Clarification on Corrosion Process Please

[caneng2002]

Some guidence would be great on this one.

I have two brine waste streams (2500 and 8400 gpm pH about 8.4, 30% TDS, Temp 65C, Alkalinity 350 ppm as CaCO3, though Alk could be higer) to be pumped below ground in carbon steel pipe. Life of pipe about 20-30 years

From these streams I intend to remove CO2 and O2 possibly with a Vac Degasifier coupled supported with a steam feed.

My first Question - What levels do CO2 need to be reduced to significantly reduce potential corrosion? I know for boilers it gets reduced to the ppb level but that seems excessive for this application.

My second question is regarding the corrision mechanics or chemistry involved.

Co2 in water converts to carbonic acid then to bicarbonate then to Carbonate as the solution pH increases. According to the charts I've seen carbon acid does not exist above pH of 8.4 or so, but as bicarbonate and carbonate. So does that mean corrosion is not a problem in this stream because of the high pH (scaling maybe a problem) and it doesn't need to be removed? Or does the stream need to be pH adjusted before the degasifier to free the CO2 for some reason that escapes me?

Thanks

 

[knowlittle]

I am not in the process industry and what you described I don't quite understand. I will just comment on CO2 chemistry. It is as you said. In high pH solution, dissolved CO2 will become CO3-- ions. Because the reaction rate is high (CO2 -> HCO3- -> CO3--), you can calculate remaining CO2 from equilibrium constants. In neutral salt water, dissolved CO2 is single digit ppm, and in high pH I would assume all are converted to CO3-- and only ppb range of dissolved CO2 still remaining in the solution. But additional CO2 can be easily dissolved if the solution comes in contact with air. I know nothing about Vac Degasifier, but it sounds like a device that keeps the fluid from contacting air. If that's the case, you will be okay as far as CO2 corrosion is concerned.

PS: I know nothing about degasification, boiler, etc.

 

[brimmer]

I would be more concerned with removing the O2 from a corrosion perspective than the CO2, if you can remove both great, but if you simply remove the O2 you will find the corrosion rate will drop greatly. If you do not remove the O2, the pipe will corrode. We operate similar brine lines, our uninhibited general rate is approx 25mpy, similar chemistry as yours, but the pitting rate reaches near 100mpy. By removing the O2, we brought the rate down to approx 2mpy. In any event, mechanical deareation can be an option here, you can also try dearation with oxygen sacavenger, both, and/or protecting the pipe with a corrosion inhibitor, or combination of scavenger and inhibitor depending on chemistries.

 

[caneng2002]

brimmer

I agree with focusing on oxygen removal.

A quick one for you. What meathod do you use?

I'm currently looking at chemical scavaging (expensive do to volume of chemical needed per year), vac degasing using either a steam source or negative suction pumps to draw off the oxygen, and the last air scubber but not hopefull with it as only brings down oxygen to equilibrium level.

What level do you remove O2 down to?

Thanks

 

[strider6]

if you have a water (brine ) line with a pH of 8.4 then the corrosion by CO2 is not a concern. As you said you'll not have any H2CO3. The issue is the oxygen content and the level of 02 to avoid corrosion is approx 8-10 ppb.
One question is: The line is already installed (Carbon Steel) or you have to select the material? Maybe a different material selction would be the best solution.

S.

http://www.corrosionist.com

 

[brimmer]

Caneng2002, I am stuck with the cost of oxygen scavengers becasue I can't deareate mechanically. I use in combination with an inhibitor via continuous injection. You will have to weigh the options of which method you might select for mechanical dearation if going that way. Removing the oxygen helps greatly, but you still have a lot of Cl in there which could lead to problems (especially if some O2 did get in somehow). The other thing is dissolved solids and the possiblilty of these settling out and causing under deposit corrosion (not sure in your case). I still would look at a filming inhibitor even if you dearate. If you are constructing the line, cement lining could be an option. My O2 levels are well below 1 ppm into the ppb range. Even an O2 level of 1 ppm is sufficient for corrosion.