Galvanic corrosion and other related subjects

[joanlluch]

Galvanic corrosion is a phenomena that occurs when two dissimilar electrode potential metals are put in contact in an electrolyte such as conductive water.

A lot of studies have been carried out in the past specially related with sea water and use of materials in boats.

Also Galvanic potential tables have been long ago published to help designers with the best choice of materials to prevent possible corrosion issues. One of the most commonly used tables is the one below:

galvanic potential by joan.lluch, on Flickr 

The unshadowed figures for Stainless Steel grades correspond to activated stainless, such as in stagnant water of when oxygen can not reach the surfaces, which to my understanding do not apply to the case I am exproring below.

My question arises from the observation in the Britannia plans that Stainless Steel is specified for the valve spindle and the bobbin, but gun metal casting is used for the valve sleeves. See this:

SS Valve by joan.lluch, on Flickr

 
These plans also show a no tolerance fit between the sleeves inner bore and the valve (both are specified at exactly 0.9375" diameter)

The combination Gun Metal/ Stainless is not the worse possible one (approximately a potential difference of 0.3 V) but it is not ether the best possible one. 

My understanding is that in most cases, at least the valve spindle is made of Stainless, and this is put in contact with another less "noble" material. However, I have never seen a corrosion problem being reported.

My question is whether should we worry about any possible corrosion effect between Stainless Steel and Gun Metal,Bronze, or Brass, in the above scenario but also in a GENERAL way. And which is the experience on such a possible issue (or lack or it).

Thanks for any comments.

[alanstepney]

Dont worry about it

In full size there were some attempts to "control" it using anodes in the water, much like they do on boats.
It wasnt worth the effort, so they discontinued the idea.


PS, it is an excellent way to remove rust and paint though!
(I've just set up my paint.rust stripping system in the garden.!)

[jma1009]

hi joan,

i have never had a problem with stainless valve and piston rods except that the standard of finish on some stuff sold commercially isnt as smooth as others.

i would never make the piston valve bobbin out of stainless having seen far too many seize up over the years. hard drawn phos bronze is a far better material in my opinion in cast gunmetal piston valve liners.

for stainless in boilers such as regulator rods and check valve balls and blowdown valves you have to be a bit careful, and i only use non-magnetic stuff. ive seen magnetic stainless corrode quite badly in these applications.

cheers,
julian

[joanlluch]

Mar 23, 2015 13:52:14 GMT jma1009 said:

hi joan,

i have never had a problem with stainless valve and piston rods except that the standard of finish on some stuff sold commercially isnt as smooth as others.

i would never make the piston valve bobbin out of stainless having seen far too many seize up over the years. hard drawn phos bronze is a far better material in my opinion in cast gunmetal piston valve liners.

for stainless in boilers such as regulator rods and check valve balls and blowdown valves you have to be a bit careful, and i only use non-magnetic stuff. ive seen magnetic stainless corrode quite badly in these applications.

cheers,
julian

Thanks Julian, Could the issue you mention with seizing up after years, be ultimately a corrosion problem?. I take your note that this will happen after years, so that may suggest that it is a slow and continuous process what is going on there...

[jma1009]

hi joan,

the seizing up of stainless piston valves hasnt been due to corrosion but due to difficulty getting the fit right. i am not alone in this observation!

'over years' not 'after years'.

cheers,
julian

[joanlluch]

Thanks Julian, I see what you mean now.

[Deleted]

(1) Galvanic corrosion is not normally going to be a problem with stainless steel piston valves in gunmetal valve liners . The presence of even a trace film of oil is usually sufficient to inhibit any corrosive action . Also the continuous movement of the valve has a natural cleaning action .

Only time any problem is likely is when engine cylinders are left without oil before a long period of storage or if some very acid water finds its way into steam .

(2) One of the reasons why piston valves sometimes seize up after a period of normal working is to do with shrinkage of the valve liner . A pressed in thin wall liner can sometimes close up on the bore over time . No one notices until the valve gets too tight in the bore and then a seizure occurs .

Stainless steel valves are more of a problem in this respect than bronze ones because stainless steel is relatively hard wearing and retains its size whereas bronze tends to wear down a bit in a tight bore and to a useful extent self correct for liner shrinkage . 
     
(3) Some grades of stainless steel don't like water droplets in steam and cavitation in water . Most common stainless steels are only stainless because an inert film forms on the surface which inhibits any further corrosion . Where there is fast flow and any scouring action the film can break down and rapid corrosion will occur .

That is why balls in clacks etc sometimes have a very short life .