ORI GIN AL PA PER

Corrosion of metal roof materials related to volcanic ashinteractions

Christopher Oze • Jim Cole • Allan Scott • Thomas Wilson •

Grant Wilson • Sally Gaw • Samuel Hampton • Colin Doyle •

Zhengwei Li

Received: 11 March 2013 / Accepted: 27 October 2013� Springer Science+Business Media Dordrecht 2013

Abstract Metal roofing material is commonly used for residential and industrial roofs in

volcanically active areas. Increased corrosion of metal roofing from chemically reactive

volcanic ash following ash deposition post-eruption is a major concern due to decreasing

the function and stability of roofs. Currently, assessment of ash-induced corrosion is

anecdotal, and quantitative data are lacking. Here, we systematically evaluate the corrosive

effects of volcanic ash, specifically ash leachates, on a variety of metal roofing materials

(i.e. weathered steel, zinc, galvanized steel, and Colorsteel�) utilizing weathering chamber

experiments and direct acid treatments. Weathering chamber tests were carried out for up

to 30 days, and visual, chemical, and surface analyses did not definitively identify sig-

nificant corrosion in any of the test roofing metal samples. Direct concentrated acid

treatments with hydrochloric (HCl), sulphuric (H2SO4), and hydrofluoric (HF) acids

demonstrate that roofing materials are chemically resilient. Our experimental results

suggest that ash-leachate-related corrosion is a longer-term process ([1 month), poten-

tially related to a multitude of factors including increased ash leachate concentrations, the

C. Oze (&) � J. Cole � T. Wilson � G. Wilson � S. HamptonDepartment of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140,New Zealande-mail: Christopher.Oze@canterbury.ac.nz

A. ScottDepartment of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800,Christchurch 8140, New Zealand

S. GawDepartment of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, NewZealand

C. DoyleResearch Centre for Surface and Materials Science, University of Auckland, Private Bag 92019,Auckland 1142, New Zealand

Z. LiBuilding Research Association of New Zealand, Private Bag 50908, Porirua 5381, New Zealand

123

Nat HazardsDOI 10.1007/s11069-013-0943-0

dissolution of the glass matrix of the ash, moisture retention at the ash-surface boundary,

and potential reactions involving photo-oxidation. Overall, corrosion is not a simple pro-

cess related to the short-term release of acid and/or salt leachates from the ash surface, but

a product of dynamic interactions involving ash and water at the surface of metal roofing

material for extended periods.

Keywords Volcanic ash � Leachates � Metal roofing materials � Corrosion

1 Introduction

Corrosion is a well-established phenomenon in volcanically active regions (Watanabe et al.

2006; Hawthorn et al. 2007; Lichti et al. 1996). Volcanoes produce a wide range of hazards

(i.e. particulate matter, aerosols, and gases) capable of leading to increased rates of cor-

rosion (Blong 1984). Specifically, widely distributed volcanic ash derived from explosive

volcanic eruptions creates both short- and long-term hazards to infrastructure including

exposed building materials such as metal roofing (e.g. Izumo et al. 1990; Blong 2003;

Watanabe et al. 2006). However, very little has been published on specific relationships,

and the reactivity and corrosion resistance of metal roofing in contact with volcanic ash,

rainfall, and atmospheric interactions remains unknown in residential building fragility

estimations (Magill et al. 2006).

Metal corrosion has been attributed to volcanic ash in several studies (Becker et al.

2001; Johnston 1997; Blong 2003; Matsumoto et al. 1988; Deguchi 1990), but these studies

are observational and are beset by limitations including (1) a lack of detail or supporting

documentation, (2) not accounting for pre-existing corrosion damage, and (3) reporting

damage where volcanic aerosols, gases, and/or acid rain may have also contributed to the

observed corrosion. For example, corrosion of galvanized steel was strongly correlated to

the amount of ash received without consideration of the effects and contributions of acid

rain and volcanic gases (Matsumoto et al. 1988). Although these studies do not quantify

corrosion, they provide sufficient evidence that volcanic ash has significant potential to

cause or increase rates of corrosion.

The focus of this study is to systematically and experimentally evaluate the corrosive effects

of volcanic ash and related volcanic acids on several types of metal roofing material, including

aged weathered steel, zinc, galvanized steel, and Colorsteel�. We evaluate both the acid and

salt components present as surface leachates on ash as well as the chemical and physical

properties of the solid ash matrix to identify both short (1 day)- and long-term (30 days) routes

related to increased corrosion. To achieve these objectives, metal roofing materials were coated

with ash, dosed with acidic water from active volcano crater lakes in the Taupo Volcanic Zone,

New Zealand. The coated samples were then placed in weathering chambers over a period of

30 days at the BRANZ facilities (Porirua, New Zealand) as well as via the direct application of

concentrated hydrochloric (HCl), sulphuric (H2SO4), and hydrofluoric (HF) acids to the roofing

material’s outer surface. Evaluating how metal roofing material may corrode in the presence of

volcanic ash will lead to better choices as to which type of metal roofing is best suited for

volcanic environments as well as for remediation techniques to minimize corrosion following a

volcanic eruption, especially as population growth, construction, and the use of metal roofing

increase in highly volcanically active regions such as Central America, equatorial Africa,

Colombia, Ecuador, and the Philippines (Horwell and Baxter 2006).

