corrosion of steel alloys in co2 environments
TRANSCRIPT
Corrosion of Steel Alloys in CO2 Environments
Corrosion of Steel Alloys in CO2 Environments
Abstract:
This report describes an experimental program to evaluate corrosion of carbon steels in CO2 environment. Coupon exposure and electrochemical tests were conducted as a part of the program. The objective of the test program was to determine the corrosion rate of steel exposed to wet CO2 conditions. Test variables were brine chemistry, CO2 partial pressure, temperature, metal alloy and exposure duration.
Introduction
This report describes an experimental program to evaluate corrosion of carbon steels in CO2 environment. Coupon exposure and electrochemical tests were conducted as a part of the program. The objective of the test program was to determine the corrosion rate of steel exposed to wet CO2 conditions. Test variables were brine chemistry, CO2 partial pressure, temperature, metal alloy and exposure duration.
The test program was performed in an effort to determine corrosion of metal alloys used in the transportation, injection and withdrawal of wet natural gas from a storage facility. Stored gas when retrieved would contain CO2 and reservoir brine over a range of temperatures and pressures.
Background
CO2 corrosion is a complex phenomenon that depends on numerous experimental variables. The key factor affecting iron dissolution in carbonic acid solutions (containing chloride) is the formation (or lack thereof) of an iron carbonate surface layer. Iron carbonate scales can be "protective" to provide low corrosion rates or "non-protective" meaning poor coverage, poor adhesion, porosity or solubility that provide higher iron dissolution rates.
The corrosion rate of iron in a CO2 environment is not constant, especially in laboratory studies in which the starting point is a bare metal surface on a test specimen. Steel, when initially exposed to a carbonic acid solution, corrodes rapidly at first and then the rate decreases as the carbonate scale forms. The final or "steady state" corrosion rate can remain high (if scaling does not occur), or it can asymptotically approach a limit that reflects ferrous ion (Fe2+) transport through the surface scale and other factors.
The dominant influence to the steady state corrosion rate of steel in CO2 environments is the aqueous phase chemistry and temperature. Low rates are promoted by conditions that favor film formation. Such conditions are not intuitive in the sense that several kinetic processes compete to produce either protective or non-protective scales. In brines, higher CO2 partial pressure, higher pH, higher temperature and low convection promote scale formation and low rates.
There is a temperature at which the CO2 corrosion rate is a maximum. The temperature at which the rate is a maximum depends on aqueous phase chemistry. When the brine phase is similar to seawater, the maximum is in the range of 60 to 150 F. Brine anions that favor scale formation and low rates are CO32-, SO42-, PO43-, NO3-, and SiO3-. Cations that favor scale formation are Ca2+, Mg2+ and Fe2+.
Experimental
The test program consisted of autoclave exposures of coupons to provide weight loss corrosion data and electrochemical corrosion rate measurements.
Apparatus
A set of eight hastelloy lined autoclave vessels with a pressure rating of 10,000 psig and a temperature rating of 550 F were employed for the 30 day coupon corrosion tests. These vessels were placed into an oven equipped with a rotisserie and were rotated end over end to determine effects of fluid velocity and to mix multiphase solutions. Illustrations of the autoclave system are provided in Figures 1and 2.
Electrochemical testing was performed in a three liter hastelloy lined autoclave with a pressure rating of 5000 psig and temperature rating of 550 F. The autoclave was equipped with pressure ports into which were mounted the electrode holders. The vessel was also equipped with a pressure balanced silver/silver chloride reference electrode system that contacted the test solution by means of a Luggin probe.
Materials
Coupons of X-42 carbon steel had dimensions of approximately 1.0 inches X 0.5 inches X 0.125 inches with two mounting holes located at one end of the coupon (See Figure 3). A section of J-55 carbon steel tubular and machined additional coupons are procured for testing. Electrochemical tests employed cylindrical electrodes machined from X-42 stock as shown in Figure 4. Two electrodes of X-42 and J-55 with dimensions of 0.250 inch diameter X 3.5 inches with one end drilled and tapped were machined for electrochemical linear polarization tests (See Figure 4). Material chemistries are listed in Table 1.
