How to Maintain a Stable Solution Chemistry when Simulating CO₂ Corrosion in a Small Volume Laboratory System
In laboratory experiments, corrosion of mild steel specimen in glass cells or autoclaves with a relatively small internal volume (of the order of 1 liter or less), will usually lead to a change in solution chemistry, (i.e., increase in ferrous ion concentration and solution pH) which will affect the corrosion product formation and ultimately the corrosion rate. However, in much larger field systems that are being simulated in the laboratory, such as for example oil and gas mild steel pipelines, the solution chemistry at any specific location does not change significantly over the same time period, since it is governed by the flow coming from further upstream. Therefore, it is very important to be able to maintain a stable solution chemistry in small scale laboratory experiments, in order to get a better simulation of corrosion seen in the field. In this work, a stable solution chemistry system was developed using ion exchange resins. H-form and K-form exchange resins were successfully tested in long term experiments aiming to keep pH and ferrous ion concentration reasonably stable. The results show that pH can be controlled within 0.02 pH units and ferrous ion concentration within 3 ppm. The results of electrochemical measurements and surface analysis show that there is a significant difference in both corrosion rate and corrosion product layer formation when a stable solution chemistry system is used.
Corrosion in Acid Gas Solutions
Abstract Corrosion in solutions of acidic gases, notably carbon dioxide and hydrogen sulfide, is very important in oil and gas production and can be expected to become important in processes for the sequestration of carbon dioxide. Carbon dioxide exhibits an equilibrium between dissolved CO 2 and carbonic acid, and this effectively buffers the hydrogen ion concentration, such that the rate of hydrogen evolution is much higher than for a solution of a strong acid with the same pH. Corrosion in CO 2 solution can be modeled using conventional electrochemical kinetic theory, and the results are in good agreement with experimental observations. Corrosion in H 2 S solution is less well understood, but models can be developed based on the premise that corrosion occurs through surface films of iron sulfide.
Electrochemical Model oF Mild Steel Corrosion in a Mixed H 2 S/CO 2 Aqueous Environment
The present study has been conducted to investigate the electrochemistry of mild steel corrosion in mixed H2S/CO2 aqueous environments and develop an electrochemical model to simulate the experimental results. The experiments were designed to determine the effect of H2S on CO2 corrosion for short term exposures of a few hours before any interference from iron sulfide corrosion product layers happened. Tests were conducted at different H2S concentrations ranging from 0 to 10% in the gas phase at 0.1 MPa total pressure at pH4 and pH5 respectively. Corrosion rates were measured by linear polarization resistance (LPR) and the corrosion mechanisms were investigated by using potentiodynamic sweeps. Results showed that the presence of H2S slowed down the charge transfer kinetics related to H2CO3 reduction and H2O reduction on the metal surface. An electrochemical model was developed for a mixed H2S/CO2 system which was calibrated with new experimental results and compared to data found in the open literature. The model predictions fit experimental data well for short exposures (measured in hours) but overestimate the experimental results for longer term exposures (measured by days and weeks) due to the formation of an iron sulfide corrosion product layer, which is not accounted for in the present model.