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.
Effect of Flow on the Formation of Iron Carbonate and Influence of Exposed Iron Carbide Layer
DI BONAVENTURA, MARIA C., M.S., August 2017, Chemical & Biomolecular Engineering Effect of Flow on the Formation of Iron Carbonate and Influence of Exposed Iron Carbide Layer Director ofThesis: Marc Singer Iron carbonate (FeCO3) is the commonest corrosion product that forms on the surface of mild steel as a by-product of the CO2 corrosion process. This FeCO3 layer slows down further corrosion by acting as a diffusion barrier, blocking corrosive species from reaching the steel surface. However, high flow velocities, which can be common in various industrial operations, have been postulated either to lead to partial mechanical removal of FeCO3 layers or to impede the nucleation of FeCO3 crystals on the steel surface altogether. In the experimental study described herein, corrosion product formation in highly turbulent conditions was investigated with surface analysis techniques. Experiments were divided in relation to three different sets of tasks focusing on high initial saturation values, low and constant saturation values, and high velocity experiments. The first set of experiments was performed in a three electrode glass cell and rotating cylinder setup and investigated the presence/attachment/adherence of FeCO3 on the steel surface in short term experiments with high initial saturation values (SFeCO3 150). The aim was to study the precipitation of FeCO3 in conditions where the bulk solution has a high concentration of ferrous ions at continuous rotational speeds, from the start to the end of each experiment. It was found that as the fluid velocity increased, there was less attachment of FeCO3, with 4 the highest velocity of 2.0 m/s (wall shear stress of 7 Pa) showing no FeCO3 formation/attachment on the metal surface. The second task focused on controlling the pH and ferrous ion concentration in solution, in order to better mimic actual field conditions. Additionally, a controlled mass transfer setup was utilized that eliminated any non-uniformity of flow and centrifugal forces often associated with rotating cylinder working electrodes. In this set of experiments, four different materials and/or microstructures were tested, namely pure Fe (99.8%), UNS G10180(1) with two different microstructures (tempered martensite and ferritic-pearlitic), and API 5L X65(2). It was observed that the carbon content and microstructure (distribution of iron carbide [Fe3C]) have a strong effect on the results with the ferritic-pearlitic steel clearly favoring FeCO3 precipitation. The third task consisted of exposing the material tested to highly turbulent conditions in a Thin Channel Flow Cell (TCFC) to identify a critical velocity for removal of Fe3C. The material chosen displayed formation of FeCO3 in task 2, which was UNS G10180 with a ferritic-pearlitic microstructure. A critical velocity for Fe3C removal was clearly identified, which further inhibited formation of FeCO3, although it is fully expected that its value should depend on the operating conditions. (1) Unified Numbering System for Metals and Alloys (UNS). UNS numbers are listed in Metals & Alloys in the Unified Numbering System, 10th ed. (Warrendale, PAE: SAE International and West Conshohocken, PA: ASTM International, 2004). (2) American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005.
Mesh-capped probe design for direct pH measurements at an actively corroding metal surface
Quantitative characterization of surface reaction species improves understanding of reaction mechanisms, especially for heterogeneous reactions such as in corrosion and electrolysis. A universal mesh-capped surface pH probe was developed in order to investigate surface proton concentrations for aqueous metal dissolution/deposition reactions. This technique can be generally applied to characterize the surface pH either for bare metal reactions or metals corroding beneath a corrosion product film. Measurements with the new probe design have shown a good degree of reproducibility. The surface pH measurements during corrosion of mild steel have shown that a higher surface pH is achieved at the steel surface as compared with that of the bulk solution, just as predicted from theory. Chemical buffering effects on surface pH were observed during corrosion by contrasting the measurements obtained in HCl deaerated solutions with those containing CO2, i.e., carbonic acid (H2CO3) and acetic acid (CH3COOH/HAc) solutions. It was found that the surface pH was mainly controlled by the water chemistry and chemical buffer capacity. The surface pH is affected by the deposited corrosion scale porosity. A higher deviation of surface pH compared to the bulk value was observed for more dense layers with lower porosity.