Corrosion of Weld Heat-Affected Zones Submersed in Seawater with Indigenous Microorganisms

The corrosion behaviors of different weld heat-affected zones (HAZs) were investigated, comprising an API 5L X65 steel immersed in seawater with an abundant native microbial community. Before tests, HAZ physical simulations were performed at the peak temperatures of 700 °C, 860 °C, 1000 °C, and 1350 °C. Throughout 14 days of immersion, various planktonic bacterial groups associated with microbiologically influenced corrosion were metabolically active in the seawater. Desulfovibrio members were present in a considerable number of OTUs. Scanning electron microscopy showed intense microbial adhesion and corrosion product deposits in all HAZ sub-regions and the control base metal coupon (BM). The electrochemical tests revealed indistinguishable corrosion behavior of all HAZ sub-regions and BM. However, topography analysis showed different behaviors of pitting corrosion. The results manifested that the pitting resistance of the microstructure composed of cementite grains (700 °C HAZ) was superior to that of other HAZ sub-regions. On the other hand, the microstructure composed of martensite and austenite grains (860 °C HAZ) exhibited the worst pitting resistance of the weld joint, leading to a maximum pit penetration of 2.3 times greater than that pit on the BM. The changes in the HAZ sub-region sensitivity to pitting corrosion were attributed to variations in the microstructural composition.


Introduction
Corrosion is a pervasive problem resulting in awful economic consequences to several industrial sectors, mainly those that richly use metal infrastructures such as oil and gas (platforms, pipelines, storage tanks), shipyards, and water utilities. In the marine environment, these infrastructures are more prone to corrosion, not only due to the damaging effects of the aggressive chloride ion content [1] but also regarding the activity and diversity of indigenous microorganisms [2,3]. This phenomenon is known as microbiologically influenced corrosion (MIC) or usually biocorrosion.
Currently, two main MIC mechanisms, metabolite MIC (M-MIC) and extracellular electron transfer MIC (EET-MIC), are widely discussed. M-MIC is caused by secreted metabolites of microbes. Corrosive metabolites are oxidant agents such as protons, organic acids, and sulfides. Examples are the MIC of steel by acetic acid secreted from acid-producing bacteria and hydrogen sulfide (H 2 S) excreted by sulfate-reducing bacteria [3,8]. On the other hand, EET-MIC commonly manifests itself as a cascade of bioelectrochemical processes by which electrons are transferred in or out of the cell wall, that is, between extracellular substrates (such as metallic surfaces) and microorganisms. EET mechanism is still divided into (i) direct electron transfer (DET), which involves physical contact with the bacterial cell membrane, and (ii) mediated electron transfer (MET) based on redox electron shuttles [9].
Many laboratory studies and field investigations report that welded joints require special attention regarding the entire metal infrastructure concerning corrosive processes [10][11][12][13]. It happens since steel microstructural evolution will occur in and around the welded area during the welding process, leading to mechanical properties substantially different from those in nonwelded areas [14,15]. From a metallurgical point of view, welding can be identified as a thermal cycle that consists of continuous heating to a specific peak temperature (T p ) and cooling down to room temperature. The weld thermal cycle changes the base metal (BM), thereby resulting in an inhomogeneous microstructure in the heat-affected zone (HAZ) [16,17]. According to previous studies, the HAZ is typically the most susceptible to potential corrosion failures, and these failures are closely correlated with the HAZ microstructure. Chaves and Melchers [18] verified the highest severity of localized corrosion (pitting) in the HAZ compared to that in the weld metal (WM) and BM in an API 5L X56 steel exposed to the Pacific Ocean water. However, these authors did not consider the participation of seawater microorganisms in the corrosion of welded joints. Hassanzadeh and Rahmani [11] investigated welded joint corrosion in an ASTM A516 Grade 70 carbon steel immersed in natural seawater. They also showed that the WM and HAZ were more susceptible to corrosion than the BM.
Previous studies were performed to obtain practical information, as close as possible, on weld biocorrosion in storage tanks (static seawater condition) [19] and oil pipelines experiencing both laminar and turbulent flows [20,21]. These studies reinforced that HAZ was the most pitting-affected area among the three weld parts (BM, HAZ, and WM) in any operational condition. Although high corrosion severity may occur in the HAZ as a whole, this zone is not metallurgically uniform [22]. The HAZ comprises several sub-regions due to the temperature gradient formed on steel during welding, providing various T p [17]. Since HAZ sub-regions are too narrow to get available test coupons, it is challenging to analyze the MIC effect on a specific HAZ sub-region using a real weldment. The suitable method currently used for preparing large test coupons is based on welding thermal simulations to identify the various HAZ sub-regions and precisely study them [23][24][25].
This study evaluated the MIC attack on different HAZ sub-regions related to the shielded metal arc welding (SMAW) process. The choice of the SMAW process was due to its wide use in the construction and repair of metal infrastructures made of various steel grades. The experiments were performed in seawater with indigenous microorganisms. MIC phenomenon of four prevalent HAZ sub-region microstructures was investigated by first performing HAZ thermo-mechanical simulations, exploring the microbial diversity of the seawater, and then monitoring the cultivable bacterial community, analyzing the corroded surface morphology, and using conventional electrochemical techniques. This may be the first report of HAZ sub-regions subjected to a marine microcosm and its proper microbial community.
To simulate HAZ sub-region microstructures, rectangular API 5L X65 coupons of 10 mm × 10 mm × 85 mm were individually subjected to heating using a Gleeble 3800 thermal-mechanical physical simulator. Computational simulations were executed previously using the JMatPro® software to provide some T p that are experienced in a real SMAW. The T p adopted was 700, 860, 1000, and 1350 °C. All coupons were cooled down to ambient temperature without treatment. Figure 1 shows the thermal cycles experienced by the different HAZ sub-regions.

