final Formula: 'Refining = pure; Corrosion = degradation; Alloy = better.'

Solved Examples

Example 1: NCERT — Electrolytic Refining (Cu)

Explain the electrolytic refining of Cu in detail.

Solution:

Setup

Required:

1. Thick strip of impure Cu — Anode (+).
2. Thin strip of pure Cu — Cathode (-).
3. Electrolyte: solution of dilute $H_2SO_4$ + $CuSO_4$.
4. A tank — to hold solution.
5. DC power source.

Electrolyte

In $CuSO_4$ solution: $$CuSO_4 \rightarrow Cu^{2+} + SO_4^{2-}$$

Reaction at Anode (+)

Impure Cu — into solution as ions: $$Cu(\text{impure}) \rightarrow Cu^{2+} + 2e^-$$

Cu²⁺ — go into solution.

Fate of impurities:

• Less reactive than Cu (Au, Ag, Pt) — fall as 'anode mud' below the Anode.
• More reactive than Cu (Fe, Zn, Ni) — go into solution as ions, but don't deposit at Cathode.

Reaction at Cathode (-)

Cu²⁺ — accept electrons from solution: $$Cu^{2+} + 2e^- \rightarrow Cu(\text{pure})$$

Pure Cu — deposits on Cathode.

Net Result

• Anode: keeps decreasing (Cu → Cu²⁺ → solution).
• Cathode: keeps increasing (Cu²⁺ → Cu → deposited).
• Solution: Cu²⁺ concentration constant.
• Anode mud: Au, Ag, Pt (valuable!).

Role of Anode Mud

Not 'unwanted waste' — highly valuable! A 'bonus' product of Cu industry.

Each year in India — significant amount of gold/silver from Cu refining.

Diagram (in words)

  | + | - |
  | A |   |   ← two electrodes
  |   |   |
  | i | p |   ← impure Cu (Anode), pure Cu (Cathode)
  | m | u |
  | p | r |
  | u | e |
  | r |   |
  | e |   |
  | Cu | Cu |
  +---+---+
  CuSO₄ + H₂SO₄ solution

  Below anode — anode mud (Au, Ag)

Purity

Finally: Cu at Cathode = 99.99% pure.

This — suitable for making electrical wires.

For Other Metals

Al, Zn, Ni — refined by similar method. For each — corresponding metal salt solution.

[NCERT textbook — every year in board]

Example 2: NCERT — Rust Experiment

What are the elements required for rust? Prove with an experiment.

Solution:

Basic Question

Rust on Fe — from water, from air, or from both?

Famous Experiment — Three Test Tubes

Setup: Three test tubes. An Fe nail in each.

Tube A:

• Fe nail + boiled water (air removed).
• Layer of oil on top (to prevent air).

Tube B:

• Fe nail + dry air.
• Below: CaCl₂ (drying agent — absorbs water).

Tube C:

• Fe nail + water + air (both available).

Observe after a few days.

Observations

Tube	Conditions	Result
A	Water, no air	No rust
B	Air, no water	No rust
C	Water + air	Rust!

Conclusion

Required for Rust:

1. Water (moisture).
2. Oxygen (air).

Not from one alone. Both needed.

Basic Reaction

$$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

Product: $Fe_2O_3 \cdot xH_2O$ = rust. Brown-red colour.

Practical Effect

This is why:

• Coastal areas have more rust (more moisture).
• Rust inside water pipes (water + small air).
• Less rust in deserts (less moisture).

Accelerating Factors

1. Salt (NaCl):

• As electrolyte.
• Rust 5-10× faster.

2. Pollution ($SO_2, NO_2$):

• Acid rain.

3. Temperature:

• More heat → more speed.

An Interesting Fact

Rust on Fe — can also be 'cyclic'. Surface that has rust — gets more rust. That is, 'autocatalytic'.

This is why — even a small rust grows a lot over time.

[NCERT textbook — every year in board]

Example 3: NCERT — Galvanisation

What is galvanisation? Explain its process and benefits.

Solution:

Definition

'Galvanisation' = process of coating Fe with Zn.

Purpose: To protect Fe from corrosion.

Method

1. Cleaning of Fe object:

• Wash with dilute acid.
• Remove surface impurities.

2. Dipping in molten Zn:

• Melting point of Zn ~420°C.
• Dip Fe object for a few seconds.

3. Cooling:

• Take out and cool.
• Thin layer of Zn settles on Fe.

Finally: Fe + Zn coating = Galvanised Iron (GI).

Principle — 'Cathodic Protection'

Zn — more reactive than Fe.

