Corrosion, Wear and Surface Engineering

Introduction and context

Engineering components rarely fail because their bulk material suddenly loses all strength. More often, damage begins at the surface: a metal dissolves electrochemically, reacts with oxygen or hot gases, loses material by sliding or impact, or develops a crack under a combination of environment and stress. Corrosion and wear therefore account for major costs through replacement, energy loss, leakage, contamination, unplanned shutdowns and safety incidents.

Surface engineering addresses these problems by controlling surface composition, structure, roughness, hardness, residual stress and interaction with the environment, while retaining the desired bulk properties. The engineer must first identify the damage mechanism, then select a compatible material, coating, inhibitor, lubricant, protection system and inspection method. A hard coating is not automatically a good wear solution; a corrosion-resistant coating may fail if porous or poorly bonded; and cathodic protection can cause hydrogen damage in susceptible steels.

This chapter treats corrosion, oxidation, tribology and environmental degradation as interacting engineering phenomena rather than isolated topics.

Learning objectives

On completing this chapter, a student should be able to:

• explain electrochemical corrosion using anodic and cathodic reactions;
• use electrode potentials, polarization and Faraday’s law to estimate corrosion behavior;
• distinguish uniform, galvanic, pitting, crevice, intergranular, erosion-corrosion and stress-corrosion damage;
• describe oxidation kinetics and the protective role of surface films;
• analyze wear using the principal tribological mechanisms;
• select lubricants, coatings, inhibitors and cathodic or anodic protection;
• compare surface-modification processes with respect to performance and limitations;
• design an inspection and life-prediction strategy;
• evaluate safety, environmental and regulatory implications of surface treatments.

1. Electrochemical foundations of corrosion

Corrosion is the deterioration of a material, usually a metal, through chemical or electrochemical interaction with its environment. In aqueous corrosion, four elements are required: an anodic site, a cathodic site, an electrolyte, and an electrically continuous path through the metal. The anodic reaction produces electrons and metal ions; the cathodic reaction consumes electrons.

For iron in an acidic aerated solution, representative half-reactions are:

and, for oxygen reduction in neutral or alkaline solution,

The overall corrosion rate is governed by the equality of anodic and cathodic current at the freely corroding potential, called the corrosion potential, E_corr. The corresponding current density, i_corr, is a useful measure of the instantaneous corrosion rate.

1.1 Electrode potential and the Nernst equation

The equilibrium potential of a half-cell depends on temperature, activity and the number of electrons transferred. The Nernst equation is

where E is electrode potential in volts, E° is the standard electrode potential, R is the gas constant (8.314 J·mol⁻¹·K⁻¹), T is absolute temperature in kelvin, n is the number of electrons transferred, F is Faraday’s constant (96 485 C·mol⁻¹), and Q is the reaction quotient based on activities. The equation applies to a reversible half-cell at electrochemical equilibrium. Activities are dimensionless; concentrations may be used as approximations only for dilute solutions.

At 25 °C, using common logarithms, the equation can be written as

Standard potentials help rank thermodynamic tendencies, but they do not alone predict practical corrosion rates. Kinetics, polarization, passive films, area ratios, solution resistance and transport must also be considered.

1.2 Polarization and mixed-potential theory

Anodic polarization makes metal dissolution more favorable, whereas cathodic polarization makes reduction more difficult. The actual corrosion potential occurs where the total anodic current equals the total cathodic current. This is the mixed-potential principle.

For a reaction controlled by charge-transfer kinetics, the Butler–Volmer equation is

where i is current density, i₀ is exchange-current density, η = E − E_eq is overpotential, α_a and α_c are anodic and cathodic transfer coefficients, and the other symbols have their usual meanings. The equation assumes a uniform electrode and charge-transfer control. At sufficiently large positive or negative overpotential, one exponential term dominates, producing the Tafel relationship:

A polarization curve can reveal active dissolution, passivation, transpassive behavior, diffusion limitation and the approximate corrosion current. Linear polarization resistance near E_corr is often expressed as

where R_p is polarization resistance, ∆E is a small potential perturbation and ∆i is the resulting current-density change. Under suitable assumptions, the Stern–Geary relationship connects corrosion current density and polarization resistance:

Here B depends on anodic and cathodic Tafel slopes. This method is rapid but becomes unreliable when the surface is nonuniform, strongly polarized, fluctuating, or affected by solution resistance.

