Preface: Bright nickel plating is widely used. It can eliminate the heavy polishing process and obtain a bright coating.
Bright nickel plating is achieved by adding brighteners to a conventional nickel plating solution. Nickel plating brighteners are classified into primary brighteners and secondary brighteners, also called softeners and main brighteners.
Primary brighteners give the nickel layer good ductility, fine crystallization, and uniform luster, but they cannot produce a mirror-bright nickel layer. To make the coating brighter, secondary brighteners must be added.
When the plating bath contains secondary brighteners, the effect of primary brighteners becomes more obvious. The bright plating range widens, and a mirror-bright coating can be obtained over a wide current density range.
The addition and control of brighteners are very important. Because the brightener content in the bath is relatively low and the control range is narrow, quality problems in bright nickel coatings are affected not only by solution composition and foreign metal impurities, but also by brighteners and their decomposition products.
Most brighteners do not yet have accurate chemical analysis methods; therefore, brightener control is calculated based on the amount of electricity passed or estimated by Hull cell tests.
Bright nickel plating process:
1. Formula and process conditions:
Nickel sulfate: 280–330 g/L
Nickel chloride: 45–60 g/L
Boric acid: 40–45 g/L
Primary brightener: 3–4 mL/L
Secondary brightener: 0.3–0.5 mL/L
pH: 4–5
Temperature: 55–65°C
Cathodic current density: 5–10 A/dm²
2. Process flow:
Pre-inspection → organic degreasing → drying → chemical degreasing → water rinse → etching → water rinse → racking → mild etching → water rinse → dull nickel plating → water rinse → mild etching → water rinse → bright nickel plating → water rinse → drying.
Common faults in bright nickel plating and troubleshooting:
Because the bath contains brighteners, adding them based only on concentration calculation or experience is not enough. Our plant uses the Hull cell test.
Since one Hull cell test can reflect the quality of the cathode deposit over a fairly wide current density range on the cathode panel, and the amount of brightener is often reflected in different conditions on the cathode panel, we usually prepare different Hull cell cathode panels with different brightener contents in the bright nickel solution as comparison standards.
In this way, when the brightener content in the bath is unknown, we can run a Hull cell test and compare it with panels of known content.
1. The coating is not bright enough.
When the coating surface is not bright enough, first check whether the current is too low. With the pH and temperature properly adjusted, increase the current and see whether the coating brightness increases. If the brightness still does not change, preliminarily determine that the secondary brightener is lacking.
In the Hull cell test, first add a small amount of secondary brightener, increasing it gradually from small to large amounts, and compare the amount added with the cathode panel of known content until the test cathode panel matches the standard panel. While observing the test cathode panel, also check whether the coating stress has increased.
2. The coating peels or flakes.
There are many causes of coating peeling, such as poor pretreatment, excessively long power interruption, improper pH, too low temperature, excessive current, or too much brightener or organic impurities. They should be eliminated one by one according to the peeling condition.
First, suspect poor pretreatment or an excessively long power interruption. Observe whether the dull nickel also peels. If the dull nickel also peels, it must be poor pretreatment. Then measure the temperature and pH, and analyze the bath composition.
If these steps do not solve the problem, then analyze the brightener content: take 1 liter of the bath and perform a dilution test. Compare the test cathode panel after each dilution with the standard cathode panel until the test panel matches the standard panel of known content.
Then use the diluted solution for a plating test to see whether the bright nickel layer still peels. If it no longer peels, it means too much brightener was added, and the solution can be adjusted according to the test result.
If it still peels, the problem may be excessive organic impurities or foreign metal impurities in the solution, and the brighteners, organic impurities, and foreign metal impurities in the bath should be treated together.
When treating metal impurities such as iron, copper, and zinc, add activated carbon, then filter. Finally, add brighteners at the low end of the standard content and electrolyze the solution.
3. The coating is hazy or mottled.
When adding brighteners to the plating tank, be sure to stir evenly. If stirring is not uniform, the coating will not only be mottled but sometimes also appear white and hazy, and may also cause brittleness and cracking.
If the coating still has mottled spots after even stirring, the primary brightener may be excessive, and the excess brightener can be removed by electrolysis. Another possibility is that the plated parts were left too long before cleaning.
4. The coating has an orange peel appearance.
Orange peel can sometimes be caused by non-uniform base material, and this is usually visible in the dull nickel layer. If the dull nickel coating is uniform, the orange peel appearance indicates that the primary brightener content is too high or the solution pH is too high.
First measure the pH. If it is abnormal, adjust the pH back. If the orange peel does not improve after pH adjustment, treat the brighteners in the solution by electrolysis or with activated carbon, then adjust the brightener content by Hull cell test.
5. The coating has pinholes.
There are many causes of pinholes in the coating. First observe whether the dull nickel has pinholes. If it does, just find the cause in the pretreatment and dull nickel solution. If the dull nickel coating is uniform, the problem is in the bright nickel solution.
The causes of pinholes in bright nickel include improper pH, organic impurities or oily substances in the bath, and the influence of foreign metals in the solution.
Therefore, the above points should be treated, then the brighteners in the solution should be readjusted and a test plating carried out.
6. The coating is rough.
Rough coating may be caused by a rough base material surface, or by etching. In addition, excessive current density, excessive foreign metal impurities, suspended solids in the solution, or incompletely dissolved replenished materials can also cause roughness.
Therefore, as long as the quality of the materials and pretreatment is controlled, the solution and anode plates are kept clean, and the current is normal, the roughness problem can be solved.
Conclusion: There may be many more problems in bright nickel plating, but these are the common faults. In the practice of recent years, avoiding detours in problem solving has not only reduced production costs but also improved product quality.