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Research Article pubs.acs.org/journal/ascecg

Screening-Level Life Cycle Assessment of Graphene-Poly(ether imide) Coatings Protecting Unalloyed Steel from Severe Atmospheric Corrosion Venkata K. K. Upadhyayula,† David E. Meyer,‡ Venkataramana Gadhamshetty,*,§,∥ and Nikhil Koratkar⊥ †

Department of Chemistry (Environmental Chemistry), Umea University, 907 36 Umeå, Sweden United States Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, United States § Civil and Environmental Engineering, and ∥Surface Engineering Research Center, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701, United States ⊥ Mechanical and Nuclear Engineering and Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States

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‡

S Supporting Information *

ABSTRACT: A major concern for exposed steel in structural applications is susceptibility to atmospheric corrosion. The International Organization for Standardization classifies atmospheric environments into six zones, C1−C5 and CX, based on factors such as humidity, airborne salinity, and acidic pollutants. The C5 and CX zones are characterized by aggressive atmospheric corrosivity that results in mass losses from steel structures. hot-dipped galvanized (HDG) zinc coatings are typically used to protect steel in C5 and CX environments. HDG coatings suffer from disadvantages related to shorter service lives and the need for frequent maintenance cycles. Graphene-reinforced poly(ether imide) (PEI) coatings have been proposed as suitable alternatives to address these issues. However, general concerns regarding the implications of nanomaterials make it necessary to understand the potential environmental impacts of these coatings. A screening-level cradle-to-grave life cycle assessment is conducted to evaluate the environmental performance of a graphene-PEI-steel structure when compared with a traditional HDG-zinc-steel structure. Impact assessment scores are calculated using the Tool for the Reduction and Assessment of Environmental and other Potential impacts v2.1 and SimaPro (v8.0.3). When considering inventory uncertainty, the graphene-PEI-steel structure yields smaller potential impacts in five of the ten categories assessed when assuming the graphene-based coating requires no maintenance during the service life of the structure. Scenario-based sensitivity studies reveal that the potential impacts are highly sensitive to the service life and maintenance needs of the coating, but insensitive to the use of thermally or chemically functionalized graphene to improve coating adhesion. Further research is needed to understand the long-term performance of the graphenebased coatings and reduce the uncertainty of the inventory. KEYWORDS: Corrosion, Graphene, Civil infrastructure, Life cycle analysis, Sustainability

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atmosphere.2 Based on these parameters, the International Organization for Standardization (ISO) 9223:20123 standard defines “corrosive atmospheres” for metals and alloys for indoor and outdoor environments, and classifies them into six categories: C1 (very low); C2 (low); C3 (medium); C4 (high); C5 (very high); and CX (extreme). For outdoor environments, a C5 atm (temperate, subtropical, and polluted climates) poses a very high risk for corrosion, whereas a CX

INTRODUCTION

Atmospheric corrosion threatens a range of steel infrastructures, including bridges, steel-reinforced concrete, fluid pipelines, military equipment, and transportation utilities. For example, atmospheric corrosion can affect the safety and readiness of military equipment (e.g., an F-14 aircraft crash due to corrosion of its landing gear),1 requiring the U.S. to spend 20 billion dollars annually to protect military infrastructure from its effects.1 The kinetics for atmospheric corrosion of metals and alloys is influenced by temperature, humidity, airborne salinity, and the concentration of sulfur dioxide (SO2), and to a lesser extent nitrogen oxides (NOx), in the © 2017 American Chemical Society

Received: December 10, 2016 Revised: January 25, 2017 Published: February 4, 2017 2656

DOI: 10.1021/acssuschemeng.6b03005 ACS Sustainable Chem. Eng. 2017, 5, 2656−2667

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ACS Sustainable Chemistry & Engineering

Figure 1. Modes of corrosion protection mechanism offered by UFG-PEI coating (based on the description provided in the open literature4,15,16,24).

