Chemistry Project to Study Effect of Metal Coupling on Corrosion
Chemistry Project to Study Effect of Metal Coupling on Corrosion
Name: Gyanesh Tiwari
Roll No.: 6261924
Class: XII (Science)
School: K.V. A.F.S Rajokri N.D. 38
Acknowledgement
It is my duty to record my sincere thanks and deep sense of gratitude to my respected teacher Mr. Sanjay Mishra for his valued guidance, interest and constant encouragement for the fulfillment of the project.
Gyanesh Tiwari Class XII (Science) Roll No. 6261924
Certificate
Certified that this is the bonafide work of Gyanesh Tiwari of class XII of K.V. A.F.S Rajokri N.D. 38. He has performed these experiments during academic year 2009-10.
The student’s initiative, cooperativeness and participation during the practical classes were excellent. His aesthetic presentation, visual appeal, expression and neatness were excellent.
His content accuracy, creativity, originality and analysis of different perception were excellent.
Teacher’s Signature: Mr. Sanjay Mishra
Aim
Aim: To study the effect of metal coupling on the rate of corrosion.
Materials Required
Apparatus: Beakers - 15, Iron sheets of 2# size - 6, Aluminium rods of 2# size - 6, Brass rods of 2# size - 6, Zinc sheets of 2# size - 6, Measuring cylinders, Chemical Balance, Weight Box.
Chemicals: Hydrochloric acid and Sodium hydroxide.
Theory
- Corrosion is a serious problem of some metals like iron, zinc, aluminium and alloys like brass which are commonly used in day to day life.
- Apart from reducing the life of articles made up of these metals or alloys, the chemical substances formed out of corrosion have serious public health problems.
- Replacement of machines or their parts and many other articles in industrial and public dealing lead to huge expenditure.
- Hence, how to reduce or avoid corrosion of articles made up of metals or alloys has been a major subject of study in the field of chemistry and electro-chemistry.
Procedure
- Mix 9 ml of conc. HCl with 241 ml of water to form 250 ml of solution.
- Take this solution in seven different beakers.
- Mark each beaker serially from 1 to 7.
- Take the weights of three iron sheets, three aluminium rods, three brass rods and three zinc sheets.
- Now keep iron sheets, aluminium rods, zinc sheets and brass rods in separate beakers.
- Then take iron + brass, iron + aluminium, iron + zinc, aluminium + zinc and brass + zinc and keep them in different beakers.
- Allow the reactions to occur for 24 hours.
- Note the maximum and minimum temperatures.
- Now at the end of reaction take out the metals and keep them in sun for sometime so that they get dried up quickly.
- Take the weights of each specimen and note the difference.
- Similarly repeat steps 1 to 8 in a basic solution.
Observations
| S.No. | Specimen (with Acid) | Initial Weight (gm) | Final Weight (gm) |
|---|---|---|---|
| 1 | Brass | 8 | 5.00 |
| 2 | Iron | 8 | 6.00 |
| 3 | Zinc | 8 | 6.50 |
| 4 | Aluminium | 8 | 7.10 |
| 5 | Iron + Aluminium | 15 | 12.30 |
| 6 | Brass + Zinc | 15 | 13.00 |
| 7 | Iron + Zinc | 15 | 14.10 |
| S.No. | Specimen (with Base) | Initial Weight (gm) | Final Weight (gm) |
|---|---|---|---|
| 8 | Brass | 8 | 5.80 |
| 9 | Zinc | 8 | 6.20 |
| 10 | Iron | 8 | 7.10 |
| 11 | Aluminium | 8 | 7.60 |
| 12 | Brass + Aluminium | 15 | 12.90 |
| 13 | Brass + Zinc | 15 | 13.60 |
| 14 | Iron + Aluminium | 15 | 14.40 |
Temperature: 21°C Time of reaction: 24 hours
Results
- The rate of corrosion observed in acidic medium, or the mass consumed during corrosion, is in the decreasing order from brass to aluminium. Brass has the highest corrosion rate while aluminium has the least corrosion rate.
