Thanks for the responses, Im afraid the solder is non-negotiable, the solder type is ASTM Alloy Grade 30A (SAE 3A). I am working on finding the exact make up of the solder which I will post as soon as I can.
The heat transfer fluid is water with a 5% organic acids as corrosion inhibitors. However they have performed poorly versus only the solder.
Here is what I get. You are starting with plain water from some source. If you use it as is you get scaling. If you add 5% organic acids you get corrosion (the lead dissolves?)
Have you tried a search for non-corroding water?
Your key might lie in control of the Saturation Index.
For solders I think I would start with Lucas Milhaupt 800 558-7006.
Were it my project I would look at automobile coolants and anti-freezes to start. You might be able to use something off the shelf.
I am actually working in the automobile coolants buisness, as part of the procedure for developing new products we need to ensure that they do not corrode a selection of metals. The coolant I am working on performs well against everything but the solder. I'm afraid I cant simply go and buy an off the shelf product as I am trying to develop a rival to said products. We have to use corrosive water (water with added sulfate, chloride and carbonate ions) for this specific test. As you say it is probably the lead that is the issue so I need to find an inhibitor for it. I can find plenty of inhibitors but none that are free from nitrite amine etc.
The acid of question is 2-ethyl hexanoic acid, however it is reacted with a base (potassium hydroxide) to for a potassium salt, so there shouldnt be much free acid in the heat transfer system.
The only use I've seen of this acid is as a salt of heavy metals. The most prevalent one is the Calcium Barium salt used as a stabilizer for plastic manufacturing. We used it a as Cadmium salt for several years. I would suspect that there is enough ionization that you are getting acid attack on the lead.
This is pure "Edisonian",I would try a phosphomolybdate as a starting point since you have excluded the normal array of corrosion inhibitors. The molybdates are quite common as inhibitors in cooling water systems.
I have no information as to the stability of your salt but based on other fatty acids it probably has some very defined temperature limitations.
Another is to try the Calcium salt in place of the potassium salt.
I would recheck your information on Calcium Hydroxide as to health or disposal risks. Lime water is used in food products and myriad of other things that are in contact with people and the environment.
I would look at the molybdates used in CTW systems as there are quite a number of formulations. We used several different formulations in our water systems until we got one with acceptable results.
In case anyone is interested and for sake of completeness the addition of Sodium Molydbate at approximately 0.1% concentration did the trick, eliminating the solder corrosion.