used in applications such as automotive body panels and HVAC ducts, a continuous HDG process integrated within steel manufacturing plants is used. For semifinished and finished steel structures (e.g., steel beams, bars, and grates), a batch galvanizing technique (a.k.a fabrication galvanizing) is preferred, as it offers thicker zinc coatings for the steel structures.8 CG is beneficial when the HDG process is impracticable due to either high cost or when the steel structures are characterized by complex geometries. However, the steel exposed to C5 and CX requires a frequent maintenance recoating, resulting in an increased zinc consumption. Currently, 58% of global zinc production (∼12 million tons) is used annually for galvanization, making the steel industry vulnerable to anticipated zinc shortages and fluctuating prices in coming decades.4 Recent studies have proposed the use of graphene and graphene-oxide (GO) composites4,9−13 as viable alternatives to zinc coatings. Graphene coatings have also been proposed for aggressive microbial conditions.13,14 Akhtar15 and Dennis et al.4 have shown that unfunctionalized graphene (UFG) can be reinforced into conducting polymers such as poly(ether imide) (PEI) to obtain UFG-PEI coatings. These coatings were reported to enhance the corrosion resistance of steel by nearly 45 times when compared to an HDG zinc coating.4 The literature suggests that conducting polymers (CP) such as PEI offer corrosion protection to metals through the

atmosphere (subtropical and tropical marine climates) is characterized as an extremely high risk.3 C5 and CX are known for high concentrations of acidic pollutants such as SO2, air borne salinity, and severe condensation of moisture, all of which accelerate corrosion.3 The steel industry currently uses a zinc coating as the primary method to protect mild, unalloyed, and low carbon steels4 against corrosion. The zinc coating offers a combination of mechanisms to inhibit atmospheric corrosion. It forms a hard, nonporous layer that serves as a physical barrier to stop corrosive agents (e.g., H2O, O2, and Cl) from contacting the steel surface (passive mechanism). Further, the coating impedes half-cell reactions through the creation of a Schottky barrier at the interface (active mechanism) and behaves like a sacrificial anode (cathodic protection).5 Zinc coatings are a logical choice for the steel industry because the numerous galvanization techniques available for their application make them suitable for a wide variety of products. For example, hot-dip galvanization (HDG) is a popular technique that enables complete immersion of steel in a bath of molten zinc at 449 °C6 to uniformly coat the exterior and interior surfaces of a complex geometrical structure while cold galvanization (CG) is a more simplistic technique that can be used to paint multiple layers of zinc onto larger steel surfaces.7 The hot-dip galvanization of steel can be operated in either a continuous or batch mode.8 For hot and cold rolled steel coils 2657


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

following four different mechanisms.16 CP coatings facilitate formation of protective metal-oxide layers on a metal surface (anodic protection). CP coatings can also alter the path of oxygen and moisture and restrict their transport to the metal surface (barrier effect). Inhibitory compounds are doped into CP coatings that are eventually released when the external environment favors defects in coatings (controlled inhibitor mechanism). Dopants (e.g., graphene) can be added to CP coatings to form a semiconducting layer on the metal surface to restrict the flow of free electrons from the metal surface to oxidizing species in an external environment (charge depletion). The corrosion protection mechanism of UFG-PEI is shown as a schematic in Figure 1. PEI films have been found to be effective in corrosion applications due to their high mechanical strength, enhanced moisture barrier properties, and high glass transition temperature.15 The PEI polymer matrix in UFG-PEI can serve as an anode by participating in corrosion reactions and impeding half-cell reactions involved in steel corrosion (active protection).13,17−22 Graphene in UFG-PEI serves as a pigment and enhances its adhesion with the steel substrate through π electron interactions. The sp2 carbon allotropes in graphene provide a barrier against aggressive environments by providing tortuous paths to permeating molecules (O2 and H2O) (passive protection).23 The motivation to commercialize graphene coatings is evident from the increasing number of relevant patents.9,25−29 However, it is necessary to understand the potential life cycle impacts30 of these coatings in order to identify any potential issues with the environmental sustainability of the nanomateri-

al-based treatment alternative. Similar to reported concerns for other nanomaterials,31−38 graphene coatings should be scrutinized with this regard to understand their potential trade-offs.39 The objective of this study is to evaluate and compare the environmental performance of a UFG-PEI coating with a commercial zinc coating used in the steel industry. Although a range of graphene composites have been demonstrated as corrosion-resistant coatings, the UFG-reinforced PEI coating was selected as the model graphene coating in this LCA study for the following reasons. The PEI polymers have been found to be resistant to SO2, UV, and atmospheric ozone.40−43 Reinforcement of the polymer with graphene has been reported to significantly reduce the effects of weathering.44 The graphene manufacturing technique (solution-phase exfoliation) described in this work4,31 is scalable for estimating mass production of UFG-PEI coatings. The availability of accelerated corrosion test results from Akhtar15 and Dennis et al.4 can provide guidance for estimating system performance.

