Stains on Plated Parts

Question:

I work for a plating company that deals with plastic plating. We have a problem with water stains on parts after they come out of our dryer. After going through the decorative chrome, the parts go through four rinse tanks. The last two are DI water, yet we still have some stains. Is there anything else that we can do to avoid these drip stains? P.B.

Answer:

You need to look at the conductivity and control method of these final rinse tanks. If the tanks really are DI water, by definition there is nothing that is in the water that will cause staining. As soon as a rack of parts touches the DI water rinse tanks though, there will be carryover of chemical and water contamination from the previous rinse tanks. It will take very little carryover to contaminate these tanks so that they are realistically no longer a DI water rinse.

My first suggestion would be to examine and document the conductivity of the bath both when you do and do not see the water staining occur. This will provide you with an idea as to what your operating window needs to be. Obviously, the conductivity will need to be below the level you read when it causes water staining but may be able to go higher than when you take a reading at the times it is not staining. You can define this as your starting operating window.

Further testing will be needed to better refine that operating window so you can balance final product quality and the economics of increased DI water usage. When you see stains re-occur, you should then lower the conductivity in the final rinse tank by overflowing it. Hopefully, your tanks are arranged and plumbed in a counterflow fashion in order to minimize water usage (if not, this is a question for a future column). Continue to lower it and document the results you get until you feel you have established a final operating window that will both minimize water usage and maximize product quality.

Once you have determined your operating window, the best way to control it automatically is by installing a conductivity controller and solenoid valve. This will allow you to minimize DI water usage while still maintaining the necessary quality of rinsing.

There are a few other considerations and options. If the rack of parts is such that spraying would be effective, you may want to consider a final fresh DI water spray rinse after the parts are removed from the final rinse tank. This would use fresh water coming right out of the DI cartridges to ensure there are no dissolved solids. This water could then be used as make-up for the final rinse tank.

Also consider the DI cylinders. It is possible that the resin is spent and needs to be recharged. If this were the case, you may never be able to get the conductivity low enough to eliminate the water staining. Most systems have lights that indicate the resistivity of the water is high enough (conductivity low). This light will go out when the resistivity gets lower than the system is rated for. You should also check to make sure you have a “mixed bed” ion exchange system. If it is not a mixed bed system, it will only remove anions (negatively charged) or cations (positively charged), but not both. If this is the case, the ions that are not removed would still leave residues that can cause the spotting.