Reactivity: $Zn > Fe$

If layer scratches and water/air reaches inside:

• Zn corrodes first (Anode).
• Fe becomes Cathode and remains protected.

That is, Zn — 'sacrificial' metal.

Practical Benefits

1. Protection even on scratches.

• Paint scratches → rust.
• Zn scratches → Fe still protected (Zn first!).

2. Long-term protection.

• For 20-50 years.

3. Simple method.

• Possible on mass scale.

4. Economical.

• Cheaper than other methods.

Practical Examples

Examples of 'Galvanised Iron' (GI):

1. Water pipes (especially taps, tanks).
2. Roofs (with asbestos).
3. Buckets, garden items.
4. Cables of bridges.
5. Some car parts.

A Comparison — Paint vs Galvanisation

Property	Paint	Galvanisation
Principle	Physical barrier	Electrochemical
Durability	Less	More
On scratches	Rust	Zn first
Cost	Low	High
Purpose	Decoration + protection	Only protection

An Interesting Demonstration

Two Fe nails:

• A: Plain.
• B: Zn coating.

Place both in saltwater. After 1 week:

• A: completely rusted.
• B: no rust (Zn coating still).

Then scratch them and check:

• A: more rust.
• B: still no rust! (Zn left).

That is, 'sacrificial protection' — works.

[NCERT — important]

Example 4: NCERT — Alloys

What is an alloy? Mention five famous alloys' name, composition, and uses.

Solution:

Definition

'Alloy' = a homogeneous mixture of two or more metals — or one metal + one non-metal.

Purpose: better properties than pure metal.

Why Alloys Are Necessary?

Weaknesses of pure metals:

• Pure Fe rusts quickly.
• Pure Au is very soft.
• Pure Al is soft.

Improvements with alloys:

• Hardness ↑
• Corrosion-resistance ↑
• Lower melting point (in some)
• More durable

Five Famous Alloys

1. Stainless Steel

Composition:

• 74% Fe
• 18% Cr (main)
• 8% Ni

Properties:

• Corrosion-resistant ($Cr_2O_3$ layer from Cr).
• Shiny.
• Strong.

Uses:

• Kitchen utensils, knives.
• Medical instruments.
• Building construction.

2. Brass

Composition:

• 70% Cu
• 30% Zn

Properties:

• Yellow colour.
• Corrosion-resistant.
• Decorative.

Uses:

• Decorative items (statues, plates).
• Musical instruments (trumpet).
• Taps, handles.

3. Bronze

Composition:

• 88% Cu
• 12% Sn

Properties:

• Hard.
• Brown colour.
• Good acoustic properties.

Uses:

• Statues (ancient).
• Medals (Olympics).
• Bells, clocks.

'Bronze Age' — an era of human history.

4. Solder

Composition:

• 50% Pb
• 50% Sn

Properties:

• Low melting point (~180°C).
• Strong bond.

Uses:

• Electrical welding.
• Wire-joining.
• Circuit boards.

5. Duralumin

Composition:

• 95% Al
• 4% Cu
• 0.5% Mg
• 0.5% Mn

Properties:

• Light (like Al).
• Strong (like steel).
• Corrosion-resistant.

Uses:

• Aircraft.
• Rockets.
• Cars.

Other Important Alloys

6. Amalgam

Composition: Hg + another metal

Uses:

• Dentistry (Hg + Ag + Sn + Cu).
• Refining of gold.

7. Magnalium

Composition: Al + 5-30% Mg

Uses: light aircraft parts.

8. 22-carat Gold

Composition: 91.7% Au + 8.3% Cu

Properties: harder than pure Au.

Uses: jewellery.

Summary Table

Alloy	Composition	Main Use
Stainless steel	Fe + Cr + Ni	Utensils, knives
Brass	Cu + Zn	Decoration
Bronze	Cu + Sn	Statues, medals
Solder	Pb + Sn	Welding
Duralumin	Al + Cu + Mg + Mn	Aircraft

[NCERT textbook — every year in board]

Example 5: NCERT — Electroplating

Describe the process of Ag plating on Fe object.

Solution:

Setup

Required:

• Fe object (the one to be Ag plated).
• Ag strip.
• $AgNO_3$ solution.
• DC power source.
• Tank.

Connections:

• Fe object = Cathode (-).
• Ag strip = Anode (+).
• Solution: AgNO₃.

Reactions

Ionisation of $AgNO_3$: $$AgNO_3 \rightarrow Ag^+ + NO_3^-$$

At Anode (+): Ag from Ag strip — into solution as ions: $$Ag(s) \rightarrow Ag^+ + e^-$$

Ag into solution.