1.3 Faraday’s law and corrosion penetration

The mass of metal dissolved by an anodic current is estimated from Faraday’s law:

where m is dissolved mass (kg or g), I is current (A), t is time (s), M is molar mass, n is electrons per metal atom, and F is Faraday’s constant. If current density i is used, I = iA, where A is exposed area.

The equivalent weight is M/n. A commonly used penetration-rate relation is

where CR is corrosion rate, K is a unit-conversion constant, i_corr is corrosion current density, EW = M/n is equivalent weight, and ρ is metal density. The value of K must match the selected units, such as mm·year⁻¹, μA·cm⁻², g·cm⁻³ and g·equivalent⁻¹. The calculation assumes uniform dissolution and a known dominant anodic reaction; localized corrosion may penetrate much faster than the calculated average.

2. Forms and mechanisms of corrosion

Uniform corrosion

Uniform corrosion removes material relatively evenly. It is comparatively predictable and can often be accommodated by a corrosion allowance. Atmospheric attack of unprotected mild steel is a familiar example. Its apparent simplicity should not lead to complacency: thinning of a pressure vessel or pipe can become critical even when no local pit is visible.

Galvanic corrosion

When dissimilar conductive materials contact each other in an electrolyte, the more active metal generally becomes anodic and corrodes faster, while the nobler metal becomes cathodic. Severity depends on potential difference, electrolyte conductivity, polarization behavior and area ratio. A small anodic area connected to a large cathodic area is particularly dangerous because the anodic current is concentrated.

Good practice includes electrically isolating dissimilar metals, selecting compatible pairs, coating the cathode rather than only the anode, using insulating washers and avoiding water traps. Coatings must be inspected for holidays because a small exposed anode beneath a large coating can suffer intense attack.

Pitting and crevice corrosion

Pitting is highly localized penetration caused by breakdown of a passive film. Chloride ions, inclusions, surface damage, stagnant conditions and unfavorable potential promote pit initiation. Once a pit forms, hydrolysis and migration of chloride can make the pit interior acidic and maintain anodic dissolution.

Crevice corrosion occurs in shielded regions such as bolted joints, lap joints, gaskets and deposits. Oxygen depletion inside the crevice creates a differential-aeration cell. Design responses include eliminating crevices, sealing joints, improving drainage, using continuous welds and preventing deposits.

Intergranular corrosion and selective attack

Intergranular corrosion follows grain boundaries when alloy chemistry or thermal history makes them electrochemically different from grain interiors. Sensitization of some stainless steels, caused by chromium-carbide precipitation during inappropriate heating, can deplete chromium near grain boundaries. Low-carbon grades, stabilized grades and proper heat treatment reduce this risk.

Selective leaching removes one alloy constituent preferentially, such as dezincification of brass. Graphitic corrosion of cast iron leaves a weak graphite network after iron dissolution.

Erosion-corrosion and cavitation

Erosion-corrosion combines mechanical removal of protective films with electrochemical attack. High fluid velocity, turbulence, entrained solids, impingement and abrupt changes in flow direction increase damage. Cavitation occurs when vapor bubbles form and collapse near a surface, producing shock waves and pitting. Solutions include reducing turbulence, changing geometry, controlling velocity, removing suspended solids and choosing resistant alloys or coatings.

Stress-corrosion cracking and hydrogen damage

Stress-corrosion cracking requires a susceptible material, a specific environment and tensile stress, often residual stress. Cracks may propagate at stresses below the yield strength. Prevention involves material selection, stress relief, environmental control, compressive surface stress and inspection.