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2 Background

2.1 Corrosion of metals by atmospheric aerosols and particulates

The interaction of corrosive ionic species, or salts, on metal surfaces is an important con-

sideration in corrosion by airborne substances. As an example, Askey et al. (1993) determined

that contamination of metal substrates (zinc and mild steel) by fly ash particulates increased in

proportion to the quantity of leachable ionic species present in the fly ash, and Lau et al. (2008)

evaluated the effect of atmospheric aerosol ionic composition related to the corrosion of mild

steel. The most rapid rates of corrosion were associated with the ionic species Na? and Cl-

and to a lesser extent SO42-, NH4

?, K?, and Mg2?, whereas, the presence of Ca2? was found

to significantly inhibit corrosion potentially due to Ca2?-forming insoluble salts such as

Ca(OH)2. Finally, Comizzoli et al. (1986) emphasized the importance of chloride as a cor-

rosive species due to its ability to form a range of soluble compounds.

The role of atmospheric conditions is another facet related to corrosion. Important

parameters include rainfall amount and composition, wind speed and direction, tempera-

ture, and relative humidity. Additionally, Cole et al. (2011) and Johnston (1997) both note

that structures such as buildings typically present a range of microclimates. For instance,

components under the eaves of a building are sheltered from rain that could remove

deposited aerosols, but exposed to winds that can increase deposition. Relative humidity

(RH) is a factor related to atmospheric conditions. Any deposited hygroscopic salts (salts

that attract moisture from the atmosphere) will become deliquescent (dissolve in moisture

from atmosphere) when the RH at the metal surface exceeds the deliquescent RH for a

particular salt. Thus, even if aerosols or particles are deposited dry, they can still promote

the formation of a conductive surface film.

2.2 Volcanic ash and its corrosive potential

2.2.1 Introduction to volcanic ash

Volcanic ash is the material produced by explosive volcanic eruptions that is \2 mm in

diameter. Fine ash is\0.063 mm, and coarse ash is between 0.063 and 2 mm. Volcanic ash

is made up of vitric (glassy, non-crystalline), crystalline, and lithic (non-magmatic) par-

ticles. The density of individual particles may vary between 700 and 1,200 kg/m3 for

pumice, 2,350 and 2,450 kg/m3 for glass shards, 2,700 and 3,300 kg/m3 for crystals, and

2,600 and 3,200 kg/m3 for lithic particles. Since coarser and denser particles are deposited

close to the source, fine glass and pumice shards are relatively enriched with ash fall

deposits at distal locations (Shipley and Sarna-Wojcicki 1982).

Additionally, vitric particles typically contain small voids or vesicles formed by

expansion of magmatic gas before the enclosing magma solidified. Ash particles have

varying degrees of vesicularity with each vesicular particles having extremely high surface

area-to-volume ratios. Exsolved magmatic gases condense onto ash particle surfaces while

they are in the conduit and ash plume (Delmelle et al. 2007).

2.2.2 Volcanic ash surface chemistry

A range of sulphate and halide (primarily chloride and fluoride) compounds may be

mobilized from fresh volcanic ash (Fruchter et al. 1980; Delmelle et al. 2007; Jones and

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Gislason 2008). These salts are formed as a consequence of rapid acid dissolution of ash

particles within eruption plumes, which is thought to supply the cations involved in the

deposition of sulphate and halide salts (Delmelle et al. 2007). In support of this contention,

Oelkers (2001) reported that selective leaching of metals from silicate lattices is coupled to

proton consumption, consistent with metal-proton exchange reactions.

While more than 50 ionic species have been reported in fresh ash leachates, the major

species found are the cations Na?, K?, Ca2?, and Mg2? and the anions Cl-, F-, and SO42-

(Witham et al. 2005; Jones and Gislason 2008). Molar ratios between ions present in

leachates suggest that in many cases these elements are present as salts such as NaCl and

CaSO4 (Witham et al. 2005; Taylor and Lichte 1980; Smith et al. 1983; Risacher and

Alonso 2001). In a sequential leaching experiment on ash from the 1980 eruption of Mount

St Helens, chloride salts were found to be the most readily soluble, followed by sulphate

salts (Taylor and Lichte 1980). Fluoride compounds are in general only sparingly soluble

(e.g. CaF2, MgF2), with the exception of fluoride salts of alkali metals. Readily soluble

fluoride may be contained within salts such as NaF and CaSiF6 (Cronin and Sharp 2002).

The pH of fresh ash leachates is highly variable. Jones and Gislason (2008) studied the

interaction of a range of different volcanic ashes with both deionized water and sea water.

