ATMOSPHERIC CORROSION SEVERITY MONITORING

Dr. Michael J. Hutchison, Excet Inc.

Dr. Derek J. Horton, U.S. Naval Research Lab

Dr. Christine E. Sanders, U.S. Naval Research Lab

Keywords: Atmospheric Corrosion, Atmospheric Monitoring

ABSTRACT

There remains a need to better understand the environmental severity conditions of specific deployment locations for infrastructure and mobile assets which vary drastically in different geographic regions and very often even de-pend on location within a particular structure. Furthermore, there continues to be a need for accelerated ageing and weather testing that best replicate variable exposure conditions to ensure that testing correlates to anticipated performance. The goal of atmospheric corrosion severity monitoring is to record the critical factors and how they vary over time that can then be correlated to the service environmental conditions (e.g., shipboard exposure) and to material or coating performance (e.g. at a test site). This work reports a comparison study of different strategies to record the atmospheric corrosion severity including deployable atmospheric corrosion monitors and characteri-zation of proposed monitoring materials.

The atmospheric exposure test site at the U.S. Naval Research Laboratory in Key West, FL (NRL-KW) provides an opportunity to conduct accelerated ageing and weathering testing of DoD systems, materials, and coatings in operationally-relevant environments. This test site makes use of current state of the art atmospheric aerosol moni -toring, sensor packages and traditional methods such as chloride candles (ASTM G140), to determine the corro-sion severity in real time. The site includes the addition of natural seawater spray racks and outdoor mechanical loading at the Center for Corrosion and Atmospheric Structural Testing (C-CoAST), NRL-KW is poised to be at the forefront of outdoor, atmospheric corrosion monitoring and assessment.

This paper provides recent updates on environmental severity and corrosion indices for various exposure condi-tions available in Key West, along with comparisons to on-ship corrosion data, and available data from compari-son test sites and other DoD installations.

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2019 Department of De-fense – Allied Nations

Technical Corrosion Con-Paper No. 2019-0000

INTRODUCTION

Weather, aerosol distribution and chemistry, seawater chemistry, spray cycle, spray interval, and UV irradiance all contribute to environmental severity. However, correlating these parameters to actual corrosion damage or coat -ing degradation for environmental severity ranking remains difficult. Different classes of materials and coating sys-tems often have differing degradation mechanisms and associated critical factors. Capturing the salient critical pa-rameters using environmental monitoring is critical to determine degradation mechanism and accelerated test de-sign. In an ideal world, there would be an accelerated laboratory test where a series of knobs could be turned to dial in a specific operating environment, and the test would give you an accelerated, accurate prediction, e.g. cor -rosion rate and morphology.

The culmination of work from Abbott and others has highlighted differences in the atmospheric corrosion rates at different DoD relevant sites and during asset duty cycles.1-3 Yet, there still remains work to be done in order to more directly correlate atmospheric parameters to degradation outcome. The atmospheric and structural test site at the U.S. Naval Research Laboratory in Key West provides the opportunity to conduct testing in a well-charac -terized moderate marine site with the ability to adjust to more severe chloride deposition and longer times of wet -ness through the addition of natural seawater spray directly to test articles as needed.

There are several different approaches to monitoring corrosion severity. The direct measurement approach is to use the corrosion rate of a metal, e.g. UNS G10100 steel, to directly determine the severity amount and compare across exposure sites. This method has been refined by using an instantaneous measure of the corrosion rate of a particular material using polarization resistance and a micro-controller and data logger. Alternative approaches directly measure specific characteristic environmental parameters. For example, chloride concentration is often measured using chloride candles (ASTM G140), but this method requires frequent upkeep and as a result only provides averages over long time increments. Increased frequency, time resolved methods are also used to as-sess aerosol constituents such as filter pack assemblies, e.g. EPA CASTNET filter pack assembly. A fully time re-solved method is achieved through use of a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) that can be used to monitor real-time contaminant levels in the ambient sampling volume. Each method has advan-tages in type of aerosol species monitored and size.

This report details a comparison study of aerosol monitoring techniques: wet and dry candle, filter pack assembly, and ToF-ACSM to determine the benefits of each technique. The outcomes will be correlated to silver and copper monitor materials as well as steel mass loss data.

