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.

Thermal sprayed ethylene methacrylic acid (EMAA) and ethylene tetrafluoroethylene (ETFE) coatings were evaluated for corrosion protection in a biochemical process to treat geothermal residues. Coupon, Atlas cell, peel strength and cathodic disbondment tests were performed in aggressive environments including geothermal sludge, hypersaline brine and sulfur oxidizing bacteria ( Thiobacillus jerrooxidans ) to determine coating suitability for protecting storage tanks and reaction vessels. It was found that the polymers were resistant to chemical attack and biodegradation at the test temperature of 55°C. The EMAA coatings protected 3l6L stainless steel from corrosion in coupon tests. However, corrosion of mild steel substrates coated with EMAA and ETFE occurred in Atlas cell tests that simulated a lined reactor operating environment and this resulted in decreased adhesive strength. Peel tests revealed that failure mode was dependent on exposure conditions. Cathodic disbondment tests in brine at room temperature indicated that EMAA coatings are resistant to disbondment at applied potentials of -780 to -1070 mV SCE for the test conditions and duration.

Thermally sprayed coatings of high performance thermoplastics are of interest especially for the chemical industry for anti-corrosion applications at elevated temperatures. In this paper coatings of polyetherether-keton (PEEK) and polyphenylen-sulphide (PPS) have been produced by simple flamespraying. They have been investigated by optical metallography, FT-IR analysis and DSC-analysis. Among the coating properties also the "in-flight" particles have been studied by wipe-tests and FT-IR analysis in order to assess possible decomposition effects during spraying.

Polymer coatings find increasing interest for anticorrosion applications and others. Thanks to its extraordinary properties (high chemical stability, low creep rate and good electrical resistivity at relatively high temperature) PEEK (Polyetheretherketone) polymer is now considered as a challenging matierial. In this work, PEEK polymer powder was thermally sprayed with different processes. Particle impacts were observed and coating was analyzed by infrared spectrometry. The temperature of the substrate was shown to play an important role in the formation of dense and continuous coatings. Thermal degradation during spraying produces new carbonyl species due to chain scissions; but the amount of decomposition could be controlled from the choice of spraying conditions.

The development of corrosion resistant sprayed coating without sealing is required to increase reliability of the thermal spray coating method and to expand the field of application for the wet corrosion environments. The conventional wire flame sprayed Al coating on the steel substrate without sealing has poor resistance against aqueous corrosion, so as to be restricted in use in practical fields. A duplex coating composed of sprayed Al on a 80Ni-20Cr alloy undercoat was proven to have sufficient resistance in a hot, near neutral aqueous environment through a trial use in a vegetable oil process. In this paper, mechanism of the corrosion resistance of the duplex coating is clarified by electrochemical measurements of the corrosion potential and the anodic polarization characteristics.

The corrosion of steel in the global reinforced concrete infrastructure is at epidemic proportions. The resulting damage is seen in highway bridges, apartments, paper mills and aquariums. In North America reinforcement corrosion in concrete bridges and parking garages generates cost estimates that range from $325 million to $1 billion per year. 1 One anode material gaining acceptance is zinc thermal spray (TS) coating. Since the first installation in 1983, zinc TSCP systems have been applied to reinforced concrete bridges, docks, parking garages and even aquariums. This paper discusses the performance of field applications of zinc thermal spray coating CP systems. The systems discussed have sprayed surface areas ranging from 280m 2 to 24,000m 2 The installations are on major concrete structures. The performance of both galvanic and impressed current systems are discussed.

Stainless steels such as AISI Type 304 SS are being used for seawater piping applications due to their desirable mechanical properties and good weldability. However, they are susceptible to pitting and crevice corrosion in chloride bearing environments. Thus a new generation of highly alloyed stainless steels such as Avesta 254 SMO with high molybdenum contents has been developed for improved localised corrosion resistance in seawater. These steels are also susceptible to the formation of undesirable secondary phases such as sigma and chi which degrades both mechanical and corrosion properties. Alternatively, the main alloying element can be surface alloyed onto the surface of a suitable substrate by laser surface alloying. In this paper, austenitic stainless steel surface alloys of varying molybdenum contents have been formed on 304 SS by laser surface alloying and characterised by optical and scanning electron microscopy. The corrosion behaviour was ascertained by electrochemical and immersion tests.