At Cathode (-): Ag⁺ — reaches Fe object: $$Ag^+ + e^- \rightarrow Ag(s)$$

Ag — as thin layer on Fe object.

Net Process

From Anode → through solution → to Cathode.

Solution concentration remains constant.

Ag strip (Anode) — slowly dissolves. Fe object (Cathode) — gets covered with Ag.

Practical Control

Electric current:

• More current → thick layer, but uneven.
• Less current → thin, uniform layer.

Time:

• More time → thicker layer.

Solution concentration:

• Must be precise.

Other Examples

Cr plating on Fe

Solution: $CrO_3$ + $H_2SO_4$. Anode: Pb (Cr is supplied from solution). Cathode: Fe object.

Use: taps, bathroom, car parts.

Sn plating on Fe (Tin Plating)

Solution: $SnSO_4$.

Use: food cans.

Sn — non-reactive with food. Fe — cheap and strong.

Advantages of Electroplating

1. Thin, uniform layer.
2. On complex shapes too.
3. For various metals.
4. Precise by electric control.

Commercial Use

On a large scale in India:

• Jewellery industry (gold/silver plating).
• Automobile industry (Cr, Ni).
• Food industry (Sn).
• Household utensils (Cr, Ni).

An Interesting Fact

'Imitation jewellery' = gold plating on cheap metal. Looks expensive, but cost 1/100. Thus — electroplating changed the world of decoration.

[NCERT — important]

Example 6: NCERT — Prevention of Corrosion

Mention 5 ways to protect metals from corrosion.

Solution:

1. Painting

Principle: Layer of paint on Fe surface.

Advantages:

• Cheap.
• Easy.
• Various colours.

Disadvantages:

• Rust on scratches.
• Periodic replacement.

Use:

• Cars.
• Windows, doors.
• Iron items at home.

2. Oiling/Greasing

Principle: Thin layer of oil.

Advantages:

• Immediate.
• Simple.
• Daily use.

Disadvantages:

• Dries up quickly.
• Reapplication needed.

Use:

• Machine parts.
• Locks.
• Tools.

3. Galvanisation

Principle: Layer of Zn on Fe.

Zn — more reactive than Fe. If layer scratches — Zn corrodes first. 'Cathodic protection'.

Advantages:

• Durable.
• Protection even on scratches.

Method:

• Dip Fe in molten Zn.
• (Hot dipping).

Use:

• Taps, pipes, buckets.
• Roofs, fences.
• 'GI sheet' — common market name.

4. Chromium Plating

Principle: Cr layer by electrolytic method.

Advantages:

• Shiny.
• Durable.
• Corrosion-resistant.

Use:

• Taps, bathroom fittings.
• Car bumpers.
• Motorcycle parts.

5. Use of Alloys

Principle: Mixing other metals into pure metal.

Example: stainless steel (Fe + Cr + Ni).

Advantages:

• Intrinsic protection.
• Protection even on scratches.
• Permanent.
• No external layer.

Use:

• Kitchen utensils.
• Medical instruments.
• Building construction.

Additional: Anodising

Principle: Thick $Al_2O_3$ layer on Al.

Only for Al. Use: window frames, utensils.

Summary Table

Method	For Whom	Durability
Paint	All	Less
Oil	All	Very less
Galvanisation	Fe	More
Cr plating	Fe	More
Alloy	Special	Very high
Anodising	Al	More

Choice — Which Method?

'Most durable — alloy.' Though initial cost is higher.

'Cheapest — paint.' But periodic replacement.

'Most popular — galvanisation.' For Fe.

[NCERT — every year in board, 5-mark]

Example 7: NCERT — A Mixed Question

(a) What is the formula of rust? (b) What is the green layer on Cu? (c) What is the black layer on Ag? (d) What is the layer on Al, and why does it protect?

Solution:

(a) Formula of Rust

Rust = Fe₂O₃·xH₂O

That is: hydrated ferric oxide. 'x' = variable number of water molecules.

Reaction: $$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

Features:

• Brown-red colour.
• Loose, porous.
• Doesn't protect Fe inside (more rust below).

This is 'autocatalytic' — promotes itself.

(b) Green Layer on Cu

Green layer on Cu over time = 'Patina'.

