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Study on Corrosion Properties of WC-Ni Cold Spray Coatings to Mitigate Flow Accelerated Corrosion e. Study on corrosion properties of WC-Ni cold spray coatings for flow-accelerated corrosion mitigation of carbon steels in nuclear power plants. On the other hand, weight loss occurs in the 20Ni and 30Ni coating and the weight of the 25Ni coating increases after 4 weeks of FAC experiments.

Introduction

Background of research

Objectives

Literature study

• Definition of flow-accelerated corrosion
• Main factor of flow-accelerated corrosion
• Flow velocity
• Temperature
• pH
• Dissolved oxygen
• Roughness
• Methods for mitigating flow-accelerated corrosion
• Limitation of control flow-accelerated corrosion factor

As mentioned above in Equation (7), the magnitude of the corrosion rate differs between laminar and turbulent flow. Therefore, high DO in the secondary system is a good process to reduce the corrosion rate of carbon steel pipes. This means that corrosion accelerated by turbulent conditions can be suppressed and the corrosion rate can be reduced.

Fig. 2.1 Fe Pourbaix diagram at 25 °C

Rationale

Methods to estimate corrosion rate

• Weight measurement
• Potentiodynamic polarization experiments

Moreover, potentiodynamic polarization experiments need less time because they artificially create oxidizing and reducing environments through potential difference between WE and CE. Therefore, estimation of corrosion properties by potentiodynamic polarization experiments are more convenient methods than weight measurements.

Cold spray coating

Minimum velocity value for adhesion to surface at substrates during collision is critical velocity and this value varies by mechanical properties, melting temperature, initial temperature of particle, temperature of carrier gas and the working environment such as distance between nozzle and substrates and diameter of nozzle. One of the advantages of low operating temperature in cold spray coating is residual stress after coating. High temperature gradient between coated samples and the external environment creates residual stress at coating layer [19, 20].

On the other hand, cold spray coating works at a relatively low temperature below 1000K, so thermal stress of substrates during operation and residual stress at coating layers after coating are reduced [22]. In thermal spray coating, a number of studies indicate that other chemical compounds, unexpected compounds such as decarburized carbide or oxide, are observed at the coating layer [26-28]. In the case of carbide, high temperature also activates decarburization, so that unexpected mechanical properties or corrosion properties are investigated [28].

However, low temperature of cold spray coating cannot earn enough energy for oxidation or decarburization.

Properties of tungsten carbideand nickel

• Tungsten carbide
• Nickel

The difference in passivity between the acidic and alkaline states is caused by the dissolution of oxides in water, and these dissolutions are produced by the following reaction [38]. In equation (19), WO3 reacts with OH- and dissolves in water, which means that WO3 is not stable in high pH solution. During the cold spray application process, WC has a high melting temperature of 3073 K and poor plastic deformation properties, so these properties are an obstacle to achieving good performance.

Moreover, the oxidation and dissolution of the surface in the alkaline solution is more severe than that in the acidic condition. To eliminate the limitations, carbide was developed with a binder metal that has a low melting temperature such as Co or Ni. It is widely used in cold spray coating with carrier gas such as N2 or He.

For example, Co, Ni, Fe or Cu is a material which is a binder metal and WC with binder metal is called cemented carbide or hard metal. The corrosion behavior of Ni investigated by the Pourbaix diagram and the literature study show that the reduction potential in alkaline solution is lower than WC. When Ni-free WC is coated with carbon steel, the weakness of corrosion resistance to alkaline solution affects the dissolution of WC and WO3.

To reduce WC dissolution, coating with a binder metal such as Ni is a useful method to create a composite compound.

Fig. 3.1 Schematics of three electrode system

Experimental

Powder production

Cold spray coating

Experimental procedures

• Potentiodynamic polarization experiments at room temperature
• FAC simulation experiments and measurements of weight change

Results

Surface and cross-section observation before experiments

Potentiodynamic polarization experiments

Surface and cross-section observation after FAC simulation experiments

X-ray photoelectron spectroscopy analysis

• X-ray photoelectron spectroscopy data of nickel
• X-ray photoelectron spectroscopy data of tungten

The detection of Ni on the surface prior to experiments without NiO or Ni(OH)2 indicates that cold spray coating reduces oxidation of powder during coating process. After 2020s etch, the fraction of Ni(OH)2 increases with Ni content because normalized counts of Ni(OH)2 increase with Ni content during coating. Compared to 2 week samples, coatings after 4 weeks experiment presented sharper and higher peak at Ni(OH)2 than coatings before experiments.

