A few winters ago I was handed a bench vice that had seized solid in a damp shed — jaws frozen, handle welded in place by rust, the whole thing one orange lump. Too good to bin, too far gone for a wipe and a squirt of oil. It came back to life in a plastic tub of washing soda and water, wired to an old battery charger, while I got on with other things. That is electrolysis, and it is the rust-removal method most people have never tried.

The pitch is simple. You pass a low-voltage direct current through a tub of mildly alkaline water, with the rusty item on one terminal and a piece of scrap steel on the other. The rust is lifted and chemically converted, and — this is the part that makes it special — the good steel underneath physically cannot be eaten by the process. Not “if you are careful”. It cannot, because of how the circuit is wired. That is the whole reason it earns a place between the gentle acids and the harsh ones.

Best practical rule: reach for electrolysis when the rust is heavy, the shape is awkward, or the item is valuable enough that you do not want any acid etching the metal underneath. For a few light-rusted spanners, a cider vinegar soak is far less faff. Electrolysis is the patient, selective option, not the quick one.

This guide covers the science properly, then the setup, the electrolyte, the safety, and the honest limits. If you want the full menu of methods first, I have a companion piece on the best ways to remove rust from metal.

The whole rig is humble: a plastic tub, washing soda, water, a scrap-steel anode and an old battery charger doing the work while you get on with something else.

⚡The Quick Version: Where It Sits and What Happens

Rust removal is a ladder, from gentle and slow to fierce and fast. Electrolysis sits slightly apart from the acids: instead of dissolving the rust with acid, it uses electric current to reduce and release it, and it protects the base metal while it works. Here is the ladder, so you can see where it fits.

In one line: gentler and far more selective than hydrochloric acid, more thorough on heavy and pitted rust than a citric acid soak — at the cost of a power supply, a bit of setup, and a part that comes out black and needs a scrub.

How the cell is wired: current runs from the sacrificial anode, through the electrolyte, to the rusty part on the negative lead — which is exactly why the sound steel underneath cannot dissolve.

🔬The Science: Why Electrolysis Only Eats the Rust

This is the part worth understanding, because once you see how the cell is wired, every rule that follows makes sense.

The cell: four things wired together

You need four ingredients. A direct-current power supply — an old battery charger at roughly 6 to 12 volts is perfect. The rusty workpiece, clipped to the negative terminal, where it becomes the cathode. A sacrificial piece of plain steel, clipped to the positive terminal, where it becomes the anode. And an electrolyte — water with a little washing soda dissolved in it — to carry the current between them. Switch it on and current flows from the anode, through the solution, to your rusty part.

Why the good steel cannot be touched

Here is the whole trick. Removing metal means oxidising it — iron atoms giving up electrons and dissolving away as iron ions. Oxidation only happens at the anode. Your rusty part is wired as the cathode, so it is the site of reduction, the opposite reaction. While the current is on, the base steel is cathodically protected: it simply cannot dissolve, because the electrons are flowing the wrong way for that. The sacrificial anode does the corroding instead, which is exactly why you feed it a lump of scrap steel.

That is the difference from every acid method. An acid dissolves the rust, then keeps right on going into the bare steel underneath — my experiment with hydrochloric acid is the cautionary tale, where the soak did as much damage as the rust. Electrolysis is self-limiting on sound metal. Leave a part in too long and you waste electricity, not steel.

At the rusty part (the cathode)

Two useful things happen at the workpiece. First, water is split at its surface and hydrogen gas is released:

2 H₂O + 2 e⁻ → H₂↑ + 2 OH⁻

In plain English: water gains electrons at the part, giving off hydrogen bubbles and leaving the surface alkaline. Those bubbles do real mechanical work, forming underneath the rust layer and levering it off. At the same time, the reducing environment converts the red rust toward black magnetite, and a little of it back toward iron:

3 Fe₂O₃ + H₂ → 2 Fe₃O₄ + H₂O

Fe₂O₃ + 3 H₂ → 2 Fe + 3 H₂O

In plain English: loose red rust (iron oxide) becomes black magnetite and a trace of metallic iron, while the rest is simply shaken loose. This is why a part comes out of an electrolysis bath black, not shiny — that black film is magnetite and loosened oxide, and it scrubs off to clean grey metal. Expect it; it is not a failure.