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LIFE CYCLE ASSESSMENT METHODOLOGY Overview of the UFG-PEI Coating. Figure 2 shows the cradle-to-grave life cycle stages for protecting an unalloyed steel surface with UFG-PEI. The life cycle is divided into eight processes: (1) production of hot rolled steel coils; (2) cold rolling of hot rolled coils; (3) surface preparation, including degreasing to remove oil impurities and dirt and shot blasting; (4) synthesis of a UFG-poly(amic acid) (PAA, a precursor of PEI) solution; (5) roller coating UFG-PAA on the steel surface; (6) thermal curing to form the UFG-PEI coating; and (7) the use of a steel sheet in a field site where C5(I) atmospheric 2658


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ACS Sustainable Chemistry & Engineering 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.50 This data set 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 association53) • Ecoinvent data set52 - sheet rolling of steel

(2) Sheet Rolling of Steel (Cold Rolling) (3) Surface preparation and shot blasting stages (4) Synthesis of UFG-PAA coated solution

(5) Roller coating of UFG-PAA solution on clean steel surface (6) Thermal curing of UFG-PAA solution and in situ imidization to form UFG-PEI on steel surface

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

• Ecoinvent data set52 - degreasing metal part in alkaline bath • LCI data for shot blasting is obtained from Peng et al.54 UFG Synthesis • Graphite is exfoliated in NMP solvent (5 g/L) using ultrasonication.47 • Energy consumption during UFG preparation is obtained from Arvidsson et al.31 PAA • Prepared from equimolar ratios of m-phenylenediamine (mPDA) and phthalic anhydride of bisphenol A (BPADA) using a method suggested in the literature.45,46 UFG-PAA Solution • UFG in PAA = 5 wt % • UFG-PAA in NMP solvent = 20 wt % • LCI data for roller coating is obtained from industrial literature.55 • Transfer efficiency of roller coaters is 83%.56 • Infrastructure potential impacts of the metal coating facility from ecoinvent52 are included. Dry Film Thickness of UFG-PEI • The coating thickness is 20 μm.4,15,57 • Imidization yield of PAA to PEI is 100% • LCI for thermal curing is obtained from industrial literature.58 • Exhaust fumes during curing are incinerated.58 • 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.

corrosivity conditions prevail; and (8) end of life treatment of the UFG-PEI coated steel scrap. The UFG-PEI synthesis is based on the procedure suggested in the literature.4,15,45,46 The PAA precursor is synthesized by reacting equimolar quantities of bisphenol dianhydride (BPADA) and m-phenylenediamine (mPDA) in the presence of N-methyl-2-pyrollidone (NMP) solvent. The solids content of this resulting mixture is assumed as 20 wt %. The UFG is obtained separately by exfoliating graphite (5 g/L) in NMP using an ultrasonic technique.47 The UFG-NMP is mixed with the viscous PAA precipitate, and the resulting solution is coated on unalloyed steel using a roller coating technique. The coated steel is thermally cured to convert UFG-PAA into UFG-PEI through in situ imidization. During curing, the volatile NMP is evaporated and collected in the exhaust fumes and incinerated. Goal and Scope Definition. The screening-level LCA work in this study is performed following the recommended practices of the ISO 14040:2006 and 14044:2006 LCA standards.48 This is a screening-level LCA because it relies on scale-up assumptions and secondary data, including patents, journal articles, and existing LCI databases to develop the inventory. The lack of primary or measured data increases the uncertainty of the model and limits the interpretation and conclusions that should be drawn from the impact results. The goal of the LCA is to quantify the cradle-to-grave environmental performance of a UFG-PEI coating used to protect unalloyed steel from C5 (I) atmospheric corrosivity conditions. These results are contrasted with a zinc coating obtained with the widely used hot-dip galvanization (HDG) process. The geographical boundary of the study is the Unites States (U.S.).

