Question: We operate a weakly acidic zinc electrolyte on our barrel system. Occasionally we get perforation marks. This is due to iron contamination, which we rectify by adding hydrogen peroxide. Recently, however, the coating has not become blotchy/dark, as in the case of Fe contamination, but gray to dark gray. We have noticed that we are no longer depositing any zinc coating at all on holes / bores. We were also able to confirm this on Hull cell sheets. Unfortunately, adding H2O2 does not bring any improvement, the coating tends to become even duller. What could cause the problem described?
Zinc anodes Answer: Weakly acidic zinc electrolytes are generally operated at pH 4.8 - 5.4. However, the addition of H2O2 to oxidize Fe2+ into Fe3+ should take place at pH 5.8. At this pH value, 10 ml of 3 % H2O2 is theoretically sufficient to oxidize 1 g of Fe2+. After a reaction time of around two hours, the iron hydroxide precipitates and can be filtered.
In practice, however, this is often done during operation, i.e. at a pH value below 5.8. The pH value is usually allowed to rise slightly beforehand by not adding hydrochloric acid. The desired effect is still achieved with the excess, but it is not nearly as efficient. Organic additives are also lost. This can be recognized by the resulting turbidity. In your description, this will be the cause of the duller layer, especially as it is most likely not iron contamination but lead contamination.
Removal of lead
Lead causes problems even at low levels (from 2 mg/L). This manifests itself in the dark layers you have observed and a lack of coverage in boreholes and corners. From 20 mg/L, the layer is almost completely uncovered.
The literature refers to the removal of lead by adding zinc dust. However, we strongly recommend selective cleaning, as known from nickel electrolytes. Unfortunately, this is not common in zinc systems, but we still advise you to add a corresponding device to the system in the medium term.
Possible causes of lead contamination
Lead contamination is rather rare nowadays, but cannot be ruled out. One possible cause is the anodes, which may be of poor quality due to a shortage of raw materials / new, less reliable suppliers. You should definitely investigate this more closely.
Another cause could be the customer material. Although this is mainly known from brass, it can also affect steel [1]. Finally, the chemistry used can of course also be affected. As with the anodes, the lack of raw materials is also a frequent cause of supplier changes and sources that would be avoided in other times.
Cadmium and nickel
Sometimes the fault pattern is also confused with cadmium or nickel contamination. Cadmium - which is very rare today - cannot be seen as a defect on the zinc coating. It only appears through the blue passivation. From 20 mg/L dark colors appear, from 50 mg/L the passivation is completely brown.
With nickel, depending on the current density, defects only appear from 3 to 6 g/L and are mainly visible on edges after passivation. The surface becomes blue-black-milky here and also does not correspond to the image described.
Organic impurities
In addition to lead impurities, organic impurities may also be present. These are mainly visible as dark coatings in the low current density range. Cleaning is carried out with activated carbon, whereby appropriate filter cartridges are usually preferred.
Literature
[1] Galvanotechnik 110 (2019) No. 10, p. 1921
