Corrosion, Oxidation, Wear and Materials Protection

Introduction and Context

Engineering materials rarely fail because they are intrinsically weak alone. They often fail because their surfaces interact with the environment, with other surfaces, or with electrical and chemical systems. Corrosion, oxidation, wear and erosion progressively alter dimensions, surface integrity, strength, reliability and appearance. In machine technology, these processes affect shafts, gears, bearings, cutting tools, pumps, boilers, pipelines, dies, fasteners and structural components.

Corrosion is the deterioration of a material, usually a metal, through chemical or electrochemical interaction with its environment. Oxidation is a chemical reaction involving loss of electrons; at high temperature it commonly produces oxide scales. Wear is the progressive loss or displacement of material from a surface because of relative motion, mechanical interaction or impact. These processes may occur separately, but they frequently interact. For example, a steel gear may experience adhesive wear, lubricant breakdown, oxidation and corrosion simultaneously.

Effective protection therefore requires more than selecting a strong material. It requires an integrated strategy involving environment characterization, material selection, design, surface engineering, lubrication, inhibitors, electrical protection, inspection and maintenance.

Learning Objectives

After studying this chapter, the student should be able to:

1. Explain the electrochemical basis of corrosion and distinguish anodic and cathodic reactions.
2. Interpret galvanic cells, polarization, passivation and corrosion-rate measurements.
3. Describe atmospheric, galvanic, pitting, crevice, erosion-corrosion, stress-corrosion and high-temperature oxidation mechanisms.
4. Classify wear mechanisms and relate them to load, speed, temperature, lubrication and surface condition.
5. Select suitable materials, coatings, inhibitors, lubricants and cathodic-protection methods.
6. Analyze a practical degradation problem and recommend a defensible protection system.
7. Recognize the limitations, environmental responsibilities and monitoring requirements of protection methods.

1. Fundamental Concepts and Terminology

The surface of a component is the boundary between the material and its environment. It has a different structure from the bulk because of machining marks, residual stress, inclusions, dislocations, oxides, adsorbed gases and contaminants. Degradation generally begins at this boundary.

Important terms include:

• Anode: Region where oxidation occurs and metal atoms lose electrons.
• Cathode: Region where reduction consumes electrons.
• Electrolyte: Ionically conducting liquid, moist film or molten salt.
• Electrode potential: Electrical potential associated with an electrode reaction.
• Polarization: Change in electrode potential caused by current flow.
• Passivation: Formation of a thin, adherent and protective surface film.
• Galvanic couple: Electrically connected dissimilar materials exposed to a common electrolyte.
• Corrosion current density: Corrosion current divided by exposed anodic area.
• Tribology: Science and engineering of interacting surfaces in relative motion, including friction, wear and lubrication.
• Friction: Resistance to relative motion between contacting surfaces.
• Lubricant: Material introduced to reduce friction, wear or heat transfer between surfaces.
• Coating: Deliberately applied surface layer that provides protection or a functional property.

The severity of degradation depends on material composition and microstructure, environment, temperature, stress, surface finish, geometry, motion, electrical condition and maintenance. A design that ignores any of these factors may remain vulnerable despite using an expensive alloy.

2. Electrochemical Corrosion

2.1 The corrosion cell

Most aqueous corrosion is electrochemical. Four elements are required:

1. An anodic region where metal dissolves.
2. A cathodic region where a reduction reaction occurs.
3. An electronic path through the metal.
4. An ionic path through an electrolyte.

At the anode, a metal atom enters solution as an ion and releases electrons. For iron:

At a neutral or alkaline aerated surface, oxygen reduction commonly occurs at the cathode:

In acidic solution, hydrogen evolution may dominate:

The electrons travel through the metal from anode to cathode, while ions move through the electrolyte to maintain charge balance. Corrosion products may form when dissolved metal ions react with hydroxide, oxygen, carbonate or other species.

For iron, an intermediate reaction is:

Further oxidation and dehydration produce mixtures commonly called rust, containing hydrated iron oxides and oxyhydroxides. Rust is generally porous and nonprotective, unlike the compact oxide films formed by aluminium, chromium and titanium under suitable conditions.

