Question: We are asked to silver-coat small orders of washers made of V2A steel for an existing customer. The order size consists of around five kilos, which are coated in a small-format drum. We used a Ni-Strike bath for this. Nickel is deposited, but even after several minutes in the pre-nickel plating process, we get blistering after silver plating. Can you tell us how long the disks have to be pre-nickel-plated before there are no more bubbles?
Answer: Blistering has several causes, especially on V2A and V4A, which is why we want to look at this holistically. In addition to the alloy, the part geometry plays a major role here.
Pre-treatment
Washers tend to stick together, which has a negative effect on pre-cleaning and coating. The low overall weight does the rest to worsen the situation. We recommend adding a few screws to the washers. Make sure that the screws have a larger diameter than the holes in the washers and that the volume fills the selected drum sufficiently and ensures good contact.
Before nickel plating, the washers are cleaned in a decoction degreaser for up to 10 minutes.
Whether and for how long pickling is necessary depends on the external condition of the material. The base material is more resistant to acid attack than steel. However, the alloy components can cause the base material to be removed differently. Nowadays, mixtures of nitric acid/sulphuric acid are used most frequently. An alternative is a mixture of sulphuric acid and hydrogen peroxide. When using hydrochloric acid, there is a risk of pitting, so preliminary tests are required before practical application.
After the discs have been electrolytically degreased, they are decapitated in a solution of sodium fluoride and sodium hydrogen sulphate for about two minutes. After thorough rinsing, the washers can be nickel-plated.
Nickel electrolyte
We recommend the following composition as a pre-nickel plating electrolyte:
- 200 g/L nickel chloride
- 200 g/L nickel chloride100 ml/L concentrated hydrochloric acid
Analysis target values:
- Nickel: 40-60 g/L
- Chloride: 50-75 g/L
The electrolyte is operated at room temperature. The current density of 3 A/dm2 is optimal for this application. Pure nickel anodes can be used as anodes. The exposure time is 5-10 minutes and should not be longer. As the layer has a high residual stress, it must not exceed 2 µm.
Pre-silver plating
Classic silver electrolytes are pre-silver-plated in order to obtain an adhesive layer. The electrolyte is composed as follows:
-
2 g/L silver as K[Ag(CN)2]
-
100 g/L potassium cyanide
-
30 g/L potassium carbonate
Working conditions
Temperature: 25 °C
Current density: 15-20 A/dm2
Exposure time: 30 seconds
The anodes are made of steel.
Coating test
The thickness of the intermediate layer does not usually need to be checked.Usually does not need to be checked. In addition to destructive methods such as cross-sectioning or coulometric methods, it can also be measured using the X-ray fluorescence method if the alloy is precisely calibrated.
In connection with the brittleness of the nickel layer, we recommend stress tests. Particular attention should be paid to the edges where the greatest layer thickness is to be expected. Depending on the material thickness, deformation tests are very suitable. The adhesive strength of the coating can be easily determined using thermal test methods. The panes are heated at 120 °C for four to eight hours and then cooled. To increase the load, it is sufficient to quench a test batch of five to ten panes in cold water directly after the oven.
If the layer flakes off during one of the tests instead of forming bubbles, this is an indication that the nickel layer is too thick. In this case, we recommend halving the deposition time in the nickel electrolyte to test whether this is sufficient for an adhesive layer.
