Overview
The −850 mV criterion is one of the best-known cathodic protection criteria used for underground or submerged steel piping systems. In practice, people often treat it like a simple pass/fail number, but the real meaning depends on how the measurement was taken, which reference electrode was used, and whether voltage error is affecting the reading.
In plain English, this criterion is intended to help determine whether the structure has reached a level of cathodic protection that is adequate for corrosion control. It is not a magic number by itself. A reading that appears more negative than −850 mV does not automatically prove that the structure is protected if the measurement method is poor or if the reading contains substantial IR drop error.
Standard Summary
What the Standard Is Referring To
The −850 mV criterion is commonly discussed in relation to a structure-to-electrolyte potential measured with respect to a saturated copper/copper sulfate reference electrode, often shortened to CSE or Cu/CuSO4.
When technicians and engineers talk about “meeting −850,” they usually mean that the measured pipe-to-soil potential is at least −850 mV versus CSE under conditions where the reading is being interpreted as a cathodic protection criterion.
Plain-English Explanation
Corrosion on steel is an electrochemical process. Cathodic protection works by shifting the steel to a more negative electrical potential so that corrosion is reduced or controlled. The −850 mV criterion is one field rule used to judge whether that shift is sufficient.
A simple way to think about it is this: if the steel is negative enough under the right measurement conditions, then the cathodic protection system may be doing its job. But the number itself is not the whole story. The meter only reports a voltage difference between the pipe and the electrolyte at the reference electrode location. That means the reading can be influenced by electrode placement, current flow through soil, coating condition, and other field factors.
Why This Criterion Exists
The reason this criterion became so widely used is that it provides a practical field method for judging cathodic protection performance on buried or submerged metallic piping systems. It gives technicians and engineers a common reference point for evaluating whether a structure has likely achieved an adequate level of protection.
It is useful because it is simple enough to apply in the field, but it still has to be used with engineering judgment. Conditions in real systems are rarely perfect. Coatings age, current distribution is uneven, soil resistivity varies, and measurements can include errors caused by voltage gradients in the electrolyte.
Field Interpretation
In the field, the main question is not just whether the meter reads −850 mV or more negative. The real question is whether that reading represents the true polarized condition of the structure closely enough to support the criterion.
For example, an ON potential can be driven more negative by voltage drop in the soil when cathodic protection current is flowing. If that voltage drop is large, the measured value may look impressive while the actual polarized pipe potential is less negative than the reading suggests.
This is why field interpretation often leads to related discussions about:
Common Misunderstandings
That is not a safe interpretation. A very negative ON reading may be influenced by IR drop. In some cases, the structure may appear to satisfy the criterion when the true polarized potential does not.
Another common mistake is treating the criterion as if it applies identically in every environment and under every measurement setup. Real corrosion control work requires judgment, correct technique, and awareness of what the meter is actually measuring.
Example Scenario
A buried steel pipeline is tested with the cathodic protection system energized. A technician records an ON potential of −0.92 V vs CSE. At first glance, this appears to satisfy the −850 mV criterion.
However, the circuit is then interrupted and an instant OFF potential of −0.78 V vs CSE is observed. That difference suggests the original ON reading included significant IR drop. The more negative ON value alone should not be used as proof that the structure met the criterion.
This is exactly why the criterion must be interpreted in context rather than treated as a simple meter-number shortcut.
Standards Context
The concepts discussed on this page originate from AMPP SP0169 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems.
These explanations are simplified educational summaries intended to help readers understand the concepts used in cathodic protection standards. They are not a substitute for the complete standard or for professional engineering training and judgment.
The official standard can be obtained from the AMPP Knowledge Hub.