2.2 Thermodynamics and kinetics

Thermodynamics indicates whether a reaction is energetically possible; kinetics determines how rapidly it proceeds. A negative Gibbs free-energy change indicates that a reaction is thermodynamically favorable, but it does not establish a practical corrosion rate. A metal may be thermodynamically unstable in water yet corrode very slowly because of passivation or a large activation barrier.

The electrode potential of a reduction reaction is described, under idealized conditions, by the Nernst equation:

Here, E is the electrode potential in volts, E° is the standard electrode potential, R is the gas constant, T is absolute temperature in kelvin, n is the number of electrons transferred, F is Faraday’s constant, and Q is the reaction quotient. Activities, rather than concentrations, are strictly required; concentrations are approximations for dilute solutions. The equation applies to a defined reversible half-cell reaction at equilibrium or near-equilibrium conditions.

Corrosion is a mixed-potential process. The metal’s anodic dissolution current equals the total cathodic reduction current at the corrosion potential, although the anodic and cathodic partial currents flow in opposite directions. The corrosion potential is therefore not necessarily the equilibrium potential of either half-reaction.

The corrosion rate can be related to corrosion current using Faraday’s law. If the metal dissolves as ions of charge number n, the mass loss is:

where m is mass dissolved in grams, I is current in amperes, t is time in seconds, M is molar mass in grams per mole, n is electrons per atom, and F ≈ 96,485 C/mol. For a corrosion current density i_corr and area A, use I = i_corr A. This relationship assumes that the measured current represents metal dissolution and that side reactions or nonuniform current distribution have been properly considered.

2.3 Polarization and corrosion-rate measurement

Polarization describes the change in electrode potential from its equilibrium value when current flows. Activation polarization results from the energy barrier of an electrode reaction. Concentration polarization results from mass-transfer limitations. Ohmic polarization results from resistance in the electrolyte or electrical connections.

The Tafel form is often used over a limited current range:

where η is overpotential, i is current density, and a and b are experimentally determined constants for a particular reaction and condition. Tafel extrapolation can estimate corrosion current, but it is unreliable if the selected linear region is too narrow, if several reactions overlap, or if the surface changes during testing.

Linear polarization resistance provides a rapid estimate near the corrosion potential. The polarization resistance is:

where ΔE is a small potential change and ΔI is the corresponding current change. The Stern–Geary relationship is commonly written:

where i_corr is corrosion current density and B depends on the anodic and cathodic Tafel slopes. This method assumes a small perturbation, approximately uniform corrosion and stable electrochemical behavior during measurement.

3. Forms of Corrosion

3.1 Uniform corrosion

Uniform corrosion removes material relatively evenly over a surface. It is common in carbon steel exposed to acids or humid atmospheres. Although visually obvious, it is often easier to predict than localized corrosion. Thickness measurements, mass-loss coupons and corrosion-rate calculations can support maintenance planning.

3.2 Galvanic corrosion

When dissimilar metals are electrically connected 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 especially dangerous because the anodic current density may be high.

Control methods include electrical insulation, compatible material pairing, protective coatings applied carefully to avoid exposing a small anode, separation from stagnant moisture, and sacrificial-anode design. Galvanic-series data must be used for the actual environment because material rankings can change with temperature, oxygen content and chemistry.

3.3 Pitting and crevice corrosion

Pitting is highly localized attack that produces cavities. It can initiate at inclusions, scratches, deposits or defects in passive films. Chloride ions are particularly damaging to many passive alloys. Once a pit forms, hydrolysis and concentration changes can make its interior more acidic and aggressive, supporting autocatalytic growth.

Crevice corrosion occurs in shielded gaps such as under gaskets, washers, lap joints and deposits. Oxygen depletion inside the crevice creates a differential-aeration cell. The crevice becomes anodic, while the external surface becomes cathodic. Good design avoids narrow stagnant gaps, uses continuous welds where appropriate, permits drainage and selects materials resistant to the service chemistry.

