ASK THE EXPERT: Solving 'Sandpaper Finishes' in Plating
30. April 2012
Q. Our company produces oven racks. The base material is a low-carbon steel (SAE 1006/SAE 1008). The racks are nickel-chromium plated, and the process we use for the plating is as follows:
- Hot degreasing
- Acidic etch
- Cathodic degreasing
- Anodic degreasing
- Acidic pickling
- Semi-bright nickel plating
- Bright nickel plate
- Chromium activation
- Chromium plating
After the process is completed, the plating has the feel and look of sandpaper, and we must have a bright, smooth finish. Can you give me any suggestions for solving this problem? F.A.
A. I agree that a sandpaper-type finish might have applications in certain areas, but not in decorative plating. Based on the information you provided, I can’t give you a definitive answer to your problem, but I certainly can give you some guidelines for troubleshooting.
The first thing I would do is inspect the parts after each step of the process. Are they smooth and not pitted after the degreasing and acid pickling steps? If yes, then these steps are not at fault. Do the same after each of the plating steps and, if you find the parts have roughness, for example, after the semi-bright nickel plating step, then that step should be investigated in more detail. If the roughness appears after your decorative chromium step, then the chromium plating bath must be investigated.
Assuming that by using this approach you can pin down the step that is giving you the roughness, you will now have to determine what the cause is. In most plating baths, roughness is usually caused by suspended materials in the bath. There are a number of possible causes for this. Start by looking at your filtration system. Filter cartridges do not last forever and must be changed on a regular basis. The cartridge you are using may not filter out the smaller particles that show up in plating baths.
If the baths are only used for short periods of time, they should be covered to keep out dust and grit that can fall into the tanks from the environment. Sometimes particulate matter is due to defective anode bags. The anodes should always be bagged and the bags inspected on a regular basis for tears. Another cause of particulate matter in the plating bath can be parts that have dropped into the tank or fallen off the rack during the plating process. If the drops are not removed promptly they may start dissolving, causing a rough deposit.
Other possible causes of rough deposits in your nickel baths are:
- Current is too high.
- Boric acid is too low.
- pH is out of control.
- Chlorides may be too low.
- Organic impurities from decomposition of brighteners and wetting agents.
If you determine that the roughness appears after the chromium plating steps, there are a couple of possible causes: Your current may be too high or suspended particulate matter may be present.