3.1.1 Corrosion—definition

Corrosion is normally defined as the “attack on a metallic material by reaction with its environment.”¹ Usually one refers to metals when describing corrosion, but nonmetals such as ceramics, plastics, rubber, and so on, can also undergo destruction or deterioration when exposed to various environments. For purposes of this review we will restrict our examination to aqueous/wet environments and methods used to mitigate their corrosive effects.

As a general review we will introduce the several types of corrosion for metals/alloys and their various forms. Corrosion can be divided into three main groups:

■ Wet corrosion

■ Corrosion in other fluids

■ Dry corrosion

“Wet corrosion” refers to metal corrosion occurring in a wet/aqueous environment.² The process is almost always electrochemical, and it occurs when two or more electrochemical reactions take place on a metal surface. When a metal is exposed to a corrosive wet/aqueous environment, the metallic nature of the metal is fundamentally changed into a nonmetallic form becoming dissolved species or solid corrosion products. The energy of the system is lowered as the metal converts to a more stable form, in this case nonmetallic corrosion products. This electrochemical process can occur uniformly or nonuniformly across the metal surface, which is called the “electrode” and the ionically conducting liquid is called the “electrolyte.” A classic example of this process is the rusting of a steel substrate: the iron metal is converted into a nonmetallic form called “rust.”

“Corrosion in other fluids” refers to the corrosion of metals/alloys in nonaqueous environments, such as fused salts sometimes referred to as molten salts.³ In addition corrosion can also occur in liquid metals.⁴ Corrosion in fused salts such as nitrates, halides, carbonates, sulfates, hydroxides, and oxides can cause profound attack on metal alloys via several mechanisms: (1) pitting because of electrochemical attack, (2) mass transport due to thermal gradients, (3) reaction of the constituents of the fused salt with the metal alloy, and/or (4) reactions with impurities present in the fused salt. In addition to fused salts, exposure of metal alloys to liquid-metal environments can cause severe corrosion. Liquid metals are used in industrial applications such as high temperature reducing agents or as coolants due to their excellent heat transfer properties. Corrosion as a result of exposure to liquid-metals can be caused by dissolution, impurity or interstitial reactions, alloying, and compound reduction.

“Dry corrosion” refers to corrosion affecting metals/alloys that are exposed to air or other aggressive gases.⁵ For a relatively small number of metals, exposure to gases does not result in corrosion; however, for most metals, exposure to very high temperature gases increases the corrosion rate resulting in corrosion failure. The figure below shows the elements necessary for corrosion under different environments (Figure 3.1).

The three main types of corrosion have been identified and described in detail; we can now turn our attention to the most common forms of corrosion, which are found in wet or aqueous environments. There are eight forms of corrosion, which can be identified based on the appearance of the corroded metal or alloy. These corrosion types are listed below:

■ Uniform or general corrosion

■ Pitting corrosion

■ Crevice corrosion (includes corrosion under tubercles or deposits and filiform and poultice corrosion)

■ Galvanic corrosion

■ Erosion-corrosion (cavitation and fretting corrosion)

■ Intergranular corrosion (sensitization and exfoliation)

■ Dealloying corrosion (dezincification and graphite corrosion)

■ Environmentally assisted cracking corrosion (stress-corrosion cracking, corrosion fatigue, and hydrogen damage)

The above listed forms of corrosion do not take place independently; there is normally overlap and concurrent forms of corrosion occurring at any time during the corrosion process. Corrosion can also occur at the macroscopic level, which is visible to the naked eye and microscopic level where corrosion occurs in minute amounts and is not visible to the naked eye. In the latter case to examine these effects requires higher magnification before the damage is so severe that it is visible to the naked eye (Figure 3.2). Structural integrity is often compromised after the effects of corrosion become visible to the naked eye.

