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AMPP Corrosion Specialist practice questions

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Question 1Corrosion Theory and Degradation Mechanisms

In an electrochemical corrosion cell, the electrode where metal dissolution occurs is called the:

  1. Anode Correct answer.
  2. Cathode
  3. Salt bridge
  4. Reference electrode

Explanation

Oxidation of metal to metal ions releases electrons at the anode, so this is the electrode that loses metal. The cathode consumes those electrons in a reduction reaction and is not where the metal dissolves.

Question 2Corrosion Theory and Degradation Mechanisms

Which of these metals is the most active according to the standard electromotive force series?

  1. Magnesium Correct answer.
  2. Zinc
  3. Copper
  4. Iron

Explanation

The standard EMF series ranks metals by standard electrode potential, and magnesium has a much more negative potential than iron, zinc, or copper. A more negative standard potential indicates a greater thermodynamic tendency to oxidize.

Question 3Corrosion Theory and Degradation Mechanisms

The corrosion resistance of stainless steel in many environments depends mainly on:

  1. A loose scale of iron oxide
  2. A thin chromium-rich oxide film Correct answer.
  3. A galvanic coating of nickel
  4. An adsorbed layer of chloride ions

Explanation

Stainless steel passivates because chromium forms a very thin, adherent, self-repairing oxide film. This film slows both anodic dissolution and cathodic reactions, giving corrosion resistance in many environments.

Question 4Corrosion Theory and Degradation Mechanisms

Faraday's law is used in corrosion testing to relate mass loss to:

  1. Solution pH
  2. Metal density alone
  3. Oxygen solubility
  4. Charge passed Correct answer.

Explanation

Faraday's law states that the amount of metal oxidized is proportional to the electric charge passed and to the electrochemical equivalent of the metal. Corrosion current integrated over time therefore gives a predictable mass loss.

Question 5Corrosion Theory and Degradation Mechanisms

A zinc electrode is immersed in 0.01 M Zn2+ at 25 C. Using E = E0 + (0.0592/2) log[Zn2+] with E0 = -0.763 V, what is the electrode potential?

  1. -0.822 V Correct answer.
  2. -0.704 V
  3. -0.763 V
  4. -0.881 V

Explanation

For a divalent ion, the Nernst slope is 0.0592/2 = 0.0296 V per decade. With log(0.01) = -2, the correction is -0.0592 V, giving -0.763 - 0.0592 = -0.822 V.

Question 6Corrosion Theory and Degradation Mechanisms

On a Pourbaix diagram for iron, the immunity region indicates:

  1. Growth of a protective oxide film
  2. Formation of soluble ferrous ions in acid
  3. The metal is thermodynamically stable Correct answer.
  4. Active pitting in chloride-containing solutions

Explanation

The immunity region represents conditions where metallic iron is thermodynamically stable relative to its dissolved ions. Corrosion and passivation regions involve soluble species or stable oxide films, respectively.

Question 7Corrosion Theory and Degradation Mechanisms

In a differential aeration cell on steel, the anodic area is:

  1. The area of highest solution pH
  2. The area of highest oxygen concentration
  3. The oxygen-depleted region Correct answer.
  4. The area with the thickest rust layer

Explanation

The oxygen-rich area supports the cathodic reduction reaction, while the oxygen-depleted area becomes anodic and dissolves. This is why steel under deposits or in crevices often corrodes preferentially.

Question 8Corrosion Theory and Degradation Mechanisms

Which metal, coupled to steel in seawater, will protect the steel by making it the cathode?

  1. Copper
  2. Zinc Correct answer.
  3. Graphite
  4. Titanium

Explanation

Zinc is anodic to steel in seawater, so zinc dissolves preferentially and the steel becomes cathodic. The more noble metals listed would instead make steel the anode and accelerate its corrosion.

Question 9Corrosion Theory and Degradation Mechanisms

Pitting corrosion of stainless steel in chloride service begins with:

  1. Localized breakdown of the passive film Correct answer.
  2. Uniform thinning of the passive film
  3. Grain-boundary chromium carbide precipitation
  4. Mechanical erosion by suspended solids

Explanation

Chloride ions can break down the passive film at discrete sites, creating small anodic pits while the surrounding film remains cathodic. The resulting small-anode, large-cathode geometry concentrates dissolution inside the pit.

