Corrosion is a natural process where a refined metal is converted into a more chemically stable form such as its oxide, hydroxide, or sulfide, leading to gradual destruction of the metal.
Why is corrosion considered an electrochemical process?
Corrosion involves oxidation and reduction reactions, where electrons are transferred between substances.
What is the most common example of corrosion?
The rusting of iron.
What two conditions are necessary for iron to rust?
The presence of both water and oxygen.
What happens at the anode during the corrosion of iron?
At the anode, iron atoms lose electrons and form iron(II) ions (Fe²⁺).
Write the oxidation reaction for the rusting of iron.
Fe(s) → Fe²⁺(aq) + 2e⁻
What happens at the cathode during the corrosion of iron?
Oxygen molecules are reduced in the presence of water to form hydroxide ions (OH⁻).
Write the reduction reaction that occurs in iron corrosion.
O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
What compound is initially formed when Fe²⁺ reacts with OH⁻?
Iron(II) hydroxide (Fe(OH)₂).
What is the final product of rusting, and what is its chemical formula?
The final product is hydrated iron(III) oxide, commonly known as rust, with the formula Fe₂O₃.xH₂O.
What is an anodic area in corrosion?
An anodic area is where oxidation occurs, and metal atoms lose electrons to form metal ions.
What is a cathodic area in corrosion?
A cathodic area is where reduction occurs, and electrons are gained by another substance, usually oxygen.
How does the presence of an electrolyte affect the rate of corrosion?
Electrolytes (such as dissolved salts or acids) increase the rate of corrosion by enhancing ion movement and electron transfer.
How does temperature affect corrosion?
Higher temperatures generally increase the rate of corrosion as chemical reactions occur faster.
What is barrier protection in corrosion prevention?
Barrier protection involves creating a physical layer between the metal and the environment to prevent corrosion.
Give an example of barrier protection.
Painting, greasing, or coating the metal with plastic.
What is sacrificial protection?
Using a more reactive metal that corrodes in place of the protected metal.
Give an example of sacrificial protection.
Galvanisation, where iron is coated with zinc.
What is the role of chromium in stainless steel?
Chromium forms a thin oxide layer that prevents further oxidation.
How does the electrochemical series help in corrosion prevention?
The electrochemical series ranks metals by their reactivity, allowing for the selection of sacrificial metals for protection.
Explanation Questions
Why does iron rust in water but not in dry air?
Rusting requires both oxygen and water to facilitate redox reactions. In dry air, water is absent, preventing corrosion.
How does the presence of salt accelerate rusting?
Salt increases the conductivity of water, allowing ions to move more freely, which speeds up the redox reactions.
Why is aluminium more corrosion-resistant than iron?
Aluminium forms a thin, non-porous oxide layer that prevents further oxidation.
Why is zinc used in galvanisation?
Zinc is more reactive than iron and oxidizes first, protecting the iron beneath.
Why do metals like gold and platinum not corrode easily?
They are lower in the electrochemical series, meaning they do not easily lose electrons.
How does sacrificial protection prevent rusting?
A more reactive metal corrodes in place of the protected metal, preventing its oxidation.
Why does stainless steel resist rusting?
Stainless steel contains chromium, which forms a protective oxide layer.
How does the electrochemical series help in selecting a sacrificial metal for corrosion protection?
The electrochemical series ranks metals based on their tendency to lose electrons. A more reactive metal (higher in the series) is used as a sacrificial anode because it oxidizes first, protecting the less reactive metal.
Why does acid rain increase the rate of corrosion?
Acid rain contains hydrogen ions (H⁺) that react with the metal, increasing the oxidation rate and accelerating corrosion.
Why is copper more corrosion-resistant than iron?
Copper is lower in the electrochemical series than iron, meaning it does not lose electrons as easily, making it more resistant to oxidation and corrosion.