October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 7 3 3 cathodic protection method [5] . Of the various techniques used to apply the anode in the cathodic protection meth- od, the most common and economical approach is thermal spray. Figure 3a shows an example of field application of a zinc coating to a concrete pillar via wire-arc spray method, which can only be used in environments that assist in conductivity of electrons between two metals to complete the circuit. Moist and immersive marine environments are highly conductive. When these con- ditions are not present, such as in ex- tremely dry locations, the resistivity of the concrete impedes the current. A galvanic coating is not useful in such a scenario. The impressed-current cathod- ic protection (ICCP) method is anoth- er technique widely used for corrosion protection of reinforced concrete. In this method, electric current is provid- ed externally using an external elec- tric-power source (alternating current rectifier). The positive terminal is con- nected to the anode and the negative terminal to the steel rebar embedded in the concrete. The anode is embedded in the concrete or applied on the concrete surface. Activated titani- ummesh coated with metal oxides is a commonly used anode. Figure 3b shows an example of titanium mesh inlaid in a mold to be cast with concrete. The anode supplies a uniform current to the steel reinforcement to prevent oxidation, which would lead to corrosion. The benefit of this meth- od over galvanic protection is that current can be easi- ly adjusted to maintain the steel-concrete potential at the desired level. The addi- tion of a corrosion inhibitor in the concrete mixture can also delay corrosion initi- ation. These can be added to fresh concrete or applied later to the hardened con- crete surface. Mixing with fresh concrete results in uniform distri- bution of the inhibitor, thus providing better protection. Although corrosion protection methods for steel and concrete struc- tures are well established, increasing harshness of the environment in coast- al areas due to SLR will accelerate the corrosion kinetics. For example, in ma- rine environments, the splash zone ex- periences the highest corrosion rates. Waves and tidal velocity are expected to increase with SLR, thereby increasing the splash zone area. Various protec- tion methods can be used in tandem for improved performance. For example, galvanically protected concrete can be coated with polyurea to reduce coating wear and permeability of chloride ions. Microbial Influenced Corrosion (MIC) . A huge biodiversity thriving in seawater exists around the globe. Aquatic microorganisms adhere and grow on surfaces of submerged struc- tures, bridges, marine vessels, and larg- er aquatic species. This undesirable accumulation and growth of aquatic microorganisms is known as biofoul- ing, which is a type of corrosion and is thus a threat to the structural integrity of submerged stationary structures. For moving vessels, it significantly increas- es the drag, thereby decreasing fuel economy and increasing the vessel’s carbon footprint [7] . Microorganisms responsible for biofouling are categorized into micro- foulers (bacterial biofilms) and mac- rofoulers (such as algae, mussels, and tubeworms). The enormous diversity of microorganisms and their variousmeta- bolic functions affect submerged mate- rials inmany ways. Themost commonly observed mechanisms of MIC are acid attack, chemical concentration cells, cathodic depolarization, fixing anod- ic sites, consumption of electrons, and phase transformations [8] . Usher et al. presented a schematic of various mech- anisms of MIC for steel pipes (Fig. 4) [8] . The main factors affecting the growth of biofilms include the temperature and Fig. 4 — Schematic of mechanisms of microbial-influenced corrosion of a steel pipe: (A) microorganism extracts electron from Fe to produce Fe 2+ ; (B) microorganism oxidizes Fe 2+ producing Fe 3+ , which precipitates as oxide; (C) anaerobic reduction of Fe 3+ to Fe 2+ ; (D) sulfate-reducing microorganism reduces sulfates; (E) acid and enzyme attack by microorganism; (F) microorganismproduces sulfuric acid; (G) iron-oxidizing bacteria [8] .

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