Question 10Corrosion Theory and Degradation Mechanisms

Crevice corrosion of stainless steel is promoted by:

  1. Oxygen depletion plus chloride build-up Correct answer.
  2. Continuous flow through the crevice
  3. High oxygen concentration inside the crevice
  4. Uniform pH throughout the crevice

Explanation

Oxygen inside a tight crevice is consumed and not readily replenished, making the crevice anodic. Chloride migrates in to balance charge, and hydrolysis lowers pH, creating an aggressive local environment that sustains attack.

Question 11Corrosion Theory and Degradation Mechanisms

A small steel rivet is coupled to a large copper plate in seawater. The likely result is:

  1. Severe galvanic attack at the rivet Correct answer.
  2. Cathodic protection of the rivet
  3. Negligible because steel is anodic
  4. Uniform corrosion over the copper plate

Explanation

Steel is anodic to copper, and a small anode coupled to a large cathode concentrates the galvanic current on a small area. The resulting high anodic current density causes rapid attack at the rivet.

Question 12Corrosion Theory and Degradation Mechanisms

Erosion-corrosion differs from pure mechanical erosion because:

  1. Corrosion and wear reinforce each other Correct answer.
  2. The metal loss equals mechanical wear alone
  3. It occurs only in dry gases
  4. It requires no fluid motion

Explanation

In erosion-corrosion, flowing fluid removes protective films or particles while electrochemical corrosion continues on the freshly exposed metal. The combined loss is greater than the sum of pure erosion and pure corrosion acting separately.

Question 13Corrosion Theory and Degradation Mechanisms

Cavitation damage to a pump impeller is caused by:

  1. Intergranular chromium depletion
  2. Crevices under gaskets
  3. Abrasion by coarse suspended sand particles
  4. Vapor bubble collapse at the surface Correct answer.

Explanation

Cavitation occurs when vapor bubbles form and then collapse against a metal surface, producing intense local pressure pulses. These pulses damage protective films and fatigue the metal, leading to pitting and material loss.

Question 14Corrosion Theory and Degradation Mechanisms

Dezincification of brass is best described as:

  1. Selective removal of copper from the alloy
  2. Hydrogen absorption into the metal lattice
  3. Selective removal of zinc Correct answer.
  4. Preferential attack at grain boundaries

Explanation

Dezincification selectively removes zinc from brass, leaving a porous copper-rich residue that is weak and often discolored. It is a form of selective leaching rather than uniform corrosion.

Question 15Corrosion Theory and Degradation Mechanisms

Graphitic corrosion of gray cast iron leaves:

  1. Formation of martensite at the surface
  2. Stress-assisted cracking along graphite flakes
  3. Iron leached out, graphite network remains Correct answer.
  4. Selective leaching of graphite from the iron

Explanation

In graphitic corrosion, the iron matrix is selectively leached away while the graphite flakes remain as a weak, porous network. The component may keep its original shape but loses strength and becomes brittle.

Question 16Corrosion Theory and Degradation Mechanisms

Sensitization of austenitic stainless steel is caused by:

  1. The alloy is quenched rapidly from solution temperature
  2. Molybdenum segregates to the grain interiors
  3. Grain-boundary chromium carbide precipitation Correct answer.
  4. Nickel-rich clusters form throughout the matrix

Explanation

When austenitic stainless steel is held or cooled slowly through the sensitization range, chromium carbides precipitate at grain boundaries. Chromium is consumed locally, leaving adjacent zones depleted in chromium and susceptible to intergranular corrosion.

Question 17Corrosion Theory and Degradation Mechanisms

Which statement best describes stress corrosion cracking?

  1. Compressive stress above the yield point
  2. Uniform metal loss over the exposed surface
  3. Sustained tensile stress plus a specific environment Correct answer.
  4. Cyclic stress in an inert environment

Explanation

Stress corrosion cracking requires a susceptible alloy, a specific corrosive environment, and sustained tensile stress. Cracks can propagate transgranularly or intergranularly with little general metal loss, leading to sudden failure.

