Faraday's Law and Corrosion Rates
Current into pounds, pounds into years - the arithmetic behind anode life, corrosion rates, and the expected-life line on every galvanic quote.
You’re back at the two tanks.
Last module, they were a problem to solve — the two-tank problem in EC-011’s Apply lesson, if you need the refresher. Two bare-steel water tanks, same design, different dirt. The hilltop sits in dry silty loam around 8,000 Ω·cm, and it got magnesium — the strong pusher, roughly six-tenths of a volt of drive, because high-resistance ground swallows gentle voltages. The creek bottom sits in wet gray clay around 400 Ω·cm, and it got zinc — a quarter-volt of steady push, plenty for soil that barely fights back. You made the call, you gave the reasoning, and it held up.
Then the operator asked the question that ends most tailgate meetings: “So how many years does each anode last?”
And we told the truth: that’s arithmetic we haven’t done yet — but it is arithmetic. Not judgment, not experience. Arithmetic. Today we do it — and by the end of this lesson, the operator’s question is one you can answer on the tailgate with a phone calculator and a straight face.
Why this module sits where it does
You’ve already held every piece of this lesson in your hands. Two modules ago you counted the electrons — two per iron atom, no exceptions. Last module you ranked the metals and sized the push; you even watched a spent 17-pound bar come out of the ground along the way. This is where the counting and the ranking turn into pounds and years — the exchange rate of the whole trade.
From counting electrons to weighing metal
You already have the bookkeeping. Every iron atom that leaves the wall leaves as Fe²⁺, and every one of them hands off exactly two electrons. Two electrons per atom — no exceptions, no partial credit. At the time, that looked like chemistry trivia. It’s actually the foundation of corrosion accounting, because if every atom ships with a fixed number of electrons, then counting charge is counting metal.
That’s Faraday’s Law. The mass of metal consumed at an anode is directly proportional to the electric charge that passed. Not roughly proportional. Proportional the way a scale ticket is proportional to what’s on the truck.
Here it is in its chemistry clothes — the only time in this module you’ll see it fully dressed:
Faraday’s constant is the same number for every metal, every couple, every soil, every job. It never changes and you never solve for it. It’s the posted conversion rate between electrons and amp-seconds, and the rest of this lesson is about spending it.
Here’s the good news. You’ll run that full equation exactly once, in the next section — and then never again in this module. The industry ran it decades ago for every metal a corrosion crew will ever meet and wrote the answers down as constants. We work the equation once so the constants aren’t magic. After that, we use the constants like everyone else in this trade does.
The twenty-pound year
Worked example — one amp-year on steel
Setup. A bare steel structure has one amp of corrosion current leaving it — continuously, around the clock — for one year. How much steel leaves with it?
Answer. Just over twenty pounds of steel, gone. That’s the one full run of the equation — from here forward, the constants carry the load.
One amp, one year, twenty pounds of steel. That’s the exchange rate — the everyday currency of corrosion work.
Sit with that number. A single amp of stray current leaving a pipeline at one spot is twenty pounds of pipe wall, every year, until somebody finds it. Half an amp is ten pounds. A hundred milliamps is two pounds — which sounds small until you remember it doesn’t take pounds to make a leak.
Because here’s what the law doesn’t tell you: where the metal comes from. Faraday counts the pounds; the current distribution picks the address. Spread one amp evenly across an entire bare tank bottom and it’s a haze of surface rust nobody will ever measure. Concentrate that same amp at three coating holidays the size of quarters, and the law delivers the same twenty pounds to three spots — and now you’re cutting out pipe. The law says how much. The circuit says where.
Run the equation for other metals and the answers land close enough to remember as a short list — the consumption rates, in pounds per amp-year, at 100% theoretical spending:
The field form of the law is just this list with the algebra pre-chewed: pounds = rate × amps × years. When a report or a design sheet writes W = K × I × T, that K is a row from the table above. Nothing new is happening — someone already did the Faraday math and folded it into one constant per metal.
