Erosion corrosion is the combined action of corrosion and
Answer: A
Rapid or turbulent flow strips away the protective film as fast as it forms, so the metal never gets the chance to passivate. It is worst at bends, orifices and pump impellers, and reducing the velocity or increasing the radius helps.
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Soil corrosion of a buried pipeline is aggravated by soil that is
Answer: B
A conductive soil lets the corrosion currents flow freely and the moisture provides the electrolyte, so resistivity surveys are used to identify the worst stretches. Differential aeration along the route sets up long-line currents in addition.
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A metal's position in the galvanic series indicates its
Answer: C
The series ranks practical alloys by their measured potentials in a particular electrolyte, most commonly sea water, so it predicts which of a pair will be attacked. It differs from the electrochemical series in listing alloys and in accounting for passive films.
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Corrosion of iron proceeds faster in a solution of sodium chloride than in pure water because the chloride
Answer: D
A more conductive electrolyte lets the corrosion current flow more easily between anode and cathode, and chloride ions penetrate and destabilise the oxide film. It is why marine and de-iced-road environments are so much more aggressive.
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The corrosion rate of most metals passes through a maximum with rising pH at
Answer: A
In acid the hydrogen evolution route is available and any protective film dissolves, while in alkaline solution iron passivates. Amphoteric metals such as aluminium and zinc are the exception, being attacked at both extremes.
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A metal with a small grain size generally corrodes
Answer: B
Correct answer: faster, having more grain boundary area
Grain boundaries are regions of disordered, higher-energy atoms, so they dissolve preferentially. Reducing the grain size multiplies the boundary area per unit volume and gives the attack more sites to work on. It is a genuine trade-off in design, since fine grains are wanted for strength and toughness and work against corrosion resistance.
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Two dissimilar metals should be joined with an insulating gasket in order to
Answer: C
Correct answer: break the electrical path between them
A galvanic cell needs four things: an anode, a cathode, an electrolyte and a metallic path for the electrons. The gasket removes the last of these, so no current can circulate however dissimilar the metals are. Insulating sleeves and washers must be used on the bolts as well, or the fastener quietly reconnects what the gasket separated.
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Cathodic protection works by making the whole structure
Answer: D
Forcing electrons into the metal suppresses its anodic dissolution, so the metal cannot corrode while the current is maintained. The oxidation has to occur somewhere, and it is transferred to a sacrificial anode or an inert one.
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In sacrificial anode protection the anode is a metal
Answer: A
Magnesium, zinc or aluminium corrodes in preference and must be replaced periodically, which is the cost of the method. No external power supply is needed, which is its great convenience for a ship's hull or a small buried tank.
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Impressed current cathodic protection differs from the sacrificial method in using
Answer: B
A rectifier drives current from a graphite or high-silicon iron anode to the structure, so the anode is barely consumed and the current can be adjusted. It suits long pipelines and large structures where sacrificial anodes would be uneconomic.
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