Zinc and aluminium coatings have been used widely to protect steel structures from corrosion in aggressive and hostile conditions. The more recent development of zinc 15wt% aluminium alloy in a wire form has demonstrated that arc-spray coatings can be produced with a resistance to red rust which is superior to that of the single metals. Competitive 'pseudo' alloy or composite coatings produced by co-spraying wires of zinc and aluminium have been shown to achieve resistance to salt spray conditions similar to this conventional alloy. Work described in this paper confirms these findings and goes on to demonstrate an additional advantage of co-spraying an aluminium -5wt% magnesium instead of aluminium with the zinc. The importance of providing a fine dispersion of the two phases in the 'pseudo' alloy is highlighted and an alternative method of providing a similar dispersion by using a 'cored' wire (e.g. Al-5wt%Mg wire in a zinc sheath) approach has been demonstrated. The importance of 'self sealing' in these coatings after the initial loss of zinc is discussed which is related to the coating microstructure and their electrochemical behaviour in chloride solutions.

The development of new spraying processes has increased the demand for high quality protecive coatings. Many thermal spraying processes have been developed to obtain coatings for a wide spectrum of materials and substrates. The High Velocity Oxygen Fuel (HVOF) process involves lower temperatures and higher velocities than those required by other techniques to obtain high density coatings. It is desirable to know which are the main factors that affect the corrosion behaviour of coated materials. The corrosion behaviour in chloride solution of a 34CrMo4 steel coated with different kinds of powder have been studied. The electrochemical corrosion of the coating-substrate system was characterised by corrosion potential measurements and potentiodynamic polarisations. Microscopic studies have also been performed by means of SEM. The corrosion tests were performed in synthetic marine water (ASTM D-1l41) in the presence of dissolved air. Polarisation resistances have been obtained from potentiodynamic studies. Measurements were carried out on two different (Ti,W)C+Ni coatings, as well as on the coatings obtained from a TiC+Ni-Ti powder which had been previously Ni coated using an electro less method. The best corrosion results were obtained from these last coatings.

Primarily thermal-spray coatings have been developed to combat excessive degradation of components due to mechanical wear. However, these coatings are increasingly being required to function in aqueous environments where corrosive attack is possible. The durability of thermal spray coatings in terms of corrosion resistance is the focus of this paper. Two coating methods were studied: high-velocity oxyfuel (HVOF) and thermal spray-fused. The HVOF coating was WC-Co-Cr and the spray-fused coatings were WC-Co based and Ni-Cr-Si-B. Samples have been exposed to aqueous environments at a range of different temperatures and of varying salinity (500ppm and 35,000ppm Total Dissolved Solids) in order to simulate freshwater and seawater environments. The detailed material loss and degradation mechanisms have been investigated using electrochemical-monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron and atomic force microscopy and x-ray microanalysis. The study has demonstrated that there is a strong dependence on coating durability for all the coatings as a function of temperature but salinity has a lesser effect. However there are interesting differences in the extent and detailed mechanisms of such effects between the different coatings.

Inconel thermal spray coatings are potential candidates for wide industrial applications. However, the coating structure resulting from thermal spray processes characterized by unmelted particles, oxides and porosity can severely hinder their corrosion resistance in aggressive media. The aim of the present work was to optimize elaboration parameters in order to enhance their chemical stability. Electrochemical criteria were used to identify the ability of those coatings to form a passive film in a sulfuric acid solution and also to determine their local corrosion resistance in a chloride solution. Results show that the passivity state is strongly conditioned by the coating microstructure. Coatings free of unmelted particles exhibit a notably reduced passive current density while the localized corrosion resistance seems conditionned by the amount of interlamellar oxide.

