marine rod plating or coating

[bdeuell]

I am working on restoring an old marine head (toilet) that utilizes a hand operated piston pump to intake fresh seawater and discharge the waste. for an idea of how the pump is designed picture a hydraulic cylinder whee the rod end is used to pump fresh water and the base end is used to pump waste. The shaft/rod for this pump is in poor condition and in need of replacement but I am not sure what the best plating /coating would be. most of the head's components are made from cast bronze including the pump rod. the original pump rod was plated with what i am guessing is chrome but this finish has almost completely flaked off in area where the seal operates and it is submerged in seawater. the rod is sealed where it enters the pump housing with a graphite lubricated packing cord. I am machining a new rod from 932 bronze and am looking for recommendations on a coating/plating, ideally in the sub $100 range. My concerns for selecting a plating/coating are galvanic compatibility (submerged in sea water with bronze), low friction on the seal (graphite cord packing), and long life (20 yrs+).

Also related to this project i am planning to let the bronze castings oxidize but am hoping to achieve a even brown appearance and avoid uneven discoloration and the blue/green oxides. any ideas on how to promote this especially in an environment where it may be splashed with water and other fluids would be appreciated.

 

[EdStainless]

You should look at using NiAlBronze for this. The long term static exposure is very aggressive.
Typically the plating would be done in two layers, but all plating is porous and will only provide limited protection. The issue is that the Cr is more corrosion resistant than the bronze, so when corrosion starts the plating actually makes it worse.
Using bare NAB might be a better option.

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P.E. Metallurgy, Plymouth Tube

 

[bdeuell]

Thank you, for your response.

I went with 932 bronze because it was the only precision ground bronze stock i could purchase from mcmaster. I wasn't able to find free access to a od/centerless grinder and being a one off part i figured i wouldn't like the price from a job shop to grind a different alloy to size. Hopefully the less ideal bronze wont preform that much worse.

My concern with leaving the bronze bare was friction on the seals and surface hardness. however the pump housing bore is left unplated so maybe i should just put it together unplated and see how it works.