Microstructural Characterization
The physically simulated HAZ sub-regions were metallographic characterized (microstructure observations) by scanning electron microscopy (SEM) (Tescan VEJA 3). Before microscopy analysis, the steel coupons were ground (sandpaper from 100 to 1200 grade), polished (diamond paste from 3 to 1 µm), and etched with 2% nital solution. The API X65 base metal microstructure without physical simulation is also shown (Fig. 2).

Experiment Overview
The experiments were performed using microcosms that simulated the marine environment. Each microcosm consisted of a 3-L glass tank filled with 2.5 L of seawater without nutrient supplementation and submitted to a slow mechanical agitation by a magnetic stirrer located at the tank bottom to promote microbial adhesion on the coupon surfaces. Our group has been using seawater collected from Guanabara Bay, Rio de Janeiro, Brazil (22° 24' S and 43° 33' W), due to its abundance of microorganisms [19,26]. The seawater was chemical and physical-chemical analyzed following the American Public Health Association methodologies [27], and the data are given in the supplementary material. Before immersion, the HAZ sub-region coupons were degreased in acetone and sterilized by spraying 75% ethanol and dried in germicidal ultraviolet light (XL-1500 Series UV Crosslinker, SPECTRONICS, New York, USA) for 30 min. For electrochemical tests, the coupons were connected with rubber-coated copper wire and sealed with an epoxy resin to expose a surface area of 1 cm 2 , whereas those coupons for corrosion morphology analysis were trapped with nylon wire. They were then accommodated in experimental microcosms for 14 days at 28 ± 3 °C. To maintain a stable volume (evaporation control), 100 mL of freshly collected seawater was added into the microcosms at day 7. Each microcosm was run in three separate experiments to ensure reproducibility.

