Anja Pfennig

Influence Of CO2 On The Corrosion Behaviour Of13Cr Martensitic Stainless Steel AISI 420 AndLow-alloyed Steel AISI 4140 Exposed To SalineAquifer Water Environment

The CCS technique involves the compression of emission gasses in deep geological layers. To guarantee the safety of the site, CO2-corrosion of the injection pipe steels has to be given special attention when engineering CCSsites. To get to know the corrosion behaviour samples of the heat treated steel AISI 4140, 42CrMo4, used for casing, and the martensitic stainless injectionpipe steel AISI 420, X46Cr13 were kept at T=60°C and p=1-60 bar for 700 h8000 h in a CO2-saturated synthetic aquifer environment similar to the geological CCS-site at Ketzin, Germany. The isothermal corrosion behaviour obtained by mass gain of the steels in the gas phase, the liquid phase and the intermediate phase gives surface corrosion rates around 0.1 to 0.8 mm/year. Severe pit corrosion with pit heights around 4.5 mm are only located on the AISI 420 steel. Main phase of the continuous complicated multi-layered carbonate/oxide structure is siderite FeCO3 in both types of steel.

Evaluation of Heat Treatment Performance of Potential Pipe Steels in CCS-Environment

To resist the corrosive geothermal environment during carbon capture and storage CCS -such as: heat, pressure, salinity of the aquifer, CO2-partial pressure, properties of pipe steels-require certain specification. For evaluation samples of differently heat treated high alloyed stainless injection-pipe steels AISI 420 X46Cr13, AISI 420J X20Cr13 as well as X5CrNiCuNb16–4 AISI 630 were kept at T=60 °C and ambient pressure as well as p=100 bar for 700 h up to 8000 h in a CO2-saturated synthetic aquifer environment similar to a possible geological situation in the northern German Basin. Corrosion rates and scale growth are lowest after long term exposure for steels hardened and tempered at 600 °C to 670 °C and pits -indicating local corrosion- decrease in diameter but increase in number as a function of carbon content of the steel. Martensitic microstructure is preferred with respect to these particular conditions.

The Role of Austenitizing Routines of Pipe Steels during CCS

Properties of pipe steels for CCS technology require resistance against the corrosive environment of a potential CCS-site (heat, pressure, salinity of the aquifer, CO2-partial pressure). The influence of austenitizing in heat treatment routines of two different injection pipe steels (1.4034, X46Cr13 and 1.4021, X20Cr13) was evaluated. Steel coupons were austenitized at different temperatures (900 – 1050 °C) for different lengths of time (30–90 min) before quenching and annealing prior to long term corrosion experiments (60°C, 100 bar, artificial brine close to a CCS-site in the Northern German Basin, Germany). In general, fewer pits are found on X46Cr13. Comparing steels with 13% chromium each the higher carbon content of X46Cr13 (0.46% C) results in a lower number of pits compared to X20Cr13 (0.20% C). It is found that neither the carbon content of the steels nor austenitizing temperature has much influence, but local corrosion behaviour is most susceptible towards austenitizing time.

Supercritical CO2-Corrosion in Heat Treated Steel Pipes during Carbon Capture and Storage CCS

Heat treatment of steels used for engineering a saline aquifer Carbon Capture and Storage (CCS) site may become an issue if handled trivially. Thus its influence on local corrosion (pitting) needs to be considered to guarantee reliability and safety during the injection of compressed emission gases (mainly containing CO2) into deep geological rock formations. 13% Chromium steel injection pipes heat treated differently (X46Cr13, 1.4034 with 0.46% Carbon and X20Cr13, 1.4021 with 0.20% Carbon) were tested in the laboratory under supercritical CO2 at 100 bar and 60 °C. Independent of the exposure time, the fewest pits were found on hardened steels with martensitic microstucture. For steels with similar Cr-content the higher C-content in 1.4034 resulted in fewer pits and lower maximum intrusion depth compared to 1.4021.

Predicting the life time of steels in CCS environment from long term local corrosion experiments

To predict the reliability and safety during the injection of compressed emission gases – mainly containing CO2 – into deep geological layers (CCS-technology, Carbon Capture and Storage), the influence of heat treatment on pit corrosion needs to be considered. Different heat treated steels used as an injection pipe with 13% chromium and 0.46% carbon (X46Cr13, 1.4034) and 0.2% carbon (X20Cr13, 1.4021) as well as 16% chromium steel X5CrNiCuNb16-4 (1.4542) were tested in laboratory experiments. The samples were exposed for up to 1 year to the distinct synthetic aquifer environment saturated with technical CO2 at a flow rate of 3 l/h. The corrosion rate generally does not exceed 0.03 mm/year. Pits with maximum pit heights around 300 µm were obtained for hardened X20Cr13 with martensitic microstructure. The least amount of pits is found on X46Cr13. The higher carbon content in, X46Cr13 (0.46% C), results in a lower amount of pits compared to X20Cr13 (0.20%).

