Is there a chemical reaction ie. electrolisis between stainless steal and bronze in a c.w. closed loop system?

have globe valves installed in a closed loop chill water system there is a nitrate added to the water for crosion control. the bronze disks of the valves are being eaten fairely quickly can this be caused by electrolisis between these dis similar metals?

Is there a chemical reaction ie. electrolisis between stainless steal and bronze in a c.w. closed loop system?
I have found that looking at a Pourbaix diagram can help in understanding what is going on when something corrodes. I have provided references for you to look at and understand. One should realize that the diagram can change depending on what is in the system. But in general the basic diagrams give a good idea.





I do not have a Pourbaix diagram for bronze but If you look at the Pourbaix diagram for copper which is best shown in the third reference I provided on the top right corner of page 3 you will see that copper is stable as copper metal below an oxidation potential of -0.6 V throughout the pH range. The greater the negative sign indicates a more reducing environment where a greater positive number indicates a more oxidative environment. For example if magnesium metal was present, magnesium metal is more reactive than copper and would have a lower potential which would keep copper from dissolving but would dissolve magnesium instead. Thus magnesium would be sacrificed during corrosion while in contact with copper, causing galvanic corrosion of magnesium.





Above a potential of -0.6 V, copper metal can be soluble starting at a pH below 7 and a pH above 9 depending on the potential. Such things can cause the potential to increase are the presence of oxidizers such as oxygen, chlorine, or less reactive metals such as 316 stainless steel.





How did I know 316SS is less reactive than copper? Well unless you have a pourbaix diagram for stainless steel you can look at a galvanic series chart like the one I provided to you in the 4th reference.





As you can see 316SS is more noble than copper. However cast iron is below copper. So if copper was in contact with cast iron, the cast iron would preferentially corrode and if in contact with 316SS the copper would preferentially corrode.





However you did not ask about copper you asked about bronze. Well if you look at the galvanic series you will see that 316SS or 304SS is more noble than bronze so the bronze will preferentially corrode.





So the options you have to prevent corrosion are the following:





To prevent galvanic corrosion, put a nonconductor break between the two metals. Companies provide non-conductive isolators for connection piping of dissimilar metals.





To prevent corrosion due to pH, try to keep the pH within the range of 7 to 9. Even if the copper was in contact with the copper the copper oxide protective layer will be stable within that pH range. Note though that some metal oxide layers are not as strong as others. Some can be removed by abrasion allowing for more corrosion to occur.

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