3.4 Intergranular corrosion

Intergranular corrosion follows grain boundaries. In austenitic stainless steels, improper exposure to an intermediate temperature range can precipitate chromium carbides at grain boundaries, creating chromium-depleted regions that lose passivity. Low-carbon grades, stabilized grades and appropriate heat treatments reduce this risk.

3.5 Erosion-corrosion and cavitation

Erosion-corrosion results from the combined action of corrosive fluid and mechanical removal of protective films. High velocity, turbulence, suspended solids and abrupt changes in flow direction increase risk. Cavitation occurs when vapor bubbles form and collapse near a surface, producing shock waves and repeated localized impacts. Prevention involves controlling velocity, improving hydraulic design, removing sharp flow disturbances, selecting resistant materials and maintaining protective films.

3.6 Corrosion fatigue and stress-corrosion cracking

Corrosion fatigue occurs when cyclic mechanical stress and a corrosive environment reduce fatigue life. Stress-corrosion cracking requires a susceptible material, a specific environment and tensile stress, which may be applied or residual. Cracks can propagate with little general metal loss, making inspection essential. Stress relief, surface improvement, environmental control, material substitution and reduction of stress concentration are possible countermeasures.

4. High-Temperature Oxidation and Degradation

At high temperatures, metals react with oxygen, sulfur, carbon, nitrogen and combustion products. Oxidation often produces an oxide scale. A useful simplified reaction is:

The protective value of a scale depends on adhesion, continuity, melting point, thermal expansion compatibility, ionic and electronic conductivity, and resistance to cracking or volatilization. A dense, adherent scale can slow further reaction. A porous, cracked or spalling scale provides little protection.

For many compact oxide scales, the growth rate is approximately parabolic:

where x is scale thickness, t is time and k_p is the parabolic rate constant at a specified temperature and gas composition. This relation is an approximation for diffusion-controlled growth of a relatively stable scale. It should not be applied when scale spallation, gas-phase transport, linear reaction control or repeated thermal cycling dominates.

Oxidation rates usually increase strongly with temperature. Alloying elements such as chromium, aluminium and silicon can form protective oxides. In nickel-based high-temperature alloys, chromium oxide or aluminium oxide may provide protection, while excessive thermal cycling can cause scale cracking and loss.

Other high-temperature mechanisms include carburization, decarburization, sulfidation, nitridation, hot corrosion and thermal fatigue. Furnace atmospheres must therefore be specified by oxygen potential, sulfur content, carbon activity, moisture and contaminants, not merely by nominal temperature.

5. Tribology and Wear Mechanisms

Tribology examines interacting surfaces in relative motion. The principal variables are normal load, contact geometry, speed, temperature, surface roughness, hardness, lubrication regime, debris and environment.

The coefficient of friction is defined as:

where μ is the coefficient of friction, F_t is tangential friction force and F_n is normal load. This relation is a useful engineering ratio, not a universal material constant. It varies with speed, pressure, temperature, surface condition and lubricant.

Wear volume is sometimes correlated with the Archard relationship:

where V is wear volume, K is a dimensionless wear coefficient, W is normal load, L is sliding distance and H is the hardness of the softer wearing body. The equation is empirical and is most applicable to adhesive or mild abrasive wear under comparable conditions. K is not universal; it must be obtained or selected for the specific materials and environment.

5.1 Adhesive wear

Adhesive wear occurs when asperities form junctions and material transfers between surfaces or tears away. It is promoted by high load, poor lubrication, similar materials, high temperature and inadequate surface films. Galling is a severe form, common in stainless-steel threads and sliding components. Dissimilar material pairs, hard coatings, polished surfaces and suitable lubricants reduce risk.

5.2 Abrasive wear

Abrasive wear occurs when hard asperities or loose particles cut, plough or gouge a softer surface. Two-body abrasion occurs when a hard rough surface slides against another; three-body abrasion occurs when loose particles are trapped between surfaces. Increased hardness, clean lubrication, filtration, surface hardening and wear-resistant coatings are common controls.

