Re: What are the possible causes of electrolysis of SS salt water piping

Hi Chris

Stainless steel is far more passive than other allows due to surface 
layers of oxides (passivation).
Thus any process that delivers hydrogen ions - tending to release nascent 
(hyperactive single atoms) at that surface is likely to be highly 
corrosive.
The presence of salt in the sea water makes it highly conducting and the 
ions (from the water) attracted to the stainless steel can reduce the 
oxide film.
There are two modes of action:
1. If there is a different metal, or semiconductor, in contact with the 
pipe elsewhere, a galvanic cell is set up. If such a cell is "short 
circuited" (allowed to produce current) then this current is due to ions 
losing or gaining electrons at the stainless steel and copper surfaces.
Corrosion will result.
2. A much faster action can take place if the current is forced through 
such a cell. For example there may be a 12 or 24 volt battery, or other 
power source, on the ship that is driving ions through the water. It 
depends how that battery is "grounded" and what the pipe is connected 
electrically to.
Any such connexion may be intermittent (sometimes connected and other 
times not: a loose connection via a bad joint) In any case once the 
passivation is removed, iron will react with the chlorine ions present in 
the solution.

Is "rapid corrosion" possible in 4 months? Yes.

Try these:
1. Turn everything electrical off. What current is now leaving the 
battery?
2. Try grounding the negative terminal of the battery (On aluminum boats 
often the positive is grounded).
3. With a multimeter set on its 2 volt range, measure the voltage between 
the pipe and the metal of the boat. Now switch to measuring ohms. Is the 
resistance the same when you swap the leads to the meter?
Try to change the pipe installation to make these resistances as high as 
possible. Ensure that no other metal contacts it at any point on its 
length.
4. Try a sacrificial anode, such a strips of magnesium attached 
(underwater) to the pipe exit.
5 Try http://www.intota.com/multisearch.asp


john