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A Screening-Level Life Cycle Assessment of Graphene-Polyetherimide Coatings Protecting Unalloyed Steel from Severe Atmospheric Corrosion Venkata Krishna Kumar Upadhyayula, David Edward Meyer, Venkataramana Gadhamshetty, and Nikhil Koratkar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b03005 • Publication Date (Web): 04 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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ACS Sustainable Chemistry & Engineering

A Screening-Level Life Cycle Assessment of Graphene-Polyetherimide Coatings Protecting Unalloyed Steel from Severe Atmospheric Corrosion Venkata K.K. Upadhyayula1, David E. Meyer2, Venkataramana Gadhamshetty3,4*, and Nikhil Koratkar5 1

Department of Chemistry (Environmental Chemistry), Umea University, 907 36 Umeå, Sweden

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United States Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio, USA.

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Civil and Environmental Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph Street, Rapid City, SD 57701, USA

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4

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Surface Engineering Research Center, South Dakota School of Mines and Technology, 501 E. St. Joseph Street, Rapid City, SD 57701, USA 5 Mechanical and Nuclear Engineering and Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA

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Corresponding Author Venkataramana Gadhamshetty, PhD Department of Civil and Environmental Engineering South Dakota School of Mines and Technology Rapid City, SDA Email: [email protected]

1 ACS Paragon Plus Environment


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Abstract

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A major concern for exposed steel in structural applications is susceptibility to atmospheric

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corrosion.

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environments into six zones, C1-C5 and CX, based on factors such as humidity, airborne salinity,

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and acidic pollutants. The C5 and CX zones are characterized by aggressive atmospheric

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corrosivity that results in mass losses from steel structures. Hot dipped galvanized (HDG) zinc

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coatings are typically used to protect steel in C5 and CX environments. HDG coatings suffer

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from disadvantages related to shorter service lives and the need for frequent maintenance cycles.

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Graphene-reinforced polyetherimide (PEI) coatings have been proposed as suitable alternatives

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to address these issues. However, general concerns regarding the implications of nanomaterials

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make it necessary to understand the potential environmental impacts of these coatings. A

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screening-level cradle-to-grave life cycle assessment is conducted to evaluate the environmental

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performance of a graphene-PEI-steel structure when compared with a traditional HDG-zinc-steel

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structure. Impact assessment scores are calculated using the Tool for the Reduction and

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Assessment of Environmental and other Potential impacts v2.1 and SimaPro (v8.0.3). When

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considering inventory uncertainty, the graphene-PEI-steel structure yields smaller potential

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impacts in five of the ten categories assessed when assuming the graphene-based coating

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requires no maintenance during the service life of the structure. Scenario-based sensitivity

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studies reveal that the potential impacts are highly sensitive to the service life and maintenance

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needs of the coating, but insensitive to the use of thermally or chemically functionalized

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graphene to improve coating adhesion. Further research is needed to understand the long-term

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performance of the graphene-based coatings and reduce the uncertainty of the inventory.

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Keywords: Corrosion, Graphene, Civil infrastructure, Life cycle analysis, Sustainability,

The

International

Organization

for

Standardization


classifies

atmospheric

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INTRODUCTION

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Atmospheric corrosion threatens a range of steel infrastructures, including bridges, steel-

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reinforced concrete, fluid pipelines, military equipment, and transportation utilities. For example,

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atmospheric corrosion can affect the safety and readiness of military equipment (e.g., an F-14

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aircraft crash due to corrosion of its landing gear)1, requiring the US to spend twenty billion

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dollars annually to protect military infrastructure from its effects1. The kinetics for atmospheric

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corrosion of metals and alloys is influenced by temperature, humidity, airborne salinity, and the

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concentration of sulfur dioxide (SO2), and to a lesser extent nitrogen oxides (NOx) in the

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atmosphere.2 Based on these parameters, the International Organization for Standardization

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(ISO) 9223:20123 standard defines “corrosive atmospheres” for metals and alloys for indoor and

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outdoor environments, and classifies them into six categories: C1, (very low); C2, (low); C3,

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(medium); C4, (high); C5, (very high); and CX, (extreme). For outdoor environments, a C5

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atmosphere (temperate, subtropical, and polluted climates) poses a very high risk for corrosion;

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whereas, a CX atmosphere (subtropical and tropical marine climates) is characterized as an

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extremely high risk3. C5 and CX are known for high concentrations of acidic pollutants such as

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SO2, air borne salinity, and severe condensation of moisture, all of which accelerate corrosion3.

