Stainless steel ball valves?

[Norman_E]

The thread on skin fitting failures reminded me that I had a ball valve break in half the first year I had the boat. It was the one that lets seawater in to cool the fridge compressor, and was probably brass. The actual skin fittings on the boat are bronze or DZR. I know that because all of them are in good condition. The ball valves fitted to them are of doubtful metallurgy, and when I needed to replace the engine sea-cock (because it was very stiff to move) I had a choice of various yellow metal ones of unspecified composition and a 316 stainless ball valve. I chose the latter after talking to the owner of a local marine service company.

I know that there will be a small galvanic cell between the bronze skin fitting and the stainless ball valve, but the brass or bronze valves all seem to have a stainless ball, so if galvanic erosion is an issue then the bronze ones must risk the ball being eaten away.

Was I right to choose the all stainless ball valve, or is it a dangerous choice? So far (3 years) the fitting looks perfectly OK.

 

[aluijten]

If I remember correctly there is a plastic jacket between the stainless ball and the housing to isolate the stainless ball from the housing.

 

[Norman_E]

If I remember correctly there is a plastic jacket between the stainless ball and the housing to isolate the stainless ball from the housing.

I thought the plastic was there to give a good seal, but unless the shaft is also plastic sleeved there will still be metal to metal contact and electrical connection. On all my valves the handle makes contact with the metal casing in both full open and full closed positions anyway, and it is a metal to metal fit to the ball.

 

[vyv_cox]

Are you sure the ball is stainless steel? Many of them are electroless nickel plated brass.

The galvanic voltage between bronze and stainless steels suggests that corrosion might occur but with a screwed coupling with PTFE tape or sealant I guess the electrical contact won't be very good and most water should be excluded.

 

[aluijten]

Could be right, yes. In that case I don't really know how corrosion is prevented. On the other hand many rudders also rely on bearings that are made from bronze & stainless, so this particular combination might not be a problem in salt water.

 

[Stu Jackson]

If you install it so that you can remove the bonnet and take out the ball for servicing or replacement, then usually the body will hold up for a good long time. I have a valve on the boat that's seen four replacement gates over the years, the body is still intact. I know, I know, gate valves don't belong on a boat, but it's not a thru hull, and I can't get a replacement valve of any kind to fit in that particular space (I've tried). The valves themselves cost only a few dollars, so I buy a few valves, toss the bodies and keep the guts for replacement.

 

[Bosun Higgs]

I know that because all of them are in good condition. .

Thats an unwarranted assumption. My valves look just like the brass ones in the YM article and they are still functioning after 14 years. I suspect that brass ones might well last that long if the boat is not electrically active and the anodes are good. The standard is 5 years min with no corrosion not 5 years max to failure.

 

[Norman_E]

Thats an unwarranted assumption. My valves look just like the brass ones in the YM article and they are still functioning after 14 years. I suspect that brass ones might well last that long if the boat is not electrically active and the anodes are good. The standard is 5 years min with no corrosion not 5 years max to failure.

I am talking of the skin fittings which show no sign of going pink, whereas the ball valve fitted to the fridge skin fitting had lost so much zinc that it snapped in half when I closed it. That particular one was clearly just common brass whereas the skin fitting itself is bronze, or possibly DZR. As all the skin fittings were given a scrape whilst the boat was out of the water and none show any sign of losing zinc I am confident that they are all bronze or DZR. Interestingly the brass ball valve that broke had no connection to any other metal except the skin fitting and neither it nor any of my other fittings are earthed, therefore its failure must have resulted from it forming a galvanic cell with the skin fitting.