The Exam Preparation Guide lists impressed current, the galvanic series, fundamentals, and SP0169. That is the whole domain, and it is scored as applied field judgment rather than definitions. You are given ON, instant-off, and depolarized numbers and asked which criterion is actually met. You are given a shunt in ohms or in mV/A and asked for amperes. You are given an anode alloy and asked for driving voltage.
Read Peabody, 3rd edition, on criteria, current requirement, anode resistance, attenuation, and interference, and the CP Tester manual on shunts, interruption, and reference electrodes. SP0169 Section 6 is the criteria chapter. Section 4 covers isolation, continuity, and test stations, which also feed Domain 7.
One distinction the old sample exams blur. NACE SP0169, the standard named in the guide, uses three criteria for steel and cast iron: a −850 mV CSE potential with the current applied only if IR drop is considered, a −850 mV polarized (instant-off) potential, and 100 mV of cathodic polarization by formation or decay. The old 300 mV shift-with-current-applied criterion was removed from SP0169. It is still written in 49 CFR 192 Appendix D, paragraph I.A(2). If the stem says SP0169, do not select 300 mV. If the stem says Appendix D, that paragraph is still there. The practice exam asks about SP0169.
The 100 mV criterion is the one structures fail the −850 mV tests and still pass. It is generally not used where stress corrosion cracking, sulfate-reducing bacteria, dissimilar metals, or elevated temperature are present. SP0169 Section 6.3 is the place that caution lives.
| Quantity | Value to know cold |
|---|
| Equivalence at the −850 mV CSE protection level | −850 mV CSE ≈ −800 mV Ag/AgCl (seawater) ≈ −780 mV SCE ≈ +250 mV vs zinc |
| Open-circuit potentials, CSE | High-potential Mg −1.75 V; standard Mg −1.55 V; zinc and typical Al alloys about −1.10 V |
| Consumption | Steel ~20 lb/A-yr; Mg 8.8 lb/A-yr theoretical, ~50% efficient; Zn ~90%; Al ~85–95%; graphite ~2 lb; HSCI ~0.75–1 lb; MMO in mg |
| Anode life | L = (W × efficiency × utilization) / (consumption rate × current in amperes). Utilization is often 0.85 because the core connection fails before the mass is gone. |
| Current density, order of magnitude | Bare steel in soil about 1–3 mA/ft²; well-coated steel in soil about 0.01–0.05 mA/ft²; bare steel in flowing seawater several mA/ft² higher |
| Rectifier ripple | Half-wave ~121%; single-phase full-wave ~48%; three-phase bridge ~4%. Efficiency climbs in the same order, roughly 40%, 60–70%, 85%+. |
Q1 · Cathodic protection · single answer
Reading a survey against the three SP0169 criteria
ON −0.920 V, instant-off −0.760 V, depolarized −0.615 V, all CSE.
What to learn
Three readings exist because they answer three different questions. ON minus instant-off is the IR drop in the soil and metallic paths while current is flowing. Here that drop is 160 mV, so the structure is not at −920 mV. Instant-off is the polarized potential, the potential across the structure-to-electrolyte interface immediately after the ohmic component disappears and before the structure has had time to depolarize. −760 mV fails the −850 mV polarized criterion. Depolarized minus instant-off, taken as a decay, is the cathodic polarization: 145 mV, which clears 100 mV.
People declare a line protected from the ON reading because −920 mV “looks like” −850 mV. SP0169 allows a current-applied reading only when voltage drops other than the interface drop are considered. A 160 mV IR error means they were not. The same data set can fail both −850 mV tests and pass 100 mV. That is normal, not a trick, and it is why the 100 mV criterion became widely used after the standard started insisting that IR drop be considered.
How the exam uses it
Expect the ON number to be the most negative and the most tempting. Compute both differences before choosing. “No criterion is met” is the answer only after the decay is also short of 100 mV.
SP0169 Section 6.2. Also Peabody on interpretation of potential measurements.