Cathodic protection of aboveground storage tank bottoms to API RP 651: whether it is needed (Section 5), the pad and soil (Table 1, 5.3.2.1), liners and double bottoms (5.4.3, 7.2), the design current, rectifier and anodes (7.3.5), permanent reference electrodes (Table 4), a potential survey judged against the three protection criteria (8.2.2, Table 3), and survey and maintenance dates (11.3)
New here? Read the User Manual — each section, the criteria with worked readings, and the design steps.
◉ API RP 651 4TH ED. (SEPTEMBER 2014) · TABLES 1 – 4 · 8.2.2 CRITERIA · NOT A REGULATORY SUBMISSION TOOL
⚠ Indicative tool only. Follows API RP 651, 4th edition (September 2014), the text on file. A 5th edition was issued in August 2024 and has not been compared — check it before relying on these values. API RP 651 is a recommended practice: its criteria are the minimum indicators of protection, other criteria may be used where control of corrosion can be shown (8.2.2), and the RP points to NACE RP0193 (now SP0193) for detail. The anode-to-earth resistance helper uses Dwight’s equation, which the RP cites (7.3.5.2.2.3 c) but does not print. Design and testing are for a qualified corrosion specialist.
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Identification
The RP gives current densities in mA/ft² and Table 4 in both m and ft. Potentials are in mV and resistivity in Ω·cm in both systems. Records are kept in SI.
Summary of checks
Enter the tank, then the pad, the design or the survey readings.
1. Tank and Need for Protection (Section 5)
2. Pad Material and Soil (Table 1, 5.3.1, 5.3.2.1)
The RP gives no consensus limits for chlorides, sulfates or sulfides — the owner specifies them (5.3.2.1 h–j). Without owner limits the tool reports the RP’s indicators as notes: pitting can start at 10 ppm chlorides, sulfates over 200 ppm often mean organic matter, sulfides over 0.10 ppm may mean sulfate-reducing bacteria.
3. System Design (Section 6, 7.3)
Without a current requirement test the RP gives the protective current density (7.3.5.2.2.1); with one, enter the current it showed. Voltage is E = I × R (7.3.5.2.2.3). The helper estimates one vertical anode with Dwight’s equation and divides by the number of anodes, which ignores their mutual interference and so understates the bed resistance — space anodes well apart or use a measured value.
Enter potentials in mV as read against the chosen electrode, negative for cathodic. Native is the potential before the system was energized, or the depolarized value after it was switched off (for the polarization criterion); On is with current applied; Instant-off is the polarized potential just after interrupting the rectifiers (8.3.4). Readings at the perimeter may not represent the centre of the bottom (8.3.1); include under-tank points from permanent electrodes or a perforated tube (8.3.2, 9.4.4).
#
Location
Position
Native mV
On mV
Instant-off mV
Result
5. Surveys, Checks and Records (11.3, 11.4)
Installation and Records Checklist (7.3.6, 9, 11.4)
Tick each item once it is confirmed. Items that don’t apply are hidden.
Save & Report
Inputs are kept on this device as you type and offered back if the page is closed before saving. A bottom with effective cathodic protection is one of the API 653 Table 6.1 credits used by the Inspection Interval tool.