Instrumented corrosion fatigue test cell with peripheral system for elevated temperatures

Abstract

Corrosion fatigue test cell (1) and peripheral system (2) with high chemical resistance for combined mechanical and corrosive stress at elevated temperatures, characterized in that the materials in contact with the medium consist exclusively of the highly resistant plastics PTFE, FKM, PVDF and glass.

Description

[0001] Compact, chemically resistant corrosion fatigue test cell without metallic materials, instrumented with electrochemical measurement technology, for temperatures up to approximately 120 °C, with pressurization and integrated temperature control, can be integrated into a vibration test system. Information on the technical field:

[0002] This utility model relates to the technical field of measurement and testing technology for material fatigue under superimposed mechanical, corrosive and thermal stress for the application-oriented determination of service life. State of the art:

[0003] Numerous materials and components are subjected not only to cyclic mechanical stresses during their use, but also to thermal and corrosive stresses. Examples include heat exchangers, injection molds, turbine components, etc. To determine the cyclic mechanical service life, fatigue tests are conducted in a vibration test system. Samples or components are mounted there and subjected to cyclic stress until failure or fracture, with the number of load cycles representing the service life. To account for corrosive stress, samples or components can be corroded prior to the fatigue test. To conduct simultaneous corrosive mechanical testing in a liquid corrosion medium, so-called corrosion fatigue test cells can be used, which are integrated into a vibration test system. This allows the sample or component to be constantly flushed with a corrosive medium.Using a standard three-electrode setup, electrochemical parameters such as the free resting potential can be recorded to characterize the corrosion mechanisms underway. If elevated temperatures and liquid corrosive media are required during fatigue testing, either hollow specimens are used, through which the corrosive and temperature-controlled medium is passed, or special autoclaves, which are integrated into a vibration test system similar to a corrosion fatigue test cell. In both cases, pressurization ensures that no vapor is generated, even if the boiling point is exceeded. Defects in the state of the art:

[0004] Corrosion occurring before the fatigue test rather than during it creates significant changes in the damage mechanisms. For example, particularly problematic forms of corrosion, such as stress and fatigue cracking, only occur under mechanically corrosive stress at elevated temperatures. This leads to errors in the determination of service life. Common corrosion fatigue test cells take the mechanically corrosive stress into account, but are not capable of measuring elevated temperatures near or above the boiling point. This is due, in part, to the materials used, such as acrylic glass, and the non-closed system, which does not allow for pressurization. At the same time, no system is known that records the temperature in the corrosion fatigue test cell and uses it for control purposes.Hollow samples are complex to manufacture due to the necessary drilling, and their damage mechanisms are not directly comparable to solid samples. Furthermore, electrochemical measurement techniques are difficult to apply. Autoclaves, on the other hand, can be equipped with a three-electrode system and can implement mechanical-corrosive stress at elevated temperatures. However, commercial autoclaves are made of metal, which can lead to electrochemical interactions, and the choice of corrosive media is limited due to chemical resistance. Due to their size, autoclaves cannot be integrated into every vibration testing system, and modifications are also very complex. Furthermore, autoclaves represent considerable costs. Technical problem to be solved:

[0005] The technical challenge is to develop a compact, chemically resistant, and cost-effective corrosion fatigue test cell. This cell should be integrable into a vibration testing system, enable the characterization of corrosion processes using electrochemical measurement technology, and be suitable for elevated temperatures above the boiling point of common corrosive media such as NaCl solutions or exhaust gas condensates. How does the invention solve the problem:

[0006] The present invention offers a compact, chemically resistant, and cost-effective solution for conducting corrosion fatigue tests under application-relevant conditions, particularly at elevated temperatures up to approximately 120 °C. In contrast to the test methods described in the prior art, the newly developed test cell provides a simple yet effective alternative. The test cell consists of a main structure milled from PTFE with complementary components made of chemically resistant materials such as FKM, PVDF, and glass. This makes it suitable for use with most corrosive media without metallic interactions affecting the electrochemical measurements. The closed-system design with a compressed air supply of only 0.5 bar prevents evaporation of the corrosion medium and allows operation above the boiling point of common water-based media.Additionally, the cell can be equipped with various electrochemical measurement techniques, including a three-electrode setup for use with a potentiostat and, if required, a pH probe. This enables continuous monitoring and analysis of corrosion behavior during the fatigue test. The integration of a glass-enclosed thermocouple or a pH probe with an integrated thermocouple enables temperature measurement within the cell, which is used to control a heating element that maintains the temperature of the corrosion medium in a flask. The sample can be monitored via an integrated glass viewing window, and an optical strain measurement can be performed during the test if necessary.Through these features, the invention offers a technically simple but highly functional test environment for corrosion fatigue investigations, which significantly simplifies both the test setup and the experimental analysis while ensuring higher chemical resistance and flexibility compared to existing solutions. Description of the advantages:

