Thermal-Sprayed Corrosion Protection for Marine Power Plants: Coating Quality and Effects on Fatigue Strength Using Standard Design Guidelines

Although having high potential of energy generation capacity, marine power plants are currently rarely used for energy production. A particular challenge is fatigue design of cast iron parts, as the corrosion allowance stemming from different regulations can lead to uneconomical designs. Therefore, fatigue tests have been conducted on spheroidal graphite cast iron samples (EN-GJS-400-18-LT) to determine the influence of surface corrosion and coating technologies (arc spraying) onto the fatigue strength of base material samples. Preliminary fatigue tests indicate that regulatory prescribed fatigue strength reduction due to contact with seawater might be too conservative and that arc spray coatings can generally be applied to spheroidal graphite cast iron without severe reduction in fatigue strength. Based on the promising fatigue test results, different thermal spray coatings preferred for use at marine power plants have been investigated regarding their general coating qualities. In this regard, coatings based on Al99, Zn99, pseudo-alloys thereof and CuSn6 were applied to cast iron materials. It became apparent, that coatings made of Al99 can be cost-efficiently applied on large areas, while showing a dense microstructure (defects ~ 1.4%) and high adhesive strength (max. 36 MPa). On the other hand, CuSn6 displayed the highest coating tensile strength and can be used in distinct areas, which might be subject to biofouling, too. Thus, both materials were applied in higher amounts in a further step to investigate fatigue strength properties of coated components on a larger scale. Additionally, these coatings were compared to specimens as used in standard regulations and others that were exposed to typical conditions such as corrosion and blasting. The results indicate that thermal spray leads to a shift in material behavior in fatigue, which should be considered when determining fatigue strength (∆σC,var = 165 N/mm2 for as-machined versus ∆σC,var = 255 N/mm2 for as-sprayed).