Formula: Several possible:

• $Cu(OH)_2 \cdot CuCO_3$ (basic Cu carbonate)
• $Cu(NO_3)_2 \cdot Cu(OH)_2$ (in urban pollution)
• $CuSO_4 \cdot Cu(OH)_2$ (from acid rain)

Reaction (main): $$2Cu + O_2 + CO_2 + H_2O \rightarrow Cu(OH)_2 \cdot CuCO_3$$

Special: Unlike rust — patina is protective! Protects inner Cu from further corrosion.

Famous example: Statue of Liberty (USA) — Cu statue, covered in green patina.

(c) Black Layer on Ag

Black layer on Ag over time = 'Tarnish'.

Formula: $Ag_2S$ (silver sulphide)

Reaction: Ag + small amount of $H_2S$ in air: $$2Ag + H_2S \rightarrow Ag_2S + H_2$$

or: $$4Ag + 2H_2S + O_2 \rightarrow 2Ag_2S + 2H_2O$$

Sources of $H_2S$:

• Rotten eggs, onions.
• Some food items.
• Sulphur-containing air pollution.

How to remove:

• Polish with toothpaste.
• Aluminium foil + soda + hot water (electrochemical).

(d) Layer on Al

Layer formed instantly on Al = $Al_2O_3$.

Reaction: $$4Al + 3O_2 \rightarrow 2Al_2O_3$$

Features:

• Thin (a few micrometers).
• Dense (pore-free).
• Transparent (Al's shine visible).
• Hard.

Why Protective?

Three reasons:

1. Dense, pore-free.

• Fe₂O₃ loose — air inside.
• Al₂O₃ dense — air doesn't penetrate.

2. Tightly attached to inner Al.

• Doesn't peel off easily.

3. 'Self-healing' on scratches.

• In air, Al immediately forms $Al_2O_3$ again.
• Cracks fill automatically.

This is why: Al is shiny and durable.

Summary

Metal	Layer	Formula	Nature
Fe	Rust	Fe₂O₃·xH₂O	Harmful
Cu	Patina	Cu(OH)₂·CuCO₃	Protective
Ag	Tarnish	Ag₂S	Surface only
Al	Oxide	Al₂O₃	Protective

Al, Cu — protective layer. Fe — harmful layer. Ag — only surface.

[Board: 5-mark]

Example 8: An Interesting — Stainless Steel

What is stainless steel? Why doesn't it rust?

Solution:

Definition

'Stainless Steel' = an alloy of Fe + Cr (at least 10%) + Ni.

Stain = mark, less = without — that is, 'mark-less steel'.

Standard Composition

Most common (304 grade):

• 70-72% Fe
• 18% Cr
• 8% Ni
• 0.08% C
• Others in small amounts

Other grades:

• 316 — medical instruments (Mo too).
• 410 — knives.

Why Doesn't It Rust?

Main reason: Cr layer.

Cr — more reactive than Fe. On stainless steel surface — Cr reacts with air immediately.

Reaction: $$4Cr + 3O_2 \rightarrow 2Cr_2O_3$$

$Cr_2O_3$ — a very thin layer (a few nanometres!).

Special Properties of This Layer

1. Dense.

• Pore-free.
• O₂, H₂O — don't penetrate.

2. Hard.

• Doesn't peel easily.

3. Transparent.

• Fe-Cr shine visible.

4. 'Self-healing'.

• If scratched → Cr forms $Cr_2O_3$ again.
• Cracks fill automatically.

That is, stainless steel — 'self-protecting'.

Role of Ni

Ni — more strength and density. Cr-Ni combination — most secure.

Without Ni — only 'martensitic' steel.

Types of Stainless Steel

1. Austenitic:

• 304, 316
• Most kitchen utensils.
• Highly corrosion-resistant.

2. Martensitic:

• 410, 420
• Hard, knives.
• Moderately corrosion-resistant.

3. Ferritic:

• 430
• Car exhaust.
• Moderate.

Practical Uses

Universal:

• Kitchen utensils (plates, bowls, spoons).
• Medical instruments.
• Building construction.
• Food industry.
• Chemical industry.
• Modern jewellery.

Stainless Steel in India

Major companies:

• TATA Steel
• Jindal Stainless
• Hindustan Stainless Steel

India — second-largest producer in the world.

An Interesting Comparison

Plain Fe vs Stainless Steel:

Property	Fe	Stainless
Rust	Quickly	Never (in normal conditions)
Shine	Lost quickly	Lasting
Maintenance	More	Less
Cost	Low	High
Suitability	Heavy work	Food, medical

A Famous Example

Iron Pillar of Delhi (near Qutub Minar):

1600 years old. No rust till today! Why? — special phosphorus-containing alloy.

