In large industrial cycles for the protection of ferrous material copper plating occupies an important place. Indeed, while not providing a protective coating in itself, it constitutes an intermediate layer great for subsequent nickel plating, so that appropriate deposits of copper-nickel-chromium-iron take on high protective value and therefore become commendable in every respect.
The protection of copper electroplated on iron is cathodic: that is, it is the soluble iron anode in the iron-copper stack that is generated by humidity. Therefore, the copper film must be relatively thick and compact, that is with porosity reduced to a minimum, in order to achieve maximum protection. Additionally, taking into account that the total deposit of copper-nickel-chromium on iron can be considered with total cathodic protection, it goes without saying that the person in charge for the electroplating should make sure to get a galvanic protective layer of the maximum compactness.
In the case of copper or nickel matt-electroplated on iron and requiring mechanical cleaning, due to the flattening and levelling of the pores, the latter greatly reduces porosity and creates a compact and resistant layer. This also explains the higher value of double copper-nickel protective deposits.
In fact, in their unified standards, many foreign specifications recommend pairing copper and nickel for the best protection of ferrous materials, provided that the thickness of copper is at least 50% of the one of nickel.
Copper plating is widely used as an intermediate layer in the nickel plating, silver plating, gold plating, etc. of aluminium and its alloys, zinc and its alloys (die cast), as well as alloys with a high percentage of lead, or tin, or antimony, where electroplating of other metals would not be possible without affecting adhesion.
Copper can be electroplated from solutions where it is bivalent (cupric solutions) and from solutions where it is monovalent (cuprous solutions). In the first case, sulphate, fluoborate, sulphamate, etc. baths are typically used; in the second case, cyanide baths.
Performing copper plating with cuprous solutions (cyanide baths) and assuming 100% cathodic current efficiency, 1 A/h would deposit 2.372 g of copper; however, a cupric solution (e.g. sulphate baths), with the same current efficiency, would only deposit 1.186 g copper, that is to say half as much, because, logically, the electrochemical equivalent is different (see Faraday’s laws).
On the other hand, while in cupric solutions the efficiency is very close to 100%, in cuprous solutions it usually drops to 70 and 50%, with the exception of some cyanide baths, very concentrated and appropriate, which, among other things, can be also used at high current density.
Currently, the most widely used baths are cyanide and sulphate. The latter are often preferable for the lower cost and the higher chemical stability. Also fluoborate, pyrophosphate, and amine-based baths are becoming more and more popular.
What, however, absolutely determines the choice of the bath is the type of work and the nature of the base metal. Therefore, most acidic cupric solutions cannot be used directly to copper-plate steel and zinc, since copper would deposit on them already by immersion and it would not adhere too well.
Often, alkaline baths diluted with cyanide are used to produce a first thin deposit, since such baths have good penetrating power and originate thin adherent deposits; then the electroplating continues with acidic cupric baths, e.g., sulphate baths, or concentrated cyanide baths (quick baths) or, better, with pyrophosphate baths.
When electroplating requires a high penetrating power from the bath, it is better to use cyanide or pyrophosphate baths.
The commonly used anodes are electrolytic and laminated, and their surface must be at most equal to the one of the cathode. They can be immersed in alkaline baths also without electrolysis; not so for acidic baths, in which the dissolution of copper would take place also without electrolysis. Nylon, polypropylene, or terylene bagging can be useful.
The tanks for alkaline baths are made of iron and suitable resin, with heaters and suction; for the other types of baths, they are made of lead, rubber, polyethylene, polyvinyl chloride, and various kind of plastics.
COMMON BATHS – They are also called diluted cyano-alkaline baths, i.e. baths with low copper content. They are widely used and their popularity is mainly due to the excellent penetration and adhesion of the deposits.
Chemically, they contain a series of complex copper cyanides as basic salts that originate with the addition of cuprous or cupric salts (cuprous cyanide) in solutions of potassium cyanide or sodium cyanide; the latter compound is the most widely used for its lower cost in respect of the potassium salt.
- Cycle with periodic reversal of the current – The anodic current dissolves the roughness and allows for smoother deposits, free of pores and almost glossy; the attack of the anodes is more uniform and high current densities can be used. The most frequently adopted cycle is 15 seconds deposit and 5 seconds reversal.
- Cycle with interrupted current – Current interruption is 2-3 seconds for 8-10 seconds deposit. The advantages of this technique are the increased gloss and the ability to use higher current densities.
Current efficiency- The current efficiency of cyano-alkaline copper plating baths at low temperature ranges between 40% and 70%; usually, it is 60%. Increasing the concentration of complex copper cyanide and the temperature (quick baths) the current efficiency can be nearly 100%.