05 · Transport of Electrical Charge in Electrolytes

Welcome to CoroZone. CoroZone is where corrosion expertise meets AI, so the hard questions in our field get faster, better answers. And yes, the voices you're hearing are AI too. Today's podcast is from the lecture "Transport of Electrical Charge in Electrolites" in our cathodic protection course. Let's dive in! So far in this course, we've talked about cathodic protection as something that happens at the metal surface. Today, I want to follow the current the rest of the way around the loop, through the soil or the water. Where does it actually go once it leaves the metal? That's exactly the right question, and it's the whole subject of this lecture. Cathodic protection works by pushing a current from an anode onto the structure, to shift its potential negative enough to stop the metal dissolving. But that current can't stay inside the wires. At some point, it has to leave the metal, travel through the environment, and come back to the structure. And that environment is the electrolyte. Right, the electrolyte. That's the soil, the seawater, fresh water, concrete pore water, any ionically conducting phase around the structure. And understanding how current moves through it isn't a side detail. It decides how much voltage your power source needs, how evenly the current spreads, what potential gradients form around the structure, and why a potential reading taken while current is flowing needs correcting before you can trust it. Okay, so let's start at the most basic level. Current in a metal and current in an electrolyte. Are those the same thing? They are fundamentally different, and that difference matters. In the metal parts, the structure, the cables, the anodes, charge is carried by free electrons moving through the metallic lattice. That's electronic conduction, and it's extremely fast and efficient. The resistance of the metal over engineering distances is typically millions or less. And in the electrolyte? In the electrolyte, it's a completely different mechanism, ionic conduction. The charge carriers are ions, dissolved positive ions, the cations, like sodium, calcium, magnesium, and hydrogen, and dissolved negative ions, the anions, like chloride, sulfate, bicarbonate, and hydroxide. When you apply an electric field, the cations drift toward the more negative region, and the anions drift toward the more positive region. Both movements add up to the same net current direction, and together, they carry the ionic current between anode and cathode. You said the difference matters in practice. Where does it bite? Two places. First, ionic conduction is far slower and less efficient than electronic conduction. Solution resistances in CP systems run from fractions of an ohm offshore in seawater to hundreds or thousands of ohms in resistive soil, vastly higher than the metal parts of the circuit. Second, ionic conduction has to be converted to and from electronic current at the two metal electrolyte interfaces. Converted how? At the anode, oxidation turns electronic current into ionic current, releasing metal ions or oxidizing water. At the cathode, reduction turns ionic current back into electronic current, consuming oxygen or protons. So the electrolyte does more than carry charge. It's the medium where the essential electrochemical reactions actually happen. You mentioned the specific ions depend on the environment. Does that change much from one setting to another? Quite a lot. Seawater is dominated by sodium chloride, with magnesium, calcium, sulfate, and potassium adding in. It's highly conductive, around five semen per meter, so CP current spreads easily over large areas at modest voltages. Freshwater has far lower dissolved ion concentrations, so it's much more resistive and variable. Its conductivity spans three orders of magnitude, depending on the geology and the catchment chemistry. And soil? Soil conducts through its pore water, so it depends on moisture content dissolved salt and temperature. Coastal or marine adjacent soils are far more conductive than inland well-drained soils. And concrete pore water is alkaline, pH 12 to 13, from dissolved calcium, potassium, and sodium hydroxide. It gives moderate conductivity, but that drops substantially as the concrete dries out or carbonates. Let's put numbers on how well it conducts. You keep using conductivity and resistivity. Can we pin those down? Yes. Conductivity, written sigma, in semen's per meter, measures how readily an electrolyte carries ionic current. At the microscopic level, it's ohms law. The local current density J equals sigma times the local electric field E. So a high conductivity means you can maintain the same current density with a smaller electric field, less voltage per unit distance. And resistivity is just the flip side? Exactly. Resistivity, row, in on meters, is one over conductivity. It's often more intuitive for CP work. Higher resistivity means more resistance to current flow, more voltage needed, and less current reaching the distant parts of a structure. Is there a simple way to turn resistivity into an actual resistance? For a uniform column of electrolyte, yes. The bulk resistance R equals row times the length L divided by the cross-sectional area A. R equals row L over A. It looks almost too simple, but it captures the two geometric levers in CP design. Longer current paths and smaller cross sections push resistance up. Wider or shorter paths bring it down. It's also why large structures, offshore platforms, long pipelines, need careful attention to the geometry of the current path. You said a practitioner needs a feel for the numbers. What's the range out in the field? It's wide. Sea water is about 0.2 ohm meters. Brackish water runs from half an ohm meter up to about five. Hard freshwater is 10 