Cathodic Protection of Tanks API 651

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$$l& voltage- and rectif&ing element to $rovide 0C out$ut. Silicon rectiers are more e is a$$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