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The steel industry currently uses a zinc coating as the primary method to protect mild,

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unalloyed, and low carbon steels4 against corrosion. The zinc coating offers a combination of

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mechanisms to inhibit atmospheric corrosion. It forms a hard, non-porous layer that serves as a

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physical barrier to stop corrosive agents (e.g. H2O, O2, and Cl) from contacting the steel surface

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(passive mechanism). Further, the coating impedes half-cell reactions through the creation of a

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Schottky barrier at the interface (active mechanism) and behaves like a sacrificial anode

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(cathodic protection).5 Zinc coatings are a logical choice for the steel industry because the



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numerous galvanization techniques available for their application make them suitable for a wide

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variety of products. For example, hot-dip galvanization (HDG) is a popular technique that

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enables complete immersion of steel in a bath of molten zinc at 449°C6 to uniformly coat the

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exterior and interior surfaces of a complex geometrical structure while cold galvanization (CG)

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is a more simplistic technique that can be used to paint multiple layers of zinc onto larger steel

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surfaces.7

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The hot dip galvanization of steel can be operated either in a continuous or batch mode.8

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For hot and cold rolled steel coils used in applications such as automotive body panels, HVAC

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ducts, a continuous HDG process integrated within steel manufacturing plants is used. For semi-

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finished and finished steel structures (e.g. steel beams, bars, and grates), a batch galvanizing

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technique (a.k.a fabrication galvanizing) is preferred as it offers thicker zinc coatings for the steel

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structures.8 CG is beneficial when the HDG process is impracticable due to either high cost or

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when the steel structures are characterized by complex geometries. However, the steel exposed

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to C5 and CX requires a frequent maintenance recoating, resulting in an increased zinc

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consumption. Currently, 58% of global zinc production (~12 million tons) is used annually for

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galvanization, making the steel industry vulnerable to anticipated zinc shortages and fluctuating

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prices in coming decades4. Recent studies have proposed the use of graphene and graphene-oxide (GO) composites4,

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9-13

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unfunctionalized graphene (UFG) can be reinforced into conducting polymers such as

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polyetherimide (PEI)) to obtain UFG-PEI coatings. These coatings were reported to enhance the

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as viable alternatives to zinc coatings. Akhtar14 and Dennis et al.4 have shown that

corrosion resistance of steel by nearly 45 times when compared to an HDG Zinc coating.4


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The literature suggests that conducting polymers (CP) such as PEI offer corrosion

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protection to metals through the following four different mechanisms.15 CP coatings facilitate

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formation of protective metal-oxide layers on metal surface (anodic protection). CP coatings can

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also alter path of oxygen and moisture and restrict their transport to metal surface (barrier effect).

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Inhibitory compounds are doped into CP coatings that are eventually released when external

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environment favors defects in coatings (controlled inhibitor mechanism). Dopants (e.g.

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graphene) can be added to CP coatings to form a semiconducting layer on metal surface to

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restrict flow of free electrons from metal surface to oxidizing species in an external environment

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(charge depletion).

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The corrosion protection mechanism of UFG-PEI is shown as a schematic in Figure 1.

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PEI films have been found to be effective in corrosion applications due to their high mechanical

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strength, enhanced moisture barrier properties, and high glass transition temperature.14 The PEI

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polymer matrix in UFG-PEI can serve as an anode by participating in corrosion reactions and

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impeding half-cell reactions involved in steel corrosion (active protection).

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UFG-PEI serves as a pigment and enhances its adhesion with the steel substrate through pi

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electron interactions. The sp2 carbon allotropes in graphene provide a barrier against aggressive

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environments by providing tortuous paths to permeating molecules (O2 and H2O) (passive

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protection)23.


16-22

Graphene in


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Polyether Imide (PEI)

Graphene (UFG)

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   

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High Molecular Weight Barrier to oxygen, moisture High glass transition temperature Electroactive Conjugate polymer (Anodic protection)

 π-π interactions  Impermeable to O2 and H2O  Strength

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PASSIVE PROTECTION

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H2O/O2

UFG-PEI coating increases tortuosity and forms impermeable barrier to H2O and O2

Impermeable UFG-PEI Steel

ACTIVE PROTECTION Semiconducting, protectiveoxide passivation layer at interface is charge depletable

UFG-PEI Steel

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Figure 1. Modes of corrosion protection mechanism offered by UFG-PEI coating (Based on the description provided in open literature.4, 14, 15, 24 )

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number of relevant patents.9, 25-29 However, it necessary to understand the potential life cycle

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impacts30 of these coatings in order to identify any potential issues with the environmental

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sustainability of the nanomaterial-based treatment alternative. Similar to reported concerns for

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other nanomaterials31-38, graphene coatings should be scrutinized with this regard to understand

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their potential trade-offs.

The motivation to commercialize graphene coatings is evident from the increasing


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The objective of this study is to evaluate and compare the environmental performance

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of a UFG-PEI coating with a commercial zinc coating used in the steel industry. Although a

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range of graphene composites have been demonstrated as corrosion-resistant coatings, the UFG-

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reinforced PEI coating was selected as the model graphene coating in this LCA study for the

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following reasons. The PEI polymers have been found to be resistant to SO2, UV and

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atmospheric ozone.39-42 Reinforcement of the polymer with graphene has been reported to

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significantly reduce the effects of weathering.43 The graphene manufacturing technique

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(solution-phase exfoliation) described in this work4, 31 is scalable for estimating mass production

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of UFG-PEI coatings. The availability of accelerated corrosion test results from Akhtar14 and

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Dennis et al.4 can provide guidance for estimating system performance.

