In our previous work, the potential of the suspension-HVOF spraying (S-HVOF) to produce dense-structured WC-12Co coatings has been shown. This contribution proposes a comparative study of the corrosion properties of the S-HVOF WC-12Co coatings and conventional sprayed HVOF coatings. The corrosion properties were evaluated at room temperature in NaCl electrolytes with different pH values and in a pH neutral 0.5 M Na 2 SO 4 solution. By varying the pH value, the corrosion mechanism of the cemented carbide coatings should be assessed more precisely, since the two components, WC and Co, show strongly different pH dependencies. The electrochemical properties of the sprayed coatings were investigated using open circuit potential measurements, linear sweep voltammetry and potentiodynamic polarization methods. Before and after corrosion tests, microstructural evaluations of the coatings were performed. Moreover, element analyses of the eluates have been performed to determine soluble corrosion products. The S-HVOF coatings show a similarly good corrosion resistance as the conventional HVOF WC-Co coatings. Generally, the coating properties, i.e. microstructure and phase compositions, as well as the electrolyte significantly influence the corrosion performance of the sprayed coatings.

Since the plasma sprayed coatings always present a limited interlamellar bonding, it is difficult for a plasma sprayed coating to be applied in corrosion environment without any post-spray treatment. In this study, a NiCr powder alloyed with boron was employed to fabricate fully dense corrosionresistant coating by plasma spraying through in-situ deoxidation effect of boron. As reported previously, plasma sprayed Ni 20 Cr 4 B coating presents fully dense microstructure with few isolated pores. Due to the oxide-free state of the inflight particles by the deoxidation effect of boron, the splats were effectively bonded upon impact so that the inter-splat boundaries were indiscernible. A long-term immersion corrosion test in NaCl solution was conducted for 80 days to confirm that the plasma sprayed Ni 20 Cr 4 B coating presents the superior resistance against the corrosion, which was comparable to the flame spray-fused NiCrBSi coating. Furthermore, the cross-sectional microstructure of the Ni 20 Cr 4 B coated Al alloy samples after 80 days immersion revealed that the plasma sprayed Ni 20 Cr 4 B coating was dense enough to completely block the penetration of corrosive substance in such an aqueous corrosion environment.

High-velocity oxyfuel (HVOF) sprayed coatings of Cr3C2-NiCr containing solid lubricants such as nickel cladded graphite and hexagonal boron nitride were successfully developed and characterised with the aim of optimizing their friction and wear behaviour. HVOF technology was used for the integration of solid lubricants to achieve strong cohesion between particles while minimizing thermal decomposition. Coating microstructure and composition were measured and correlated to the results of tribological and corrosion tests. The integration of the solid lubricant greatly reduced friction and wear volume at room temperature, but the lubricating effect was highly dependent on atmosphere and temperature. Cr3C2-NiCr with hBN, however, tends to exhibit more stable wear resistance over a wider temperature range and can be used at temperatures beyond 450 °C.

Thermal sprayed marine coatings in the marine environment usually encounter chloride-induced corrosion and microbiologically induced corrosion. Formation of microbial biofilm is crucial for subsequent attachment of large fouler and understanding the initiation and growth of the biofilm is essential for possibly controlling the occurring of biofouling. This paper reports the formation of Bacillus sp. bacterial biofilm on arc sprayed aluminum coatings and its effect on the corrosion behaviors of the coatings. Results show fast and pronounced attachment and colonization of the bacteria on aluminum coatings. The bacterial biofilm was systematically examined by CLSM, FESEM, and Raman spectroscopy. Electrochemical assessment revealed that the aluminum coating immersed in the bacteria-containing media showed higher corrosion resistance than the sterile samples. A model was proposed to explain how the microorganisms and their metabolic by-products protect the coatings against penetration of corrosive media. The results would give insight into design and fabrication of thermal sprayed coatings for enhanced anti-biocorrosion performances in the marine environment.