Amp-hours per pound: the anode’s fuel gauge
For sacrificial anodes, the same constant is friendlier flipped upside down. Instead of asking how many pounds an amp consumes in a year, ask how many amp-hours of charge live inside one pound of metal. That’s a capacity — a battery rating for a bar of metal — and it’s how anode suppliers publish their numbers.
Theoretical capacity comes straight off Faraday’s Law: about 1,000 amp-hours per pound for magnesium, about 372 for zinc.
But a buried anode doesn’t spend every electron on your structure. Some of its surface runs little corrosion cells of its own — metal dissolving locally, electrons round-tripping right there on the bar, doing nothing for the tank at the end of the wire. The trade calls the honest fraction current efficiency, and the difference between theoretical and actual is the tax the anode pays for being active enough to do the job at all.
Magnesium gives you half of what the chemistry promises. Zinc gives you ninety cents on the dollar. Neither number is a defect — it’s a property of each metal, and it’s already folded into every honest anode-life estimate in the industry.
Quote anode life off actual capacity, never theoretical. An estimate built on theoretical magnesium is wrong by a factor of two — and wrong in the direction that gets remembered, because the system dies ten years before the paperwork said it would.
Back at the hilltop: the arithmetic, start to finish
Now the operator gets an answer.
The hilltop tank’s magnesium string is built from standard 17-pound bars — the same size bar you’ve already watched come out of the ground, eaten down to a stub around its steel core. At the test station, put your multimeter across the bar’s shunt and let Ohm’s Law do the rest: 5.0 mV across a 0.1-Ω shunt is 50 milliamps, and the polarity tells you which way it’s flowing. (An amp clamp gets you the same number when there’s one on the truck.) Fifty milliamps is a modest output, and modest is the point: that dry 8,000 Ω·cm loam is exactly why the strong pusher got this job, and even magnesium only moves a trickle through it.
Worked example — the operator’s answer
Setup. One 17-lb standard-alloy magnesium anode, actual capacity 500 A-hr/lb, measured output 50 mA. How many years?
Answer. On the tailgate, out loud: “Call it nineteen years, give or take how wet the seasons run.”
Three honest footnotes come with that answer. First — run the same chain on theoretical capacity and you’d promise 39 years. That’s the efficiency tax in one comparison: the difference between an estimate and an apology is one factor of two, applied up front.
Second — the bar doesn’t die on schedule at 19.4 years. Output wanders with soil moisture and season; the draw you measure in April isn’t the draw in August. The number is a design estimate, not an expiration date. It tells you which decade to plan the replacement in, and that’s its job.
Third — crews don’t wait for zero. As an anode gets down toward its last stretch — a common planning figure is around 85% consumed — the remaining stub can’t hold dependable output, and replacement gets scheduled. Design sheets carry that as a utilization factor. For now it’s enough to recognize the idea: the last bite of the bar never gets spent.
And the creek-bottom zinc? Same chain, different numbers — 335 amp-hours per pound in the bank, and a bigger draw, because wet clay at 400 Ω·cm barely resists and zinc runs steadier and harder there. We could work it right here, but you’d learn more running it yourself. It’s waiting in Lesson 3 with fresh numbers — and this time, you’re the one at the tailgate.
Running the law in reverse: rates and current density
Everything so far ran forward: current in, pounds out. The law runs backward just as well — grams of metal lost convert to the current that must have carried them out. The industry uses both directions, and which one you meet depends on which side of the pipe wall you work.
The backward direction is a lab move. Weigh a coupon before and after exposure, walk the mass loss through density, area, and time, and out comes a corrosion rate in mils per year. That workflow belongs to internal corrosion work, where coupons ride inside the line and come out on a schedule with retrieval tools. If your career runs down the IC path, you’ll meet it properly there.
External corrosion coupons live a different life. They go in the ground beside the pipe, wired to it through the test station — and they stay buried. Nobody’s mailing them to a lab. What they give you, they give you alive: a bare piece of steel with one property no real holiday ever has — a known surface area. Read the current on the coupon’s shunt, divide by that area, and you’re holding current density — the number CP design speaks in — measured, not assumed. The same coupon gives you honest potentials too, one reason we’ve called coupons the closest thing to asking the steel directly.