EXPERIMENTAL PROCEDURES

Testing Environment

Key West is located at the bottom of the Florida Keys off the bottom tip of Florida in the Straits of Florida. The wa -ter, which flows through the Keys, brought up from a deep-water upwelling in the Florida straits, is due partially to the Florida loop current. This water has low levels of pollutants and is representative of the water seen in “blue ocean” conditions. Furthermore, there is no upstream pollutant sources (wind or air) for the test site in Key West. In addition, the UV loading in Key West is very strong since the site is so far south. The UV index in Key West is also similar to many of the operational theaters of the DoD, specifically of the Navy. All of the tests performed in this study were conducted at the C-CoAST site, 50 m from the waterline as depicted in Figure 1, beginning in De-cember 2018.

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Figure 1. Location of the test site at NRL – Key West in relation to the loop currents from the Gulf of Mexico that pass Key West before Heading up the Eastern Coast. The Center for Corrosion and Atmo-spheric Structural Testing (C-Coast) is located at building F1 and a pier based test site is located at F-14.

Direct Deposition Monitoring

Wet Candle

The chloride candle or commonly called “wet candle” chloride monitoring method is prescribed by ASTM standard G140. Briefly, medical gauze is wrapped around an inverted glass test tube secured with a modified rubber stop-per with center hole to accommodate the test tube and flattened sides for the gauze ends. Both ends of the wrapped gauze are brought through the top of a standard 500 mL Erlenmeyer flask filled with deionized water (DI, 18.2 MΩ-cm). The gauze is kept continuously wet through capillary action from the flask water reservoir. Biweekly refills were needed to maintain the water reservoir level. After a set time interval of one month, the gauze is soaked in the remaining reservoir water (~300 mL) and DI water is added as needed. The gauze is soaked overnight and the solution is chemically analyzed for chloride content with ion chromatography. Figure 2 below shows an example wet candle being exposed in a marine shoreline location.

Figure 2. Image of a wet candle prepared according to ASTM G140 sheltered and placed near the marine shoreline.

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Dry Candle

According to procedures from Hawthorn and Hihara4, a dry candle was prepared and deployed alongside the standard ASTM G140 wet candle. Dry medical gauze was wrapped in identical fashion as ASTM G140 around a glass test tube which was supported with a cut rubber flask stopper. The dry candle was not kept in any fixed moisture condition as imposed with the wet candle reservoir and was subjected directly to the ambient environ-ment in order to compare dry deposition rates vice the wet candle method.

Corrosion Surrogates

Cu Tubing

Copper has multiple known chloride-based corrosion products with relatively low corrosion rates. It has been known to be used as a corrosion sensor1. Cu cylindrical samples in triplicate were exposed as a surrogate chlo-ride deposition monitor. Segments of commercially available residential Cu tubing (> 95 wt% Cu) with an exterior diameter of 0.5” were cleaned and polished to a surface finished of 600 grit. Samples were then rinsed with DI water, degreased with acetone, and dried with compressed lab air. Cylindrical samples were secured in a vertical orientation at a height of (6 ft above ground level) using nylon ¼-20 threaded rod and compressed in series be -tween flat nylon washers secured at segment series ends with nylon nuts.

Samples were exposed, sheltered from direct sun and rainwater, alongside other wet and dry candle chloride monitors for a period of one month. Following exposure, samples were individually placed in 30 mL plastic con-tainers with 25 mL of 0.1 M acetic acid. Containers were then shaken at 300 rpm for 15 minutes using a shaker plate, sonicated in an ultrasonic bath for 30 min and stored overnight at 4˚C. The solution was then sampled for chloride content using ion chromatography. Acetic acid was used to chemically dissolve Cu corrosion products from the surface. Chloride-containing Cu corrosion products are formed on the outer layer of corrosion products in many environments5. Acetic acid was selected as the acetate ion does not interfere with chloride for the ion chro-matography eluent and column selection. Chloride was estimated using a rider peak subtraction from the asym-metrical tail from the eluted acetate. This chemical extraction method was adopted in lieu of an electrochemical reduction as prior experience of this technique demonstrated that the carbonaceous species and/or corrosion products inhibited electrical contact between the Cu substrate and the reducible corrosion products.6

Chloride-based corrosion products of copper are expected to form on the surface of the copper tubing. The cylin-drical geometry facilitates chloride pickup via corrosion from all directions, in contrast to conventional flat panels which are typically oriented at 45° facing south. The advantage of this design is that it allows the chloride deposi -tion rate to be independent from the direction of exposure which would be more representative of a real-world ge-ometry.