Ash from the 2000 eruption of Hekla was found to be most acidic, with an initial pH of 3.5

in contact with deionized water. Hekla ash was also very high in fluoride, which in turn led

to higher bulk mineral dissolution rates (Frogner Kockum et al. 2006; Wolff-Boenisch

et al. 2004). Ash from the 2005 eruption of Galeras, the 2003 dome collapse of Soufriere

Hills volcano, Monsterrat, and a 1994 eruption of Sakura-jima volcano, Japan, had similar

initial pH values of approximately 4.6. Other studies have also reported neutral to basic ash

leachates. Gislason et al. (2011) reported that the pH of ash leachate from the explosive

phase of the April 2010 Eyjafjallajokull eruption was slightly basic at pH 8. However, the

ash from the later phase of the eruption was more acidic, with a pH of approximately 5.1,

implying the presence of acid salts on the ash surface. In a study of fresh ash leachates

from the 1993 eruption of Lascar, Chile, Risacher and Alonso (2001) reported pH values

were as high as 10.5 for ash leachates.

The release of salts from freshly fallen ash is rapid (Taylor and Lichte 1980; Witham

et al. 2005; Jones and Gislason 2008; Gislason et al. 2011). Gislason et al. (2011) reported

that for the Eyjafjallajokull ash, surface salts dissolved rapidly, in less than 15 min. Jones

and Gislason (2008) noted that ash surface coatings appeared unstable, decaying in situ

even if kept unhydrated. Fewer studies have been concerned with the longer-term release

of soluble material from deposited ash. Jones and Gislason (2008) noted that ash leachate

dissolution occurs several orders of magnitude faster than dissolution of volcanic glass

(Wolff-Boenisch et al. 2004). Oelkers (2001) provides a useful general review of multi-

oxide silicate mineral and glass dissolution. He notes that multioxide dissolution proceeds

via a series of metal-proton exchange reactions, whereby alkali metals are exchanged first,

followed by calcium, magnesium, aluminium, and finally by the breaking of Si–O bonds.

3 Materials and methods

3.1 Roofing materials

Four types of roofing material were utilized in our experiments and include old steel (x),

zinc (z), galvanized steel (g), and Colorsteel� (s). Samples of used corrugated iron

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(c. 50 years old) were sourced from a farm in mid-Canterbury, New Zealand. Samples of

zinc sheeting (Zn *99 wt%, Cu 0.08–1.0 wt%, Ti 0.07–0.20 wt%, and Al 0–0.015 wt%)

are from MICO Metals. The zinc used is referred to as ‘Anthra Zinc’ and is pre-weathered

to provide the appearance of a natural patina. Normal galvanized steel (bulk composition

of Fe *99 wt%, C 0.12 wt%, Mn 0.5 wt%, S 0.035 wt%, and P 0.03 wt%) and Color-

steel� (Fe *99 wt%, C 0.05 wt%, Mn 0.2 wt%) are from Steel and Tube Ltd. The

Colorsteel� has a metallic coating of 43.3 Zn wt%, 55 Al wt%, and 1.5 Si wt% with the

paint being a pigmented organic resin. Samples were cut into 100 mm 9 100 mm squares

using hydraulic sheet metal shears and engraved on the underside with the sample code.

3.2 Ash and acid dosing

Characterization of metallic roof material vulnerability to volcanic ash ideally relies on

using fresh volcanic ash, as acidic soluble salts attached to recently erupted ash are

hypothesized to be the main driver for corrosion at the metallic surface. Given the high

solubility of salts, it is vital ash used for laboratory testing has not been rain washed or

been on the ground for more than a few hours (ideally not at all). These requirements create

considerable logistical and financial challenges to collect sufficient ash for laboratory

testing. The locations of active volcanoes are not always conducive to collection of a

sufficient quantity of ash needed for the study, and even if the ash could be collected and

transported, there are biosecurity issues that make utilizing the ash for the study

impractical.

Due to these issues, a ‘pseudo-ash’ was synthesized and utilized rather than relying on

fresh volcanic ash. Large quantities of basalt from Banks Peninsula, New Zealand, were

prepared and designed to have the same geotechnical characteristics (size, grain-shape,

density, etc.) of ash from the Taupo Volcanic Zone (TVZ), New Zealand. Crater lake water

from both White Island and Ruapehu was utilized to dose the raw pseudo-ash at a ratio of

4:1 (20 mL ash to 5 mL of crater lake solution with the crater lake water diluted to 20 %)

to simulate the chemical/leachate properties of freshly erupted ash. Based on development

and utilization over the past three years (Broom, 2010; Wardman et al. 2012, Wilson et al.

2012a), this combination was determined to be the most representative of the leachate

chemistry in natural ash samples and falls within the range of ash leachate levels presented

by Ayris and Delmelle (2013). For example, the release and concentration of sulphate

(SO42-), chloride (Cl-), and fluoride (F-) from the pseudo-ash compared very well to

‘pristine’ ash collected from the 1995 Mt Ruapehu eruption and the 1980 Mt St Helens

eruption with values of *4,000 mg kg-1 Cl-, *900 mg kg-1 SO42-, and *25 mg kg-1

F- (Witham et al. 2005).

