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Qwestrum Engineering360 · Mechanical Engineering · Material Science

Corrosion and Prevention

Corrosion is electrochemical metal loss; the rate (mpy) = (K·W)/(A·T·ρ). Protection uses coatings, cathodic protection, inhibitors, and alloy selection, per material-science texts.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Pilling-Bedworth ratio for oxide film stability
  • Crevice and pitting corrosion in stagnant chloride environments
  • Passivation: Cr in stainless steel forms protective Cr₂O₃

Topic details

Introduction

Corrosion causes enormous economic loss, so its mechanisms and prevention are essential engineering knowledge. Indian material-science courses cover the electrochemical basis, corrosion forms, and protection methods.

Scope in B.Tech and GATE syllabus

Corrosion needs an anode, cathode, electrolyte, and metallic path — a galvanic cell. The more active (anodic) metal dissolves; the galvanic series ranks metals by tendency to corrode. Forms include uniform, galvanic, pitting, crevice, intergranular, and stress-corrosion cracking.

Why this topic matters in practice

Prevention manipulates the cell: barrier coatings, sacrificial (galvanic) or impressed-current cathodic protection, inhibitors that slow the reactions, and choosing corrosion-resistant alloys (stainless steel's passive film). Computing corrosion rate and selecting a protection method are the exam tasks.

Key relations & formulas

Corrosionrate=(K⋅W)(A⋅T⋅D)Corrosion rate = \frac{(K\cdot W)}{(A\cdot T\cdot D)}
(mpy, mils per year)
Ecell=Ecathode−EanodeE_{cell} = E_{cathode} - E_{anode}
(galvanic series driving force)
icorrfromTafelextrapolationi_{corr} from Tafel extrapolation
(polarisation curves)

Formulas (Indian textbook notation)

  • Protection:sacrificialanode,impressedcurrentcathodicprotectionProtection: sacrificial anode, impressed current cathodic protection

Notation and sign conventions

Relation 1 —
Corrosionrate=Corrosion rate =
Corrosionrate=(K⋅W)(A⋅T⋅D)Corrosion rate = \frac{(K\cdot W)}{(A\cdot T\cdot D)}
(mpy, mils per year)
Write this relation with symbols exactly as in Materials Science & Engineering — V. Raghavan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Ecell=Ecathode−EanodeE_{cell} = E_{cathode} - E_{anode}
Ecell=Ecathode−EanodeE_{cell} = E_{cathode} - E_{anode}
(galvanic series driving force)
Write this relation with symbols exactly as in Materials Science & Engineering — V. Raghavan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
icorrfromTafelextrapolationi_{corr} from Tafel extrapolation
icorrfromTafelextrapolationi_{corr} from Tafel extrapolation
(polarisation curves)
Write this relation with symbols exactly as in Materials Science & Engineering — V. Raghavan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 4 —
Protection:sacrificialanode,impressedcurrentcathodicprotectionProtection: sacrificial anode, impressed current cathodic protection

Formulas (Indian textbook notation)

  • Protection:sacrificialanode,impressedcurrentcathodicprotectionProtection: sacrificial anode, impressed current cathodic protection
Write this relation with symbols exactly as in Materials Science & Engineering — V. Raghavan before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Fundamentals and definitions

Corrosion is an electrochemical process: at the anode metal oxidises (M → Mⁿ⁺ + ne⁻), at the cathode a reduction (oxygen or hydrogen) consumes the electrons, with an electrolyte completing the circuit. The anodic metal is lost.

Governing relations in practice

Corrosion rate is expressed in penetration units: mpy = (534·W)/(D·A·T) with W in mg, D density, A area, T hours — proportional to mass loss and inversely to area and time. Faraday's law links mass loss to current.

Design and analysis considerations

Galvanic coupling of dissimilar metals accelerates corrosion of the more active one; a small anode with a large cathode is especially damaging (unfavourable area ratio). Localised forms (pitting, crevice) are dangerous because they concentrate attack.

Advanced theory and extensions

Prevention: barrier coatings (paint, plating) isolate the metal; cathodic protection makes the structure the cathode using a sacrificial anode (Zn, Mg) or impressed current; inhibitors reduce reaction rates; and passivating alloys (stainless steels) self-heal a protective oxide. Selecting the method to the environment is the design decision.