Ag Panel Coupons

Triplicate pure Ag (99.99%) samples 0.5” x 3” x 0.011” were affixed with nylon hardware. The samples were boldly exposed facing southward without sheltering at a 45˚ angle from horizontal. Details on exposure methods can be found from Sanders et al.7 Exposed samples were evaluated for chloride and sulfide content using electro-chemical reduction, in accordance with ASTM B825-13. A solution of 0.1 M Na2SO4 was deaerated by bubbling 99.9% N2 gas for 2 hours. Cells were deaerated for 10 min prior to solution introduction. The deaerated solution was hydraulically transferred to cell to avoid exposure to lab air. Ag samples were galvanostatically held at a ca-thodic current density of -0.1 mA/cm2. The potential was monitored using a mercury/mercurous sulfate electrode in saturated potassium sulfate solution with equipped with a Luggin tip. The exposed area was calculated from im-ages of the electrochemically-affected area (via color-change) using an analytical microscope. The reduction charge from AgCl was determined using the time spent at the reduction potential of AgCl; in this electrochemical environment the reduction potential lies between -0.25 and -0.4 VMMSE. The deposition velocity of chloride on Ag was determined with Equation 1.

C l−¿=

iappt red (MW AgCl )( MCl

MAgCl )dnF

¿ Equation 1

Where iapp is the applied current density, tred is the reduction time for AgCl, MWAgCl is the molecular weight, and MCl

and MAgCl are the unit masses for chlorine and silver chloride, respectively. The duration of exposure is given by d, the valence of AgCl is given by n, and F is Faraday’s constant.

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Steel Panel

The general corrosivity of the environment was determined using C1010 steel mass loss coupons. 3” x 3” x 1/16” steel coupons, glass bead blasted were pre-weighed and boldly exposed facing southward without sheltering at a 45˚ angle from horizontal. Following exposure, samples were removed from fixtures and corrosion products were removed using a glass-bead blasting media. Following surface cleaning, samples were then weighed to the near -est 0.0001 g and recorded. The area used for calculating the mass loss of the samples included the back face of the samples, which was elevated above the support platform by 0.5”. The corrosion penetration rate in mils per year (mpy) was determined using Equation 2.

mpy=1.825∆mAd Equation 2

where Δm is the mass change before exposure and after sandblasting in grams, A is the exposed area in m2 and d is the duration of exposure in days. The factor 1.825 encompasses the unit conversion from metric to imperial and the density of steel.

Ambient Concentration

Coarse Aerosols: Filter Pack Assembly

An assembly of multiple filters was deployed to collect ambient aerosols with an effective diameter of 1.0 µm or greater. Ambient aerosols were sampled using a constant flow of 1.0 L/min was passed through polytetrafluo-roethylene (PTFE) filters 2” diameter x 0.014” for a period of one week each. PTFE filters had a pore diameter of 1.0 µm. Filter cartridges were stacked sequentially and the flow path was automatically switched using an on -board microcontroller and solenoids. Following the sampling campaign, filters were removed, placed in 30 mL polyethylene plastic bottles with 25 mL of DI water. The bottles were then shaken at 300 rpm for 15 min on a shaker plate, then transferred to an ultrasonic bath for 30 min. Following sonication, samples were stored at 4 ˚C overnight. The extraction solution was then chemically analyzed for chloride using ion chromatography. The ambi-ent chemical composition was determined using Equation 3.

C l−¿=CVdr ¿ Equation 3

Where C is the cumulative analyte concentration extracted from filters, V is the extraction solution volume (25 mL), d is the sampling duration, and r is the volumetric sampling air flow rate.

Figure 3. Major components of filter pack assembly. Filters are stored in the orange cartridges (Inset A) and selec-tively switched using the black cylindrical solenoids (B) and a computer and microcontroller (C). The flow rate im-posed using a small air pump (D) and is measured with a flow controller (E).

