T/CNS 6-2018 in English

Scope

In the process of writing this standard, references were made to national standards such as GB/T 10123-2001 "Corrosion of Metals and Alloys - Basic Terms and Definitions", GB/T 24196-2009 "Corrosion of Metals and Alloys - Electrochemical Test Methods - Guide for Polarization Measurement by Potentiodynamic and Impedance Methods", as well as ASTM standards like ASTM G3-14 "Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing", ASTM G5-14 "Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements", ASTM G31-72 "Standard Practice for Lab Immersion Corrosion Testing of Metals", and ASTM G61-86 "Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys". These standards provide relevant provisions on electrochemical tests. Additionally, the technical documents for electrochemical test methods for nuclear water reactor materials in high-temperature and high-pressure water environments developed by the Metal Research Institute of the Chinese Academy of Sciences were also referenced. Combined with the experience of implementing high-temperature and high-pressure water electrochemical tests at the Metal Research Institute of the Chinese Academy of Sciences and relevant research institutions abroad, this standard was formulated. The following are specific technical content explanations:

(1) Due to the special nature of high-temperature electrochemical testing, working electrode samples should not be encapsulated in any form; the bare metal surface directly participates in the test. Precise control over sample dimensions and surface roughness must be maintained.

(2) To ensure the validity of electrochemical test data, each test point generally needs to be repeated three times.

(3) To facilitate the extraction of electrical signals from samples in high-temperature and high-pressure water environments, it is recommended to use metal wires of the same or similar material as the sample. These wires should have a thermally shrinked PTFE tube for insulation and be connected to the sample via spot welding; it is recommended that the spot weld position be on the side of the sample.

(4) To ensure real-time control and monitoring of water chemical parameters in high-temperature and high-pressure water solutions, electrochemical test equipment must include a recirculating water loop. This includes elements such as storage tanks, circulating pumps, high-pressure pumps, heat exchangers, pre-heaters, condensers, backpressure valves, ion-exchange resins, etc. The outlet of the storage tank is connected to the circulating pump via piping; the outlet of the circulating pump is split into two paths: one for water chemistry monitoring and another for the high-temperature/high-pressure water loop. Water chemistry monitoring equipment includes conductivity probes, dissolved oxygen probes, pH probes, ion-exchange resins, etc., which then return to the storage tank. The high-temperature/high-pressure water loop consists of a high-pressure pump, heat exchanger, sample stand, pressure vessel, condenser, backpressure valve, etc., with piping leading out to the storage tank.

(5) To ensure that typical light-water reactor nuclear power station circulating water environments can be simulated, it is specified that the pressure vessel must remain sealed and operate stably under high-temperature (280-325℃), high-pressure (8-16.5 MPa) water conditions for a long time.

(6) To ensure key water chemistry parameters in the test environment, real-time precise monitoring and control of dissolved oxygen content in the circulating water is required.

(7) To ensure the stability of external reference electrodes, it is recommended to replace the electrolyte solution and calibrate their potential regularly. A recommended replacement cycle is every two weeks.

(8) To ensure the success rate of electrochemical tests in high-temperature and high-pressure water environments, insulation testing should be conducted after loading samples and pressurizing to ensure electrical isolation between the sample and the pressure vessel body.

(9) To ensure water chemistry parameters within the high-temperature and high-pressure water inside the pressure vessel, it is stipulated that the flow rate of circulating water must allow for replacement of high-temperature and high-pressure water in the pressure vessel once per hour.

(10) To ensure smooth, safe, and effective electrochemical tests under high-temperature and high-pressure water environments, operators are required to strictly follow operating procedures when conducting tests.

(11) To ensure safety, it is stipulated that the electrochemical testing apparatus should automatically alarm and shut down the system if the outflow temperature is too high. A bursting valve is installed at the inlet of the pressure vessel so that the pressure inside the vessel can be vented in case of a fault increase; an automatic alarm and shutdown occur when the temperature inside the pressure vessel is too high, when there are leaks in the pressure vessel, pre-heater, or heat exchanger components, or when cooling water supply is interrupted.