Aluminium togehter with Copper
Aluminium togehter with Copper
(OP)
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 ?





RE: Aluminium togehter with Copper
Anyway, if you can mix SS with Al you should be able to use Cu too. Both of them are quite cathodic to Al.
RE: Aluminium togehter with Copper
RE: Aluminium togehter with Copper
To answer your question: Different metals can be used with enough corrosion inhibitor in the water (e.g., coat the entire wetted surface with a silicate film) and/or with electrical isolation between them (think of dielectric pipe unions or radiator hoses or lots of Teflon(R) tape on threads).
RE: Aluminium togehter with Copper
A once thru raw water system it is going to be extremely hard to protect the components without inhibitors. It can be done, we once used a continual injection of sodium silicate (desired 7-9 ppm) to a extremely pure raw water system to increase the hardness which prevented dissolution of the Cu pipe.
I hope this is not a potable system, an entirely different game.
RE: Aluminium togehter with Copper
http://isaksen.home.cern.ch/isaksen/
RE: Aluminium togehter with Copper
You have to keep the flow velocity low with Cu, so if you can't you might see if Cu-Ni is avail. for what you need. Cr-plated brass might also work OK, but Cr is difficult to plate without lots of fine cracks.
RE: Aluminium togehter with Copper
As I understand it an inhibitor would be prohibitive in your system. Demineralized water will corrode Cu. The mechanism is the simple dissolving of the Cu to achieve an equilibrium with the water. I checked some piping codes/practices that I’ve used in the past on extremely low conductivity pure and demineralized water. All the piping and components in these systems were Al or S/S. No plating allowed. One reason is that Cr plating a complex object will probably leave porosity, though minute, a pore can become extremely active.
What type of separator are you trying to use?
Do you mind losing very small amounts of your demineralized water?
You could put a S/S separator, similar to a steam purifier at the high point and take the air off the top and dump through a air eliminator. This will dump a very small amount of Demineralized water. We have done this.
Can you use a separator prior to the demineralizer?
RE: Aluminium togehter with Copper
Just a pedantic pull up
Aluminum bronzes are corrosion resistant to both in atmospheric and water conditions. They have a nominal composition of 90%Cu and 10%Al. Although I fully agree at the aluminum rich end of the spectrum they have huge corrosion problems, at the copper end, they have good corrosion resistance.
RE: Aluminium togehter with Copper
RE: Aluminium togehter with Copper
RE: Aluminium togehter with Copper
RE: Aluminium togehter with Copper
Copper is noble to aluminum and will be the cathode. Aluminum is the anode and will dissolve. If the aluminum area is large and the copper area is small, and/or the water is not very corrosive/conductive, and/or they are not in electric contact, you might get away with the two metals in the same system.
In short, the answer is in the details.
Robert
RE: Aluminium togehter with Copper
Is copper similarly corroded by steam condensate? (i.e. very hot de-mineralized water?) If so, what about all the miles of copper steam tracing out there in industry? Is there some volatile additive in everyone's boiler feed water which renders the copper exposed to it passive against attack by condensate? Or is condensate different somehow?
Copper is widely used in domestic hot/cold water and hydronic heating tubing/piping and lasts a very long time indeed in those services.
If the mechanism of corrosion in deionized water is, as a previous poster put it, "...the simple dissolving of the Cu to achieve an equilibrium with the water...", then surely the same would be true in more conductive but essentially copper-free water? Surely some copper may dissolve and result in an increase of the conductivity of the de-min water (possibly rendering it unfit for its intended use), but does this happen fast enough to lead to failure of the copper components?
Other posters have indicated that silicate passivates copper. Surely there's not that much silicate in potable water? Is there something else which is present in most potable water which renders copper passive? I've read that "hardness" (carbonate) precipitation can result in an enhanced corrosion of copper.
RE: Aluminium togehter with Copper
This was our problem in real life. I don’t remember the conductivity of the water but it was very low. At the time there was no detectable minerals in the H2O. No one was concerned until we started having leaks in the potable water system, very large, the plant at that time covered 45 acres. Metallurgical analysis revealed small areas of island type corrosion within an area of general thinning. We begin to pick up Cu in the water at the distant stations from the source and not as many leaks. The source water had no Cu. After several phone discussions my boss and several big names in the corrosion business concluded that the cause was there was no Cu ions in the water so there was an equilibrium being established. As I stated the end result was to add sodium silicate at ppm range to establish a silicate film on the pipe. At the time the plant was constructed the piping code for pure water and deionized water was Aluminum or S/S.