The typical practice followed by the steel industry is to apply the initial treatment of the corrosion-resistant coating prior to installation and subsequently perform recoating maintenance at predefined intervals within the expected service. Figure S1 shows the decision making process used to identify the recoating requirements (See Supporting Information (SI)). Functional Unit. The equivalent functional unit (FU) is a rectangular unalloyed steel sheet with dimensions of 12 mm (thickness) × 180 mm (width) × 6 m (length) and a mass of 102 kg. The total surface requiring protection from atmospheric corrosion is 2.31 m2. The specified steel sheet is available as a commercial product.49 The selected coatings are intended to provide corrosion resistance during the steel’s sixty-year design life. Life Cycle Inventory (LCI) Data for UFG-PEI Coatings. The SimaPro LCA software package (version 8.0.3)29 was used to build the life cycle model. The foreground LCI data pertaining to the life cycle stages of the UFG-PEI system were obtained from academic and patent literature. The background data were obtained from the World Steel Association (the LCI data for hot rolled steel plate including end of life credits),50 the U.S. LCI database51 maintained by the National Renewable Energy Laboratory (NREL), and the ecoinvent database (v.3.2) after modifying for U.S. conditions (i.e., the energy consumption was calculated according to the U.S. national grid mix).52 Key parameters and major assumptions made while compiling the UFG-PEI LCI are summarized and presented in Table 1. Assumptions for Conventional Galvanized Zinc Coatings. A detailed description of the life cycle of a steel sheet coated with HDG zinc is provided in Section S3 of the SI. The 2659


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Table 2. Key Parameters and Major Assumptions 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.2,59−61 • Maintenance activities after the 15th 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 is 14.28 kg. • 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.62 • The dezinced steel scrap follows the end-of-life steel management practice specified by the world steel association.53,62 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 the use stage

Time to first maintenance and maintenance recoating

Mass loss of a steel due to corrosion during the use stage of HDG coated steel sheet Dezincing of galvanized steel scrap at the end of life

conditions and the predicted corrosion rates are higher than the actual rates.67,68 However, there is a lack of consensus supporting this conclusion, with other researchers concluding that it is difficult to establish a true correlation between predicted and observed corrosion rates. We addressed this uncertainty by considering three different scenarios representing one, two, and three cycles of maintenance. A maintenance cycle includes the following steps: (i) remove the steel from service, (ii) transport it to a workshop for degreasing the surface followed by an application of a 20-μm coat of UFG-PEI, (iii) transport the coated steel to the field for reinstallation. The maintenance cycles occur at uniform intervals (i.e., 30 years for one cycle, 20 years for two cycles, and 15 years for three cycles). The mass of steel lost during an interval due to rust formation was considered similar to the HDG system. The second scenario was examined to study the sensitivity of potential environmental impacts when UFG is replaced with functionalized graphene during the preparation of the UFG-PEI solution. Functionalization of graphene can be important for two reasons. First, a poor interfacial bonding between the coating and metal surface can result in the premature failure of the coating exposed to atmospheric corrosion. Thermal or chemical methods can be used to obtain functionalized graphene that can be reinforced within the conducting polymer matrix.69 This process can improve the adhesive properties of the UFG-PEI coating and may ensure a sixty-year service life. With regard to solution preparation, loading graphene in a functionalized form enables better dispersion in the polymer matrix and decreases agglomeration.57 Although functionalization normally requires additional material and energy, the enhanced dispersive properties can reduce the graphene loading requirements (5−20 wt % with UFG compared to 1 wt % with functionalized graphene),24 enhancing the overall environmental performance of graphene-PEI coatings. This study considered the scenario analysis with both thermally functionalized graphene (i.e., oxidation and thermal expansion of graphite oxide) (TFG) and chemically functionalized graphene (CFG). The LCI data for TFG and CFG were constructed based on synthesis procedures described in the open literature.57,70−72 The modeling parameters and assumptions were identical to the UFG-PEI system except that the graphene loading in TFG or CFG is reduced to 1 wt %. Details about the LCI data used for the synthesis of TFG and CFG are shown in Section S4 of the SI.

