Dissimilar/Similar Pipe Connections 1

[wwtpguru]

In a Stainless Steel line, is it neccessary to have cathodic protection between carbon steel to SS connections or SS to iron bodied valves.
I have tried to research on dissimilar metals but all I can find is info regarding galvanic materials to other metals. Is SS (304 or 316) and ductile Iron or steel considered similar enough metals not to require cathodic protection? Does anyone know of a table out there that shows which metals you need cathodic protection and which metals you don't?

 

[waterworker]

You probably know all of this , but:

http://www.azom.com/details.asp?articleID=1177

“This effect is also dependent upon the relative areas of the dissimilar metals. If the area of the less noble material (the anodic material, further towards the right in Figure 2) is large compared to that of the more noble (cathodic) the corrosive effect is greatly reduced, and may in fact become negligible. Conversely a large area of noble metal in contact with a small area of less noble will accelerate the galvanic corrosion rate. "

It is a common practise of ours to refurbish /excavate iron and or cast iron body valves and replace all of the bolts ( 12 to 20 bolts) with stainless steel bolts ( small volume of noble metal compared to the large volume of less noble metal in the valve) ) and attach a 9 pound magnesium anode under one bolt head with a wire loop to protect the less noble valve body.

We don't cathodically insulate the stainless bolts from the valve body because the area of the cathode ( 316 stainless bolts ) is small compared to the area/volume of the less noble valve ( the anode ). If the situation were reversed- If a large area of stainless was in contact with a small area of carbon mild steel or iron, then dissimiliar metal galvanic corrosion would possibly become a problem. The metals are dissimiliar, but comparative volume is imortant. Some links that talk around the edges of your question:

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

http://www.diveweb.com/maritech/features/uw-su99.01.htm

http://corrosion.ksc.nasa.gov/html/galcorr.htm

 

[briand2]

I'd say it also depends upon the pipe contents. In a "hard water" area, for instance, the scale deposition can greatly reduce the rate of corrosion. That's why, in London (UK), there are many domestic hot water systems comprising a mixture of copper and galvanised pipes yet they don't generally suffer from excessive corrosion.

Regards,

Brian