Hydrogen can enter metals during pickling, electroplating, corrosion or cathodic protection. In high-strength steels it may cause delayed brittle cracking. Baking after suitable electroplating, controlling cathodic potential and avoiding excessive protection are common safeguards. Hydrogen embrittlement is a serious reason why protection systems must be designed rather than applied indiscriminately.

3. Oxidation and high-temperature degradation

High-temperature oxidation is a chemical reaction between a material and an oxidizing gas, commonly oxygen. A compact, adherent and slow-growing oxide can protect the substrate. Porous, cracked, volatile or poorly adherent oxides provide little protection.

For a planar oxide layer, the parabolic law is frequently used when diffusion through the oxide controls growth:

where x is oxide thickness, x₀ is initial thickness, t is time and k_p is the parabolic rate constant. The law assumes a continuous scale, approximately constant temperature and diffusion-controlled growth. For a linear reaction controlled by surface reaction or repeated scale loss, x ≈ k_l t; for an initially protective film, logarithmic laws may apply over limited ranges.

Oxidation resistance is improved by alloying elements such as chromium, aluminium or silicon, which form stable oxides. Thermal cycling can cause cracking because the oxide and substrate have different thermal expansion coefficients. Hot corrosion, carburization, sulfidation and nitridation may be more damaging than simple oxidation, especially in turbines, furnaces and petrochemical equipment.

4. Tribology and wear

Tribology is the science and engineering of interacting surfaces in relative motion, including friction, lubrication and wear. Wear is material loss or functional damage caused by mechanical action, often assisted by chemical reactions.

4.1 Friction and contact

The coefficient of friction is

where μ is dimensionless coefficient of friction, F_t is tangential friction force and F_n is normal load. This relation is a definition, not a universal material constant. It changes with speed, temperature, surface roughness, lubricant, load, contamination and wear state.

Real contact occurs at microscopic asperities rather than across the apparent area. High local pressures can cause plastic deformation, adhesive junctions and fatigue. Contact analysis may require Hertzian theory, but that theory assumes idealized smooth elastic bodies and should not be applied without checking geometry, hardness and plasticity.

4.2 Principal wear mechanisms

Adhesive wear occurs when asperities weld locally and then tear material from one surface to the other. It is reduced by compatible material pairs, lubrication, increased hardness and lower contact stress.

Abrasive wear results when hard asperities or particles cut or plough a softer surface. Two-body abrasion involves a hard counterface; three-body abrasion involves loose particles. Hard coatings, filtration and contamination control are important countermeasures.

Surface fatigue develops under repeated rolling or sliding contact. Subsurface cracks can produce pitting or spalling, as in bearings and gears. Proper hardness gradients, clean steel, alignment and lubrication improve life.

Erosive wear is caused by high-speed particles or droplets striking a surface. Impact angle, particle hardness, velocity and ductility determine the mechanism.

Fretting wear occurs under small-amplitude oscillatory motion, often with oxidation debris. It is common in bolted joints, splines and turbine connections. Increasing clamping, eliminating micro-motion, applying suitable coatings and controlling debris can help.

Tribocorrosion is the combined action of wear and corrosion. Wear removes passive films; corrosion weakens the surface, so the combined damage may exceed the sum of separate effects.

4.3 Archard wear model

A useful first estimate for adhesive or abrasive wear is Archard’s relationship:

where V is wear volume, W is normal load, L is sliding distance, H is representative hardness of the softer surface, and K_w is a dimensionless wear coefficient. If wear depth h is required over an apparent area A, h = V/A. The model assumes approximately steady sliding and an empirically determined coefficient. K_w may vary by orders of magnitude with lubrication, debris, temperature and surface condition; therefore it should be obtained from representative tests rather than treated as a universal constant.

5. Lubrication and environmental control

Lubrication separates surfaces, lowers friction, removes heat and carries away debris. Regimes include:

• Boundary lubrication: surfaces remain partly in contact; additives form protective films.
• Mixed lubrication: some load is carried by the fluid film and some by asperities.
• Hydrodynamic lubrication: relative motion generates a full fluid film; load is supported by pressure in the lubricant.
• Elastohydrodynamic lubrication: elastic deformation and pressure-dependent viscosity are important, especially in rolling contacts.

