Corrosion caused by deionized water

A. I can only respond with limited knowledge. DI Water is "Hungry"; it is so clean that it is aggressive. In resin exchange, I have noted that since I am removing more cation (Ca, Mg, etc,) than anion that my final pH is <7. I think others can shed more light on this.

A. If you are recirculating the fluid, chances are the deionized water is dissolving enough of the copper and or stainless to make itself less corrosive to the stainless. However, if you are exchanging the water or you are seeing corrosion (look at the welds for the first sign of damage), adding a bit of sodium carbonate / washing soda [affil links] or another non-halogen soluble salt to the water should abate any corrosion in this situation. (you don't need very much to virtually eliminate the corrosive nature of water). If you require a food-grade additive for FDA approval, there are many to choose from. As stated before, avoid chlorides, fluorides, iodides & bromides, and try to keep the pH neutral or slightly basic.

A. Michael,

It sure seems that Dale and Jon are right. D.I water is very aggressive and will leach out impurities with ease.

May I suggest you go to the local supplier/manufacturer of your D.I. water and find out what THEY USE ... and I think you might find it is PVC.

There are, of course, better plastics than PVC but why buy a Rolls when a Mini will do?

A. Typically distilled and/or deionized water can be effectively stored in tin-plated [substrate] (put any kind of steel or ceramic here). The old standby tin auto-oxidizes & forms a surface which has has a very low solubility index in water. As long as the pH is neutral, long term storage in tin is common. There are a lot of platers represented here on the Finishing.com website who can do tin plating.

Glass is also effective for storage of distilled and/or deionized water. The solubility of glass is extremely low and even ultrapure water will reach a saturation quickly at parts per billion--not really a scaling issue at all.

3XX Stainless is perfectly suited for storing distilled water--the only susceptible points in the stream might be the welds. Very smooth, clean and very passivated welds should be fine. Electropolished contact surfaces are even better. Storing very pure waters in PVC has limitations, even PVC listed as DWC or NSF-DW. There is still a problem with leaching, especially over time, notably on heating.

A. potassium permanganate [on eBay or Amazon] stabilizes pH in Deionized water.......

A. Clyde, the conductivity in your tank is increasing because of the absorption of CO2 from air. The CO2 converts to carbonic acid in the water which causes the conductivity to increase and pH to go down to around 5.5.

A. Michael and All

The DI water will corrode the Copper and lower grades of Stainless and can even leach the Chlorides from PVC Plastics. If your application only requires a low conductivity you could use RO water or a 2-bed DI system which produces water in the range of 10 - 20 ppm. Dale is also correct in the CO₂ absorption this can be stopped through the use of a Nitrogen Blanket type system

A. Hi Dave. This NASA document indicated no dissolution of titanium, type 1100 aluminum, or type 304 stainless steel in DI water. .
Unfortunately, it doesn't seem to indicate anything about the conductivity of the DI water used in the test, and I suspect (but don't know) that it makes a difference. The "DI water" available in supermarkets is probably no more corrosive than ordinary tap water, and I'm confident of the NASA results for such mundane water. 18 megohm ultra purity DI water for integrated circuit fabrication might well be "hungry" and aggressive as some people say, but I think as long as you stay away from copper countertops you'll be okay. There are countless references which claim that 3xx series stainless is fine.

Regards,

A. I just stumbled upon this conversation and thought I would add to it a little.
The materials used for Deionized water very much depends on the quality of water needed in your system.
For example, certain high purity water applications can only use ultra high purity PTFE/ PFA materials with zero metal contact allowed. The downside being though that these materials are soft and limit your max pressure and temperature capability. For applications that are "one step lower" if you will, the choice material is 316SS but, this has to be electropolished/ passivated. Of note, be cautious when considering the different grades of Stainless Steel. For example, "equivalent to" ASTM A217 CA-15 chrome steel (high nickel carbon steel) is considered as a "higher" grade 400 series stainless steel but actually has a lower chemical compatibility rating for DI water than 316SS. I have never personally considered using 304SS for DI water as I simply prefer the lower % Chromium (ion) and Molybdenum content of 316SS.
There is a whole water grading system (ASTM, ISO, CLSI standards) which is the guideline/ standard of water purity. Google search ASTM D1193 and then go through the various types and grades.
In short, unless you're in a Lab setting dealing with bacteria, it all comes down to what is the max level of conductivity that is acceptable in your particular system.
Every square inch of a system needs to have the appropriate materials and finishing as just one small component (i.e. weld joint not properly electropolished) can leach enough ions (increase conductivity level) to substantially degrade the purity grade of the water needed for your system. Investigate/ design the whole system for any potential areas of impurity/ leaching which will keep the costs lower by way of any elaborate filtration.
Even the old NASA study focus was on the physical corrosion of metals and they do indeed highlight that chemical analysis is much more accurate measurement.
In short, unless there is some extenuating circumstance, 316SS electropolished for DI water is the way to go. My two cents! Cheers.