key parameters and major assumptions for this system are summarized in Table 2. Life Cycle Impact Assessment. The Tool for the Reduction and Assessment of Chemical and other environmental impacts (TRACI 2.1)63,64 was used to assess the ten impact categories: global warming potential (GWP, kg CO2 eq); acidification potential (AP, kg SO2 eq); potential human health impact due to carcinogens (HH-CP) and noncarcinogens (HH-NCP) as CTUh; respiratory effects potential (REP, kg PM2.5 eq); eutrophication potential (EP, kg N eq); ozone depletion potential (ODP kg CFC-11 equiv); potential for ecotoxicity (ETXP CTUe); smog formation potential (SFP kg O3 eq); and fossil fuel depletion potential (FFDP MJ surplus). Given the intent of this LCA is to investigate the potential impacts of UFG-PEI within a U.S. context, TRACI is the most appropriate impact method because it is a North American midpoint impact assessment method developed by the U.S. Environmental Protection Agency (EPA).63 TRACI 2.1 incorporates the USEtox environmental model for characterization of potential human health and ecotoxicity impacts.65,66 It is important to note that the release of graphene and its subsequent exposure to both humans and species in the environment was not modeled in this study because of a lack of material flow tracking data describing releases for inventory modeling and characterization factors to enable use of such data in the impact models Scenario Analysis and Sensitivity. A scenario-based sensitivity analysis was performed to study the effects of changes in the key modeling parameters on the potential environmental impacts of the UFG-PEI coating. We examined the following two scenarios: (i) the effect of reducing the UFGPEI coating’s service life, and (ii) the effect of replacing UFG with functionalized graphene during the preparation of the UFG-PEI solution. A key assumption in this study is the sixty-year service life of the UFG-PEI coating. The corrosion rates for steel coated with UFG-PEI were based on the studies using immersion corrosion testing with 3.5% NaCl solution. These studies may not provide a true representation of atmospheric corrosion, as they do not take into account the effects of atmospheric factors related to wet and dry cycles of climate, air pollutants (e.g., SO2 and salinity), and ozone and UV radiation. Some researchers have concluded that immersion corrosion tests simulate more aggressive exposure conditions when compared to field 2660


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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.

Uncertainty Analysis. In addition to the scenario-based sensitivity analysis, uncertainty analysis of the UFG-PEI system inventory was performed in SimaPro using Monte Carlo simulation with 5000 steps and a 95% level of confidence. The full uncertainty of the impact scores was not quantified because the uncertainty of the impact models could not be included based on a lack of necessary data describing distributions for the characterization factors. Background LCI from ecoinvent already included standard deviation estimates for most data. The inventory uncertainty in the foreground data was mainly associated with the preparation of the UFG-PAA coating solution and the thermal curing of the UFG-PEI. Ranges were established for key quantities in the inventory using reported experimental results. For example, the reported concentration of UFG in the UFG-PAA solution ranged from 515 to 20 wt %,4 and the thickness of the applied coating varied between 15 and 30 μm.73 Energy use during surface preparation of the steel sheet was bounded between the baseline scenario value and the maximum reported value74 of 3 kWh/m2 for cases when sand blasting is used instead of shot blasting. The oven curing stage also introduces uncertainty with respect to energy consumption. Thermal curing can be replaced with an energy-efficient, UV curing process that results in lower VOC emissions.58 Energy use and emission ranges during curing were assigned to the inventory to reflect curing uncertainty. To support comparative interpretation of the results, the same uncertainty analysis was applied to the HDG-coated steel sheet.