We would like to emphasize that one major limitation of the pseudo-ash is related to

amount of crystalline to glassy components. Although the bulk composition of ash is

similar to fresh ash, it contains less glass and more crystalline components (i.e. feldspar

and mafic minerals) which may have direct implications on how the ash may dissolve/

weather over longer durations. As the weathering experiments (described in the next

section) are designed to test the short-term impact (\1 month) of volcanic ash and its

easily released leachates, this ash should be suitable for this study.

3.3 Weathering chamber

Testing of roofing samples at the BRANZ facilities, Moonshine Road, Porirua, commenced

in November 2010. A programme was set up in which the four different types of roofing

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material were coated with *5 mm of pseudo-ash (Fig. 1a, b), which had been dosed with

either Crater Lake, Ruapehu, or White Island crater lake waters, and subjected to alter-

nation of mist and drying in a Q-fog chamber (Fig. 1c) for a period of 1 day, 1 week, or

1 month. The operating conditions are summarized as follows: (1) First Stage: water

spraying, 6 h, 25 �C, 0.3–0.4 mm/h (deionized water \10 ls/cm); (2) Second Stage: air

drying, 18 h, 35 �C; and (3) total time 24, 168, and 720 h. This wet-drying protocol was

established by BRANZ to investigate the corrosion performance of metallic fasteners in

treated timbers exposed to the atmosphere. The amount of water sprayed, time duration,

and temperature were selected based on climate data (e.g. ambient temperature, rainfall) at

the BRANZ Porirua facilities. In accelerated corrosion testing such as salt spray, cyclic

testing with drying and wetting subcycles is normally accepted to be more reliable than

continuous spraying.

The position (and orientation) of the sample trays in the chamber was randomly changed

every week. Each sample tray had samples of one water type (in order not to cross-

contaminate), two different roofing types, and three duplicate samples (to test variability)

for a total six samples per tray (Fig. 1b). Four trays were used for each time period. The

total number of samples tested was 72. After removal from the Q-fog chamber, excess ash

was removed by tapping the side with a wooden spatula. Samples were transferred from

BRANZ to the University of Canterbury in specially designed boxes (Fig. 1d) to ensure

that no contamination occurred between samples.

Fig. 1 Images of the BRANZ testing facility and experiments are shown demonstrating: a and b theapplication of pseudo-ash to the roof materials, c samples placed in the Q-fog chamber, and d shipmentboxes utilized to prevent cross-contamination

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3.4 Sample preparation for microanalytical analyses

A third of the exposed metal samples were chosen for microanalytical analyses. This

included a sample from each metal type, each duration, and each ash type. Eight slugs

(10 mm in diameter) were cut from each sample using a hand-operated press, with care taken

not to damage the exposed surface. Half of the slugs were washed with a soft bristle brush in

deionized water and dried in a 50 �C oven overnight to remove ash and expose the metal

surface for analysis. Each slug was labelled with a numerical code and placed inside a

corresponding sample bag that was enclosed in a plastic container for transport and analysis.

3.5 Direct interaction of metals with concentrated acids

In order to evaluate the reactivity of individual acids common in ash leachates, each roofing

material was exposed to concentrated HF, H2SO4, and HCl acids. Three 10-mm-diameter

slugs of each metal type (galvanized steel, old steel, zinc, and Colorsteel�) were placed in

upturned plastic lids and dosed with trace metal grade ultrapure HF, H2SO4, and HCl. One

drop of each acid was placed directly onto the metal surface and left to evaporate in a fume

hood. Once the acid had evaporated, each sample was washed with ultrapure water and was

dried at room temperature before being placed in sample bags ready for analysis.

3.6 Scanning electron microscopy

An FEI Quanta 200F scanning electron microscope (SEM) with energy-dispersive X-ray

spectroscopy (EDS) capabilities at the University of Auckland was utilized for the char-

acterization of roofing and ash materials. Voltage, spot size, magnification, etc., for each

sample are displayed in the data bar of each image.

3.7 X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos Axis UltraDLD

instrument equipped with a hemispherical electron energy analyser. Spectra were excited using

monochromatic Al Ka X-rays (1,486.69 eV) with the X-ray source operating at 150 W. This

instrument illuminates a large area on the surface, and then using the hybrid (magnetic and

electrostatic) lens, system collects photoelectrons from a desired location on the surface. In this

case, the slot aperture defined an analysis area, which was 300 by 700 microns. The mea-

surements were carried out in a normal emission geometry. A charge neutralization system was

used to alleviate sample charge build-up, resulting in a shift of approximately 3 eV to lower

binding energy. Survey scans were collected with a 160 eV pass energy. The analysis chamber

was at pressures in the 10-9 torr range throughout the data collection. Data analysis was

performed using CasaXPS. Shirley backgrounds were used in the peak fitting. Quantification of

survey scans utilized relative sensitivity factors supplied with the instrument.

