API 651 Cathodic Protection of
Aboveground Petroleum Storage Tanks
1.0 Scope:
This covers the procedures. Practices and guidance for effective
corrosion control of the above ground hydrocarbon storage
tank bottoms using cathodic protection to new and existing
storage tanks.
4. Corro
Corrosion
sion of Steel Storag
Storage
e Tanks
Tanks
Corrosion is the deterioration of metal due to reaction
r eaction !ith the
environment. Corrosion occurs !hen"
a. Areas !ith di#erent electrical $otentials e%ist on metal surface
b. These areas are
ar e electricall& connected
c. Areas are in contact !ith electrol&te like moist soil in contact !ith
tank bottom. 'ater and sludge are the electrol&tes for internal
bottom surface.
(orms of Corrosion)
*eneral Corrosion leading to general metal loss a nd thinning
Pitting due to locali+ed actions , metal loss ma& be concentrated
!ithin relativel& small area and other near area ma& be una#ected-
API 651 Cathodic Protection of
Aboveground Petroleum Storage
Tanks
4.1
Condition !hich inuence Corrosion
a. Com$osition of /etal
b. 0i#erential electrochemical $otential
c. 0i#erence bet!een the !eld bead heat
a#ected +one and $arent metal
d. Ph&sical and chemical $ro$erties of the
electrol&te
e. 0i#erence o%&gen concentration
f. Soil characteristics , dissolved salts
moisture contents $2g. Cla& and debris in bottom contact
sus$ended solids in $roduct stored $2 level
dissolved gases like 23S C3 or 3.
API 651 Cathodic Protection of
Aboveground Petroleum Storage
Tanks
6. Cathodic Protection /ethods
Cathodic $rotection is the techniue for $reventing corrosion b& making entire
surface of the metal to act as cathode of an electrol&te cell. There are t!o
mathods commonl& used"
a. Sacricial Anode
b. Im$ressed current
6.3 Sacricial Anode S&stem ,*alvanic S&stem-)
This s&stem involves an anode buried in soil but electricall& connected to
the structure ,cathode-. The anode is thus corroded ,sacriced- and metal
surface is $rotected. /etals commonl& used as anodes are magnesium and
+inc , cast or ribbon t&$e-. The& are either buried beneath the bottom are
distributed around the $erimeter of the tank.
Advantages)
a. 7o e%ternal $o!er su$$l& is reuired
b. 8as& to install
c. 9o! ca$ital investment
d. /inimum maintenance cost
e. Interference $roblems are less
f. 9ess monitoring reuired
0isadvantages)
a. 0riving $otential is limited
b. Current out$ut is lo!
c. Suitable onl& for lo! :esistivit& soil
d. 7ot ver& suitable for large bare structures
API 651 Cathodic Protection of
Aboveground Petroleum Storage
Tanks
6.;
Im$ressed Current S&stem
This s&stem uses 0irect Current ,0C- usuall& $rovided b& a
rectier. 0C o!s from the rectier to the buried im$ressed
current anode.
Po!er source include a ste$ do!n transformer ,reduces AC su$$l&
voltage- and rectif&ing element to $rovide 0C out$ut.
Silicon rectiers are more e<cient. Selenium rectiers are al so
used but have limited life s$an.
Im$ressed current anodes are of gra$hite steel high silicon cast
iron or mi%ed metal o%ide on titanium. The& are usuall& buried in
conducting coke bree+e backll ,to reduce circuit resistance- in
remote ground beds distributed around and under the tank or
installed in dee$ ground beds.
Advantages)
a. 9arge driving $otential is available
b. 2igh current out$ut for $rotecting large structures
c. Ca$able for variable current out$ut
d. A$$licable to almost an& soil resistivit&.
2o!ever the& are costl& more freuent monitoring is reuired and
loss of AC $o!er ma& cause loss to $rotection. Solar $o!ered
s&stems are also available.
API 651 Cathodic Protection of
Aboveground Petroleum Storage
Tanks
=. 0esign Criteria for Cathodic
Protection
A negative ,cathodic- $otential of at least =5
m> is a$$lied for the cathodic current.
A minimum of 1 m> of cathodic $olari+ation
shall be measured bet!een the tank bottom
metallic surface and the stable reference
electrode contacting the electrol&te.
The standard method of determining the
e#ectiveness of cathodic $rotection on a tank
bottom is the tank to soil $otential
measurement. /easurement is $erformed
using a high im$edance ,resistancevoltmeter and stable reference electrode
contacting the electrol&te.
API 651 Cathodic Protection of
Aboveground Petroleum Storage
Tanks
11.
$eration and /aintenance of CP S&stems
This s&stem shall be sub?ect to eld ins$ection surve&s to
establish the e#ectiveness at intervals.
Conditions that e#ect $rotection are sub?ect to change !ith
time. 2ence changes !ill be reuired in $arameters that
give $rotection.
If tank is em$t& large area under bottom ma& not be in
contact !ith underneath soil hence $otential surve& in
such conditions ma& be misleading. Such surve&s have to
be done !hen tank is under load and ma%imum bottom
cushion contacts have been made.
Initial surve&s of ne! installation shall not be conducted until
adeuate $olari+ation has occurred !hich generall& occurs
after 6@1= months of s&stem energi+ation.
Surve&s shall include one or more of)
Structure to soil $otential Anode current 7egative
structure to soil $otential Pi$ing to tank isolation , if
$rotected se$aratel&- e#ect of ad?acent structure rectier
0C volts 0C Am$s e<cienc& and ta$ settings.
Pro$er records have to be maintained revie!ed and
referred.