**5.3.4 Hot corrosion test**

The hot corrosion test results are shown in Fig. 12. This chart compares the weight loss data obtained on weld overlay coating (with and without salt), 304 stainless steel (304 SS) sample and the coatings – NiCr by arc spray, NiCr and NiCr + SiO2 by hybrid gun . The chromia stabilized alumina embedded coatings were not included in the test due to their unfavorable oxidation results presented in Fig. 9. NiCr + SiO2 coatings showed the lowest weight loss compared to all the other samples. Plain NiCr coating by hybrid spray also exhibited lesser weight loss compared to the arc spray coating and this could be attributed to the improved density of the hybrid spray coatings. The superior corrosion resistance of the NiCr + SiO2 coating is possibly due to the enhanced stability of the chromia scale and the improved oxidation resistance caused by SiO2. Weld overlay coating showed least weight loss in the absence of the salt; however, when salt was present, it showed poor corrosion resistance compared to the hybrid spray coatings.

Fig. 12. Weight loss measured in hot corrosion test.

The case study demonstrates that even the base NiCr hybrid spray coatings outperformed the alloy 625 coatings in the presence of corrosive salts. The presence of second phase particles, especially SiO2, showed improved oxidation and corrosion characteristics. Incorporation of ultrafine and nano sized oxide particles is expected to improve the creep properties by pinning the splat boundaries and reduce the oxidation rate. Chromia addition by itself did not help improve the properties significantly. However, it could act as a

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## **6. Summary**

It is apparent from the brief review and case study that oxide systems could be quite helpful to reduce or slow down the corrosion phenomenon of metallic systems. However, it could be dependent on the oxide system employed. Importantly the stability of oxide phases is very vital as it could undergo several changes during the operation depending on the temperatures of application environment. Dispersion of SiO2 showed marked enhancement in the oxidation and corrosion resistance of the hybrid coatings at high temperatures.

### **7. References**


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It is apparent from the brief review and case study that oxide systems could be quite helpful to reduce or slow down the corrosion phenomenon of metallic systems. However, it could be dependent on the oxide system employed. Importantly the stability of oxide phases is very vital as it could undergo several changes during the operation depending on the temperatures of application environment. Dispersion of SiO2 showed marked enhancement

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**13** 

Dimitar Krastev

*Bulgaria* 

**Improvement of Corrosion Resistance** 

The corrosion of metals is a destructive process regarding to the basic modern constructional material with a great importance for the nowadays industry and in many cases represents an enormous economic loss. Therefore, it is not a surprise that the research on the corrosion and corrosion protection of metallic materials is developed on a large scale in different directions and a wide range of engineering decisions. For all that, the improvement of corrosion behaviour of metals and alloys still stays as one of the most important engineering problems in the area of materials application and it is one of the

Special attention is usually focused on the corrosion behaviour of steels as the most commonly used engineering material, because of the limited corrosion resistance for many basic types of these alloys. In more cases they are selected not for their corrosion resistance and important properties are strength, easy fabrication and cost, but there are a lot of exploitation conditions requiring high corrosion resistance. For such a purpose is developed the special group of stainless steels which covers with a high level of certainty these requirements. The stainless steels have an excellent corrosion resistance, but it is not always attended with high strength, hardness and wear resistance. Together with the higher price of the high-alloy steels these are the main restriction for many applications and open up a wide field of opportunities for the surface modification as a method for combination of

Surface modification in a wider sense includes all types of surface treatments and coatings that result in change in composition and microstructure of the surface layer. There are different methods for modifying the surfaces of structural alloys, dictated by the performance requirements of the alloy in its service environment. One of the approaches, traditional for the steels, is to modify the surface region of engineering alloys via diffusion of different elements and forming a layer with determinate chemical composition, microstructure and properties. These are the commonly used in practice methods for thermochemical treatment of metals which extended with the methods for physical vapor deposition and chemical vapor deposition form the basic modern techniques for surface engineering regarding to metals. Another approach involves coating of alloy surfaces via plasma spraying, electrospark deposition, modifying the surface by ion implantation or sputter deposition of selected elements and compounds, etc. In recent years a particular

corrosion resistance along with high strength, hardness and wear resistance.

**1. Introduction** 

fundamental parts of modern surface engineering.

**of Steels by Surface Modification** 

*University of Chemical Technology and Metallurgy* 

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