4 Results

4.1 BRANZ weathering chamber roofing samples

Based on cursory visual inspections, no corrosion is visibly present on any sample based on

the lack of discoloration, indicating (oxy)hydroxide formation (i.e. metal staining). Metal

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release, metal (oxy)hydroxide formation, surface precipitates, and/or a change in surface

topography in relation to the controls are an expected indication of corrosion. Ash adhering

to the roofing surfaces provides some discoloration and topography on the metal surface.

When the ash is removed with a brush, the roof surfaces appear identical to their original

condition.

Scanning electron microscopic images of each roofing material prior to ‘reacting’ them

in the weathering chamber are shown with increasing magnification in Fig. 2. The surface

of old steel is highly corroded and comparatively displays the greatest surface topography.

(b)(b) Zinc (z) Zinc (z)

(d)(d) Colorsteel (s) Colorsteel (s)

(c)(c) Galvanized (g) Galvanized (g)

(a)(a) Old Steel (x) Old Steel (x)

©

Fig. 2 SEM images of a old steel (x), b zinc (z), c galvanized (g) steel, and Colorsteel� (s) at threemagnifications (9100, 9800, and 93,000) from left to right are shown

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Zinc appears to have linear streaks at low magnification and a bladed metal texture that

protrudes from the surface at higher magnification. Galvanized steel has the least topo-

graphic surface; however, at higher magnification, pock marks are present in rounded

protrusions that make topographic highs on the metal’s surface. Flow ridges demonstrate

the application of the resin on Colorsteel� at low magnification, and the metal oxides that

comprise the resin are visible at higher magnification. Surface chemistry analyses via EDS

for each metal surface collected at the same time as SEM imaging are shown in Table 1.

Scanning electron microscopic images of the roofing surfaces after 20–30 days are

shown in Fig. 3. Despite physical removal of the ash, ash is present as a superfluous

surface product on top of all roof surfaces examined. Ash appears to adhere best to the

highly topographic surface of the old steel and less so compared to the newer and non-

corroded roof materials. The ash on the Zn surface is entrained in between the microblades

where the lack of microtopography in the galvanized and Colorsteel� steel does not

provide ample sites for the ash to adhere to the metal surface. More important than ash

adherence, no new corrosion is physically (images) or chemically (EDS analyses) evident

in any of the roofing samples compared to their respective standards. Only the ash and/or

the roof surface was visually seen in SEM images, and these observations were supported

with EDS analyses (Fig. 3) where we interpret the analyses to represent a combination of

the ash and the roofing surface. No evidence is present to support the formation of any

secondary phases that might indicate corrosion.

X-ray photoelectron spectroscopic analyses based on atomic percentage for all the metal

roofing materials (washed and unwashed) with respect to the ash type (Ruapehu or White

Island) over time (1 day, 1 week, and 1 month) are shown in Fig. 4. Untreated samples

serve as a standard to evaluate potential ash–metal surface interactions that may indicate

corrosion. For all samples, carbon decreases and oxygen increases or remains at similar

Table 1 EDS analyses of roofing materials as shown in Fig. 2

Element Old steel Zinc Galvanized steel Colorsteel�Atomic percentage

C 33.8 78.2 77.0

O 39.1 12.2 10.9 16.5

Mg 0.3

Al 1.8 2.8

Si 4.3 1.4 1.3

P 0.9 2.8

Si 0.6

Cl 0.3

K 0.7

Ca 0.5

Ti 1.4

V 0.1

Cr 2.6

Fe 0.3

Ni 0.2

Cu 1.1

Zn 17.4 5.3 86.3

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(b)(b) Zinc (z)Zinc (z)

(d)(d) Colorsteel (s)Colorsteel (s)

(c)(c) Galvanized (g)Galvanized (g)

(a)(a) Old Steel (x)Old Steel (x)

©

Sample G21R

Element Atomic %

O 8.34

Al 2.31

Si 0.9

P 2.71

Cl 0.17

Ca 0.13

Fe 0.2

Zn 85.22

Total 100

Sample Z22W

Element Atomic %

C 68.61

O 17.79

Mg 0.56

Al 0.66

Si 3.12

P 2.87

Ca 0.41

Fe 0.36

Co 0.09

Zn 5.53

Total 100

Sample X31R

Element Atomic %

C 37.66

O 39.22

Mg 0.66

Al 1.94

Si 4.73

S 0.52

Cl 0.13

Ca 0.6

Fe 0.66

Zn 13.88

Total 100

Sample S31R

Element Atomic %

C 64.68

O 22.61

Mg 0.54

Al 1.11

Si 3.71

Pt 0.24

Cl 0.09

Ca 0.38

Ti 1.63

V 0.03

Cr 2.96

Fe 0.81

Cu 1.21

Total 100

Fig. 3 SEM images of a old steel (x), b zinc (z), c galvanized steel (g), and colour steel (s) after being reactedwith doped pseudo-ash after varying durations with EDS analyses (star) adjacent to each appropriate image

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levels with respect to time. Additionally, all the samples except for the Colorsteel� have

Zn values that generally increase with respect to time in the weathering chamber.