Peer-to-Peer Lecture Films in a First Year Laboratory Material Science Course

Material science is believed to be one of the more complicated subjects for first year students of mechanical engineering because the scientific background is generally not taught at school or during job training. First year students of mechanical and automotive engineering at HTW Berlin are required to take 2 classes in material science with laboratory exercises accompanying the education. Still, basic knowledge upon theory is necessary to work practically during lab sessions and hand out are given to the students. Additionally lecture films show the laborato20thry routine prior to lab hours and show students what they are going to experience and learn. These films were initially inspired by students and conducted during a one term semester project supervised by lecturers and film experts (peer-to-peer approach). It was found that students watching the films were prepared better and gained more knowledge during practical work than those who did not have access to the films. Watching the introductory films lead to more download activity and actual studying of the lectures provided to prepare the experiments and furthermore lead to slightly better testing results.

Impact of saline aquifer water on surface and shallow pit corrosion of martensitic stainless steels during exposure to CO2 environment (CCS)

Pipe steels suitable for carbon capture and storage technology (CCS) require resistance against the corrosive environment of a potential CCS-site, e.g. heat, pressure, salinity of the aquifer, CO2-partial pressure. Samples of different mild and high alloyed stainless injection-pipe steels partially heat treated: 42CrMo4, X20Cr13, X46Cr13, X35CrMo4 as well as X5CrNiCuNb16-4 were kept at T=60 °C and ambient pressure as well as p=100 bar for 700 h - 8000 h in a CO2-saturated synthetic aquifer environment similar to possible geological on-shore CCS-sites in the northern German Basin. Main corrosion products are FeCO3 and FeOOH. Corrosion rates obtained at 100 bar are generally much lower than those measured at ambient pressure. Highest surface corrosion rates are 0.8 mm/year for 42CrMo4 and lowest 0.01 mm/year for X5CrNiCuNb16-4 in the vapour phase at ambient pressure. At 100 bar the highest corrosion rates are 0.01 mm/year for 42CrMo4, X20Cr13 (liquid phase), X46Cr13 and less than 0.01 mm/year for X35CrMo4 and X5CrNiCuNb16-4 after 8000 h of exposure with no regard to atmosphere. Martensitic microstructure offers good corrosion resistance.

Flipping the classroom and turning the grades – a solution to teach unbeloved phase diagrams to engineering students

Phase diagrams may simply be described as alloying maps in material science. However, the required thermodynamic background knowledge should be high level and understanding of the cooling procedure of metal melts as well as microstructure of metal alloys is challenging. Common teaching material presents results, but not how to get there and leaves frustrated first year engineering students behind. Knowledge on “how to read” phase diagrams is expected from teachers in advanced courses, but requirements are seldom met by the students. Teaching phase diagrams in an “inverted classroom” scenario is a method to let the students study the science on their own and then take the time to discuss their questions and do extended hands on lectures or exercises in class. Implementing the inverted classroom approach has been proven to be successful in terms of learning outcome, problem solving skills related to phase diagrams and in improving grades. Although the time of preparation is raised by a factor of approximately 4 for 2 four-hour classroom sessions, the positive and sustainable learning outcomes make fun to teach and worth the effort.

Improvement of Learning Outcome in Material Science through Inverted Classroom Techniques and Alternative Course Assessment

Material Science is known to first year mechanical engineering students as one of the fundamental courses with high work load. The knowledge of the complex science of materials enables students to select appropriate engineering materials in different designs due to acquired knowledge on the correlation of materials properties, microstructure and their intended manipulation. These abilities are not well constituted in one final exam. Therefore peer-to-peer lecture film supported inverted classroom scenarios were established to work in the course. These were accompanied by a newly developed Moodle course following the blended learning approach that gives students the chance to cumulative accomplish micro-grades via multiple activities, such as tests, lectures, presentations, forum discussions, written homework and glossary entries. These grades are summed to obtain the overall course grade. An improved learning outcome is demonstrated in high quality class discussions and most -important to students- in better grades (average 43/60=B) compared to those being assessed by one final exam only (average 39/69=C + ). The majority of students agreed on enhanced study skills when forced to study throughout the entire semester instead of learning intensely towards the end of the semester. This paper introduces the learning structure as well as graded activities, evaluates the course and compares activity results to former class results. The Original Paper No of the HEAD`17 Conference is: 4936 (not as stated with the submission: 4935)

Corrosion Fatigue Of X46Cr13 in CCS Environment

During CCS components are exposed to a corrosive environment and mechanical stress which results in corrosion fatigue and is inevitably followed by the lifetime reduction of these components. The lifetime reduction of the cyclically loaded high alloyed stainless injection-pipe steel AISI 420C (X46Cr13, 1.4034) constantly exposed to highly corrosive CO2-saturated hot thermal water is demonstrated in in-situ-laboratory experiments (T = 60 °C, brine: Stuttgart Aquifer, flow rate: 30 l/h, CO2) in an environment similar to the on-shore CCS-site in the Northern German Bassin. In-situ tension-compression experiments were established simultaneously along with electrochemical measurements using a newly designed corrosion chamber in a resonant testing machine at a frequency as low as 30 – 40 Hz. In addition technical CO2 was introduced into the closed system at a rate close to 9 L/h. S-N plots, micrographic analysis and surface analysis of the fracture surface are demonstrated. X46Cr13 (surface roughness Rz = 4) reached the maximum number of cycles (12.5 x 106) at stress amplitude of 173 MPa producing a low scatter range of 1:3.5. Hydroxide and siderite layers were found on pits and crack surfaces. No typical fatigue limit exists. Pit corrosion prior to crack initiation may be identified as failure cause.