Case Study: Corrosion Behavior of High Velocity Air Fuel (HVAF) Thermal Spray Coatings

Kermetico is a company specializing in high-velocity oxy-fuel (HVOF) and high-velocity air-fuel (HVAF) spray technology, manufacturing high-speed flame spray guns and equipment. The coatings produced with these specially designed guns exhibit high density, high hardness, and superior corrosion and wear resistance. Developed as an alternative to conventional HVOF processes, HVAF spray guns use air instead of oxygen, enabling both economically and metallurgically superior performance coatings. Additionally, coating deposition efficiency and speed provide superior performance compared to many competing products.

A study on the production of iron-based metallic amorphous coatings via HVAF process and comparative investigation of their corrosion performance:

Metallic glasses have attracted intense engineering interest by combining superior corrosion resistance, wear resistance, high elastic modulus and strength properties due to the absence of grain boundaries and dislocations in their microstructure. Today, metallic glass structures can be obtained in Ti-, Zr-Fe- and Cu-based alloy systems. Due to limited glass-forming ability in bulk form, thermal spray technology offers the opportunity to obtain an effective amorphous form in coating form for engineering applications, expanding industrial application areas. Plasma spraying, arc spraying, laser surface treatment, and high-velocity oxy/air-fuel spray (HVOF/HVAF) methods have previously been used to obtain amorphous glass coatings. Among these methods, HVOF spray, known as high-velocity flame spray technique, exhibits significant advantages in producing amorphous coatings due to its high spray rates and high cooling rates. High-velocity flame spray processes exist in two types: HVOF and HVAF, competing with each other depending on oxygen and air fuel gas usage. Particularly, the HVAF process, having lower spraying temperatures, causes less phase transformation and structural changes compared to HVOF process.

The use of air instead of oxygen in the HVAF process also substantially eliminates structural problems due to oxidation. Coating performance and durability increase. With the HVAF process, spray rates exceeding 700 m/s enable obtaining coatings with lower porosity rates.

Another important competitive advantage is that coating production costs can be substantially reduced by using air instead of oxygen. To date, various applications of amorphous coating production using HVAF process have been observed in both academic studies and industrial fields. The performance of amorphous coatings produced using both HVOF and HVAF processes was compared in NaCl solution. Studies demonstrate that coatings produced with HVAF process are expected to expand their application areas in industrial use due to both economic benefits and high coating performance. In research on metallic glasses, particularly Fe-based amorphous metallic glasses are selected as candidate coating materials because they have relatively high crystallization temperatures, superior corrosion resistance, good magnetic properties, and relatively low costs compared to other compositions. Within the scope of this study, a coating composition based on Fe42.87-Cr15.98-Mo16.33-C15.94-B8.88 (atomic %) was selected and applied to metal surfaces via HVAF coating process, after which corrosion performance was observed in different solutions. Microstructural characterization of the coatings was determined by examining cross-sectional microstructures. As can be seen in Figure 1 from the work of Gang Wang and colleagues (Ref: J. of Man. Proc. 22 (2016)), the Fe-Cr-Mo-C-B based alloy powders are clearly produced by gas atomization technique and have an average powder size of 35 μm. As can be understood from Figure 2a showing the cross-section of the coating produced via HVAF process and the top surface image (b), a dense coating layer exists, and black-colored regions indicate porosities present in the coating microstructure. Porosity ratio below 1.5% was detected in the coating layer, and coating thickness is approximately 100 μm. Examining the top surface image, it can be understood that during coating, splats impact the surface rapidly and flatten, enabling the achievement of a lamellar and dense structure. The top surface microstructure also indicates that splats show good melting and excellent wetting and adhesion on the surface. Figure 3 shows the XRD patterns of coating powder and coating. Phase analysis reveals that a high degree of amorphous structure was obtained via HVAF process (spray rate >700 m/s and cooling rate >107 K/s).

Figure 4 shows potentiodynamic polarization curves of coatings produced via HVAF process in different solutions (for corrosion performance) compared with stainless steel material. All curves exhibit similar polarization behavior.

When compared with stainless steel (grade 304), it can be seen that HVAF coating has higher corrosion resistance. Figure 5 compares the top surface morphology of HVAF coating after corrosion testing with stainless steel. Effects of corrosion are observed following testing in NaOH and NaCl solutions. In NaOH and NaCl solutions, pitting corrosion effects and corrosion cracks are clearly visible on stainless steel surfaces. In the amorphous coating, however, high levels of corrosive effects are not observed. When microstructural examinations are evaluated overall, in line with the results obtained, it is seen that a dense amorphous HVAF coating with low porosity has higher corrosion resistance. The morphological properties of the coating are directly related to the high cooling rate during HVAF process in addition to the effect of glass-forming alloy elements. Fe-based coating provides relatively superior corrosion resistance in NaOH and NaCl solutions compared to stainless steel. However, it was found that the amorphous coating exhibited lower corrosion resistance in NaOH solution compared to NaCl solution.

General Conclusion

The most important feature of Kermetico spray guns, which have many advantages in terms of quality and process efficiency, is that they have higher spraying capability with lower combustion temperature compared to conventional HVOF process, particularly reducing the risk of phase transformation occurring in carbide-based coating production. Depending on air-gas fuel mixture, in addition to fuel economy, axial powder feeding through the spray gun nozzle provides homogeneous melting capability and high deposition efficiency. The nozzle inner wall allows particles to be accelerated without interaction. This enables spray rates to reach 800 or even 1,000 m/s. Kermetico spray guns have 5-6 times higher spray rates and 20-30% higher deposition efficiency compared to HVOF spray guns. Dr. Ekrem Altuncu

References: • KERMETICO; www.kermetico.com • G. Wang et al. / Spraying of Fe-based amorphous coating with high corrosion resistance by HVAF, Journal of Manufacturing Processes 22 (2016) 34–38. • Ye FX, Chen Y, Yu P, Luo Q, Qu SJ, Shen J. Structured analysis of ironbased amorphous alloy coating deposited by AC-HVAF spray. Acta Phys Sin 2014;63: 78101–78106. • Zhou Z, Wang L, Wang FC. Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying. Surf Coat Tech 2009; 204: 563–70. • Wang SL, Cheng JC, Yi SH, Ke LM. Corrosion resistance of Fe-based amorphous metallic matrix coating fabricated by HVOF thermal spraying. Trans Nonferrous Met Soc China 2014;24:146–51. • Guo RQ, Zhang C, Chen Q, Yang Y, Li N, Liu LW. Study of structure and corrosion resistance of Fe-basedamorphous coatings prepared by HVAF and HVOF. Corros Sci 2011;53:2351–6.