Corrosion and Its Prevention

Corrosion is the gradual deterioration of metals through chemical or electrochemical reactions with the environment. Refined metals are converted into more stable compounds such as oxides, hydroxides, or sulfides — often causing loss of strength, appearance, or function. Corrosion and Its Prevention

Types of Corrosion

• Uniform Corrosion: Even attack over the surface.
• Galvanic Corrosion: Occurs when two dissimilar metals are electrically connected in an electrolyte; the more reactive (anodic) metal corrodes.
• Pitting Corrosion: Localized, small but deep pits; hard to detect until failure.
• Crevice Corrosion: In confined spaces/crevices where solution becomes stagnant and aggressive.
• Intergranular Corrosion: Attack along grain boundaries (usually due to segregation or precipitates).
• Stress Corrosion Cracking (SCC): Cracking caused by tensile stress combined with a corrosive environment.

Causes of Corrosion

Corrosion is accelerated by oxygen, moisture, electrolytes (salts), acids, pollutants (SOx, NOx), and temperature. Electrochemical corrosion requires anodic and cathodic sites, an electrical connection, and an electrolyte to complete the circuit.

Electrochemical Process of Corrosion

At anodic sites metal atoms oxidize to ions (M → Mⁿ⁺ + ne⁻). Electrons travel to cathodic sites where reduction occurs (commonly O₂ + 2H₂O + 4e⁻ → 4OH⁻). Metal ions combine with anions (e.g., OH⁻) to form corrosion products.

Effects of Corrosion

• Loss of structural integrity and possible catastrophic failure.
• Economic cost — repairs, replacements, downtime.
• Safety hazards — bridges, pipelines, pressure vessels.
• Aesthetic degradation and decreased lifetime of components.

Methods of Corrosion Prevention

1. Protective Coatings

Paints, varnishes, polymer coatings and metallic platings form a barrier between metal and corrosive environment. Proper surface preparation is critical for longevity.

2. Cathodic Protection

Make the structure the cathode so it cannot oxidize.

• Sacrificial Anode: Attach a more reactive metal (e.g., zinc, magnesium) which corrodes preferentially.
• Impressed Current: Use an external power source to supply protective current.

3. Alloying

Alloys such as stainless steel (Fe–Cr–Ni) form passive oxide layers (e.g., Cr₂O₃) that slow corrosion significantly.

4. Corrosion Inhibitors

Chemicals added to environments (e.g., cooling water) that adsorb on metal surfaces, form protective films, or neutralize aggressive species.

5. Proper Design & Maintenance

Design to avoid crevices, ensure drainage, prevent stagnation, provide access for inspection, and schedule regular maintenance and monitoring.

Industrial Importance of Corrosion Control

Controlling corrosion extends infrastructure lifespan (bridges, pipelines, storage tanks), reduces operational cost, improves safety, and prevents environmental incidents (e.g., leaks). Sectors that benefit include marine, oil & gas, transportation, and construction.

MCQs — Self-test

1. Q1. Which of the following best describes galvanic corrosion?
   Options: (A) Uniform surface attack   (B) Localized pitting   (C) Two dissimilar metals connected in electrolyte where the anodic metal corrodes   (D) Crack propagation due to stress Answer Correct: (C). Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte; the more active metal becomes anodic and corrodes.
2. Q2. Which is NOT a cathodic protection method?
   Options: (A) Sacrificial anode   (B) Impressed current   (C) Paint coating   (D) Both A and B Answer Correct: (C). Paint coating is a protective barrier method, not a cathodic protection technique.
3. Q3. Pitting corrosion is dangerous because:
   Options: (A) It is uniformly visible   (B) It causes deep, hard-to-detect holes   (C) It can be easily repaired   (D) It only occurs on stainless steel Answer Correct: (B). Pitting creates deep localized holes that are often hidden under corrosion products and can lead to sudden failure.
4. Q4. Alloying helps corrosion resistance primarily by:
   Options: (A) Reducing cost   (B) Forming protective oxide layers   (C) Increasing electrical conductivity   (D) Making materials softer Answer Correct: (B). Many corrosion-resistant alloys form passive oxide layers (e.g., chromium oxide) that protect the underlying metal.
5. Q5. Stress corrosion cracking (SCC) requires:
   Options: (A) High tensile stress   (B) Specific corrosive environment   (C) Susceptible material   (D) All of the above Answer Correct: (D). SCC occurs when high tensile stress, a corrosive environment, and a susceptible material are present together.

Quiz — Short Answer (Answers & Explanation)

1. Define corrosion and explain its main cause. Ideal answer & explanation Corrosion is the gradual degradation of metals due to chemical or electrochemical reactions with the environment; the principal cause is exposure to oxygen and moisture, often accelerated by electrolytes or pollutants that facilitate electrochemical cells on metal surfaces.
2. List three common types of corrosion and one preventive measure for each. Ideal answer & explanation Uniform: Prevent with coatings/painting.
   Galvanic: Prevent by electrical isolation or using similar metals / sacrificial anode.
   Pitting: Prevent by material choice (pitting-resistant alloys) and controlling chlorides.
3. How does cathodic protection work? Ideal answer & explanation Cathodic protection supplies electrons to a metal structure so that it becomes cathodic and cannot oxidize. This can be achieved by attaching a sacrificial anode that corrodes instead, or by applying an impressed current from a power source.

Frequently Asked Questions

How often should cathodic protection systems be inspected?

Inspection frequency depends on environment and criticality. Typical intervals: annually for many structures; more frequently for harsh marine or industrial environments. Always follow standards (e.g., NACE) and your company policy.

Can stainless steel corrode?

Yes — stainless steels resist corrosion via passive oxide layers but can still corrode (e.g., pitting, crevice corrosion) under aggressive chloride environments or if passive film is damaged.

Are corrosion inhibitors safe to use?

Many inhibitors are safe when handled per instructions, but some are toxic or hazardous; always refer to Safety Data Sheets (SDS) and regulatory guidance before use.

• Corrosion — Britannica

Published by CHEMASH.

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