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Submicron Aerosols: Atmospheric Chemical Speciation Monitor – ACSM

An atmospheric chemical speciation monitor manufactured by Aerodyne Research Inc. was used to monitor the sub-micron fraction of aerosols in the ambient environment. Details of instrument operation can be found from ref-erence8 and a schematic is given in Figure 4. Briefly, ambient aerosols are extracted from a 3.0 L/m airstream via an isokinetic sampler. Aerosols are passed through a 100 µm diameter critical orifice and then condensed to a beam with an aerodynamic lens configured to capture aerosols with an effective diameter of 100 nm to 1 µm. The aerosol beam contacts a vaporizer (600 ˚C) and electron impact ionizer (W-filament). The ionized aerosols are then directed to a time-of-flight mass spectrometer (ToF-MS). Standard fragmentation tables according to Ng et al.8 were used to evaluate the chemistry of aerosols. The airbeam prior to isokinetic sampling was dried using an in-line Nafion® dryer. The background was subtracted using a filtered airstream using automatic switching valve. Large particulate matter with diameters greater than 2.5 µm are filtered out using a cyclone filter.

Figure 4. Schematic diagram of the atmospheric chemical speciation monitor (ACSM) setup. Figure modified from Fröhlich, R. et al 9.

A comparison of all of the chloride monitoring techniques used in this study is given in Table 1. In addition, some details about the weather data, water quality logging, and co-located corrosion sensors are listed. Data from the weather station were sampled every 15 minutes, water quality was logged every 30 minutes and water composi-tion was analyzed via ion chromatography at least once per month.

Table 1. Comparison of Chloride Monitoring Methods

Monitoring Method Measurement Sampling

RateParameters

which Influence Corrosion

Notes Size

Atmospheric Chemical Specia-tion Monitor (ACSM)

Chemical species and concentration of Aerosols

Minutes/Hourly Cl-, SO42-, NH4

Live site monitor-ing, mass spec-trometry.

100 nm – 1 µm

Filter Pack Assembly (FPA)

Chemical species and concentration of aerosols

Weekly Cl-, SO42-, NH4

Dry Deposition, Ion Chromatography

> 1.0 µm

Weather Station Weather and ambi-ent conditions Minutes/hourly Tair, UV, RH, TOW,

Wind Campbell Scientific

Wet CandleChemical species and concentration of aerosols

Monthly Cl-, SO42- Wet Deposition, Ion

Chromatography

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Dry CandleChemical species and concentration of aerosols

Monthly Cl-, SO42- Dry Deposition, Ion

Chromatography

Ag Panel Chloride deposi-tion, corrosion Monthly

Oxidizing Power (Ag2O), corrosivity via Cl(AgCl)

Wet Deposition*, Coulometric Re-duction

Cu PanelChloride and Sul-fate deposition, Carbonates, Ni-trates.

Monthly Cl-, SO42-

Wet Deposition*, Acetic Acid diges-tion, Ion Chro-matography

Water Quality Logging

Water chemistry and ambient condi-tions

Hourly; Monthly

Twater, pH, DO2, Salinity; [Cl-], [SO4

2-],Seabird

Steel Panel Mass Loss (Corro-sion Rate) Monthly Direct Corrosion

Rates (Datum)

Corrosion Sensor Thickness Loss (Corrosion rate) Minutes/Hours

Instantaneous Corrosion Rates, real-time refer-ence

Luna, Inc LS2A

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RESULTS

Deposition and Surface Loading Rates of Chlorides

It was theorized that the deposition rate and efficiency of chloride onto active (metallic), wet passive, and dry pas -sive surfaces would be significantly different. Representative data from the galvanostatic reduction of the silver is given in Figure 5. The top plateau (-0.4 VMMSE) present in all three traces is attributed to the electrochemical reduc-tion of AgCl to metallic Ag, the second plateau (-1.2V MMSE) is attributed to the reduction of Ag2S to metallic Ag, and the final two plateaus are associated with the hydrogen evolution reaction (HER) and the artifact described at -1.6 and -1.8 VMMSE, as described by Frankel et al.10 Table 2 gives a comparison of the chloride deposition rates between the Ag and Cu coupons and the wet and dry chloride candles. In all cases, the measured chloride con -tent is significantly higher using either candle method than the indirect measures using monitor material.

Figure 5. Example of Ag panel electrochemical reduction curves. Decreasing plateaus correspond to specific elec-trochemical reactions: AgCl -> Ag, Ag2S->Ag, HER artifact (see Lin and Frankel10 and Abbott10) and HER respec-tively. HER indicates the hydrogen evolution reaction signifying the conclusion of corrosion product reduction and electrolyte breakdown.

