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Uniform corrosion of zirconium alloy

When the zirconium alloy corrodes in high temperature water and steam, Zr combines with the oxygen in the water molecules to form ZrO2, and at the same time release hydrogen, the chemical reaction is:
Zr+2H2O→ZrO2+2H2
Part of the generated hydrogen is absorbed by the substrate, and ZrO2 forms an oxide film. Since the molar volume of the oxide is 1.56 times the molar volume of the metal, the oxide has a large internal stress, and the thickness of the oxide film is one third higher. The original metal surface.
The oxide film produced in the early stage of corrosion is dense and firmly attached to the surface of the zirconium alloy.
Zirconium alloy is an excellent electrical insulator. The standard electrode potential is -1.539V. It is easy to form an oxide film in the air and aqueous solution, and it adheres to the metal surface densely and firmly, which is the basis for the corrosion resistance of zirconium.
In the initial stage of oxidation, the controlling factors are mainly electronic defects and voids. As the oxide film thickens, the diffusion process plays an increasingly important role and becomes the main controlling factor for continued oxidation.
The oxidation process of zirconium alloy: electrons migrate outward, oxygen diffuses inward through the crystal lattice and grain boundaries, and reacts on the metal/oxide film to form a protective oxide film. The growth of oxide is the diffusion of oxygen ions through the oxide film to the oxide film/metal, and reacts with zirconium to produce ZrO2, which is an internal oxidation type.
This is an electrochemical process in which O2 diffuses through the anode reaction, and ZrO2 is generated through the oxide film and the Zr matrix. The cathode reacts, and electrons react with H+ to form H2 at the interface of the medium/oxide film. The anode process is the control step of the corrosion process. The diffusion path of O2- in the oxide film is defects such as grain boundaries and zirconium, and the diffusion path of electrons in the oxide film is the metal inclusions embedded in the oxide film and the second phase.
As the oxide film thickens, the stress between the film and the metal causes the oxide layer to form cracks. The corrosive medium reaches the metal surface through the cracks, shortening the corrosion path and accelerating corrosion. This is the corrosion turning point of zirconium alloys. Before the turning point, the corrosion rate is low. A thin, protective, black oxide film adhered to the substrate is formed. This dense oxide film is rich in metastable tetragonal zirconia (t-ZrO2) and cubic zirconia (c-Zr02), which obeys the parabola Or cubic law. When the oxidation weight gain reaches 30-50mg/dm3 or the oxide film thickness reaches 2-3μm, the transition occurs. After the transition, the corrosion kinetics shows a linear law, and the metastable t-ZrO2 or c-ZrO2 transforms into monoclinic zirconia (m -ZrO2), the oxide film is loose, and the color of the oxide film becomes grayish brown.
After the transition, the oxide film changes from black to white and corrodes in a linear law. The oxidation law of zirconium alloy before and after the turning point can be expressed as: W=Ktn(3-1)
Where:
W is the weight gain per unit area (Mg/ dm2);
t a time (days);
K is the corrosion constant.
Before the turning point, the oxide film of zirconium alloy smoothly and continuously adheres to the metal surface. It is a dense black oxide film, which has a good protective effect. The index n in the oxidation law before the transition is between 0.25 and 0.6. The transition process usually occurs when the oxide film thickness is in the range of 2 μm to 7 μm.
The off-white oxide formed during the transition is loosely attached to the metal surface. The turning point means that the protective oxide film begins to damage and corrosion accelerates. The speed of the transition process is related to factors such as temperature, pressure, and geometry. The diffusion rate of oxygen ions increases with the increase of hole concentration and increases with the increase of mobility. The diffusion process of oxygen ions is related to the temperature, so as the temperature increases, the corrosion accelerates and the transition advances. After the transition stage, the oxide film is chemically proportioned, the oxide film changes from black to gray and white; peeling occurs when the film grows to a certain thickness.
After the transition, the oxidation rate is approximately a constant, that is, it is approximately a linear law. The oxidation law can be expressed mathematically as follows: W=Kt(3-2)
The new Zirlo alloy and M5 alloy have optimized chemical composition and adopted low-temperature processing technology to obtain a high-precision microstructure. Compared with Zr-4 alloy, the corrosion resistance is greatly improved, and there is no obvious corrosion transition.
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