Aluminium togehter with Copper 1

[lasselur]

I have a cooling circuit where some aluminium is used, this have forced me to use Stainless steel and not copper everywhere else du to elecromagnetic corrosion. The problem i now have is that certain components I need does not exist in SS. Is it ok to use Crome plated Carbon steel or Brass togeter with aluminium. The liquid running inside the pipes is water. Could normal corbon steel be used with Aluminium ?

 

[kenvlach]

We've been a little derelict in not mentioning the importance of maintaining the correct pH.
Besides adding a little sodium silicate, adjusting the pH to 7.8-8.0 will minimize corrosion. Cu has a solubility minimum of ~0.05 ppm at pH ~8.5; see

http://www.finishing.com/Library/metalpH.html

but Al starts to dissolve as pH increases above 8.

Most municipal water supplies are kept at a pH of 7.8 and a zero or neutral Langelier index (enough hardness to prevent dissolution, but without causing CaCO₃ deposits). The comments on copper pipes in homes reminded me of some lawsuits here in California. A builder supplied an entire housing subdivision with slightly acidic well water; all copper plumbing had to be replaced inside of 3 years. Lots of interior damage.

 

[moltenmetal]

Thanks, kenvlach! That was useful information and a good site to refer to! Impure water with low pH will definitely shorten the service life of copper, as in your well-water example. But I'm concerned particularly about the apparent difference between condensate and deionized water- two things which ideally should be identical, but one of which apparently eats copper while the other clearly doesn't at an appreciable rate.

Absolutely pure water will of course have a pH of 7, where the graph from finishing.com shows a solubility for copper ions of ~ 4 mg/L- so in the absence of a passive film, the copper will dissolve until this concentration is achieved. But clearly, copper doesn't dissolve rapidly in condensate, otherwise you couldn't keep 0.032" thick copper heat trace tubing in service for any appreciable length of time. Condensate certainly isn't alkaline, so why doesn't the copper dissolve away to keep the condensate saturated with copper?

A material may survive a pH at which its bulk metal species are soluble if it forms a self-healing passive film on its surface- surely this is true of copper, which is actually listed as resistant to dilute non-oxidizing acids like sulphuric, phosphoric and acetic. What I'm trying to understand is how deionized water apparently affects the stability of the passive film on the surface of copper, whereas condensate does not seem to disrupt or prevent the healing of this film. Could dissolved oxygen content play a role?

 

[unclesyd]

metalguy,
Went and checked the recent failure I mentioned in my post. The failure/hole is 1 ½" below the tee that allows an air chamber in the line. The branch line is good until about 18" from the tee where about 3" to 4" is affected to about ½ wall thickness. The Cu pipe that I was able to examined was all affected in some degree.
Another puzzling aspect is in the initial failure being upstream of a glassed lined water heater that is being dissolved why wasn’t there enough other ions to protect the Cu in this area.

Molten metal,
We attributed to the difference between the purer waters and condensate to the presence of Fe in the condensate. A couple of boiler upsets and you have iron in the condensate. We had trouble with condensate system early on in the form of Dezincification of brass compression fittings and valves. There was also some Graphitization of CI valves, no failures that I know of, in the raw water headers. There was valve problems in the potable water system. I’m still trying to get some detailed information on the original problems and hopefully some numbers. I don’t have access to labs old files.

Hopefully I be able to get more information.

 

[Metalguy]

Unclesyd,
Corrosion FA's are always fun-when you are able to solve them!

I think it's generally acknowledges that Cu pipe/tubing/etc &quot;like&quot; hard, high-mineral water, whereas Al is just the opposite. But because of the mechanical nature of the protective oxides/sulfates, metals like Ti, SS and Al are able to withstand fairly high water velocities. But Fe and Cu cannot, so all of the Cu corrosion problems I have seen have occured just past elbows and T's--except for silt-caused inlet-end erosion of 90/10 CuNi in some large condensers-were retubed with AL6XN SS at $$$$$$$ cost. (the US Navy, being somewhat smarter than whoever spec'd. 90/10, uses 70/30<g>).

But I have never seen general corrosion or ID pitting of straight sections of Cu. Doesn't mean it can't happen, that's for sure, but I'd suspect a soft, low pH water could do it, depending on the acid. There is a &quot;standard&quot; pre-use treatment for Cu/Cu alloy tubing which involves a wash with ferrous sulfate, IIRC. Greatly helps establish a good protective sulfate layer on the new, bare Cu.

 

[unclesyd]

moltenmetal,metalguy,
Most of the condensate returned to the powerhouse came from the 300# steam system (the largest system) by way of a 175# flash tank. All our condenste from the 175# flash tank and the 300# system were reused. The other systems 75# and 25# were either sent to the ditch or to the cooling tower make up system. Nearly all trace steam was trapped to the ditch.
As I mentioned I think there were enough boiler upsets to keep the condensate system protected.
As mention we did have trouble in the tracing at the fittings and valves.
The silicate level was 3-5 ppm silicate in the form of sodium silicate.
As all know in industry once you resolve a problem there is no more money for study.
Don't criticize our steam system as we were paying $0.04/therm for natural gas at the time.