Presence of orange tubercles does not always indicate accelerated low water corrosion

Abstract

The rapid degradation of marine infrastructure at the low tide level due to 18 accelerated low water corrosion (ALWC) is a problem encountered worldwide. Despite 19 this, there is limited understanding of the microbial communities involved in this 20 process. We obtained samples of the orange-coloured tubercles commonly associated 21 with ALWC from two different types of steel sheet piling, located adjacent to each other 22 but with different levels of localised corrosion, at a seaside harbour. The microbial 23 communities from the outer and inner layers of the orange tubercles, and from adjacent 24 seawater, were studied by pure culture isolation and metabarcoding of the 16S rRNA 25 genes. A collection of 119 bacterial isolates was obtained from one orange tubercle 26 sample, using a range of media with anaerobic and aerobic conditions. The 27 metabarcoding results showed that sulfur and iron oxidisers were more abundant on the 28 outer section of the orange tubercles compared to the inner layers, where 29 Deltaproteobacteria (which includes many sulfate reducers) were more abundant. The 30 microbial communities varied significantly between the inner and outer layers of the 31 orange tubercles and also with the seawater, but overall did not differ significantly 32 between the two steel sheet types. Metallurgical analysis found differences in 33 composition, grain size, ferrite-pearlite ratio and the extent of inclusions present 34 between the two steel types investigated. 35 IMPORTANCEThe presence of orange tubercles on marine steel pilings is often used 36 as an indication that accelerated low water corrosion is taking place. We studied the 37 microbial communities in attached orange tubercles on two closely located sheet pilings 38 that were of different steel types. The attached orange tubercles were visually similar, 39 but the extent of underlying corrosion on the different steel surfaces were substantially 40 Phan et al., Applied and Environmental Microbiology 3 different. No clear difference was found between the microbial communities present on 41 the two different types of sheet piling. However, there were clear differences in the 42 microbial communities in the corrosion layers of tubercles, which were also different to 43 the microbes present in adjacent seawater. The overall results suggest that the 44 presence of orange tubercles, a single measurement of water quality, or the detection of 45 certain general types of microbes (e.g. sulfate reducing bacteria) should not be taken 46 alone as definitive indications of accelerated corrosion. 47 48 Microbiologically/microbially influenced corrosion (MIC) refers to the deterioration of 49 materials due to the presence of microbes and/or biofilms attached to the surface (1). A 50 special case of MIC is accelerated low water corrosion (ALWC), which is associated 51 with the rapid corrosion of metallic structures at the low tide water level (Fig. 1) (2). The 52 corrosion rates due to ALWC can be up to several mm/yr, compared to around 53 0.05 mm/yr which i...