Now the block above has a job. Say the coupon is 10 square centimeters and its shunt works out to 22 microamps: that’s 2.2 µA/cm² — call it a mil a year in steel language. On a freely corroding bare spot, that’s the wall it would lose. On a protected coupon, that’s the bill your anodes are paying on its behalf. Same number, read as a threat or as a receipt — and Faraday’s Law is what makes the translation honest.
The full coupon procedures — retrieval tools, lab reports, pitting rates — live where they belong, later in the catalog and over on the internal side. What matters now is smaller and sturdier: a corrosion rate is a current, a current density is a rate-in-waiting, and every mil of wall that ever leaves is carried out by electrons somebody could have metered.
Why cathodic protection works, by the pound
Which brings us to the quiet idea underneath this whole trade.
If a structure’s corrosion cells are drawing current off it — pounds leaving on schedule, per the exchange rate — then supplying that current from somewhere else means the structure stops paying. The pounds don’t vanish. They move. Faraday’s Law keeps the books balanced either way.
Cathodic protection doesn’t cancel corrosion. It redirects the bill — to metal you chose, priced, and planned to replace.
With galvanic anodes, the payer is magnesium or zinc, budgeted in advance — that’s literally what the “expected life” line on a galvanic CP quote is: Faraday’s Law, worked out the way we just did, priced by the pound. With impressed current systems, a rectifier drives the circuit and the anodes are materials picked because they spend almost nothing — high-silicon cast iron, mixed metal oxide, graphite — each with its own small published consumption rate. Same law, different bill structure. The design details of both live in later modules; the accounting principle is yours today.
And that principle is why this module matters more than its four formula blocks suggest. Every galvanic-anode quote our office writes ends in a Faraday line item — anode count, anode size, expected life. Every internal-corrosion coupon report our lab produces converts mass to rate with the same law. The tech who can run the chain reads those documents instead of taking them on faith — checks them, catches the theoretical-capacity mistake before it ships, answers the operator without calling the office. That’s the difference between running the survey and understanding the system. It’s the first step from technician thinking toward designer thinking, and it costs four lines of arithmetic.
The road ahead — you can answer the operator
EC-012 is module 5 of 7 in the Electrochemistry & the Galvanic Series series. You came in able to call the anode in any couple; you leave able to bill it — current into pounds, pounds into years, forward for anode life and backward for corrosion rates. Every number today leaned on measured potentials, and EC-013 covers the thing those measurements trust: the reference electrodes — why a copper-sulfate cell holds still while the world shifts around it, and why the same protected pipe can read two completely different numbers against two different cells. Then EC-014 closes the set with polarization: what potentials actually do while current is flowing.
Finish all seven modules in this set and the certificate for Electrochemistry & the Galvanic Series is yours. Two to go after today.
Key takeaways
- Faraday’s Law is exact bookkeeping — metal consumed is directly proportional to charge passed: m = ( I × t × M ) ÷ ( n × F ), with F = 96,485 coulombs per mole of electrons. You ran it once; the industry’s constants carry it from there.
- One amp-year ≈ 20 pounds of steel — and the four rates worth knowing cold, in lb per amp-year theoretical: steel ≈ 20.1, zinc ≈ 23.6, magnesium ≈ 8.8, aluminum ≈ 6.5.
- The law counts pounds; the circuit picks the address — spread, it’s a haze of rust; concentrated at a few holidays, it’s a leak.
- Anode capacity is the same constant flipped — magnesium ≈ 500 A-hr/lb actual (about 50% efficient), zinc ≈ 335 (about 90%). Quote life off actual, never theoretical — theoretical magnesium overpromises by 2×.
- The chain — pounds × capacity = the bank · divide by the draw = hours · divide by 8,760 = years. One 17-lb magnesium bar at 50 mA: 8,500 A-hr → about 19.4 years.
- Anodes retire before zero — around 85% consumed the stub can’t hold dependable output; design sheets call it utilization.