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LIFE CYCLE ASSESSMENT METHODOLOGY

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Overview of the UFG-PEI Coating. Figure 2 shows the cradle-to-grave life cycle stages

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for protecting an unalloyed steel surface with UFG-PEI. The life cycle is divided into eight

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processes: (1) production of hot rolled steel coils; (2) cold rolling of hot rolled coils; (3) surface

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preparation including degreasing to remove oil impurities and dirt and shot blasting; (4)

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synthesis of a UFG- Poly(amic acid) (PAA, a precursor of PEI) solution; (5) roller coating UFG-

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PAA on the steel surface; (6) thermal curing to form the UFG-PEI coating; and (7) the use of

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steel sheet in a field site where C5(I) atmospheric corrosivity conditions prevail; and (8) end of

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life treatment of the UFG-PEI coated steel scrap.

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The UFG-PEI synthesis is based on the procedure suggested in literature.4, 14, 44, 45 The

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PAA precursor is synthesized by reacting equimolar quantities of bisphenol dianhydride

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(BPADA) and m-phenylenediamine (mPDA) in the presence of N-methyl-2-pyrollidone (NMP)



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solvent. The solids content of this resulting mixture is assumed as 20 wt%. The UFG is obtained

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separately by exfoliating graphite (5 g/l) in NMP using an ultrasonic technique46. The UFG-NMP

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is mixed with the viscous PAA precipitate and the resulting solution is coated on unalloyed steel

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using roller coating technique.

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Figure 2. Block diagram representing the cradle-to-grave life cycle of a steel sheet coated with UFG-PEI.


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The coated steel is thermally cured to convert UFG-PAA into UFG-PEI through in situ

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imidization. During curing, the volatile NMP is evaporated and collected in the exhaust fumes

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and incinerated.

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Goal and Scope Definition. The screening-level LCA work in this study is performed

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following the recommended practices of the ISO 14040:2006 and 14044:2006 LCA standards47.

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This is a screening-level LCA because it relies on scale-up assumptions and secondary data,

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including patents, journal articles, and existing LCI databases to develop the inventory. The lack

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of primary or measured data increases the uncertainty of the model and limits the interpretation

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and conclusions that should be drawn from the impact results. The goal of the LCA is to quantify

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the cradle-to-grave environmental performance of a UFG-PEI coating used to protect unalloyed

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steel from C5 (I) atmospheric corrosivity conditions. These results are contrasted with a zinc

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coating obtained with the widely used hot-dip galvanization (HDG) process. The geographical

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boundary of the study is the Unites States (U.S.). The typical practice followed by the steel

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industry is to apply the initial treatment of the corrosion-resistant coating prior to installation and

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subsequently perform recoating maintenance at predefined intervals within the expected service.

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Figure S1 shows the decision making process used to identify the recoating requirements (See

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Supporting Information (SI)).

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Functional Unit. The equivalent functional unit (FU) is a rectangular unalloyed steel sheet

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with dimensions of 12 mm (thickness) x 180 mm (width) x 6 meters (length) and a mass of 102

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kg. The total surface requiring protection from atmospheric corrosion is 2.31 m2. The specified

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steel sheet is available as a commercial product.48 The selected coatings are intended to provide

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corrosion resistance during the steel’s sixty-year design life.



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Life Cycle Inventory (LCI) Data for UFG-PEI Coatings. The SimaPro LCA software

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package (version 8.0.3)29 was used to build the life cycle model. The foreground LCI data

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pertaining to the life cycle stages of the UFG-PEI system were obtained from academic and

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patent literature. The background data were obtained from the World Steel Association (the LCI

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data for hot rolled steel plate including end of life credits)49, US LCI database50 maintained by

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the National Renewable Energy Laboratory (NREL), and the ecoinvent database (v.3.2) after

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modifying for U.S. conditions (i.e. the energy consumption was calculated according to the U.S.

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national grid mix).51 Key parameters and major assumptions made while compiling the UFG-PEI

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LCI are summarized and presented in Table 1.

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Table 1. Key parameters and assumptions when modeling the UFG-PEI coating system Life Cycle Processes Major Assumptions Made & Data Sources Used (1) Production of hot rolled steel and end of life recycling of UFG-coated steel scrap

 LCI data for a flat steel sheet rolled on a hot mill as supplied by the World Steel Association.49 This dataset is cradle-to-gate, but includes recycling credits for the end-of-life scrap.  The recycling rate of steel scrap at the end of life is 85% (according to the world steel association52)

(2) Sheet Rolling of Steel (Cold Rolling) (3) Surface preparation & shot blasting stages