3.1.2 Costs of metallic corrosion/prevention

Metallic corrosion affects almost every U.S. industrial sector ranging from infrastructure to manufacturing.⁶ Aging infrastructure is currently the most important sector affected by corrosion. Recently state and federal agencies have made it a priority to extend their useful lifetimes.⁷ Additional sectors (including infrastructure) in today’s modern society affected by corrosion are:

■ Infrastructure—highway bridges, gas/liquid transmission pipelines, waterways/ports, airports, railroads

■ Utilities—gas distribution, drinking water/sewer systems, electrical utilities, telecommunication

■ Transportation—motor vehicles, ships, aircraft, railroad cars, hazardous materials transport

■ Production/Manufacturing—oil/gas exploration/production, mining, petroleum refining, chemical/petrochemical/pharmaceutical production, pulp/paper, agricultural production, food processing, electronics, home appliances

■ Government—defense, nuclear waste storage

The cost for corrosion repair, maintenance, and replacement affects all facets of the United States and the industrialized world’s infrastructure. These costs are not only limited to severe damage compromising public safety, but also environmental damage. Corrosion can disrupt operations requiring extensive repair and replacement of failed assets. Direct corrosion costs in the United States have been estimated at ~$300 billion U.S. dollars annually, accounting for nearly 3.1% of gross domestic product (GDP).⁸ Extrapolating to the industrialized world, corrosion costs account for nearly $2.2 trillion U.S. dollars, which represents over 3% of the world’s GDP.

3.1.3 Corrosion costs to national economies

As was detailed above in Section 3.1.2, corrosion costs are estimated at ~$300 billion U.S. dollars annually for the United States. This often cited number produced by the National Association of Corrosion Engineers (NACE) Corrosion study of 1998 has been reviewed and updated.⁷ G2MT Laboratories have updated this estimate by including both the direct and indirect costs of corrosion and have estimated that all corrosion costs to the U.S. economy are over $1 trillion U.S. dollars for 2013.⁹ Additional studies have been conducted throughout the industrialized world. Similar percentage costs to GDP (2-5%) have been found across various economies.¹⁰ Table 3.1 lists several national economies and their associated corrosion costs.

In order to reduce the costs of corrosion affecting various national economies, effective corrosion strategies need to be employed. These existing corrosion-inhibiting technologies include: (1) proper design; (2) proper selection of materials (alloys, metals, plastics, and so on); (3) cathodic protection; and (4) coatings, inhibitors, and surface treatment. The focus of this review is to provide the reader with an overview of “smart pretreatment coatings,” a necessary first step in the prevention and control of corrosion for metals/alloys.

3.3.1 Selecting the proper metal alloy

Prior to depositing the pretreatment coating, the proper metal alloy must be selected. Different metal alloys are known to have various susceptibilities to corrosion depending on their composition. In order to mitigate these effects, different levels of corrosion protection are needed. The selection criteria are based on the most optimal material required, but this is never a straightforward process. There is the balance of adequate performance versus cost-effectiveness. The selection process is dependent on the type of industry and the corrosive environment that the metal/alloy will be exposed to. Figure 3.3 provides a flow diagram to follow for material selection criteria with the caveat that costs often outweigh the purchase of highly corrosion-resistant metal/alloys in most cases.¹⁹

3.3.2 Surface modification

Before we can discuss various “smart pretreatment coatings” on metals/alloys, we must first examine the methods to clean a surface. Before a coating is applied, the surface must be rendered suitable for the pretreatment coating (e.g., conversion coating). This process involves several steps: (1) assessment of surface condition; (2) inspecting for fabrication defects (3) pre-cleaning; and (4) final surface inspection.²⁰

The assessment of the surface condition requires inspection of the entire surface to determine the level of contaminants present. These contaminants can be smut, grease, oil, dirt, rust, corrosion products, salt deposits, and die-release compounds. The presence of these contaminants can dramatically alter the adhesion properties of applied pretreatment coats and subsequent primers and topcoats.

Prior to surface cleaning, any design and/or fabrication defect in the part or substrate to be coated must be removed or repaired. These defects can include excessive pitting, dents, or defects in the metal/alloy component. There are several methods available to remove contaminants from the surface of metals/alloys, which include (1) solvent cleaning, (2) etching cleaners (alkaline and acid etch), (3) desmut or deoxidation, (4) physical cleaning (blasting, abrasion, and/or polishing), (5) acid pickling, and (6) rinsing.¹⁸ Solvent cleaning removes the bulk of smut, grease, oil, dirt, rust, corrosion products, salt deposits, and die-release compounds, but usually requires an additional step such as acid or alkaline etch to remove reactive species. Further cleaning via physical methods is required for the harder steel surfaces where thick or scaly residues are present. This process is not normally used for softer aluminum surfaces. After multiple cleaning processes are complete, a desmut, deoxidation, or pickling step is sometimes used on the metal/alloy to remove any contaminating species rendering an acceptable surface. This provides a surface without weak boundary layers or damage in a reasonable time and cost. A rinsing step is often employed in industrial settings to remove excess reagent(s) and/or solvent(s) from various bath steps and to prevent cross-contamination of the baths. Now that we have reviewed corrosion, metal alloy selection, and cleaning procedures, our next step is to focus on the core of this chapter, which are the various pretreatment coatings commercially available and under development that can act as “smart coatings” as the first line of defense for corrosion inhibition.

