Demineralised water - Effect on 304 Stainless Steel 5

[Graham58]

We have a customer who is requesting that we remove a 304 Stainless Steel ring main and replace for plastic due to corrosion issues associated with passing Demineralised water through the pipe work. I have always been of the thinking that Stainless should be fine for this application. Does anyone have any comments? The quality of the water will be in the region of 1 - 5 Microseimens. Many thanks

 

[bimr]

Demineralized water will not be corrosiive in the absence of oxygen. With oxygen present, the water will tend to be somewhat corrosive. However, you should still expect a reasonable service life from stainless. 316 stainless steel will be a better choice.

 

[CorBlimeyLimey]

See also thread338-206134 for some more details.

 

[mxmaciek]

Carbon dioxide (if present) would cause corrosion as well (5uS is deionized water rather than demineralized one, so I'd expect some sodium which can mask the CO2 in pH reading).

 

[IRstuff]

This report from NASA is pretty interesting. They claim that 304SS does not corrode in DI water, but they ran their test only for about a month, and there's not much in the way of details about the DI water.

TTFN
faq731-376

 

[stanier]

Have you researched

http://www.nidi.org

on this matter. Nalco's Guide to Boiler Failure Analysis may also cover it. this book is avaialbale in electronic form in a number of institution virtual libraries.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[mxmaciek]

From my experience, the standard material would be:
raw water (inland, river, lake, pond) - carbon steel;
raw water (brackish, estuary, sea) - 304/316/PP/PE
decationized water (after cation exchanger, strong, pH around 3-4) - PVC/PP/PE (temperature depending: PVC below 40C, PP/PE up to 80C, steel above - can be lined)
demineralized water (after cation-anion or RO, conductivity up to 5uS, pH 7-9.5) - as above, but if steel then 316
deionized water (conductivity up to 1uS, pH 6.5-8) - plastic or lined steel
last two of above, thermally degassed to oxygen content below 100ppb as micrograms of O2 - plastic, if temperature would allow.

In general, indoor pipelines with temperature below 40C - PVC, 40-80: PE/PP, above 80C - steel.
Last but not leastlastic is far cheaper, have 20years of lifetime, and is much cheaper and easier to modify or repair.

 

[Graham58]

Many thanks to all of you who have responded to my (first) thread. I have now decided that we will replace the 314 ring main with Class E ABS. Thanks again for your input

 

[bimr]

Demineralized water and deionized water are the same thing.

There will be no carbon dioxide in demineralized water. The anion will remove the carbon dioxide. The pH of demineralized water is normally a pH of 10 since the efflluent is in the form of sodium hydroxide.

Some power plants are successfully operating carbon steel boiler systems with demineralized water. The secret is the water must be oxygen free.

 

[bimr]

Demineralized water and deionized water are basically the same thing. There is no carbon dioxide present in demineralized water because carbon dioxide is removed by the anion unit. The pH of demineralized water is 10 because the demineralized effluent is in the form of sodium hydroxide.

Some power plants operate carbon steel piping systems with demineralized water. The reason that this works is that there is zero oxygen presnt.

 

[IRstuff]

Unless the water loops are completely sealed, it's pretty hard to have no air in the system. Most DI water systems run the water into a holding tank, which can have some air in it, resulting in a slightly acidic water from the dissolved CO2. Note that DI water is ph 7, i.e. it is neutral.

TTFN
faq731-376

 

[mxmaciek]

@bimr:
deionized water: water after one or two stages of RO, typically with conductivity around 5uS. Same is if the final stage is anion exchanger only, without mixed bed or EDI.
demineralized water: water after mixed bed ion exchanger or EDI, typically with direct conductivity below 1uS.

Same time, demineralized water pH would be around of 7. Deionized water ph will be alkaline ONLY after the anion exchanger, after RO it will be close to neutral.

Both water: in case of storage in tank which is not fit with carbon dioxide trap (e.g. with Sofnolime), it will absorb carbon dioxide from atmospheric air.

About the "oxygen free" water for use with boiler - especially for the low pressure sections - it is a simplest way to introduce flow accelerated corrosion in the system. Please refer for example to "Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment, EPRI, Palo Alto, CA:2005. 1004925."

From my practice, to avoid FAC in LP systems (both, in HRSG and "normal" coal boilers) it is necessary to introduce 2% Cr steel to exposed regions and/or to DOSE oxygen to keep it from 20-100ppb.

 

[bimr]

mxmaciek,

With due respects, the post above was meant to correct some of the previous posts. Instead, additional further erroneous posts are added.

Water treatment professionals would not equate RO effluent and demineralized water. They are different.

RO effluent typically has a lower pH because carbon dioxide passes through the membrane.

For example, "Demineralisation" Any process used to remove minerals from water, however, commonly the term is restricted to ion exchange processes.

http://www.lenntech.com/application...ionised-demineralised-water.htm#ixzz2DMwlS81d

RO effluent is not considered to be demineralized water. There are too many ionic elements still present in the RO effluent since RO process are generally guaranteed to remove about 90% of the ionic elements whereas ion exchange systems remove about 100%. The effluent from a cation unit is also not demineralized water either, since you have removed just 50% of the ionic parameters.

Definition of demineralization from the Environmental Engineering Dictionary:

Water which has been passed through a mixed-bed ion exchanger to remove soluble ionic impurities. Nonelectrolytes and Colloids are not removed from water so treated. Also referred to as Deionized Water.

http://www.ecologydictionary.org/DEMINERALIZED_WATER

Regarding absorption of Carbon Dioxide from air. If this is a concern, many facilities use nitrogen blanketing of storage tanks to prevent this from occuring. If carbon dioxide was present in demineralized water, one would assume that you have demineralized water that has been contaminanted.