This — 'Indian' technology before stainless steel. Our ancestors knew the secrets of alloys.

[Board + General Knowledge]

Example 9: NCERT — Comparison of Fe and Cu Refining

Differences between methods of refining Fe and Cu.

Solution:

Refining of Fe

'Pig Iron' (4% C, impurities) from Blast Furnace.

Main methods:

1. Bessemer Process

In egg-shaped 'converter'. Air blown through molten pig iron.

Impurities removed: $$C + O_2 \rightarrow CO_2 \uparrow$$ $$Si + O_2 \rightarrow SiO_2$$ $$Mn + O_2 \rightarrow MnO_2$$ $$P + O_2 \rightarrow P_2O_5$$

Impurities — as slag.

2. Open-hearth Process

In a large open furnace. Fe oxide + impurities. More controlled.

3. Electric Steel Furnace

For very pure steel. For special alloys.

Finally: steel with controlled C.

Refining of Cu

'Blister Copper' (~99%, but has impurities) from Cu pyrites.

Method: Electrolytic Refining.

Setup:

• Anode: thick strip of impure Cu.
• Cathode: thin strip of pure Cu.
• Electrolyte: $CuSO_4$ + dilute $H_2SO_4$.

Reactions: Anode: $Cu \rightarrow Cu^{2+} + 2e^-$ Cathode: $Cu^{2+} + 2e^- \rightarrow Cu$ (pure)

Impurities:

• Au, Ag, Pt: anode mud.
• Fe, Zn: in solution.

Finally: 99.99% pure Cu.

Comparative Table

Property	Fe	Cu
Initial	Pig Iron (4% C)	Blister Cu (99%)
Main impurities	C, S, P, Si	Fe, Zn, Au, Ag
Method	Bessemer / Open-hearth	Electrolytic
Final	Steel	Pure Cu
Purity	Controlled C	99.99%
Product impurity	Some (controlled)	Very low

Why the Difference?

Fe:

• More production (lakhs of tonnes each year).
• Electrolytic impractical (very expensive).
• 'Bessemer' — cheap.
• Need controlled C in steel.

Cu:

• Less production (thousands of tonnes).
• 99.99% purity from electrolytic.
• Required for electrical wires.
• Bonus gold from anode mud.

An Interesting Point

That is — refining method depends on final use of the product.

Fe → steel (controlled C — Bessemer). Cu → wires (very pure — electrolytic).

[NCERT — important]

Example 10: A Numerical — Electrolytic Refining

How much electric charge is needed to reduce 10 g of Cu²⁺ ions? (Cu = 63.5)

Solution:

Reaction (at Cathode): $$Cu^{2+} + 2e^- \rightarrow Cu$$

Basic Principle: 1 mol Cu = 63.5 g. 2 mol electrons for 1 mol Cu. Charge of 1 mol e⁻ = 1 Faraday = 96500 C.

Calculation:

Moles in 10 g Cu: $$\text{mol Cu} = \frac{10}{63.5} = 0.157 \text{ mol}$$

Quantity of electrons: $$\text{mol e}^- = 2 \times 0.157 = 0.314 \text{ mol}$$

Total charge: $$Q = 0.314 \times 96500 = 30301 \text{ C}$$

≈ 30,300 C (Coulombs).

Answer: About 30,300 C of charge needed.

Additional Calculation

If current is 5 A — what time?

$$t = \frac{Q}{I} = \frac{30300}{5} = 6060 \text{ s} = 101 \text{ min}$$

That is ~1 hr 41 min.

Practical Relevance

In Cu industry:

• Hundreds of Cu strips in one tank.
• Currents of thousands of amperes.
• Runs for many days.
• Active day and night.

Energy use:

• 1 kg Cu refining ≈ 200-300 kWh.
• Less than Al (Al — 14000 kWh/ton).
• Cu — relatively 'cheap'.

Faraday's Law

This is Faraday's First Law:

'Amount of metal deposited is proportional to charge passed.'

$$m = \frac{ZIt}{1}$$

Where Z = electrochemical equivalent.

Key Insight

Electrolytic refining = precise control. Current, time → amount of pure metal.

[Board: 3-mark numerical]

Example 11: An Interesting Question — Anode Mud

What is in the anode mud during electrolytic refining of Cu? Is it 'waste' or 'valuable'?

Solution:

What's in Anode Mud?

Impurities in thick strip of Cu (Anode):

• Cu (main)
• Less reactive metals:
• Gold (Au)
• Silver (Ag)
• Platinum (Pt)
• Iridium (Ir)
• More reactive metals:
• Fe, Zn, Ni

What Happens in Electrolysis?