Moreover, the difference in the Ni(OH)2 peak between 2 weeks and 4 weeks indicates that the oxide layer becomes thick and the dissolution of the coating layer is smaller than the oxidation of the coating layer. On the surface of the coating, WC and WO3 coexist and a noticeable change of XPS results is observed with 25Ni coating. The ratio of WC in 25Ni coating is increased after 4 weeks, but other coatings have no significant change in the chemical composition of WC and WO3.

Other oxidized or decarburized products such as W2C, W or WO2 were not detected on the surface before the experiments. The low temperature of cold spraying affects the decarburization or oxidation of the WC during the coating application process and reduces the deformation of the powder. The increase in WO3 for the 30Ni coating after 4 weeks compared to 2 weeks is greater than for the 20Ni and 25Ni coatings.

Fig. 5.1 SEM image of surface after coating (a: 20Ni, b: 25Ni and c: 30Ni)

Discussions

• Chemical composition
• Weight change
• Oxidation of nickel and tungsten carbide
• Corrosion properties

The decrease indicates that galvanic corrosion of Ni is the dominant reaction and Ni is dissolved in water. Therefore, the increase of Ni content in the coating has an effect on the corrosion and change of the weight of the coating in the early stage of oxidation. However, due to the lower Ni content in the 20Ni coating than the other coatings, the galvanic corrosion of Ni in the 20Ni coating occurs less than that in the other coating.

In the case of the 20Ni coating, the low Ni content of the coating accelerates the oxidation of WC before 2 weeks, so that the oxidation products, WO3, dissolve in water in 4 weeks. However, the higher Ni content in coatings other than the 20Ni coating delays the oxidation of WC and this effect is observed at 4 weeks. Consequently, the ratio of W in the 25Ni and 30Ni coating decreased or increased after 4 weeks.

The dissolution of WO3 in the 20Ni coating caused by the lack of Ni matrix occurs in 4 weeks and the percentage of WC compared to WO3 increases. However, the increase in WO3 in the 25Ni and 30Ni coating indicates the change in coating weight after 4 weeks FAC experiments are the result of WC oxidation. Experimental results after FAC simulation test and mpy calculation from potentiodynamic polarization experiments explain that the coating has better corrosion properties than carbon steel.

In addition, compared to P22 as a substitute for carbon steel, the mpy of 25Ni coating in Fig.

Fig. 6.1 Schematic view of corrosion at surface

Conclusion

원자력에 대해 잘 알지 못함에도 불구하고, UNIST 대학원 진학 이후에도 계속 지도해주신 지도교수님 김지현 교수님께서는 연구 결과뿐만 아니라 졸업 후 해야 할 일에 대해서도 많은 조언을 해주셨습니다. 석사 학위. 석사논문 심사를 맡아주신 안상준 교수님, 최성열 교수님께 감사의 말씀을 전하고 싶습니다. 그리고 다른 후배들과는 다르게 선배 후배들을 데리고 와준 우리 연구실 대학원생 김태호, 김승현, 최상일, 고광범, 유승창, 이정현 학생에게도 늘 감사하다는 말씀 전하고 싶습니다. . 만약 그들이 후배임에도 불구하고 나이가 많아서 제대로 지적하지 못한 부분도 많았고, 연구실 생활에서도 답답한 부분이 많았을 것 같아요.

일일이 이름을 지을 수는 없지만, 지도교수였던 대학원생 김승현 학생에게 늘 미안한 마음을 갖고 있었기 때문에 서로 조심했던 것 같아요. 하지만 연구와 대학원 공부에 있어 연구실 대학원생들의 도움을 많이 받았기 때문에 무사히 마칠 수 있었습니다. 그리고 또래였던 최경준 학생이 제가 입사한 후 연구실에 적응할 수 있도록 선배로서도, 친구로서도 친절한 말씀을 많이 해주셔서 무사히 마칠 수 있었습니다. 내 졸업 후 생활은 영원히 행복해요.

합류한 이후 대학원생 함준혁, 김태용, 송인영을 많이 가르쳐주지는 못했지만 많은 것을 배울 수 있어 감사하다. 제가 힘들 때 옆에 있어주시고, 덕분에 여기까지 올 수 있었습니다. 그리고 마음 속으로 아끼는 동생에게 지금까지 잘 해왔고 앞으로도 좋은 일만 생길 거라는 말을 전하고 싶습니다.

마지막으로, 본 연구를 가능하게 해주신 모든 분들께 감사의 말씀을 전하고 싶습니다.