At the sacrificial anode

The scrap steel on the positive terminal is where the metal loss happens, on purpose:

Fe → Fe²⁺ + 2 e⁻

4 OH⁻ → O₂↑ + 2 H₂O + 4 e⁻

In plain English: the anode dissolves and gives off oxygen, caking itself in sludge so your good part does not have to. The anode does the suffering, which is why it crusts over and why you scrub it clean every so often — a caked anode chokes the current and the job slows to a crawl.

The gases, and why ventilation is not optional

Step back and you are partly electrolysing water itself — hydrogen coming off the workpiece, oxygen off the anode:

2 H₂O → 2 H₂↑ + O₂↑

Hydrogen and oxygen together are a flammable mix. This is the single biggest safety point of the whole method: run it outdoors or in strong ventilation, with no flames, sparks or smoking nearby, and never in a sealed container or closed space where the gas can collect.

🧪Why Washing Soda, and Never Salt

Washing soda is sodium carbonate. Its only job is to make the water conductive by providing ions, so the current can flow. It is not consumed in any real quantity, and it is not an acid, so it cannot etch your steel — it just carries charge. It is cheap, safe to handle, and alkaline, which is exactly what you want.

A sensible starting ratio is about one tablespoon of washing soda per 4 litres of water (roughly 5 grams per litre). More than that will not speed things up much. No washing soda in the cupboard? You can make it from baking soda — ordinary sodium bicarbonate loses water and carbon dioxide in a hot oven and turns into washing soda:

2 NaHCO₃ → Na₂CO₃ + H₂O↑ + CO₂↑

Spread baking soda on a tray, bake it at around 200°C for an hour, and what comes out is washing soda, ready for the tub.

🧰How to Set It Up, Step by Step

You need: a plastic or other non-metal tub; washing soda; water; one or more mild-steel anodes (rebar, angle iron, an old steel baking tray — never stainless); a manual battery charger at around 6 to 12 volts; insulated leads and clips; some wire to suspend the part; gloves and eye protection; and a wire brush or nylon pad for afterwards.

1. Mix the electrolyte — about a tablespoon of washing soda per 4 litres of water — in the tub.
2. Degrease the part first. Current and chemistry cannot work through grease or grime; methylated spirits makes a handy degreaser, though test it on any paint you want to keep.
3. Suspend the rusty part in the tub so it is fully submerged but not touching the anode.
4. Place the anode, or several anodes, around the part. Surrounding it gives better “line of sight” and cleans more evenly.
5. Connect black (negative) to the workpiece and red (positive) to the anode. Get this backwards and you will turn your good part into the sacrificial one — check it twice.
6. Switch on. Within a minute you should see a fine stream of bubbles rising off the part. That is the method working.
7. Leave it to run — a few hours for light rust, overnight for heavy. Top up the water as it evaporates, and scrub the caked anode if the current drops away.
8. Switch the power off before you put your hands in. Lift the part out and scrub off the black film with a wire brush or nylon pad.
9. Rinse, dry it completely, and protect it straight away (more on that below).
10. Let the spent electrolyte settle, bag the iron sludge, and dispose of it responsibly — not down the stormwater drain.

🆚Electrolysis vs Vinegar, Citric and the Strong Stuff

Where electrolysis really shows its worth is on the jobs the acids handle badly. A cider vinegar bath or a citric acid soak is perfect for a box of lightly rusted spanners: cheap, simple, no power needed. But on a deeply pitted casting, a valuable plane body, or a cast-iron pan you do not want etched, the acids face a problem — the moment they finish the rust, they start on the metal. Electrolysis does not, so you can leave it running without watching the clock.

It also reaches into the awkward recesses an acid would over-etch on the outside before it ever cleaned the inside. The trade is real, though: electrolysis needs a charger, a bit of setup, and patience, and it leaves a black film to scrub off. For speed on rough, low-value steel, hydrochloric acid still wins on pace — it is just unforgiving in every other way. The full comparison lives in the best ways to remove rust from metal.

⛔What Electrolysis Can’t Do (and When to Skip It)

For all its elegance, electrolysis has firm limits. Be honest about them and you will not be disappointed.

