Super Duplex Stainless Steel Surfaces and their Effects on Marine Biofouling

Abstract: Some of the world’s most ancient, but still viable, organisms have since the beginning of maritime caused problems for the industry. The problems affect both the longlivety and efficiency of ships which is caused by the mere presence of organisms attached to the ship hulls. The organisms, called biofoulers, causes problems with longlivety related to moisture and crevice corrosion which break down the hull material. The problem regarding efficiency of the ship is related to the added hydrodynamical resistance that the biofoulers cause. To limit fouling in the marine industry, paint poisonous to the biofoulers is applied to prohibit growth. Until recent the paint seemed to be a long-term solution but severe damage to the sea life has been traced to the use of antifouling paint. This master thesis aims on exploring one putative solution to the problems related to biofouling. In a maritime perspective, advanced stainless steels are modern materials with use limited to fittings and certain high strength parts. However, in 2014 a small ship constructed completely in super duplex stainless steel 2507 was launched. Immediately the longlivety of the ship increased by several times. The approach was that no antifouling paint was necessary, but biofoulers will grow on the now non-poisonous surface. Surprisingly, in some areas of the ship the biofoulers adhered seemed to detach when driving the ship in certain speeds. This lead to the initiative to examine this mechanism further in the form of this master thesis. The master thesis was held at Sandvik Materials TechnologyAB. The main hypothesis was that adhesion of biofoulers will decrease with decreasing surface roughness. Few studies on the subject stainless steel, biofouling and surface roughness have been performed. Even fewer studies on stainless steel with metallic surface coatings and biofoulers have been performed why another hypothesis was driven: There are surface coatings which will affect growth and adhesion of biofoulers. 22 different stainless steel 2507, 3207 and 316L surface setups were produced by either polishing, coating, bending or magnetizing. Plates were analysed before being immersed in natural seawater in Brest, France for 70 days. After 70 days, the plates were taken up and two major tests were performed at site; fouling amount rank analysis and barnacle adhesion strength measurements. SEM and GDOES were used in the post-experment analysis. No sign of corrosion on the plates were found. The data was processed and results were obtained: Maximum corrosion potential, surface roughness, barnacle adhesion strength and biofouling has quite strong or strong correlations. The lowest barnacle mean adhesion strength was measured to 0.02 MPa. In practical, based on experimental formulated formulas, the low adhesion barnacles would detach in a water flow of 11 m/s.