5.3 Fatigue wear

Repeated rolling or sliding contact can initiate subsurface cracks. Pitting and spalling in bearings and gears are typical examples. The mechanism is influenced by contact stress, material cleanliness, hardness gradient, lubrication film thickness and alignment. Proper heat treatment, accurate geometry, clean lubricant and avoidance of overload are important.

5.4 Fretting wear

Fretting results from small-amplitude oscillatory motion between contacting surfaces, often under vibration. It may produce oxidized debris, surface notches and fatigue cracks. Clamping changes, improved fit, damping, compatible coatings and anti-fretting compounds can help.

5.5 Erosive and impact wear

Erosive wear is caused by particles or droplets striking a surface. Its severity depends on velocity, particle hardness, angle of impact, particle size and surface toughness. Ductile materials may resist normal impact but wear rapidly at shallow angles; brittle materials may behave differently. Flow redirection, particle removal and replaceable liners are practical solutions.

5.6 Lubrication regimes

In boundary lubrication, surfaces are separated mainly by adsorbed or chemically formed films, so asperity contact remains significant. In mixed lubrication, both asperity contact and fluid-film support occur. In hydrodynamic lubrication, a full fluid film supports the load. Elastohydrodynamic lubrication occurs where elastic deformation and pressure-viscosity effects are important, especially in rolling contacts.

Lubricant selection must consider viscosity, viscosity index, temperature, load, speed, oxidation stability, water contamination, compatibility with seals and environmental requirements. Excess lubricant can be as harmful as insufficient lubricant because it may increase churning losses and temperature.

6. Materials and Surface Protection Strategies

6.1 Design and material selection

Protection begins at the design stage. Designers should avoid water traps, sharp crevices, incompatible metal couples, inaccessible surfaces and unnecessary stress concentrations. Drainage, ventilation, inspection access and replaceable wear components should be incorporated.

Material selection should consider corrosion potential, passive-film stability, strength, toughness, hardness, temperature capability, weldability, cost, availability and environmental compatibility. Examples include:

No material is universally corrosion-proof. Stainless steel can pit in chlorides; aluminium can suffer galvanic attack; ceramics may be brittle; polymers may swell or soften; coatings may fail at defects.

6.2 Metallic coatings

Zinc coatings protect steel by barrier action and sacrificial action. Even if a scratch exposes steel, nearby zinc can act anodically in many environments. Nickel and chromium coatings provide barrier, hardness and appearance, but pores or cracks can create localized galvanic conditions if the substrate is less noble.

Thermal spraying, hot dipping, electroplating, cladding and weld overlays are selected according to substrate, geometry, temperature, required thickness and repairability. Surface preparation is critical; oil, rust, salts and weak oxide layers reduce adhesion.

6.3 Organic and inorganic coatings

Paint systems commonly include a surface-preparation stage, primer, intermediate barrier coat and topcoat. Primers may contain corrosion-inhibiting pigments or promote adhesion. Barrier coatings reduce water and ion transport. Topcoats provide ultraviolet, chemical and weather resistance.

Inorganic coatings include conversion coatings, enamels, ceramics and anodized layers. Anodizing thickens the protective oxide on aluminium; sealing improves resistance by reducing pore activity. Coating selection must account for edge coverage, mechanical damage, permeability, thermal expansion and maintenance access.

6.4 Inhibitors

Corrosion inhibitors are chemicals added in small concentrations to reduce anodic dissolution, cathodic reaction, or both. Anodic inhibitors promote passivation but may be dangerous if added below a critical concentration because localized attack can intensify. Cathodic inhibitors reduce oxygen reduction or hydrogen evolution. Adsorption inhibitors form films on the metal surface.

Inhibitor performance depends on concentration, temperature, flow, contamination, metallurgy and residence time. A laboratory test in clean solution cannot guarantee field performance. Environmental toxicity, discharge limits, worker exposure and biodegradability must be considered.

6.5 Cathodic protection

Cathodic protection forces the protected structure to behave as a cathode. In sacrificial-anode systems, a more active material such as zinc, aluminium alloy or magnesium supplies current while it dissolves. In impressed-current systems, a controlled DC power source supplies current through inert or slowly consumed anodes.

