Plating

The bath

Ingredient	Per litre	Role
Copper sulphate, pentahydrate	200 g	Metal source
Sulphuric acid, concentrated	30 ml	Conductivity, throwing power
Hydrochloric acid 30 %	0.2 ml	Chloride, about 60 mg/l — needed for the anode to dissolve evenly
Deionised water	to 1 l	Carrier

About the chloride

Those two tenths of a millilitre look like a typo but are decisive. Without chloride the phosphorised copper anode passivates, stops dissolving evenly, and the bath loses copper. Too much chloride gives a pitted, matt deposit instead. The window is roughly 40–80 mg/l.

Brighteners

Without additives the copper comes out dull and coarse-grained. Three components are used industrially: a carrier (polyethylene glycol), a brightener (SPS, bis-sodium sulphopropyl disulphide) and a leveller (Janus Green B). Buy a ready-made additive set for circuit boards — mixing them yourself is not worth the trouble.

An honest home alternative: a few drops of molasses or a pinch of gelatin per litre gives a fine-grained, semi-bright deposit. Not a mirror, but far better than nothing.

Use for

• A levelling base layer under nickel and chrome. Copper is soft and fills microscopic unevenness.
• Building up worn parts that are then turned or ground to size.
• Electroforming — building a free-standing copper shell on a model that is then removed.
• A conductive layer on plastic after graphiting.

The bath

Ingredient	Per litre	Role
Nickel sulphate, hexahydrate	280 g	Metal source
Nickel chloride, hexahydrate	45 g	Dissolves the anode, raises conductivity
Boric acid	40 g	Buffer at the cathode surface
Deionised water	to 1 l	Carrier

The boric acid is not optional

Hydrogen ions are consumed at the cathode surface, so the pH rises locally even if the bath as a whole is correct. Without a buffer, nickel hydroxide precipitates into the deposit and you get a grey, brittle, poorly adhering layer. The boric acid holds the pH down exactly where it counts. It is the ingredient most often left out of home recipes and the commonest reason they do not work.

Bright or matt

• Without additives the nickel comes out semi-matt and silver-grey, softer and more ductile. That is often what you want on a functional part.
• Bright nickel plating needs two additives: saccharin or sodium saccharinate as the primary brightener, and a secondary one such as butynediol. The result is mirror-bright but harder and more brittle, and it contains sulphur — which lowers the corrosion resistance.
• A wetting agent, sodium lauryl sulphate at a very low concentration, lowers the surface tension so hydrogen bubbles release from the surface rather than leaving pinholes.

Maintenance

• The pH rises while running. Bring it down with dilute sulphuric acid, up with nickel carbonate. Check it every time you run.
• The anode should be bagged in polypropylene. Nickel anodes shed a sludge that otherwise ends up in the deposit.
• Filter between runs. Pinholes are nearly always down to particles.
• Iron contamination from dropped work makes the bath cloudy. Raise the pH to 5, aerate, filter off the precipitated iron hydroxide, and bring the pH back down.

Why it is interesting

Sodium hypophosphite reduces the nickel ions chemically, without external current. That means the layer comes out exactly the same thickness everywhere — in threads, in drilled holes, inside tubes, on the back. No current density, no throwing power, no anode geometry. For a part of complex shape it is often the only thing that works.

Ingredient	Per litre	Role
Nickel sulphate, hexahydrate	25 g	Metal source
Sodium hypophosphite	25 g	Reducing agent
Sodium citrate	15 g	Complexing agent, keeps the nickel in solution
Sodium acetate	10 g	Buffer
Stabiliser, bismuth or antimony	trace	Stops the whole bath precipitating

Heat treatment at 400 °C for an hour precipitates nickel phosphide and raises the hardness from about 550 to over 1000 HV. That makes it harder than hard chrome.

The sacrificial anode principle

Zinc is less noble than iron. If the coating is scratched the zinc goes on corroding instead of the steel, and the exposed steel is protected electrochemically, not merely mechanically. That is the difference from nickel and chrome, which protect only as long as they are unbroken — a scratch through nickel accelerates the rust underneath.

Ingredient	Per litre	Role
Zinc chloride	70 g	Metal source
Potassium chloride	200 g	Conductivity
Boric acid	25 g	Buffer
Brightener for zinc	per manufacturer	Without it the zinc goes grey and powdery

Passivation — the step that actually gives the rust protection

Bare zinc oxidises to white rust within weeks. Passivation lays a thin chromate film over the zinc, and that film is what multiplies the life. Use trivalent chromium, Cr(III), sold ready-made in the hobby trade. Hexavalent Cr(VI) gives the classic yellow rainbow but is carcinogenic and requires an authorisation.

The hours refer to salt spray to ISO 9227 until first white rust, and should be read as a ranking rather than as a promise about years outdoors.