- The law runs both ways — on iron, 1 mpy ≡ 2.17 × 10⁻⁶ A/cm² ≡ about 2 mA/ft². An external coupon’s known area turns a shunt reading into measured current density; mass-loss lab reports live on the internal side.
- CP moves the bill, it doesn’t erase it — the “expected life” line on a galvanic quote is this module’s arithmetic, priced by the pound.
References & further reading
- Evaluating Galvanic Anode Beds — Field Notes from RCS article on reading the health and remaining life of an aging galvanic system in the field.
- External Corrosion Coupons — Field Notes from RCS article on what buried coupons measure: honest potentials and current density over a known area.
- Electrochemical Basics Part 2 — Field Notes from RCS companion on current flow in corrosion cells and what it does to the metal.
- Corrosion Basics: An Introduction — foundational text on the electrochemistry of corrosion, including Faraday’s Law and its applications.
- Peabody’s Control of Pipeline Corrosion — long-standing field reference on galvanic-anode characteristics, capacities, and the rate-to-current equivalence.
- Cathodic Protection Training Materials — industry credential materials covering consumption rates, current efficiency, and anode-life estimating at the CP-technician level.
- AUCSC Short Course Materials — pipeline corrosion technician training curriculum, including the consumption-rate tables used in this module.
- NACE SP0169 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems.
Listen — narrated walkthrough
Faraday’s Law and Corrosion Rates
Same scope as the read — the law that turns charge into metal, the twenty-pound year, capacity and the efficiency tax, the full chain from a shunt reading to an anode-life estimate, and the law run in reverse through an external coupon’s known area — walked through with the two tanks as the anchor, from the operator’s question to the answer you can give on the tailgate.
Narrated by Mike Roberts · ~18 min
Listen on the drive in or between test stations. Come back for the deck or the worked problems whenever you want.
Once you’ve worked the audio or the deck, the Apply lesson runs three field problems — the creek-bottom zinc finally settles up, a records review ends in the sentence this set promised you’d get to say, and one meter reading writes two bills at a freeloading tie-in — and the quiz after that closes out the module. From there the set turns to the measuring sticks themselves: the reference electrodes every number in this module leaned on. The Electrochemistry & the Galvanic Series certificate posts to your profile when you complete the full set (EC-008 through EC-014).
Apply · three problems
Run the chain
Three problems, one chain — run forward, run from the middle, then run in both directions at once. In the first, the creek-bottom tank from the Read lesson finally settles up. In the second, a records review ends in the exact sentence this set once promised you’d be able to say. In the third, one meter reading writes two bills, and you get to read both.
Work each one on paper (or in your head on the tailgate) before you open the reveal. The reveals walk the reasoning step by step — the point isn’t the answer, it’s the path.
The creek bottom settles up
0.1 Ω shunt in the lead. Kneel at the test head, put the meter across the first bar’s shunt: 12.0 mV.
A — What’s this bar’s output, and which way is the current headed?
Show solution — part A
Ohm’s Law, straight off the shunt:
And the polarity of that drop is information, not decoration: it tells you the current is flowing from the bar to the tank — the anode is feeding the structure, not the other way around. If that sign ever flips on you, stop trusting the hookup and find out why before you write anything down.
B — Run the chain: how many years does this bar have?
Show solution — part B
Pounds to the bank, bank to hours, hours to years:
Tailgate answer: “Call it nine and a half — ten if the summers run dry.”
Put it next to the hilltop and the pattern shows: the magnesium up there promised about nineteen years on 50 mA. This bar carries nearly two and a half times the current, because wet clay barely resists. Wet ground works its anodes harder — more metal per year is the price of easy soil.
C — The operator remembers the hilltop’s nineteen years: “Should we have put magnesium down here instead?”
Show solution — part C
Run it before you answer. A hilltop-size 17-lb magnesium bar asked to carry this same 120 mA:
Shorter, not longer — the smaller bank empties faster at the bigger draw. And in practice it’s worse than that: magnesium’s stronger push in soil this easy wouldn’t hold 120 mA — it would drive more, spend itself faster still, and feed the tank current it doesn’t need.