Microbiological Measurements
The microbial community composition of the Guanabara Bay water was assessed by high-throughput DNA sequencing analysis. The phylogenetic diversity was addressed at phylum and genus levels. The DNA was extracted using a commercial kit (DNeasy PowerSoil Pro, Qiagen), as reported previously [21]. Sequencing analyses were performed using the MiSeq platform (Illumina, Inc., USA). The primers 515 forward (5′-GTG YCA GCMGCC GCG GTA-3′) and 928 reverse (5′-CCC CGY CAA TTC MTTT RAG T-3′) were used for the amplification of the V4 and V5 regions of the 16S rRNA gene [28,29]. The primers set used amplifies 16S rRNA genes from a broad range of the two prokaryotic domains, Archaea and Bacteria. The open-source software QIIME 2 (Quantitative Insights Into Microbial Ecology) was used for sequence read analysis. High-quality sequences were processed to generate operational taxonomic units (OTUs) at a 97% sequence similarity threshold. The complete dataset was deposited in the NCBI Sequence Read Archive (SRA) database (SUB11495967). It is available under the Bioproject ID PRJNA839443.
To monitor the cultivable planktonic bacteria over the experiment, quantitative determinations were performed using the most probable number (MPN) technique. Heterotrophic aerobic bacteria (HAB), iron-oxidizing bacteria (IOB), acid-producing aerobic bacteria (APAB), and sulfate-reducing bacteria (SRB) were enumerated at the beginning, at day 7, and at day 14 of the experiment using nutrient broth, ferric ammonium citrate broth, phenol red broth supplemented with 1% (w/v) glucose, and Postgate E medium, respectively [19,30,31]. All culture media were prepared with seawater as a diluent to keep the salinity proper to the indigenous microorganisms and sterilized by autoclaving at 121 °C for 20 min. Incubation conditions were established according to the requirements of each bacterial population [19,26].

Surface Examination
After immersion, the corrosion products and biofilm on the coupon surfaces were removed using a rust remover (1000 ml hydrochloric acid, 20 g of antimony trioxide, and 50 g of stannous chloride). Then, the corrosion morphology was investigated using a three-dimensional measurement system with a super depth of field (Zeiss Smartzoom 5 microscope) to simulate pit profiles. Ten random pits on each coupon were analyzed to determine the maximum pitting corrosion penetration depth.

Electrochemical Tests
Electrochemical investigations were carried out in a classical three-electrode set: (i) a working electrode with unique HAZ sub-region microstructure, (ii) a saturated calomel electrode (SCE) worked as the reference electrode (Sigma-Aldrich Z113085, USA), and (iii) a stainless steel sheet as a counter electrode. Linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and potential dynamic polarization measurements were obtained with a potentiostat (Metrohm Autolab PGSTAT302N, Switzerland). LRP and EIS were performed daily, whereas potentiodynamic polarization curves were obtained at the last experiment day. LPR tests were recorded at a scan rate of 1 mV/s in the range of − 10 to + 10 mV versus E OCP . EIS was performed under a steady-state E OCP using a 10 mV amplitude sinusoidal wave ranging from 10 5 to 10 -2 Hz, acquiring 8 points per frequency decade. Potentiodynamic polarization scans were performed after 14 days of immersion, under stable OCP, at a scan rate of 0.5 mV/s from − 150 to + 200 mV versus E OCP .

Data Analysis
All the experiments were performed in triplicate, and the data were subjected to statistical study applying analysis of variance (ANOVA). Results were considered significantly different for p value < 0.05.

Microstructure Examinations
The BM and prevalent HAZ sub-regions of the API 5L X65 steel obtained by physical simulation are shown in Fig. 2. As expected, the BM microstructure with a majority of cementite grains was transformed into variable microstructures according to the specific T p applied by the Gleeble simulator. Four T p was simulated, representing the prevalent HAZ sub-regions found in the SMAW (Fig. 1). The T p of 700 °C created a subcritical heat-affected zone (SC-HAZ), promoting a partial transformation of the BM microstructure; just a slight decrease in the number of cementite (or iron carbide) grains was noticed. The T p of 860 °C created an intercritical heat-affected zone (IC-HAZ), changing the BM microstructure to grains of austenite and martensite. The T p of 1000 °C and 1350 °C formed only austenite grains of different sizes. At 1000 °C, the HAZ sub-region was classified as