A simplified design estimate for sacrificial-anode mass is:

where m is required anode mass, I is protective current in amperes, t is design time in hours or seconds consistent with Q, u is utilization factor, and Q is practical electrochemical capacity in ampere-hours per unit mass or coulombs per unit mass. The value of Q must be obtained from the selected anode material and service conditions. Utilization accounts for unusable residual anode, current-distribution limitations and design practice.

Cathodic protection is widely used for buried pipelines, storage tanks, marine structures and water heaters. Excessive protection can cause hydrogen generation, coating disbondment or hydrogen embrittlement in susceptible high-strength materials. Systems require reference electrodes, electrical continuity, current control and periodic surveys.

7. Monitoring, Inspection and Failure Analysis

A protection system is incomplete without verification. Common methods include visual inspection, mass-loss coupons, electrical resistance probes, ultrasonic thickness measurement, linear polarization resistance, electrochemical impedance spectroscopy, coating-thickness measurement, hardness testing, microscopy and lubricant analysis.

Failure analysis should proceed systematically:

1. Define the component function and service history.
2. Record environment, temperature, loads, motion, chemicals and maintenance.
3. Identify damage morphology: pits, grooves, transfer layers, cracks, scale or spalls.
4. Obtain representative samples without destroying evidence.
5. Examine macrostructure, microstructure, deposits and fracture surfaces.
6. Test material composition, hardness, coating condition and lubricant.
7. Reconstruct the degradation mechanism.
8. Recommend controls and validate them through testing or monitoring.

Artificially accelerated tests are useful for comparison but may not reproduce field mechanisms. Salt-spray testing, for example, is not a direct prediction of service life for every coating system. Field exposure and mechanistic understanding remain essential.

8. Illustrative Case Study: Corrosion and Wear in a Cooling-Water Pump

Background

A carbon-steel pump handling aerated cooling water develops leakage near the shaft sleeve after eight months. Maintenance reports a roughened sleeve surface, reddish deposits, and increased vibration. The pump is connected to a stainless-steel piping section, and water contains chlorides and suspended solids.

Problem

The equipment experiences both material loss and performance deterioration. Replacing the sleeve alone may not solve the problem because the degradation mechanisms may be coupled.

Analysis

Inspection identifies shallow pits near the junction with the stainless-steel component, circumferential grooves at the seal contact, and erosion marks near a flow disturbance. The likely mechanisms are:

• galvanic corrosion between carbon steel and stainless steel;
• pitting assisted by chloride ions and deposits;
• abrasive or erosion-corrosion wear from suspended solids;
• fretting or seal-related wear caused by vibration and misalignment.

The small exposed carbon-steel sleeve adjacent to a relatively large stainless-steel cathodic area creates an unfavorable area ratio. Deposits retain moisture and particles. Vibration damages protective films and accelerates seal wear.

Approach and solution

The redesign uses a corrosion-resistant, wear-compatible sleeve material selected for the actual chloride concentration. An electrically insulating joint is installed where appropriate, while ensuring that the pump remains safely grounded. The flow path is modified to reduce turbulence, and a strainer limits particle entry. Shaft alignment and seal compression are corrected. A compatible lubricant is selected for the seal and bearing system. A coating is considered only after confirming adhesion, edge coverage and resistance to the sliding contact.

A monitoring plan includes monthly vibration measurements, periodic shaft-sleeve inspection, water-chemistry records, corrosion coupons and ultrasonic checks of vulnerable sections. If galvanic coupling is retained, the electrical potential and current distribution are assessed rather than assumed.

Outcome

The combined intervention reduces vibration, eliminates the unfavorable small-anode/large-cathode arrangement, lowers particle loading and improves inspection. The principal lesson is that material substitution alone would not address the hydraulic, electrical and mechanical causes.

Learning points

1. Corrosion and wear may accelerate one another.
2. Geometry and area ratio can be as important as nominal material grade.
3. Coatings cannot compensate for poor surface preparation or unsuitable contact mechanics.
4. Protection should be validated by condition monitoring and not judged only by initial appearance.