Table 2. Comparison of Chloride Deposition Rates between Monitoring Methods Deployed in Key West, FL. Error values for replicated samples indicate standard error of the mean, n=3.

Method Chloride Deposition Rate (mg/m 2 /d) December 2018 January 2019 February 2019

Wet Candle 77.4 76.0 45.1

Dry Candle 37.1 37.8 44.3

Ag Panel 1.1 ± 0.1 2.3 ± 0.7 4.4 ± 0.1

Cu Tube Segment 23.6 ± 3.2 11.7 ± 1.1 9.3 ± 0.4

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Ambient Aerosol Chemistry and Concentrations

A comparison between the concentration of aerosol species measured using the filter pack assembly and the ACSM is shown Figure 6. In the filter pack measurements an appreciable quantity of both chloride and sulfide were measured at deposition rates on the order of a few µg/m3 per sampling interval. In the case of the ACSM, sulfate was measured at about 1/3 the total of the filter pack assembly (summed over the same sampling interval). Chloride was not measured, but other chemical species were resolved.

Figure 6. Ambient concentrations and aerosol chemical speciation determined from FPA measured from PTFE fil-ters analytes [solid symbols] and from ACSM instrument measurements [lines without symbols] in Key West, FL. Mass spectrum fragmentation tables used for chemical assignments are given in Ng8.

DISCUSSION

Comparison of Chloride Monitoring Methods

Wet vs. Dry Candle

Previous data presented by Hawthorn and Hihara4 showed that in the severe marine environments in Oahu there was a good correlation between wet candles and dry candles. So the same technique was invoked here in order to understand if the same correlation would hold. It turns out that, for Key West, using a dry candle is not consis-tently a good proxy to the wet candle measurement. For the Key West environment, in two months of exposure there was a much higher deposition of chloride onto the wet candle versus the dry candle. This is consistent with different sticking coefficients for sea spray aerosols onto wet and dry surfaces,11 and suggests that the difference between Oahu and Key West may be due to a characteristic difference in the aerosols and environmental factors such as temperature and relative humidity at these sites.

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Figure 7. Chloride deposition rate values of dry and wet candle chloride values obtained from Oahu, HI and Key West, FL. Oahu, HI values taken from Hawthorn and Hihara4.

Silver Chloride vs. Wet Candle

Previous work by Abbott and Hihara’s group4 has shown varying levels of success when correlating the deposition of chloride onto Ag coupons and wet candles. Figure 8 shows the chloride content measured by silver chloride film thickness formed on silver compared to chloride content measured by the wet candle method for several at -mospheric test sites in. Unfortunately there is no clear correlation between sites or even within a given site. An empirical based, regression model was proposed by Abott, Equation 4, to convert the electrochemically-reduced AgCl film thickness (Å) to the chloride concentration measured by the wet candle method (mg/m 2/day), based on results from both measured at the same sites1. The regression model does not appear to extend to other test sites. Both Oahu and Key West show a significant deviation from the predicted trend. Interestingly, the Oahu data shows significantly increased silver chloride film thickness while the Key West data and some of Daytona data have increased Wet Candle measured chloride content relative to the film thickness.

ln [WetCandle ]=1.654∗ln [AgCl ]−4.10 Equation 4

The passive chloride deposition measurements from this study are compared in Figure 9. Each measurement de-tected significantly lower chloride than the Wet Candle method. The relative quantity of chloride measured is con -sistent across the two exposure periods. The Wet Candle method measured the highest chloride content. The Dry candle measured the second highest, followed by the copper deposits and then the silver chloride film thickness. However, the difference in quantity of chloride measured was not consistent across these techniques and no clear passive method directly correlates to the Wet Candle method.

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Figure 8. Chloride deposition rate comparison between silver chloride reduction and wet candle methods. Chloride content directly measured from samples (a) and wet candle equivalent (converted) silver chloride measurements using the regression model by Abbott1 (b). Measurements of silver chloride in WP-AFB (Wright-Patterson Air-Force Base, OH) and Daytona, FL are taken from Abbott1. Measurements of chloride content with silver coupons and wet candles in Oahu from Hawthorn and Hihara4.