Question 18Corrosion Theory and Degradation Mechanisms

Hydrogen embrittlement of high-strength steel is:

  1. Ductility loss from absorbed atomic hydrogen Correct answer.
  2. Grain-boundary melting at elevated temperature
  3. A surface film that accelerates general corrosion
  4. Anodic dissolution that removes the passive film

Explanation

Atomic hydrogen absorbed into high-strength steel can diffuse to stress concentrations and reduce ductility, leading to brittle cracking under sustained or cyclic load. The damage occurs below the surface and is not simply a corrosion film effect.

Question 19Corrosion Theory and Degradation Mechanisms

Corrosion caused by sulfate-reducing bacteria typically produces:

  1. Uniform removal of mill scale from the steel
  2. High oxygen concentrations at the metal surface
  3. Nitrate-rich passivating films on the metal
  4. Sulfide and locally aggressive conditions Correct answer.

Explanation

Sulfate-reducing bacteria generate sulfide, which can stimulate anodic dissolution and shift the local environment toward aggressive conditions. Their activity is especially associated with oxygen-depleted soils and waters.

Question 20Corrosion Theory and Degradation Mechanisms

A corrosion rate of 1 mil per year (mpy) is approximately equal to:

  1. 0.25 mm/yr
  2. 2.5 mm/yr
  3. 0.025 mm/yr Correct answer.
  4. 0.0025 mm/yr

Explanation

One mil is one thousandth of an inch, or about 0.0254 mm. Therefore 1 mpy is approximately 0.025 mm/yr, which is a useful conversion for comparing corrosion rates in SI units.

Question 21Corrosion Theory and Degradation Mechanisms

A buried steel pipeline crosses from aerobic soil into anaerobic soil. The anode forms in:

  1. The aerobic soil
  2. The dry gravel backfill
  3. The anaerobic soil Correct answer.
  4. The weld metal only

Explanation

The aerobic soil supplies oxygen for cathodic reduction, while the anaerobic soil becomes oxygen-depleted and anodic. The pipeline therefore corrodes preferentially in the anaerobic section.

Question 22Corrosion Theory and Degradation Mechanisms

Atmospheric corrosion of steel rises sharply above a critical relative humidity because:

  1. Oxygen diffusion to the surface stops
  2. Chloride ions evaporate from the surface
  3. The steel surface becomes passive
  4. A continuous electrolyte film forms Correct answer.

Explanation

Above the critical relative humidity, enough moisture condenses on the steel to form a continuous electrolyte film. That film allows ionic current to flow between anodic and cathodic sites, so the corrosion rate increases sharply.

Question 23Corrosion Theory and Degradation Mechanisms

On a mixed-potential diagram, the corrosion potential is where:

  1. Anodic and cathodic currents are equal Correct answer.
  2. The anodic current is zero
  3. The cathodic current is zero
  4. The metal begins to passivate

Explanation

At the corrosion potential, the total anodic oxidation current equals the total cathodic reduction current, so the net external current is zero. This mixed potential sets the corrosion rate on the polarization diagram.

Question 24Corrosion Theory and Degradation Mechanisms

For an activation-controlled corrosion reaction, raising temperature generally:

  1. Decreases the corrosion rate
  2. Has no measurable effect
  3. Stops oxygen reduction completely
  4. Increases the corrosion rate Correct answer.

Explanation

Activation-controlled reactions follow Arrhenius behavior, so raising temperature increases the reaction rate constants. In corrosion, this usually raises both anodic and cathodic kinetics and therefore the corrosion rate, unless passivation or oxygen solubility changes dominate.

Question 25Corrosion Theory and Degradation Mechanisms

Which condition indicates cathodic overprotection of a coated steel pipeline?

  1. A stable passive film with no current flow
  2. Potentials below the hydrogen evolution potential Correct answer.
  3. Potentials at the free-corrosion potential
  4. Potentials slightly positive to the open-circuit potential

Explanation

Overprotection occurs when cathodic polarization is so strong that the steel potential falls below the hydrogen evolution potential. Hydrogen bubbles can then form at the coating interface and cause disbondment or blistering.

Question 26Corrosion Theory and Degradation Mechanisms

Which statement best describes uniform corrosion of a metal?

  1. It initiates at discrete sites and penetrates rapidly with little general metal loss.
  2. It occurs only where a protective oxide film is absent and is self-limiting.
  3. It proceeds at a similar rate over the entire exposed surface, gradually thinning the metal. Correct answer.
  4. It is concentrated at grain boundaries and leaves a network of shallow grooves.

