Suitable time of immersion to measure the corrosion rates of steel in an autoclave containing high-pressure CO2-bicarbonate solutions. If we have around 20 coupons, and we want to measure their corrosion rates over different periods of time, what are the suitable ranges?
you should expose at least 4 weeks with weekly measurements. If possible some cupons should remain for 6 months.
What is the temperature condition? In my opinion it should be minimum of one month before an appreciable amount of corrosion can be noticed. I think the temperature has to be very high in the conditions being mentioned otherwise it has to be 6 months or more before you could measure this.
I agree with Railph and Gurmeet regard the time and temperature. I think also you should maintain the pH value constant during exposure for long time.
Dear Faysal Fayez,
Is there any sdtandard method to measure the rate of steel corrosion? Is there any standard steel material to whom other materrials should be compares? If the questions to both questions is no, then just perform your experiment until you observe something. After all the results are qualitative.
Goo luck!
Chicgoua
PS: If there is a standard material perform parallel experiments. If there is no standard material, 'create' your 'operational standard' by choosing a well-characterized material (an compare it behaviour to the speciment to be investiagated).
Each set of coupons was exposed in the autoclave for 24 hours in the following reference:
Das, G.S. and A. S. Khanna. “Parametric Study of Co2/H2s Corrosion of Carbon Steel Used for Pipeline Application.” International Symposium of Research Students on Materials Science and Engineering, December 20-22, 2004, Chennai, India
Also, more than 20 coupons would be needed to make statistically significant conclusions...Regards.
In closed system with only CO2/bicarbonates, you may expect initial low pH conditions with high corrosion rates, depending on T°C. You should be able to measure it. On coupons of 1mm to 2 mm thick, I would recommend not to wait too long before taking out the first sample (< 1 month if T°C > 50°C).
With time, pH should increase and corrosion rate decrease accordingly, because of passivation layers. If possible, sample some fluid when getting the coupon (pH...).
I am sure you thought of a system which avoid CO2 leaks when sampling ?
Thank you all Ralph, Gurmeet, M. Morks, Sebastián, Chicgoua, Daniel, Nicolas, for all your valuable answers. Some of them raised questions about the standards and the environmental conditions. In regard to temperature, a range of temperatures is to be considered (between 20 to 120 C). Thank you also again for the valuable information that emphasized on the roles and importance of pH and pH control. Good luck to you all with your research!! :)
Thank you Abdulaziz for your input, I actually won't apply any stresses during a high-pressure autoclave testing. I am investigating the general corrosion behavior where the three main factors will ultimately affect the corrosion rates and interchangeably the corrosion products' properties, which are the period of immersion, temperature, and pH (controlled, or being kept unbuffered).
Thank you again.
Faysal
You need to think about what you are trying to find out. If you want to know how the metals corrode in stagnant solution of limited volume, then you want to let the system seek its chemical equilibrium and pH. If you want to know how the metals corrode in flowing solution of a fixed chemistry, then you need to periodically check and refresh the solution and/or its pH or use flowing solution. The experiments should be run until you reach steady state; and even then, extrapolation into the future assumes an uncertainty due to the assumption of no change in the rate limiting processes. Of course, all factors are offset by cost and time considerations. In our case, we were looking at fire suppression storage container alloys and the possibility of using fluidized sodium bicarbonate as a non CFC fire suppressant.
Thank you Richard for your very valuable input. It raises important considerations about the chemistry and the chemical equilibrium of the test fluids with respect to their (small) volume and the test time periods.
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