• It does not restore lost metal. Where rust has eaten pits into a tool, those pits stay. You are removing the rust and converting some of it to black oxide, not regrowing steel. Cleaning reveals the damage; it cannot undo it.
• The part comes out black. That magnetite film is normal and scrubs off to clean metal — but if you wanted shiny straight out of the tub, you will be doing some elbow work.
• It can embrittle hardened steel. The hydrogen produced at the cathode can drive hydrogen embrittlement in hardened or high-tensile parts — springs, blades, high-grade bolts. Keep runs sensible, and for critical hardened parts consider a different method or a low-temperature bake afterward to drive the hydrogen out.
• It only works in line of sight. Current cleans where the anode can “see” the part. Shadowed faces and deep recesses clean slowly — reposition the part or surround it with anodes.
• It is for iron and steel only. Do not use it on aluminium, brass or other non-ferrous metals; the alkaline electrolyte can attack aluminium in particular.
• It is overkill for light rust. A few orange spanners do not justify the rig. Save it for the jobs that earn it.

🧯Safety: Gas, Chlorine, and the Stainless Trap

• Ventilate, always. The hydrogen and oxygen given off are a flammable mix. Work outside or in strong airflow, keep flames, sparks and cigarettes away, and never run it in a sealed container or closed cupboard.
• Washing soda only — never salt. Salt makes toxic chlorine gas at the anode and pits your steel.
• Never use a stainless-steel anode. Under current, stainless leaches chromium compounds — including carcinogenic hexavalent chromium — into the electrolyte, turning your tub into hazardous waste. Use plain mild steel for the anode, every time.
• Treat it as live equipment. The voltage is low, but switch the charger off before reaching into the tub, and never let the anode and the workpiece touch and short out.
• Wear gloves and eye protection. The electrolyte is alkaline and a mild irritant.
• Dispose of it sensibly. Let the iron sludge settle, bag the solids, and keep the lot out of waterways.

🛡️After-Care: Bare, Reduced Steel Flash-Rusts Fast

A part fresh out of an electrolysis bath is chemically naked — no oxide layer, no oil, and still damp with alkaline solution. That is a recipe for flash rust, the faint orange haze that can appear within minutes on a warm day. So the moment you have scrubbed off the black film, the clock is running: rinse it, dry it completely — towel first, then heat or compressed air into threads, hinges, pits and stamped marks where water hides — and get oil or wax onto it straight away.

This after-care matters as much as the soak, and I have written it up in full: how to stop rust coming back after you remove it covers drying, the right oil or wax for each job, and storing tools so a damp shed does not undo your work.

❓Frequently Asked Questions

Does electrolysis damage the metal underneath the rust?

No. The rusty part is wired as the cathode, so it is the site of reduction, not oxidation. It is cathodically protected and cannot be dissolved by the process — only the rust is reduced and lifted, while the sacrificial anode does the corroding.

Can I use salt instead of washing soda?

No. Salt is sodium chloride, and under current the chloride produces toxic chlorine gas and pits the steel. Use washing soda (sodium carbonate), which carries the current without those problems.

Why is my part black after electrolysis?

That black film is magnetite and loosened oxide, and it is completely normal. Scrub it off with a wire brush or nylon pad and clean grey metal is underneath.

What kind of battery charger works for electrolysis?

A manual charger of around 6 to 12 volts. Many modern smart chargers will not output current unless they detect a battery, so they sit idle on a tub of water; an older manual charger or a plain DC power supply is the answer.

Does electrolysis get rust out of pits and threads?

Only where the current can reach. Electrolysis works in line of sight, so deep recesses and shadowed faces clean slowly. Surround the part with anodes or reposition it partway through.

Is electrolysis better than vinegar or citric acid?

For heavy, pitted or valuable items, yes — it is selective and will not etch the base metal. For light rust on cheap tools, a cider vinegar or citric acid soak is simpler and needs no power supply.

This sits alongside apple cider vinegar and citric acid rust removal for lighter jobs, and the hydrochloric acid cautionary tale for when speed tempts you — electrolysis is the selective middle path. Whatever lifts the rust, stopping it coming back is the other half of the job. For the full map of methods, start at the best ways to remove rust from metal.