This paper reviews some data regarding the combined effect of erosion and corrosion of thermal spray ceramicmetallic coatings and their potential application in sea water and production systems for the oil industry. The paper also presents recent research results from SINTEF Corrosion and Surface Technology with respect to development of new and better ceramic-metallic coatings from the Norwegian coating industry. It has been shown that taking new high velocity spraying technology into use in combination with recent developed powders, the result is a more corrosion and erosion resistant coating than the traditional and most often specified type Praxair LW45.

It is pointed out that properties corresponding to 3 to 5 % of gross domestic products are lost by corrosion in every year in advanced countries. Corrosion including oxidation is still one of the biggest technical problems which human beings are facing. The application of thermal spray coatings is one of the strongest weapons to prevent corrosion of steel and iron structures. The thermal spray coatings, however, are not panaceas to prevent corrosion. They involve many problems and it is important to understand the proper ways to apply the thermal spray coatings for corrosion resistance. In this paper, a state-of-the-art review on the science and technology against corrosion, oxidation and hot corrosion by the thermal spray coatings is presented.

Different WC-based coatings have been sprayed by the HVOF process and characterised by different methods. The influence of different powder characteristics on the wear and corrosion performance was investigated. Powder characteristics investigated include: i) average WC particle size, ii) matrix composition and iii) powder grain size distribution. The results show that the coating properties are affected by the different powder characteristics. Small WC particles were found to be beneficial as regards the erosion and erosion-corrosion resistance. An increase of the Cr and Mo content increased the corrosion and erosion-corrosion resistance. Powders with narrow grain size distributions gave coatings of higher quality than powders with wider grain size distributions.

The aim of this study is to compare the quality of corrosion-resistant thermal spray coatings depending on where and how they are applied. ZnAl 15 coatings were applied to S235 steel substrates by wire arc spraying in keeping with standard (EN ISO 2063) practice. The coatings were sprayed under factory conditions and in a simulated onsite environment using manual and automated methods. As-sprayed and sprayed-and-sealed coating samples were subjected to climate-cycle and corrosion testing. Coatings applied onsite varied in quality to a much greater extent than those sprayed under workshop conditions. Poor quality, and implicitly a lower corrosion resistance, was widely detected in coatings where ZnAl was manually sprayed under simulated onsite conditions. The implications of the study are particularly relevant to the sustainability of coastal and offshore wind power generators due to maintenance challenges.

Thermal spray coatings are widely used in sealed constructions where a specific seal material is sliding on the coating surface in aqueous conditions. Such applications are often highly corrosive and therefore limit the lifetime and increase the wear of the seal and the coating. In this work special test equipment was manufactured to study corrosion and wear performance of the specific test materials used under chlorine containing conditions at low pH-values. In the test procedure, a rotating seal material was pressed with certain pressure against the thermal spray coating material in the test solution and the wear of the seal and the weight loss of the coating material was measured during the test. Coating samples for the tests were prepared using HVAF, HVOF, twin wire arc spray and atmospheric plasma thermal spray techniques. Corrosion resistant stainless steel material EN1.4404 was tested as a reference together with the thermal sprayed coatings. Altogether four different seal materials were tested and the seal material was found to affect the weight loss of the tested base material and also to the coated material during the corrosion wear tests. Pure graphite seals were seen to accelerate the wear rate of both bulk stainless steel material and the thermal spray coatings, as compared to the other seal materials involved in the tests. The HDPE polymer seal gave the lowest weight loss in the tests. Clear differences in corrosion wear resistance were found between tested thermal spray coatings. Clearly the best performance was achieved with plasma sprayed chromium oxide coating compared to the other tested coatings when glass fibre reinforced teflon was used as the seal material.

Titanium grade 2 as well as steel 446 and 316L powders are applied for production of X-HVOF coatings on mild steel substrates. Deposition efficiency is determined by process parameters. Microstructural investigations are carried out by means of optical microscopy, SEM and XRD. In addition comparisons of oxygen and nitrogen content in titanium coatings and powder feedstock are drawn. Corrosion protection capability of produced coatings is studied by current density-potential measurements and by salt fog tests. Depending on the process parameters increase of oxygen and nitrogen content can be restricted to factor of 2 compared to the powder feedstock. Coatings showing nearly theoretical density in metallographical inspections are possible. In direct comparison to wrought titanium grade 2 material the corrosion behavior of the titanium coatings is very promising. Keeping in mind that coatings have been produced under atmospheric conditions the observed increase of the corrosion current density by factor four is regarded an excellent result. During the corrosion tests no damage, neither to the surface nor the substrate - indicated by rust precipitates on the specimen surface, is observed. So penetration of corrosive medium to the substrate is securely avoided.