Explanation

Uniform corrosion attacks the whole exposed surface at a comparable rate, so metal loss is spread out and wall thinning is the main consequence. It is distinguished from localized forms such as pitting, crevice attack, and intergranular corrosion. The rate is controlled by the environment and the metal's electrochemical kinetics rather than by a small number of isolated sites.

Question 27Corrosion Theory and Degradation Mechanisms

Which statement about the galvanic series is correct?

  1. It includes only noble metals and deliberately excludes active-passive alloys.
  2. It ranks pure metals by standard electrode potentials in pure water at unit activity.
  3. It is fixed and independent of electrolyte composition, temperature, and dissolved oxygen.
  4. It ranks alloys by measured potentials in a specific environment such as seawater. Correct answer.

Explanation

The galvanic series is empirical: it orders materials by their measured potentials in a specified electrolyte, usually seawater. Because potentials depend on the environment, the ranking can change with electrolyte composition, temperature, and oxygen content.

Question 28Corrosion Theory and Degradation Mechanisms

The reddish-brown rust that forms on steel is primarily composed of:

  1. Mixed iron(II,III) oxide, commonly called magnetite (Fe3O4)
  2. Iron(II) oxide (FeO)
  3. Iron sulfide corrosion product (FeS), typical of sulfate-reducing bacteria
  4. Hydrated iron(III) oxide (FeOOH or Fe2O3·nH2O) Correct answer.

Explanation

Rust is not a single compound; the reddish-brown product is hydrated ferric oxide, often written FeOOH or Fe2O3·nH2O. It forms as iron dissolves and ferrous ions oxidize further in the presence of water and oxygen.

Question 29Corrosion Theory and Degradation Mechanisms

A metal is said to be passivated when:

  1. Its surface is mechanically polished to remove all mill scale.
  2. It forms a thin, adherent surface film that reduces the corrosion rate. Correct answer.
  3. Its open-circuit corrosion potential shifts to a substantially more active value.
  4. It is coupled electrically to a sacrificial anode.

Explanation

Passivation is the formation of a thin, adherent, often oxide-rich film that separates the metal from the environment and lowers the corrosion rate. The film can be air-formed or grown anodically, and its breakdown leads to localized attack.

Question 30Corrosion Theory and Degradation Mechanisms

In a corrosion cell, the electrode at which oxidation occurs is the:

  1. anode, where metal dissolution releases electrons Correct answer.
  2. reference electrode, which supplies current to the cell
  3. electrolyte, which carries electrons between electrodes
  4. cathode, where reduction consumes electrons

Explanation

Oxidation is the loss of electrons, and in an electrochemical corrosion cell it occurs at the anode. The cathode consumes those electrons in a reduction reaction, while the electrolyte carries ionic current between the electrodes.

Question 31Corrosion Theory and Degradation Mechanisms

A steel coupon has an anodic current density of 1 mA/cm². Using Faraday's law with an equivalent weight of 27.9 g/eq and a density of 7.87 g/cm³, what approximate corrosion rate results?

  1. About 1.2 mm/y
  2. About 12 mm/y Correct answer.
  3. About 0.12 mm/y
  4. About 120 mm/y

Explanation

Using Faraday's law, a current density of 1 mA/cm² on steel corresponds to about 11.6 mm/y with the stated equivalent weight and density. The calculation converts charge to mass loss and then divides by density to obtain penetration depth.

Question 32Corrosion Theory and Degradation Mechanisms

Which statement distinguishes activation polarization from concentration polarization?

  1. Activation polarization applies only to cathodic reactions, while concentration polarization applies only to anodic reactions.
  2. Activation polarization occurs only in stirred solutions, while concentration polarization occurs only in stagnant solutions.
  3. Activation polarization is independent of electrode potential, while concentration polarization depends on flow.
  4. Activation polarization stems from slow charge transfer; concentration polarization stems from mass-transport limitation. Correct answer.

Explanation

Activation polarization is caused by a slow charge-transfer step and follows an exponential Tafel relationship with potential. Concentration polarization appears when the reaction is limited by arrival of reactants or removal of products, so it depends strongly on flow and diffusion.

