Carbon Steel Corrosion

Electrochemical behaviour of carbon steel in aqueous amine solvents for post combustion carbon capture

Background

An investigation of the corrosion behaviour of carbon steel in two aqueous amine solvents used for post-combustion CO2 capture (PCCC), via amine scrubbing, was conducted in one of our past PhD projects, carried out by Dr Samara Sadeek. PCCC via amine scrubbing remains one of the few CO2 emission mitigation strategies that is commercially deployed to date. However, its wide-scale adoption as an abatement strategy is yet to be achieved due to the high capital and operating costs. Therefore, our goal was to evaluate a corrosion mitigation strategy to enable the use of economical carbon steel in lieu of costly stainless steel and in so doing, increase the chance of wide-scale deployment of PCCC.

Scanning electron microscopy of carbon steel after one week of immersion in monoethanolamine saturated with CO2 at 80 °C.

The benchmark solvent for PCCC, monoethanolamine (MEA), becomes corrosive when saturated with CO2 and heated to elevated temperatures required for the CO2 capture process. The image on the left shows a corroded carbon steel surface after one week of immersion in MEA .

 

Scanning electron microscopy of carbon steel after one week of immersion in methyldiethanolamine saturated with CO2 at 80 °C.

By contrast, methyldiethanolamine (MDEA), a solvent used for amine scrubbing in natural gas processing, has been demonstrated to facilitate the formation of FeCO3 on carbon steel in aqueous CO2 environments, which is thought to protect the underlying steel from corrosion. Therefore, the corrosion mitigation strategy investigated in this study was the pre-treatment of carbon steel through the initial use of methyldiethanolamine to induce the formation of a protective FeCO3 film. To investigate the viability of this corrosion mitigation strategy, the project involved:

  • Establishing the effect of amine type on the corrosion of carbon steel.
  • Determining the surface reactions on carbon steel in CO2 saturated aqueous MEA and MDEA solutions.
  • Assessing the protectiveness and composition of surface films formed on carbon steel in CO2 saturated aqueous MDEA solutions.
  • Investigating the electrical resistivity and stability of surface films in CO2 saturated MEA.

Investigation

A host of electrochemical techniques was used to investigate the varied behaviour exhibited by carbon steel in MEA and MDEA.

Electrochemical set up used for laboratory scale experiments.

Cyclic voltammetry was used to determine the varied surface reactions occurring on iron in the two amine solvents. Chronoamperometry was used to investigate the FeCO3 formation reaction and was interpreted using local speciation predictions made from a diffusion reaction model. Electrochemical impedance spectroscopy (EIS), along with open circuit potential measurements, was used to assess the protectiveness and stability of FeCO3 surface films on carbon steel.

Bespoke electrode holder with attached pre-treated coupon used for pilot-scale experiments.

The stability of the surface films formed via pre-treatment of carbon steel coupons in methyldiethanolamine was also assessed in the CO2 capture pilot plant in the Chemical Engineering department at Imperial College London. An electrode holder was designed to embed the coupons in the plant at a fixed orientation and to enable continuous open circuit potential measurements to be made, which were thought to be a sensitive indicator of film breakdown. From this investigation we concluded that protective FeCO3 films formed on carbon steel in MDEA, and were relatively stable in MEA under anoxic conditions, but could not recommend pre-treatment as a reliable corrosion mitigation strategy in environments containing dissolved O2.

 

Coupons installed in the CO2 Capture Pilot Plant in the Department of Chemical Engineering at Imperial College London.

 

 Publications

The work of Dr Samara Sadeek on carbon steel behaviour in monoethanolamine (MEA) and methyldiethanolamine (MDEA):