only 10% smaller for FFDP. These observed differences are mainly related to the actual coating and maintenance of the steel sheet. To better understand these differences, a comparative assessment of the two systems while excluding steel processing is reported in Figure S6. Interestingly, the potential impacts attributed to the steel (not shown) for HDG zinc are 15% higher than UFG-PEI because the metal losses from rusting imply that less steel scrap is available for recycling at the end of life. The use stage results in a loss of 14.28 kg of steel due to the atmospheric corrosion. The recycling rate of steel scrap at end of the life is adjusted to 71% (instead of 85%) after discounting 14.28 kg of the steel lost during the use stage of the HDG coated steel sheet (see Sections S3.3 and S3.4). Contribution Analysis. The performance of the UFG-PEI steel was further analyzed using contribution analysis to identify hotspots for potential improvement (Figure 3). The potential impacts of steel preparation contribute to at least 50−65% of GWP, AP, SFP, and FFDP, with the majority of the remaining 35% coming from cold rolling and the UFG-PAA coating solution. Cold rolling is the dominant contributor to HH-CP and ETXP at 93% and 81%, respectively. EP is primarily driven by the UFG-PAA coating solution (55%) and sheet rolling (46%). For the coating solution, both NMP solvent and BPADA are known to directly cause eutrophication in water bodies75−77 while the nitrogenous waste released from the upstream production of m-PDA also induces eutrophication.78 Similarly, cold rolling and the coating solution account for a combined 83% of ODP and 80% of REP. The UFG-PAA coating solution contributes 41% of ODP because of electricity consumption during the production of UFG and the use of NMP solvent during synthesis. The same sources are the drivers for the contributions of UFG-PAA to REP, as well as FFD. HH-NC is more equally distributed across steel preparation, cold rolling, and the coating solution, with each contributing around 30%. It should be noted that solvent fumes

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RESULTS AND DISCUSSION When compared with HDG zinc, the UFG-PEI coating exhibits lower environmental potential impacts in all the ten impact categories (Figure S5). The UFG-PEI potential impacts are 93% smaller for HH-NCP; 68% smaller for AP; 57% smaller for ETXP; 45−48% smaller for REP and EP; 38% smaller for HHCP and ODP; 32% smaller for SFP; 21% smaller for GWP; and 2661


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Figure 4. Potential impacts of UFG-PEI steel compared with HDG zinc steel when one, two, and three cycles of maintenance are included for UFGPEI. The UFG-PEI performance under the three scenarios is compared to the HDG coating with a 15-year lifespan.

Figure 5. Potential impacts of functionalized (TFG-PEI and CFG-PEI) and unfunctionalized (UFG-PEI) polymer-coated steel compared with HDG zinc-coated steel.

to the potential release and surface runoff of zinc from galvanized steel structures during their usage.80,81 The larger AP impact originates from the acid pretreatment associated with the HDG process. As opposed to the baseline “No Maintenance” scenario, the potential impacts of the UFG-PEI-coated steel with three maintenance cycles are the largest for GWP, FFDP, SFP, ODP, EP, and REP. With two maintenance cycles, the GWP, FFDP, ODP, and EP potential impacts of UFG-PEI remain larger than HDG zinc. However, the SFP of the two systems is approximately equal and the REP impact of the UFG-PEI

during curing were assumed to be incinerated as opposed to being collected and recycled, which could reduce the NMPrelated potential impacts. Sensitivity Analysis. Results for the sensitivity analysis of service life are shown in Figure 4. Even if three maintenance cycles are applied to the UFG-PEI coating, the HDG zinc coating still has the highest impact scores in HH-CP, HH-NCP, ETXP, and AP, which should be expected. The metal fumes of zinc oxide and particulate dust released from galvanizing plants cause carcinogenic and noncarcinogenic (e.g., metal fume fever) effects in humans.79 The ETXP impact is the highest due 2662


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Figure 6. Comparison of the internally normalized impact scores for a steel sheet coated with either UFG-PEI or HDG zinc when considering inventory uncertainty.

higher consumption of solvent by the UFG-PEI-steel system for exfoliation. One limitation of this analysis is that the existing data does not describe how functionalization can affect key variables such as service life and corrosion rate. If functionalization extends service life or reduces corrosive losses, the sensitivity of the impact results may be much different. Further research is needed to establish this data. Uncertainty Analysis. The comparison of the internally normalized impact scores for the baseline UFG-PEI and HDG zinc coatings when considering uncertainty is shown in Figure 6. The error bars correspond to 95% confidence intervals and ±1 standard deviation (SD) around the mean. For UFG-PEI, EP and ODP vary by 28% and 18%, respectively. The other impact categories except for HH-CP, which has an uncertainty range of 39%, have relatively minor uncertainty ranges of less than 10%. For HDG zinc, the ranges of uncertainty for the potential impacts can be substantial: 79% for HH-NCP; 61% for ETXP; 53% for HH-CP; 48% for EP; 24% for ODP; 16% for AP, SFP, and REP; and less than 10% for GWP and FFDP. Based on the overlap of uncertainty ranges, the only meaningful differences in potential impacts are for GWP, AP, HH-NCP, REP, and SFP. For the other impact categories, it will be necessary to refine the inventories and reduce the uncertainties before drawing any definitive conclusions regarding the comparative performance of the two coatings performs. This study suggests UFG-PEI coatings may yield reduced environmental impacts for protecting steel substrates from atmospheric corrosion when compared to the HDG coatings. However, several technological and modeling challenges must be resolved to validate these findings. The LCA performance of the UFG-PEI coatings should be evaluated using the corrosion rates determined under actual field conditions as opposed to the immersion tests used in this study. The values for the corrosion rates affect the service life and the corresponding LCA performance of the UFG-PEI coating. A decreased service