Question 33Corrosion Theory and Degradation Mechanisms

Anodic protection is most suitable for which situation?

  1. Carbon steel in dilute sulfuric acid, because it dissolves actively and cannot be passivated.
  2. Zinc in neutral soil, because it is galvanically sacrificial to steel and dissolves uniformly.
  3. Stainless steel in sulfuric acid, where the potential can be held in the passive range. Correct answer.
  4. Copper in seawater, because it is cathodic to steel and therefore easy to protect anodically.

Explanation

Anodic protection holds the metal in its passive potential range by applying a small anodic current, so it works only for active-passive metals in environments where the passive film is stable. Stainless steel in sulfuric acid is the classic example; carbon steel, copper, and zinc do not form a suitable stable passive film under those conditions.

Question 34Corrosion Theory and Degradation Mechanisms

Which condition most directly causes intergranular corrosion of austenitic stainless steel?

  1. Preferential dissolution of molybdenum from the alloy matrix.
  2. Formation of a thick, porous iron oxide scale over the entire surface.
  3. Nickel enrichment along grain boundaries, leaving the matrix depleted in nickel.
  4. Chromium carbide precipitation at grain boundaries with local chromium depletion. Correct answer.

Explanation

When austenitic stainless steel is heated through the sensitization range, chromium carbide precipitates at grain boundaries and consumes chromium from the adjacent matrix. The chromium-depleted zones become anodic to the grain interiors, so corrosion runs along the boundaries.

Question 35Corrosion Theory and Degradation Mechanisms

Selective leaching of an alloy is favored when:

  1. The alloy contains only one metal, so no galvanic couple exists.
  2. The alloy is single phase and both elements have nearly equal nobility.
  3. The environment is strongly oxidizing and quickly passivates the alloy.
  4. One alloying element is appreciably more active and can dissolve selectively. Correct answer.

Explanation

Dealloying occurs when one component of an alloy is significantly less noble than the others and dissolves preferentially, leaving a porous or chemically altered residue. The driving force is the difference in nobility between the alloy constituents, not the presence of a single metal.

Question 36Corrosion Theory and Degradation Mechanisms

Stray-current corrosion occurs where:

  1. Direct current enters a buried structure from the electrolyte.
  2. Alternating current leaves a buried structure to enter the electrolyte.
  3. The structure is electrically isolated from all grounding systems.
  4. Direct current leaves a buried structure to enter the electrolyte. Correct answer.

Explanation

Stray-current corrosion is driven by direct current that leaves a metallic structure and enters the surrounding electrolyte, making that discharge point anodic. Alternating current is much less efficient at driving metal dissolution, and current entering the structure would make it cathodic rather than anodic.

Question 37Corrosion Theory and Degradation Mechanisms

Which environment is most conducive to microbiologically influenced corrosion by sulfate-reducing bacteria?

  1. Dry desert air with very low relative humidity and no free water.
  2. Strongly acidic, oxidizing process solution with a high redox potential.
  3. Anaerobic, low-oxygen soil or sediment containing organic matter. Correct answer.
  4. Aerobic, fast-flowing chlorinated water with a high dissolved-oxygen content.

Explanation

Sulfate-reducing bacteria thrive in anaerobic, low-oxygen environments that contain organic matter and sulfate, such as waterlogged soil, sediments, and stagnant water. Their metabolism produces sulfide and can depolarize the cathodic reaction, accelerating corrosion.

Question 38Corrosion Theory and Degradation Mechanisms

A Pourbaix diagram for a metal indicates a passivation region, yet the metal corrodes rapidly in service. Which explanation is most consistent?

  1. The diagram is based on pure water at 25°C and does not account for aggressive ions such as chloride that can break down the passive film. Correct answer.
  2. The potential and pH of the service environment cannot be plotted on a Pourbaix diagram.
  3. The diagram shows thermodynamic equilibrium only, so rapid corrosion is impossible in a passivation region under any conditions.
  4. The passivation region always corresponds to a high corrosion rate because passive films are electronic conductors.

Explanation

Pourbaix diagrams are computed from thermodynamic data for specified species, usually pure water at 25°C, and they do not include the effects of aggressive ions or kinetic barriers. Chloride can lower the pitting potential and destroy passivity, so a region labeled passive in the diagram may not be protective in real service.