 Ecoinvent dataset51 - sheet rolling of steel

(4) Synthesis of UFG-PAA coated solution

UFG Synthesis  Graphite is exfoliated in NMP solvent (5 g/L) using ultrasonication.46  Energy consumption during UFG preparation is obtained from Arviddson et al.31 PAA  Prepared from equimolar ratios of m-phenylenediamine (mPDA) and phthalic anhydride of bisphenol A (BPADA) using a method suggested in literature.44, 45 UFG-PAA Solution  UFG in PAA = 5 wt.%  UFG-PAA in NMP solvent = 20 wt.%

(5) Roller coating of UFGPAA solution on clean steel surface

  

 Ecoinvent dataset51 - degreasing metal part in alkaline bath  LCI data for shot blasting is obtained from Peng et al.53

LCI data for roller coating is obtained from industrial literature.54 Transfer efficiency of roller coaters is 83%.55 Infrastructure potential impacts of the metal coating facility from ecoinvent51 are included.


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(6) Thermal curing of UFGPAA solution & in-situ imidization to form UFG-PEI on steel surface

Dry Film Thickness of UFG-PEI  The coating thickness is 20 µm.4, 14, 56  Imidization yield of PAA to PEI is 100%  LCI for thermal curing is obtained from industrial literature.57  Exhaust fumes during curing are incinerated.57

(7) Use Stage of UFG-PEI coated steel substrate

  

The transportation distance from the workshop to the field site is 100 km (one-way). Annual corrosion rate for the UFG-PEI coated steel sheet is 0.846 µm/year. This is based on the experimental data reported by Dennis et al.4 Thickness of the steel sheet is 12 mm. Maximum allowance of thickness loss due to corrosion (5% red rust formation) before the time to first maintenance = 12 mm*0.05 = 600 µm. The thickness lost due to corrosion of UFG-PEI coated steel sheet = (0.846 µm/yr)*(60 yrs) = 50.76 µm. Hence, no maintenance recoating is required.

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Assumptions for Conventional Galvanized Zinc Coatings. A detailed description of

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the life cycle of a steel sheet coated with HDG zinc is provided in Section S3 of the SI. The key

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parameters and major assumptions for this system are summarized in Table 2.

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Table 2. Key parameters and major assumption related to the life cycle of a steel sheet coated with HDG Zinc Key Modeling Parameter

Assumptions

Coating thickness and service life

 The HDG coating has a thickness of 130 µm  The service life for this thickness in a C5(I) atmosphere is 15 years.2, 58-60

Time to first maintenance and maintenance recoating

 Maintenance activities after the fifteenth and forty-fifth years involve onsite touch-up of 5% of the total surface area (0.115 m2 per touchup) with zinc-rich paint.  Maintenance after the thirtieth year includes decommissioning from the field site, transportation to a workshop, regalvanization, transportation back to the field, and reinstallation.  The transportation distance from the workshop to the field site is 100 km (one way).  A total of 14% of the steel surface area is lost as rust (due to atmospheric corrosion) over 60 years of design life (either localized or uniformly spread).  The total mass loss of steel as rust after 60 years 14.28 kg.

Mass loss of a steel due to corrosion during the use stage of HDG coated steel sheet



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Dezincing of galvanized steel scrap at the end of life

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 The steel sheet is removed from service and sent to a dezincing facility for chemical recovery and recycle of the zinc.  A credit is assigned to the recovered zinc for avoided primary production of zinc.  LCI for the dezincing process is consistent with the study performed by Viklund-White.61  The dezinced steel scrap follows the end-of-life steel management practice specified by the world steel association.52, 61 However the recycling rate of dezinced steel scrap at the end of life is adjusted to 71% after discounting the amount of steel lost (14.28 kg) due to atmospheric corrosion during use stage

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Life Cycle Impact Assessment. The Tool for the Reduction and Assessment of

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Chemical and other environmental impacts (TRACI 2.1)62, 63 was used to assess the ten impact

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categories: global warming potential (GWP, kg CO2 eq.); acidification potential (AP, kg SO2

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eq.); potential human health impacts due to carcinogens (HH-CP) and non-carcinogens (HH-

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NCP) as CTUh; respiratory effects potential (REP, kg PM2.5 eq); eutrophication potential (EP, kg

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N eq); ozone depletion potential (ODP kg CFC-11 eq); potential for ecotoxicity (ETXP CTUe);

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smog formation potential (SFP kg O3 eq); and fossil fuel depletion potential (FFDP MJ surplus).

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Given the intent of this LCA is to investigate the potential impacts of UFG-PEI within a U.S.

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context, TRACI is the most appropriate impact method because it is a North American mid-point

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impact assessment method developed by the U.S. Environmental Protection Agency (EPA)62.

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TRACI 2.1 incorporates the USEtox environmental model for characterization of potential

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human health and ecotoxicity impacts.64, 65 It is important to note that the release of graphene and

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its subsequent exposure to both humans and species in the environment was not modeled in this

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study because of a lack of material flow tracking data describing releases for inventory modeling

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and characterization factors to enable use of such data in the impact models


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Scenario Analysis and Sensitivity. A scenario-based sensitivity analysis was performed

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to study the effects of changes in the key modeling parameters on the potential environmental

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impacts of the UFG-PEI coating. We examined the following two scenarios: (i) the effect of

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reducing the UFG-PEI coating’s service life, and (ii) the effect of replacing UFG with

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functionalized graphene during the preparation of the UFG-PEI solution.