4.2 Corrosion with concentrated acids

Roof surfaces treated with concentrated HCl, HF, and H2SO4 acids provide an opportunity

to separately examine the reactivity of acids present as surface leachates on volcanic ash

surfaces. The SEM investigation of the roof surfaces after reacting with each acid only

shows nondescript efflorescences. As an example, SEM images of galvanized steel with

respect to each acid treatment are shown in Fig. 5 where only the efflorescences are

apparent. X-ray photoelectron spectroscopic analyses for each roofing material and acid

1 Month Unwashed1 Month Washed

1 Week Washed1 Day Unwashed

1 Day WashedOld Steel:Standard

0

20

40

60

Al

Au C

Ca

Cl

Cr

Cu F

Fe K

Mg

Mn N Na

Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Old Steel:Standard1 Day Washed1 Day Unwashed

1 Week Washed1 WeekUnwashed1 Month Washed1 Month Unwashed

0

20

40

60

Al

Au C Ca

Cl

Cr

Cu F

Fe K N

Na

Ni O P S S

iS

nZ

n

Ato

mic

Per

cen

t

Element

Zinc: Standard1 Day Washed1 Day Unwashed1 Week Washed1 Week Unwashed1 Month Washed1 Month Unwashed

0

50

100

Al

Au C

Ca Cl

Cr

Cu F

Fe K

Mg

Mn N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Zinc: Standard1 Day Washed1 Day Unwashed1 Week Washed1 Month Washed1 Month Unwashed

0

50

100

Al

Au C

Ca Cl

Cr

Cu F

Fe K

Mg

Mn N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Galvernised Iron: Standard1 Day Washed

1 Week Washed1 Week Unwashed1 Month Washed1 Month Unwashed

0

20

40

60

80

Al

Au C

Ca Cl

Cr

Cu F

Fe K

Mg

Mn N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Galvernised Iron: Standard1 Day Washed

1 Day Unwashed1 Week Washed1 Month Washed1 Month Unwashed

0

20

40

60

80

Al

Au C

Ca Cl

Cr

Cu F

Fe K

Mg

Mn N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Coloursteel:Standard1 Day Washed1 Week Washed1 Month Washed1 Month Unwashed

0

20

40

60

80

Al

Au C

Ca Cl

Cr

Cu F

Fe K

Mg

Mn N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

Coloursteel:Standard1 Day Washed1 Week Washed1 Month Washed

0

20

40

60

80

Al

Au C

Ca Cl

Cr

Cu F

Fe K N

Na Ni

O P S Si

Sn

Zn

Ato

mic

Per

cen

t

Element

(b)(b) Zinc (z)Zinc (z)

(d)(d) Colorsteel (s)Colorsteel (s)

(c)(c) Galvanized (g)Galvanized (g)

(a)(a) Old Steel (x)Old Steel (x)

©

Mg

Mn

Mg

Mn

Ruapehu Pseudo-Ash White Island Pseudo-Ash

Ruapehu Pseudo-Ash White Island Pseudo-Ash

Ruapehu Pseudo-Ash White Island Pseudo-Ash

Ruapehu Pseudo-Ash White Island Pseudo-Ash

Fig. 4 XPS analyses provided in atomic percentage of a old steel (x), b zinc (z), c galvanized steel (g), andcolour steel (s) before and after being reacted with pseudo-ash doped with Ruapehu and White Island water

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combination are shown in Fig. 6. As the acids are trace metal grade, elemental concen-

trations (specifically the metals) likely represent elements that were released from the roof

surface and subsequently incorporated into the resulting efflorescence. Metal values in the

old steel and Colorsteel� are not significantly different from those in the untreated stan-

dard. Silica concentrations are higher in the efflorescences than in the carbon surface film

originally present on the Zn and galvanized steel roofing surfaces.

5 Discussion

Our results show that no significant corrosion was observed macroscopically, micro-

scopically, or chemically. This result is surprising due to the observed corrosion after a

HClHCl

Galvanized (g)

HFHF H SOH SO2 4

Fig. 5 SEM images of galvanized (g) steel after being reacted with concentrated HCl, HF, and H2SO4

0

10

20

30

40

50

60

Al C Ca Cl F Fe Mn N O P S Si Zn

0

10

20

30

40

50

60

70

80

Al C Ca Cl F Fe Mn N O P S Si Zn

(d)(d) Colorsteel (s)Colorsteel (s)

(c)(c) Galvanized (g)Galvanized (g)

©

Element

Element

Ato

mic

Per

cen

tA

tom

ic P

erce

nt

HClStandard

HFH SO

HClStandard

HFH SO0

51015202530354045

Al C Ca Cl F Fe Mn N O P S Si Zn

0102030405060708090

Al C Ca Cl F Fe Mn N O P S Si Zn

(b)(b) Zinc (z)Zinc (z)

(a)(a) Old Steel (x)Old Steel (x)

Element

Element

Ato

mic

Per

cen

tA

tom

ic P

erce

nt

HClStandard

HFH SO

HClStandard

HFH SO

2 4

2 42 4

2 4

Fig. 6 XPS analyses provided in atomic percentage of a old steel (x), b zinc (z), c galvanized steel (g), andcolour steel (s) before and after being reacted with concentrated HCl, HF, and H2SO4

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number of eruptive events or in other experiments. Some metal–fluid interactions did

occur, with changes of C, Si, and Zn interactions present at the surface, but it was clearly

insufficient to induce major corrosion within the time frame of the experimental work.