system is reduced below that of HDG zinc. For one maintenance cycle, which is equivalent to the single full recoating modeled for the HDG zinc system at 30 years, the two systems have nearly identical scores in GWP and ODP and the UFG-PEI system has lower potential impacts in all other categories except FFDP. The ODP and EP potential impacts increase by 21% for each additional maintenance cycle, which is consistent with the increased consumption of NMP solvent, BPADA, and mPDA required for the additional coatings. The scores for REP and FFDP increase 17−18% per additional maintenance cycle through the combined effect of decreasing the amount of steel for recycling after corrosive losses and increasing the consumption of NMP solvent to produce more coating. Increasing the required amount of coating also increases electricity consumption during the synthesis of UFG, but this only has a minor effect on the potential impacts because UFG is a mere 5 wt % of the coating solution. The effects of electricity consumption may become more significant if the weight percent of UFG is increased. GWP and SFP increase by 13% per maintenance cycle for similar reasons. These results suggest solvent recovery and reuse should be considered as a means to improve the environmental performance of UFG-PEI coatings. Results for the sensitivity analysis of functionalization are shown in Figure 5. Despite the reduction of graphene loading to 1 wt % for functionalized materials, the potential impacts are not sensitive to this change based on the near identical profiles in the plots. Given graphene accounts for less than 20% of the coating solution preparation impacts, the lack of sensitivity is reasonable. This is especially true if one considers that as the weight percent of graphene reduces the consumption of BPADA and mPDA increases correspondingly and counteracts the potential benefit. The lone exception when comparing the polymer systems is that the functionalized options (TFG-PEI and CFG-PEI) reduce REP by 14%. This is mainly attributed to 2663


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itself is relatively lower than CNTs.88 Graphene materials have also been reported to induce genotoxic effects (e.g., chromosomal aberrations and fragmentation of DNA in stem cells) even at a low concentration.89 The recent literature sheds light on toxic effects of graphene on other species. For example, graphene has been reported to exhibit toxicological effects on Chlorella pyrenoidosa due to oxidative stress and physical penetration. Compared to CNTs, graphene has been reported to induce severe toxic effects to Chlorella pyrenoidosa.90 However, several other studies30,91 have reported that graphene does not induce any toxicity effects on several soil-borne species (e.g., Caenorhabditis elegans) and water-borne species (e.g., zebrafish embryos and Artemia salina crustaceans) even at higher concentrations (1 mg/L). Although there is no consensus regarding the toxic effects of graphene-based materials, many researcher can agree that the toxicity is dose dependent86 and that factors such as size, shape, degree of agglomeration, presence of functional groups on the surface, etc.85,86,89 determine the level of interactions with biological cells and ultimately decide its potency effect. Once all of this knowledge is available, the behavior of emerging graphene coatings during the course of steel scrap recycling and end-of-life treatment processes, particularly in an electric arc furnace (EAF), will need to be addressed. Recently, some groups studied the effects of incinerating carbon nanomaterial laden waste.92,93 The studies concluded that the incineration of waste containing carbon nanomaterials in the combustion zone might aggravate formation of hazardous pollutants such as polyaromatic hydrocarbons (PAHs), chlorinated dioxins, and furans. The operational temperature of an EAF is higher than incinerators, but the graphene nanomaterials are expected to persist in the combustion zone. Just as the carbon nanomaterials may end up in the ash after incineration and be contained by pollution control equipment such as an electrostatic precipitator (ESP) or cyclone separator,92,93 graphene nanomaterials might accumulate in the ash if the steel scrap and residual UFG-PEI are treated in an EAF. Thus, further studies are warranted to investigate the behavior of graphene-laden EAF dust subjected to landfilling.

