Marine Grade Molybdenum based stainless steel resist corrosion in marine structures. Therefore, is there any need of coatings ? Under any circumstances, is plain carbon steel used as marine structure instead of marine grade ?
Due to the weldability and low cost ship are made of plain carbon steel. Its corrosion protection is obtained by coating and Mg-blocks. Marine steel is too expensive to be used for the construction.
Just to improve the mechanical properties with a cheaper solution. Erosion -Corrosion along with cavitation is a big problem in marine applications, so it's always better to have the material with higher resistance to tackle this phenomenon. Coating comes into play because you don't need to modify the bulk, rather just the surface.
Of course carbon steel is used for ship construction (and maybe for steel structures) and its corrosion is mitigated by painting and cathodic protection. On the contrary, carbon steel is not a good choice for pressure piping/equipment submerged, splash exposed or in sea water service. In these cases also Mo Stainless Steel (e.g. 316 grades) can suffer pitting & crevice corrosion and Chloride SCC (if skin metal temperature is higher than 50-60 °C); Duplex and SuperDuplex stainless steel can be considered as well as higher alloys (superaustenitic SS and/or nickel alloys). As alternative, CS could be considered if lined CRA or coated with specific high level coatings and a risk assessment is carried out.
Hello
The presence of mosquito protection is a must. It should be as long as possible with the longest possible time. Such coatings are Zn coatings.
Recent studies [1] show that the resistance to Zn coating can increase up to 30 years.
[1] https://www.researchgate.net/publication/320195662_CORROSION_PROPERTIES_OF_DIFFUSIONAL_ZINC_COATINGS_OBTAINED_BY_NANOGALVANIZING
With respect
Emil Yankov
Article CORROSION PROPERTIES OF DIFFUSIONAL ZINC COATINGS OBTAINED B...
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