These results suggest corrosion must be time dependent or potentially related to a factor

not examined in our experiments. It is, therefore, important to consider how corrosion will

occur over a longer time frame ([30 days), and what other factors could influence this.

Naturally, volcanic ash on roofs will be able to retain water for longer than without ash,

thereby increasing the time that potentially acidic solutions are in contact with roofing

material. This is one alternative to how ash might provide a long-term source of acidity.

However, other factors may alter the pH of rainwater and serve as a continual source of

corrosion capable of corroding metal roof materials.

5.1 Other potential routes of increasing corrosion

5.1.1 Colour

Colour is a factor capable of influencing the temperature of a material when exposed to

sunlight. Darker ashes will retain more heat compared to analogous ones with lighter

colours. Likewise, fluids adsorbed onto ash surfaces and/or pore spaces will also be sub-

jected to temperature changes experienced by an ash. As the temperature of water (pH 7 at

25 �C) increases, the pH of the water will decrease such that at 50 �C water will have a pH

of 6.55. Based on this line of reasoning, darker-coloured ashes are more capable at low-

ering the pH of rainwater; however, this is probably not a significant mechanism leading to

enhanced corrosion compared to other potential factors.

5.1.2 Atmospheric interactions with CO2 and O2

Water interacting with the atmosphere is subject to a wide variety of chemical reactions

involving carbon and oxygen. Water in equilibrium with atmospheric CO2 (pCO2 = 10-3.5

atm) has a pH of 5.66 due to the formation of carbonic acid, discounting the effects of

industrial and volcanic inputs that may create acid rain. As the XPS data show, all roof

samples reacted have carbon as a main constituent comprising the surface film and support

that carbon is involved in reactions occurring at the metal surface. In addition to increasing

the acidity of ash–water interactions, O2 will provide an oxidizing influence capable of

increasing the potential of corrosion. As our BRANZ weathering experiments allowed

atmospheric interactions, a longer time period must be needed for these interactions to

induce corrosion.

5.1.3 Weathering of ashes and acid-forming cations

Exchangeable and soluble Al3? released during ash dissolution has a strong tendency to

split water molecules into H? and OH- via hydrolysation. Aluminium combines with OH-

ions, resulting in excess H? that will lower the pH of the solution. Soluble Al3? from

minerals and the glassy matrix will serve as a long-term source of acidity (pH values

between 4.5 and 5) until the ash is completely removed, counter to the argument that the

ash should consume acidity (McBride 1994).

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5.1.4 Photo-oxidation

A further corrosion mechanism not addressed in our study is the photocatalysed degra-

dation of protective organic coatings on roofing materials induced by the ash layer. Our

experiments were carried out in the dark, preventing any potential photocatalytic effects

from being observed. Volcanic ash contains between 0.2 and 2 % of the photocatalyst

titanium dioxide (TiO2) (Nockolds 1954). Titanium dioxide reacts with water in the pre-

sence of UV light and oxygen to produce hydroxyl radicals that non-selectively and

aggressively oxidize a wide range of organic compounds including polymers (Malato et al.

2009; Hidaka et al. 1996; Chen et al. 2007).

Two forms of TiO2, rutile and anatase, are photocatalytically active with the anatase

form being more active. Titanium-containing volcanic ash has been suggested as an

inexpensive source of photocatalytic material and has been demonstrated to successfully

degrade phenol in solution (Borges et al. 2008). There is evidence to suggest that Ti-

containing volcanic ash could induce corrosion of roof materials by degrading protective

coatings. An investigation into unexpected and rapid deterioration of surface coatings of

pre-painted steel roofs in Australia found that the damage was caused by contact with

sunscreens that contained TiO2 nanoparticles as UV absorbers. The sunscreens were worn

by the roofing contractors during installation and were inadvertently transferred to the

roofing materials. Subsequent investigations found that the sunscreens containing titanium

oxide nanoparticles could increase degradation rates of surface coatings by a factor of 100

and were able to degrade a range of organic polymer surface coatings including polyester-

melamine-, polyester-urethane-, and polyvinylidene difluoride (PVDF)-based polymers

(Barker and Branch 2008). As TiO2 is chemically and thermally stable and resistant to

chemical degradation (Chong et al. 2010), any induced photocatalysis would occur until

the ash had been removed from the roof surface. This photocatalytic effect could poten-

tially occur over a longer time frame than effects due to dissolution of acidic salts.

5.2 Ash interactions and adhesion related to long-term ([30 days) corrosion

The experiments indicate that either more time or a greater thermodynamic drive for

corrosion was needed. From a macroscopic perspective, one way to increase this chemical

drive would be to increase the ash layer thickness. Simply, this would provide more ash

leachates to be delivered to the roof surface. From a microscopic perspective, increasing

the topography of the surface would allow more sites/places for the ash to adhere and react

on the roofing surface.