9. Contemporary Developments and Professional Responsibilities

Modern protection increasingly combines materials engineering with sensors, digital maintenance and life-cycle analysis. Embedded corrosion sensors, wireless vibration monitoring, machine-vision inspection and data-driven remaining-life estimates can identify changing conditions earlier. Artificial intelligence can assist with image classification, anomaly detection and maintenance prioritization, but its predictions depend on representative data and expert validation. It should support, not replace, engineering judgment and safety procedures.

Environmentally responsible practice favors lower-toxicity inhibitors, reduced volatile-organic-compound coatings, recyclable materials, efficient lubrication, controlled waste treatment and designs that permit repair rather than premature replacement. Regulations may govern hazardous pigments, chromates, heavy metals, chemical discharge, confined-space work and cathodic-protection systems. Engineers must consult applicable national standards, company procedures and environmental legislation.

A technically successful treatment may still be unacceptable if it creates toxic waste, exposes workers to hazardous chemicals, causes hydrogen embrittlement or transfers pollution to another part of the life cycle. Sustainable protection therefore considers performance, cost, safety, carbon impact, maintainability and end-of-life disposal.

Best-Practice Framework

A robust degradation-control program follows this sequence:

1. Characterize the environment and operating cycle.
2. Identify likely corrosion, oxidation and wear mechanisms.
3. Select materials and geometry together.
4. Specify surface preparation, coating, lubricant or inhibitor controls.
5. Design electrical isolation or cathodic protection where required.
6. Establish measurable acceptance criteria.
7. Monitor degradation using suitable inspection methods.
8. Review failures and update the design or maintenance plan.

The most economical solution is often prevention at the design stage, because inaccessible corrosion, unexpected downtime and secondary damage are expensive to correct.

Chapter Summary

Corrosion is an electrochemical or chemical degradation process, while oxidation is especially important in high-temperature environments. Aqueous corrosion requires anodic dissolution, cathodic reduction, electronic conduction and ionic conduction. Thermodynamics indicates reaction feasibility, whereas kinetics, polarization and passivation control practical rate. Major corrosion forms include uniform, galvanic, pitting, crevice, intergranular, erosion-corrosion, corrosion fatigue and stress-corrosion cracking.

High-temperature protection depends on forming stable, adherent and slow-growing scales. Wear is a tribological process involving interacting surfaces and may be adhesive, abrasive, fatigue, fretting, erosive or impact-based. Lubrication regime, hardness, load, speed, temperature and contamination strongly influence wear.

Protection is integrated rather than singular. Material selection, sound geometry, coatings, inhibitors, lubrication, cathodic protection and inspection must be matched to the mechanism and service environment. Monitoring and failure analysis are essential because laboratory tests and initial specifications cannot guarantee field performance. Sustainable engineering also considers worker safety, environmental impact, regulatory requirements and life-cycle cost.

Key Terms

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

Oxidation: Loss of electrons or reaction with oxygen or another oxidizing species.

Anode: Region at which oxidation and, commonly, metal dissolution occur.

Cathode: Region at which reduction occurs.

Electrolyte: Medium that conducts ions.

Passivation: Formation of a protective surface film that reduces reaction rate.

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

Pitting: Localized corrosion producing cavities or holes.

Crevice corrosion: Localized corrosion in shielded, stagnant gaps.

Stress-corrosion cracking: Cracking caused by tensile stress and a specific corrosive environment.

Oxide scale: Surface oxide layer formed during oxidation, especially at elevated temperature.

Tribology: Study of friction, wear and lubrication of interacting surfaces.

Adhesive wear: Wear caused by junction formation and material transfer between surfaces.

Abrasive wear: Wear caused by hard asperities or particles cutting or ploughing a surface.

Fretting: Damage caused by small-amplitude oscillatory motion.

Passivation: Surface-film condition that greatly reduces anodic dissolution.

Inhibitor: Chemical added to reduce corrosion reactions.