Assumptions and validity limits

State assumptions explicitly before using any relation for corrosion and prevention — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Material Science viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Material Science papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to corrosion and prevention.
4. Use equation 1:
Corrosionrate=Corrosion rate =
.
5. Use equation 2:
Ecell=Ecathode−EanodeE_{cell} = E_{cathode} - E_{anode}
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Corrosion and Prevention appears in material selection and heat treatment. In Indian mechanical curricula this topic is tested because it connects theory to structure–property relationships in materials.
GATE and semester exams often combine corrosion and prevention with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use corrosion and prevention?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Reversing anode and cathode (the anode corrodes)
• Ignoring the unfavourable small-anode/large-cathode area ratio
• Confusing sacrificial (galvanic) with impressed-current cathodic protection
• Unit errors in the corrosion-rate formula (mass, area, time, density)

Quick revision checklist

Before attempting corrosion and prevention problems, confirm you can:
1. Pilling-Bedworth ratio for oxide film stability
2. Crevice and pitting corrosion in stagnant chloride environments
3. Passivation: Cr in stainless steel forms protective Cr₂O₃
Revise the solved examples in Materials Science & Engineering — V. Raghavan and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Galvanic couple direction

Problem

Steel is fastened with copper rivets in a marine environment. Which metal corrodes and how can it be prevented?

Solution

Steel is more active (anodic) than copper, so the steel corrodes preferentially. Prevention: coat/insulate the joint, or add a sacrificial zinc anode (cathodic protection) to protect the steel.

Conceptual check — Corrosion and Prevention

Problem

In a Material Science semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of corrosion and prevention." What should a complete answer include?

Practice questions

Most-asked interview and GATE questions for this topic — expand any item for a model answer.

  1. 1
    What is Corrosion and Prevention, and why does it appear in B.Tech / GATE syllabi?

    Model answer

    Corrosion is electrochemical metal loss; the rate (mpy) = (K·W)/(A·T·ρ). Protection uses coatings, cathodic protection, inhibitors, and alloy selection, per material-science texts.
  2. 2
    State the relation Corrosion rate = and name each symbol.

    Model answer

    The governing relation is Corrosionrate=Corrosion rate =. Write every symbol with SI units before substituting numbers.
  3. 3
    State the relation E_cell = E_cathode − E_anode and name each symbol.

    Model answer

    The governing relation is Ecell=Ecathode−EanodeE_{cell} = E_{cathode} - E_{anode}. Write every symbol with SI units before substituting numbers.
  4. 4
    State the relation i_corr from Tafel extrapolation and name each symbol.

    Model answer

    The governing relation is icorrfromTafelextrapolationi_{corr} from Tafel extrapolation. Write every symbol with SI units before substituting numbers.
  5. 5
    State the relation Protection: sacrificial anode, impressed current cathodic protection and name each symbol.

    Model answer

    The governing relation is Protection:sacrificialanode,impressedcurrentcathodicprotectionProtection: sacrificial anode, impressed current cathodic protection. Write every symbol with SI units before substituting numbers.
  6. 6
    Explain: Pilling-Bedworth ratio for oxide film stability

    Model answer

    Pilling-Bedworth ratio for oxide film stability — state the assumption range and one exam trap linked to this point.
  7. 7
    Explain: Crevice and pitting corrosion in stagnant chloride environments

    Model answer

    Crevice and pitting corrosion in stagnant chloride environments — state the assumption range and one exam trap linked to this point.
  8. 8
    Explain: Passivation: Cr in stainless steel forms protective Cr₂O₃

    Model answer

    Passivation: Cr in stainless steel forms protective Cr₂O₃ — state the assumption range and one exam trap linked to this point.
  9. 9
    How would you correct this error in a viva: Reversing anode and cathode (the anode corrodes)?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  10. 10
    How would you correct this error in a viva: Ignoring the unfavourable small-anode/large-cathode area ratio?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  11. 11
    How would you correct this error in a viva: Confusing sacrificial (galvanic) with impressed-current cathodic protection?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.
  12. 12
    How would you correct this error in a viva: Unit errors in the corrosion-rate formula (mass, area, time, density)?

    Model answer

    Identify the wrong assumption or unit mix-up, rewrite the correct relation, and recompute with a one-line sanity check.

Exams & GATE

  • 1
    Raghavan Ch. 14 — galvanic series ranking for dissimilar metal contact.
  • 2
    Avoid: Reversing anode and cathode (the anode corrodes)
  • 3
    Avoid: Ignoring the unfavourable small-anode/large-cathode area ratio
  • 4
    Avoid: Confusing sacrificial (galvanic) with impressed-current cathodic protection

📖 Standard books (India)

  • Materials Science & Engineering — V. Raghavan

    Read: Syllabus unit

    Structure, properties, and phase diagrams

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