API 651 - 3 (TCorrosion of Aboveground Steel Storage Tanks)

Question 1

Corrosion may be defined as the deterioration of a:

Answer 1

metal due to a reaction with its environment

Question 2

there are four components in each corrosion cell .they are :

Answer 2

anode,cathode, a connecting metallic path, & an electrolyte

Question 3

at the anode, the base metal______ by releasing electrons & forming positive metal ions..

Answer 3

corrodes

Question 4

_______ corrosion takes place at the cathode

Answer 4

No

Question 5

the electrolyte for external tank bottoms is usually

Answer 5

moist soil

Question 6

the two most common types of corrosion relative to tank bottoms are

Answer 6

general and pitting corrosion

Question 7

individual corrosion cells are large and a distinct anode & cathode area can be identified in ________ corrosion

Answer 7

pitting

Question 8

thousands of mircoscopic corrosion cells occur on an area of the metal surface & results in relatively uniform metal loss. This type of corrosion is referred to as ______ corrosion .

Answer 8

general

Question 9

in ___________ corrosion, the individual corrosion cells are larger & distinct anode and cathode areas can be identified

Answer 9

pitting

Question 10

differences between weld metal, heat affected zones, and parent metal can cause

Answer 10

corrosion

Question 11

Physical and chemical properties of the electrolyte may influence the location of cathodic areas on the metal surface.Areas of lower oxygen concentrations become anodic and areas with higher oxygen concentrations become anodic and areas with higher oxygen concentrations become cathodic.This can cause corrosion in areas :

Answer 11

where clay or other debris contracts a steel tank bottom constructed on a sand cushion foundation

Question 12

Dissolved salts in the soil that supports a tank bottom influence the:

Answer 12

current carrying capacity of electrolyte

Question 13

Interference currents travel through the soil electrolyte and on to structures for which they are not intended.These currents are also known as :

Answer 13

stray currents

Question 14

The most common stray current and potentially most damaging is :

Answer 14

direct current

Question 15

Direct stray currents are generated from grounded DC electric power systems such as :

Answer 15

electric railroads,Welding machines,and other cathodic protection systems

Question 16

Separation and routing of the interfering and affected structures and location of interfering current source,the magnitude and density of current,the quality of or adsence of coating on affected structures,and the presence and location of mechanical joints having high electrical resistance all contribute to:

Answer 16

the severity of corrosion resulting from interference currents

Question 17

Electric currents takes the path of ______ resistance

Answer 17

least

Question 18

an example of galvanic corrosion is:

Answer 18

when a stainless steel or copper pipe is connected to a steel tank

Question 19

the two most significant factors in galvanic corrosion in the corrosion of a tank bottom when a stainless steel or a copper pipe is connected to a tank are

Answer 19

the relative areas of the cathode & anode and the relative position of the cathode & anode in galvanic series

Question 20

the corrosion that occurs when two metals with different compositions are connected in an electrolyte( usually soil) is called

Answer 20

galvanic corrosion

Question 21

current , in galvanic corrosion, is considered to flow from the:

Answer 21

more active metal( anode) to the less active metal ( cathode)

Question 22

the most significant factor in the amount of galvanic corrosion that takes place is the relative

Answer 22

areas of the cathode & anode and the relative position of the two materials in the galvanic series

Question 23

we learned earlier in section 4 that there are 2 major types of corrosion, general and pitting .To a lesser extent there is another problem that can be considered a manner of corrosion mechanisms . what is it?

Answer 23

enviromental cracking