The electroforming of nickel is usually carried out with electrolyte temperatures ranging from 40°C to 65°C and with a density ranging from 28° to 35° Baumé.
High current and low temperature electroforming of the electrolyte cannot coexist. At low temperatures, in fact, metallic ions become less mobile, and if the current is high, the following are generated:
- mechanical stress, which can cause deformations or bending of the part being electroformed at the cathode. In more severe cases, the layer of nickel being deposited, especially if it is still thin, can be lifted or detached from the substrate;
- poor quality of the deposit, with a rough and uneven surface.
Another issue caused by a particularly high current, regardless of the temperature of the electrolyte, is the formation of gas bubbles, namely hydrogen, but especially oxygen at the anode. In practice, with a remarkable potential difference, the electrolysis of the water contained in the electrolyte is also triggered, a reaction that leads to the release of these gases.
These micro-bubbles can interfere with the quality of the deposit, making it porous. If they do not burst spontaneously, they can remain attached to a specific point of the part being electroformed and block conductivity at that point. As a result, nickel will no longer deposit at that point, and a small hole will form (starting from the thickness reached at that moment onwards) on the surface of the part.
The holes that have formed in the final layers of electroforming, usually at very high currents, are common and generally harmless. The holes that have instead formed closer to the initial layers of electroforming, where the groove structure is present, can create audible defects.
To prevent these problems, the electroforming process always starts at low voltage and low current, gradually increasing them as the thickness of the electroformed part increases.
Another commonly used approach to reduce the adhesion of micro-bubbles to the cathode part is the use of surfactants (also known as "anti-dotting agents") as mild additives to the electrolyte. These substances make the electrolyte slightly more slippery and, consequently, reduce the likelihood of gas bubbles adhering to the cathode part.
Even in this case, the use of this additive must be carried out with some caution, as it weakens the crystalline structure of the deposit and makes the electroformed part slightly less elastic.
Electrolyte temperatures exceeding 65°C should be avoided, as they can lead to undesirable secondary reactions and, above all, to the partial decomposition of the solution.
The pH of the electrolyte has a very significant impact on both the quality and the morphology of the layer that is electroformed.
If the pH is too alkaline (above 4), there is a risk of electrolyte contamination due to the spontaneous precipitation of nickel hydroxide. Conversely, if the pH is too acidic (below 2.5), the concentration of H+ ions in solution increases, and these ions can compete with metal ions during the reduction process at the cathode, causing irregularities in the deposit or – in this case as well – small holes.
Among these values, there is a theoretical range of 1.5 acidity points. The commonly used pH values, moreover, have an even narrower range, ranging from 2.8 to 3.8.
A more alkaline electrolyte causes the electroforming of larger and less uniform crystals, while a more acidic electrolyte produces smaller and more uniform crystals. The difference lies mainly in elasticity, which is a crucial factor in this type of electroforming, as – for example – a mould that is mounted on a press must be centred and preformed (thus bent multiple times) before it is actually mounted, and subsequently it is subject to continuous contraction and expansion due to the thermal excursion it undergoes in each cycle, in addition to the mechanical stress caused by the 100T force of the press.
The pH of the electrolyte would naturally tend to become increasingly alkaline, due to the evaporation of its acidic component caused by relatively high temperatures, and the subsequent addition of water to maintain a constant level of the electrolyte in the tank. The correction can be made simply by dissolving sulfamic acid in the water being added, or directly into the solution.
A "buffer" solution can also be used to "buffer" the variation in pH. This type of solution is also presented as a simple additive to the electrolyte and is usually based on substances such as boric acid and sodium hydroxide.
Temperature, density and pH