In conventional all-volatile treatment (AVT) for boilers, the water quality is adjusted using ammonia to control pH and hydrazine as a deoxidant. Because dissolved oxygen is thought to be a corrosive component, its concentration is minimized and the boiler feed-water pH is adjusted to prevent
corrosion. Oxygenated treatment (OT), on the other hand, is based on the theory that slightly soluble oxides adhered to the surface of steel can prevent steel corrosion and elute corrosion products into water. OT includes neutral water treatment (NWT), in which dissolved oxygen is allowed to
coexist in neutral water, and CWT, in which dissolved oxygen is allowed to coexist in weak alkaline water adjusted to a range of pH 8.0 to 9.3 by ammonia. Boiler piping systems for these systems utilize carbon steel piping.

Finally, the effluent of a demineralizer is 10 (NOT pH 7) because the demineralized effluent is in the form of sodium hydroxide ionic species. Sodium leakage occurs from the cation unit and hydroxide leakage occurs from the anion unit.

 

[mxmaciek]

@bimr: thanks for the explanation, first, it seems that during last 22 years I was doing everything completely wrong!!!
What is more, I know several organizations, which should now shut their water treatment down as they are using wrong equipment.

But, following the above, could you please explain me how are you obtaining the sodium hydroxide in the final effluent of mixed bed (typical in my, again: totally wrong, setups final stage of demineralization)?
I see only two alternatives:
1. to bypass mixed bed at all;
2. to regenerate the anion exchanger poorly (e.g. by not finishing the mixed bed final flushing in recirc mode).

Anyway, I'm happy that you discovered a way to avoid ammonia dosing in your installations, as you stated above: demin plant effluent pH value is 10, so please compare it with pH requirements for feedwater and cycle params.
It seems that most of Utilities are wasting ammonia and phosphate, huh? Instead, maybe they should start acid dosing to decrease pH of feed water from mentioned by you 10 to around 9?

Finally, "Oxygenated treatment (OT), on the other hand, is based on the theory that slightly soluble oxides adhered to the surface of steel can prevent steel corrosion and elute corrosion products into water." - it is wrong.
In AVT-R regime you have protective layer of magnetite (Fe3O4). In some systems, where two phases or single phase turbulent, directed flow occurs (e.g. in bends, header inlets etc) in specific pressure/temperature conditions (again: typically in LP systems), the mmagnetite layer is stripped out. In AVT-R regime, there is no conditions to restore protective layer, therefore lower pH and some oxygen injection is applied to promote oxides (mixture of hematite and magnetite) growth. In addition to that, many of HRSG owners have replaced the exposed locations to 2-3% Cr steels, as FAC occurs ONLY in carbon steel (of course: too much chromium would cause SCC in such chemical conditions, but this is another story).

To shorten, it seems that you (surprisingly) have not too much experience with professional generation utilities? I'd be curious if you could give the example of power station where the final stage of makeup water treatment is NOT the mixed bed or EDI, but to shorten the thread - please direct it as PM.

 

[bimr]

You seem to be extrapolating based. Why are you assuming that this is a power plant when the original post only mentions a demineralizer? A better assumption would be that this is some type of ultrapure water system loop.

Assume that the effluent from a demineralizer is less than 1 ppm. Would not that 1 ppm be sodium hydroxide? If not, what is it? Would also expect you to inderstand that there is essentially no buffering capacity in demineralized water.

Don't believe that I mentioned anything about condensate polishing in the ammonia cycle in any of the above comments either.

The original poster's enquiry was about corrosion of a piping system with demineralized water. The only thing that can be stated with the known information is that the corrosion is probably caused by oxygen.

 

[mxmaciek]

Sorry, but it seems that you are adopting the reality to your theories.
First, it can be everything, starting from the traces of chlorides from cation exchanger, via the slip of carbon dioxide, silica, ammonia or sodium.

But OK, lets follow your assumption that it is 1ppm of sodium.
Therefore, from the commonly known table, assuming that there is absence of other impurities, you will have: pH=9.4, conductivity of 6.2uS/cm.
Sorry, but it is NOT ultrapure water for whatever reason you'd like to use it.
Lets go further, pH of 10, you said? so again, lets assume, as you want, that all of it is caused by sodium only.
Therefore, the amount of sodium would be 4ppm, with the conductivity of 24.8uS/cm (the conductivity, contrary to pH, is directly proportional to sodium ions concentration).

If you could not find the tables, although commonly available, here is a set of graphs:

 

[IRstuff]

I'm not sure where this discussion is going, but DI water, as used by the semiconductor industry, is run through resin beds, RO, UV, and polishing filters. The end result is 18 megohm-cm, pH 7 water. The 18 megohm-cm translates to 0.06 uS/cm. Anything worse than that is unusable for semiconductor processing. We would have thrown product into the trashcan if the resistivity ever dropped below 18 megohm-cm. Based on your graph, that would be 0.1 ppm Na or less. Anything more than that, the sodium contamination would render the transistors useless.

TTFN
faq731-376

 

[mxmaciek]

@Rstuff - this is exactly what I mean (sorry if not explicated precisely enough - English is not my primary language), that so called "ultrapure" water NEEDS polishing and in any case, on the makeup plant outlet cannot have pH around 10.

 

[bimr]

Thank you for the clarification. I understand now that the issue is really your english, not water treatment and I can't help you with that. My point all along is that you have been posting inaccurate descriptions, comments and replies. You may understand water treatment but lack the ability to communicate it. Enough said.

 

[mxmaciek]

So finally we have the consensus. I have some difficulties with English, you have some difficulties with water treatment, especially: polishing, knowledge what the final effluent parameters requirements are and last but not least: basic chemistry principles.
As result, you were posting grammatically accurate descriptions, comments and replies, which were completely inaccurate from process point of view.