At Anode:

• Cu — into solution as ions.
• Fe, Zn, Ni — also into solution (more reactive).
• Au, Ag, Pt — not into solution! (less reactive — don't form ions).
• They fall below the Anode.

At Cathode:

• Cu²⁺ — deposited as Cu.
• Fe²⁺, Zn²⁺ — above Cu in ionic series — don't deposit.
• Stay in solution.

Anode Mud — Analysis

Muddy sediment at the bottom of the tank below the anode — 'anode mud'.

Typical composition (~%):

• Au: 0.5-2%
• Ag: 5-15%
• Pt, Pd: 0.1-1%
• Cu (small): few%
• Others: selenium, tellurium

Valuable!

This is not 'waste' — highly valuable!

Economic value:

• 1 ton of anode mud has ~5-10 kg Au, 50-150 kg Ag.
• Value: lakhs of rupees.

In Cu industry:

• Profit from selling Cu.
• Additional profit from selling anode mud.
• 'Double profit'.

How to Extract Metals from Anode Mud?

Method:

1. Collect anode mud.
2. Smelting:

• Melting at high temperature.
• Cu and others separated in first phase.

1. Chemical treatment:

• Au, Ag — cyanide leaching.
• Pt — special solvents.

1. Electrolytic refining again:

• Pure Au, Ag, Pt.

Commercial Importance

Worldwide:

• ~20% profit of Cu industry — from anode mud.
• India: Hindustan Copper Limited — commercial production from anode mud.

'Gold mines' — old. Now a big share of Au — from Cu refining!

An Interesting Fact

Amount of Au extracted from Cu refining in a year — more than several 'gold mines'!

That is — Cu industry = indirectly Au industry.

Key Insight

'Waste' and 'valuable' — a matter of perspective. Anode mud is a famous example. This is — modern 'circular economy' principle.

[Board + General Knowledge]

Example 12: NCERT — A Comparative Question

(a) What is patina (green layer)? Why is it protective? (b) What is tarnish (black layer)? How to remove?

Solution:

(a) Patina

Definition: Green layer formed on Cu over time.

Formula (main): $Cu(OH)_2 \cdot CuCO_3$

That is: basic copper carbonate.

Reaction: $$2Cu + O_2 + CO_2 + H_2O \rightarrow Cu(OH)_2 \cdot CuCO_3$$

This is a slow reaction — visible over many years.

Why Protective?

Three reasons:

1. Dense layer.

• O₂, CO₂ don't penetrate.

2. Tightly held to inner Cu.

• Doesn't peel off easily.

3. Lasts for generations.

• 100+ years.

This — opposite to rust on Fe. Rust loose, more rust inside; patina dense, protective.

Famous Example

Statue of Liberty (USA):

• Cu statue, installed 1886.
• Initially shiny red-brown.
• After ~30 years — completely covered in green patina.
• Cu inside still safe today!

Others:

• Cu doors of old temples.
• Copper statues (ancient).
• Some roofs of Cu sheets.

(b) Tarnish

Definition: Black layer formed on Ag over time.

Formula: $Ag_2S$ (silver sulphide)

Reaction: $$2Ag + H_2S \rightarrow Ag_2S + H_2$$

or (with O₂ in air with H₂S): $$4Ag + 2H_2S + O_2 \rightarrow 2Ag_2S + 2H_2O$$

Sources of $H_2S$:

• Rotten eggs, onions, garlic.
• Some food items.
• Sulphur-containing air pollution.
• Some rubber items.

How to Remove?

Method 1: Mechanical (Polish)

• Toothpaste + soft cloth.
• Silver polish.
• Simple, but some Ag also removed.

Method 2: Chemical — Aluminium Foil Method

This is an electrochemical method.

Materials:

• Al foil (a piece).
• Hot water.
• Soda (NaHCO₃ or Na₂CO₃).

Method:

1. Put Al foil at the bottom of a container.
2. Place tarnished Ag on Al foil.
3. Pour soda + hot water.
4. Wait a few minutes.

Reaction: $$3Ag_2S + 2Al \rightarrow 6Ag + Al_2S_3$$

That is:

• Al displaced Ag (Al > Ag in reactivity).
• Ag freed — shiny back.
• Al — as Al₂S₃ on the foil.

This is a displacement reaction.

Why Al?

Al — more reactive than Ag. Forming Al₂S₃ — easy. Ag freed.

