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Because of the complexity involved in evaluating corrosion inhibitors, the availability of sophisticated methodologies to evaluate inhibitors, the cost associated with screening and using inhibitors to control internal corrosion of pipelines, and the widespread use of inhibitors to manage risk where public safety is involved, it is necessary, firstly, to standardize the methodologies that are used to evaluate and qualify inhibitors and, secondly, to use the standardized methods.

In this paper, the various steps CANMET undertook to standardise a methodology are described.

In order to assess the needs of the industry and the state of advancement in the industry, more than 40 industrial personnel were interviewed. Their inputs are summarized as follows:

• The most relevant types of corrosion in flowing wet sour environments containing both H₂S and CO₂ are pitting corrosion and general weight loss corrosion.

• The types of measurement most commonly used for inhibitor evaluation are weight loss of coupons, linear polarization, and hydrogen permeation using foils.

• More experience is required with the advanced electrochemical techniques - electrochemical noise and electrochemical impedance spectroscopy (EIS) - before the industry as a whole is sufficiently confident to use this technology extensively.

• Analysis of the flow regime is essential, and the laboratory methodologies that are designed must reflect the flow parameters that exist in the field.

In response to the needs of the industry, the CANMET/Industry Consortium on Development of Standardized Methodologies for Evaluation and Qualification of Inhibitors for Sour Service was established. The concept of the Consortium, the research to be carried out, and the specific interests of participants were established (Fig.1).

The main objectives of the Consortium were:

• To compare quantitatively the results of tests currently used for inhibitor qualification;

• To develop methodologies to be standardized for inhibitor evaluation and qualification for sour service; and

• To submit the agreed-upon methodologies to appropriate standards-making organizations for consideration as recommended and standardized practices.

In order to attain the objectives the following approach was adopted.

• From published literature and data supplied by the project sponsors, a database of more than 750 references in Reference Manager^TM on current test methodologies, their reproducibility and effectiveness, and their accuracy for predicting field performance was developed.

• Specific qualification procedures were evaluated in the laboratory. Round robin tests were carried out in three laboratories to assess the reproducibility of the wheel test. In total, twelve (12) laboratory methodologies were evaluated.

• Field monitoring was carried out to support the laboratory evaluation and to define the conditions under which specific laboratory techniques can be used with the confidence that the laboratory data will predict field performance.

Based on the database, the start-of-the-art of selecting inhibitors was critically reviewed. The main outcomes were:

• Although wheel and bubble tests are popular laboratory methodologies for corrosion inhibitor evaluation in the laboratory, neither methodology simulates flow conditions in the field.

• Rotating cylinder electrode and jet impingement methodologies are becoming more widely used. These methodologies have well established flow patterns, and methods to determine the hydrodynamics parameters (wall shear stress and mass-transfer coefficient, and Reynolds number) are well established. Both methodologies are carried out only at atmospheric pressure.

• The rotating disc electrode has a well-established flow pattern and is a very popular laboratory methodology for electrochemical studies, but is not used for corrosion inhibitor evaluation.

• The high-temperature, high-pressure rotating cage is also used, but the flow characteristics are not known.

• The types of measurement most commonly used for inhibitor evaluation in the field are weight loss of coupons, linear polarization resistance, and hydrogen permeation using foils. There is a need for considerably more experience with the advanced electrochemical techniques - electrochemical noise and electrochemical impedance spectroscopy (EIS) - before the industry as a whole is sufficiently confident to use this technology extensively.

High-Temperature, High-Pressure Rotating Electrode: All the results on rotating electrode systems reported have been carried out at atmospheric pressure or at pressures only slightly above atmospheric. Conventional rotating electrode systems can function up to only 70EC and atmospheric pressure. Great care must be taken when using these systems in corrosive environments, such as hydrogen sulphide. In order to overcome these obstacles and also to perform continuous electrochemical experiments at elevated temperature and pressure using rotating electrodes, CANMET has designed and constructed a high-temperature, high-pressure rotating electrode (HTHPRE) system. Using this system, it is feasible to carry out experiments with rotating cylinder (or disk) electrodes at high temperature and high pressure (Fig.2).