Thermal spray committee of Japan Association of Corrosion Control (JACC) has been conducting a corrosion test of thermal sprayed Zn, Al and Zn-Al coatings at coastal area since 1985. Twelve kinds of sprayed coating were deposited onto steel pipe by arc- and flame-spraying at varied thickness and subjected to various post-spray treatment. The samples were set vertically into the seawater at a port 80 km south from Tokyo. Corrosion performance were inspected annually by recording the appearance and coatings' thickness at sea air-, splash- and tidal-zones. No significant changes were observed for five years exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, Al and Zn-Al coatings still exhibit superb corrosion performance. The test will be continued till 2001 to finish a 15 years field test. This paper reports the summary of corrosion performance of sprayed coating during the first 10 year period.

This work examines the corrosion behavior of cobalt and nickel base coatings produced by APS and HVOF spraying. Laser fusing and sealing are also assessed for comparison as are the corrosion properties of tungsten carbide, chromium carbide, and chromium oxide. All coatings and processes are analyzed by cyclic polarization testing and optical and scanning electron microscopy. Test results are presented and discussed along with the relative merits of each process.

This work assesses various ways of improving the corrosion protection and service life of thermally sprayed coatings used in offshore structures. Test results show that improvement is possible even in the case of Zn-Al15 spray coatings and, that in some cases, coating thickness can be reduced without loss of corrosion protection. Test samples were prepared by arc and cold-gas spraying and some of the coatings were sealed. The protective characteristics of the various coating systems were evaluated in a 25-week cyclic aging test as defined in ISO 20340. The results are compared based on the degree of blistering, rusting, and corrosion penetration at the scratch as well as adhesion to the substrate.

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.

This study examines the structural and corrosion properties of Ni and NiCu coatings produced by high-pressure cold spraying. It also assesses the effect of heat treatment. FE-SEM images of coating cross-sections show tightly bonded particles with only a minor presence of open or oxidized boundaries. Polarization measurements in alkaline salt and acid solutions show that the Ni and NiCu coatings have good potential for corrosion protection applications. Corrosion behaviors of as-sprayed and heat treated coatings are compared with corresponding properties of bulk and substrate materials.

In this study, iron-based coatings are deposited on stainless steel substrates by HVOF and HVAF spraying and are evaluated based on SEM examination, hardness measurements, and corrosion and wear testing and by comparison with WC-CoCr and CrC-NiCr reference coatings. The results indicate that corrosion resistance is insufficient if the coating is not fully dense and has open porosity. During spraying, the particles must be totally melted and rapidly solidified to achieve uniform coating composition. Open porosity and nonuniform distribution of alloy elements, particularly chromium, is seen to induce crevice corrosion in iron-based coatings.

The present work evaluates the oxidation and hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H). The coated as well as uncoated specimens were exposed to air and molten salt (Na 2 SO 4 -25%NaCl) environment at 800 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The corrosion products were characterized using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro analyser (EPMA). The WC-NiCrFeSiB coating provides necessary resistance against oxidation and hot corrosion to both the nickel and iron based superalloys in the given environmental conditions at 800 °C. The oxides of active elements of the coatings, formed in the surface scale as well as at the boundaries of nickel and tungsten rich splats, have contributed for the oxidation and hot corrosion resistance of WC-NiCrFeSiB coatings, as these oxides act as barriers for the diffusion/penetration of the corrosive species through the coatings.

Al-based coating was prepared with Al-based cored wire by arc spraying. The properties of high-temperature oxidation resistance and corrosion resistance of Al-based coating were compared with those of Fe-Al coating and 45CT coating. The property of H 2 S corrosion resistance of Al-based coating was compared with that of A304 stainless steel and Fe-Al coating. The results suggest that the Al-based coating has better properties of high-temperature oxidation resistance and corrosion resistance than Fe-Al coating and 45CT coating. Al-based coating has better property of H 2 S corrosion resistance than A304 stainless steel and Fe-Al coating.

Aluminum coatings were deposited onto Al7075 T651 structural alloy using both cold spraying and arc spraying. Arc spray coatings were produced using optimized parameters for two atomizing gases, namely air and nitrogen. Cold spray coatings were produced using a low pressure system with air and nitrogen as propelling gases. Six surface preparation procedures prior to deposition were evaluated. Interface quality of as-deposited coatings was investigated by means of fluorescent dye interface penetration technique, bond strength testing and backscattered electron microscopy. Environmentally assisted cracking tests were performed to study the corrosion protection capability of the resulting coatings for structural applications. Micrographs of samples taken before and after cyclic load testing in salt water immersion were compared. The results demonstrated that the Al coatings produced by both arc spray and cold spray provide to Al7075 alloy a cathodic protection against cracking and localized corrosion. However, to obtain such coating properties arc spray technique required advanced surface preparation prior to deposition. For cold spray, the surface preparation has minimal influence on the coating properties thus making this process more advantageous than arc spraying for this application.