[0007] The invention thus offers a number of decisive advantages over existing solutions, which are listed below. 1. Extended temperature range: By applying slight pressures of 0.5 bar, the test cell can be operated at temperatures above the boiling point of common corrosive media without the need for hollow samples or an autoclave. 2. High chemical resistance: The absence of metals and the exclusive use of chemically resistant materials such as PTFE, FKM, PVDF and glass make the test cell suitable for a wide range of aggressive media. 3. Compact and cost-effective design: The test cell can be constructed using simple laboratory equipment, does not require costly autoclaving technology, and can be integrated into existing vibration testing systems with minimal effort. 4. Advanced measurement technology: The ability to simultaneously use a three-electrode system, a pH probe, and a thermocouple allows detailed monitoring of the corrosion process and environmental conditions. 5. Optical analysis options: The integrated glass viewing window enables the use of optical strain measurement methods in addition to the control function.

[0008] These advantages provide a significantly improved opportunity to investigate corrosion fatigue mechanisms under practical conditions as a technically and economically attractive alternative to existing testing systems.

[0009] An embodiment of the invention is described with reference to Fig. 1 to Fig. 3 explained. Fig. 1: Structure of the corrosion fatigue test cell on a cylindrical specimen with electrodes for electrochemical measurement technology. Fig. 2: Side views of corrosion fatigue test cell without sample and electrodes with glass cover. Fig. 3: Peripheral system consisting of heating mantle with glass flask, laboratory bench regulator, pressure reducer, safety valve, diaphragm pump and thermocouple. Explanation of the solution:

[0010] The utility model represents a corrosion fatigue test cell (1) made of PTFE, including a peripheral system (2), in which a cylindrical sample (3) can be tested. A glass lid (4) is mounted on the corrosion fatigue test cell as a viewing window, and FKM O-rings are inserted into the designated grooves (5, 6) for sealing. The corrosion fatigue test cell also features a tapered connection point for a glass-encased thermocouple or, alternatively, for a pH electrode with integrated thermocouple (7). Connection points are also provided for a Haber-Luggin capillary (8), into which a reference electrode is inserted, and a counter electrode (9), which is sealed by an FKM cork (10) in a PVDF double nipple (11). The PTFE hoses (12) are connected to the corrosion fatigue test cell using PVDF hose nozzles (13).The peripheral system includes a glass flask (14) with a heating element (15), which is controlled by a benchtop controller (16) and a thermocouple (17). A diaphragm pump (18) is used to circulate the medium. A pressure reducer (19) is used to pressurize the medium. For safety purposes, a pressure relief valve (20) must be installed to protect the entire system from excessive pressure and the resulting risk of bursting in the event of an unplanned exceedance of the boiling point. List of designations 1 corrosion fatigue test cell 2 Peripheral system 3 Cylindrical sample 4 glass lids 5 Groove for axial sealing FKM O-ring 6 grooves for radial sealing FKM O-rings on the sample 7 Connection point for glass-encased thermocouple alternatively for pH electrode with integrated thermocouple 8 Haber-Luggin capillary 9 Counter electrode 10 corks made of FKM 11 double nipples made of PVDF 12 PTFE tubes 13 PVDF hose nozzle 14 glass bulbs or reservoirs for the corrosion medium 15 patio heaters 16 laboratory bench controllers 17 Thermocouple 18 Diaphragm pump 19 pressure reducers 20 Safety valve

Claims

[1] Corrosion fatigue test cell (1) and peripheral system (2) with high chemical resistance for combined mechanical and corrosive stress at elevated temperatures, characterized by that the materials that come into contact with the medium consist exclusively of the highly resistant plastics PTFE, FKM, PVDF and glass. [2] Corrosion fatigue test cell (1) and peripheral system (2) according to claim 1, characterized by that a glass-encased thermocouple or a pH probe with integrated thermocouple is used in the corrosion fatigue test cell (1) to control the temperature in the reservoir (14) via a heating mantle (15). [3] Corrosion fatigue test cell (1) and peripheral system (2) according to claims 1 and 2, characterized bythat a closed system is used which, with the aid of a slight pressurisation of standard laboratory compressed air at 0.5 bar at the reservoir (14), enables a corrosion fatigue test in an aqueous medium up to approximately 120 °C without steam generation. [4] Corrosion fatigue test cell (1) and peripheral system (2) according to claims 1 to 3 with extended electrochemical instrumentation, characterized by that sealed connection points for a thermocouple or a pH probe with thermocouple (7), a counter electrode (9) and a Haber-Luggin capillary (8) for a reference electrode are arranged in such a way that they record the measured variables locally near the sample surface without restricting the view through the glass cover (4).

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