The main factors which affect corroision are

Options

A. Position of metal in electrochemical series
B. Presence of `CO_(2)` in water
C. Presenc of impurities in metals
D. All of these

Answer

### Step-by-Step Solution

1. **Definition of Corrosion**:
- Corrosion is a process where metals degrade or "eat away" due to chemical reactions, primarily with atmospheric gases and moisture.

2. **Reactivity of Metals**:
- The reactivity of a metal plays a crucial role in its susceptibility to corrosion. More reactive metals tend to oxidize more readily, leading to higher chances of corrosion.

3. **Presence of Impurities**:
- The presence of impurities in water can enhance the corrosion process. Impurities can set up a voltage cell, which accelerates the corrosion rate.

4. **Role of Electrolytes**:
- Electrolytes in water, such as salts, can increase the conductivity of water, facilitating the corrosion process. The presence of carbon dioxide (CO2) in water can lead to the formation of carbonic acid, which further increases the rate of corrosion.

5. **Temperature Effects**:
- An increase in temperature generally increases the rate of corrosion. Higher temperatures can accelerate the chemical reactions involved in the corrosion process.

6. **Protective Coatings**:
- Coating a metal with a more reactive metal can reduce the rate of corrosion. This is because the more reactive metal will oxidize preferentially, protecting the underlying metal.

7. **Overall Reaction of Corrosion**:
- For example, in the case of iron corrosion, iron (Fe) oxidizes to form iron ions (Fe²⁺), while oxygen (O₂) reacts with water (H₂O) to form hydroxide ions (OH⁻). This can be summarized in the overall reaction:
\[ 2Fe(s) + O_2(g) + 2H_2O(l) \rightarrow 2Fe(OH)_2(s) \]

8. **Conclusion**:
- The main factors affecting corrosion include the reactivity of the metal, the presence of impurities and electrolytes in water, temperature, and protective coatings. Therefore, the correct answer to the question is "All of these."