Microbial Community of the Seawater
Sequences of the 16S rDNA gene by the Illumina platform produced 1515 good-quality OTUs. The microbial community structure at phylum and genus levels of the Guanabara Bay seawater is shown in Fig. 3. Analysis of the representatives at phylum shows that Proteobacteria predominates, representing 62% of all identified OTUs. The second most represented phylum was Firmicutes, with 32% of the OTUs. The phyla Spirochaetae and Bacteroidetes represented 3% and 2%, respectively. Then, with much more modest numbers of OTUs, Tenericutes and Synergistetes gathered together only 1% of representativeness.
The dominance of Proteobacteria is justified because this phylum clustered 7 representative genera (Desulfovibrio, Marinobacterium, Thioclava, Vibrio, Marinomonas, Pseudoalteromonas, and Pseudovibrio). Desulfovibrio was predominant with 20.4% of relative abundance. The second most abundant genus in the Guanabara Bay seawater was Thioclava (15%). Clostridium sensu stricto 9 was the third largest genus (13%). Additionally, Clostridium was represented by other two genera, sensu stricto 7 (8%) and sensu stricto (3%). It demonstrates that related Clostridium members belong to a complex phylogenetically heterogeneous group. These Clostridium genera were the only representatives of the Firmicutes phylum in this seawater. The presence of the nutritionally versatile genera Pseudoalteromonas, Marinobacterium, and Marinomonas represented 5%, 4%, and 0.5% of all identified OTUs, respectively. Other genera that showed representativeness in the Guanabara Bay water include Vibrio (1.5%) and Pseudovibrio (0.6%). Figure 4 shows the counting of four cultivable bacterial groups widely associated with MIC in seawater. At the beginning of the experiment, the analysis revealed mostly HAB (log 7) and similar low concentrations of IOB, APAB, and SRB, varying between log 2 and log 3. At day 7, the HAB abundance remained statistically equal (p > 0.05), whereas the IOB and SRB increased their population by almost 2 logs (p > 0.05) and APAB decreased by 1 log (p > 0.05). At the end, all bacterial populations remained numerically identical to day 7 (p > 0.05).

Corrosion Morphology of the HAZ Sub-regions
In order to determine whether microbial colonization of the coupon surfaces resulted in pitting corrosion, analyses of corrosion morphology were performed after 14 days of immersion. Values of maximum pit depth are shown in Fig. 5, which measurement is among the most important MIC metrics. The BM coupon was smooth, and a few relatively shallow pits were observed on the surface. The maximum pit penetration depth was only 22.3 μm (Fig. 5A). In contrast, larger and deeper pits were visible on HAZ sub-region coupons regardless of the T p simulated. On the 700 °C HAZ coupon (Fig. 5B), the maximum pit depth was 10 μm deeper than BM. On the other hand, 860 °C HAZ (Fig. 5C) coupon showed the most significant pit penetration, 29.1 μm deeper than that pit on BM. The 1000 °C HAZ (Fig. 5D) coupon exhibited a maximum pit depth slightly shallower than the 860 °C HAZ. The maximum pit depth on the 1350 °C HAZ (Fig. 5E) coupon was 16 μm deeper than the BM but 12.3 μm shallower than the 860 °C HAZ.

