Cathodic protection for Duplex Stai

[bababode]

Cathodic protection for Duplex Stainless Steel (2205)

I'm trying to design a CP system for the internal of a vessel built with 2205. The content is 26wt% NaCl brine in water. This is more corrosive than seawater which is about 2.7% of Chloride.
Do anyone know the resistivity of this concentration or conductance?

 

[PVRV]

1. If the service temperature is high duplex ss will fall
apart. (Severe Pitting)

2. Why are you considering the use of sacraficial
anodes; if an investment has been made to use
an alloyed material. (contingency ?)

3. Resistivity of this concentration; ASM (Vol.13)

 

[mcguire]

Let's get a few more facts. 2205 in brine is in danger, depending on temperature. What is the temperature? This is a curious design. I think someone left you to hold the bag.

 

[bababode]

The service temperature is 228 degree F. The vessel is already in service but internal inspection lately revealed localised pitting on some areas. I'm only try to save the vessel. The material selection on the whole system is poor from the begining

My preference is power impressed current system but this is going to be limited by a number of factors: Anodes position, cabling etc.

Also I sleep and wake up with Vol. 13 but resisivity or conductance of this concentration is not there unless I have not looked well, if you know the page please let me have it. What i've seen in Vol. 13 is seawater resistivity and conductance meanwhile seawater is about 2.7% by volume which is fa far lower than 26wt% NaCl brine in water. Density of this fluid is about 71.1lb/Cubic.ft.

Thanks
Thanks

 

[mcguire]

A small bit of help, I hope. My chart from CRP Handbook stops at 30C, but at that temperature a saturated solution of NaCl of 0.2774 mho/cm. At 20C it is 0.2259 and at 10C it is 0.1779. It's a long extrapolation, but that's all I have.

 

[bababode]

Thanks Mcguire. Which book is that? IS it corrosion resistance tables?
Bababode

 

[PVRV]

a) Consider an internal coating system (lower half).
+
b) Take a look at Galvalum III
+
c) Internally line/coat all nozzles (lower half)


General:
Are you referring to RCP-anodes?
I dont sleep/wake up with ASM Vol.13 Ill take a
look at it though

Cheers

 

[kenvlach]

bababode,
Re concentration: "26wt% NaCl brine in water. Density … 71.1lb/Cubic.ft"
I presume this means 26 wt% NaCl? (NOT water + 26 wt% brine solution)

Taking the density of a 26 wt% NaCl solution at 20 C, 1.1972 Kg/L = 74.58 lb/cu.ft, from CRC Handbook of Chemistry & Physics and adjusting for temperature, I estimated 70.72 lb/cu.ft at 228 F (109 C). Is this how you got your figure, or did you measure?

Re conductivity. For strongly electrolytes such as NaCl solutions, conductivity increases linearly with the square root of concentration, and linearly with temperature at a given concentration. However, it seems that the solubility at 20 C is almost exactly 26 wt% NaCl, so no correction for concentration is needed.

The CRC table for NaCl + water solutions at 20 C indicates the solubility limit is between 26 & 27 wt% NaCl [& the solubility limit at 25 C is 6.144 molality = 26.42 wt% from Journal of Electrochemical Society, Vol. 143, p. 1789 (1996).

http://www.jhinst.cas.cz/~rheyrovs/JES1996.htm

]

From data for NaCl in the following reference, I took the variation of conductivity with temperature for a 26 wt% solution as 2.10% per degree C.

http://global.horiba.com/story_e/conductivity/conductivity_04.htm

So, for 26 wt% NaCl solution at 109 C,

k(@109 C) = k (@20 C) x [ 1 + 0.0210 x (109 – 20 C)]
= 0.2259 x 2.869
= 0.6481 mho/cm
[Without the correction, a linear extrapolation gives a slightly high value of 0.6687 mho/cm at 109 C.]

I hope this is of use for cathodic protection, but frankly, this case is beyond me. Maybe mcguire, PVRV, others, can give you further advice, but I’d start checking compatibilities & costs of coating/lining with Derakane, Hypalon, PVDF, Viton, etc.

Reference: ‘The Story of Conductivity’

http://global.horiba.com/story_e/conductivity/index.htm

 

[mcguire]

Bababode
As Kenvlach cites, the book is the Handbook of Chemistry and Physics published by the Chemical Rubber&Publishing Company ( really!). I don't know if they still exist. My copy is the 1955 edition. Older than most of us.

 

[bababode]

Thanks so much Kenvlach, those values are good approximate, I can now easily get outmy resistivity and finish up my design. However the first link you gave www. jhinst...... is not working.

Thanks everybody

 

[kenvlach]

Re non-working link: Seems like their server is down, might start working Monday. Or, maybe the Electrochemical Society clamped down on free distribution of copyrighted material.

It was only used indirectly, to show a solubility limit of 26 wt% at 20 C is reasonable (so could use mcguires's 20 C conductivity), since it gave a 25 C value of 6.144 molality = 26.42 wt%. Anyhow, the reference is there, and maybe you can find a copy in some library.

I don't understand PVRV's post; are these PV's only filled 1/2 full? Also, Galvalum III sounds like Zn+Al; is that a sacrificial anode material?

 

[PVRV]

kenvlach regarding the PV being 1/2 full in service,

a)
What is the purpose of the pressure vessel in question?
Where is 26wt% NaCl brine in water coming from? and what is raising service temperature to 228 degree F?

I assumed the PV to be a crude desalter pressure vessel and I may be wrong it was a mistake to assume the PV to be half full with brine and the other half to be a non-corrosive fluid. Apologise for not outlinning this bababode and thank you for highlighting the same to me.

b)
bababode mentioned,

"power impressed current system & CP system for the internal of a vessel"

Impressed current to protect a pressure vessel internally against the service fluid. Please elborate on this issue I find it very interesting.

What CP system would you recommend kenvlach ?

Appreciate your insights and valuable comments on such issues.

 

[kenvlach]

PVRV,
I was merely asking in order to understand and learn, not making criticism.
As I said earlier, "...this case is beyond me. Maybe mcguire, PVRV, others, can give you further advice..."

 

[PVRV]

kenvlach,

Not at all, I appreciate your feedback thats why I stated

"Apologise for not outlinning this bababode and thank you for highlighting the same to me."

In addition to,

What CP system would you recommend kenvlach ?

"Appreciate your insights and valuable comments on such issues."

I read a report for Bp off-shore PV with similar problem; they used plastic lining as you outlined previously. Yes; I came across impressed current system for internal protection but there was internal access since it was a open roof tank (brakish water-fire tank) and the potential was readily read from the reference electrodes.

As for Galvalum III, its service temperature is around the operating temperature stated by bababode.


You are a solid engineer kenvlach and I do appreciate all your insights.