4.4.1 Applications of Electroplating

Steel objects are often protected from corrosion by electroplating with zinc, tin, and chromium.

Zinc Plating

Purpose: To protect steel from corrosion.

How it works:

• Zinc plating on steel is done by using zinc metal as anode.
• A solution of potassium zinc cyanide K₂[Zn(CN)₄] containing a little sodium cyanide is used as an electrolyte.
• The steel object is made the cathode.
• During the electrolysis:
• Zinc at the anode enters the solution as Zn²⁺ ions.
• These ions are deposited at the cathode.

Electrolyte ionization:

K₂[Zn(CN)₄]_(aq) → 2K⁺_(aq) + [Zn(CN)₄]^2-_(aq)

[Zn(CN)₄]^2-_(aq) → Zn²⁺_(aq) + 4CN^-_(aq)

Electrode reactions:

At anode (oxidation): Zn_(s) → Zn²⁺_(aq) + 2e^-

At cathode (reduction): Zn²⁺_(aq) + 2e^- → Zn_(s)

Role of sodium cyanide: Sodium cyanide prevents the hydrolysis of electrolytes.

Awareness against Pollution:

Cyanide ions are extremely toxic. Therefore, a solution containing cyanide ions must not be dropped into rivers and streams. This is responsible for killing fish and other animals.

Tin Plating

Purpose: Food cans are generally tin-plated.

Society, Technology and Science: Tin-plated steel is used to make cans. Food and beverages industries use tinned steel cans. This is because the components of food, beverages, and preservatives contain organic acids or their salts. They may form toxic substances by reacting with iron. These acids and salts are corrosive. Tin plating is non-poisonous and prevents corrosion.

How it works:

• Tin plating on steel is done by using tin as an anode.
• A solution of stannous sulphate (SnSO₄) is used as the electrolyte.
• A few drops of dilute H₂SO₄ are added to the electrolyte to prevent its hydrolysis.
• The steel object is made the cathode.

Electrolyte ionization:

SnSO_4(aq) ⇌ Sn²⁺_(aq) + SO₄^2-_(aq)

Electrode reactions:

At anode (oxidation): Sn_(s) → Sn²⁺_(aq) + 2e^-

At cathode (reduction): Sn²⁺_(aq) + 2e^- → Sn_(s)

Chromium Plating

Purpose: Chromium plating is used to make automobile parts.

Important note: Since chromium metal does not adhere strongly to steel, steel is first plated with copper or nickel and then with chromium.

How it works:

• Chromium metal is used as the anode.
• Chromium sulphate Cr₂(SO₄)₃ is used as the electrolyte.
• A few drops of dilute H₂SO₄ are added to the electrolyte to prevent its hydrolysis.
• The steel object is made the cathode.

Electrolyte ionization:

Cr₂(SO₄)_3(aq) → 2Cr³⁺_(aq) + 3SO₄^2-_(aq)

Electrode reactions:

At anode (oxidation): Cr_(s) → Cr³⁺_(aq) + 3e^-

At cathode (reduction): Cr³⁺_(aq) + 3e^- → Cr_(s)

Application: Chromium-plated steel is used to make automobile parts.

Summary Table

Intermediate Level

Steps in electroplating:

• The object to be plated is made the cathode.
• The plating metal is made the anode.
• The electrolyte contains ions of the plating metal.
• When electricity is passed:
• A thin layer of the plating metal is formed on the object.

Key point:

• The anode is the plating metal.
• The cathode is the object to be plated.
• The electrolyte contains ions of the plating metal.

Examples:

• Zinc plating on steel → zinc anode, K₂[Zn(CN)₄] electrolyte.
• Tin plating on steel → tin anode, SnSO₄ electrolyte.
• Chromium plating on steel → chromium anode, Cr₂(SO₄)₃ electrolyte.

Important Points

• - Electroplating = electrolytic process used to deposit one metal on another metal.
• - The object to be plated is made the cathode.
• - The plating metal is made the anode.
• - The electrolyte contains ions of the plating metal.
• - Zinc plating:
• - Anode: Zinc
• - Electrolyte: K₂[Zn(CN)₄] + NaCN
• - Cathode: Steel object
• - Purpose: Protect steel from corrosion
• - Tin plating:
• - Anode: Tin
• - Electrolyte: SnSO₄ + dilute H₂SO₄
• - Cathode: Steel object
• - Purpose: Food cans; non-poisonous, prevents corrosion
• - Chromium plating:
• - Anode: Chromium
• - Electrolyte: Cr₂(SO₄)₃ + dilute H₂SO₄
• - Cathode: Steel object (pre-plated with Cu or Ni)
• - Purpose: Automobile parts
• - Cyanide ions are extremely toxic — must not be dropped into rivers and streams.

Remember

• - Electroplating = coating one metal with another using electricity.
• - Object to be plated = cathode.
• - Plating metal = anode.
• - Electrolyte = contains ions of the plating metal.
• - Zinc plating → protects steel from corrosion.
• - Tin plating → food cans; non-poisonous.
• - Chromium plating → automobile parts; pre-plate with Cu or Ni.
• - Cyanide ions are toxic → do not pollute water.

Common Mistakes

Quick Revision

Electroplating: The electrolytic process used to deposit one metal on another metal is called electroplating. An object can be electroplated by making it cathode in an electrolytic tank containing ions of the plating metal. The plating metal is made anode. On passing electricity through the electrolytic tanks, a thin layer of anode metal is deposited on the surface of the object.

4.4.1 Applications of Electroplating:

Steel objects are often protected from corrosion by electroplating with zinc, tin, and chromium.

Zinc Plating:

• Zinc plating on steel is done by using zinc metal as anode.
• A solution of potassium zinc cyanide K₂[Zn(CN)₄] containing little sodium cyanide is used as an electrolyte.
• The steel object is made the cathode.
• During the electrolysis zinc at the anode enters the solution as Zn²⁺ ions, which are deposited at the cathode.
• Sodium cyanide prevents the hydrolysis of electrolytes.

Awareness against pollution: Cyanide ions are extremely toxic. Therefore, a solution containing cyanide ions must not be dropped into rivers and streams. This is responsible for killing fish and other animals.

Tin Plating:

• Food cans are generally tin-plated.
• Tin plating on steel is done by using tin as an anode and a solution of stannous sulphate (SnSO₄) as electrolyte.
• A few drops of dilute H₂SO₄ are added to the electrolyte to prevent its hydrolysis.
• Tin-plated steel is used to make cans. Food and beverages industries use tinned steel cans, this is because the components of food, beverages, and preservatives contain organic acids or their salts. They may form toxic substances by reacting with iron. These acids and salts are corrosive. Tin plating is non-poisonous and prevents corrosion.

Chromium Plating:

• Since chromium metal does not adhere strongly to the steel, therefore steel is first plated with copper or nickel and then with chromium.
• For electroplating chromium, chromium metal is used as an anode and chromium sulphate, Cr₂(SO₄)₃ as an electrolyte.
• A few drops of dilute H₂SO₄ are added to the electrolyte to prevent its hydrolysis.
• Chromium-plated steel is used to make automobile parts.

60-Second Revision

• Electroplating = coating one metal with another using electricity.
• Object to be plated = cathode.
• Plating metal = anode.
• Electrolyte = contains ions of the plating metal.
• Zinc plating: protects steel from corrosion.
• Tin plating: food cans; non-poisonous.
• Chromium plating: automobile parts; pre-plate with Cu or Ni.
• Cyanide ions are extremely toxic → do not pollute water.
• ⚠️ Warning: the object to be plated is always the cathode, and the plating metal is always the anode.