Corrosion and Its Control Questions and Answers

Answer: C

Correct answer: poorly supplied with oxygen

This is the result that seems backwards until the cell is drawn out. The well-aerated area supports the cathodic reduction of oxygen, so the poorly aerated area is left to be the anode, and it is the anode that dissolves. Crevices, gasket faces, the underside of a deposit and the metal below the waterline are all attacked for exactly this reason.

Answer: D

Correct answer: more anodic of the two

The anodic metal is the one that gives up electrons, so it dissolves while the cathodic metal is protected. Which is which is read off the galvanic series in the electrolyte concerned. The area ratio matters greatly but does not decide the direction: a small anode joined to a large cathode simply concentrates the same current into less metal, and corrodes very much faster.

Answer: A

The whole current generated by a large cathode is concentrated on a small anode, so its penetration rate is enormous. A steel rivet in a copper plate perforates quickly, whereas copper rivets in a steel plate cause only slight general attack.

Answer: B

A tank may leak while more than ninety-nine per cent of its wall remains sound, so weight loss measurements badly understate the damage. Once a pit starts, its oxygen-starved base stays anodic and it deepens autocatalytically.

Answer: C

A local discontinuity becomes a small oxygen-starved anode surrounded by a large aerated cathode, and chloride ions prevent the film from healing. Polishing the surface and excluding chloride are both genuine preventive measures.

Answer: D

Diffusion into the crevice is too slow to replace the oxygen consumed, so the gap turns anodic and the acidity and chloride concentration inside both climb. Continuous welds in place of lap joints and gaskets is the design remedy.

Answer: A

The metal at the surface has abundant oxygen and is cathodic, and the metal well below has neither oxygen nor a nearby cathode, so the narrow band beneath the line takes the attack. A distinct groove forms at that level in ships and storage tanks.

Answer: B

Chromium carbide forming in the heat-affected zone leaves the adjacent metal too poor in chromium to passivate, so a narrow path corrodes preferentially. Low-carbon or stabilised grades, or a post-weld solution treatment, prevent the sensitisation.

Answer: C

Neither factor alone would cause failure, and the environment must be one to which that alloy is susceptible, as brass is to ammonia and austenitic steel to chloride. Stress relieving the component removes half the requirement.

Answer: D

Free alkali concentrating at riveted and stressed joints cracks the metal along its grain boundaries with no appreciable loss of thickness. Adding tannin, lignin or phosphate to the boiler water is the standard preventive.