Corrosion problems and solutions

Question 1: What is corrosion?

Solution: Corrosion is a natural process that converts a refined metal into a more chemically stable form such as its oxide, hydroxide, or sulfide. It’s the gradual destruction of materials by chemical and/or electrochemical reaction with their environment.

Question 2: What is the chemical formula for rust?

Solution: The chemical formula for rust is Fe₂O₃.xH₂O, where ‘x’ represents the number of water molecules associated with iron(III) oxide.

Question 3: Name the most common form of corrosion.

Solution: The most common form of corrosion is known as “Uniform or General Corrosion.”

Question 4: What is galvanic corrosion?

Solution: Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.

Question 5: What is the principle behind sacrificial anodes used in corrosion control?

Solution: Sacrificial anodes work on the principle of galvanic corrosion. Here, a metal that is more easily oxidized than the one it protects is used. This more reactive metal acts as the anode and corrodes first, sparing the protected metal.

Question 6: What is passivation in the context of corrosion?

Solution: Passivation is the process of making a material “passive,” usually by the deposition of a layer of oxide that adheres to the metal surface. This layer prevents further corrosion by blocking oxygen diffusion to the metal surface.

Question 7: What is pitting corrosion and why is it dangerous?

Solution: Pitting corrosion is a localized form of corrosion where cavities or “holes” are produced in the material. It’s dangerous because it can lead to the failure of a system with very little total metal loss.

Question 8: What is the redox reaction that occurs in the rusting of iron?

Solution: The redox reaction that occurs in the rusting of iron is as follows:

Oxidation: 2Fe(s) → 2Fe²⁺(aq) + 4e⁻

Reduction: O₂(g) + 4e⁻ + 2H₂O(l) → 4OH⁻(aq)

Question 9: What is crevice corrosion and where is it most likely to occur?

Solution: Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working fluid from the environment is limited. It’s most likely to occur in areas such as under gaskets, washers, insulation material, fastener heads, surface deposits, and crevices in welds.

Question 10: How does the process of galvanization protect against corrosion?

Solution: Galvanization, typically involving the use of zinc, provides a protective layer on the surface of the metal (usually steel or iron). Zinc is more likely to oxidize than the underlying metal, so it corrodes first, thereby protecting the metal beneath.

Question 11: What is stress corrosion cracking (SCC)?

Solution: Stress Corrosion Cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected sudden failure of normally ductile metals subjected to a tensile stress.

Question 12: How does temperature influence the rate of corrosion?

Solution: Generally, the rate of corrosion increases with temperature for most metals in contact with oxygen and water. Increased temperature can increase the rate of chemical reactions and the amount of moisture in the air, both of which can increase corrosion rates.

Question 13: How does pH influence corrosion?

Solution: The corrosion rate of metals tends to increase with decreasing pH (i.e., an increase in acidity). However, for some metals like zinc and aluminum, corrosion rates can increase at both high and low pH levels.

Question 14: How can the rate of corrosion be measured?

Solution: The rate of corrosion can be measured through several methods, including weight loss methods, electrochemical methods (like linear polarization resistance), and direct inspection.

Question 15: Why is stainless steel resistant to corrosion?

Solution: Stainless steel is resistant to corrosion because it contains a minimum of 10.5% chromium. The chromium in the alloy forms a self-healing oxide layer that provides protection against corrosive substances.

Question 16: What is intergranular corrosion?

Solution: Intergranular corrosion is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their interiors.

Question 17: What is the primary role of a corrosion inhibitor?

Solution: The primary role of a corrosion inhibitor is to decrease the corrosion rate of a metal or an alloy.

Question 18: What is erosion corrosion?

Solution: Erosion corrosion is an accelerated form of corrosion that occurs at locations where the metal surfaces are exposed to relative movement against a fluid or suspended particles.

Question 19: Why are copper pipes used instead of iron for water supply?

Solution: Copper pipes are used instead of iron because copper is more resistant to corrosion, has a longer life, and is less prone to build-up inside the pipe which can affect water flow.

Question 20: How does salinity affect the rate of corrosion?

Solution: Salinity can increase the rate of corrosion, particularly for metals like steel. Salts, particularly chloride ions, can break down the passive oxide layers on the metal surface, which leads to an increased corrosion rate.

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