Question 39Corrosion Theory and Degradation Mechanisms

A large copper cathode is coupled to a small steel anode in seawater. Compared with equal areas, the steel corrosion rate will:

  1. Decrease, because the cathode polarizes less and draws less current from the anode.
  2. Remain unchanged, because corrosion rate depends only on the steel's exchange current density.
  3. Increase, because the large cathode can support a higher anodic current with little cathodic polarization. Correct answer.
  4. Become zero, because copper cathodically protects the steel.

Explanation

In a galvanic couple, the larger area electrode polarizes less because the same current is spread over more surface. A large cathode coupled to a small anode therefore draws a high anodic current density from the small anode and accelerates its corrosion.

Question 40Corrosion Theory and Degradation Mechanisms

For a cathodic reaction controlled by oxygen diffusion, increasing the flow velocity of the electrolyte generally will:

  1. Have no effect, because oxygen diffusion through the boundary layer is independent of flow velocity.
  2. Shift the cathodic reaction from oxygen reduction to hydrogen evolution.
  3. Decrease the limiting current density and lower the corrosion rate.
  4. Increase the limiting current density and can raise the corrosion rate of the anode. Correct answer.

Explanation

For a diffusion-limited cathodic reaction, the limiting current density increases with flow because the diffusion boundary layer becomes thinner. A higher cathodic limiting current can support a higher anodic dissolution rate, so the corrosion rate of the anode may rise.

Question 41Corrosion Theory and Degradation Mechanisms

A metal has a negative standard reduction potential, yet it shows a very low corrosion rate in a given environment. The best explanation is:

  1. A negative standard potential proves that the metal cannot corrode under any practical conditions.
  2. The standard potential already includes the stabilizing effect of the passive surface film.
  3. The metal must be receiving cathodic protection from an impressed-current power source.
  4. Kinetics, not thermodynamics, controls rate; a passive film or slow reaction can inhibit corrosion. Correct answer.

Explanation

A negative reduction potential shows a thermodynamic tendency to oxidize, but corrosion rate is governed by kinetics and by the presence of protective films. Aluminum, for example, is thermodynamically active yet resists corrosion because of a passive oxide film.

Question 42Corrosion Theory and Degradation Mechanisms

For the half-reaction Zn²⁺ + 2e⁻ ⇌ Zn, decreasing the Zn²⁺ activity from 1 M to 0.01 M at 25°C will:

  1. Shift the equilibrium potential by about -59 mV. Correct answer.
  2. Leave the equilibrium potential unchanged.
  3. Shift the equilibrium potential by about -118 mV.
  4. Shift the equilibrium potential by about +59 mV.

Explanation

The Nernst equation for this half-cell is E = E° + (0.059/2) log[Zn²⁺]. Lowering the activity from 1 to 0.01 changes the log term by -2, giving a shift of about -0.059 V, or -59 mV.

Question 43Corrosion Theory and Degradation Mechanisms

In a cyclic polarization test, the repassivation potential is:

  1. The potential at which visible hydrogen evolution begins on the cathode surface of the test cell.
  2. The potential below which the metal becomes thermodynamically immune to all forms of pitting attack.
  3. The potential at which the anodic current density first increases sharply during the forward polarization scan.
  4. The potential on the reverse scan where current falls to a low value, below which pits stop propagating. Correct answer.

Explanation

In a cyclic polarization curve, the forward scan shows the breakdown or pitting potential where current rises sharply. On the reverse scan, the current falls to a low value at the repassivation potential; below that potential existing pits do not propagate.

Question 44Corrosion Theory and Degradation Mechanisms

Which pair is most likely to be compatible when joined in a marine environment?

  1. Carbon steel and copper.
  2. 316 stainless steel and copper. Correct answer.
  3. Aluminum and copper.
  4. Carbon steel and 316 stainless steel.

Explanation

Galvanic compatibility depends on the potential difference between the joined materials in the service electrolyte. Copper and 316 stainless steel are both relatively noble in seawater, whereas couples with carbon steel or aluminum create a strong anode-cathode mismatch.

