Industrial silver plating: Typical general-purpose cyanide silver-plating solutions contain around 90 to 120 g free alkali cyanide per litre and about 25 to 40 g metallic silver per litre, added as the equivalent alkali silver cyanide or as AgCN. Sodium cyanide was once the standard, but due to the greater solubility of most potassium salts, potassium cyanide has generally supplanted it. Carbonate develops gradually in alkaline cyanide systems due to hydrolysis, and at concentrations more than roughly 90 g/L, occasionally less, it obstructs anode dissolving and produces deposit roughness.
Excess carbonate in sodium cyanide-based solutions can be “frozen out” by cooling the solution to around 3°C and sifting it. In potassium-based solutions, this is not practicable since they must be treated with calcium cyanide or barium cyanide. Thus, some brightener systems, notably those based on selenium or sulphur, are more effective in sodium-based solutions.
A study of new brighteners for industrial silver plating in cyanide solution
In a silver bath, free cyanide serves numerous purposes. It corrodes the anodes, solubilises the silver, and acts as an electrolyte. The cyanide ion, like gold, is very surface-active and necessitates thorough washing after plating. Carbonate is occasionally added at makeup to improve conductivity and minimise the amount of free cyanide in the solution; however, because carbonate develops as the solution is treated, this is rarely necessary. Both nitrate and alkali hydroxide have been employed to speed up the corrosion of anodes and, in the case of hydroxide, to delay the cyanide’s breakdown.
In any case, cyanide silver processes’ anode efficiency usually never reaches 100%, and it’s required to refill a percentage of the plated-out silver with alkali silver cyanide or AgCN regularly. Antimony, bismuth, selenium, and sulfur-based compounds are among the brightening agents for silver. The majority of them are exclusive. Sulfur-bearing materials, in particular, are frequently complicated, resulting from reactions between different organic molecules and carbon disulfide, sodium thiosulfate, or other reagents.
As a result, they’re frequently referred to as “organic” brightening agents, yet the active activity is nearly always derived from sulphur. For cyanide silver solutions, brightener systems nearly usually comprise both the primary brightener and a surface-active ingredient, the role of which is unknown.
For cyanide silver solutions, brightener systems nearly usually comprise both the primary brightener and a surface-active ingredient, the role of which is unknown. One proposed method is for the surfactant to allow the main brightener to adsorb onto the crystallite surface in a specific orientation. Another option is to adjust the viscosity of the diffusion layer to control the primary brightener’s access to the surface. In any case, different surfactants tend to be best for other main brighteners, and the benefits of the additional surfactants are most noticeable at lower current densities.
In addition, brightened silver deposits of all sorts are much more challenging than annealed, wrought silver in their as-plated state. After plating, the hardness of unannealed silver deposits usually diminishes gradually over time, owing to recrystallisation and grain development. Antimony-brightened deposits, on the other hand, gradually loosen with time and are regarded as permanently hardened.
There are two types of electrolytes for high-speed plating of cyanide silver: 1) solutions containing a significant amount of free cyanide, in which the silver is replenished at least partially by anodic dissolution, and 2) solutions designed for use with insoluble anodes and containing only as much free cyanide as is required to prevent AgCN precipitation at the anodes. The first kind of solution is widely used for wire plating and overall or controlled-depth plating of electronic components.
The second type of solution is utilised in applications that need high solution velocity or impingement, such as spot or stripe plating. In the cyanide system, silver is electrochemically noble compared to most other metals, including gold. In the cyanide system, silver is electrochemically noble compared to most other metals, including gold. As a result, it’s prone to forming immersion deposits on less noble surfaces, a problem that may be addressed to some extent by keeping a relatively high amount of free cyanide.
Immersion is highly prevalent in high-velocity, low-free-cyanide solutions, mainly because they are frequently used at high metal concentrations and high temperatures. Before entering these solutions, it is standard practice to use a pretreatment to passivate the substrate surface partly. If permitted, a general silver strike is a viable option. Some low-free-cyanide solutions also include components that help to reduce immersion deposition.
For typical uses, cyanide silver strike solutions generally include 2 to 2.5 g silver metal/L and 90 to 105 g/L of free alkali cyanide. They are set to run at room temperature and with a low tank voltage. Before silver plating, you may wish to use a mixed silver-copper strike solution followed by a pure silver strike for passive-prone metals, especially carbon steels. An alternative to mixed silver-copper striking is a sulfamate nickel strike without chloride or bromide.