Open circuit potential

date2026-07-24tags:chem:

Explanation for forgetful Vishakh

You have a metal sample in a beaker of conductive liquid. The liquid can be salt water, acid, or body fluid. Nothing connects to the metal. No potentiostat or wires touch it. The metal just sits in the liquid.

Is anything happening? Yes. The surface hosts a crowded set of electrochemical reactions. Metal atoms dissolve. Oxygen molecules reduce. Hydrogen ions may also react, depending on the pH. Each reaction has its own preferred electrical potential. The metal can have only one potential at a time. All reactions compete for that potential. They settle at the value where total current leaving the metal equals total current entering it. Anodic reactions like dissolution push current out. Cathodic reactions like reduction pull current in. That equilibrium point is the open circuit potential, E_OC.

E_OC is not something you apply. The system decides it. Your job is to listen to what the system tells you.

What shifts E_OC

The OCP is a fingerprint of the electrode electrolyte pair at that moment. It moves when anything about that pair changes. The big four knobs are:

Electrode material: Every metal or alloy has its own characteristic reactivity. The standard electrode potential of iron differs from copper and platinum. Their OCPs in the same solution differ accordingly. This is just the Nernst equation playing out at rest. See the Electrochemistry hub for the Nernst statement and the Corrosion notes for the full derivation. The practical consequence is that E_OC is already a useful diagnostic. If you swap two nominally identical samples and their OCPs diverge after minutes of stabilization, one of them is different.

Electrolyte composition: Dissolved species identity and concentration shift the redox equilibria at the surface via Nernst. This moves E_OC even for the same electrode material. Chloride, sulfate, and nitrate interact differently with the surface and with dissolution kinetics. pH is the biggest lever. The Nernstian slope of 59 mV per pH unit for many oxide equilibria means a two pH unit change can shift E_OC by over 100 mV. That is just the thermodynamic contribution before kinetic effects.

Aeration: This catches people out most often. Dissolved oxygen adds a cathodic reaction. Oxygen reduction follows this pattern: O2 + 2H2O + 4e- forms 4OH- in neutral solution, or O2 + 4H+ + 4e- forms 2H2O in acidic solution. This reaction is unavailable in a deaerated solution. An aerated and deaerated OCP for the same sample can differ by hundreds of millivolts. The extra cathodic reaction shifts the mixed potential balance. Every serious electrochemical experiment reports whether the solution was aerated, deaerated, or sparged. Be suspicious of any OCP value that does not state this.

Passivation: A surface can spontaneously form a protective oxide or hydroxide film. E_OC tends to drift toward more noble or positive values as the film grows. A continuously rising E_OC over minutes to hours suggests film formation. A stable E_OC that suddenly drops suggests film breakdown. This applies to any material that can form a passive film. Examples include stainless steel, titanium, aluminum, and cobalt chromium alloys.

E_OC as a technique gate

Apply any perturbation only after you let the cell sit and watch E_OC. You can run a CV sweep, an EIS signal, or an LPR ramp. ASTM G 59 requires this waiting step. The deeper reason is more fundamental. Running a perturbation experiment on a drifting baseline wastes time. The signal you measure will be mostly drift. The interpretation will be wrong.

How long to wait depends on the system. A freshly polished electrode in a well aerated simple electrolyte may stabilize in 10 to 15 minutes. A rough surface with a slow forming passive film may take hours. A concrete embedded rebar may take days. The criterion is not a fixed time. It is a fixed drift rate. A typical target is plus or minus 5 mV over 5 to 10 minutes for most lab experiments. The acceptable window depends on the measurement amplitude. An LPR sweep applies plus or minus 10 mV. A drift of plus or minus 5 mV is half your signal. Wait longer in that case.

E_OC is also a measurement modality in its own right. It is not just a gate. Tracking E_OC continuously under mechanical or flow disturbance reveals how repeated local damage to a passive film affects the resting potential. Cavitation, erosion, and flow impingement all cause this damage. The pattern of a sudden drop followed by gradual recovery is general. Erosion corrosion and tribocorrosion studies all use it. The drop removes the film. The recovery signals repassivation. This pattern holds for Stellite, 316L stainless steel, and titanium alloys alike.

Mixed potential origin

E_OC is a mixed potential. Wagner and Traud formalized this concept in 1938. A corroding surface hosts at least two independent partial reactions. The reactions run simultaneously at the same interface. E_OC is the potential at which total anodic current equals total cathodic current. This is not a hand wavy approximation. It is the observable consequence of charge conservation on a metal that cannot accumulate net charge.