life of the UFG-PEI coating results in a potential trade-off (as discussed in the scenario based sensitivity analysis) under the FFDP, ODP, EP, and GWP categories. The FFDP impacts for the UFG-PEI surpass HDG by 11% when the service life of the former is decreased to 30 years; by 28% when the service life is decreased to 20 years; and by 45% when decreased to 15 years. When the service life of the UFG-PEI coatings is decreased to 20 years, the UFG-PEI has 19% and 40% higher potential impacts for the ODP category. When the service life is decreased to 15 years, the UFG-PEI coatings exhibit 14% and 35% higher potential impacts for the EP category; and 10% and 23% higher potential GWP impacts. The potential SFP and RE impacts increased by only 14% and 19% when the service life was decreased to 15 years. The remaining four impacts (AP, HH-C, HH-NC, and ETXP) remained insensitive to the service life of the UFG-PEI coating. It is important to evaluate potential risks for a premature failure of the coatings when exposed to excessive levels of UV radiation, atmospheric ozone, and humidity. A recent tribological study conducted by Dong and Qi82 has determined that friction and wearing of polymers can be lowered by the addition of graphene. Similar studies are required to resolve uncertainty of the inventory related to the service life and coating properties (e.g., thickness and composition) of the UFG-PEI coatings. It is also important to minimize modeling uncertainties that influence the quantifiable values for the ecotoxicity and human health impacts of UFG-PEI coatings. The impact characterization factors (CFs) that would enable inclusion of graphene materials for this study would depend on two elements: (a) the fate and transport of graphene materials released across the lifecycle of the UFG-PEI coated steel (specifically during the use stage); and (b) the cytotoxic effects of the graphene materials when they interact with biological systems. To the best of the authors’ knowledge, only one study83 has reported an aquatic ecotoxicity CF for graphene-based nanomaterials. This study focused on graphene oxide (GO) and provided three key insights. First, GO behaves as a colloidal particle in aquatic environments. Second, bioaccumulation of GO nanoparticles themselves is negligible. However, some studies have implied GO nanoparticles as carriers of persistent organic pollutants and enhance their bioaccumulation potential.84 Third, a large uncertainty is associated with the effect factor because information on toxicity of graphene materials is sparse. The resulting aquatic ecotoxicity CFs reported for GO nanoparticles were found to vary by a large magnitude, and additional data points would be needed to resolve the issue for precise impact assessment. Furthermore, the CFs reported by Deng et al.83 have been explicitly developed for GO particles, and they cannot be translated to the UFG-PEI coatings considered in this study. The toxicology of nanomaterials from the graphene family is yet to be thoroughly understood. However, few studies conclude that graphene materials (graphene, GO, and reduced-GO) in their unmodified form are more potent in inducing cytotoxic and genotoxic effects in living cells compared to functionalized (surface modified) graphene materials.85,86 A recent study has reported that graphene nanoparticles can accumulate in tracheobronchial airways and cause lung complications in human beings.87 Another study confirmed that graphene materials can induce cytotoxic effects that are similar to carbon nanotubes (CNTs) in terms of shape and concentration dependency, but that the cytotoxic effect

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03005. Details on acronyms and terminology, global assumptions for LCA modeling, process descriptions and flowcharts, material-specific assumptions, recoat-maintenance calculations, and detailed life cycle inventories for the six protection systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Venkataramana Gadhamshetty, Ph.D.) Department of Civil and Environmental Engineering, South Dakota School of Mines and Technology, Rapid City, SD. E-mail: Venkata. [email protected]. ORCID

Venkataramana Gadhamshetty: 0000-0002-8418-3515 2664


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ACS Sustainable Chemistry & Engineering Notes

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Disclaimer. The views expressed in this article are those of the authors and do not necessarily represent the views or policies of the US Environmental Protection Agency. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

V.G. acknowledges the funding support from National Science Foundation CAREER Award (#1454102), NASA (NNX16AQ98A), and the South Dakota Board of Regents under the auspices of the Surface Engineering Research Center (SERC).

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