LESSON 5
PRACTICE EXERCISE
ANSWER KEY AND FEEDBACK
Item
Correct Answer
1.
D.
When metals cannot be identified by visual examination or mechanical testing use
an acid spot test. (page 5-22, para 5-5c)
2.
B.
A Corrosive attack begin on a metal surface that is exposed to a corrosive
environment. If allowed to progress, corrosion works down into the core of the
metal's material. Since corrosion never originates in the core, there will always be
evidence on the surface when an attack is in progress. (pages 5-3 and 5-4, para 5-
lb[2])
3.
C.
The cathodic protection method uses an electrode that is referred to as a sacrificial
anode which corrodes to protect a structure. (page 5-16, para 5-4b[1])
4.
D.
A good example of stress corrosion is the use of cold-worked rivets to join steel
sheets used in storage tank construction. (page 5-7, para 5-2d[3])
5.
A.
An alternating current source is required with this protection method. In addition,
a rectifier is necessary to obtain the required direct-current potential. (page 5-17,
para 5-4b[21
6.
A.
Four conditions must exist before electrochemical corrosion can take place;
namely, the presence of a metal anode, a cathode, an electrolyte, and a conductor.
(page 5-3, para 5-1b[(1])
7.
D.
By misidentifying metal, you may waste your efforts, damage material, or cause
corrosion. (page 5-18, para 5-5)
8.
B.
If a metal cannot be positively identified by visual examination, see if it is
attracted by a magnet. Perform a spark test if the metal is magnetic. (page 5-21,
para 5-5b)
9.
C.
Stray currents, many of which are direct causes of corrosion, are usually direct-
current circuits that pass in and out of an electrolyte. This condition poses the
greatest problem in the vicinity of electrical-transportation systems, electrified
coal mines, or manufacturing plants where their direct-current distribution system
requires a ground as a complete or partial circuit return. (page 5-12, para 5-3e)
EN0562
5-28
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