Thermal-sprayed titanium coatings were investigated as anodes for impressed current cathodic protection systems for steel reinforced concrete structures. The coatings were applied by twin-wire thermal-spraying using air and nitrogen as atomizing gases. The coatings were non-homogeneous due to oxidation and nitridation of the molten titanium with the atmospheric gases oxygen and nitrogen. The primary coating constituents were α-Ti (containing interstitial nitrogen and oxygen), γ-TiO and TiN. Nitrogen atomization produced coatings with less cracking, more uniform chemistry, and lower resistivity than air atomization.

Steel-reinforced concrete slabs coated with a thermal-sprayed titanium anode were used to simulate impressed current cathodic protection systems. The titanium anodes were activated with a cobalt nitrate catalyst and subjected to accelerated electrochemical aging representing approximately 23 years at 0.00215 A/m 2 (0.2 mA/ft 2 ). During the aging experiment, current was kept constant at 0.0215 A/m 2 (2 mA/ft 2 ), voltages were recorded, and water was applied periodically when voltages exceeded compliance levels. At the end of the experiment, coating resistivity, adhesion strength, and titanium-concrete interfacial chemistry were determined. Results show that the coating resistivity increases and adhesion strength decreases with electrochemical aging. Voltages for the slabs varied with the relative humidity. Electrochemical reactions at the titanium-concrete interface caused deterioration of the cement paste by leaching of calcium compounds. Accelerated aging results are compared to similar ones for an uncatalyzed titanium anode and to results from the Depoe Bay Bridge.

Many of the state-of-the-art thermal-spray coatings (e.g. plasma, HVOF) have been developed with wear resistance as a primary aim. However, these coatings are increasingly being required to function in environments where corrosive attack is possible. This paper comprises a description of a study of the corrosion a WC-based coating containing 10%Co and 4%Cr as the metallic binder. The coating, in the form of test coupons, involving a substrate of superduplex stainless steel, has been exposed to seawater at ambient temperature (18°C) and 50°C. The corrosion behaviour and detailed corrosion mechanisms have been investigated using electrochemical monitoring techniques supported by precise post-test microscopical examination using light microscopy, scanning electron microscopy, atomic force microscopy and x-ray microanalysis. Results have shown the corrosion resistance of the coating material to be critically dependent on the temperature of the solution and that important changes in corrosion mechanisms arise as a function of the temperature.

This paper focuses on the influence and role of galvanic interactions in the corrosion behaviour of thermally-sprayed coated components. Coatings, of different chemistry and applied by various processes (including HVOF) to substrates of carbon steel or stainless steel, have been utilised to facilitate study of galvanic corrosion phenomena both between coating and substrate and also within the coating itself. The experiments have involved the measurement of galvanic currents between separate specimens and also the microscopical examination of galvanic interactions on single specimens. Galvanic corrosion effects, on both a macroscale or microscale, have been observed and the implications of these for coating and coating/substrate integrity are discussed.

For very large structures and parts in critical environments, a materials solution often cannot be found by using one material. The specific desired properties for those structures, like stiffness, ductility, high temperature stability, corrosion resistance, etc. are difficult to fulfill with only one material. In this case a solution may be found by using coatings and design their specific properties to replenish each other by their combination. The Thermal Spraying processes offer the necessary flexibility of producing thin to thick, ductile, soft to hard coatings while due to the wide range of process temperatures it is possible to process a wide range of materials, both as coating and structure. In this paper the some recent and important developments in Thermal Spraying to produce coatings for technical demanding structures will be described. These developments consist of High Power Plasma Spraying, powder- and process control development. To ensure process consistency during long spraying times and to apply reproducible coating quality a suitable process control is of great importance and the development of temperature control by Pyrometry and Thermography will be presented. The example will be drawn according to the application of a coating on a ball valve for off-shore and ship diesel engine parts (piston and valve).

Thermal spraying has been used to protect many steel structures from aqueous corrosion using Zinc and Aluminium, and to some extent their alloy coatings to provide galvanic protection. The lifetimes of the coatings can approach 50 years even when exposed in severe marine environments. Zinc coatings work by continuously sacrificing themselves and slowly dissipating over time. Aluminium coatings passivate more readily and form a barrier layer, the passivity makes them less able to protect damaged areas and to self heal. A new ternary coating system involving Aluminium, Zinc and Magnesium has been shown to be capable of providing both a passive barrier layer as well as being able to give galvanically active protection. Salt spray tests have shown that the resistance to red rust of these new coatings increases by 300% over similar thicknesses of the separate metal coatings. Processing by arcspray is straightforward and both adhesion and deposition efficiency are better than where Zinc is sprayed alone.

Low-velocity oxyfuel-spray and arc-spray coatings of Zn, Zn-Al, Al, and various Al-Si and Al-Mg alloys were tested in immersion and salt-spray conditions with artificial sea water for up to 6,000 hours. Coatings were tested as-sprayed or sealed with fluorocarbon, epoxy or silicone sealants. Comparison and overview of coatings recommended by international and Japanese standards are considered. Coatings Al 99.8 outperformed Zn 99.9 and Zn-13A1 ones while Al-(2.5~5.2)Mg and Al-(5~6)Si showed better corrosion resistance than Al 99.8 coatings. The silicone sealant offered better resistance than fluorocarbon and epoxy organic sealants.

Fusible Ni-B-Si alloys with a variety of alloy additions (Cr, Mo, Cu etc.) have been in service for many years as fused coatings with moderate corrosion resistance. Both gas- and water-atomised powders have been used with the spray and fuse and with the plasma transferred arc process to produce coatings. As the severity of corrosive industrial environments has increased, for example in waste burning boilers, existing alloys have not provided the desired service performance. This study was undertaken to develop a new family of alloys with improved corrosion resistance without sacrificing usability, wear resistance or cost effectiveness. A range of compositions was prepared and evaluated for deposition characteristic, microstructure, hardness, wear resistance and corrosion resistance in various media. The resulting alloy has an exceptional combination of wear and corrosion resistance in comparison to conventional alloys, when tested under comparable conditions.

Corrosion behavior of a flame sprayed titanium coating sealed by some resins was investigated in 3.5% NaCl solution by an electrochemical polarization measurement and an immersion test. The composition and structure of the sprayed film was also analyzed by SEM and EPMA. Although an as-sprayed titanium had no resistance to the corrosion because of its porosity, the sprayed titanium sealed with epoxy or silicon resin showed an excellent resistivity against the chloride corrosion. In spite that almost half amount of the titanium changed to oxide, nitride and carbide through the gas flame spraying, the conversion of the metal to the compounds had little effect to decrease the corrosion resistivity. The sprayed and sealed titanium coating obtained by a conventional onsite thermal spraying is expected as an economical material for chloride containing environments.