Figure 1 - Rotating Autoclave Apparatus
Figure - 2 Autoclave Configuration
Figure 3 - Coupon Dimensions
Figure 4 - Electrochemical Test Specimen
A - 0.25" ?? 0.01" Dia.B - 3.50" ?? ; 0.01"C - 0.50" ?? 0.05"Note: The specimen is finished to surface roughness of 32 Micro-inch or finer
Table 1Material ChemistryGradeCarbon, maxManganesePhosphorousSulfur, maxCb, minTi, min
minmaxminmax
X-420.29--1.25--0.040.05----
J-55--------0.030.03----
Corrosion of Steel Alloys in CO2 Environments
EnvironmentsThe environments for the autoclave exposures and electrochemical test were identical. The brine solutions consisted of two concentrations that were laboratory formulated to simulate field conditions. Table 2 provides the chemical composition of the laboratory formulations. A third brine used in testing was provided by a third party company.The gas phase of the tests consisted of carbon dioxide at partial pressures of 25, 140, and 250 psia respectively. Test temperature was 80 F with the exception of the final test which was 125 F. A total test pressure of 3500 psig at room temperature was obtained with methane. The test matrix for the autoclave and electrochemical exposures are provided in Tables 3 and 4.Some tests (see Tables 3 and 4) included 10 percent by volume of jet fuel (similar to diesel). The hydrocarbon composition is provided in Table 4. Jet fuel was chosen because it most closely matched the paraffin content of hydrocarbon phase in the reservoir.Autoclave Test ProcedureDuplicate coupons of carbon steel X-42 were used in tests 1-11 and duplicate carbon steel coupons of J-55 were used in tests 12-13. Prior to testing the coupons were cleaned in solvent, dried with acetone and weighed to the nearest 0.1 mg. The coupons were fastened to one end of a teflon rod with nylon nuts and bolts. A teflon liner was inserted into each autoclave to provide galvanic isolation (See >Figure 2).Brine solutions "A" and "B" were laboratory formulated according to the specifications previously mentioned (See Table 2). Prior to testing, the batch solutions were dearated with nitrogen and analyzed for alkalinity and pH. A sample of the brines were analysed.The coupons and 550 ml of brine was added to each autoclave, sealed and a vacuum was pulled. Each vessel was pressure tested to 4000 psig and the pressure was bled off slowly. When the vessel reached ambient pressure another vacuum was pulled.Carbon dioxide gas was added to each vessel to the corresponding partial pressure (See Test Matrix Table 3) and allowed to equilibrate. Each vessel was brought to a total pressure, at room temperature, of 3500 psig with methane. The vessels were placed into the rotisserie oven and heated to test temperature by means of electrical resistance heaters and maintained with a digital proportional temperature controller. Test durations were 15 and 30 days respectively.At the completion of the test exposure, the vessels were allowed to cool and the pressure was bled off slowly. The vessels were opened sequentially and immediately analyzed for alkalinity and pH.Corrosion data were determined from the following equation:
R = 534 WD A T
where:534 = constantW = weight loss (mg)D = density (g/cm3)A = area (in2)T = time (hours)
Data generated from these tests were weight loss values and visual observations. Surface analysis of the scale corrosion was also performed.
Electrochemical Test Procedure
A total of 13 linear polarization tests were performed in a 3 liter Hastelloy C-276 lined autoclave equipped for high pressure electrochemical studies (See Figure 3). A pressure balanced silver/silver chloride reference system using 0.1 N potassium chloride as a bridge was utilized.For each experiment, two electrodes were tested. One electrode was tested in the as machined condition and the second was coated with a heat shrink teflon sleeve over the upper two thirds of the electrode surface to mask the solution/vapor interface.Solutions were formulated and dearated with nitrogen and subsequently analyzed for alkalinity and pH prior to the addition to the autoclave. Solution was added to provide a known surface area. The autoclave was sealed and a vacuum was pulled. The vessel was pressure tested to 4000 psig and the pressure was bled off slowly. When the vessel reached ambient pressure, another vacuum was pulled.Carbon dioxide gas was added to the autoclave to the corresponding partial pressure (See Matrix Table 4) and allowed to equilibrate. The vessel was brought to a total room temperature pressure of 3500 psig with methane. The vessel was brought to test temperature and maintained with a digital proportional temperature controller.Linear polarization tests were performed on each electrode at intervals of approximately 1, 6, 24, and 48 hours and corrosion rates determined. Parameters for the linear polarization tests were as follows:Scan rate = 0.2 mv/secScan region = 20 mv ?? EcorrEquivalent weight = 28 for FeDensity = 7.8 g/cm3Tafel constant A = 120Tafel constant B = 120At the completion of each test, the solution was analyzed for alkalinity and pH.