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A key assumption in this study is the sixty-year service life of the UFG-PEI coating. The

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corrosion rates for steel coated with UFG-PEI were based on the studies using immersion

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corrosion testing with 3.5% NaCl solution. These studies may not provide a true representation

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of atmospheric corrosion as they do not take into account the effects of atmospheric factors

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related to wet and dry cycles of climate, air pollutants (e.g., SO2 and salinity), and ozone and UV

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radiation. Some researchers have concluded that immersion corrosion tests simulate more

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aggressive exposure conditions when compared to field conditions and the predicted corrosion

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rates are higher than the actual rates.66, 67 However, there is a lack of consensus supporting this

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conclusion, with other researchers concluding that it is difficult to establish a true correlation

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between predicted and observed corrosion rates. We addressed this uncertainty by considering

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three different scenarios representing one, two and three cycles of maintenance. A maintenance

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cycle includes the following steps: (i) remove the steel from service, (ii) transport it to a

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workshop for degreasing the surface followed by an application of a 20-m coat of UFG-PEI,

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(iii) transport the coated steel to the field for reinstallation. The maintenance cycles occur at

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uniform intervals (i.e., thirty years for one cycle, twenty years for two cycles, and fifteen years

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for three cycles). The mass of steel lost during an interval due to rust formation was considered

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similar to the HDG system.



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The second scenario was examined to study the sensitivity of potential environmental

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impacts when UFG is replaced with functionalized graphene during the preparation of the UFG-

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PEI solution. Functionalization of graphene can be important for two reasons. First, a poor

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interfacial bonding between the coating and metal surface can result in the premature failure of

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the coating exposed to atmospheric corrosion. Thermal or chemical methods can be used to

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obtain functionalized graphene that can be reinforced within the conducting polymer matrix.68

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This process can improve the adhesive properties of the UFG-PEI coating and may ensure a

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sixty-year service life. With regard to solution preparation, loading graphene in a functionalized

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form enables better dispersion in the polymer matrix and decreases agglomeration.56 Although

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functionalization normally requires additional material and energy, the enhanced dispersive

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properties can reduce the graphene loading requirements (5-20 wt% with UFG compared to 1

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wt.% with functionalized graphene),24 enhancing the overall environmental performance of

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graphene-PEI coatings. This study considered the scenario analysis with both thermally

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functionalized graphene (i.e. oxidation and thermal expansion of graphite oxide) (TFG) and

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chemically functionalized graphene (CFG). The LCI data for TFG and CFG were constructed

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based on synthesis procedures described in the open literature.56, 69-71 The modeling parameters

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and assumptions were identical to the UFG-PEI system except that the graphene loading in TFG

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or CFG is reduced to 1 wt%. Details about the LCI data used for the synthesis of TFG and CFG

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are shown in Section S4 of the SI.

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Uncertainty Analysis. In addition to the scenario-based sensitivity analysis, uncertainty

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analysis of the UFG-PEI system inventory was performed in SimaPro using Monte Carlo

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simulation with 5000 steps and a 95% level of confidence. The full uncertainty of the impact

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scores was not quantified because the uncertainty of the impact models could not be included


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based on a lack of necessary data describing distributions for the characterization factors.

272

Background LCI from ecoinvent already included standard deviation estimates for most data.

273

The inventory uncertainty in the foreground data was mainly associated with the preparation of

274

the UFG-PAA coating solution and the thermal curing of the UFG-PEI. Ranges were established

275

for key quantities in the inventory using reported experimental results. For example, the reported

276

concentration of UFG in the UFG-PAA solution ranged from 514 to 20 wt%4, and the thickness

277

of the applied coating varied between 15 and 30 µm.72 Energy use during surface preparation of

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the steel sheet was bounded between the baseline scenario value and the maximum reported

279

value73 of 3 kWh/m2 for cases when sand blasting is used instead of shot blasting. The oven

280

curing stage also introduces uncertainty with respect to energy consumption. Thermal curing can

281

be replaced with an energy-efficient, UV curing process that results in lower VOC emissions.57

282

Energy use and emission ranges during curing were assigned to the inventory to reflect curing

283

uncertainty. To support comparative interpretation of the results the same uncertainty analysis

284

was applied to the HDG-coated steel sheet.

