News, views and contacts from the global Offshore industry
 

Corrosion Correction

9 September 2009 Carolyn Johlman




Corrosion inhibitors are used to protect oil and gas pipelines made of carbon steel that transport CO2 or H2S containing wet hydrocarbons. Here, ConocoPhillips Company US's Mohsen Achour, David Blumer and Carolyn Johlman describe an in-house developed analytical method to determine the partitioning of the corrosion inhibitor between the oil, water and solid phases for a given system.


Because water and oil are usually present in the pipeline, it is important to understand the partitioning of the corrosion inhibitor between the two phases in order to assess its performance. The inhibitor's efficiency and performance are highly dependent on its capability to be present in the water phase and reach the pipe wall.

Therefore, for a given corrosion inhibitor dosage, there is a need to determine the amounts of the active components present in the water and those in the oil phase as well as any loss to the solid surfaces.

The partitioning of the inhibitor is evaluated using an LC-MS (liquid chromatography-mass spectroscopy) analytical method to trace the key active components of the inhibitor in the various phases. Such partitioning information can be used to choose the optimum inhibitor dosage and to monitor corrosion in the pipeline.

In wet hydrocarbon systems, especially those containing corrosive gases such as CO2, H2S and organic acids, internal corrosion can be a significant problem and might heavily and rapidly damage the inside wall of the pipeline. Corrosion inhibitors (CI) are often used to mitigate destructive effects.

"The partitioning of a corrosion inhibitor is evaluated using an analytical system called liquid chromatography-mass spectroscopy (LC-MS)."

In most applications where they are used to lessen CO2 or H2S corrosion, more than a single phase is present in the system. Therefore, when the inhibitor is applied, its components are inevitably distributed between the phases – mainly the oil, brine and solids phases.

As corrosion occurs on water-wetted metal surfaces, the adequate amount and type of the inhibitor's active components in the water phase is vital to provide protection. Solids can interfere with performance in several ways. When present on the pipe wall they constitute an extra physical layer the inhibitor has to overcome in order to reach the surface, and therefore mass transfer limitations can occur.

These solids will also adsorb the inhibitor's active components and may cause underdosage of the inhibitor if these losses are not considered.

Moreover, the accumulation of solids can constitute a localised cell providing ideal conditions for bacteria to grow. Controlling the partitioning of the inhibitor in an oil/water/solids system will enable the selection of the adequate dosage required for protection.

All these concerns drive the need for a reliable method to quantify the presence of the active components of a given corrosion inhibitor between the various phases present in a wet hydrocarbon system. It is within this framework that this article describes an analytical method used to provide such information.

Description of the LC-MS analytical method

The LC-MS system is mainly composed of a liquid chromatography column coupled to a mass spectroscopy unit. The various components of the inhibitors are identified by passing a carefully prepared sample through the two units.

Sample preparation: Samples are prepared for LC-MS analysis by solid phase extraction (SPE) using a proprietary method, mainly involving a special cocktail of solvent to dissolve all possible active components of the inhibitor. The samples are centrifuged and then a portion of the sample is transferred to the SPE cartridges.

"Samples are prepared for LC-MS analysis by solid-phase extraction."

The solids remaining in the sample container are extracted with a solution of 2% formic acid in a proprietary solvent mixture in an ultrasonic bath. Once the sample is ultrasonicated, the container is placed in a centrifuge for 20 minutes at 3,000rpm.

The extract is removed and the solids extraction process is repeated twice more. The SPE extracts and extracts of the solids are analysed by liquid chromatography/mass spectrometry, using a proprietary method.

The instrument, a Thermo Deca XP Plus, is equipped with an electro spray interface (LC/ESI-MS), operated in the positive ion mode, and a Thermo Surveyor liquid chromatograph (LC).

Calibration standards: Calibration standards are prepared from a variety of sources including the corrosion inhibitor of interest, pure chemicals, and synthesised intermediates. Stock standards are prepared at a concentration of 10,000ppmV and then diluted concentrations ranging from 0.5 to 50ppmV.

Sample extracts are diluted and re-analysed if the response exceeds the upper range of the calibration curve.

Data reporting: In order to simplify the results, the concentrations of the individual compounds are grouped into three families of components and the average concentration calculated. This approach is justified because the components in each group belong to the same chemical family.

The results are then reported as a percentage of the total inhibitor used in the partitioning study.

Data report and analysis

Corrosion rate measurements: Before proceeding to partitioning data, it is common to perform a standard bubble test where a linear polarisation resistance (LPR) curve is first determined for a given inhibitor to assess its capability to protect the line and ensure low corrosion rates. Sampling of the water and oil phases and their analysis for inhibitor residuals can be vital to understand the good or bad performances of various inhibitors.

Corrosion inhibitor characterisation: Before analysing for the inhibitor residuals in the samples, the active components of the inhibitor should be identified and quantified using LC/ESI-MS. Corrosion inhibitors may be complex mixtures of compounds; however, for the inhibitor used in this study, most of the compounds can be classified into three main chemical families or groups.

To simplify the results, the measured concentrations of the individual compounds in a particular chemical family are averaged.

"Solids constitute an extra physical layer that the inhibitor has to overcome in order to get to the surface and therefore mass transfer limitations can occur."

Corrosion inhibitor residual analysis: Once the corrosion inhibitor is characterised and its individual components are identified, the individual species are measured in the oil, brine and solid samples collected from the partitioning test. The main compounds are grouped into three categories (comprised of three chemical families) and reported as an average concentration.

Corrosion monitoring using CI residual analysis

For a given pipeline, monitoring the concentration of the corrosion inhibitor active components present in the brine at the sludge catcher level will help indicate if the corrosion inhibitor is being transported all along the pipeline. The absence of actives in the outlet brine is a warning that the corrosion inhibitor could be consumed and/or trapped along the path.

The objective of this is to detect a certain trend or consistency in residual values. The detection of hills and valleys in the results should be carefully assessed.

Conclusions

A bubble test is used to test the performance and partitioning of corrosion inhibitors between phases – oil, brine and solids – present in pipelines carrying wet hydrocarbons containing CO2 and H2S. The corrosion rate measured using the LPR technique is determined and residual analyses of the corrosion inhibitor active components in all phases are also provided using an LC-MS analytical method.