[Cele55]

I have a cooling situation where are Copper and Aluminium used in the same water loop. I know it is bad, but I can not avoid it at the moment. There is visible corrosion taking in place and over 5-6 months theres plenty of blue jelly like substance floating around that jams the pump.

Question: what substance I am exactly dealing with and what I can use to clean the system from it without damaging copper and aluminium pipes?

[hobobot]

Is it just pure water? Or are there some corrosion inhibitors, anti-freeze agents, or anything else?

[Cele55]

Is it just pure water? Or are there some corrosion inhibitors, anti-freeze agents, or anything else?

Pure destilled water.
Its a small loop, total ammount of water is around 2 liters. Water temperature is between 18-30 degrees C.

[hobobot]

I think it's copper (ii) hydroxide.

I found this pdf on a search for corrosion inhibitors. I have to admit I don't know too much about them, so maybe someone else knows another good solution.

http://www.electrochemsci.org/papers/vol3/3010001.pdf

[Woofuls]

It most certainly is not copper. At least in sea water, Al is far less noble than Cu (see galvanic series).

200 ppm of sodium molybdate at a pH of 7-8 will slow the corrosion. The pH MUST be controlled.

[hobobot]

It most certainly is not copper. At least in sea water, Al is far less noble than Cu (see galvanic series).

200 ppm of sodium molybdate at a pH of 7-8 will slow the corrosion. The pH MUST be controlled.

You're probably right. However, I was just drawing on experience dealing with Cu(OH)2.  I know it as the blue, viscous kind of material Cele55 was describing.

[Woofuls]

However, I was just drawing on experience dealing with Cu(OH)2.  I know it as the blue, viscous kind of material Cele55 was describing.

Very true... but then I can't explain this galvanic corrosion... unless these pipes are alloyed.

A simple test would be to take this jelly and dissolve it in ammonium hydroxide. See if tetraamminecopper(II) forms.

[cil-baha]

I don't follow how would you get the tetraamincopper(II) complex? I have seen a similar situation where the pH was later (correctively) controlled by adding citric acid/citrate buffer, perhaps both the Al &Cu dissolve at lower pH and then Al dissolves further at an increased pH (alkaline) forming the Al(OH)3 + Cu(OH)2 gel.