Electrochemical Measurements
EIS results of the HAZ sub-region coupons throughout 14 days of immersion in seawater are shown in Fig. 6. According to the Nyquist plots, capacitive arc diameters were higher on the first day, with values ranging from 6000 to 12,000 Ohm cm 2 . The BM coupon presented the larger capacitive arc and, consequently, better stability against corrosion, while the 1350 °C HAZ coupon showed the lowest corrosion resistance. Capacitive arc diameters steeply decreased to a maximum of 1500 Ohm cm 2 at day 4 and 1000 Ohm cm 2 at the last day. A noteworthy phenomenon was the progressive dispersion reduction between different HAZ sub-region curves until they were indistinguishable from each other at day 14. Thus, the HAZ sub-regions acted electrochemically differently only in the early days of immersion.
The formation of a film (biotic and abiotic deposits) on the HAZ sub-region surfaces was better estimated by analyzing the Bode plots (Fig. 7). On the first day of immersion, all samples showed phase angle peaks with 70° on intermediate frequency ranges, corresponding to the freely corroding metal. The peak intensity decreased to lower frequencies on the following days, while an even lower peak appeared in the original frequency range. Furthermore, EIS results were fitted to equivalent circuits considering the presence and absence of a deposit layer on the steel surface, as shown in the supplementary material. However, the conclusions remained the same as the ones attained by analyzing the Nyquist and Bode plots.