The increase of commodity prices for classical corrosion protection materials such as WC/Co and Cr 3 C 2 /NiCr are leading to significantly increased coating costs. This reduces the competitiveness when compared to other coating technologies such as electroplating, particularly if carbide coatings are representing an over engineered solution for the considered application. A promising economic alternative is the employment of advanced ferrous materials. Fe-base coatings have the potential for very good corrosion protection and favourable wear properties combined with a low cost level due to relatively low priced alloy materials. In this study three different Fe-base materials have been sprayed by liquid and gaseous fuel HVOF spraying and analysed with respect to microstructure, hardness and corrosion resistance. Corrosion tests have been carried out with sulphuric acid as well as artificial seawater.

In this paper, Ni-based cored wires were prepared by using NiCr strip to wrap metal powders of Ni, Cr, Mo, CrB. Ni-based coatings were prepared by electric arc spraying process. The microstructures of Ni-Cr-Mo and Ni-Cr-B coatings were investigated by means of scanning electron microscopy (SEM) and energy-dispersive analysis (EDAX) and X-ray diffraction (XRD) analysis. The coatings have compact surface and presented the bonding strength higher than 40MPa. Corrosion-resistance performance of coatings was evaluated by salt-spray- test and electrochemical corrosion test. The results showed that Ni-based coatings containing Mo (5%) element or B element (2-4%) had better anti-chlorine ion corrosion performance than that of Ni-based coatings without Mo element, and PS45 (Ni-Cr-Ti) coating. The anti-chlorine ion corrosion coatings could be used for resolving the corrosion protection problem of the equipments and piping contacting sour, alkali, salt liquid in the petrochemical engineering.

This investigation deals with a study of the friction, wear and corrosion behavior of vacuum plasma sprayed quasicrystalline (QC) Ti 41.5 Zr 41.5 Ni 17 coatings. During pin on disc experiments, a change in the mode of wear has been found to occur with corresponding changes in normal load and sliding velocity. The low thermal conductivity of quasicrystals and its brittleness play a vital role in determining the friction and wear behavior of such materials. When these coatings are subjected to rubbing for a longer period of time, wear occurs by subsurface crack propagation and subsequent delamination within the coated layer. By comparing the QC to its polycrystalline counterpart during potentiodynamic measurements according to ASTM G 31, higher currents were found over the whole range of potentials for QC when immersed in 1MHCl solution.

In the continuous hot-dip galvanization process, the corrosion and chemical stability of the sink roll in the galvanizing bath are important issues that affect quality and productivity. In order to protect the sink roll, carbide cermet and/or ceramic coatings were deposited on the surface of the sink roll. The WC-, Cr 3 C 2 - cermet coatings were deposited by high velocity oxygen fuel (HVOF) spray, respectively. The coating samples were immersed in molten Zn-alloy containing 50wt.% aluminum at 833K for 24 hr and 144 hr, respectively. The inter-diffusion and inter-reaction of Zn, Al and elements in the coating and corrosion behavior of these coatings were investigated by XRD, SEM and EPMA, etc. The corrosion mechanisms of the carbide cermet coatings and ceramic coatings in molten High Al-Zn-alloy were proposed.

Hot corrosion tests have been conducted on Ni- and Cr-based laser coatings, a high-velocity oxy-fuel (HVOF) sprayed coating and various wrought alloys covered with a synthetic salt of Na 2 SO 4 -V 2 O 5 and exposed at 650°C for 1000 h in air. Coating microstructures and reaction product layers were analyzed with scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The hot corrosion resistance of tested specimen was evaluated by measuring its mean thickness loss. Generally, wrought alloys, HVOF coating and Cr-based laser coatings suffered from selective corrosion beneath salt film, that is, distinct Cr-depleted layer was formed at alloy/salt interface. Cr-based laser coatings exhibited extended solid solubility and they transformed towards equilibrium condition. Cr-rich phases enriched further with Cr and they were prone to corrosion. Low diluted laser coatings and HVOF coating were more resistant to hot corrosion than commonly used industrial standard alloy, Nimonic 80A. Ni-based laser coating exhibited resistance equivalent to Cr-based coatings and superior to corresponding wrought alloy.