Lubricant selection considers viscosity, viscosity–temperature behavior, load, speed, temperature, chemical compatibility, water tolerance, oxidation stability, fire risk and environmental disposal. Excessive viscosity increases churning losses; insufficient viscosity allows asperity contact. Additives may include antiwear, extreme-pressure, antioxidant, detergent, dispersant and corrosion-inhibiting species, but additives can be incompatible with seals or yellow metals.

Environmental control is often more economical than changing the entire material. Dehumidification, drainage, filtration, removal of chlorides, control of pH, reduction of oxygen ingress and prevention of deposits can substantially reduce damage.

6. Corrosion protection and surface engineering

6.1 Material and design selection

The first protection decision is often design rather than coating. Avoid stagnant zones, sharp crevices, dissimilar-metal contact, water traps and inaccessible surfaces. Select alloys with appropriate passivity, strength and environmental compatibility. Specify corrosion allowance only when the mechanism is sufficiently uniform and measurable.

6.2 Inhibitors

Inhibitors reduce corrosion by adsorbing on the surface, changing anodic or cathodic kinetics, forming precipitated films, scavenging oxygen or modifying the solution. Anodic inhibitors can be dangerous if used below the concentration needed for complete protection because they may promote localized attack. Cathodic inhibitors reduce the cathodic reaction, while mixed inhibitors affect both branches. Inhibitor selection must consider toxicity, biodegradability, concentration control, temperature, flow and compatibility with downstream processes.

6.3 Cathodic and anodic protection

Cathodic protection forces the protected structure to act as a cathode. It can use sacrificial anodes, commonly zinc, aluminium or magnesium, or an impressed-current system using an external DC source and inert or slowly consuming anodes. The protected potential must be sufficiently negative to suppress anodic dissolution but not so negative that coating disbondment or hydrogen damage occurs.

Anodic protection maintains a metal in its passive potential range. It is applicable only to materials and environments that exhibit stable passivation. It requires careful monitoring because loss of control can cause rapid active dissolution.

6.4 Coatings and surface modification

Coatings function as barriers, sacrificial systems, inhibitors or combinations of these. Important classes include:

Surface modification includes carburizing, nitriding, carbonitriding, induction hardening, shot peening, laser treatment and additive surface deposition. Nitriding can produce a hard diffusion zone with limited distortion; carburizing produces a hard case and tough core; shot peening introduces beneficial compressive residual stress but may increase roughness or damage sensitive surfaces.

Coating design must specify substrate preparation, cleanliness, roughness, thickness, adhesion, porosity, residual stress, edge coverage, service temperature and repair procedure. Inspection may include visual examination, dry-film thickness, adhesion testing, holiday detection, metallography, hardness, roughness and electrochemical testing.

7. Inspection, diagnosis and life prediction

Effective maintenance begins with mechanism-based diagnosis. A useful workflow is:

1. Define the function, material, load, environment and consequence of failure.
2. Examine damage morphology: uniform thinning, pits, grooves, cracks, spalls, discoloration or transfer films.
3. Collect deposits, lubricant and process-fluid samples without contaminating them.
4. Measure thickness, pit depth, roughness, hardness, crack dimensions and coating condition.
5. Confirm the mechanism through microscopy, chemistry, electrochemical tests or wear tests.
6. Select mitigation and establish a monitoring interval.

For approximately uniform wall thinning, remaining life can be estimated as

where t_rem is estimated remaining time, t_meas is measured wall thickness, t_min is minimum allowable thickness and r is the metal-loss rate in the same thickness-per-time units. If two measurements are available,

provided the dates and measurements are comparable. A conservative assessment should include measurement uncertainty, changing service conditions, localized attack and an appropriate safety margin. This simple equation is not valid for crack growth or rapidly changing corrosion mechanisms.

