Questions about Caustic service (Materials selection) 1

[kissjxp]

I started my work as a Materials Engineer three months ago. Now I have questions on materials selection for drums and heat exchangers in caustic service.

My questions:

1) What is the exact definition of the caustic service.
2) In NACE RP0403-2003 Figure 1, the materials selection and PWHT requirement are determined by the temperature and concetraion of NaOH (wt%). But my supervisor said that the concetraion of caustic included NaOH and other sodium salts (Na2S...). Is he correct?

For example:
The design temperature: 80C
NaOH: 0.5 wt%, other sodium salts: 9.5 wt%
what is the caustic concentraion I should use: 0.5 wt% or 10 wt%.

 

[robsalv]

Can't speak specifically about the other contaminations - however, if they were to be synergistic, then caution should be the order of the day.

Also, if there is any potential for the caustic concentration to locally concentrate up, I'd definitely be tending towards the cautious side. Based on the curve, at 80degrees C, it wouldn't take much for a little concentrating up to result in caustic SCC of CS weldaments.

Out of curiousity, why are you doubting your supervisor?



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[EdStainless]

You can go through the exercise of looking at each of the salts and converting the concentration into an equivalent NaOH.
Anything that raises the pH will need to be counted. Caution is good.

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Still trying to help you stop corrosion.
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or edstainless@earthlink.net

 

[TomEun]

Industrial definition of caustic service may be limited to NaOH and KOH. Normally NaOH is slightly more corrosive than KOH. As a practical guidance, we may be able to use the NACE RP0403-2003 Figure 1 (which is based on the pure NaOH) with combined concentration in mixture service.

Meanwhile the corrosion in caustic service is greatly affected from some impurities, such as chlorate (as chloride) and oxidizable sulfur compounds (i.e. H2S, mercaptans, or sodium sulfide). In this case, we cannot use the NACE curve above but search the field or Lab data.

For instance,
1. Austenitic stainless steels may not be useful in chloride bearing caustic service above 120F regardless of caustic concentration.
2. Comparison between pure and impurity
; Corrosion rate of Nickel 200 in 75wt% pure NaOH @ 266F : 0.6 mpy (0.015 mm/year)
; Corrosion rate of Nickel 200 in 75wt% pure NaOH plus 0.75wt% Na2S @ 266F : 22.8 mpy (0.57 mm/year)