API 651 - 3 (TCorrosion of Aboveground Steel Storage Tanks) Flashcards
Corrosion may be defined as the deterioration of a:
metal due to a reaction with its environment
there are four components in each corrosion cell .they are :
anode,cathode, a connecting metallic path, & an electrolyte
at the anode, the base metal______ by releasing electrons & forming positive metal ions..
corrodes
_______ corrosion takes place at the cathode
No
the electrolyte for external tank bottoms is usually
moist soil
the two most common types of corrosion relative to tank bottoms are
general and pitting corrosion
individual corrosion cells are large and a distinct anode & cathode area can be identified in ________ corrosion
pitting
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 .
general
in ___________ corrosion, the individual corrosion cells are larger & distinct anode and cathode areas can be identified
pitting
differences between weld metal, heat affected zones, and parent metal can cause
corrosion
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 :
where clay or other debris contracts a steel tank bottom constructed on a sand cushion foundation
Dissolved salts in the soil that supports a tank bottom influence the:
current carrying capacity of electrolyte
Interference currents travel through the soil electrolyte and on to structures for which they are not intended.These currents are also known as :
stray currents
The most common stray current and potentially most damaging is :
direct current
Direct stray currents are generated from grounded DC electric power systems such as :
electric railroads,Welding machines,and other cathodic protection systems
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:
the severity of corrosion resulting from interference currents
Electric currents takes the path of ______ resistance
least
an example of galvanic corrosion is:
when a stainless steel or copper pipe is connected to a steel tank
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
the relative areas of the cathode & anode and the relative position of the cathode & anode in galvanic series
the corrosion that occurs when two metals with different compositions are connected in an electrolyte( usually soil) is called
galvanic corrosion
current , in galvanic corrosion, is considered to flow from the:
more active metal( anode) to the less active metal ( cathode)
the most significant factor in the amount of galvanic corrosion that takes place is the relative
areas of the cathode & anode and the relative position of the two materials in the galvanic series
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?
enviromental cracking