Question 45Corrosion Theory and Degradation Mechanisms

In buried pipeline corrosion, low soil resistivity generally indicates:

  1. Lower corrosivity, because ions are less mobile in the soil.
  2. No relationship to corrosivity, because soil resistivity affects only coatings.
  3. Higher corrosivity, because the electrolyte can carry more current. Correct answer.
  4. A completely dry, noncorrosive soil with no electrolyte.

Explanation

Low soil resistivity means the soil electrolyte has a high ionic content and can carry current more readily, so galvanic and stray-current corrosion cells are more active. Resistivity is therefore a useful, though not complete, indicator of soil corrosivity.

Question 46Corrosion Theory and Degradation Mechanisms

In an open water system, increasing temperature typically has what net effect on the corrosion of steel?

  1. It always decreases corrosion, because oxygen solubility falls with increasing temperature at every temperature.
  2. It has no effect, because corrosion in open water is controlled solely by the bulk pH of the solution.
  3. It first raises corrosion by accelerating kinetics, but lower oxygen solubility can later dominate and reduce the rate. Correct answer.
  4. It eliminates corrosion completely by driving all dissolved oxygen and carbon dioxide out of the water.

Explanation

Raising temperature accelerates electrochemical reaction rates, so corrosion often increases initially. In open systems, however, oxygen solubility decreases with temperature, and above a certain point the reduced oxygen supply can lower the cathodic reaction rate and reduce corrosion.

Question 47Corrosion Theory and Degradation Mechanisms

Which statement about the effect of flow velocity on corrosion is correct?

  1. Velocity has no effect on diffusion-limited cathodic reactions in any aqueous environment.
  2. Velocity affects only impressed-current cathodic protection systems, not natural corrosion mechanisms.
  3. Moderate flow can raise corrosion by supplying more oxygen, while high flow can cause erosion-corrosion. Correct answer.
  4. Increasing velocity always decreases corrosion by sweeping away all surface deposits and corrosion products.

Explanation

Flow velocity affects corrosion through mass transport and mechanical damage. Moderate flow can increase the cathodic oxygen supply and raise corrosion, while very high flow can strip protective films and cause erosion-corrosion.

Question 48Corrosion Theory and Degradation Mechanisms

Which statement best distinguishes hydrogen-induced cracking (HIC) from hydrogen embrittlement?

  1. HIC involves brittle fracture from absorbed hydrogen under stress, while hydrogen embrittlement is internal blistering from hydrogen recombination.
  2. Hydrogen embrittlement is stress-assisted ductility loss and cracking; HIC can occur without stress at internal inclusions. Correct answer.
  3. HIC is caused by cathodic protection, while hydrogen embrittlement is caused by anodic dissolution.
  4. HIC occurs only above 500°C, while hydrogen embrittlement occurs only at room temperature.

Explanation

Hydrogen embrittlement is a stress-assisted loss of ductility and cracking caused by absorbed hydrogen. Hydrogen-induced cracking can occur without external stress when hydrogen recombines at internal inclusions or laminations and builds high local pressure.

Question 49Corrosion Theory and Degradation Mechanisms

Corrosion fatigue differs from pure mechanical fatigue because:

  1. It affects only nonmetallic materials and never occurs in engineering alloys.
  2. It occurs only in dry air and always displays a well-defined fatigue endurance limit.
  3. It needs a corrosive environment and lacks a clear endurance limit, with cracks starting at corrosion pits. Correct answer.
  4. It is caused solely by absorbed hydrogen and is completely independent of cyclic stress.

Explanation

Corrosion fatigue is cyclic loading in a corrosive environment, and the environment prevents the formation of a true endurance limit while pits act as crack starters. Pure mechanical fatigue in dry air often shows a clear endurance limit.

Question 50Corrosion Theory and Degradation Mechanisms

In a marine atmosphere, chloride deposition on steel most strongly affects corrosion by:

  1. Forming a protective chloride film that reduces corrosion.
  2. Increasing the conductivity of the surface electrolyte and promoting breakdown of protective films. Correct answer.
  3. Raising the pH of the surface moisture to strongly alkaline values above pH 12.
  4. Removing oxygen from the metal surface.

Explanation

Chloride deposited from marine air raises the conductivity of the thin moisture film and attacks passive films, so it promotes both general corrosion and pitting. Chloride does not form a protective film or remove oxygen from the surface.