Cathodic Protection Manual

Contents

About this tool

The Cathodic Protection calculator follows API RP 651 for the soil side of aboveground storage tank bottoms: whether protection is needed, the pad and soil, liners and replacement bottoms, the design current, rectifier and anodes, the permanent reference electrodes, a potential survey judged against the three protection criteria, and the survey and maintenance dates.

Edition

It follows the 4th edition (September 2014), the text on file. A 5th edition was issued in August 2024 and has not been compared yet. API RP 651 is a recommended practice: its criteria are minimum indicators, and others may be used where corrosion control can be shown (8.2.2). Design and testing are for a qualified corrosion specialist.

Quick start

Two samples: a new 36 m tank on a sand pad over a liner with an impressed-current design, and an existing 24 m tank with an annual survey that has one unprotected reading, overdue checks and pad chemistry worth a look.

1
Section 1: D, new or existing, temperature, the pad, any liner or second bottom, internal lining.
2
Section 2: pad and soil resistivity, pH, chlorides, sulfates, sulfides; owner limits if set.
3
Section 3: impressed current or galvanic, the current density (or a test current), rectifier margin, anode output and resistance.
4
Section 4: the reference electrode and the readings: native, on and instant-off at each point, perimeter or under the tank.
5
Section 5: dates of energizing, the last rectifier check and the last annual survey.

Pad and soil (Table 1, 5.3)

Resistivity, Ω·cm (Table 1)	Potential corrosion activity
under 500	very corrosive
500 to 1000	corrosive
1000 to 2000	moderately corrosive
2000 to 10,000	mildly corrosive
over 10,000	progressively less corrosive

Resistivity alone should not judge corrosivity (5.3.1.2); the tool reads it with the chemistry.
pH (5.3.2.1 g): corrosion fairly independent of pH from 5.0 to 8.0, appreciably higher below 5.0, lower above 8.0.
Chlorides (h): pitting can begin at 10 ppm, and chlorides raise the current needed. Sulfates (i) over 200 ppm often mean organic matter. Sulfides (j) over 0.10 ppm may mean sulfate-reducing bacteria.
There is no consensus limit for chlorides, sulfates or sulfides: the owner specifies them. Without owner limits the tool shows the RP indicators as Notes; with them, a value over the limit fails.
Concrete and asphalt pads can block the current while water under the bottom still corrodes it (5.3.3, 5.3.6); a liner under the tank means the anodes must be between the liner and the bottom (5.4.3, 7.2.3, Figure 7); a new bottom over an old one needs the CP in the space between them (7.2.4, Figure 8).

Design current, rectifier and anodes (7.3.5)

Step	Rule	Design sample
Bottom area	πD²/4	36 m → 1,017.9 m²
Current density (7.3.5.2.2.1)	without a test: 1 – 2 mA/ft² ambient, 0.5 mA/ft² close-coupled anodes, 2 – 8 mA/ft² elevated temperature; blank uses the top of the range	2 mA/ft² = 21.53 mA/m²
Current	I = A × i, or the current found by a current requirement test (Figure 9)	1,017.9 × 21.53 / 1000 = 21.91 A
Rectifier (7.3.5.2.3 c)	excess capacity typically 10 to 50 %	+25 % → 27.39 A
Anodes (7.3.5.2.1.4)	total current ÷ rated output per anode, plus more for distribution and failures	21.91 ÷ 4 A → 6 anodes
Anode resistance	Dwight, one vertical anode: R = ρ/(2πL)·(ln(8L/d) − 1), ρ in Ω·m — cited by the RP (7.3.5.2.2.3 c) but not printed	30 Ω·m, 2.1 m, 0.25 m → 7.293 Ω
Voltage (7.3.5.2.2.3)	E = I × R	7.293/6 + 0.5 = 1.716 Ω; 27.39 A × 1.716 = 47.0 V

Dividing one anode’s resistance by the number of anodes ignores their mutual interference: it understates the bed resistance. Use a measured value where you can.
Galvanic anodes drive only about 0.25 V (zinc) to 0.75 – 0.9 V (magnesium) against protected steel (Table 2): the tool fails a galvanic design whose E = I × R exceeds 0.9 V.
Permanent reference electrodes (Table 4): 3 for 6 – 12 m, 4 for 12 – 18 m, 5 for 18 – 30 m, 6 for 30 – 45 m, 7 for 45 – 76 m, 10 for 76 – 107 m, equally spaced across the diameter.