Due to the porosity and relatively high surface area of volcanic ash, water can be

adsorbed on the ash surface for extended periods of time and then released/flushed with

periodic precipitation. This cycle may continue on indefinitely allowing a wide variety of

atmospheric interactions as well as dissolving ash inputs such as Al until the ash is

removed (physically or via dissolution). Another consideration is the residence time and

flow rate of fluid. Longer residence times and/or slower flow rates of water will only

increase or enhance chemical reactions occurring at the surface. This residence time is then

directly related to the ash thickness as well as the ability of ash to adhere to surface of the

roof. One way to directly address this issue is to make roofing material as smooth and as

chemically inert as possible. An alternative is to simply remove the ash from the roof in

less than 1 month after deposition. The caveat of ash removal is that it needs to be

completely removed, which requires more than sweeping it off or rinsing a roof down with

water. Gentle sweeping that minimizes abrasion followed by power washing with water

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would probably be the most effective way to ensure that ash was removed as much as

possible. If power washing is undertaken, it may create significant demands on water

supply; therefore, a phased cleaning of neighbourhoods may be appropriate. Down pipes

should also be disconnected from wastewater systems to avoid ash ingestion which may

cause blockage or damage to wastewater treatment plants (Wilson et al. 2012b).

5.3 Zinc layer considerations

The Pourbaix diagram for zinc shows active corrosion for a pH of less than 7 at 25 �C with

Zn2? being the stable species (Beverskog and Puigdomenech 1997). The rate of corrosion

is known to increase with both a decrease in the pH and an increase in the concentration of

chlorides. A drop in pH from 7 to 5, for instance, can result in reduction in the service life

for a 75-lm zinc coating from approximately 150–25 years (American Galvanizers

Association 2011). Where chlorides are present, the reduction can be even greater. Thomas

et al. (2012) determined the corrosion current density for zinc in a solution of 0.1 M NaCl

with a pH of 3 to be approximately 50 lA/cm2, which is equal to a penetration depth of

60 lm over a 1-month exposure period. Given that the zinc layer of samples under

investigation was *38 lm, it is not surprising the underlying steel remained protected

since the pH of the leachate only decreased for a relatively short period of time and the

chloride concentration would have been well below 0.1 M.

5.4 Hazard implications

The results presented here suggest that there is relatively little risk of significant loss of

cross section of the roofing material due to tephra-induced corrosion that would lead to

immediate significant damage or loss of structural integrity for exposure periods of less

than one month. We, therefore, suggest that the primary concern for metal roofing material

immediately after an ash fall should be the static load caused by ash accumulation, par-

ticularly for roofs with a shallow pitch (Spence et al. 2005). Further investigation is

required to determine the impact of long-term exposure to volcanic ash in addition to

quantitative mass loss measurements to determine the loss of future service life, but these

are unlikely to affect the short-term structural performance of the roofing material.

Therefore, these results suggest that urgent cleaning of roofs is not required to mitigate

roof damage. Injuries caused by falls during roof cleaning after an ash fall are common

(Magill et al. 2013), particularly on the slippery ash-covered surfaces. Urgent cleaning of

thick ash deposits ([100 mm) may be unavoidable to reduce the structural load. However,

ash falls \100 mm may be left for several days to weeks until individuals have the

necessary safety equipment for safe roof cleaning. Further investigation of cleaning

methods that minimize roof surface damage is also recommended. It is unknown whether

ash cleaning methods, such as firm sweeping, may cause excessive abrasion to metal roof

surfaces.

6 Conclusions

These experiments attempted to provide quantitative information with regard to the rates of

corrosion of different types of metal roof materials when subjected to coating by a volcanic

ash proxy. However, they demonstrate that no significant corrosion was macroscopically or

microscopically present on any of the roofing surfaces despite the presence of corrosive

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salts after a duration of 30 days and suggest ash-leachate-related corrosion is not a major or

immediate concern in the short term (\1 month). Despite our best attempt to create

environmental conditions to promote corrosion, one possibility is that our laboratory

experiments could not recreate all the proper conditions that may be responsible for ash-

leachate-related corrosion. Anecdotal evidence from areas experiencing volcanic ash

deposition on metal surfaces over extended periods ([1 month–years) is that corrosion can

become a major problem. For example, results from Sword-Daniels et al. (2013) and

Wilson et al. (2012b) suggest that a corrosion assessment should be part of a long-term

approach unless the corrosive conditions greatly exceed that presented in this study.

Sweeping or rinsing a roof down with water is unlikely to be effective as ash is still able to

adhere to the roofing surface. A more rigorous method of cleaning roof material (without

damaging the roof) is advisable.

Acknowledgments The authors would like to gratefully acknowledge funding from Earthquake Com-mission (EQC) [Contract: 10/SP607] and the MSI Natural Hazard Research Platform [Subcontract:C05X0804] for financial assistance with this project. We also thank Carol Steward and two anonymousreviewers for their input and comments.

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