The belt guard off the 1943 South Bend was sheet steel, 0.048 inch, rusted the same on both faces by what looks like twenty years of a leaking roof. No paint left, no oil, no pit deep enough to go through. It was the most uniform piece of rust I have ever had in my hands, so instead of cleaning it I cut it up.
Five coupons, 4 by 6 inches, off the shear, stamped 1 through 5. Each weighed on a jeweler's scale reading 0.01 gram, between 147.8 and 148.6 grams. Thickness at five punch marked spots per coupon with a micrometer, recorded to the tenth, meaning 0.0001 inch. Same tap water for every rinse, same drying, which was compressed air and then twenty minutes at 200 F in the toaster oven I keep for powder coat and absolutely not for food.
One variable each. What each coupon got, and what is going on in the pan while it happens.
Coupon 1, electrolysis. Washing soda, which is sodium carbonate, one tablespoon per gallon, 14 gallons in an open tote. The soda removes no rust at all. Its only job is to carry current. The part is wired as the cathode and a piece of quarter inch mild steel four inches away is the anode. Current splits water at the part, which is where the hydrogen comes from, and the bubbling lifts and loosens the oxide while some of it gets reduced back toward magnetite. The anode gives up iron instead of your part, which is why it ends up looking like a swamp. It is also line of sight, and will not clean the inside of a hole it cannot see.
Coupon 2, chelation. Evapo-Rust, room temperature, plastic tub with a loose cover. It runs at a pH between 6.1 and 7, so it is not an acid and does not etch. A chelating molecule grabs iron out of iron oxide and holds it in solution, and a sulfur bearing molecule carries it off as a soluble complex. It stops dead at bare metal because iron in steel is held far more tightly than iron in rust, and the chelator cannot pull it out. No fumes, no gloves, no attention.
Coupon 3, phosphoric acid, about 30 percent, brushed on twice. It turns red oxide into iron phosphate, a hard dark layer that stays put, which makes it a converter rather than a remover. Very good under primer on a casting. Wrong anywhere a surface has to slide, seat or be measured, because you have added a skin of something that is not steel.
Coupon 4, oxalic acid, three ounces of powder to a gallon of hot tap water. It turns iron oxide into iron oxalate, which dissolves in water, so it rinses off and leaves steel almost white. Fast, and impressive coming out of the bucket. It also deserves more respect than shop videos give it: harmful or fatal swallowed, hard on the kidneys because it pulls calcium out of your blood as calcium oxalate, and the dry powder is a respiratory irritant. Dust mask and eye protection to mix it, and never keep it in anything that has held food or ever will.
Coupon 5, white vinegar, 5 percent acetic acid, out of the jug. The internet's favorite and the worst of the five. Acetic acid dissolves the oxide and then keeps right on going, because nothing tells it to stop at bare steel.
| Method | Clock | Cost, this job | Mass off | Thickness lost | Flash rust | Verdict |
|---|---|---|---|---|---|---|
| Electrolysis, washing soda | 6 h plus 25 min scrubbing | 60 cents of soda and power | 6.2 g | none I can measure | 20 min | good, if you enjoy tending equipment |
| Chelation, Evapo-Rust | 8 h, 3 min rinse | 7 dollars out of a 29 dollar gallon, reused twice since | 6.0 g | none I can measure | none in the tank | winner |
| Phosphoric acid, 30 percent | 40 min, 15 min brushing | 2 dollars | 4.1 g | none I can measure | none, that is the point | painted castings only |
| Oxalic acid, 3 oz per gallon | 50 min, 20 min handling | a dollar fifty | 6.4 g | 1 tenth at one spot | 4 min | fast and mean |
| White vinegar | 24 h, 35 min scrubbing | 3 dollars | 7.9 g | 4 tenths at three spots | 10 min | quit using it |
The flash rust on coupon 4 is not a rumor. It came out of the rinse bright and had a brown bloom inside four minutes while I stood there with the air hose in my hand.
Coupon 5 gave up a gram and a half more than the coupons that only lost rust, and the micrometer found 4 tenths gone at three of the five marked spots. That is not rust leaving, that is the part leaving. It came out dull and smutted, it needed a brass brush, and the pits it already had were wider than before. Vinegar is cheap because you pay for it with your part instead of your wallet.
Electrolysis makes hydrogen at the part and oxygen at the anode, in the same bucket, in the ratio that makes chemists go quiet. Hydrogen burns in air from about 4 percent by volume. Never put a lid on the tank, never run it in a closed cabinet or a closet or a basement corner, never in a room with a water heater pilot light or somebody grinding. Mine runs outdoors on gravel with the charger six feet back, and the power goes off before my hands go near the tank.
Never use a stainless anode. The chromium in stainless oxidizes to hexavalent chromium, which the International Agency for Research on Cancer puts in Group 1, carcinogenic to humans. It goes into your solution, onto your gloves, and then wherever you pour that solution out. Mild steel only, cut from stock you bought as mild steel. Plain iron sludge goes in the trash, never down a drain or into a storm sewer.
Never use table salt as the electrolyte. Chlorine gas comes off the anode. Washing soda costs three dollars a box and does the job better.
Hardened steel under stress absorbs hydrogen at the cathode and gets brittle from it. Springs, saw plate, taps, anything up around 44 Rockwell C and above stays out of the tank and goes in the chelation tub instead. If something hardened has already had a long soak, give it weeks before you load it, because the hydrogen does leave on its own.
tank 18 gal plastic tote, no lid, outdoors on gravel
solution 14 gal water, 14 tbsp washing soda (sodium carbonate)
anode 1/4 x 4 x 10 in mild steel, 4 in from the part, swapped when thin
part cathode, hung on 12 gauge solid copper, connections above water
supply old manual 12 V battery charger, ran 4.0 to 4.6 A for six hours
gotcha a modern smart charger may refuse to start because it cannot see a
battery on the clips. Use an old manual one or a bench supply.
The routine, which does not vary:
Spent oxalic and phosphoric get neutralized with washing soda until they quit fizzing, then go wherever your county says. Mine takes them at the household hazardous waste day in April. The chelation fluid is water based and neutral, and the maker says spent fluid is fine for normal disposal, which I believe and still check the local rules on. It took a year and eleven jobs to wear a gallon out.
Chelation won and it is the most boring answer on the list. Highest cost per gallon, slowest, and it asks nothing of me. It does not etch, it does not flash rust in the tank, it does not care if I forget it until Tuesday, and no step in it requires me to be clever with chemistry inside a building. Anything that has to fit, slide, seat or get measured goes in that tub.
Electrolysis for the big greasy iron, where filling a tote with chelation fluid would run four hundred dollars. Cabinet bases, wheels, gear guards, the tailstock casting off that same lathe. Phosphoric acid only where paint goes on within the hour and nothing will ever slide on it.
I spent my thirties being pleased with clever. Somewhere in my forties I noticed the clever methods were the ones I had scars from, and the good ones mostly involve leaving something alone in a tub and going to bed. @ferment/i-made-garum-for-six-months is a much slower kind of waiting than mine and it made me feel better about the eight hours.