Corrosiveness of Deionized vs Distilled water for use in Laboratory Water Bath [duplicate]








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[Is pure water very corrosive?](/questions/7467/is-pure-water-very-corrosive)

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I've read conflicting descriptions of the corrosiveness (or lack thereof) of very pure water. For a precision water bath, it's stated that one should fill the bath with distilled water rather than deionized of a certain purity [as it would be corrosive](https://tools.thermofisher.com/content/sfs/manuals/D01203%7E.pdf):



> 
> 5.2 Filling Bath  
> 
> NOTE: DISTILLED WATER IS RECOMMENDED; IT WILL
> NOT CORRODE THE BATH CHAMBER
> AND IT REDUCES THE NEED FOR FREQUENT
> BATH CLEANING.
> 
> 
> DO NOT USE 18 MEG DEIONIZED WATER.  
> 
> NOTE: If this is the only
> source of treated water available; mix 50% with regular tap water.
> 
> 
> 


Why is this? I'm struggling to find a specific mechanism by which purer water would corrode the bath.




You can stop struggling. Your chemical instincts are correct; the purer water is, the less corrosive.


The main factor in the corrosivity of water solutions is the ion content, the ability of the water to carry a current in galvanic reactions. Lower ions, less current, less corrosion. The second factor is the presence of dissolved ions etc. to participate in various and sundry reactions. The less of these the fewer possible reactions. It is best to use water of the highest quality available in water baths, humidifiers, drip coffee makers, CPAP devices, plating baths, even laboratory dishwashers, and definitely as a chemical reactant or solvent. Printed circuit board manufacturing uses only the highest quality DI water in any aqueous process [and they probably wish they could get rid of the pesky 10^-7M H3O+ and OH-]. [Which they do by inventing these super nonionic detergents and no residue fluxes and, once upon a time, freon rinses.]


The increased corrosiveness of pure[r] water is a myth that I think originated with the water supply companies that found that certain plumbing structures literally fell apart when high calcium water was replaced by softened water. The pipes were partially held together by buildup of CaCO3 deposits and the common ion effect prevented dissolution of the deposits. Remove Ca++ from the water the deposits dissolved and everything leaked or even fell apart. This was probably part of the problem in Flint, MI. A change in water disturbed decades of protective coating in the pipes; the problem was deeper than removal of a coating.


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NASA published a 23-paged reported in the 60s on the corrosion of metals in deionized water at 38 $^o$C. The report is titled "Corrosion of Metals in Deionized Water at 38° C (100° F)" By Barbara Alice Johnson, 1969.


As one can see, we cannot make sweeping statements about corrosion in deionized water. Some metals will corrode and some will not in deionized water. Long time ago, someone told a story of detecting measurable amounts molybdenum in human blood. They were using very sensitive analytical technique and it puzzled everyone as to why Mo is being detected. It turned out that the metallic syringe needle leached a few atoms of Mo in a very short period of drawing blood. Yes, deionized or distilled water will leach some metallic ions but corrosion means *measurable metal loss to the extent that it causes economic damage*.


Thus, corrosiveness distilled vs. deionized water is like a popular urban myth. What the manufacturer probably want is some level of slow scaling in the bath to "protect" metallic heaters.


[![enter image description here](https://i.stack.imgur.com/fXhj9.png)](https://i.stack.imgur.com/fXhj9.png)


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