RE: Aluminium togehter with Copper
At that time the CTW was treated with chromates, so no corrosion.
I just call and talked to the water chemist that came in about the time the leaks started and he said that the leaking started when the piping was about 7 years of age.
In this area most houses are built on a slab with embedded Cu pipe. There are a couple of plumbing companies that make a living rerouting the water piping overhead in the house. Most of the first to go is the exit line at the hot water heater. The water from their wells is also very pure, the only thing is that they have 1-2 ppm CO2. There is sometimes a CU carbonate film, but usually this is scoured off. This weekend I was shown a section of 3/8 Cu tubing from the exit of a water heater with a hole just below the tee. The recommendation was to repipe the hot water system and bing me a sample of the cold water. The house is 6 years old.
RE: Aluminium togehter with Copper
RE: Aluminium togehter with Copper
There a three Princeton Potentiostats sitting idle at the old place, maybe I should try to get some time on one.
The statement about design was made totally with out facts, a big problem at times. This plant happened to be designed by engineers from two very large chemical companies along with a competent core of engineers for the new plant. Even though they might not have had the computational power available today they were at least slide rule accurate and if warranted there were logarithms. The degree of accuracy was always to the positive side because we always rounded up. The system was designed to supply 10,000 people and never over 7700 availed themselves of the potable water.
I can assure you this is not a case of poor design. My example was happened to a 3/8" pipe off a local heater for an upstairs bath room. I can also state that I don't think the owner was shorted on anything in the plumbing line. He owns the supply house
How is water no solid impurities at this pressure probably, 50 psig at the most, and velocity the shower heads are of the 1 ½ gal type, toilets are tankless, don't know about the tub. I'll let you know next week how much pipe is affected.
Just for my own edification how much pure or low conductivity water piping have you been involved with. Though not directly involved with the water system I saw all the failures of any kind on this site for 42 years. Every failure regardless of origin from day one on this site passed through the Metallurgical laboratory. We were doing RCFA a long time ago.
RE: Aluminium togehter with Copper
Anyway, excessive water velocity has caused many (probably millions) of Cu pipe/tubing leaks, usually just downstream of elbows, tees, etc. It frequently leaves "islands" of Cu just like you describe. The rest of the system is unaffected-just the turbulent zones get attacked.
One case I did the FA on involved a very large engr. co.-probably the largest in the US for nuclear-related construction. Doesn't mean that even the giants don't sometimes make mistakes.
I should have used the word "builders" instead of contractors, esp. home builders. They just love the slightly cheaper thin type M Cu tubing instead of type L. I lived in a devel. in AZ where most of the houses, mine included, had leaking pipes buried in the slabs. Mine had the pitting as described above. Also performed a FA for a huge VA hospital with leaks all over their CU potable system. They couldn't understand why no leaks for the 1st 20 years or so, then leaks all over. I collected many samples-all had the leaks right after fittings. No straight tubing problems. I asked about increased water usage. They answered with "problem started after they added a new hosp. wing". I told them that made sense, because of higher/longer flow. They saw the light and agreed-thought I was a god for coming up with their answer. I love it when people think like that!
RE: Aluminium togehter with Copper
Though I'd never use copper for demin process feed water, I still don't know if I should be concerned about spec'ing copper for BFW, condensate handling or trace service, or not- clearly, you can get away with it sometimes and not others. Copper corrosion has got to be worth megabucks- surely this has been studied on more than just an anecdotal basis?
RE: Aluminium togehter with Copper
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...
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.
RE: Aluminium togehter with Copper
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?
RE: Aluminium togehter with Copper
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.
RE: Aluminium togehter with Copper
Corrosion FA's are always fun-when you are able to solve them!
I think it's generally acknowledges that Cu pipe/tubing/etc "like" 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 "standard" 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.
RE: Aluminium togehter with Copper
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.