3.4.1 Conversion coatings

Conversion coatings include chromate conversion coatings (CCCs), phosphate conversion coating, and lanthanide-based conversion coating and are deposited onto various metal/alloy substrates such as steel, zinc, aluminum, magnesium, copper, tin, silver, and nickel. Chemical conversion coatings processes are classified by their primary constituent. For example CCCs are mainly composed of chromates, whereas phosphate conversion coatings have as their main component phosphates.³³ The metal/alloy surface is converted into coatings via a chemical or electrochemical process. These types of coatings are used for corrosion protection, increased surface hardness, and added decorative finish and/or as paint primers. Conversion coatings can be very thin 0.00001″ (0.254 μm) or thick coatings 0.002″ (50.8 μm) and can be grown either anodically or by repeated exposure in conversion baths.

Conversion coatings are adherent, insoluble, inorganic crystalline, or amorphous surface coatings that are an integral part of the metal/metal oxide surface. In such films a portion of the base metal is converted into a metal oxide that is less reactive to corrosion than the original metal and improves adhesion for primers. Conversion coatings have gained widespread industrial use due to their strong adhesion properties, high speed of coating formation, and very favorable economics, while providing reproducible and consistent coatings for large-scale industrial use.³⁴

3.5.1 Hybrid sol-gel coatings

Over the past two decades sol-gel-based thin films have been investigated by several groups for corrosion inhibition of metal alloys. This research has been spurred by the need for environmentally friendly and nontoxic alternatives to CCCs.^{28,46–48,114–116} Significant efforts have been employed to develop sol-gel coatings that can act as adhesion promoters between metal alloys and organic primers/topcoats as well as provide corrosion protection.¹¹⁷

There are two methods available to prepare sol-gel coatings via either a hydrolytic method in aqueous media or a nonhydrolytic process in organic media. Either method can produce various sol-gel coatings with different properties. These properties can be controlled via adjusting the composition of the reactive species, organic functionalization, temperature, time, and pH, affecting their potential applications for corrosion protection.¹¹⁸ Sol-gel based pretreatment coatings have been studied on various metals/alloys, such as copper,^119,120 aluminum,¹²¹ magnesium,¹²² and carbon steel.¹²³ These studies have shown that sol-gel pretreatment coatings exhibit corrosion- inhibiting properties.

The sol-gel process is based on the hydrolysis and condensation reactions of metal alkoxides [M(OR)_n] where M = Si, Ti, Zr, or Al; and R = alkyl group (methyl, ethyl, butyl, and so on) (Figure 3.4).^115,124 The sol-gel process involves two distinct and separate methods for the preparation of the “particles or films,” which consists of either an inorganic or organic approach, and the latter is the preferred method. The sol-gel process involves the initial formation of a colloidal suspension or solution referred to as the “sol,” which is followed by the formation of the integrated network referred to as the “gel” to give either discrete particles or network polymers.

The sol-gel process typically occurs in four stages:

■ hydrolysis

■ condensation and polymerization of monomers to form chains or particles

■ growth of the particles or chains

■ finally, agglomeration of the networks, thickening and formation of the gel^115,118,124

Inorganic sol-gel pretreatment coatings have been investigated for their corrosion-inhibiting properties, but have suffered from cracking for thick coatings ≥ 200 nm. Cracking of the sol-gel film is formed during the heating stage when shrinkage of the gel occurs during solvent evaporation.¹²⁵ In order for crack-free sol-gel films to be formed, the thickness of the pretreatment film must be ≤ 100 nm, but their barrier properties are compromised due to presence of micropores in the film. Films produced by the sol-gel method show excellent adhesion between the alloy/metal substrate and primer/paint system.¹²⁶