Figure 2: Schematic Diagram of High-Temperature, High-Pressure Rotating Electrode

High-Temperature, High-Pressure Jet Impingement: Analysis of the flow regimes produced in various systems indicates that the impinging jet simulates most closely the variety of flow conditions in operating pipelines. A new design of impinging jet has been developed and evaluated at the CANMET laboratories. The instrument consists of a central cell with four arms containing the nozzles (Fig.3). Using this system, experiments can be carried out simultaneously using four (4) samples, and measurements can be performed using electrochemical and/or weight loss coupons.

Figure 3: High-Temperature, High-Pressure Jet Impingement Apparatus

Hydrodynamics of Rotating Cage: Several papers have reported results from the rotating cage, but methods to calculate the hydrodynamics of this system, to optimize apparatus dimensions, and to assess their effects on flow are not available. Although coupons in the rotating cage apparatus have been shown to be undergoing severe corrosion as well as pitting corrosion, the conditions prevailing during the experiments are not defined. Most of the experiments using the rotating cage have been carried out in an autoclave at elevated pressure and temperature. To perform rotating cage experiments under known hydrodynamic conditions and under the conditions of a given pipeline, CANMET studied the fluid characteristics in a rotating cage (Fig.4) and developed a method to calculate the wall shear stress. The flow pattern and calculation are valid only for the dimensions of the CANMET rotating cage, but the method can be used to characterise the flow pattern in systems of other dimensions. Based on the studies, the wall shear stress of rotating cage has been defined as:

t_RC = 0.0791 Re^-0.3rr²w^2.3 (1)

Where t_RC is wall shear stress; Re, Reynolds number; r, density; r, radius of rotating cage; and w, angular velocity.

Figure 4: Rotating Cage

Inhibitor Selection Software: The laboratory test methodologies to evaluate inhibitors for a particular field should be carried out so that the field conditions of the pipeline are simulated. To predict the corrosion rates in the field from laboratory test data, geometry-independent fluid flow parameters common to all hydrodynamic systems must be evaluated. The fundamental assumption to develop a correlation between laboratory and field corrosion rates is that when the hydrodynamic parameters of different geometries are the same, then the corrosion rates will be similar. The program determines the conditions to be used for evaluating inhibitors using RDE, RCE, RC, and JI, based on the field conditions.

The Consortium recognised the importance of comparing the results obtained in the field with those obtained in the laboratory using the wheel test, a standard test used in many companies. Therefore, a round robin of wheel tests was carried out, using formation water and formation water containing either continuous or batch inhibitors.

Of the four inhibitors used, two are water-soluble (Inhibitor #1 and #3), one is oil-soluble water-dispersible (Inhibitor #2), and one is oil-soluble (Inhibitor #4). Inhibitors #1 and #3 are intended for use in continuous inhibition, whereas Inhibitor #4 is a batch inhibitor. Inhibitor #2 can be used both as a continuous and as a batch inhibitor. Experiments were carried out using three continuous inhibitors, #1, #2, and #3, at 50, 100, and 200 ppm concentrations and two batch inhibitors, #2 and #4, at 500, 1000 and 2000 ppm concentration. Three laboratories have participated in these tests. Common experimental procedures were developed and used.

With the current level of standardization of the wheel test, the results obtained in each laboratory should be considered as indications of trends. Although the results obtained by different laboratories are not quantitatively comparable, differences are a function of the corrosion rate.

For example, in plain formation water, the corrosion rates are larger and vary considerably. On the other hand, in formation water containing continuous inhibitors, the corrosion rates are lower and there is better consistency of results from the different laboratories.

The main benefits of conducting the round robin wheel tests are in recognizing and understanding the limitations of the wheel test in that with the level of standardisation of wheel test, it may be possible to differentiate a good inhibitor from a poor inhibitor, but not a better inhibitor among good inhibitors. In other words, the wheel test may be an effective screening test.