This is 'sacrificial' method — Al sacrificed, Ag saved.

Comparison — Patina vs Tarnish

Property	Patina	Tarnish
Metal	Cu	Ag
Formula	Cu(OH)₂·CuCO₃	Ag₂S
Colour	Green	Black
Nature	Protective	Surface only
Removal	Unnecessary	Toothpaste / Al foil

Key Insight

Different layers on different metals — different behaviour. Some protective (Cu, Al), some harmful (Fe), some surface (Ag).

[Board + Practical]

Example 13: NCERT — A Challenge

State true/false for the following — with reasons:

(a) Alloys are weaker than pure metals. (b) In galvanisation, Cu coating is given on Fe. (c) Only water is required for rust. (d) Anode mud is waste of Cu industry.

Solution:

(a) Alloys weaker — False ✗

Truth: Alloys are mostly stronger than pure metals.

Reasons:

• Increased hardness (other atoms create obstruction in 'crystal lattice').
• Corrosion-resistance.
• Durability.

Examples:

• Stainless steel (Fe + Cr + Ni) — much stronger than Fe.
• Duralumin (Al alloy) — 4× stronger than Al.
• 22-carat Au — harder than pure Au.

Exception: Some alloys — less in electrical conductivity. Example: steel compared to Cu.

(b) Cu on Fe — False ✗

Truth: In galvanisation, Zn layer on Fe.

Why Zn, not Cu?

Reactivity: $Zn > Fe > Cu$

• Zn — more reactive than Fe.
• On scratch, Zn corrodes first.
• Fe protected ('cathodic protection').

If Cu layer:

• Cu, less reactive than Fe.
• On scratch — Fe corrodes first!
• Reverse — no protection of Fe, more damage.

Hence Cu layer — harmful.

(c) Only water for rust — False ✗

Truth: For rust both water and air (O₂) are required.

Evidence — three test tubes experiment:

• A: water + no air → no rust.
• B: air + no water → no rust.
• C: water + air → rust!

Basic Reaction: $$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

All three in reaction — Fe, O₂, H₂O.

Not just water. Not just air either.

(d) Anode mud waste — False ✗

Truth: Anode mud — highly valuable.

Why? Anode mud has:

• Gold (Au)
• Silver (Ag)
• Platinum (Pt)

These metals less reactive than Cu — don't form ions in electrolysis — fall below Anode.

Economic value:

• 1 ton anode mud has 5-10 kg Au + 50-150 kg Ag.
• Price: lakhs of rupees.

Commercial use:

• ~20% profit of Cu industry — from anode mud.
• Separate companies — just for extracting metal from anode mud.

That is, 'waste' — a 'treasure'!

Summary

Statement	T/F	Reality
Alloy weaker	✗	Stronger
Cu on Fe	✗	Zn coating
Only water rust	✗	Water + O₂
Anode mud waste	✗	Valuable

All four false — learn the correct answers.

[Board: 5-mark logical]

Example 14: A Numerical — Composition of Brass

50 g of brass (Cu + Zn) has 70% Cu. Quantity of each metal?

Solution:

Given:

• Total brass = 50 g
• Cu content = 70%
• Zn content = 30% (remainder)

Quantity of Cu: $$\text{Cu} = 70\% \times 50 = 0.70 \times 50 = 35 \text{ g}$$

Quantity of Zn: $$\text{Zn} = 30\% \times 50 = 0.30 \times 50 = 15 \text{ g}$$

Verification: 35 + 15 = 50 g ✓

Answer:

• Cu = 35 g
• Zn = 15 g

Additional Calculation

Quantity in moles:

• Cu: $\frac{35}{63.5} = 0.551$ mol
• Zn: $\frac{15}{65.4} = 0.229$ mol

Mole ratio: Cu : Zn = 0.551 : 0.229 = 2.4 : 1

That is, ~2.4 atoms of Cu for every 1 atom of Zn.

Types of Brass

70:30 — common brass (Brass 70/30). Use: cartridges, jewellery.

Other types:

Composition	Type	Use
95% Cu, 5% Zn	Gilding metal	Decoration
70% Cu, 30% Zn	Cartridge brass	Bullets
60% Cu, 40% Zn	Muntz metal	Building
90% Cu, 10% Zn	Commercial bronze	Utensils

Why This Ratio?

More Cu → soft, less corrosion. More Zn → hard, but brittle. 70:30 — balance.

An Interesting Demonstration

If 30% Cu and 70% Zn:

• Colour: silvery
• Properties: like Zn.
• Now this is not 'brass' — different name.