In order to produce crack-free films, researchers have developed hybrid sol-gel-derived coatings that combine the unique properties of both the inorganics/ceramics with organic polymeric materials.¹²⁷ The inorganic/ceramic coatings provide scratch resistance, durability, and improved adhesion between the metal/alloy substrate and organic primer/paint system, whereas the organic components increase flexibility and functional compatibility with the organic primer/paint system.^128,129 There are two classes of organically functionalized sol-gel coatings: (1) nonfunctional organoalkoxysilane and (2) organo-functional alkoxysilanes. Nonfunctional organoalkoxysilane sol-gel coatings are prepared using methyl groups as the organic part for the preparation of the hybrid coatings,¹³⁰ whereas, organo-functional alkoxysilanes incorporate such functionalities as epoxy,^131,132 methacrylic,^133,134 acrylics,¹³⁵ amino,^136,137 allyl,¹³⁸ pyridine,^139,140 or vinyl/phenyl moieties ^138,141,142 as the precursor groups for sol-gel formation. These additional functional groups are also available for further polymerization producing sol-gel films with high crosslink densities, improved mechanical properties, and tailored compatibility of the sol-gel pretreatment coating to the organic primer/paint system. Corrosion resistance using sol-gel films has been dependent on obtaining crack-free films. The hybrid sol-gel films offers several advantages, but still has limitations associated with reduced wear resistance and lower mechanical properties. Incorporation of nanoparticles into a sol-gel network can improve their mechanical properties while providing for corrosion resistance of metals/alloys.^143–146 Further improvements in hybrid sol-gel networks’ corrosion-inhibiting properties have focused on introducing corrosion inhibitors with various degrees of success. Incorporation of inorganic inhibitors that are non-Cr(VI) such as phosphates,¹⁴⁷ V,¹⁴⁸ Ce,^{147,149–153} and Mo^154,155 compounds as well as organic corrosion inhibitors such as phenylphosphonic acid,¹⁵⁶ mecaptobenzothiazole,^149,157 benzotriazole,¹⁵⁸ and 8-hydroxyquinoline¹⁵⁹ have been investigated for their corrosion-inhibiting properties.

3.5.2 Conductive polymer coatings

Over the past three decades, evidence that CPs such as polyaniline (PANI) and polypyrrole (PPy) could inhibit corrosion has come from the pioneering work of Mengoli,¹⁶⁰ DeBerry,¹⁶¹ and MacDiarmid.^162,163 Mengoli showed that electrochemically deposited CP coatings on iron (Fe) anodes resulted in an adherent and corrosion-inhibiting film.¹⁶⁰ Further proof was obtained by DeBerry in 1985¹⁶¹ who showed that PANI electrochemically deposited onto stainless steel showed a change in its corrosion behavior in a sulfuric acid solution, which provided concrete evidence that the PANI films provided anodic protection, thus maintaining a native passive film on the steel metal. There are numerous studies using CPs deposited onto metals/alloys under a variety of conditions such as electropolymerization, solvent-casting, water-dispersive formulations, spray, and dip-coating. These CPs are new “smart coatings” for corrosion protection of ferrous and nonferrous alloys. CPs can function by an anodic protection mechanism for ferrous alloys and therefore can potentially replace such corrosion-inhibiting compounds as Cr(VI) and cadmium (Cd), which are known carcinogenic and environmentally hazardous materials.^{42–48,164–166}

The corrosion mechanism for ferrous alloys as mentioned in the preceding paragraph is via anodic protection for steel alloys, passivation for stainless steels,^167–169 and released dopant ions for mild steels.^170,171 The corrosion protection mechanism for aluminum alloys is still under intense investigation. There are several proposed mechanism(s) by which CPs can potentially inhibit corrosion of aluminum alloys in corrosive aqueous environments.¹⁷² These proposed mechanism(s) are as follows: (1) ennobling of the aluminum alloy,¹⁷³ (2) anodic protection/passivation,¹⁷⁴ (3) galvanic and ion-exchange release of oxygen reduction inhibitors,¹⁷⁵ and (4) barrier protection by the reduced form of the CP following an initial galvanic coupling.¹⁷⁶

Most of the investigations into the corrosion-inhibiting properties of CPs have contained either a nonconductive primer and/or topcoat for enhanced corrosion protection.¹⁷⁷ Additional work has focused on the corrosion mechanism of CPs via electrochemical and spectroscopic techniques that was also described in the preceding paragraph. There are fewer reports in the literature that focus exclusively on the corrosion-inhibiting properties of CPs as thin films with direct comparisons to the corrosion-inhibiting properties and performance of CCCs. Several recent reports highlight the corrosion-inhibiting properties of PANI, PPy, and PEDOT thin films (< 1.0-2.5 μm) deposited on steel alloys with and without topcoats.^178–181