Table 2
Chemical Analysis of Brines used in the TestsComponentBrine Form. "A", (mg/l)Brine Form. "B", (mg/l)Actual Brine, (mg/l)
Na+100901009017,895
K+5042504215,509
Ca2+8058180377,545
Mg2+99079910,021
Sr2+252225222,028
Cl-19091422696221,898
SO42-39639660
CO3--500500--
Table 3
Test Matrix and Test DataAutoclave ExposuresTest(1)StartFinishDuration (days)Temperature (F)CO2 (psia)BrineAlki (mg/l)Alkf (mg/l)pHipHfCorrosion Rate (mpy)
1-312/5 (6:30pm)3/8 (1:00 pm)318025A14008.85.25.16
2-312/5 (6:30 pm)3/8 (1:00 pm)318025B24014009.06.42.15
3-312/5 (6:30 pm)3/8 (1:00 pm)3180250A14008.85.010.13
4-312/5 (6:30 pm)3/8 (1:00 pm)3180250B24020009.06.13.78
1-142/5 (6:30 pm)2/19 (1:00 pm)148025A14008.85.23.15
2-142/5 (6:30 pm)2/19 (1:00 pm)148025B24011609.06.41.89
3-142/5 (6:30 pm)2/19 (1:00 pm)1480250A14008.84.96.95
4-142/5 (6:30 pm)2/19 (1:00 pm)1480250B24011409.06.27.51
5-15-H2/22 (4:00 pm)3/9 (1:00 pm)158025A16008.85.2--
6-15-H2/22 (4:00 pm)3/9 (1:00 pm)158025B>40018008.96.4--
7-15-H2/22 (4:00 pm)3/9 (1:00 pm)1580250A16008.84.9--
8-15-H2/22 (4:00 pm)3/9 (1:00 pm)1580250B>40020008.95.9--
5-30-H3/12 (4:00 pm)4/12 (2:00 pm)318025A18009.14.55.93
6-30-H3/12 (4:00 pm)4/12 (2:00 pm)318025B84020008.96.12.44
7-30-H3/12 (4:00 pm)4/12 (2:00 pm)3180250A18009.14.412.8
8-30-H3/12 (4:00 pm)4/12 (2:00 pm)3180250B84021008.95.85.5
9-30-A3/12 (4:00 pm)4/12 (2:00 pm)318025Act3607205.75.13.19
10-30-A3/12 (4:00 pm)4/12 (2:00 pm)3180250Act3606405.74.62.13
11-30-A3/12 (4:00 pm)4/12 (2:00 pm)3180140Act3607605.74.41.27
12-30-A3/12 (4:00 pm)4/12 (2:00 pm)3180250Act36012005.74.71.62
13-30-A4/16 (4:00 pm)5/17 (2:00 pm)31125250Act2468405.54.910.19
(1)H indicates tests with hydrocarbon
Corrosion of Steel Alloys in CO2 Environments
Results and Discussion
The two brines that were used in testing were distinguished by their scaling tendency with brine A having a high scaling tendency and high chloride and brine B having a low scaling tendency (also low chloride). It was postulated that corrosion rates would be lower in the brine (A) that possessed the higher scaling tendency.The time dependence of corrosion rate must be considered when viewing the rate data generated by autoclave (weight loss) methods and electrochemical methods. As illustrated in Figure 5, the general (theoretical) form of the corrosion function is logarithmic with time reaching a limiting value asymptotically. Electrochemical data points fall on the initial steep portion of the curve and are instantaneous, i.e. the rate at the instant measure. The normal variation in electrochemical rates is approximately 20 percent. Autoclave corrosion rates, on the other hand, are averages of the rate over the time period measured. These points aid rationalization of the corrosion data provided in Tables 3 and 4.Autoclave Test DataBrineHigher corrosion rates (See Table 3) were obtained for (high scaling) Brine A in autoclave exposures suggesting that the postulate of low scaling brines being more corrosive may not always be true. This result may have been due to the higher content of aggressive ions such as chloride in Brine A.HydrocarbonHydrocarbon in equilibrium with the brine had little effect on test outcome.Test DurationNo major difference in rate was measured in autoclave tests with duration of 14 days as opposed to tests with a 30 day duration.CO2 Partial PressureBrines in equilibrium with 250 psi CO2 are approximately twice as corrosive as brines in equilibrium with 25 psia CO2.Corrosion RatesGenerally, corrosion rates in this system were not excessive. All rates measured in autoclave tests were less than 12 mpy. Actual brines produced corrosion rates less than 4 mpy at 80 F and approximately 