285 286

RESULTS AND DISCUSSION

287

When compared with HDG zinc, the UFG-PEI coating exhibits lower environmental

288

potential impacts in all the ten impact categories (Figure S5). The UFG-PEI potential impacts are

289

93% smaller for HH-NCP; 68% smaller for AP; 57% smaller for ETXP; 45-48% smaller for

290

REP & EP; 38% smaller for HH-CP and ODP; 32% smaller for SFP; 21% smaller for GWP; and

291

only 10% smaller for FFDP. These observed differences are mainly related to the actual coating

292

and maintenance of the steel sheet. To better understand these differences, a comparative

293

assessment of the two systems while excluding steel processing is reported in Figure S6.



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Interestingly, the potential impacts attributed to the steel (not shown) for HDG zinc are 15%

295

higher than UFG-PEI because the metal losses from rusting implies that less steel scrap is

296

available for recycling at the end of life. The use stage results in a loss of 14.28 kg of steel due to

297

the atmospheric corrosion. The recycling rate of steel scrap at end of the life is adjusted to 71%

298

(instead of 85%) after discounting 14.28 kg of the steel lost during the use stage of HDG coated

299

steel sheet (See Section S3.3 and S3.4).

300

Contribution Analysis. The performance of the UFG-PEI steel was further analyzed

301

using contribution analysis to identify hotspots for potential improvement (Figure 3). The

302

potential impacts of steel preparation contribute to at least 50-65% of GWP, AP, SFP and FFDP,

303

with the majority of the remaining 35% coming from cold rolling and the UFG-PAA coating

304

solution. Cold rolling is the dominant contributor to HH-CP and ETXP at 93% and 81%,

305

respectively. EP is primarily driven by the UFG-PAA coating solution (55%) and sheet rolling

306

(46%). For the coating solution, both NMP solvent and BPADA are known to directly cause

307

eutrophication in water bodies74-76 while the nitrogenous waste released from the upstream

308

production of m-PDA also induces eutrophication.77 Similarly, cold rolling and the coating

309

solution account for a combined 83% of ODP and 80% of REP. The UFG-PAA coating solution

310

contributes 41% of ODP because of electricity consumption during the production of UFG and

311

the use of NMP solvent during synthesis. The same sources are the drivers for the contributions

312

of UFG-PAA to REP, as well as FFD. HH-NC is more equally distributed across steel

313

preparation, cold rolling, and the coating solution, with each contributing around 30%. It should

314

be noted that solvent fumes during curing were assumed to be incinerated as opposed to being

315

collected and recycled, which could reduce the NMP-related potential impacts.


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316 317 318 319 320 321


Figure 3. Contribution analysis showing potential impacts of the various life cycle stages of the UFG-PEI steel system. The highest score in each impact category is listed along the top. Sensitivity analysis. Results for the sensitivity analysis of service life are shown in Figure

322

4. Even if three maintenance cycles are applied to the UFG-PEI coating, the HDG Zinc coating

323

still has the highest impact scores in HH-CP, HH-NCP, ETXP and AP, which should be

324

expected. The metal fumes of zinc oxide and particulate dust released from galvanizing plants

325

causes carcinogenic and non carcinogenic (e.g. metal fume fever) effects in humans.78 The ETXP

326

impact is the highest due to the potential release and surface runoff of zinc from galvanized steel

327

structures during their usage.79, 80 The larger AP impact originates from the acid pretreatment

328

associated with the HDG process.

329

As opposed to the baseline “No Maintenance” scenario, the potential impacts of the UFG-

330

PEI-coated steel with three maintenance cycles are the largest for GWP, FFDP, SFP, ODP, EP,



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and REP. With two maintenance cycles, the GWP, FFDP, ODP, and EP potential impacts of

332

UFG-PEI remain larger than HDG Zinc. However, the SFP of the two systems is approximately

333

equal and the REP impact of the UFG-PEI system is reduced below that of HDG Zinc. For one

334

maintenance cycle, which is equivalent to the single full recoating modeled for the HDG Zinc

335

system at 30 years, the two systems have nearly identical scores in GWP and ODP and the UFG-

336

PEI system has lower potential impacts in all other categories except FFDP. The ODP and EP

337

potential impacts increase by 21% for each additional maintenance cycle, which is consistent

338

with the increased consumption of NMP solvent, BPADA and mPDA required for the additional

339

coatings. The scores for REP and FFDP increase 17-18% per additional maintenance cycle

340

through the combined effect of decreasing the amount of steel for recycling after corrosive losses

341

and increasing the consumption of NMP solvent to produce more coating. Increasing the

342

required amount of coating also increases electricity consumption during the synthesis of UFG,

343

but this only has a minor effect on the potential impacts because UFG is a mere 5 wt.% of the

344

coating solution. The effects of electricity consumption may become more significant if the

345

weight percent of UFG is increased. GWP and SFP increase by 13% per maintenance cycle for

346

similar reasons. These results suggest solvent recovery and reuse should be considered as a

347

means to improve the environmental performance of UFG-PEI coatings.