Ingredient	Per litre	Role
Tin(II) sulphate	40 g	Metal source
Sulphuric acid, concentrated	100 ml	Conductivity
Phenolsulphonic acid	20 ml	Stops Sn(II) oxidising to Sn(IV)
Additive for tin	per manufacturer	Without it the layer comes out coarse-grained

The bath ages by oxidation

Divalent tin is oxidised by atmospheric oxygen to tetravalent, which precipitates as a white sludge and cannot be plated. A tin bath left open for a few weeks is spent. Keep the lid on, stir no more than necessary, and count on a shorter life than the nickel bath.

Silver has the highest electrical conductivity of all metals, which is the whole reason it is plated onto RF conductors and contacts rather than for looks.

The cyanide bath is the classic and gives the best result, but is ruled out here. Cyanide-free alternatives exist as ready-made concentrates — thiosulphate- and succinimide-based — and work well on a small scale. Mixing them from scratch is rarely worth the trouble: the silver salts dominate the cost, and the ready-made bath contains complexing agents that are hard to substitute.

Silver tarnishes from sulphur in the air. Follow with a tarnish inhibitor, or accept it and clean it with recipe 18 in Chemicals, which restores the silver without abrading it away.

Gold is plated in fractions of a micrometre — a connector typically has 0.2 to 0.8 µm. The cost is in the metal, so the layer is kept as thin as the wear allows.

A nickel barrier underneath is mandatory on copper. Without it, copper diffuses through the thin gold layer within months and the surface goes dull and discoloured. Electronics often uses ENIG — electroless nickel with thin immersion gold — for exactly that reason.

The rule that makes the thickness predictable

The American rule of thumb is called the 720 rule and is reckoned in amperes per square foot: current density times minutes, divided by 720, gives the thickness in thousandths of an inch. Converted to metric units it becomes

1.5 A/dm² for 45 minutes therefore gives about 22 µm, which is a normal thickness for type II. Double the time gives double the thickness. The relationship holds as long as the bath is kept at the right temperature — if it gets too warm the oxide dissolves as fast as it forms and the rule stops applying.

The sequence

1. Degrease alkaline, rinse.
2. Etch in 50 g/l sodium hydroxide, 50 °C, 1–3 minutes. Gives the matt satin finish. Skip it if you want to keep a polished surface.
3. Desmut in 10 % nitric acid, 1 minute. Removes the black alloy layer the caustic leaves behind.
4. Anodise as in the table. Bubbles rise from the work throughout.
5. Rinse in deionised water.
6. Dye in a 60 °C dye bath, 5–15 minutes. The pores draw it in.
7. Seal in boiling deionised water, 20–30 minutes. The oxide converts to boehmite, swells and closes the pores.

Dyeing

• Textile dye for polyester works surprisingly well and is the cheapest. RIT and similar. The lightfastness against sun is poor, though.
• Anodising dyes bought for the purpose are lightfast and give a deeper colour.
• Black is the simplest and most durable. Red and yellow fade fastest in UV.
• Electrolytic colouring with a tin salt and alternating current gives bronze shades that are completely lightfast — it is how architectural facades are done.

The same electrolyte, but close to zero degrees. The cold stops the acid dissolving the oxide while it forms, so the layer comes out much denser and harder. The price is that it turns grey or dark brown of its own accord and can barely be dyed in light shades.

Mind the dimensions. Roughly half the layer thickness grows outwards from the original surface. 50 µm of hard anodising makes the part 50 µm larger in diameter. On a fit, that matters.

The cooling is the real difficulty at home. Without active cooling of the bath you get type II with delusions of grandeur.

Electrolyte

Trisodium phosphate, 50 g per litre, is the standard and gives clean colours. Alternatives that work: bicarbonate 50 g/l, borax, or Coca-Cola — the last is not a joke but a consequence of its phosphoric acid content, though it gives muddier shades. Avoid chloride electrolytes such as table salt: the chlorides attack the oxide and give patches.

The sequence

1. Clean thoroughly. Degrease with alkali, rinse, and do not touch the surface afterwards. Fingerprints become permanent markings in the colour.
2. Pickling in dilute hydrofluoric acid gives the cleanest colours, but hydrofluoric acid has no place in a home — the burns are not felt for hours. Mechanical cleaning and thorough degreasing is enough.
3. Set the voltage before you dip. The colour is decided the moment the work goes in.
4. Dip for 10–60 seconds until the current has fallen towards zero.
5. Rinse and dry. No sealing is needed — the layer is dense.

What it is used for

• Jewellery — titanium and niobium are hypoallergenic, unlike nickel.
• Surgical implants and instruments, where the colour is used for size marking.
• Bicycle and motorcycle parts, screws and bolts.
• Marking tools that have to withstand solvents — the colour is oxide and cannot be wiped off.

Ingredient	Per litre	Role
Phosphoric acid 85 %	700 ml	Forms the viscous layer at the surface
Sulphuric acid, concentrated	200 ml	Conductivity
Deionised water	100 ml	Carrier

The mechanism is that a viscous, metal-saturated layer settles in the valleys and insulates them, while the peaks stick up through it and dissolve. The result is a bright surface and passivation into the bargain.

Works best on stainless. Carbon steel needs other electrolytes and rarely gives a true mirror. Aluminium is electropolished in a different mixture entirely.