To define the conditions under which specific laboratory techniques can be used with the confidence that the laboratory data will predict field performance, field trials were carried out at three (3) locations with different operating conditions:

• Oily-gas field
• Gassy-oil field
• Oil-transmission pipeline

The field loop was installed at the wellhead (Fig.5). The monitoring was done mainly using four three-pair, tree-coupon ladders and two-strip coupon ladders. The tree ladders were made such that electrical connection could be made to any one of the coupons mounted on it. Three of the six coupons on each ladder were wired for electrochemical monitoring.

Figure 5: Schematic Diagram of Pipe Loop

The hydrogen flux across the test spool was monitored using Beta Foils™. The Beta foils were located at twelve positions.

Monitoring was carried out using:

• Weight loss
• Linear polarisation resistance (LPR)
• Electrochemical Impedance Spectroscopy (EIS)
• Electrochemical Noise (EN)
• Hydrogen permeation (Beta foil)

The pipe loop has two sections: Replaceable and non-replaceable (static) sections. The replaceable section was changed after each field trial; i.e., three separate replaceable sections were used in each field. The pipe pit distribution was compared with those of the coupons.

In the laboratory, experiments were carried out using twelve (12) methods under the conditions of the fields.

• Round robin wheel test
• Bubble test
• Static test
• High-temperature, high-pressure static test (HTHPStatic)
• Rotating disk electrode (RDE)
• High-temperature, high-pressure rotating disk electrode (HTHPRDE)
• Rotating cylinder electrode (RCE)
• High-temperature, high-pressure rotating cylinder electrode (HTHPRCE)
• Jet Impingement (JI)
• High-temperature, high-pressure jet impingement (HTHPJI)
• Rotating cage (RC)
• High-temperature, high-pressure rotating cage (HTHPRC)

The ranking of the laboratory methodologies is based on the following considerations:

• Bottom coupons undergo higher corrosion and exhibit extensive pitting corrosion;

• For comparison purposes, the corrosion rates and pitting corrosion rates from the weight loss measurements from the field are used;

• The CANMET field loop used in the Kaybob field contains 6" and 10" pipes. The general and pitting corrosion rates of the bottom coupons are used.

• The field loop contains both standard and pipe coupons, whereas, in the laboratory experiments, only pipe coupons were used. To compare with laboratory corrosion rates, only the data from the pipe coupons are used.

• Because the deepest pits lead to failure, the pit depth, rather than pit density, is considered.

By comparing the results of the field and laboratory experiments at the same inhibitor concentrations, a preliminary ranking of test methodologies at each inhibitor concentration was developed.

The approaches used to calculate the ranking of the laboratory methodologies were:

1. Comparison of the logarithm of the ratio of the laboratory corrosion rate to the corrosion rate of the bottom coupon in the field;

2. Comparison of the logarithm of the ratio of the laboratory pitting corrosion rate to the pitting corrosion rate of the bottom coupon in the field; and

3. Comparison of the percent inhibition (calculated from both corrosion rate and pitting corrosion rate) in the laboratory and in the field.

Based on three field conditions using 6 inhibitors (3 continuous and 3 batch) and 68 individual comparisons, the final ranking of laboratory methodologies (Table 1) was developed.

Table 1: Ranking of Laboratory Methodologies Based

In the field, corrosion rates were measured using various techniques, including weight loss, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), electrochemical noise (EN), and externally mounted hydrogen permeation foils (Beta foils). In order to develop a real-time corrosion monitoring technique for assessing inhibitor performance in the field, the measuring techniques were ranked. The method for ranking the measuring techniques was as follows:

• The corrosion rates are expressed in various units, e.g., mils per year for weight loss, LPR, EIS, and noise, and kPa/year for Beta Foils™.

• The trend in the variation of corrosion rate (rather than the comparison of absolute corrosion rates) with inhibitor concentration was used as an appropriate method to rank the techniques.

