Magnetic Moments Revisited

Q. Our company recently has become involved with a technical problem that we are unable to resolve. We supply a number of companies with zinc plated steel components. One of these components comes back from our plating vendor with non-plated areas and roughness on certain surfaces. The parts are zinc-plated and treated with a RoHS-compliant chromium conversion coating. The parts are machined at one facility and zinc plated at another.

When discussing this with the manufacturing company and the plating shop, we receive the usual finger-pointing. The plating shop says the parts are magnetic when they receive them and the machining operation says they are not magnetic when shipped to the plater.

Do you have any words of wisdom as to how we might be able to resolve this problem? F.P.

 

A. The finger-pointing you mention is a fairly standard procedure in the metal finishing industry (actually in most aspects of modern life). As we all have learned from experience, this approach to problem-solving usually does not work. So what does work?

Let’s start with some well-known information about magnetic parts. If steel components are magnetic, roughness is certainly one of the results of plating these parts. What causes this roughness is if a plating bath contains small amounts of iron, and many do, the iron is attracted to the magnetic surfaces of the parts and a rough is obtained where these iron particles have collected. This roughness is usually found on all surfaces of the plated parts. To eliminate this, parts must be degaussed (demagnetized) prior to plating. Better filtration of the plating bath can also help.

Your e-mail suggests there are areas on the surface of the parts that do not plate. I know there have been some reports of this type of problem, but usually roughness is the “smoking gun.” A more likely cause of this is probably inadequate cleaning of the steel prior to the plating step.
What might be a cause of this magnetism? Steel parts, if plated in a barrel, may become slightly magnetic from the tumbling action in the barrel. If the particular parts that show this problem are plated in barrels as opposed to racks, the parts could take on a small amount of magnetism during the actual plating process. Parts can also become magnetic during the machining process.

The first thing you should do is talk to the machining company and determine how they certify parts being magnetic free. You must also talk to your plating vendor and find out how the parts are being handled when they are received from the manufacturer. Are they being plated on racks or in barrels? What type of cleaning process is being used to clean the steel prior to the plating step?

The next step would be to determine if the parts are really magnetic when they are received from the machining company. How does the plating shop determine that parts are magnetic? I am not sure what the best method is for determining whether a part is magnetic.

One method I have seen used is similar to a grade school science class experiment. A magnet is covered with a piece of paper and a small amount of fine iron filings are scattered on top of the paper. The paper is gently tapped and then patterns in the magnetic filings are looked at. You should see some of the filings lining up and that indicates magnetism. If this simple test shows signs of magnetic fields a portion of the parts should be degaussed and plated separately from any other parts. An inspection of the parts after the plating step should show greatly reduced roughness or complete elimination of the roughness problem. If the roughness has disappeared with this experiment, then the parts must be degaussed prior to the plating process.
If you still have unplated areas, then the cleaning process must be investigated. Perhaps the manufacturer is using machining fluids that require a modified cleaning process. Once again, it is most important that you get past the finger-pointing stage and obtain some real information.

Additional Information:
“Gauss meters are available from several sources on the Internet. A cheap analog-style meter can be purchased for less than $40 and will detect magnetism/gauss down low enough that if the meter doesn’t wiggle, there isn’t any magnetism in it. The only magnetism that could be in a part but can’t be measured, is if it is circular magnetism in a sphere. A ball bearing could be magnetic, and the only way you would know is if it cracked, you would be able to find the N/S poles across the crack.” (Google “gauss meter” to find vendors.)

The information about magnetism in a sphere was something I did not know about. It’s always great to learn something new! Thanks to Mike Janes for taking the time to send along the additional information.