That is — Cu predominance defines brass.

Practical Use

70:30 brass:

• Decorative statues.
• Plates, bowls.
• Musical instruments (trumpet, bell).
• Taps, handles.

Most 'brass utensils' — this composition.

[Board: 3-mark numerical]

Example 15: A Mixed — An Interesting Experiment

Three Fe nails:

• A: Plain.
• B: Zn coating (galvanised).
• C: Oiled.

All three placed in salt water. After 1 week — what?

Solution:

Analysis

Nail A — Plain

Result: Completely rusted.

Reason:

• Salt water = electrolyte.
• Both O₂ and H₂O available.
• No protection.
• Fe → Fe₂O₃·xH₂O (rust).

Rust 5-10× faster in presence of salt.

Nail B — Galvanised

Result: No rust!

Reason:

• Zn layer — active.
• Zn — more reactive than Fe.
• If any corrosion — Zn first.
• Fe protected (Cathodic protection).

At end of week:

• Zn layer slightly reduced.
• Fe — shiny, no rust.

Nail C — Oiled

Result: Some rust.

Reason:

• Oil — physical barrier.
• Slight protection from O₂ and H₂O.
• But oil dissolves slightly in water or evaporates.
• Over time, oil is removed.
• Some Fe exposed.

At end of week:

• Some part rust-free.
• Some part rusted.
• Better than A, worse than B.

Comparative Table

Nail	Protection	Result	Durability
A (plain)	None	Full rust	—
B (galvanised)	Zn coating	No rust	Highest
C (oiled)	Physical	Some rust	Less

Conclusion

Best protection: B (galvanisation).

Zn layer — most effective.

Practical Insight

If we want to keep Fe object in salt water (sea) for a long time:

1. Best: Galvanised + additional paint. 2. Moderate: Galvanised. 3. Less: Oil. 4. Worst: No protection.

A Question: Why Do Items Rust Quickly on Coasts?

Answer:

• More moisture.
• NaCl (salt) — electrolyte.
• Cl⁻ ions — attack Fe surface.
• Rust 5-10× faster.

Hence — special paint and alloys on sea ships. Coastal cities of India — Kolkata, Mumbai, Chennai — Fe items rust quickly.

Final Formula

'Sea + Fe = ideal for rust.' Hence in marine environment: galvanisation + paint + special alloys.

[Board + General Knowledge]

Example 16: A Concluding Question

(a) Process of electrolytic refining. (b) Required elements for rust. (c) Why is galvanisation useful? (d) Names and compositions of 5 famous alloys.

Solution:

(a) Electrolytic Refining

Setup (Cu example):

• Anode (+): thick strip of impure Cu.
• Cathode (-): thin strip of pure Cu.
• Electrolyte: $CuSO_4$ + dilute $H_2SO_4$.

Reactions: At Anode: $Cu(\text{impure}) \rightarrow Cu^{2+} + 2e^-$ At Cathode: $Cu^{2+} + 2e^- \rightarrow Cu(\text{pure})$

Fate of impurities:

• Au, Ag, Pt → anode mud.
• Fe, Zn → in solution.

Final: 99.99% pure Cu at Cathode.

(b) Required Elements for Rust

Two required elements:

1. Water (H₂O / moisture).
2. Oxygen (O₂).

Not from one alone. Both needed.

Reaction: $$4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$$

Evidence:

• A test tube: water (no air) — no rust.
• B test tube: air (no water) — no rust.
• C test tube: both — rust.

(c) Why is Galvanisation Useful?

1. Cathodic Protection:

• Zn — more reactive than Fe.
• On scratch, Zn corrodes first.
• Fe protected.

2. Long Durability:

• 20-50 years.

3. Protection on Scratches:

• Unlike other methods.

4. Economical:

• Cheap on mass scale.

Use: taps, pipes, buckets, roofs.

(d) 5 Famous Alloys

1. Stainless Steel

• Composition: 74% Fe + 18% Cr + 8% Ni
• Use: kitchen utensils, knives.

2. Brass

• Composition: 70% Cu + 30% Zn
• Use: decoration, utensils, musical instruments.

3. Bronze

• Composition: 88% Cu + 12% Sn
• Use: statues, medals.

4. Solder

• Composition: 50% Pb + 50% Sn
• Use: electrical welding.

5. Duralumin

• Composition: 95% Al + 4% Cu + 0.5% Mg + 0.5% Mn
• Use: aircraft, cars.

Final Formula

'Refining + Protection + Alloys = Modern metal industry.'

[Board: 5-mark mixed question]