Researchers at the Naval Air Warfare Center Weapons Division, China Lake, CA synthesized,^182–184 coated, and evaluated a CPs performance both in laboratory^103,185,186 and field testing environments,¹⁸⁷ which was directly compared to CCC. This compound, poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene) (BAM-PPV), (Figure 3.5) was shown to have promise as a viable nontoxic, environmentally friendly alternative to CCCs as reported by Zarras et al.^182–188 The neutral salt fog laboratory results showed that BAM-PPV coated on aluminum alloy could meet the military requirement as a replacement for CCCs. Electrochemical noise method analysis of BAM-PPV coatings showed evidence of a surface passivation mechanism similar to other CPs.¹⁸⁶

However, when BAM-PPV was coated on aluminum alloy with a non-Cr(VI) primer and topcoat, the neutral salt fog survivability did not match a fully Cr(VI) military coating system [CCC + Cr(VI) epoxy primer + polyurethane topcoat]. A 1-year Air Force field test of BAM-PPV incorporated into a full military coating [BAM-PPV + non-Cr(VI) epoxy primer + polyurethane topcoat] on the C-5 cargo plane’s rear hatch door showed equal performance to the fully Cr(VI) military coating.¹⁸⁷ There are several recent reviews available to the reader to get a more comprehensive understanding of CP coatings and their ability to act as “smart coatings” in a variety of corrosive environments.^{109,189–191}

3.5.3 Self-assembling pretreatment coatings

Self-assembling (SA) molecules are compounds that provide surface protection for the underlying metal or alloy from corrosive environments via the formation of thin films. These SA films can be very thin (~ 100 Ǻ) and produce flexible, densely packed, stable films capable of blocking electron transfer or inhibiting the transport of corrosive species to the underlying metal or alloy.¹⁹²

SA films are generated when molecules with specific chemical groups are deposited onto a surface have strong affinities for the surface and undergo ordering due to chemical interactions between substrate and SA molecules. In addition to the chemical or physical affinities necessary to form strong interactions on the substrate surface, the type and strength of the intermolecular interactions between the SA molecules that hold the assembly together are equally as important.¹⁹³ The SA molecules that are deposited onto substrates (metal or alloy surfaces) are either physically or chemically adsorbed. A significant portion of the research investigating SA films have focused primarily on the self-assembly of n-alkanethiolates and related structures on gold substrates. These well-ordered SA films are easily formed under ambient conditions due to the chemical inertness of the gold substrate and the affinity of thiol compounds to chemisorb onto gold.¹⁹⁴

SA molecules have also been studied for their corrosion-inhibiting properties on various alloys. These SA molecules are specific for certain metals such as alkanethiols for gold,^192–195 silver,¹⁹⁶ and copper¹⁹⁷; alcohols; amines for platinum¹⁹⁸; and phosphonate compounds for aluminum.¹⁹⁹ The fabrication of SA films onto either a metal or alloy consists of finding suitable functional groups sandwiched between an aliphatic spacer unit capable of adhering onto the surface of the metal or alloy. The opposite functional group can then react further with the primer layer to form a more robust and durable film (Figure 3.6).

SA films can provide effective barriers against the diffusion of oxygen,²⁰⁰ and/or water.²⁰¹ Cross-linking reactions can further enhance the durability and corrosion- inhibiting properties of the SA film.²⁰² There are several studies of SA films for corrosion inhibition of copper metal using sodium 1-octadecyl-1H-benzimidazole,²⁰³ diethydithiocarbamate,²⁰⁴ which provided cathodic inhibitor protection, and hexane-1,6-diamine and 2-mercapto-ethanol, which provided evidence for enhanced corrosion inhibition on copper metals.²⁰⁵ Corrosion inhibition of aluminum metal using octadecylphosphonic acid monolayers was investigated by Grundmeier et al.²⁰⁶ The 1-tetradecylphosphonic acid SA films deposited on Al 2024 and 1060 showed corrosion inhibition during initial corrosion monitoring,²⁰⁷ and alkane diphosphonate monolayers on 1050 aluminum showed improved corrosion resistance when the aluminum surface was exposed to oxide formation treatments.²⁰⁸ Recent work on SA films coated onto iron and steel substrates has been investigated by several groups.^209,210 Adipic acid SA molecules formed compact films onto carbon steel exhibiting reduced electrochemistry on the steel surface²⁰⁹; compact and superior seawater stability were found with SA polydopamine/dodecanethiol films on 304 stainless steel²¹¹; and triazinethiol SA monolayers coated onto zinc treated steel substrates showed improved corrosion resistance but poor water repellency.²¹² Brass alloys and magnesium alloys have also been investigated using SA films. In the case of brass alloys several silane SA films inhibited corrosion in 0.2 M NaCl,²¹³ and SA nanophase particle films provided corrosion protection for AZ31B magnesium alloy in 0.005 M NaCl solution for 354 h.²¹⁴