In this paper the oxidation behaviour of WC, Cr 3 C 2 , and TiC and their composites with binder metals is compared on the basis of literature data and some oxidation experiments. It was found that the oxidation mechanisms in air, which are more or less equal to service conditions of coatings, seem to be totally different from those in the spray process in the case of WC, but similar in the case of TiC and Cr 3 C 2 . Oxide layer scale adhesion strengths and, possibly, the high volatility of WO 3 seem to be responsible for this difference. It can also be assumed, that the problem of oxidation of TiC-based materials and Cr3C2-NiCr is that of local oxidation on the surface of the spray powder particles during the spray process. Cr 3 C 2 -NiCr and TiC-based materials are characterized by a high oxidation resistance in air. Plain WC-Co oxidizes readily in the temperature range 400-600°C, but alloying with Cr, as for instance in WC-CoCr and WC-(W,Cr) 2 -Ni spray powders increases the oxidation resistance.

Dilute aluminium alloys with additions of tin and indium when deposited by thermal spraying no longer behave as barrier coatings but demonstrate sacrificial corrosion properties when they exist on corrodible substrates. The degree to which the sacrificial attack occurs depends upon the spraying conditions and the tin or indium contents of the coating. The form in which the tin and/or indium exists in these coatings has not been specified but both elements are known to be sparingly soluble in aluminium. A series of experiments have been carried out using Al-12wt%Sn alloy powder as a feedstock for high velocity oxy-fuel (HVOF) spraying on to a steel substrate. The as-sprayed coatings were highly reactive in distilled water and dissolved in a few minutes. Heat-treatment of the coatings at 450°C for increasing amounts of time up to 20 hours reduced the reactivity to water but did not influence the corrosion rate in 0.1M NaCl solution. SEM/TEM observations on the coating provided evidence of the coarsening of tin particles from 15nm (as sprayed) to 0.5-2µm (as heat-treated). A second alloy with a copper addition i.e. Al-12wt%Sn-1wt%Cu was also sprayed to form coatings. The copper addition prevented reaction in water but did not influence the high corrosion rate of the as-sprayed coating in 0.1M NaCl. Heat treatment at 450°C reduced the corrosion rate and allowed passive films to form over limited ranges of electrode potential. The size and distribution of the tin phase was different in the copper containing coatings and this influenced the corrosion rate.

A reaction-formed NiAl intermetallic compound (IMC) powder has been deposited as a coating onto low carbon steel test coupons by the High Velocity Oxy-Fuel (HVOF) process using both gaseous and liquid fuels. The microstructure of this coating has been examined using scanning electron microscopy and x-ray diffraction and was found to depend on spraying conditions. Oxidation tests on the coating in air, between the temperatures of 800°C-1200°C, revealed that an α-alumina (Al 2 O 3 ) scale formed on the coating's surface. At 1200°C, a nickel spinel (NiO/NiAl 2 O 4 ) and haematite (Fe 2 O 3 ) phases were observed. Diffusion studies were performed to calculate an activation energy for iron ion diffusion in NiAl.

Until now the use of thermal sprayed metallic coatings for wet corrosion protection is limited to applications where the coating (Al-alloys, Zn-alloys and their mixtures) acts as an anode to protect the substrate or special cases, where thick cold sprayed metallic layers give good results. Other atmospheric cold sprayed layers made of corrosion resistant Ni, Co, Cu or Fe base alloys have their limitations due to the process related discontinuities like pores and oxide films. In more aggressive environments thermal sprayed and fused layers made of so called self-fluxing Ni and Co based alloys are commonly applied. Also in some applications the use of specially designed gas shrouds or of spraying techniques running in inert gas atmospheres or vacuum can yield protective coating solutions. However, these techniques have high investment or service costs or the size of the parts to be coated is restricted. Recent developments in HVOF-spraying open new possibilities in applying cold sprayed coatings on site with good corrosion resistance. The aim of this paper is on one hand to give a comparative overview about the wet corrosion behaviour of thermally sprayed metallic coatings using different spraying techniques related to the performance of the corresponding bulk materials and on the other hand to demonstrate the potential of a new generation of coatings to protect or repair structures exposed to aggressive environments. Keywords: corrosion behavior, thermal sprayed metallic coatings, cermet coatings HVOF, HVIF, PTAW.