electrolytic plating 1

[chtimi]

I Would like to know if someone have an experience with an electrolytic plating type chrome on a piece in Monel.
Is it possible?
It is for a pump shaft, and an application in sea water . An error was made during machining, we have 2 solutions:

first : electroplating
second: Reject the shaft.

Thank your for your help.

JL

 

[kenvlach]

I would not recommend chrome plating as a long-term solution due to galvanic incompatibility in seawater. Chromium is anodic vs. Monel®, and chrome plating is normally micro-cracked, so eventually the chrome would dissolve. However, I do not know your design. Possibly, applying a sealant where the propeller hub fits over the shaft and using sealed, lubricated bearings will suffice if exposed shaft areas do not matter.

For a supporting reference, see MIL-STD-889B, Table II Galvanic Series of Metal in Seawater: Note the wide separation between Chromium plating and Monel. Also, see Table 1 Guide to Use of Joined Metals (most recent version is in Notice 3). The ‘I’ in the center of the cell where the chromium row intersects the Monel column indicates galvanic incompatibility in seawater. Note the ‘C’ in the lower left corner, which indicates compatibility in marine atmospheres (non-immersion).
Corrosion minimization treatments are given in Appendix A. Resin coating of the chromium plating is one method. MIL-STD-889B is available at

http://assist1.daps.dla.mil/quicksearch/

Another idea would be to rebuild the surface by alternatingly plating thin layers of Ni and Cu, of about 2:1 thickness ratio, followed by a diffusion anneal at 800 or 900^oC in a protective atmosphere. However, the cost and risk of shaft warping are probably enough to justify ordering a new shaft.

Hope this helps,
Ken

 

[chtimi]

Thank you for help and links.

JLuc