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348 349 350 351 352 353 354


Figure 4: The potential impacts of UFG-PEI steel compared with HDG zinc steel when one, two, and three cycles of maintenance are included for UFG-PEI. The UFG-PEI performance under the three scenarios is compared to the HDG coating with a 15-year lifespan. Results for the sensitivity analysis of functionalization are shown in Figure 5. Despite the

355

reduction of graphene loading to 1 wt% for functionalized materials, the potential impacts are

356

not sensitive to this change based on the near identical profiles in the plots. Given graphene

357

accounts for less than 20% of the coating solution preparation impacts, the lack of sensitivity is

358

reasonable. This is especially true if one considers that as the weight percent of graphene reduces

359

the consumption of BPADA and mPDA increases correspondingly and counteracts the potential

360

benefit. The lone exception when comparing the polymer systems is that the functionalized

361

options (TFG-PEI and CFG-PEI) reduce REP by 14%. This is mainly attributed to higher

362

consumption of solvent by the UFG-PEI-steel system for exfoliation. One limitation of this

363

analysis is that the existing data does not describe how functionalization can affect key variables



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such as service life and corrosion rate. If functionalization extends service life or reduces

365

corrosive losses, the sensitivity of the impact results may be much different. Further research is

366

needed to establish this data.

367 368 369 370 371

Figure 5. The potential impacts of functionalized (TFG-PEI and CFG-PEI) and unfunctionalized (UFG-PEI) polymer-coated steel compared with HDG Zinc-coated steel. Uncertainty Analysis. The comparison of the internally normalized impact scores for the

372

baseline UFG-PEI and HDG zinc coatings when considering uncertainty are shown in Figure 6.

373

The error bars correspond to 95% confidence intervals and +/- 1 Standard deviation (SD) around

374

the mean. For UFG-PEI, EP and ODP vary by 28% and 18%, respectively. The other impact

375

categories except for HH-CP, which has an uncertainty range of 39%, have relatively minor

376

uncertainty ranges of less than 10%. For HDG zinc, the ranges of uncertainty for the potential

377

impacts can be substantial: 79% for HH-NCP; 61% for ETXP; 53% for HH-CP; 48% for EP;

378

24% for ODP; 16% for AP, SFP and REP; and less than 10% for GWP and FFDP. Based on the


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379

overlap of uncertainty ranges, the only meaningful differences in potential impacts are for GWP,

380

AP, HH-NCP, REP, and SFP. For the other impact categories, it will be necessary to refine the

381

inventories and reduce the uncertainties before drawing any definitive conclusions regarding the

382

comparative performance of the two coatings performs.

383

384 385 386 387 388

Figure 6. A comparison of the internally normalized impact scores for a steel sheet coated with either UFG-PEI or HDG Zinc when considering inventory uncertainty. This study suggests UFG-PEI coatings may yield reduced environmental impacts for

389

protecting steel substrates from atmospheric corrosion when compared to the HDG coatings.

390

However, several technological and modeling challenges must be resolved to validate these

391

findings. The LCA performance of the UFG-PEI coatings should be evaluated using the

392

corrosion rates determined under actual field conditions as opposed to the immersion tests used

393

in this study. The values for the corrosion rates affect the service life and the corresponding LCA



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394

performance of the UFG-PEI coating. A decreased service life of the UFG-PEI coating results in

395

a potential tradeoff (as discussed in the scenario based sensitivity analysis) under the FFDP,

396

ODP, EP, and GWP categories. The FFDP impacts for the UFG-PEI surpasses HDG by 11%

397

when the service life of the former is decreased to 30 years; by 28% when the service life is

398

decreased to 20 years; and by 45% when decreased to 15 years. When the service life of the

399

UFG-PEI coatings is decreased to 20 years, the UFG-PEI has 19% & 40% higher potential

400

impacts for ODP category. When the service life is decreased to 15 years, the UFG-PEI coatings

401

exhibits 14% and 35% higher potential impacts for EP category; and 10% and 23% higher

402

potential GWP impacts. The potential SFP and RE impacts increased by only 14% and 19%

403

when the service life was decreased to 15 years. The remaining four impacts (AP, HH-C, HH-

404

NC and ETXP) remained insensitive to the service life of UFG-PEI coating.

405

It is important to evaluate potential risks for a premature failure of the coatings when

406

exposed to excessive levels of UV radiation, atmospheric ozone, and humidity. A recent

407

tribological study conducted by Dong and Qi81 has determined that friction and wearing of

408

polymers can be lowered by the addition of graphene. Similar studies are required to resolve

409

uncertainty of the inventory related to the service life and coating properties (e.g., thickness and

410

composition) of the UFG-PEI coatings.

411

It is also important to minimize modeling uncertainties that influence the quantifiable

412

values for the ecotoxicity and human health impacts of UFG-PEI coatings. The impact

413

characterization factors (CFs) that would enable inclusion of graphene materials for this study

414

would depend on two elements: (a) the fate & transport of graphene materials released across the

415

lifecycle of the UFG-PEI coated steel (specifically during the use stage); and (b) the cytotoxic

416

effects of the graphene materials when they interact with biological systems. To the best of


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417

authors knowledge, only one study82 has reported an aquatic ecotoxicity CF for graphene-based

418

nanomaterials. This study focused on graphene oxide (GO) and provided the three key insights.

