Corrosion behaviour and biofouling characteristics of structural steel in the coastal waters of the Gulf of Mannar (Bay of Bengal), India

Study on corrosion behavior and mechanism of AISI 4135 steel in marine environments based on field exposure experiment

The application of high-strength steels in marine engineering is gaining importance because of their high performance and ability to help save resources. However, detailed and systematic information about the corrosion behavior of high-strength steels in different marine corrosion zones is still limited. This study aimed to investigate and compare the corrosion behavior of AISI 4135 high-strength steel in marine atmospheric, splash, tidal, and immersion zones, focusing on rust layer characteristics, corrosion form and electrochemical corrosion behavior. Corrosion exposure experiments were performed in a specific sea area, and the recovered steel samples were characterized by Raman spectroscopy, confocal laser scanning microscopy, nitrogen adsorption analysis, etc. Results showed that the rust layer formed on the surface of the steel in all corrosion zones had component delamination. The steel samples in the atmospheric, splash, and tidal zones were characterized by pitting corrosion, where the average depths of the corrosion pits were 56.1 ± 4.7 μm, 199.5 ± 12.6 μm, 108.1 ± 11.0 μm, respectively, whereas those in the immersion zone were characterized by general corrosion. Meanwhile, electrochemical tests were performed on the electrode samples during exposure. Results showed that the corrosion of the steel progressed from slow to fast in the atmospheric, splash, and tidal zones, whereas it was relatively steady in the immersion zone. Differentiated models of the corrosion evolution of steel under wet-dry cycle and immersion conditions were presented. This discrepancy is related to the varying degrees of accumulation of ionic corrosion products at the metal/oxide interface, which are determined by the mean pore access diameter of the rust layer and the corrosion environment. This study is highly significant for the design of marine engineering considering the safety applications of high-strength steel structures in harsh marine environments.

Evaluation of Hydrogen Permeation into High-Strength Steel during Corrosion in Different Marine Corrosion Zones

Hydrogen permeation into high-strength steel during the corrosion process can deteriorate their mechanical properties, thus seriously threatening the safety of steel structures. However, the hydrogen permeation behavior of steels in corrosive marine environments is not well understood. In this study, the hydrogen permeation behavior and mechanism of AISI 4135 steel in different marine corrosion zones was investigated for the first time using an in situ hydrogen permeation monitoring system via outdoor and indoor tests. The three-month outdoor hydrogen permeation test showed that the diffusible hydrogen content of the steels exposed to the marine atmospheric, splash, tidal and immersion zone was 3.15 × 10−3, 7.00 × 10−2, 2.06 × 10−2 and 3.33 × 10−2 wt ppm, respectively. Meanwhile, results showed that the hydrogen permeation current density was positively correlated with the corrosion rate of the steel in the marine environments. This research is of great significance for guiding the safe application of high-strength steel in the marine environments.

Bacterial community structure of early-stage biofilms is dictated by temporal succession rather than substrate types in the southern coastal seawater of India

Bacterial communities colonized on submerged substrata are recognized as a key factor in the formation of complex biofouling phenomenon in the marine environment. Despite massive maritime activities and a large industrial sector in the nearshore of the Laccadive Sea, studies describing pioneer bacterial colonizers and community succession during the early-stage biofilm are scarce. We investigated the biofilm-forming bacterial community succession on three substrata viz. stainless steel, high-density polyethylene, and titanium over 15 days of immersion in the seawater intake area of a power plant, located in the southern coastal region of India. The bacterial community composition of biofilms and peripheral seawater were analyzed by Illumina MiSeq sequenced 16S rRNA gene amplicons. The obtained metataxonomic results indicated a profound influence of temporal succession over substrate type on the early-stage biofilm-forming microbiota. Bacterial communities showed vivid temporal dynamics that involved variations in abundant bacterial groups. The proportion of dominant phyla viz. Proteobacteria decreased over biofilm succession days, while Bacteroidetes increased, suggesting their role as initial and late colonizers, respectively. A rapid fluctuation in the proportion of two bacterial orders viz. Alteromonadales and Vibrionales were observed throughout the successional stages. LEfSe analysis identified specific bacterial groups at all stages of biofilm development, whereas no substrata type-specific groups were observed. Furthermore, the results of PCoA and UPGMA hierarchical clustering demonstrated that the biofilm-forming community varied considerably from the planktonic community. Phylum Proteobacteria preponderated the biofilm-forming community, while the Bacteroidetes, Cyanobacteria, and Actinobacteria dominated the planktonic community. Overall, our results refute the common assumption that substrate material has a decisive impact on biofilm formation; rather, it portrayed that the temporal succession overshadowed the influence of the substrate material. Our findings provide a scientific understanding of the factors shaping initial biofilm development in the marine environment and will help in designing efficient site-specific anti-biofouling strategies.

Influence of Crassostrea gigas on the permeability and microstructure of the surface layer of concrete exposed to the tidal zone of the Yellow Sea

Concrete exposed to the tidal zone of the Yellow Sea and bearing Crassostrea gigas (CG) with differing areal coverages was investigated for evidence of biologically induced corrosion prevention. The experimental results indicated that both the chloride ion profile and the neutralization depth of the concrete decreased with increasing CG coverage. Moreover, the water absorption rate and the chloride ion permeability of concrete with the original surface intact also declined with increasing degrees of CG coverage. However, the water absorption rates of three concrete samples with 2 mm of the surface layer removed were similar, as was their chloride ion permeability. Mercury intrusion porosimetry tests indicated that CG significantly reduced the pore structure of the concrete surface layer. SEM observation revealed that the CG cementation membrane and left valve were tightly glued to the concrete surface and had a dense structure. Concrete durability indices showed that high CG coverage greatly improved concrete durability.