3.5.4 Polyelectrolyte multilayer films

Polyelectrolyte (PE) films are fabricated by a layer-by-layer (LbL) approach using alternating assemblies of polyelectrolyte pairs consisting of a polyanion and polycation (Figure 3.7).²¹⁵ The adsorption of the alternating PE multiple layers is driven by electrostatic interactions, and assemblies of hundreds of alternating polyanions and polycations are possible.²¹⁶ The LbL approach for multilayer PE films allows for the precise control over the physical properties of the film (e.g., film thickness and morphology).²¹⁷

The deposition by the LbL approach affords nanometer-scale precision with multifunctional capabilities for the PE multilayer coating, which can exhibit excellent adhesion to the metal or alloy and can “self-heal” to seal surface defects.²¹⁸ The PE multilayer films exhibit pH-buffering activity and can stabilize the pH between 5 and 7.5 at the metal surface in a corrosive environment. The PE multiple layer coatings are relatively mobile and also have the ability to seal and eliminate mechanical cracks in the coating via a “self-healing” mechanism.²¹⁹ Additionally, active inhibitors can be trapped in the PE multilayer films. The inhibitor is trapped to the layers closest to the substrate for release of inhibitors while maintaining effective substrate barrier properties for maximum corrosion efficiency.^220–223

3.5.5 Controlled release coatings containing inhibitor-loaded nanocontainers

Controlled release and encapsulation of various materials within an inert host containing the active ingredient, such as drugs, oils, perfumes, and so on, have been studied for several decades.^224–226 Recent efforts have focused on developing encapsulated inhibitors via LbL, polyelectrolytes, copolymer vesicles and interfacial polymerization methods for corrosion inhibition on various metals and alloys.²²⁷ These inhibitor microcapsules can be formulated into coating systems for controlled release of the inhibitor triggered by corrosion to provide a “self-healing” mechanism.^228,229 The size of the microencapsulated inhibitor containers are normally too large (≥ 1 μm) to be effective in pretreatment, conversion, or multilayered coatings.

The development of “smart nanocontainers” offers a method by, which nanostructured materials can store the inhibitors, providing a new corrosion-inhibiting system based on a “passive matrix/active container structure” for use in pretreatment, conversion, or multilayered coating systems.²³⁰ Corrosion processes are normally accompanied by local pH changes on the surface of the metal or alloy. Nanocontainers placed near these surfaces can be activated via pH changes or mechanical damage to release the encapsulated inhibitor effectively inhibiting corrosion via a self-healing mechanism. There are numerous literature reports on nanocontainers based on cyclodextrins,²³¹ mesoporous silica,^232,233 halloysite clays,²³⁴ carbon nanotubes,²³⁵ layered double hydroxides,²³⁶and titanium dioxide,²³⁷ which have been synthesized and tested for their corrosion-inhibiting properties.¹¹²

3.5.6 Biofilms as pretreatment coatings

Corrosion control using beneficial biofilms has recently gained interest as a corrosion-inhibiting strategy that is promoted as an environmentally green and nontoxic alternative to current corrosion control methods.²³⁸ Biofilm formation is a natural process, which consists of a highly organized bacterial community with cells entrapped in a matrix formed by extracellular polymer.²³⁹ The formation of biofilms may impede or accelerate the corrosion process on metals or alloys.²⁴⁰ The factor that promotes corrosion is the bacteria colonization process on metal substrates. This colonization is nonuniform, producing colonies that are thinner or thicker resulting in anodic and cathodic regions promoting corrosion on the substrate. Additionally corrosion can be impeded if corrosion products formed by the biofilm produce passive layers.²⁴¹ The interaction between the metal substrate and biofilm formation depends solely on the type of metal and the degree of microbial activity. The inhibition of corrosion on various metals and alloys using biofilms can be accomplished via several strategies: (1) biofilms secreting antimicrobials,^113,242,243 (2) formation of protective layers,^244,245 (3) biofilms secreting corrosion inhibitors,^246,247 and (4) beneficial biofilms.^248,249