Cleaning Q&A: Coating a Threaded Area

Why won’t this conversion coating take to a threaded area?


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Q. My company is currently zinc phosphate coating a part made out of AISI 1018 steel per the TT-C-490E specification by immersion application for the painting. We have no problem meeting our panel weights during our setup procedure, however, the parts themselves have a threaded area where the phosphate does not seem to be taking. Between 3 and 10 percent of the parts that come out of our phosphate process immediately flash rust in the threaded area, regardless of how we dry them. It seems there is no conversion taking place in this area.

The speculation in the shop is that we don’t have enough agitation during the process and my assumption is that the fallout rate would surpass 10 percent if there was a flaw in the process. Why won’t this conversion coating take to a threaded area?

A. To verify the presence of a conversion coating, submit a threaded section of the part through a scanning electron microscopy (SEM) to confirm the presence of the crystalline deposit. For reference, compare the flat area with a good phosphate coating to the threaded areas that aren’t taking. Many contract laboratories have an SEM to assist you.

If there is not much difference between the two areas, there may be a significant amount of free acid trapped in the screw threads. If so, a stable phosphate coating is unlikely. Also, the part may not rinse well, so it may carry over quite a bit of the free acid through the remaining stages and flash rust quickly when exiting the phosphate system.

If you do see a significant difference, there could be a couple potential reasons. One reason is that the threaded area may have a relatively smoother surface finish than the balance of the part. The smoother surface finish will generally lead to a worse phosphate coating with a lower coating weight. This could be due to the threadmaking process locally forming the material, creating some variance between parts depending on the base material and tool maintenance.

Another reason for the variable coating could be the residual chromium and nickel left from the tool steel. Both elements are frequently found in tool steels that could be used for the thread cutting and forming, and are known to inhibit the initiation and growth of the zinc phosphate coating. There would be little or no phosphate coating in the area where these deposits are embedded in the surface.

One potential solution would be cleaning these parts in an acid to ensure any tool steel residue is removed prior to phosphating. Alternately, you can discuss this with the department or supplier performing the thread cutting. An alternate steel, such as a high carbon, surface hardened tool, may not have as good a tool life, but could eliminate these sporadic problems.

 

Originally published in the June 2016 issue.  

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