A research on Thermal Spraying Coating for High Temperature Corrosion Wear Protection of Boiler

(1)Thermal spraying material:

NiCr 80/20 wire, FeCrAl wire, NiCrTi wire.

NiCr 80/20 wire. When NiCr 80/20 is in the selective oxidation process of the heating and oxidation condition, due to the affinity of chromium and oxygen, it can form a dense chromium oxide surface thereby preventing further oxidation of the metal. However, different chromium content, alloy oxidation resistance is different, when the NiCr80/20 has 20% of chromium, in high temperature environment, the nickel-chromium alloy can form a continuous dense chromium oxide protective work piece. When the oxide film is destroyed, the chrome oxide film is formed again because of the high content of chromium in the alloy, so its oxidation rate is very small. A nickel-chromium-titanium alloy (NiCrTi , equal to 45CT) made of a titanium element added to a nickel-chromium alloy is widely used as a high-temperature corrosion-resistant material

2) FeCrAl wire. Chromium can improve the oxidation of iron alloy, because chromium is more easily oxidized than iron. Chromium-rich oxide is not only better at oxidation but also better at the oxidation protection than iron.

Chromium in the role of iron-based alloys: when chromium is in high-temperature oxidation, it can generate Cr2O3 film. The membrane structure of the film is dense,  and has excellent corrosion resistance character.

The oxidation of Al is much stronger than the oxidation of Fe, and its oxide Al2O3 forms a very dense oxide layer on the outer surface of the substrate, which itself is wear-resistant. In addition, even if this layer is destroyed, the inner layer of Al will soon form a new oxide film, the outside and the substrate was separately, so as to achieve the purpose of protecting the substrate. In the FeCrAl alloy, with the increase of chromium content, the content of aluminum needed for selective oxidation in the alloy decreases.

(2) The structure of the coating

 NiCr 80/20 wire, FeCrAl wire, NiCrTi wire coating particles are small, the coating is dense, and the porosity is low. In the case of the same magnification (X500), NiCrTi, NiCr 80/20 coating is more dense than FeCrAl coating.

(3) Performance test.

1) The high temperature corrosion resistance of FeCrAl is not as good as NiCr 80/20 and NiCrTi. But the high temperature corrosion resistance of FeCrAl can be improved by spraying aluminum coating on the FeCrAl coating

2) Thermal shock performance
NiCr 80/20 impact resistance is worse compared to several other materials, FeCrAl followed, the best is NiCrTi.

3) Anti-erosion performance
NiCr 80/20, NiCrTi, FeCrAl, the three kinds of coating erosion test results is in the following table, we can see that the erosion of FeCrAl this material better than other materials

Erosion resistance test results

4) Bonding strength.

The bonding strength of the coating is slightly different between the three materials. The bond strength of NiCr 80/20 coating was lowest, and the bonding strength of FeCrAl coating was in the middle, and the bond strength of NiCrTi coating was the highest. The bonding strength of the three coatings is as follows:

Coating Bonding strength

(4) Test conclusion

The results show that the NiCr 80/20 coating, NiCrTi coating is superior to the high temperature corrosion resistance of the FeCrAl coating. FeCrAl coating has a large area of rust, and shedding phenomenon, indicating that the iron alloy composition of FeCrAl coating in high temperature environment loss is relatively large, the high temperature corrosion resistance of this material is not ideal. From the thermal shock test, FeCrAl, NiCrTi coating, the impact resistance is relatively strong, NiCr 80/20 is relatively weak. The bonding strength between coating and substrate from large to small is NiCrTi, FeCrAl, NiCr 80/20 coating. The hardness of the three coatings is almost the same.

Considering all kinds of factors, the bonding strength and microhardness of NiCrTi coating are relatively high, and its impact resistance, erosion resistance and high temperature corrosion resistance are all good. It is a good material for high temperature corrosion resistance. FeCrAl thermal spray materials in the overall performance is slightly less than NiCrTi, the high temperature corrosion resistance is also worse than NiCr 80/20 But it’s stronger than NiCr 80/20 at the impact resistance, and the micro-hardness and bonding strength is also superior than NiCr 80/20. The erosion resistance of FeCrAl were quite good and it ‘s more suitable for wear and tear as the main destructive factors of the boiler. NiCr 80/20 coating has high hardness, good corrosion resistance, the thermal shock resistance and the erosion resistance is poor, it’s more suitable for corrosion of the main destructive factors of the boiler.Different materials of the coating is suitable for different working conditions of the boiler for corrosion and wear resistance requirements, We have to take into account the practical application of cost-effective, in order to select the appropriate coating material.

                                                 Kim zheng

                                                2016.11.18