Crack-like flaws require fracture-mechanics or fitness-for-service assessment. Fatigue crack growth is commonly represented over a suitable regime by the Paris law:

where a is crack length, N is load-cycle count, C and m are experimentally determined constants, and ∆K is stress-intensity-factor range. The law applies only within its valid crack-growth regime and does not represent threshold, near-instability or environmental acceleration automatically.

Risk-based inspection combines probability of failure, consequence of failure, detectability and degradation rate. Inspection methods include visual examination, ultrasonic thickness measurement, radiography, eddy-current testing, magnetic-particle testing, dye penetrant testing, acoustic emission and replica metallography. Method selection depends on whether the damage is volumetric, surface-breaking, magnetic, electrically conductive or geometrically accessible.

8. Illustrative case study: corrosion and wear in a cooling-water pump

Background. A carbon-steel impeller in a cooling-water pump experiences reduced flow after nine months. The water contains chloride, suspended sand and dissolved oxygen. A stainless-steel sleeve is coupled to the impeller assembly.

Problem. Inspection finds pits near the impeller eye, grooving at the leading edges and a roughened sleeve. Vibration has increased, and the pump efficiency has fallen.

Analysis. The pits indicate localized corrosion promoted by chloride and stagnant regions. The grooves indicate erosion-corrosion caused by high local velocity and entrained particles. The dissimilar steel/sleeve interface creates a galvanic risk. Wear debris and roughness increase turbulence, which further accelerates attack: this is a tribocorrosion interaction.

Approach. The engineering team maps pit depth and measures wall thickness ultrasonically. Water chemistry is reviewed, and the pump is checked for cavitation, misalignment and excessive velocity. The redesign uses a more suitable impeller alloy or a qualified erosion-resistant coating, improves hydraulic geometry, installs upstream filtration, eliminates a crevice at the sleeve, and applies a compatible corrosion-control program. Sacrificial anodes or impressed-current protection are considered only after checking the materials and hydrogen risk. The coating specification includes blast preparation, thickness, adhesion, holiday testing and repair criteria.

Outcome. A subsequent inspection interval is based on measured thinning and pit growth rather than a generic calendar period. Vibration and efficiency are trended with water chemistry and flow rate. If the measured uniform loss is 0.18 mm·year⁻¹, current thickness is 8.2 mm and the allowable minimum is 6.0 mm, the simple remaining-life estimate is:

This value is not accepted without adjustment: the deepest pit, uncertainty, possible acceleration and the consequence of impeller failure must be considered. The result illustrates why an average corrosion rate cannot substitute for localized-damage assessment.

Learning points. Failure analysis must integrate electrochemistry, flow, contact mechanics, materials compatibility and inspection. A coating alone cannot correct cavitation, poor geometry or contaminated water. Monitoring should measure both damage and the variables that cause it.

9. Contemporary developments and professional responsibilities

Modern surface engineering increasingly uses high-entropy alloys, nanostructured coatings, cold spray, laser cladding, solid lubricants, environmentally preferable conversion coatings and digital condition monitoring. Machine-learning systems can assist anomaly detection by combining vibration, acoustic emission, temperature, lubricant chemistry and inspection images. However, AI predictions require representative training data, traceable sensor calibration and human review. A model should not be used to override a safety limit without validated uncertainty and engineering authority.

Environmental and ethical responsibilities include controlling hexavalent chromium and other hazardous plating chemicals, managing abrasive and coating waste, preventing volatile-organic emissions, protecting workers during blasting and spraying, and ensuring that inhibitors do not create unacceptable aquatic toxicity. Regulations may govern pressure equipment, hazardous chemicals, electrical cathodic-protection systems, confined-space work and waste disposal. Documentation should identify materials, coating batches, surface preparation, inspection results, deviations and repair history.

