Question: We put our first rack system into operation at the beginning of the year. Previously, we had nickel-plated, tin-plated or silver-plated non-ferrous metals in manual electroplating. We rarely encountered problems with pre-treatment or coating there, which is why we also use the same chemicals with identical parameters in the rack system. However, there are considerable problems with the system, particularly with regard to adhesion. Some of the layers are peeling off the base material and some are peeling off the nickel-plated surface. We also have the impression that these difficulties increase in the summer heat. In order to remain able to deliver, we are currently plating the majority of the goods by hand again. However, as the machine is not intended to be a decoration, but to be used productively on a permanent basis, we would like to rectify the causes as quickly as possible and would ask for your support.
Answer: In practice, such problems are well known and almost always prove to be very complex. It often starts with fundamental mistakes being made at the design stage - especially if there is no practical experience. One of the classic mistakes is to transfer the processes and parameters of manual electroplating 1:1 to an automated process.
Due to this complexity, we can only formulate thoughts and suggestions below, from which you must derive your own tests and measures.
Differences to manual electroplating
The main difference lies in the process speed. Rinsing by hand is much faster, the transfer times are significantly shorter and there are often no significant dripping times at all. An automated system, on the other hand, works with fixed cycle times - and even if these are reduced to a minimum, mechanical limits are reached.
Added to this is the greater flexibility of manual electroplating, which a classic automatic system can never achieve. Experienced employees recognize the condition of the surface and react immediately: If, for example, the water film breaks after pre-treatment, the process step is repeated - by returning to degreasing, for example. An automatic machine, on the other hand, stubbornly executes the programmed processes.
It can be assumed that the same problems would occur if the employees in the manual electroplating process - with identical parameters and chemical concentrations - behaved in exactly the same way as the machine.
If this is not the case, two other influencing factors come into question:
- Differences in water quality
- Differences in room temperature and/or air properties
Differences in water quality can usually be explained by various sources, such as different ion exchanger systems. In this context, conductivity, pH value, impurities and, if necessary, the pipes used should be checked.
The room temperature can be decisive - for example, if the rack system is located on the sunny side of the building and the manual electroplating system is in the shade. In addition to the temperature, differences in the air result from different capacities in supply and exhaust air systems [1]. It may also play a role whether production is open (e.g. through windows, doors or open roofs), so that air draughts reach the rack parts and dry them more quickly, causing a light passive layer to form. Overheating and dripping times have a particularly negative effect here.
Possible causes
It is advisable to turn everything upside down, at least mentally. It is also possible that the process in manual electroplating is not optimal, but is compensated for by well-trained employees who think for themselves.
This raises the question of whether the pre-treatment is really optimally matched to the customer material in terms of chemistry, concentration, temperature and times. Numerous suggestions can be found here [2, 3]. If there are problems with adhesion, practitioners often tend to increase the intensity of the pre-treatment, which can lead to further defects, such as surface attack. The old saying "a lot helps a lot" should therefore be taken with a grain of salt. Depending on the composition of the degreasing agent, excessive chemical attack can also be inhibited, for example by adding sodium silicate. However, care must be taken to ensure that adhering silicon compounds are then completely removed from the surface.
In addition to the rinsing water quality already mentioned, decapping and possible cross-transformers also play an important role. In the coating processes described, two-stream systems are common, whereby separation takes place either directly after pre-treatment or after nickel plating. In both cases, the water in the cross-converter should be slightly acidified. If this is not possible, for example because it is circulating water, activation is absolutely essential before further coating.
The acids used are hydrochloric acid, sulphuric acid or a fluoride-containing dry acid - usually a mixture of sodium fluoride and sodium hydrogen sulphate. Fluoride-containing decapsulations also help to remove poorly soluble surface films such as silicates, borates and lead compounds. The effectiveness can be significantly increased by cathodic or anodic polishing. This is also suitable for activating passive nickel layers. Graphite is preferably used as the counter electrode. Depending on the non-ferrous metal, the treatment time is between 30 and 120 seconds. If the parts are cathodically activated, the current density is around 2.5 A/dm2.
Influence of the frames and temperature
The racks used can have a further influence - both in terms of the type of suspension and their processing and material. In manual electroplating, racks are generally much smaller. Material defects and unfavorable contact points have less of an effect there than in a system where many times more parts are passed through each rack. This can already have a negative impact during electrolytic degreasing, but also during the actual coating process. It is advisable to measure the electrical contacts and systematically check the coating thickness distribution on the rack.
Unfortunately, problems caused by excessive room temperatures under the conditions described are not uncommon, but they are time-consuming and costly to rectify. If all other causes can be ruled out, attempts must inevitably be made in this direction. Often the only option is to wait for cooler days. Night-time test runs often fail because the production hall does not cool down sufficiently to detect significant differences. One reason for this is that not only the air, but also unheated or uncooled tubs - especially sinks - cool down much more slowly than the surrounding area. The warmer water, in combination with other non-optimal parameters (such as insufficient water quality), can lead to problems - such as faster drying of the workpieces with subsequent passive layer formation. Cooling the rinsing water, for example the circulation water, could have a positive effect here.
Conclusion
If you want to automate, you have to do more than just copy chemicals and parameters. A process that appears to run smoothly in manual electroplating can fail in an automated system - not because the system is bad, but because the conditions are fundamentally different. So instead of treating individual symptoms, it makes more sense to scrutinize the system as a whole.
Further information
[1] Environmental technology part 2 - Energy and recycling technology; https://www.galvanotechnik-for-you.de/uebersicht-kurse/umwelttechnik-teil-2-energie-und-recyclingtechnik/
[2] Pretreatment of non-ferrous metals; Galvanotechnik 112 (2020), No. 9, page 1335
[3] Book "From practice - for practice"; Eugen G. Leuze Verlag GmbH & Co. KG; ISBN 978-3-87480-390-8
