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Corrosion on plated 'sintered' bushings
July 6, 2011
Q. Hello,
My name is Michael Wardas and I'm a mechanical designer. I've looked through your site many times and really appreciate the thoroughness of your replies!
Background:
I have a sintered steel Browning Split Taper Bushing 1008 and it is showing signs of corrosion. The environment where this is happening is on our factory floor! This is a controlled environment and much nicer than the paper mill environments where this component will be going. The part is located in a drive system and is moderately protected by purge air and should not see any direct contact with water or chemicals. Just the humid and chemical cloud that is sometimes present in a paper mill. The other corrosion resistant materials and finishes which we specify on our equipment are: SS 304, 18-8, anodized aluminum, powder coated steel & aluminum and zinc plating.
Attempted solutions:
We have tried zinc plating these bushings and the zinc plating looks blotchy on some parts and is fine on others. We zinc plate the solid steel pulleys with good success.
I also tried flash chrome plating 0.0005" thick (I've read your Introduction to Chrome Plating page) and that seems to produce a decent finish with inconsequential dimensional changes to the part. This is what we got from the vendor: 1008 BUSHING 1 3/8" OD X 7/8". ID 3/4". FLASH CHROME CONSISTENT LAYER OF 0.0005" ALL OVER. OD AND FACE IS IMPORTANT SO NEEDS FULL COVERAGE.
So my questions are these:
1) Was the 'blotchiness' of the zinc plating characteristic of sintered steel or is it a processing issue?
2) Is the flash chrome plating a good solution with the spec above?
Many thanks for your prompt reply.
Best regards,
Mechanical Designer - N. Vancouver, BC, Canada
A. Hi, Michael.
It can be problematic to plate directly on sintered metal because it is so porous that it absorbs water and plating solutions, which later weep out or out-gas. This porosity is probably at least part of the reason that the component rusts so fast on the factory floor.
Successful plating probably will involve impregnating against the porosity before plating. Try to find a plating shop that does vacuum impregnation, and then I think simple zinc plating and chromating will solve the problem for relatively benign exposure like on your plant floor. But for papermill applications you may find it necessary to electroless nickel plate the component unless you have found zinc plating to be satisfactory.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 6, 2011
Q. Thanks for the prompt reply.
We have tried 'flash chroming' these sintered parts with some apparent success. Do you think we will see corrosion occurring due to the chemicals used in the plating process 'seeping' through?
Thanks again.
Mechanical Designer - N. Vancouver, BC, Canada
July 12, 2011
July 12, 2011
A. Hi, Michael.
1/2 a mil of plating sounds about 10X heavier than a "flash", but no big deal. If nothing has seeped out after several days, I think it probably won't. But I'm not too confident of chrome plating over sintered metal holding up in a paper mill ... chrome is usually "cracked" and doesn't protect the substrate well, and I would expect a paper mill to be sulfide ridden. You might have a couple of parts "Kesternich" tested. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Hi Michael
Perhaps I am not understanding the function of this bush but sintered bearings are normally used to provide a long lasting reservoir of lubricant held in the pores. They are designed to be highly porous and pose a different plating problem to sintered functional parts which are intended to have minimum porosity - which the plater can deal with.
If you try to plate porous parts, there is unlikely to be any significant deposit within the body of the part and removing the plating chemicals is next to impossible.
It is entirely to be expected that your plating is uneven - and zinc is a very poor bearing metal.
I am a little surprised that you are using sintered steel. Sintered bronze bearings would be the expected choice and these would be impregnated with oil either during manufacture or on assembly. If you must use steel, I suggest soaking them in oil before assembly.
Geoff Smith
Hampshire, England
July 14, 2011
Taperlock Sprocket
on
Amazon
(affil links)
Taperlock Bushing
on
Amazon
(affil links)
Hi Geoff. We are probably having one of those British English vs. American English moments. Split tapered bushings are a way to lock a component like a pulley or sprocket onto a shaft while offering some advantages over boring-to-size and cutting keyways like holding on better and not rolling the keys.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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