Discussion
The present study evaluated the corrosion behavior of weld HAZ sub-regions in real seawater. The microbial profile of the Guanabara Bay seawater was mostly dominated by Proteobacteria, Bacteroidetes, and Firmicutes representatives. The phylum Proteobacteria implicates many marine bacteria, while Bacteroidetes and Firmicutes typically govern copiotroph bacteria [32]. Their presence is because Guanabara Bay receives massive anthropogenic impacts and shows high degrees of pollution.
Desulfovibrio were the most dominant planktonic species detected by the 16S rRNA analysis. This genus is tightly associated with sulfate-reducing bacteria (SRB) and is frequently identified in MIC samples from marine environments due to the rich sulfate content in seawater. Besides natural environments, SRB is a morphologically and physiologically diverse group widely spread in industrial areas. Traditionally, SRB has been reported to release H 2 S, a strongly corrosive metabolite, which can cause pitting corrosion in infrastructures made of several types of steel [13,33,34]. Our previous study showed by biomolecular tools a huge SRB diversity associated with weld corrosion in API X65 steel exposed to seawater from the same site [21]. Additionally, some SRB can directly use metallic iron as an electron donor. For instance, Desulfovibrio ferrophilus strain IS5 is a lithotrophic organism isolated from marine sediment capable of EET mechanism [35]. Desulfovibrio vulgaris also accelerated corrosion via EET in the presence of electron mediators (riboflavin and flavin adenine dinucleotide) [36]; however, there has been no direct evidence of natural secretion of electron mediators by SRB.
Among the species with the highest number of OTUs present in this seawater, the genus Thioclava attracted attention. Its members do not secrete any known corrosive metabolite, and their role in MIC was recently reported using Thioclava electrotropha ElOx9 [37]. Electrochemical investigation demonstrates that this isolated sulfur-oxidizing marine bacterium is capable of extracellular electron uptake from solidstate surfaces and coupling this functionality to nitrate respiration. Unlike D. ferrophilus that performs outward EET, T. electrotropha ElOx9 may corrode steels by the inward ETT mechanism. In general, the pathways of EET can be outward, that is, a flow of electrons toward an electrode (from a "microbial bioanode"), or inward, harvesting electrons from an electrode (to a "microbial biocathode") [38]. Members of Clostridium genus were also identified in our analyses. They have been reported in environmental corrosive biofilms by other studies as well [21,39]. Monroy and colleagues [39] detected lower corrosion rates for Clostridium celerecrescens than that corrosion rate for SRB. The newest reports suggest that Clostridium species can secrete enzymes and other small molecules to enhance their electron uptake from extracellular electron donors [40]; thus, they may cooperate for metallic dissolution. The presence of Pseudoalteromonas, Marinobacterium, and Marinomonas was detected in the Guanabara Bay seawater. Novel insights can explain the multiple effects of these microorganisms on corrosion in marine environments [41][42][43]. Guo et al. [43] described that Pseudoalteromonas piscicida accelerates pitting corrosion due to the secretion of pyomelanin, a bacterial pigment that may act as an electron acceptor or mediator. Hirano et al. [41] firstly reported that flavins secreted by Marinobacterium sp. strain DMS-S1 served as photosensitizers to help the cells assimilate oceanic organic sulfur compounds. Nowadays, it is well-known that flavins also work as an electron shuttle in MIC [38]. Mugge et al. [44] corroborate that Marinomonas species are typically found in marine biofilms. They analyzed biofilms grown over C1020 steel coupons within different marine microcosms Vibrio genus is commonly reported in biofouling studies on various metal alloys in quiescent or flowless seawater conditions. Cai et al. [45] suggested that the synergism of Vibrio sp. with other microbial species within biofilms can increase the corrosion current densities in carbon steels. Conversely, Pseudovibrio has not been reported in MIC studies. Its species have attracted attention because they are part of the microbiome of healthy marine sponges by secreting biologically active secondary metabolites with antimicrobial activity [46]. Since planktonic Pseudovibrio species are often detected in seawater, new studies should focus on whether those metabolites are also active electron shuttles in marine corrosion processes or not. Even though the members of some genera found in this study are not directly related to metallic corrosion, they can play an essential role in maintaining the biofilm integrity and creating favorable conditions for corrosion reactions.
The high number of HAB in the seawater may have provided a favorable environment to accelerate the corrosive process of the HAZ surfaces. Previous studies reported similar HAB counting for seawater samples from different points of the Guanabara Bay [7,19,26,47]. Most of these bacteria are described as pioneer colonizers during biofilm formation over surfaces by producing extracellular polymeric substances (EPS). During the experiments, the IOB growth may have been stimulated by the release of ferrous ions (Fe 2+ ) from HAZ sub-region corrosion to the bulk seawater. This lithotrophic group rapidly oxidizes dissolved Fe 2+ to insoluble ferric ions (Fe 3+ ). Results confirmed that planktonic SRB could grow in aquatic environments with high dissolved oxygen (7.4 ± 0.2 mg L −1 ). The SRB number may have also increased due to cell detachment from the biofilm formed over the HAZ sub-region coupons. On the other hand, the slight decrease in the APAB population can be justified by the rising concentration of iron (Fe 2+ and Fe 3+ ) and sulfide (S 2− ) in the microcosm, which may have promoted an inhibitory effect on its metabolism. The planktonic cell counting indicates that the cultivable microbial community remained metabolically active throughout the experiment, promoting the occurrence of MIC.
On the whole, illumina sequencing and MPN method pointed out this seawater as a natural medium with a complex microbial community assembled by multiple groups of microorganisms that coexist in Guanabara Bay. Most of these microbial groups can promote both M-MIC and EET-MIC, whether alone or synergistically with others. Additionally, the microbial colonization of the coupon surfaces after 14 days of immersion in this seawater was corroborated by scanning electron microscopy (SEM). All coupons exhibited microbial cells embedded in corrosion products. Since SEM images shared the same visual information, regardless of the HAZ sub-region analyzed, representative images with different magnitudes are shown in the supplementary material. Characterization of the sessile microbial community on the coupon surfaces by high-throughput DNA sequencing was not performed here. Our previous study [21] showed rather similarity between planktonic and sessile microbiota in the Guanabara Bay.
The high diversity, abundance, and activity of microorganisms in the seawater were the greatest factor in promoting pitting corrosion on HAZ surfaces. Corrosion susceptibility was in the following order: BM > 700 °C HAZ > 1350 °C HAZ > 1000 °C > 860 °C HAZ. Results indicated that austenite and martensite grains seen in the 860 °C HAZ (Fig. 2C) might be less resistant to pitting corrosion than cementite grains seen in the BM ( Fig. 2A). It may have happened because 860 °C HAZ showed a varied microstructure, while the BM showed a uniform microstructure. The size of the grains also may influence their sensitivity to corrosion. Although the 1000 °C HAZ (Fig. 5D) and 1350 °C HAZ (Fig. 5E) coupons were composed of only austenite grains in their microstructures, the HAZ sub-region with fine grains (1000 °C) exhibited pitting corrosion of 18.5% deeper than that HAZ sub-region with coarse grains (1350 °C). It is important to highlight that there was no significant variation in the chemical composition of the HAZ sub-regions (data not shown).
Electrochemical tests demonstrated that the corrosion behaviors in the early days were mainly associated with the differences in granulometry and microstructure, as seen in Figs. 2 and 6, when the whole surface was not covered by biofilm or a significant amount of corrosion products. The decrescent EIS values (Fig. 6) and smaller peaks on intermediate frequencies by the Bode plots (Fig. 7) can be associated with the cell attachment and progressive biofilm formation on the HAZ sub-region surfaces during the experiments. On the other hand, the one shifted to lower frequencies may be related to the electrochemical reactions developing at slower rates due to a higher mass transfer resistance caused by the deposit. Lv et al. [48] also suggested that phase angle peaks shifting to lower frequencies indicated the formation of denser deposit layers. According to Fig. 7, dense layers (biofilm and abiotic deposit) were already formed on the BM, 700 °C HAZ, 860 °C HAZ, and 1000 °C HAZ coupons from the fourth day onward, while the 1350 °C HAZ coupon caused two-time constants within day 6 and day 9. Despite the different behavior, there is no indication that the HAZ heated at 1350 °C could induce a different electrochemical response due to its particular microstructure.
As all HAZ sub-regions exhibited identical behavior after day 4, the electrochemical tests indicated that the biofilm formation on the coupons could "standardize" the corrosion reactions and their velocities on surfaces (Fig. 6, 7, and 8), regardless of the HAZ sub-region microstructures investigated. However, these tests were not suitable enough to detect the differences in pitting corrosion promoted by the activity of seawater microorganisms, which pit penetration was particular to each microstructure (Fig. 5). In a condition under the same water salinity but without microorganisms, Zhao and coworkers [16] investigated the corrosion behavior of five HAZ sub-regions (T p of 750, 850, 1000, 1150, and 1350 °C) formed during the welding of an FH32 carbon steel. The authors described considerable differences in the corrosion behaviors of the different HAZ sub-regions and that this behavior is related to changes in the microstructural sensitivity. The findings of Zhao et al. [16] are corroborated by other authors using diverse types of steel.