419

First, GO behaves as a colloidal particle in aquatic. Second, bioaccumulation of GO

420

nanoparticles themselves is negligible. However some studies have implied GO nanoparticles as

421

carriers of persistent organic pollutants and enhance their bioaccumulation potential;83 Third, a

422

large uncertainty is associated with the effect factor because information on toxicity of graphene

423

materials is sparse. The resulting aquatic ecotoxicity CFs reported for GO nanoparticles were

424

found to vary by a large magnitude and additional data points would be needed to resolve the

425

issue for precise impact assessment. Furthermore the CFs reported by Deng et al.82 have been

426

explicitly developed for GO particles and they cannot be translated to the UFG-PEI coatings

427

considered in this study.

428

The toxicology of nanomaterials from the graphene family is yet to be thoroughly

429

understood. However, few studies conclude that graphene materials (graphene, GO, and reduced-

430

GO) in their unmodified form are more potent in inducing cytotoxic and genotoxic effects in

431

living cells compared to functionalized (surface modified) graphene materials.84,

432

study has reported that graphene nanoparticles can accumulate in tracheobronchial airways and

433

cause lung complications in human beings.86 Another study confirmed that graphene materials

434

can induce cytotoxic effects that are similar to carbon nanotubes (CNTs) in terms of shape and

435

concentration dependency, but that the cytotoxic effect itself is relatively lower than CNTs.87 ,

436

Graphene materials have also been reported to induce genotoxic effects (e.g., chromosomal

437

aberrations and fragmentation of DNA in stem cells) even at a low concentration.88 The recent

438

literature sheds light on toxic effects of graphene on other species. For example, graphene has

439

been reported to exhibit toxicological effects on Chlorella pyrenoidosa due to oxidative stress


85

A recent


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Page 24 of 31

440

and physical penetration. Compared to CNTs, graphene has been reported to induce severe toxic

441

effects to Chlorella pyrenoidosa.89 However, several other studies30,

442

graphene does not induce any toxicity effects on several soil-borne species (e.g., Caenorhabditis

443

elegans) and water-borne species (e.g., zebrafish embryos and Artemia salina crustaceans) even

444

at higher concentrations (1 mg/l). Although there is no consensus regarding the toxic effects of

445

graphene-based materials, many researcher can agree that the toxicity is dose dependent85, and

446

that factors such as size, shape, degree of agglomeration, presence of functional groups on

447

surface etc.84, 85, 88 determine the level of interactions with biological cells and ultimately decide

448

its potency effect.

90

have reported that

449

Once all of this knowledge is available, the behavior of emerging graphene coatings during

450

the course of steel scrap recycling and end-of-life treatment processes, particularly in an electric

451

arc furnace (EAF), will need to be addressed. Recently, some groups studied the effects of

452

incinerating carbon nanomaterial laden waste.91, 92 The studies concluded that the incineration of

453

waste containing carbon nanomaterials in the combustion zone might aggravate formation of

454

hazardous pollutants such as polyaromatic hydrocarbons (PAHs), chlorinated dioxins and furans.

455

The operational temperature of an EAF is higher than incinerators, but the graphene

456

nanomaterials are expected to persist in the combustion zone. Just as the carbon nanomaterials

457

may end up in the ash after incineration and be contained by pollution control equipment such as

458

an electrostatic precipitator (ESP) or cyclone separator91,

459

accumulate in the ash if the steel scrap and residual UFG-PEI are treated in an EAF. Thus,

460

further studies are warranted to investigate the behavior of graphene-laden EAF dust subjected to

461

landfilling.

92

, graphene nanomaterials might


Page 25 of 31

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462

Supporting information includes details on acronyms and terminology, global assumptions for

463

LCA modeling, process descriptions and flow charts, material-specific assumptions, recoat-

464

maintenance calculations, and detailed life cycle inventories for the six protection systems.

465 466

Acknowledgements

467

Gadhamshetty acknowledges the funding support from National Science Foundation CAREER

468

Award (#1454102), NASA (NNX16AQ98A) and the South Dakota Board of Regents under

469

the auspices of the Surface Engineering Research Center (SERC).

470 471

Disclaimer

472

The views expressed in this article are those of the authors and do not necessarily represent the

473

views or policies of the US Environmental Protection Agency.

474 475

References

476 477 478 479 480 481 482 483 484 485 486 487 488 489

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A Screening-Level Life Cycle Assessment of Graphene-Polyetherimide Coatings Protecting Unalloyed Steel from Severe Atmospheric Corrosion

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Venkata K.K. Upadhyayula1, David E. Meyer2, Venkataramana Gadhamshetty3,4*, and Nikhil Koratkar5

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Synopsis: Graphene composites exhibit potentially enhanced sustainability impacts for protecting steel infrastructure against atmospheric corrosion when compared to zinc coatings.

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