The present study compares prerequisites for cavitation-resistant bronzes production by different coating techniques, namely cold spraying, HVOF spraying, warm spraying and arc spraying. If optimized to maximum cavitation resistance, the deposited coatings can increase the service life of ship rudders significantly. Furthermore, these methods could enable repair processes for ship propellers. This study is meant to help selecting the right coating technology to achieve best cavitation protection for a given set of requirements. Using high-pressure warm spraying and cold spraying, properties similar to those of cast nickel aluminum bronze are achieved. Also, coatings produced by using HVOF and arc spraying have erosion rates that are only about four, respectively, three times higher as compared to cast nickel aluminum bronze, while by far outperforming bulk shipbuilding steel. Their properties should be sufficient for longer service life, i.e., less docking events for ship rudder repair. Hence, with respect to costs, HVOF and arc spraying could represent a good compromise to reach the specified coating properties needed in application, potentially even for propeller repair.

The influence of thermally sprayed aluminum coatings (Al99%; arc spraying) on the fatigue strength of gas metal arc welded (GMAW) non-alloyed structural steel specimens with respect to foundations for offshore wind turbines was investigated. Additionally, the corrosion protection effect of such coatings for water conditions similar to the Baltic Sea was determined. Wöhler tests were carried out on test specimens with different weld details in the as-welded condition as well as in the thermal spray coat under the consideration of different kinds of surface preparation (blast cleaning with corundum and grit). Substrate and coating were characterized by scanning electron microscopy and the influence on the residual stress states was determined. Corrosion rate monitoring via LPR measurements was carried out as well as the monitoring of the galvanic current between coated and uncoated steel to characterize the coatings’ sacrificial capability for minor defects. Fatigue strength was significantly increased through thermal spraying, especially for test specimens with welded transverse stiffeners (Δσ_c,var = 127 MPa after coating compared to Δσ_c,var = 89 MPa as welded). With a characteristic value of the stress range of Δσ_c,var = 153 MPa, the welded butt joint specimens already exhibited a high fatigue strength in the as-welded condition. The corrosion studies demonstrated that thermally sprayed Al99% coatings have a high resistance to corrosion in seawater environments and are suitable as planar sacrificial anodes sufficiently polarizing bare steel below 0.8 V. The combination of fatigue strength improvement and corrosion protection makes the thermally sprayed Al coatings promising for design and operation of e.g., offshore structures.

Arc spraying is a cost-effective technology, which is determined by a few key factors. It is already established for corrosion protection of large structures and thus considered for restoration of eroded ship rudders and propellers by depositing typical propeller materials. The main parameters like arc current, gas type/flow or process kinematics strongly influence the residual stresses in these coatings, which in turn affect coating properties. In past investigations, it was shown that this impact could be modified by using alternative gas mixtures or changing the heat input in the process. However, the mentioned studies neglect the influence of the substrate, since solely steel substrates were used. In consequence, propeller alloys CuAl9Ni5Fe4Mn (Ni-Al bronze) and CuMn13Al8Fe3Ni2 (Mn-Al bronze) were now arc-sprayed onto bronze substrates while using the same parameters and kinematics as in the past. For reproducible results, the residual stresses within the coatings were measured by novel incremental hole drilling method based on electronic speckle pattern interferometry (ESPI) and correlated with the other coating properties. In comparison with spraying onto steel, the same conditions led to reduced Young’s moduli, lower tensile stresses and improved cavitation erosion resistance, while other properties like hardness and electrical conductivity showed varying trends.

In the current work, a typical NiCrAlY alloy and two novel amorphous Fe-based alloys were arc-sprayed for the desired application as insulative coatings in cryogenic environments such as for the inner walls of LH2 tanks. For the experiments, nitrogen was used as process gas, while the stand-off distance and number of passes were varied. The deposition efficiency, tensile adhesive strength, hardness, durability under cryogenic conditions, and thermal diffusivity were investigated. Another focus included the visual and chemical characterization of the microstructures and potential changes in these due to the harsh temperature regime. Additionally, the residual stresses within the coatings were measured by the incremental hole drilling method based on electronic speckle pattern interferometry and correlated with the other coating properties. This was done to estimate the influence of the stress state on possible coating delamination in the tanks over a longer period. The results demonstrate coatings with low, but varying porosity and oxide content as well as material-dependent correlations to electrical conductivity. In particular, the amorphous Fe-based coatings reveal homogeneous coating structures and promising properties in terms of insulation capacity for use in various cryogenic applications.