• The ranking was done individually at each position, i.e., top, middle, and bottom positions.

• The first step was to develop a "standard trend" to which the trend from each measuring technique could be compared.

• The standard trend was taken as the "most repeated trend" from different measuring techniques.

• The "most repeated trend (MRT)" was deduced as follows. The corrosion rates (at different inhibitor concentrations) were arranged in decreasing order. For example, at one position the corrosion rates (as measured by one technique) were: 50 at 0 ppm, 20 at 10 ppm, 10 at 20 ppm. Then the trend in the measuring technique was: 0, 10, and 20 ppm.

• In this manner, the trend was deduced for each measuring technique.

• From this trend, the most repeated trend was deduced (as the number of times the trend was repeated by different measuring techniques). For example in Table XIX.4, two measuring techniques (Wt.loss and pitting) show the trend: 0 > 50 > 100 > 200 ppm, whereas the third technique (beta foil) shows a trend: 50 > 0 > 100 > 200 ppm. In this case, the MRT is: 0 > 50 > 100 > 200 ppm.

• The trends exhibited by the techniques were then compared with the M.R.T. In the above example, weight loss agree with the M.R.T. four times out of four comparisons (100%); where as the beta foil agrees two times out of four comparisons (50%).

It is assumed that the technique with the closest agreement with the M.R.T. indicates the real life scenario inside the pipe. Based on the comparison of general corrosion rates, the field measuring techniques are ranked as:

• It is recommended that the rotating cage be standardized as a methodology for evaluating and qualifying inhibitors for pipeline applications.

• The results of this project were published widely. All papers from this project are listed in the references.

• Based on the experience gained from this consortium, an ASTM Document was prepared.

The authors would like to acknowledge the helpful discussions and financial support from the members of the CANMET/Industry Consortium on Development of Standardized Methodologies for Evaluation and Qualification of Inhibitors for Sour Service. Financial support was also provided by the Federal Interdepartmental Program of Energy R&D (PERD). In addition, the authors would like to acknowledge the help of the numerous staff members at CANMET who have participated in the project including, from the CANMET Western Research Centre, A.Teclemariam, K. Kar, W. Friesen, T. Dabros, H.Sun, and S.Jefferies; from the CANMET Materials Technology Laboratory, Y. Lafreniere, J.McKinnon, and W. Zheng; and from CANMET Engineering and Technical Services, Ed. Kelly and the late K. Grenzowski.

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3. S. Papavinasm, R.W. Revie, M. Attard, and K.M.L. Ma, Hydrodynamics of Rotating Cage: The Top Ranked Methodology for Inhibitor Evaluation, First Internet Conference on Pipeline Reliability, November 13-19, 2000.

4. S.Papavinasam, R.W.Revie, M.Attard, A.Demoz, D.C.Donini, and K.Michaelian, "Standardized Methodology For Inhibitor Evaluation And Qualification For Pipeline Applications", IPC2000, Vol.1, 901, October 1-5, 2000, ASME, Calgary, AB, Canada.

5. S. Papavinasam and R.W. Revie High-Temperature, High-Pressure Rotating Electrode System: A Compact Laboratory Methodology to Study Combined Effect of Variables Influencing Corrosion Rates, First Internet Conference on Pipeline Reliability, September 11-18, 2000.

6. S.Papavinasam, R.W.Revie, M.Attard, A.Demoz, H.Sun, J.C.Donini, and K.Michaelian, "Inhibitor Selection for Internal Corrosion Control of Pipelines: Comparison of Rates of General Corrosion And Pitting Corrosion Under Oil-Transmission Pipeline Conditions in the Laboratory and in the Field" EUROCORR 2000, September 10-14, 2000, University of London, London, U.K.

7. S.Papavinasam, R.W.Revi, A. Demoz, J.C. Donini, and K. Michaelian, Ranking of Field Techniques for Monitoring General Corrosion Rates inside Pipelines, First Internet Conference on Pipeline Reliability, September 4-10, 2000.