The protection criteria (8.2.2) and the survey

Criterion	Meets when	Example (mV CSE)
8.2.2.1 — current applied	on-potential at least −850 mV, with voltage drops other than at the bottom considered	on −880, no instant-off: on only (a Note: consider the IR drop)
8.2.2.2 — polarized	instant-off at least −850 mV	off −920: protected
8.2.2.3 — polarization	at least 100 mV between native (or depolarized) and instant-off	native −575, off −790: shift 215 mV, protected
None	none of the above	native −550, on −930, off −620: shift 70 mV, not protected

Once an instant-off value is measured it is the IR-free potential, so a reading whose on-potential passes but whose instant-off misses both −850 mV and 100 mV is not protected.
−850 mV polarized may be impractical on a bare bottom (8.2.2.2); use the 100 mV criterion with native or depolarized values.
Other electrodes (Table 3): −850 mV CSE equals −780 mV saturated calomel, −800 mV silver/silver chloride, +250 mV zinc. Readings are converted to CSE before judging.
Perimeter readings may not represent the centre of the bottom (8.3.1): without an under-tank reading the tool adds a Note. Potentials also vary with the liquid level (8.3.5).

Surveys, checks and records (11.3, 11.4)

Item	Interval
Impressed current sources	checked at intervals not exceeding two months (11.3.2.2)
CP survey; impressed current facilities inspected	annually (11.3.1.2, 11.3.2.3)
After energizing	right away, and again after polarization has stabilised (may take several months) (11.3.1.1)
Records of CP effectiveness	five years unless regulation allows less; records of need kept while the tank is in service (11.4.7)

The tool gives the next due dates from the last dates entered and fails a check that is overdue on the assessment date.

The checks

Check	Clause	Result
Barrier and access to the bottom	5.4.3, 7.2	a Note with a liner or second bottom
Pad resistivity, pH	Table 1, 5.3.2.1 g	a Note when corrosive or below pH 5; fails under an owner minimum
Chlorides, sulfates, sulfides	5.3.2.1 h – j	a Note above the RP indicators; fails over owner limits
Number of anodes	7.3.5.2.1.4	fails when fewer than the current needs
Galvanic driving voltage	Table 2, 6.2.3	fails over 0.9 V
Protection criteria	8.2.2, Table 3	fails if any reading meets no criterion; a Note for on-only readings
Readings under the tank	8.3.1, 9.4.4	a Note with perimeter readings only
Rectifier check, annual survey	11.3	fails when overdue
Installation and records checklist	7.3.6, 9, 11.4	a Note until every item is ticked

How the numbers were checked

Tables 1 – 4, the criteria of 8.2.2, the current densities of 7.3.5.2.2.1 and the intervals of 11.3 were read from the 4th edition text.
The design sample is recomputed in this manual’s build; the test suite (26 checks) covers Table 3 conversions, Table 1 and Table 4 bands, Dwight’s equation, each criterion, owner limits, due dates in two time zones, save, reopen and the report.

Saving, reopening and reports

Inputs, including the survey readings, are kept on this device as you type. Calculate & Save stores the check in the cathodicProtectionCalcs collection; Show saved checks lists them for the tank.
Print / PDF report: summary, need, pad, design, the survey table and chart, due dates and the checklist.
Effective cathodic protection is one of the API 653 Table 6.1 credits in the Inspection Interval tool.

Limits and interpretations

Built on the 2014 edition; the 2024 edition has not been compared.
Internal (product-side) cathodic protection (7.4) and interference testing (Section 10) are not calculated.
The anode-bed resistance from Dwight ÷ n is a lower bound.
No over-protection limit is given by the RP; follow the coating or lining manufacturer where one applies.

Glossary

CSE	Saturated copper/copper sulfate reference electrode.
Instant-off	The potential read just after interrupting the protective current: the polarized potential without the IR drop.
Native / depolarized	The potential before current is applied, or after it has been off long enough to decay.
IR drop	The voltage drop in the soil between the reference electrode and the bottom while current flows.