Question: Like so many other electroplating companies, we are currently working on switching from chromium(VI) to chromium(III) electrolytes. Our requirements are primarily decorative. As far as we have been informed, there are electrolytes based on sulphate and chloride, whereby - as we have been told - chloride electrolytes have prevailed. Unfortunately, we do not have any further information and we do not know whether tests with sulphate-based electrolytes are worthwhile at all. In your opinion, should we test the whole spectrum or should our preliminary tests be limited to chloride electrolytes?
Answer: There are some good reasons for chloride-based baths [1], but sulphate-based electrolytes are also used [2]. The three main criteria are economy, corrosion resistance and appearance. You would need to discuss this - also in consultation with customers - and set the priorities.
Cost-effectiveness primarily relates to the coating speed, as such electrolytes generally have to be integrated into existing systems. Electrolytes that work more slowly than your current chromium(VI) processes could cause problems. Most chloride-based electrolytes work about twice as fast as sulphate-based electrolytes. This means you can achieve the same speed as with conventional processes and do not need any additional stations or adjustments to the control system.
" Color and brightness are seen as the most important optical criteria. "
Visually, the electrolytes of the current generation of chromium(VI) processes are hardly inferior, if at all. For years, both types of electrolyte have been in a neck-and-neck race to come as close as possible to the "original". Color and brightness are seen as the most important optical criteria. Depending on the electrolyte, a whole range of colors can even be covered [3]. Samples from suppliers can already help to make initial decisions here. However, there is also the factor of color stability, which can decrease over the years. Tests carried out between 2015 and 2018 indicate that sulphate-based electrolytes lose more color stability over the years [4]. However, this does not necessarily apply to the current generation of electrolytes. However, it is in the nature of the tests that it is difficult to provide reliable information on this at present.
In the corrosion tests, chloride-based electrolytes also appear to be ahead [4]. As these are very complex relationships, current preliminary tests and corrosion tests would be advisable, regardless of possibly outdated studies [5].
In addition to the three factors mentioned above, there is wastewater treatment, which can be regarded as a sub-item of economic efficiency. Here too, preliminary in-house tests are of decisive importance.
If we were you, we would proceed as follows: We would contact all relevant suppliers in German-speaking countries and ask for all data sheets. Based on this information, we should be able to check the cost-effectiveness. All electrolytes that do not meet the required criteria are eliminated. We would request color samples and specimens of the remaining electrolytes for initial laboratory tests. Here, the handling can be tested with the first test parts and the appearance can be checked with the color samples and the current chromium(VI) electrolyte. As the chemistry is already in-house, tests can also be carried out with the waste water. Of course, you should bear in mind that such tests will only show trends and not absolute values.
Once the field has been narrowed down further, the next step is to carry out tests on a larger scale. As a rule, such tests are carried out in smaller tubs with a volume of 20 to 100 liters. This allows customer parts to be chromium-plated on a small scale. Corrosion tests can be carried out on some of the samples, while another part is sent to the customer for assessment.
In the end, there will be two or three electrolytes that meet all the requirements, at least for the most part. If these are also at the same price level, the decision will depend on which supplier and electrolyte type you trust the most in the long term.
Literature
[1] How to Choose Between Sulfate and Chloride-Based Trivalent Chromium; Products Finishing; https://www.pfonline.com/articles/how-to-choose-between-sulfate-and-chloride-based-trivalent-chromium
[2] Chromium(VI) goes, chromium(III) comes; Galvanotechnik 1, (2021), p. 49
[3] High-performance chromium(III) process; Galvanotechnik 5, (2020), p. 738
[4] Products Finishing; https://www.pfonline.com/articles/how-to-choose-between-sulfate-and-chloride-based-trivalent-chromium
[5] Electrochemical simulation of the CASS test; Galvanotechnik 6, (2023), p. 689