10 mpy at 125 F (on J-55).Electrochemical DataBrineElectrochemical corrosion rates measured for low scaling brine (B) were generally larger than for the high scaling brine which is opposite to the trend measured in autoclave tests. These higher initial rates indicate that scale formation is aided by a high initial rate. Similar trends have been measured for CO2 corrosion. A high initial rate produces saturation of the interfacial region (metal/solution) with ferrous ion giving rise to a high degree of scale surface coverage that eventually (t > 2 - 3 days) results in low rates.HydrocarbonHydrocarbon did not have a major influence on corrosion rate.Test DurationTests were conducted at 1, 6, 24 and 48 hours (approximately). Electrochemical corrosion rates are plotted versus time in Figures6, 7 and 8 for tests without hydrocarbon, with hydrocarbon and with field brine, respectively.In general, the trends (of rate vs. time) are downward as expected. The high scaling brine (A) reached much lower rates in 48 hours than did the low scaling brine (B). This observation is in line with expectation and is likely due to the faster rate of scale formation for brine A.CO2 Partial PressureNo trend of rate vs. partial pressure of CO2 was apparent.Corrosion RatesCorrosion rates for the field brine (C) and high scaling brine (A) measured electrochemically are less than 10 mpy and correlate reasonably well with the autoclave test data. The higher rates measured for brine B indicate scaling is slower.
Conclusions
In the system examined, CO2 corrosion of X-42 steel was measured as less than 10 mpy in autoclave and electrochemical tests. Of the test variables examined, CO2 partial pressure and brine chemistry (scaling tendency) have the greatest influence on rate.The initial corrosion rate (less than 48 hours) as measured by electrochemical methods is significantly higher for the low scaling brine (B), however, the steady state rate, as measured by autoclave tests was lower. This observation is consistent with the known mechanism of CO2 corrosion. Autoclave experiments in which the rate is obtained from mass loss over a 30 day period indicate that the high scaling brine A will produce the higher metal dissolution rates. The field brine which is similar in composition to brine A, produced low rates (less than 5 mpy).Test parameters that were insignificant were metal alloy and presence of hydrocarbon. The J-55 alloy produced a corrosion rate similar to X-42 at 80 F (2 mpy) and 10 mpy at 125 F.
Table 4
Test Matrix and Test DataElectrochemical ExposuresTest(1)StartFinishDuration (days)Temp (F)CO2 (psia)BrineAlki (mg/l)Alkf (mg/l)pHipHfInitial Corr Rate (mpy)Final Corr Ratef (mpy)12/3 (11:00 am)2/5 (10:00am)28025A10008.25.34.783.2022/8 (10:00 am)2/10 (9:00 am)28025B>400>4008.65.968.5724.3532/10 (12:00 pm)2/12 (11:00 am)280250A12008.24.621.578.9042/16 (10:00 am)2/18 (9:00 am)280250B>400>4008.65.957.7713.025-H2/18 (12:00 pm)2/20 (11:00am)28025A12008.24.022.026.946-H2/22 (10:00 am)2/24 (9:00 am)28025B100010008.66.230.1037.067-H2/24 (12:00 am)2/26 (11:00 am)280250A10008.24.818.774.118-H3/1 (10:00 am)3/3 (9:00 am)280250B100016008.66.135.0815.8493/15 (11:00 am)3/17 (10:00 am)28025Act3604805.55.07.131.47103/22 (11:00 am)3/24 (10:00 am)280140Act2403604.94.420.571.61113/24 (2:00 pm)3/26 (1:00 pm)280250Act3604005.24.43.931.74*123/29 (11:00 am)3/31 (10:00 am)280250Act2402405.24.44.663.19*133/31 (2:00 pm)4/2 (1:00 pm)2125250Act2402405.04.424.556.52(1)H indicates tests with hydrocarbon
Table 5Hydrocarbon Analysis
Identification: Jet Fuel "A" (Los Angeles)Composition
P
O
A
N
24.8
55.1
18.6
1.2
P = ParaffinsO = Olefin-CyclicsA = AromaticsN = Napthalenes