Question: We received several tons of nickel-plated steel screws that we were allowed to de-nickel and re-nickel plate. The screws were produced and coated in Asia (we were not told exactly where). After transportation and storage, the parts show blisters, overlaps, corrosion and regular chips of nickel. The coating thickness varies greatly and is in the range of 2-20 µm, which is why the fine thread is not common in some cases. As we are particularly concerned about the thread, we would like to know which decoating process you would recommend.
Answer: In the case of the problem described, attention must be paid not only to the technical side, but also to the commercial side. This depends on the amount of work involved, the conditions in the electroplating shop and ultimately the disposal costs. Nickel can be electrolytically and chemically stripped from steel.
Electrolytic process
A sulphuric acid solution with a concentration of 700 g/L is used. The current density is 2-10 A/dm2, depending on the geometry, drum and rectifier. The electrolyte is operated at room temperature.
Handling the concentrated sulphuric acid can be problematic in view of the mass described. The same applies to the temperature. Cooling - to maintain constant conditions - is recommended. If the screws are clean, they can be immersed directly in the solution. If the surface is dirty, it must be cleaned first. It is essential to ensure that the immersion is not wet on wet, as an exothermic reaction of the sulphuric acid takes place. This means that you must allow for intermediate drying.
Chemical process
A cyanide solution is used here.
- 45 g/L 3-nitrobenzoic acid
- 100 g/L sodium cyanide
- 10 g/L sodium hydroxide
The process can be operated at room temperature, but the dissolution of nickel is massively increased by higher temperatures. At RT, 3 µm per hour are dissolved, at 75-80 °C 1 µm per minute. The speed of dissolution decreases as the amount of metal removed increases. The water is preheated to 40 °C in order to dissolve the salts well. In many cases, this is the working temperature.
Important: Potassium cyanide must not be used for the preparation, as this can lead to precipitation and render the stripper unusable.
The nitrobenzoic acid decomposes as the temperature rises. For this reason, the solution should not be heated during breaks, such as weekends. If a large amount of sludge starts to form during nickel removal, this means that a new batch needs to be prepared. With an average layer thickness of 15 µm nickel, approximately 10m2 can be stripped with 100 liters of this solution. The cyanide content should be kept as constant as possible. Both higher and lower concentrations can cause pores in the steel. This risk can be counteracted somewhat by convection.
Handling
Good extraction is a basic requirement in both cases. In the sulphuric acid process, we only recommend drums that are additionally covered to prevent the solution from splashing out. Units whose rotation speed can be regulated are preferable. Keep the speed as low as possible to protect the thread.
Drums are also well suited for the cyanide-containing solution. Smaller quantities can also be treated in bells, which makes intermediate checks easier.
The screws must be rinsed after treatment. In the sulphuric acid process, a final rinse in diluted caustic soda is advisable to minimize flash rust during drying and interim storage if the parts are not immediately nickel-plated again.
Controls
The fluctuations you mentioned increase the inspection effort. Intermediate and, above all, final inspections are important. In most cases, a visual inspection is sufficient, as the nickel should be clearly distinguishable from the steel. To increase process reliability, a few parts are usually placed under an XRF and the spectrum is scanned. If no XRF is available, a quick test for nickel can help.
Disposal
The used solutions are concentrates and should be treated accordingly. We recommend - especially in the case of the cyanide solution - having it disposed of externally. In practice, this often proves to be a problem as disposal companies want to have a sample beforehand in order to quote a price. However, you cannot start work before the price because you have to submit a quote for the service (stripping and recoating) in advance. Compromise: Except for the disposal, you can calculate the price relatively well. Pass this on to the customer with the note that you will pass on the disposal costs incurred 1:1. This should be the best solution for both sides.
Note: This question was asked in the online forum at www.gtfy.de.
