This study assesses the applicability of high-speed laser cladding for producing iron-based alloy coatings, in particular, CrNi duplex steel, FeCrV, and FeCrNiB. Process parameters are optimized for 150 µm thick claddings on mild steel using different laser power levels, surface speeds, and preheating temperatures. Claddings are also produced on cylindrical substrates of different diameters to investigate dependency on component geometry. Duplex steel was found to be highly processable by high-speed laser cladding. In contrast, crack-free FeCrV claddings can only be produced on small diameter surfaces, and only with preheating, while FeCrNiB could not be applied at all without cracking.

The degradation of pump components by corrosion and complex damage mechanisms, e.g. erosion and cavitation leads to high costs through replacement and maintenance of parts. To increase the lifetime of cost-efficient components with superior casting properties, gray cast iron parts are surfaced with duplex stainless steel using an inert shielding gas metal arc welding process. The dilution of the surfacing increases with both increasing heat input and increasing thermal conductivity of the shielding gas. The microstructure is highly affected by the cooling conditions that may enhance diffusion processes and eventually lead to precipitation of deleterious carbides. Higher heat input and prolonged cooling duration during surfacing lead to high dilution and a pronounced carbide network and thus, substantially reduced corrosion resistance in artificial seawater. The corrosion of the surfacings in the potentiodynamic polarization test is driven by selective corrosion of the phase boundary between carbides and chromium-depleted austenite. Passive behavior is observed for coatings with low dilution and higher cooling rates, which showed homogeneous chromium distribution and no interconnected carbide networks. In conclusion, the corrosion behavior of gray cast iron was improved by surfacing with duplex stainless steel.

This study assesses the effectiveness of wire arc sprayed aluminum coatings for protecting welded super duplex stainless steel (UNS S32750) in subsea applications. Pitting and crevice corrosion tests were conducted at different potentials in recirculated synthetic seawater maintained at 90 °C with an acidity of 7.5-8.1 pH. After 90 days, the samples were examined, showing no signs of corrosion even in areas where coating defects were present.

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.

Coatings have been prepared using the Diamond Jet hybrid and JP5000 high velocity oxyfuel (HVOF) systems with the objectives of improving corrosion resistance and reducing costs through increasing deposition efficiency. Models relating deposition efficiency, coating oxygen content and corrosion resistance to process parameters including fuel flow rate, oxygen flow rate and stand-off distance have been developed. A corrosion test cell has been designed and a procedure determined for studying the corrosion behaviour of large numbers of thermally sprayed coatings in an efficient manner. A significant improvement to the corrosion resistance of HVOF sprayed coatings has been achieved by spraying parameter optimisation and investigation of powder size and distribution. The project has also investigated the influence of spray angle on coating performance with a view to future onsite application. Coating materials tested and compared include nickel alloys Hastelloy C276 and 59, cobalt alloy Ultimet, duplex stainless steel S32750 and an experimental iron-based spray-fuse composition.

Water turbine parts damaged by cavitation erosion (CE) and/or slurry erosion (SE) may cause excessive operational costs for plants worldwide. The damages can be reduced by choosing more resistant materials and right technology in the first-production or at repair and overhaul. Thermal spray technologies have a great potential in the field of repairing works. Thick multilayered coatings deposited by wire electric arc spraying (WAS) has been developed and applied as CE and SE protection at the repair of stationary Francis turbine blades. Repair technology by WAS was performed on large eroded areas (1-3 m 3 ) of preguide blades of Francis turbine: 1) local damaged depths 30-35 mm maximum were repaired by sprayed materials, 2) subsequently wire arc spraying of functional coating was applied. Three types of functional coatings with total thickness 10 mm a) duplex high - Cr stainless steel with NiAl bond coat, b) graded NiAl - Cr stainless steel coatings, and c) multilayered graded NiAl - Cr stainless steel coatings were compared by means of stress measurements and structural analysis. The coating structure influences very strongly the residual stress level and adhesive-cohesive strength. Multilayered graded NiAl - Cr stainless steel coatings showed the best results and were sprayed on water turbine blades in 4 Czech water power station during regular cut-off repair periods. After 30 - 36 months' continuous operation, Francis turbine blades repaired by WAS technology show better behaviour in comparison with original material from the point of wear resistance, reliability, cost-effect and life-time.