8. S.Papavinasam, R.W.Revie, M.Attard, A.Demoz, H.Sun, J.C.Donini, and K.Michaelian; "Inhibitor Selection for Internal Corrosion Control of Pipelines: Comparison of Rates of General Corrosion And Pitting Corrosion Under Oily-Gas Pipeline Conditions in the Laboratory and in the Field", 9th European Symposium on Corrosion Inhibitors, University of Ferrara, Ferrara, Italy, September 4-8, 2000.

9. S.Papavinasam and R.W.Revie, Inhibitor Selection for Internal Corrosion Control of Pipelines, First Internet Conference on Pipeline Reliability, August 28-September 3, 2000

10. M.Attard, S.Papavinasam, R.W.Revie, A.Demoz, H.Sun, J.C.Donini, and K.Michaelian Inhibitor Selection for Internal Corrosion Control of Pipelines: Comparison of Pitting Corrosion Rates of Coupons and Pipe, First Internet Conference on Pipeline Reliability, July 17-23, 2000.

11. A.Demoz, H.Sun, J.C.Donini, and K.Michaelian, S.Papavinasam, R.W.Revie, Inhibitor Selection for Internal Corrosion Control of Pipelines: Experience with Field Monitoring and Measurement, First Internet Conference on Pipeline Reliability, July 10-16, 2000.

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13. S. Papavinasam, R.W. Revie, M. Attard, A. Demoz, H. Sun, J.C. Donini, and K.H. Michaelian, "Laboratory Methodologies for Corrosion Inhibitor Selection" Materials Performance, Vol. 39, No.8, p.58, (2000).

14. S. Papavinasam, R.W. Revie, M. Attard, A. Demoz, H. Sun, J.C. Donini, and K. Michaelian, "Inhibitor Selection for Internal Corrosion Control of Pipelines: Comparison of Rates of General Corrosion and Pitting Corrosion under Gassy-Oil Pipeline Conditions in the Laboratory and in the Field", NACE/2000, Paper No. 55, Orlando, March 2000.

15. S. Papavinasam, R. W. Revie, M. Attard, A. Demoz, H. Sun, J. C. Donini, and K. Michaelian, Inhibitor Selection for Internal Corrosion Control of Pipelines: 1. Laboratory Methodologies, Paper No. 1, CORROSION/99, NACE International, Houston, Texas.

16. S.Papavinasam and R.W.Revie , "Synergistic Effect of Pressure And Flow on Corrosion Rates: Studies Using High-temperature, High-pressure Rotating Electrode System", Paper No. 30, CORROSION/99, NACE International, Houston, Texas.

17. S.Papavinasam, R.W.Revie, A.Demoz and K.Michaelian, "Inhibitor Selection For Internal Corrosion Control of Pipelines: 2. Measuring Techniques" Northern Area Western Conference, Calgary, March 1999.

18. Alebachew Demoz, Kirk H. Michaelian, John Donini, S. Papavinasam and R. Winston Revie, "Inhibitor Selection for Internal Corrosion Control of Pipelines: Experience with Field Monitoring and Measurement", Northern Area Eastern Conference and Exhibition, Ottawa, Ontario, Canada, October 24-27, 1999, Paper No. 2A.4.

19. M. Attard, S. Papavinasam, R.W. Revie, A. Demoz, J.C. Donini and K. Michaelian, "Inhibitor Selection for Internal Corrosion Control of Pipelines: Comparison of Pitting Corrosion Rates of Coupons and Pipe", Northern Area Eastern Conference and Exhibition, Ottawa, Ontario, Canada, October 24-27, 1999, Paper No. 2A.3.

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22. A.Demoz, J.Donini,S. Papavinasam, and W. Revie, "Lessons learned from an instrumented field corrosion test loop" International Pipeline Conference, Calgary, Alberta, Canada, American Society of Mechanical Engineers, New York, 1998, Vol. 1, p. 521.