Question: For a few months now, brass parts have been electroplated in our manual electroplating shop using a barrel and rack. Most of the parts are nickel-plated and tin-plated, while some items are nickel-plated and silver-plated.
Sometimes the coated parts show streaks and irregular roughness. In some places, it looks as if fingerprints can be seen; these also occur with drum goods and it should be possible to remove the corresponding marks.
Initially, we assumed that there was a problem with the nickel electrolyte, but we were unable to detect any deviations here. The Hull cell sheets look just as good as the steel parts that are coated in the same electrolyte. We currently suspect that the cause lies in the pre-treatment, which consists of three stages:
- Cyanide boil-off degreasing
- Electrolytic degreasing
- Fluoride content decapitation
Only non-ferrous metals are pretreated in these three tanks. As far as we can see, the defects already appear in a weakened form after decapitation. Especially after tinning, the defects are much more obvious. We do not have any adhesion problems, it is purely an optical effect. Nevertheless, the customer insists on a perfect appearance for the technical parts. Reworking is not worthwhile, as the result is the same afterwards.
Answer: The main difficulty with pre-treatment problems is that different causes can lead to the same - or similar - defects. In the case of non-ferrous metals - and here again preferably brass - it depends on the alloy, the processing, the materials used and much more.
"Fingerprints" defect pattern
This defect can actually come from real fingerprints. If the brass parts are touched by your employees or the customer without gloves, the hand perspiration can react with the surface. It is not the grease that is decisive here, but the chemical reaction with the metal surface. This only occurs rarely and only a few days a month. Even fabric gloves are not sufficient protection; employees must wear rubber gloves when handling the parts.
There is therefore no point in discarding the pre-treatment or reinforcing it further. The reaction is already complete before the parts reach the decoction degreaser. On the contrary, you may even intensify the degreasing and cause additional problems. You may have done this because of the first defect, causing the other difficulties.
The reason why this defect does not always occur is due to process variations and, above all, the base material and its processing. The surface of oily parts is not affected as much by aggressive degreasing as that of dry, oil-free surfaces. The grain structure of the surface can also have an influence, to name just one example.
"Streaks and roughness" defect pattern
This sounds like a typical defect pattern caused by overly aggressive degreasing. As electroplaters, we tend to continuously increase the pre-treatment with sodium hydroxide and complexing agents. True to the motto: "A lot helps a lot!" This can severely damage the surface of brass in particular. Due to the water film, this attack is barely visible even on frame parts, but it is clearly visible after electroplating and drying. Some people refer to the coating as a "magnifying glass" in reference to the clear defect.
Alkalis generate a high pH value and good conductivity to easily saponify fats. The cyanide serves as a complexing agent, but also removes copper in combination with hydroxides. As zinc is an amphoteric metal, it also dissolves. Together, this results in streaky, rough surfaces.
If at some point you coat brass alloys with a high lead content, this can even lead to adhesion problems. Then you have created a technical problem from a purely visual one.
Cyanide can be replaced or at least supplemented by phosphates, which - apart from the toxicity - only reduces the problem in waste water treatment. It does not have such a great influence on the surface, as phosphates also have a complexing effect, but significantly more on calcium and magnesium ions.
Carbonates and borates are used in degreasers to stabilize the pH value, but are often omitted in proprietary formulations of highly alkaline cleaners. It should be noted at this point that, in our opinion, you do not need a highly alkaline cleaner for brass.
Surfactants have the task of dissolving fats and oils from the surface and then emulsifying them in the cleaning solution to prevent them from being drawn onto the cleaned component. As a rule, they have no influence on the defect pattern. However, the surfactant content could be increased to reduce alkalinity. Silicates are much more important. They have a good dirt-carrying capacity and at the same time have a protective effect on the surface by forming a very thin covering layer. The silicates are added as sodium silicate (sodium water glass).
The perfect degreaser
There is no cleaning solution that is perfect for all requirements. We can only give you tips to help you find the right mixture for your products and requirements.
Depending on the degree of soiling, the pH value should be around 8-9.5. Degreasers for brass tend to have a slightly lower pH value (approx. 8.5) and higher temperatures (70-85 °C). Above all, make sure that the cloud point is not exceeded. Otherwise the degreasing additives will start to oil out and become ineffective. The cloud point depends on the degreasing additive used and the salt load of the degreaser. The higher the salt load, the lower the cloud point.
In addition to a surfactant mixture, borates, phosphates and silicates are added. We recommend using this opportunity to find a mixture that will allow you to completely dispense with the use of cyanide in the future. Gluconates, phosphates and phosphonates are suitable as soft complexing agents. Stronger complexing agents (and therefore more problematic for wastewater treatment) are alkanolamines, citrates, polycarboxylic acid, polyoxicarboxylic acid and NTA. Try it first without complexing agents (except for phosphates) and then with the soft complexing agents before shooting sparrows with cannons. A high proportion of complexing agents is generally only required if non-ferrous metals and steel are also treated in the same degreasing process. In such cases, it is advisable to use a highly alkaline amine-containing system.
A slight removal of the surface (we are talking about a few atomic layers here) is desirable during pre-treatment in order to obtain an active surface. Nevertheless, the following rule applies to all pre-treatment steps: only pre-treat as much as is absolutely necessary!
