I’d previously thought about copper-plating everything in the house like US pennies for antibacterial reasons, but it seemed like it would be a huge hassle, what with cyanides and electric current and so on. But it turns out that iron in particular has an extremely easy electroless way to plate it, discovered by alchemists many generations ago: immersion in solution of blue vitriol oxidizes some iron on the surface into green vitriol, replacing it with copper.
This might be interesting for decorative purposes as well, including selectively marking part of an iron or steel surface.
Nowadays this is called “immersion plating” in the "metal finishing" trade, whose bible is the Metal Finishing Guidebook, or sometimes “displacement plating”. There’s a standard test for cleanliness using 2 oz./gal. of copper sulfate with 0.1 oz./gal. of H₂SO₄ RT, I suppose to immersion-plate copper wherever the steel is clean. A bronze-displacement-plating solution uses 7.5 g/ℓ stannous sulfate, 7.5 g/ℓ copper sulfate, and 10–30 g/ℓ sulfuric acid at 20° for 5 minutes.
Stannous sulfate is deliquescent but relatively nontoxic; you can make it by a displacement reaction with tin and copper sulfate, so it isn’t necessary to buy it. I suspect you can make it with tin-lead solder and copper sulfate; if lead sulfate forms, it should be nearly insoluble.
At times this copper deposits in a powdery fashion, and apparently this also works on zinc, or more generally can displace any less noble metal with any more noble metal; reading on the “galvanic series” or “seawater series” or “electrolytic series” is suggested. Instructables has photos of the powdery copper deposit and reports better results with some electroplating. Caustic cleaning and an acid dip ahead of time are reported to give better results; electroplating success with salt and ethylene glycol was also reported to work, with the electrolyte recipe 100 cc white vinegar, 1 heaping teaspoon kosher salt, 3–6 cc of ethylene glycol.
Professional metal finishers don’t recommend trying to do this, although they’re concerned about questions like durability and pore-freeness as well as appearance, antisepsis, and adherence. They explain that the reason copper cyanide or copper pyrophosphate works better for electroplating is that the copper is “tightly complexed”, so it won’t plate out onto the steel without a current applied. “Finishing.com has been on the air 20 years now and no hobbyist has ever reported back that they achieved robust copper plating on steel from kitchen or hardware store chemistry. Maybe you’ll be first; I hope so,” immediately followed by someone reporting success in electroplating with copper sulfate from an electrolytic-copper anode, sulfuric acid, phosphoric acid, and dish detergent as a brightener.
The electrode potentials are confusing. More noble metals seem to be more positive, while more reactive metals are more negative. Zinc makes its first appearance at -1.2, and iron at -1.2 also (but with cyanide), with tin at -1.1, alkaline iron at -0.9, silver (with sulfide) at -0.7, gold (with cyanide) at -0.6. Silver’s last appearance is at +2.0. Copper can be displacement-plated with tin, although it’s not trivial, and tin can also be displacement-coated with copper (with just copper sulfate).
In particular, though, it seems like immersion plating of silver and gold onto copper or onto nickel are popular; I’m not finding much information about immersion-gold-onto-copper (ECIG and related processes) though. One paper on immersion-gold-onto-nickel gives the recipe of “immersion gold bath…at 80 °C, where 2 g ℓ⁻¹ Na₃Au(SO₃)₂, 40 g ℓ⁻¹ Na₂SO₃, 15 g ℓ⁻¹ NaS₂SO₃, and 10 g ℓ⁻¹ Na₂B₄O₇·10H₂O were contained”.
The important thing about the potentials, I think, is that iron is more noble than zinc, tin and copper are more noble than iron, silver is more noble than tin and copper, and gold is more noble than silver.
Popular Mechanics May 1906 gives the recipe of 1 oz. blue vitriol dissolved in 6 oz. water with ½ oz. sulfuric acid, but I think it was suggesting using that as the electrolyte for a copper-zinc battery for electroplating.
The ASTM test A-239 describes how to use the Preece Test to find the thinnest spot in a galvanized coating. The current version is A-239-14. The 1995 version A-239-95 prescribes 36 g of copper sulfate pentahydrate per 100 g of distilled water, plus an excess of Cu(OH)₂, about 1 g/ℓ, enough to not dissolve completely, or CuO of 0.8 g/ℓ; the solution should be 1.186 g/cc at 18°. The result is that this removes the zinc coating and, when it gets down to the steel underneath, deposits “bright, adherent copper deposits” that cannot be removed “with an ink eraser”.
Instructables has a CC BY-NC-SA copper acetate electroplating recipe, using acetate from vinegar and 3% H₂O₂. It comes with an excellent introduction to electroplating. (Copper acetate is somewhat toxic. It bioaccumulates with a biological concentration factor of over 100, but it was used as a green food coloring in the 19th century and had a large death toll.)
Copper sulfate is sold as fertilizer and fungicide for AR$145 per kg (US$9.50/kg) by Cristina at e-Moyos on MercadoLibre. This is the pentahydrate, which is 25.5% copper by mass (because it’s CuSO₄, 159.6 g/mol, water is 18.02 g/mol for a total molar mass of 249.7 g/mol, and copper is 63.55 g/mol), so this works out to US$37/kg for the copper. This is not a good price for copper; the USGS average 2015 published price for copper is 277¢/lb., which works out to US$6.11/kg. But it’s high by less than an order of magnitude. The Amazon price is US$33.40/5 lb., which is more expensive at US$14.70/kg.
(It loses four of its water molecules, amounting to 28.9% of its mass, upon heating to 109°, and reabsorbs them at 63°, so it might be useful as a powerful and compact desiccant; however, upon heating to 650° it emits SO₃, so it may not be very safe, depending on how well-controlled the regeneration temperature is. But this is beside the point.)
Machinery’s Shop Receipts and Formulas gives several recipes for such things; one of them suggests “brassing” with “a quart of water and ½ ounce each of sulphate of copper and pro-tochloride of tin”, which would have the great advantage that brass is both more yellow (less red) and a lot easier to keep polished. Nowadays tin protochloride, which is what’s used for tin-plating cans, is called stannous chloride or tin(II) chloride, and it’s a relatively safe chemical (in fact it’s E512), but apparently a little bit tricky to deal with in solution. It’s a lot harder to find than blue vitriol.
Other recipes from the collection suggest using e.g. “hydrochloric acid diluted with three times its volume of water, in which a few drops of a solution of sulphate of copper is poured”. Others suggest using e.g. zinc chloride: “saturated solution of zinc chloride with a very little copper sulphate added, say a half-dozen drops of copper sulphate to a spoonful of zinc chloride solution”. Zinc chloride is a deadly corrosive deliquescent salt of zinc that is considerably more easily available than stannous chloride because of its wide use as an acid welding flux.
Another recipe from the collection suggested caustic cleaning followed by charcoal cleaning, presumably to remove the caustic.
US patent 3,715,289 from the early 1970s gives a formula for a rather complicated brightener for copper electroplating; it says previous brighteners have included “casein, animal glue, sugar, urea and thiourea and their derivatives and polyvinyl alcohol.”