Question: Among other things, aluminum tubes (h:400 mm; d:150 mm) are copper-plated and silver-plated on the outside of our rack system. The tubes are covered with lacquer on the inside. After the zincate pickling, they are chemically nickel-plated and then coated 200-250 µm thick in a sulphuric acid copper electrolyte. Before the tubes are silver-plated, we grind the copper layer flat. Recently, we have noticed that the tubes are showing more and more burns. In some cases, the copper layer on the outside is so bad that it is crumbling/tearing off down to the nickel.
The copper content is analyzed and supplemented daily. We check the sulphuric acid content three times a week. This is relatively constant within the bath load. The copper content, on the other hand, fluctuates quite strongly, although the anodes are topped up regularly.
The copper content is set to 20 g/L. The sulphuric acid content is 100 ml/L. The aluminum tubes were copper-plated at 4 A/dm2, but we have now halved this.
The scorching increases the mechanical effort and internal rework massively. Due to the lower current density, metal deposition is becoming increasingly uneconomical. What causes and measures could there be for this problem?
Answer: As is so often the case, there are several reasons for the problem. It is striking that you do not mention an important value in the analyses given: the chloride!
In addition, at a current density of 4 A/dm2 with 20 g/L copper, the electrolyte is working in the limit range. 2 A/dm2 is the usual setting for the metal content. At this cathodic current density, the deposition rate is around 0.44 µm/min. Although you have set the cathodic current density so high, the deposition problems should not be so severe. The anodic current density should be 0.5-2.5 A/dm2.
Chloride content
The chloride content should be 50-80 mg/L and is usually set at 60 mg/L. Chloride supports the operation of the copper bath additives and ensures uniform dissolution of the anode. If the content is too low, the growth of dendrites in the high current density range increases.
A chloride content that is too high is more problematic. As it should be in the milligram range, overdosing can quickly occur here. Values above 100 mg/L are critical in sulphuric acid copper electrolytes. An excessively high chloride content causes anode polarization, which is visible as a white anode coating.
The polarization leads to a low solubility of the anodes, which results in metal depletion. As their copper content is 20 g/L, at 4 A/dm2 you quickly get into a range where the burning you describe can occur. This is not surprising, as with an average layer thickness of 220 µm, approximately 371 g of copper is deposited per aluminum tube.
When using new anodes, increased chloride consumption is to be expected during the forming phase. The chloride content should be analyzed and corrected regularly during this phase.
The chloride content can be determined by potentiometric analysis. By adding 2.5 ml of concentrated hydrochloric acid per 100 L bath, the chloride content is increased by 10 mg/L.
Anodes
Phosphorus-containing copper anodes with a phosphorus content of 0.04-0.06 % are used, which must be encased in anode bags made of suitable, acid-resistant material. The anode surface should always be at least as large as the fabric surface.
For newly inserted, unused anodes, it is recommended to work through the bath for a few hours at room temperature and a current density of approx. 1 A/dm², as new, unused anodes can tend to form a fine sludge that can also penetrate the anode bags.
If the anodic current density is too high (> 2.5 A/dm2), passivity can also occur with the consequences described above. If this is the case, the anode surface area must be increased. The same can happen if the sulphate content is too high.
Even if the chloride content is OK or slightly elevated according to analysis, we strongly recommend that the anodes are checked thoroughly and cleaned if necessary.
For the sake of completeness, it should be noted that scorching can also occur if there is a lack of additives and the electrolyte temperature is low. Based on the description of the problem, however, we rule this out.
Bath care and maintenance
The chloride content should be analyzed and replenished at the same intervals as copper and sulphuric acid. We also recommend a Hull cell sheet every week and a transmitted light test or examination using an angle cathode at least every two weeks.
The transmitted light test is carried out in a Plexiglas test cell (volume 1.25 liters) with air injection. The current density is 1 A/dm2 and the exposure time is 5 minutes.
Two plates are usually run with an angled cathode. One at 1 A/dm2 and 5 minutes and a second at 2 A/dm2 and 60 minutes. In addition to the appearance and layer thickness distribution, the ductility of the first sheet can be tested excellently. The second sheet provides information about the layer structure at high layer thickness. The average coating thickness here is around 26 µm.
Higher currents are not useful, as it is difficult to simulate the flow conditions and thus the subsequent supply of metal ions in the small volume. More precise information about this behavior can be determined approximately using the Hull cell sheet. Polished brass plates are used for this purpose. With a bath volume of 250 ml, metallization is carried out at 1.5 A for 15 minutes. The burn-on zone is approximately 5 mm in the high current density range (locally the current density is 10 A). This means that scorching is normal in this zone. The rest of the surface must be even and, depending on the additives, even shiny.
We recommend increasing the copper content to 25 g/L in order to achieve greater safety in the high current density range. For air movement in the electrolyte, we recommend an air volume of 3-6m3/ h /m pipe length. Filtration must be continuous, with at least 2-3 bath volumes per hour.
