Corrosion and Its Control Questions and Answers

Answer: A

An anodic inhibitor at insufficient concentration films most of the surface but leaves small bare anodes against a large cathode, concentrating the whole current on them. Either dose properly or use a cathodic inhibitor instead.

Answer: B

Penetration rate follows current density rather than total current, so the same current on a tenth of the area does ten times the local damage. It is why steel fasteners in a copper assembly fail so much faster than the reverse arrangement.

Answer: C

The oxide is indeed the less reactive species, but a porous, cracked or volatile film leaves the metal exposed and protects nothing, as magnesium and molybdenum show. Protection depends on the film's physical character, not merely its chemistry.

Answer: D

Its simplicity suits a hull or a small buried tank, while the anodes must be renewed and the current cannot be varied. An impressed current system covers long pipelines and lets the current be tuned, at the cost of a rectifier and a supply.

Answer: A

Correct answer: a zinc coating

Only zinc is anodic to steel and therefore sacrificial. Tin, copper and nickel are all cathodic to it, so each protects only as a barrier and each accelerates the attack at any break in the coating, where a small exposed anode faces a large cathode. That is the practical distinction between a sacrificial coating and a noble one.

Answer: D

Liquid metal corrosion is not a wet corrosion process. It is attack of a solid metal by a molten metal, with no aqueous electrolyte involved, and is grouped with dry or direct chemical corrosion. Wet corrosion needs an electrolyte and works by electrochemical cells: soil corrosion (buried metal in moist soil), pitting corrosion (localised pits under a wet film) and galvanic corrosion (two dissimilar metals in contact in an electrolyte) are all of this type.