Brass > Iron > Zinc > Aluminium
- When coupling of these metals was done, each couple showed some difference in their corrosion with respect to each metal kept alone. Iron + Aluminium couple has the highest rate of corrosion while Iron + Zinc couple has the lowest rate of corrosion.
Iron + Aluminium > Brass + Zinc > Iron + Zinc
- Rate of corrosion in basic medium is in the decreasing order from Brass to Aluminium:
Brass > Zinc > Iron > Aluminium
- When these metals were coupled, the rate of corrosion was in the decreasing order:
Brass + Aluminium > Brass + Zinc > Iron + Aluminium
- Temperature and time of reaction were constant - temperature was 21°C and time of reaction was 24 hours.
Conclusions
- Corrosion is a serious problem of some metals like iron, zinc, aluminium and alloys like brass which are commonly used in day to day life.
- Apart from reducing the life of articles made up of these metals or alloys, the chemical substances formed out of corrosion have serious public health problems.
- Replacement of machines or their parts and many other articles in industrial and public dealing lead to huge expenditure.
- Hence, how to reduce or avoid corrosion of articles made up of metals or alloys has been a major subject of study in the field of chemistry and electro-chemistry.
- The study of the rate of corrosion of different metals or alloys showed gradual decrease in their masses in acidic medium. The decrease is in the order of brass, iron, zinc, aluminium.
- The present experiments are in full agreement with the well known electro-chemical reactions. Some of the typical reactions as occur with iron are illustrated below.
Electrode Reactions (Iron)
(a) The reactions at respective electrodes are:
At cathode:
Fe → Fe²⁺ + 2e⁻
In acid the equilibrium is:
HCl → H⁺ + Cl⁻
At anode:
The water which is in equilibrium:
H₂O → H⁺ + OH⁻
Here the Fe²⁺ cation will readily take Cl⁻ and form FeCl₃. While H⁺ of acid will be reacting with another H⁺ of water and will form H₂ gas. While OH⁻ anion will also react with some of the iron and will form Fe(OH)₃ which is observed in the form of rust.
(b) The e.m.f. of these metals are in the order of Al : Zn : Fe. The values are:
| Reaction | e.m.f. |
|---|---|
| Al → Al³⁺ + 3e⁻ | 1.66 V |
| Zn → Zn²⁺ + 2e⁻ | 0.76 V |
| Fe → Fe²⁺ + 2e⁻ | 0.44 V |
Brass, which is an alloy of zinc and copper, has the e.m.f. 0.42 V during the forward (oxidation) reaction. During the backward reaction the e.m.f. value is -0.42 V. This is because during oxidation reaction the e.m.f. values of zinc and copper are -0.76 V and +0.34 V respectively, which is why the value differs.
© In acid there are replaceable H⁺ ions which react with metals and H₂ gas is evolved. This is because all the metals are highly electronegative in nature. When these two come in contact they react very easily and form stable compounds. Thus the rate of corrosion is very high.
Corrosion in Basic Medium
The rate of corrosion in basic medium is very less as compared to acidic medium. This is shown because of the following factors:
- For example, sodium hydroxide (NaOH) is in equilibrium with Na⁺ and OH⁻ ions:
NaOH → Na⁺ + OH⁻
When NaOH comes in contact with water the two ions immediately dissociate. The hydrated Na⁺ ions will take the H⁺ ion. The electropositive character here will be the main factor in the slow rate of corrosion. Na being more electropositive than the metals mentioned above, most of OH⁻ ions will be taken by Na⁺ when compared to the other metals - i.e., the rate of corrosion is slow.
While: H⁺ + e⁻ = H, and H + H = H₂ gas.
- The availability of electrons is very less for the conversion of H⁺ to H₂ gas. That is why there will not be replaceable H⁺ ions. If there is no replaceable H⁺ ion then corrosion will not be possible. Hence the rate of corrosion is very slow.
Bibliography
- NCERT Textbook
- Wikipedia
- APC Lab Manual