Best-practice design checklist

• Identify the dominant damage mechanism before selecting a treatment.
• Separate material compatibility from coating performance; both must be satisfactory.
• Control water, oxygen, chlorides, deposits, temperature and flow where possible.
• Use representative laboratory or field tests to determine wear coefficients and coating life.
• Specify surface preparation and inspection as carefully as the coating chemistry.
• Avoid small-anode/large-cathode galvanic arrangements.
• Check hydrogen risk before applying electroplating or strong cathodic protection.
• Treat localized corrosion and cracks separately from uniform thinning.
• Trend condition data and revise predictions when new evidence appears.
• Include worker safety, environmental compliance and end-of-life disposal in the design decision.

Chapter Summary

Corrosion is an electrochemical process involving anodic metal dissolution and cathodic reduction. Electrode potentials provide thermodynamic guidance, while polarization and mixed-potential theory describe actual corrosion currents. Faraday’s law relates charge transfer to material loss, but localized attack requires additional inspection and conservative judgment. Uniform, galvanic, pitting, crevice, intergranular, erosion-corrosion, cavitation and stress-corrosion damage have distinct causes and controls.

High-temperature oxidation depends on the formation, transport and stability of oxide scales; parabolic growth is appropriate only under diffusion-controlled conditions. Tribology examines friction, lubrication and wear. Adhesive, abrasive, fatigue, erosive, fretting and tribocorrosive wear require different countermeasures. Archard’s relationship is a useful empirical estimate, not a universal prediction.

Protection combines good design, compatible materials, inhibitors, cathodic or anodic protection, lubrication, coatings and surface modification. Carburizing, nitriding, shot peening, thermal spraying, plating, PVD/CVD and laser cladding modify surfaces in different ways and must be specified with preparation and inspection requirements. Remaining-life calculations are useful for uniform thinning but must not be applied uncritically to pits or cracks. Safe practice integrates diagnosis, monitoring, uncertainty, environmental compliance and professional judgment.

Key Terms

Corrosion: Deterioration caused by chemical or electrochemical interaction with an environment.

Anode: Electrode region where oxidation occurs.

Cathode: Electrode region where reduction occurs.

Mixed-potential theory: Principle that anodic and cathodic currents balance at the corrosion potential.

Polarization: Change in electrode potential caused by current flow.

Passivation: Formation of a protective surface film that greatly reduces dissolution.

Galvanic corrosion: Corrosion caused by electrical coupling of dissimilar materials in an electrolyte.

Pitting: Highly localized corrosion producing cavities or holes.

Crevice corrosion: Localized attack in shielded, stagnant regions.

Stress-corrosion cracking: Crack growth caused by combined tensile stress and a specific environment.

Tribology: Study of friction, lubrication and wear.

Adhesive wear: Material transfer caused by local adhesion and junction failure.

Abrasive wear: Material removal by hard asperities or particles.

Fretting: Wear caused by small-amplitude oscillatory motion.

Tribocorrosion: Synergistic damage from wear and corrosion.

Inhibitor: Chemical that reduces the rate of corrosion.

Cathodic protection: Protection achieved by forcing a structure to act as a cathode.

Surface engineering: Deliberate modification of a surface to improve functional performance.

Holiday: A discontinuity or pinhole in a protective coating.

Remaining life: Estimated time until a component reaches a defined allowable condition.

Review Questions

Short Answer Questions

1. What four conditions are required for aqueous electrochemical corrosion?
2. Distinguish corrosion potential from corrosion current density.
3. Why can a small anodic area coupled to a large cathodic area be dangerous?
4. Name three principal wear mechanisms and one control measure for each.
5. State one important limitation of estimating remaining life from uniform wall thinning.

Long Answer Questions

1. Explain mixed-potential theory and describe how polarization curves can be used to estimate corrosion behavior.
2. Compare pitting, crevice corrosion, galvanic corrosion and stress-corrosion cracking in terms of mechanism, appearance and prevention.
3. Discuss the assumptions and engineering limitations of Archard’s wear relationship.
4. Compare sacrificial-anode cathodic protection, impressed-current cathodic protection and anodic protection.
5. Develop a surface-engineering specification for a steel shaft exposed to abrasive particles, moisture and cyclic loading.