Conclusions
The microbial abundance and community composition in the seawater, including many bacterial genera associated with M-MIC and EET-MIC, make Guanabara Bay a highly biocorrosive environment. Thus, the results showed that all API X65 coupons deteriorated at different levels. The simulated HAZ sub-regions were more and more susceptible to pitting penetration than the area away from weldment (base metal). The weld-HAZ with a peak temperature of 860 °C exhibited the worst corrosion resistance in the weld joint, showing a pit penetration depth 2.3 times greater than that pit on the base metal. This behavior may be attributed to the microstructure composed of austenite and martensite of fine grains, while the base metal microstructure was formed by cementite grains. The results also indicated that increased peak temperature did not create HAZ sub-regions more susceptible to corrosion in the API X65 steel. It is highlighted that differences in corrosion severity were observed just by surface topography analysis since electrochemical measurements indicated indistinguishable corrosion behavior among the HAZ sub-regions.
Acknowledgements V.S.L. thanks the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro-FAPERJ-for providing financial support.
Author contribution VSL was responsible for conceptualization, methodology, investigation, formal analysis, writing, and editing. GBL carried out investigation, formal analysis, and writing. SLB and JPF were involved in conceptualization and formal analysis. EFCS was involved in conceptualization and funding acquisition.
Funding Not applicable.
Data Availability All data generated or analyzed during this study are included in this published article (and its supplementary information files).

Competing interests
The authors declare no competing interests.