Aluminum coatings applied by thermal spraying are widely used for protection against marine corrosion. The HVOF process produces aluminum coatings with high particle velocity, at a higher temperature for longer, permitting a high adhesion among the deposited particles. The objective of this work was to analyze how the microstructure of the aluminum coating sprayed by HVOF was formed on different complex surfaces. The influence of different deposition process parameters on the coating, varying the angle of incidence during deposition, substrate preheating and nitrogen flow was studied. Outer corner and convex surfaces presented greater uniformity in the formed microstructures. The angle of incidence was the most influential parameter on the coatings, filling complex profiles better, with 60° of incidence, although the 90° of incidence increased the adhesion. The potentiodynamic polarization test was performed to evaluate the corrosion resistance of coatings. The higher preheating and lower angle of incidence increased the corrosion resistance of coatings. Corrosion tests in salt spray are in progress in order to compare with continuous results.

It is well known that the presence of KCl deposited on superheater tubes in biomass- and waste-fired boilers leads to a severe corrosion and premature damage. In order to protect such critical components which are routinely exposed to aggressive environments, thermal sprayings are frequently proposed as a potential solution. By virtue of the techno-commercial benefits that provides as a direct outcome of its ability to cost-effectively deposit coatings virtually free of porosity and in situ formed oxides, the high velocity air-fuel (HVAF) process offers a particularly attractive approach. In the present work, the influence of KCl on the oxidation behavior of four HVAF-sprayed Ni-based coatings (Ni21Cr, Ni5Al, Ni21Cr7Al1Y, and Ni21Cr9Mo) has been investigated. The coatings were deposited onto specimens of 16Mo3 steel, a widely used boiler tube material. High temperature corrosion tests were carried out in ambient air at 600°C, with 0.1 mg/cm2 KCl being sprayed onto the samples prior to the exposure. Uncoated substrates and an identical test environment without KCl were used as reference. SEM/EDS and XRD techniques were utilized to characterize the as-sprayed and exposed samples. The results showed that the small addition of KCl significantly accelerated damage to the coatings. It was further revealed that the alumina-forming NiAl coating was capable of forming a more protective oxide scale compared to other chromia and mixed-oxide scale forming coatings. In general, the oxidation resistance of the coatings based on the kinetic studies had the following ranking (from the best to the worst): NiAl >NiCr> NiCrAlY> NiCrMo.

This study assesses the effect of acid corrosion on the luminescence of YAG:Ce coatings. The feedstock powder is prepared by high-temperature solid phase synthesis and the coatings are deposited by air plasma spraying. Microstructure and phase composition are characterized and the effect of acid immersion duration on luminescent intensity is measured. It is found that the luminescent properties of YAG:Ce 3+ coatings have a tendency to fluctuate with immersion time, which appears to be related to phase composition.

Thermally sprayed aluminum (TSA) has been used in offshore applications for decades, protecting steel structures from seawater corrosion. However, very little work is reported on the performance of TSA when damaged, particularly in deep sea applications. This paper presents the results of a study in which an arc-sprayed aluminum-coated steel sample was subjected to synthetic seawater at 5 °C for 30 days in an autoclave at 50 MPa to simulate 5000 m of water pressure. Discontinuities or “holidays” amounting to 3% of the sample area were drilled into the coatings, exposing the underlying steel to direct attack by the synthetic seawater. After testing, SEM and EDX analysis revealed the formation of a protective Mg-based layer on the exposed steel with negligeable calcium content and no visible corrosion products. The results indicate that TSA coatings can protect steel in deep sea environments even when damaged.

In this investigation, 5083 aluminum alloy coatings were deposited on substrates of the same material by high-pressure cold spraying. Spray trials were carried out using powders with size ranges of 5-20 µm and 20-44 µm, gas temperatures of 673 K and 773 K, and nitrogen and helium process gases. Coatings and coating-substrate interfaces were evaluated primarily by SEM and EDS, while XRD was used to examine coating stresses and oxidation effects. Corrosion protection was assessed by electrochemical potentiodynamic measurements in synthetic seawater and Knoop indentations tests were conducted as a measure of work-hardening and mechanical integrity of the coatings. Test results are presented and correlated with spray parameters.