Zinc was the right call twice over: the gentle pusher matched to easy dirt when the string was chosen, and now the arithmetic that proves it. Same verdict, two kinds of evidence.
Wet ground works its anodes harder. The right metal isn’t the strongest pusher — it’s the one whose push matches the dirt, and the chain turns that judgment into a number you can put in a report.
Eleven more years
17-lb standard-alloy magnesium anodes since a project five years ago. The annual reads have held steady around 60 mA per bar. The operator’s question this time isn’t about new anodes — it’s about the ones already in the ground: “How much life is left?”
A — What’s already been spent, and what’s left in the bank?
Show solution — part A
Five years of steady draw, converted to amp-hours:
One honest footnote: this assumes the output really did hold near 60 the whole time — which is exactly what the annual reads are for. Records turn an assumption into an estimate.
B — At today’s draw, how many more years?
Show solution — part B
So the records review closes with one sentence, said out loud: “That anode lasts eleven more years.”
Current into pounds, pounds into years — the whole module in one line on a tailgate. That sentence is the skill.
C — So the replacement dig goes on the eleven-year plan?
Show solution — part C
No — eleven is when the math says empty, not when the plan says dig. Anodes come out around the 85% line, because the last stretch of a bar can’t hold dependable output:
Put the dig in the eight-to-nine-year window. You’d rather replace on schedule than on failure — and the schedule is the same chain with one planning factor on top.
Remaining life is the same chain entered from the middle: subtract what’s spent, divide by the draw. The 85% line is what turns the estimate into a schedule.
Same current, two bills
250 mA flowing off your system into the freeloader, and the paper trail says the tie was probably missed on a project three years ago. The anode string on this segment is zinc.
A — What has three years of freeloading cost the anode side?
Show solution — part A
Convert the draw to amp-hours per year, then to zinc:
Call it twenty pounds of zinc — bought, buried, and spent on a line nobody uses.
B — Translate the same 250 mA to the steel side: what corrosion bill has the system been paying off?
Show solution — part B
Notice the trade is not pound-for-pound: the system spent about twenty pounds of zinc covering roughly fifteen pounds’ worth of steel demand. That’s normal, and it’s the whole idea — zinc pounds are budgeted and replaceable; steel pounds are pipe.
C — Why would those fifteen pounds matter far more if the current had been leaving the coated main at a few holidays, instead of feeding a bare abandoned line?
Show solution — part C
No arithmetic in this one — just the principle that decides whether pounds become paperwork or become a leak. On a bare line, demand spreads across the whole surface: fifteen pounds over hundreds of square feet is a haze nobody will ever measure. On a coated main, everything funnels to the few square inches where the coating’s gone — the same pounds delivered to three quarter-size addresses is pitting, and pitting on a schedule is a leak.
The law counts the pounds; the circuit picks the address. That’s why the freeloader hunt is worth the survey day — the bill always lands somewhere, and the somewhere is what leaks.
One shunt reading, two invoices — anode pounds and steel pounds, priced by the same law. Cathodic protection is a transfer of the bill; the survey’s job is knowing whose account it lands on.
What this Apply lesson was after. Three runs of one chain. Forward at the creek bottom — shunt to years, with the numbers backing last module’s metal call. From the middle on a records review — what’s spent, what’s left, and the difference between when the math says empty and when the plan says dig. And both directions at once on a freeloader — the same 250 milliamps written as a zinc bill and a steel bill, which is the entire idea of cathodic protection in one meter reading.
Every problem started at a shunt, because that’s where the field hands you the number. The law does the rest — and now, so can you. The quiz is ten questions, eighty percent to pass, and you’ve already done everything it asks. After that, the reference electrodes are waiting.
Faraday's Law and Corrosion Rates
Foundation tier · EC TRACK · ELECTROCHEMISTRY & THE GALVANIC SERIES
One module done. Keep going — you'll earn the certificate when you finish this section, and the Foundation medal when you complete every section in the tier.
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