Cathodic protection: Method that makes the protected structure function as a cathode.

Polarization: Change in electrode potential resulting from current flow.

Wear coefficient: Empirical parameter describing wear severity under specified conditions.

Review Questions

Short Answer Questions

1. What four elements are required for an aqueous electrochemical corrosion cell?
2. Distinguish an anodic reaction from a cathodic reaction.
3. Why can a small anodic area connected to a large cathodic area be dangerous?
4. Define passivation and give one engineering example.
5. Name two differences between adhesive wear and abrasive wear.

Long Answer Questions

1. Explain the electrochemical mechanism of iron corrosion in aerated neutral water, including the anodic and cathodic reactions.
2. Discuss the causes, characteristics and prevention of pitting and crevice corrosion.
3. Explain the formation and protective significance of high-temperature oxide scales.
4. Describe the principal lubrication regimes and the factors governing lubricant selection.
5. Compare sacrificial-anode and impressed-current cathodic protection, including advantages and limitations.

Analytical / Application Questions

1. A carbon-steel fastener is installed in a large aluminium component exposed to rainwater. Analyze the likely galvanic behavior and propose at least four design or maintenance controls.
2. A bearing shows surface spalling, oxidized debris and increased temperature after lubricant contamination by abrasive particles. Identify the interacting wear mechanisms and recommend a diagnostic and corrective plan.
3. A coated steel tank develops corrosion only beneath damaged coating areas and around a gasketed joint. Explain the electrochemical reasons for the localized attack and design an improved protection and inspection strategy.

Multiple Choice Questions with Answers

Question 1

Which reaction represents anodic dissolution of iron?

A. Fe → Fe^{2+} + 2e^−

B. O_2 + 2H_2O + 4e^− → 4OH^−

C. 2H^+ + 2e^− → H_2

D. Fe^{2+} + 2OH^− → Fe(OH)_2

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

Question 2

Which condition generally produces the highest galvanic-corrosion risk?

A. Large anode connected to a small cathode

B. Small anode connected to a large cathode

C. Two electrically isolated metals

D. A dry, nonconductive environment

Correct Answer: B. Small anode connected to a large cathode

Question 3

The Nernst equation primarily relates electrode potential to which factor?

A. Reaction quotient and temperature

B. Surface hardness and sliding distance

C. Coating thickness and roughness

D. Bearing clearance and viscosity only

Correct Answer: A. Reaction quotient and temperature

Question 4

Which wear mechanism involves hard particles cutting or ploughing a surface?

A. Adhesive wear

B. Abrasive wear

C. Fatigue wear

D. Fretting corrosion only

Correct Answer: B. Abrasive wear

Question 5

In hydrodynamic lubrication, the load is mainly supported by:

A. Direct metal-to-metal asperity contact

B. A full fluid film

C. Loose abrasive particles

D. A sacrificial metal coating

Correct Answer: B. A full fluid film

Question 6

Cathodic protection operates by making the protected structure function as the:

A. Anode

B. Electrolyte

C. Cathode

D. Insulator

Correct Answer: C. Cathode

Question 7

Which corrosion form is especially associated with narrow stagnant gaps under gaskets?

A. Crevice corrosion

B. Uniform corrosion

C. High-temperature oxidation

D. Erosive wear

Correct Answer: A. Crevice corrosion

Question 8

The simplified Archard relationship predicts wear volume as proportional to load and sliding distance and inversely proportional to:

A. Temperature

B. Hardness

C. Electrical resistance

D. Electrode potential

Correct Answer: B. Hardness

Question 9

A compact, adherent and continuous oxide scale generally:

A. Accelerates diffusion of oxygen indefinitely

B. Reduces further oxidation rate

C. Eliminates all thermal stresses

D. Guarantees resistance to chloride pitting

Correct Answer: B. Reduces further oxidation rate

Question 10

Which practice best supports reliable degradation control?

A. Selecting material without considering the environment

B. Applying a coating without surface preparation

C. Combining design, protection and condition monitoring

D. Using the hardest material in every application

Correct Answer: C. Combining design, protection and condition monitoring