Due to its low processing temperature, cold-sprayed coatings tend to exhibit many advantages over traditional thermal sprayed coatings, such as, lower porosity, oxidation and residual stress levels. These characteristics impart the improved anti-corrosion performance of the cold-sprayed coatings. However, it is not known how much these factors influence the corrosion performance of them. In addition, there are few results on the electrochemical behavior difference between cold and thermally sprayed coatings. In this article, as-deposited cold-sprayed copper, aluminum bulk material, arc-sprayed copper and pure copper are tested with electrochemical methods to compare their electrochemical behaviors in natural seawater. The electrochemical methods included the potential-dynamic polarization curve and electrochemical impedance spectrum. The results indicated that there is no significant difference in the corrosion potential between cold-sprayed and bulk copper materials. The potential of cold-sprayed copper is much different from that of the arc-sprayed one. Polarization curves indicated that the corrosion rate is significantly higher for arc-sprayed copper. The electrochemical impedance spectrum shows that there is no difference in electrode interface structure between cold-sprayed and bulk copper materials. Based on these results it is inferred that the higher corrosion rate for arc-sprayed copper was caused by its high porosity level and oxidation degree.

In this paper evaluation of sealed and unsealed thermally sprayed aluminum (TSA) for the protection of 22%Cr duplex stainless steel (DSS) from corrosion in aerated, elevated temperature synthetic seawater is presented. The assessments involved general and pitting corrosion tests, external chloride stress corrosion cracking (SCC), and Hydrogen induced stress cracking (HISC). These tests indicate that DSS samples which would otherwise fail on its own in a few days do not show pitting or fail under chloride SCC and HISC conditions when coated with TSA (with or without a sealant). TSA-coated specimens failed only at very high stresses (>120% proof stress). In general, TSA offered protection to the underlying or exposed steel by cathodically polarizing it and forming a calcareous deposit in synthetic seawater. The morphology of the calcareous deposit was found to be temperature dependent and in general is of duplex nature. The free corrosion rate of TSA in synthetic seawater was measured to be ~5-8 µm/year at ~18°C and ~6-7 µm/year at 80°C.

A series of Ni-based cored wires with different boron contents were designed to prepare corrosion-resistant coatings by two-roll wire-arc spraying. These coatings were evaluated for their potential to provide added protection and reduced maintenance for applications in waste-to-energy (WTE) plants. The as-deposited coatings, which primarily are composed of nanocrystalline particles, exhibit uniform and dense layered structures with porosity of about 3%. The investigators selected thermo-gravimetric techniques to evaluate the high-temperature corrosion behavior of the coatings in molten salt environment (Na 2 SO 4 -10 wt% NaCl) at 800°C. The coated surfaces exhibited significantly reduced corrosion rates in comparison to those of the SA 213-T 2 substrate during all tests. These results were due to the formation in the coatings of composite surface oxide films, including Cr 2 O 3 and NiCr 2 O 4 , which serve to prevent the diffusion or penetration of corrosive species. Furthermore, the boron content appears to have a significant influence on the corrosion behavior of the designed coatings: the coating with the best performance had 16 at. % B added. The wire-arc sprayed Ni-based coatings could be an effective and economical treatment to prevent corrosion and extend the lifetime of super-heater tubes in WTE plants.

Zirconium (Zr) metal is of interest for chemical corrosion protection and nuclear reactor core applications. Inert chamber plasma spraying has been used to produce thin Zr coatings on stainless steel (SS) substrates. The coatings were deposited while using transferred arc (TA) cleaning/heating at 5 different current levels. In order to better understand thermal diffusion governed processes, the coating porosity, grain size and interdiffusion with the substrate were measured as a function of TA current. Low porosity (3.5% to < 0.5%), recrystallization with fine equiaxed grain size (3-8 µm diameter) and varying elemental diffusion distance (0-50 µm) from the coating substrate interface were observed. In addition, the coatings were low in oxygen content compared to the wrought SS substrates. The Zr coatings sprayed under these conditions look promising for highly demanding applications.

Zinc coatings are widely adopted for cathodic corrosion protection. Mostly the process of choice is hot-dip galvanizing but due to limitations regarding component size and composition of the galvanizing bath it is not always practicable. In the present paper zinc coatings alloyed with Al, Sn, Mg and Cr are applied by twin wire arc spraying to enhance the corrosion protection ability of zinc thermal sprayed coatings. The alloys were characterized and investigated using salt spray test and by means of electrochemical corrosion. Corrosion damage and products were investigated by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and electron probe microanalysis (EPMA).

