Materials Science practice questions

Answer: B) 30 years

• AIncorrect. That divides 3 by 0.5.
• BCorrect. Rate = 0.1 mm/yr, and 3 ÷ 0.1 = 30.
• CIncorrect. That halves it.
• DIncorrect. That isn't the calculation.

Answer: B) Low carbon, which reduces sensitization during welding

• AIncorrect. That isn't the meaning.
• BCorrect. Less carbon means less chromium carbide at grain boundaries.
• CIncorrect. Chromium content is similar.
• DIncorrect. These are steels.

Answer: A) It forms a compact, adherent TiO2 passive film that re-forms quickly and resists breakdown by chloride ions

Titanium's corrosion resistance in seawater comes from passivity: a thin, stable, self-healing TiO2 film that chlorides do not readily break down, unlike the films on many stainless steels.

• ACorrect. Titanium is a very reactive metal, but it immediately forms a thin, stable TiO2 passive film. The film repairs itself in oxidizing water and resists chloride attack, so titanium avoids the pitting and crevice corrosion that can affect stainless steels in seawater.
• BIncorrect. Passivated titanium sits near the noble end of the seawater galvanic series. It is not a sacrificial anode; zinc, aluminum, and magnesium play that role.
• CIncorrect. Cr and Mo are what give stainless steels their pitting resistance. Titanium's resistance comes from its own oxide film; commercially pure titanium contains essentially no Cr or Mo.
• DIncorrect. Titanium reacts strongly with oxygen. It resists corrosion because that reaction quickly forms a protective oxide film (passivity), not because the metal is inherently unreactive.

Answer: A) Extra wall thickness added to the calculated pressure-design thickness to cover expected metal loss in service

A corrosion allowance is extra wall thickness added beyond what the pressure-design formula requires, so the vessel keeps adequate strength as metal corrodes away over its service life.

• ACorrect. The corrosion allowance is extra metal added on top of the thickness the design formula requires. For example, 46 CFR 54.01-35 normally calls for 1/6 of the calculated thickness or 1/16 in., whichever is smaller, so the vessel still has enough thickness for pressure as it corrodes.
• BIncorrect. Allowable stresses and design margins deal with strength uncertainty. Corrosion is handled separately by adding thickness to the calculated value, not by lowering the allowable stress.
• CIncorrect. A corrosion allowance is a thickness (for example, inches or mm), not a rate. Inspectors compare measured corrosion rates with the remaining allowance to estimate remaining life, but the allowance itself is not a rate limit.
• DIncorrect. Cladding or corrosion-resistant materials are an alternative to a corrosion allowance; the regulation lets them replace the added thickness. The allowance itself is extra wall metal, not a lining.

Answer: B) Undergo large, reversible elastic deformation

• AIncorrect. They're highly elastic.
• BCorrect. They recover their shape after compression.
• CIncorrect. They're chosen for elasticity.
• DIncorrect. They're soft and elastic.

Answer: A) Beneath flange gaskets, lap joints, and surface deposits where solution stagnates

Crevice corrosion forms in shielded, stagnant gaps such as under gaskets, lap joints and deposits. Oxygen runs out there, and chloride builds up and acidifies the trapped solution.

• ACorrect. In shielded gaps the trapped solution stagnates and runs out of oxygen. Chloride concentrates and the solution acidifies, so the metal inside the crevice corrodes as the anode of a local concentration cell.
• BIncorrect. Fast flow over exposed surfaces causes erosion corrosion. Crevice corrosion needs a shielded, stagnant zone, which is the opposite condition.
• CIncorrect. That describes intergranular corrosion of sensitized stainless steel, caused by chromium carbides forming at grain boundaries. It is not crevice corrosion.
• DIncorrect. That is galvanic corrosion, driven by the difference in electrode potential between two metals. Crevice corrosion can occur on a single metal.

Answer: A) Quenched-and-tempered high-strength steel bolts after acid pickling and electroplating

Hydrogen embrittlement mainly affects high-strength steels that pick up atomic hydrogen, for example from pickling, electroplating, cathodic protection or sour service. The other options describe different cracking mechanisms.

• ACorrect. Pickling and electroplating put atomic hydrogen into the steel. High-strength (martensitic) steels are the most susceptible, and the risk rises as strength increases, which can lead to delayed brittle fracture under load.
• BIncorrect. The usual concern here is chloride stress-corrosion cracking. Annealed austenitic stainless steel is relatively resistant to hydrogen embrittlement.
• CIncorrect. Dry nitrogen supplies no atomic hydrogen, and low-strength ductile steel has little susceptibility, so embrittlement is unlikely.
• DIncorrect. This is the classic setting for season cracking, a type of stress-corrosion cracking in copper alloys, not hydrogen embrittlement.

Answer: B) Molybdenum

• AIncorrect. Chromium content is similar.
• BCorrect. Mo improves chloride pitting resistance.
• CIncorrect. That isn't added.
• DIncorrect. Carbon doesn't help pitting.

Answer: D) Softens it and restores ductility

• AIncorrect. Annealing lowers it.
• BIncorrect. That's what cold work does.
• CIncorrect. That's galvanizing.
• DCorrect. Recrystallization removes the strain hardening.

Answer: A) At elbows and pump impellers where turbulent, high-velocity flow strikes the metal

Erosion–corrosion happens where fast or impinging flow keeps stripping protective surface films, such as at elbows and pump impellers. Stagnant crevices, dissimilar-metal joints and stressed welds point to other corrosion mechanisms.

• ACorrect. High shear and impingement remove the protective corrosion-product film, which exposes fresh metal to repeated corrosion. Elbows, tees, pump impellers and areas downstream of restrictions are typical sites.
• BIncorrect. Stagnant, occluded spaces lead to crevice (or under-deposit) corrosion, a mechanism driven by local chemistry differences, not flow.
• CIncorrect. An electrical coupling of dissimilar metals in an electrolyte causes galvanic corrosion of the more active metal. Flow is not the driver.
• DIncorrect. Tensile stress combined with a specific corrosive environment points to stress-corrosion cracking, not flow-driven metal loss.