The full treatment includes Wagner Traud theory. It shows how two Tafel lines on an Evans diagram intersect at E_corr. It explains why E_OC approximately equals E_corr for most freely corroding systems. That treatment lives on the Corrosion page. It is the same conceptual engine that drives LPR and Tafel extrapolation. What matters here is that E_OC is physically meaningful. It emerges from real electrochemistry. It does not come from a poorly zeroed instrument.

Worked example: Stellite 6, cast vs HIPed, in 3.5% NaCl

This is one worked example of OCP behavior. The same measurement logic applies to any electrode electrolyte pair. I include it because it illustrates several general patterns in a concrete published study: the drift toward noble potentials, sensitivity to processing route, and an open question at the end.

¹1Unresolved citation key: rosalbinoCorrosionBehaviourAssessment2013 describes the following experiment. The working electrolyte was naturally aerated 3.5% NaCl at pH 6.5. The temperature was 25 plus or minus 1 C. The solution was quiescent. Freshly polished Stellite 6 samples were immersed. E_OC was monitored continuously for 168 hours.

The cast alloy started at about minus 325 mV vs SCE immediately after immersion. It drifted to minus 180 mV over 24 hours. It crept slowly to minus 165 mV at 168 hours. That is a total noble shift of about 160 mV over the week.

The HIPed alloy, which stands for hot isostatically pressed, started more noble at minus 240 mV. It reached minus 110 mV after 24 hours. It stabilized at roughly minus 100 mV by 168 hours. The HIPed alloy started higher, stabilized faster, and ended 65 mV more noble than the cast alloy.

The drift toward less negative potentials in both cases indicates spontaneous passivation in the chloride solution. The surface forms an oxide film. The film progressively reduces the anodic dissolution rate and forces the mixed potential higher. The HIPed alloy shows a greater tendency to form a spontaneous oxide. Its film provides better corrosion protection in this solution.

Open question: why does processing route affect passivation?

I do not have a settled answer. Neither does the literature, beyond appealing to microstructure. Both cast and HIPed Stellite 6 have the same nominal composition. It is roughly Co 28Cr 4W 1.1C with some Ni, Fe, Si, Mo, and Mn. The difference is purely in processing. The likely explanation applies generally to any two differently processed samples of the same alloy:

Porosity: Cast materials typically contain microporosity from solidification shrinkage. HIPing closes that porosity. A porous surface has more nucleation sites for oxide films. It also has more defect sites where the film is thinner, weaker, or discontinuous. Which effect dominates depends on pore size, distribution, and film growth kinetics.
Inclusion density: Non metallic inclusions like carbides, oxides, and sulfides act as local cathodes or weak points in the passive film. HIPing can redistribute or break up inclusion clusters. It can also create new ones if the encapsulation material reacts during processing.
Grain size and grain boundary character: Finer grains mean more grain boundary length per unit area. Grain boundaries are generally more reactive. They can be sites of enhanced passivation from more rapid oxide nucleation or enhanced dissolution from preferential boundary attack. The material and environment decide which.

The general lesson: if you measure significantly different OCP behavior for two samples of the same nominal composition, examine the processing history before examining the chemistry. The composition tells you what is possible. The microstructure tells you what actually happens.

Practical notes for measuring E_OC

Stabilization is not optional. A drifting E_OC means the system is not at steady state. A perturbation experiment on a drifting baseline measures the drift, not the material. This is a requirement for physical interpretability.
Report the reference electrode. An OCP value without the reference electrode stated is meaningless. A value of minus 200 mV could be vs SHE, SCE, Ag/AgCl, or a pseudo reference. Those differ by hundreds of millivolts. Always write minus 200 mV vs SCE or equivalent.
Report aeration status. State whether the solution was aerated, deaerated, or sparged. Identify the gas and note the flow rate. These matter as much as the solution composition.
Track the trend, not just the final value. A single OCP reading shows where the potential is now. A time series shows whether the surface passivates, activates, or stays stable. The drift rate and direction are often more informative than the absolute number.
Three replicates minimum. OCP is a mixed potential. Mixed potentials are sensitive to minor surface variations. A single measurement is a hint. Three measurements that agree are a result. The standard advice for corrosion measurements applies here too. Fifteen percent relative standard deviation is considered excellent.