to 100. Soft freshwater is 100 to 1000. High conductivity soil is around 2 to 10 ohm meters, average soil 10 to 100, and resistive soil or rock 100 up past 10,000. Saturated concrete is 50 to 200, and dry concrete 1000 to 10,000. That's an enormous spread. What does it mean for a system that's designed for one and dropped into another? It means a CP system that works easily in sea water can be completely inadequate, as designed, in resistive inland soil. To put a number on it, the same current needs about 25 times more driving voltage in a soil at 50 ohm meters than in sea water at 0.2. Does temperature move these values around? It does. Ionic conductivity increases with temperature, because warmer water is less viscous, and the ions can migrate more freely. The rule of thumb for aqueous electrolytes near ambient is roughly a 2 to 3% increase in conductivity per degree Celsius. What does that add up to over a realistic temperature swing? Cold sea water at 0 degrees has roughly half the conductivity of warm sea water at 25 degrees. Seasonal swings in soil temperature shift resistivity measurably, too. So you size the system for the worst case, which usually means the most resistive environment, the coldest and driest. If resistivity drives so much, how do you actually measure it on site? The standard technique is the Wiener 4-pin method. You drive four electrodes into the ground at equal spacing, call that spacing A in meters, pass a small AC current between the two outer electrodes, and measure the voltage difference between the two inner ones. The apparent resistivity row is 2 pi times A times the ratio of voltage to current. Row equals 2 pi A times V over i. And by changing the spacing, you change what you're seeing? Right. Variing the electrode spacing builds up a depth-averaged resistivity profile, so you learn how the soil is layered and how it changes with depth. That measurement is a fundamental input to CP design. It directly sets the rectifier voltage you'll need and the ground-bed resistance calculations. Let's move to where the current actually goes. In an ordinary circuit, it's stocking a wire, but in soil or water, it isn't, is it? No, and that's the key difference. Once current leaves the metal and enters the electrolyte, it can spread through a three-dimensional volume of conducting medium. The pattern of those current paths decides which parts of the structure get protected and how uniform that protection is. Is there a rule for which way it goes? A simple one. Current follows the path of least resistance. In a uniform electrolyte, it spreads radially from the anode, taking the shortest available routes to the structure. In non-uniform ground, where resistivity varies with location, depth, moisture, or rock type, the current concentrates in the low resistivity zones and gets deflected around the high resistivity ones. Picture a buried anode and a pipeline. What does the flow look like? For a cylindrical anode in soil, current leaves the anode surface radially and fans out in every direction. As it spreads further out, it meets ever larger cross-sectional areas, so the current density drops. Eventually, it reaches the pipeline, which sits at a lower potential and converges onto the pipe surface to supply the cathodic current that protects it. Earlier, you hinted that most of the resistance lives near the anode. Can you unpack that? That's the idea of spreading resistance, also called anode to earth resistance. It's the resistance of current spreading away from the anode into the electrolyte. For an idealized hemispherical anode of radius r in a semi-infinite medium, the spreading resistance is row divided by 2 pi r, r equals row over 2 pi r. And the punchline of that formula? The resistance, and therefore the voltage drop, is heavily concentrated near the anode. Most of the resistive drop happens within a few anode radii of the surface. Beyond 5 to 10 anode radii, each extra meter of electrolyte adds almost nothing to the resistance. What does that change about how you build a ground bed? Several practical things, enlarging the anode diameter dramatically cuts spreading resistance. Double the radius, and you have the resistance. Using a low resistivity backfill, like carbonaceous coke breeze, around a ground bed electrode, reduces the near electrode resistance significantly. The far-field soil between the anode and the structure matters less to the total than that near anode zone. And the potential gradients are steepest right near the anode, not spread evenly along the path. Real systems aren't all buried pipelines, though. Does the geometry change the picture a lot? It does. Let's run through a few. Offshore in seawater, the low resistivity, around 0.2 ohm meters, means small anodes can drive current over large areas. There, the limiting factor is usually geometric. Crevises, internal compartments, and shielded surfaces make it too little current. Not because of electrolyte resistance, but because the current path is physically blocked. And a buried coated pipeline? Most of the pipe is coated, so CP current concentrates at the coating holidays, the defects. In resistive soil, the IR drop from anode to holiday is significant, and the path length is large, meters to tens of meters, which amplifies how much the soil resistivity drives up the required voltage. What about steel in concrete? Concrete has moderate resistivity, and you can only reach the reinforcement through the concrete cover. As the concrete dries or carbonates, its conductivity falls further, so pushing the required current through gets progressively harder, sometimes needing 5 to 50 volts for an impressed current system. You mentioned shielding. Is that just high resistance or something else? Something else, and it's important. A shielded zone is where the