**1. Introduction**

Microbiologically influenced corrosion (MIC) of carbon steel is a major cause of metal corrosion and pipeline failure [1–3]. It is estimated that MIC accounts for about 20% of the corrosion damage in the oil and gas sector [4]. Despite the tremendous e fforts made thus far to improve corrosion management, MIC has remained a pressing issue for the oil/gas sector, where there is exposure of metals to bacteria found in water. Several types of microorganism are responsible for MIC, including sulfate-reducing bacteria (SRB), iron-oxidizing bacteria (IOB), slime-forming bacteria, and iron-reducing bacteria (IRB). Amongst those, SRB are the main microorganisms responsible for MIC by generating sulfide species anaerobically, which causes progressive biocorrosion in the water transport systems [5,6]. The SRB strains produce hydrogen sulfides (H2S), metal sulfides, and sulfates as a result of biogenic oxidation/reduction reactions [7,8]. In particular, the production of H2S at elevated concentrations creates intrinsic heterogeneity, which accelerates the corrosion process by favoring electrochemical reactions [9–11].

Biocorrosion control methods are mainly based on either inhibiting the metabolic/growth activities or altering the corrosive conditions to reduce the adaptation of microorganisms. Di fferent types of approach, such as cathodic protection [12], protective coatings [13], corrosion inhibitors [14], and biocides [15], have been used to control/minimize biocorrosion. Oil/gas industries usually need high concentrations of biocides for water disinfection and controlling SRB biofilm formation thereby reducing biocorrosion [16]. However, the use of conventional biocides may cause harmful impact to the environment since it produces disinfection byproducts in addition to the low e fficiency against biofilms, and high operational cost [17]. Di fferent nanomaterials demonstrate strong antimicrobial activities, rendering them potential alternatives to conventional biocides [18–22]. Nanomaterials, such as AgNPs [23], ZnONPs [24], TiO2NPs [4], FeNPs [25], graphene [26], CuONPs [27], and metal-nanocomposites [22], have been used for the inhibition of SRB-induced biofilm and subsequent MIC. However, the environmental impact of nanomaterials due to their biological toxicity has restricted their use in practical applications [28,29]. The use of green biocides with lower toxicity, environmentally benign, and ease of use can overcome these issues [30].

Chitosan (CS) is a biodegradable polymer abundant in nature with high hydrophilicity, nontoxicity, antimicrobial properties, and low cost [31]. The antimicrobial activity of CS has been widely established against many microorganisms and it shows a high inhibition rate against both Gram-positive and Gram-negative bacteria [32–35]. CS also displays anti-biofilm activities with a high ability to damage biofilms formed by microbes [36–38]. Due to its cationic nature, CS has been able to penetrate biofilms by disrupting negatively charged cell membranes through electrostatic interaction when microbes settle on the surface [36]. Recently, our research group used ZnO-interlinked chitosan nanoparticles (CZNC-10) as stable biocide formulations against SRBs from industrial waste sludge [24]. The nanoparticles achieved a concentration-dependent SRBs inhibition with over 73% e fficiency at 250 <sup>μ</sup>g·mL−1. Also, CZNC-10 demonstrated 74% MIC inhibition on carbon steel [39]. The dose of CZNC was limited to 250 <sup>μ</sup>g·mL−<sup>1</sup> due to the ZnO content in the CZNC biocide considering the environmental toxicity issues. In order to develop a more "green" and e fficient chitosan-based biocide, the metal or metal oxide nanoparticles need to be replaced with more environmentally benign alternatives.

Lignin is an abundant natural resource that has been widely used as a potential source for fuel and chemical production [40]. Lignin can be incorporated into di fferent polymeric systems such as dispersants, bioadhesives, biosurfactants, polyurethane foams, and epoxy resins, etc., depending on its solubility and reactivity characteristics [41]. Lignosulfonate (LS) exhibits good water-solubility and anionic characteristic [42]. LS also exhibits antioxidant and antimicrobial properties that extend its potential applications to di fferent fields [43,44]. Both CS and LS are good bactericidal agents, and therefore it is expected that CS-LS complexes can be used as highly e fficient and environmentally friendly chitosan-based biocides against SRB induced biocorrosion.

The synthesis of CS-LS polyelectrolyte complexes is mainly based on ionic interaction or ultrasonic homogenization [45,46]. CS cross-linked graphene oxide (GO)/LS composite aerogels have been synthesized by the simple mixing of GO, LS, and CS solutions [47]. The aerogels demonstrated 3D porous structure. These CS-LS hybrids showed non-uniform sizes/shapes and instability above pH 4.5, which restricts their practical applications. To solve these issues, we have introduced a new crosslinking strategy towards the synthesis of stable cross-linked chitosan-lignosulfonate (CS/LS) nanospheres [31]. The optimum composite structure was formed at 1:1 ratio of CS:LS. These 150–200 nm nanospheres demonstrated the highest thermal, mechanical, and bactericidal e ffect against aerobic *Escherichia coli* (*E. coli*) and *Bacillus subtilis* (*B. subtilis*) bacteria as well as anaerobic SRB. As a proof of concept, 100 mg/<sup>L</sup> CS/LS-1:1 was able to inhibit SRB growth, as demonstrated by 48.8% lower sulfate reduction [31].

Here, we have investigated the ability of the new "green" CS/LS nanospheres with an optimal 1:1 (CS:LS) ratio to treat SRB induced MIC on SS400 carbon steel. The nature and kinetics of SRB inhibition induced by the CS/LS are thoroughly studied with electrochemical impedance spectroscopy (EIS) and surface characterization methods.

#### **2. Materials and Methods**
