Summary for di water stainless steel Discussion
DI water on stainless steel is the central concern here: Chua Eng Koon asked whether DI water at 0.2 Mohms/cm resistivity and pH 5.5 would be corrosive to 303/304 stainless in a heat exchanger cooling loop. Marlon L. Cordez replied that a pH of 5.5 suggests the DI system may have a problem, noting that in his own work DI water is kept around pH 6.8 to 7.3. A later answer explained that very pure water can have a pH near 6.5 and that small pH shifts at this purity are often not very meaningful. It added that the main issue with pure water is its strong ability to dissolve materials over time, including some ions from stainless steel, but said the corrosion rate is usually very low. The practical advice was that corrosion is unlikely to be a major problem, though a trace corrosion inhibitor could be used if the process allows it.
Hi
I am working with a customer to introduce a heat exchanger to cool DI water from 35 Deg. C to 15 Deg. C I understand that DI-water is stripped of its ions and will corrode stainless steel. But what does it means if the DI water has a resistivity of 0.2 Mohms/cm and pH value of 5.5. Is this considered corrosive? How does DI water with these properties effect stainless steel (ASIS 303/304)?
Thank you.
Chua Eng Koon
Heating/Cooling control – Singapore
2004
What? DI water with pH of 5.5? seems to me that the DI water generating machine have some problem. Normally in our case DI water does not corrode stainless steel because we maintain the pH at 6.8 to 7.3
Marlon L. Cordez
Surface treatment – Sta. Rosa, Laguna, Philippines
2004
DI Water, at its perfect balance of H2O, with no contaminants, will have a pH of around 6.5. Your water, at that purity, most likely has an imbalance of H to OH ions caused by one side of the DI system being slightly more efficient than the other. At this level of purity, the difference is normally negligible as far as pH goes.
The corrosive nature of pure water comes from the fact that the water has HUGE solubility capacity, and will take almost anything into solution given time. This includes certain ions in the stainless steel. I seriously doubt that you will ever see a corrosion problem in your system, but the way to be sure is to run a minute amount of a corrosion inhibitor in the loop if you can get away with that in your process. At any rate, the corrosion rate of DI water on Stainless steel is very low, so you may not have anything real to worry about.
Jeff Watson
– Pearland, Texas
2004
Q. Water is known to penetrate material including stainless steel. The rate of penetration is up to 10s of mil/yr.
What I like to know is how temperature of DI water affects the leaching ions from SS. I am interested in the data/info for hot/boiling water.
Al Badakhshan
– Los Angeles, California, USA
2005
A. 316SS is commonly used to handle DI water. It shows little to no corrosion form the water even at high temperatures. As far as your pH and resistivity goes, I could speculate that CO2 has dissolved into the water and hydrolyzed to carbonic acid. This would explain the low pH and any capacity for current. Do you have a DI column purifying the water?
Dave Robinson
– Bristol, Rhode Island, USA
June 30, 2014