Analytical / Application Questions

1. A steel pipe has measured thickness 10.4 mm, an allowable minimum thickness of 7.0 mm, and a measured uniform loss rate of 0.24 mm·year⁻¹. Calculate the simple remaining-life estimate and identify at least three reasons why the estimate should be reviewed before use.
2. A coated steel assembly joins a small exposed steel fastener to a large stainless-steel plate in seawater. Analyze the likely galvanic risk and propose at least four design or maintenance controls.
3. A bearing shows pitting, lubricant discoloration and increased vibration. Construct a diagnostic plan that distinguishes surface fatigue, abrasive contamination, inadequate lubrication and tribocorrosion, and recommend inspection data for life prediction.

Multiple Choice Questions with Answers

Question 1

Which reaction is anodic during corrosion of iron?

A. Fe → Fe^{2+} + 2e^−

B. O₂ + 2H₂O + 4e^− → 4OH^−

C. 4OH^− → O₂ + 2H₂O + 4e^−

D. Fe^{2+} + 2e^− → Fe

Correct Answer: A. Fe → Fe^{2+} + 2e^−

Question 2

At the freely corroding condition, which statement is correct?

A. Anodic current is zero.

B. Cathodic current is zero.

C. Total anodic and cathodic currents are equal in magnitude.

D. The electrode must be at its standard potential.

Correct Answer: C. Total anodic and cathodic currents are equal in magnitude.

Question 3

Which situation most strongly promotes galvanic corrosion of the anodic metal?

A. Large anodic area coupled to a small cathodic area

B. Small anodic area coupled to a large cathodic area

C. Two electrically isolated metals in dry air

D. Two identical metals in a nonconductive coating

Correct Answer: B. Small anodic area coupled to a large cathodic area

Question 4

A corrosion current density is 10 μA·cm⁻², equivalent weight is 27.9 g·equivalent⁻¹, density is 7.8 g·cm⁻³, and K = 0.00327 for mm·year⁻¹ units. What is the approximate corrosion rate?

A. 0.00117 mm·year⁻¹

B. 0.0117 mm·year⁻¹

C. 0.117 mm·year⁻¹

D. 1.17 mm·year⁻¹

Correct Answer: C. 0.117 mm·year⁻¹

Question 5

Which condition is most characteristic of pitting corrosion?

A. Uniform removal over the entire surface

B. Localized penetration after passive-film breakdown

C. Wear caused only by loose abrasive particles

D. Crack growth requiring no environmental influence

Correct Answer: B. Localized penetration after passive-film breakdown

Question 6

Under the assumptions of Archard’s model, wear volume increases directly with which pair of variables?

A. Hardness and viscosity

B. Normal load and sliding distance

C. Density and coating thickness

D. Temperature and yield strength

Correct Answer: B. Normal load and sliding distance

Question 7

Which lubrication regime carries load mainly through a continuous fluid film generated by relative motion?

A. Boundary lubrication

B. Mixed lubrication

C. Hydrodynamic lubrication

D. Dry abrasive lubrication

Correct Answer: C. Hydrodynamic lubrication

Question 8

What is the primary function of cathodic protection?

A. Force the protected structure to act as an anode

B. Force the protected structure to act as a cathode

C. Increase anodic dissolution uniformly

D. Remove all residual stresses from the structure

Correct Answer: B. Force the protected structure to act as a cathode

Question 9

A component has thickness 8.2 mm, minimum allowable thickness 6.0 mm, and uniform loss rate 0.18 mm·year⁻¹. What is the simple remaining-life estimate?

A. 2.2 years

B. 8.2 years

C. 12.2 years

D. 45.6 years

Correct Answer: C. 12.2 years

Question 10

Which surface treatment primarily introduces beneficial compressive residual stress to improve fatigue resistance?

A. Shot peening

B. Acid pickling only

C. Solvent cleaning

D. Unsealed barrier painting

Correct Answer: A. Shot peening