Titanium exhibits very good corrosion resistance property because of the formation of very dense oxide coating. Especially the good corrosion against Cl- solution for titanium material makes it wide applications in sea industry. It is very difficult to deposit titanium coating under atmospheric condition due to the strong affinity with oxygen and nitrogen especially in high temperature plume. Except the expensive LPPS process, much attention has been paid to the newly developed cold spraying. Unfortunately the stringent requirement for the starting power and low production efficiency limit the application of the cold spraying. A modified HVOF process was developed by reducing the outlet diameter of chamber and by directly introducing water into chamber, therefore lower plume temperature and higher chamber pressure than conventional HVOF process can be achieved. Attempts to deposit Titanium coating were carried out, and immersion of Titanium coated A3 steel into artificial seawater was performed in order to evaluate the density of as-sprayed Titanium coating. The results showed that dense Titanium coating could be obtained after parameter optimization and very few corrosion spot was observed on the surface of Titanium coated A3 steel after immersion into artificial seawater for 120 h.

In this investigation, aluminum-copper, aluminum-zinc, and zinc coatings were flame sprayed onto a sand-blasted mild steel substrate. The coatings were immersed in seawater and were examined on a monthly basis using EDS and XRD analysis to assess corrosion and marine fouling behaviors. The spraying and test procedures are described and the results are presented and discussed. Zinc and aluminum-zinc coatings with high zinc content proved to have good anti-corrosion and anti-fouling properties, but the aluminum-copper coatings did not.

Nanocomposite epoxies are novel sealants developed especially for sealing metalized coatings. In order to test the corrosion protection performance of arc-sprayed aluminum coatings plus this sealer, steel panels were coated and placed in a corrosion test site on the East China Sea. Test panels were mounted in a marine atmosphere zone, seawater splash zone, tidal zone, and full-immersion zone. Several tests were conducted including corrosion and coating adhesion tests. This paper presents the results obtained from composite-coated steel panels after three years of seawater exposure.

In severe corrosive or abrasive environments, steel is rarely used since the range of properties available, in existing steels, are insufficient, resulting in the prevalent usage of either corrosion resistant materials like nickel based superalloys or abrasion resistant materials like tungsten carbide based hardmetals. Recently, a host of carbide based alloys including WC-Co-Cr, NiCr-Cr 3 C 2 , WC-WB-Co etc. have been developed in an attempt to bridge the gap between providing both wear and corrosion protection. Data will be presented showing how a newly developed steel coating, SAM2X5, with an amorphous / nanocomposite structure can bridge the gap between conventional metallic alloys and ceramic hardmetal performance with excellent combinations of properties including corrosion resistance superior to nickel base superalloys in seawater / chloride environments and wear resistance approaching that of tungsten carbide. The unique combination of damage tolerance developed should be especially applicable for the replacement of electrolytic hard chromium coatings.

Cored wires and high velocity arc spraying technique (HVAS) were used to produce high Mg content Zn-Al-Mg alloy coatings on low carbon steel substrates. The microstructures, mechanical properties and electrochemical corrosion behaviors of the Zn-Al-Mg coatings were investigated comparing with Zn and Zn-Al alloy coatings. And the electrochemical corrosion mechanisms of the coatings were discussed. The coatings show a typical aspect of layered thermal sprayed material structure. Chemical analysis of the coating indicated the composition to be Zn-14.9Al-5.9Mg-3.0O (wt.%). The main phases in the coatings are Zn, Mg 2 Zn 11 , Al 12 Mg 17 and MgAl 2 O 4 , together with a little Al 2 O 3 and ZnO. The Zn-Al-Mg coatings show higher electrochemical corrosion resistance in salt solution than Zn-Al coatings. The corrosion potential of Zn-Al and Zn-Al-Mg coatings decreased a little and then increased towards the noble potential. The analysis of XRD and Electrochemical impedance spectroscopy (EIS) shows that, with addition of Mg, the corrosion products can block off the pores in the Zn-Al-Mg coating, which is so-called self sealing, and thus prevent attack on the underlying steel substrate.

Titanium has an excellent corrosion property in chloride containing environments such as seawater. A modified HVOF spray process was developed by introducing a mixing chamber between the combustion chamber and the powder feed port. Nitrogen gas was fed into the mixing chamber to control the temperature of the combustion gas generated in the combustion chamber. By controlling the flow rate of nitrogen, various Ti coatings with different degree of oxidation and porosity could be fabricated. The densest coating produced by this process with surface polishing treatment maintained excellent corrosion protection over a steel substrate in artificial seawater in a laboratory test over 1 month.