Corrosion, Stainless Steel &

Tin Plating

XII CHEMISTRY CHAPTER 5 SIDRA JAVED

Co

rro

sio

n A harmful and unwanted reaction

of a metal as a result of

exposure to atmosphere or any

chemical agent.

Almost all metals except noble

metals (Ag, Au, Pt) show great

tendency to corrode at

different rates.

Types of corrosion

1.Atmospheric Corrosion:

By the action of air

2.Corrosion in Liquids:

Due to reaction of a

liquid on metal surface

Disadvantage of

Corrosion

About 1/5th of Iron in use

is lost annually

Reduction in strength of

iron structures

Loss of Billions of dollars

Methods to Prevent

Corrosion

Protective Metallic Coating

Tin Plating or Galvanization

(Zinc Coated iron)

Electroplating

Plating of noble metals

Ag, Au, Pt etc

Alloying of metals

Coating with non

metallic materials

Stainless SteelStainless

Steel

Stainless steel

Alloys of iron that are corrosion resistant.

Formed by addition of chromium or chromium and nickel into

steel.

They are oxidizable to such slight degree thata thin layer of

oxide is formed on the surface which prevents further

corrosion.

Stainless

Steel

Iron based alloys which show

resistance to corrosion

Formed by the addition of Chromium

or Chromium and Nickel in steel

Types

1. Stainless steel containing 13% Cr and 0.1 to 0.4

% C

2. Stainless steel containing 17% Cr and 2% Ni

3. Stainless steel containing 18% Cr and 6% Ni

Corrosion resistance

All types of steel are corrosion resistant alloys

Only oxidizable to such slight degree that a thin layer of oxide is

formed on surface of metal which resist further corrosion.

The oxide layer is so thin that the apperance and color of basic

material looks like unchanged.

Uses of stainless steel

House hold utensils

Cutlery

Decorative objects

Industrail uses

Tin plating

Tin plating

An art of making coat of

metallic tin on baser

metals such as iron and

copper for their

protection from corrosion

and poisoning.

Mechanical tin plating

Iron sheets are cleaned and dipped in warm dilute

sulphuric acid to remove oxide films

Sheets are washed with water and dried.

Dried sheets are dipped in molten tin.

The sheets are then rolled to remove excess tin.

Mechanical tin plating

Copper and brass utensils are heated and rubbed with

NH4Cl to remove oxide layer

Tin metal is rubbed over hot surface after sprinkling

NH4Cl.

Utensils are polished with rag until a uniform layer of

tin is obtained.

Electrolytic tin

plating

Iron sheets are cleaned, washed,

dried and then suspended into a

bath containing Stannous chloride

and HCl.

Anode is made up of pure tin and

iron sheets are made the cathode.

On passing current throught the

bath, tin starts depositing on iron

sheets.

End of lesson