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I need a coating for a reciprocating rod

I am currently using a Melonite (or similar process) to plate a rod I use in a reciprocating pump application. The application pumps mostly water and no other chemicals. I have used 304SS as a base material and the Melonite will cause the 304SS to rust causing failure. Electroless nickel, chrome plate and TiN have been tried not found to be unacceptable due to eventual surface failure (EN & CP) or too expensive to process and too hard (surface) that wore away the seals (TiN-PVD). Ceramic is a very expensive option that is impractical. Plain stainless is too soft, galls, and the seal becomes damaged. Melonite works fantastic interms of wear, but will fail a soak test in plain water. Nitronic 60 will work well as the base material and not corrode, but is very difficult to source and machine.

The rod is 3/8" diameter x 2" long. I will process 5,000-10,000/year.

Ed James
Pump Manufacturer - Mpls, MN, USA
2007

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Maybe you need to review your seal design instead (material and shape). Too little contact area avoids the formation of a hydrodynamic water film that is needed for seal durability. Also, improper material selection may end up in excessive friction (affinity between rod and seal surfaces) and melting of the seal material.

Guillermo Marrufo
Monterrey, NL, Mexico
2007

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Regarding your use of PVD-TiN, remember that there are many subsets of PVD, with different characteristics. We found that when it's applied by a cathodic arc method, we'll get droplets and mist depositing on the surface along with the coating. This gives a rough surface that certainly can chew up seals. When we have parts coated using other methods like EB, there are no droplets and mist, so the coating is far smoother.

Lee Gearhart
metallurgist - E. Aurora, New York
2007

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2007

Tin nickel electroplate, while I don't know if anyone has a plating tank of this fluoride bath anymore, has some interesting properties; Lou Morin used it for coating valves in the 1970's in New York. I don't know if anyone has the tribological properties, but I found some references on the internet for some articles. The coating is hard and glass-like according to Morin's audio file he recorded.

  
Tom Pullizzi
Falls Township, Pennsylvania

Ed. note: There is also an article entitled "Electrodeposition of Tin-Nickel Coatings on Steel from an Acid Chloride Electrolyte", Ibrahim et al, in the Journal of Applied Surface Finishing, Vol. 1, No. 4, Oct-Dec 2006 (available at www.nasf.org).

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2007

You could use a material for the rod which has inherent adequate corrosion resistance and strength for the job, instead of using a material that you acknowledge is inadequate then trying to protect its surface.

The higher-strength higher-corrosion-resistance stainless steels could be considered, such as 2205 or 2507. They have properties far superior to those of the old 304 and 316 types.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.