Pitting corrosion on 316 in chlorinated RO water heater 3

[Jaspor]

Looking for information on corrosivity (crevice & pitting) on 316SS plate heat exchanger being used to heat RO permeate water.
The permeate has been hardened by addition of lime (about 30ppm hardness & 90ppm alk, pH 8), but has up to 2ppm free chlorine from hypo addition. Temp is 55-65 degC.
Look forward to seeing if anybody has documented data.
Cheers

 

[gr2vessels]

2 ppm Chlorides at 65 deg C may produce pitting or crevice corrosion, but in relative long period of exposure. You need a hundred chlorides ppm at 65 deg C to create a damaging environment for your plate exchanger. Look up in the Outokumpu 'Corrosion handbook'.

 

[Jaspor]

Thanks gr2vessels.
My concern is that I have 2ppm of free chlorine, not just chlorides.
There does not seem to be any literature that sites the effect of the free chlorine on pitting.
There are references to it in chlorination of sea water for scum removal/control in offshore rig water circuits, but not for clean, soft water. This water is used in food industry application, hence the hypo dosing.

 

[EdStainless]

If this water is low oxygen and there is little or no flow then there is a real risk of crevice corrosion.
If you have continuous flow then I wouldn't be too worried.

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Plymouth Tube

 

[Jaspor]

Thanks for the information Edstainless.
The flow is quite high rate, not sure what the o2 content. Though I thought that crevice corrosion would be worse if it was high o2 content. WShat mechanism are you thinking for that reasoning?
Recent HX maintenance has shown extensive crevice corrosion between plates after 6 months operation & we are looking for a fix (without changing the plates for Ti). This is a food contact application.

 

[EdStainless]

SS will tend to resist pitting and crevice corrosion when there is enough oxygen present to help the material re-passivate. Down inside of crevices you get oxygen depletion, concentration of impurities, and lower pH. The result is a high tendency for corrosion.
If you have had 316 crevice plates, then you may need to move to a better alloy. Can you get 2205 plates?
You might also look into the gaskets. It could be the type of elastomer that they used.

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Plymouth Tube

 

[Jaspor]

Hi Ed,
There is also some pitting in some of the 316SS pipe lines where pipes were not properly flushed before service (pitting in bottom 1/2 of pipe only).
As to gaskets, they are nitrile and they have seen degradation in that the are loosing their filler, so rubbing black on your hands. How do you think the elastomer may be affecting the crevice corrosion?

 

[EdStainless]

yes.
They should be perfectly intact. 65C in water should have zero effect on the rubber.

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Plymouth Tube

 

[stanier]

http://www.nidi.org

has wealth of information