Figure 9. Visual comparison of various tested chloride deposition measurement methods compared to the wet can-dle method.

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Direct Deposition vs. Ambient Concentration

The two different aerosol characterization methods resolve aerosols of two different size ranges. The filter pack assembly can detect aerosols greater than 1 µm, while the ACSM detects particles below 1 µm. Figure 10 shows the cumulative total concentration over a 3 month exposure period. While the ACSM is capable of time resolved measurements, chloride was not detected because the chloride aerosols predominantly have greater than the 1 µm diameter or the vaporizer temperature too low (600°C) to instantly vaporize MexCly compounds. Ongoing work is studying if the chloride deposition rate is proportional to the measured concentrations a detectable aerosol species such as sulfate or organics. The filter pack assembly captures both chloride and sulfate would be suffi -cient for site characterization, however it is worth noting that the deposition rate is lower than the wet candle method.

Figure 10. Integrated cumulative total of greater than 1 micron sized species captured by the filter pack assembly and submicron aerosol species collected in Key West, FL using the ASCM instrument. Mass spectrum fragmenta-tion tables used for chemical assignments are given in Ng8.

Comparison of Key West to a Navy ShipFor context, data collected from September 2015-January 2017 compared the cumulative severity of AgCl film thickness between NRL-KW and five locations on one Navy ship, where it is likely that ship operations would in-crease the exposure to atmospheric aerosols. The locations of the boards are given in Table 3. Boards 1, 2, and 4 were all located on the same deck but were oriented facing different directions. The ship exposures were per -formed twice, the first set were predominantly exposed while the ship was at port (with some limited time away from port but nearby). The second set was on the ship during deployment. The AgCl data for both sets is shown in Figure 11 along with a year’s worth of exposure data at Key West (over the same time period). While the ship was predominantly docked, the AgCl thickness was less than that from Key West. However, once the ship was de-ployed, the AgCl film was thicker on the samples exposed on the 04 level vs Key West and lower on the flight deck and well decks. Overall, the film thickness in the well deck was the lowest which is consistent with the low ambient light levels in this area of the ship even though there is a higher amount of seawater spray present. This observation supports the previous theories12 that sunlight is needed to catalyze the corrosion of silver in a pre-dominantly marine environment.

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Table 3. Locations of Boards on a Navy Surface Ship

Location

Board 1 04 Level, Inboard Bulkhead

Board 2 04 Level, Aft Bulkhead

Board 3 Well Deck

Board 4 04 Level, Forward Bulkhead

Board 5 Flight Deck (sheltered)

Figure 11. Average AgCl film thickness (nm) of Ag samples exposed in Key West, FL (black line) and on-board a US Navy ship (colored points). The samples exposed for ~180 days were exposed from SEP2015-MAR2016. The subsequent samples exposed for ~320 days were exposed from MAR2016-JAN2017. These were 2 separate sets of samples and were not exposed during the same time.

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SUMMARY

A suite of sensors and experimental methods to assess site to site variability in exposure severity remains an im -portant and challenging goal. Accessibility, cost and robustness are critical limitations of site characterization. Un-fortunately the most readily deployable methods are monitor materials or sensors. The surrogate monitor materi-als do not correlate very well to other monitoring methods. Environmental factors preclude current regression-based correction models of chloride deposition from monitoring materials (Ag, Cu). Sensors, like the filter pack as-sembly here, are not ideal for long term no-upkeep deployment, due to unexpected equipment failures.

All evaluated aerosol capture methods are less effective at capturing chloride than the wet candle method, which raises the question if passive deposition is important. Monitor materials and ambient concentration monitor meth-ods did not yield equivalent values to the wet candle, indicating missing chloride aerosols or plumes. However, there are biogenic sources of potentially corrosive species (e.g., organics/sulfides) that are not be sampled via the wet candle method but are present in the ambient environment as measured using the aerosol chemistry specia-tion monitor.

Some ongoing work involves correlating chloride deposition to the real time measurements of species detectable by the aerosol chemistry speciation monitor. Increasing the flow rate and decreasing the filter sizes of the filter pack assembly (100 nm) is a likely path to improve the ability of these sensors to measure both chloride and con-tent. If consistent wet candle measurements are infeasible, the filter pack assembly is the best option for consis -tent assessment of the chloride and sulfate content and their relative changes weekly.

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