path from anode to structure is blocked or severely restricted by a non-conducting barrier. A rock, an insulating coating on a foreign structure, an isolated section of piping. That blocked zone can receive zero CP current, no matter how much you apply everywhere else. Let's bring it back to the power source. How do all these resistances show up as a voltage demand? Through the IR drop, when a CP current I flows through a segment of electrolyte with resistance R, a voltage drop develops across it. Delta V equals I times R. That's the omic drop, and it appears in the total voltage budget of the circuit as a demand on the driving source. Whether that source is a sacrificial anode's electrochemical driving voltage or a rectifier's output. So what does the total driving voltage have to cover? Four things. One, the potential shift needed at the structure surface to reach the protection level. Two, the IR drop through the electrolyte from anode to structure. Three, the IR drop through the connecting cables and bonds, and four, the polarization losses at the anode. Which of those tends to dominate? It depends on the environment. In high resistivity soils, the electrolyte IR drop can dominate the whole budget. Rectifiers may need to deliver 10 to 50 volts or more, just to push modest protective currents. In low resistivity seawater, the electrolyte IR drop may be negligible, and the dominant requirement is the anode driving voltage. The difference between the anode's open circuit potential and the target structure potential. You keep coming back to potential gradients. Why do they matter so much? Because wherever current flows in a resistive electrolyte, a potential gradient must exist. It's the driving force for the current. The gradient is steepest where current density is highest, near anodes, near coating holidays, and it falls off with distance. And that has several real consequences. Start with measurement. The first is a measurement artifact, the IR error. A reference electrode placed in the soil between the anode and the structure picks up not just the structure's true polarized potential, but also a contribution from the potential gradient in the soil. That makes the measured on potential systematically more negative than the true steel to electrolyte potential. You have to remove that IR error by IR correction or an off potential measurement before comparing readings against protection criteria. The course comes back to that correction in detail later. And the second consequence? Interference with neighboring structures. The potential gradient around a CP anode extends over a considerable volume of electrolyte. Other metal structures within that field get shifted. One's closer to the anode are driven more positive, anodic. And one's further out, but still in the cathodic gradient may pick up unintended CP current. That interference is the basis of stray current corrosion from impressed current systems, which the course covers separately. Is there an upside to these gradients? There is, there are the physical basis of key survey methods, DC voltage gradient surveys, DCVG, measure the gradients directly to locate coding holidays on pipelines and close interval potential surveys. C.I.P.S. map the on and off potentials at regular intervals to check whether the structure is polarized to the required level along its whole length. Before we close, can we make the IR drop concrete with a number? Let's do the lectures worked example. A buried, impressed current anode supplies 50 milliamps to a holiday in a pipeline coding, about 3 meters away, in soil with a measured resistivity of 50 ohm meters. If you crudely approximate the path as a cylinder, 3 meters long, with an effective cross section of 0.05 square meters, then R equals rho L over A, 50 times 3 over 0.05, which is 3000 ohms. The IR drop is I times R, 50 milliamps times 3000 ohms. That's 150 volts. That's huge. Is that realistic? No, and that's the point of the exercise. It's clearly an overestimate because the simple cylinder model ignores the spreading at both the anode and the holiday. If instead you use the hemispherical spreading resistance formula for an anode of radius 0.05 meters, you get rho over 2 pi R, about 160 ohms, and the IR drop for 50 milliamps comes out around 8 volts. The exact answer depends sensitively on geometry, but two lessons hold. The cylinder formula overestimates, and even 8 volts of IR drop is significant. You have to account for it both in sizing the rectifier and in interpreting your potential measurements. So if we step back, what's the one thing to hold on to from this lecture? That the electrolyte is not a passive conduit. It's an active part of the CP circuit. It shapes the current distribution, it consumes driving voltage, and it creates potential gradients throughout the environment. The charge transport in it is ionic and far less efficient than in metal. Conductivity and resistivity measure that efficiency, bulk resistance scales as rho L over A with most of it piling up near the anode. Current takes all the three-dimensional paths that can find, the IR drop eats your voltage budget and distorts your readings, and the potential gradients are unavoidable wherever current flows through resistive ground. That reframes the whole environment as a circuit element, not just the stuff the pipe sits in. Thanks for walking it through. My pleasure. Keep the electrolyte in mind, and the rest of CP design makes a lot more sense. That's our look at transport of electrical charge in electrolytes, hopefully a bit clearer than when we started. You'll find this and more at coreozone.com. But the real invitation is this, bring us your corrosion challenge. Whether it's an industrial headache or a research problem, come and tell us about it. See you next time on Coreozone.