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Materials Chemistry and Physics 105 (2007) 260–267

Corrosivity of long-term fire retardant Fire Trol® 931 andFire Foam® 103 on aluminium alloys and steel

Stefano Rossi a,∗, Marielle Eyraud b, Florence Vacandio b, Yvan Massiani b

a Department of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38050 Trento, Italyb MADIREL, UMR 6131, Universite de Provence, Centre de Saint Jerome, 13397 Marseille Cedex 20, France

Received 4 October 2006; received in revised form 16 April 2007; accepted 19 April 2007

bstract

Fire retardants and foams are rapidly gaining acceptance as effective and efficient tools by fire management agencies. There is a lack of studybout the corrosive effect of fire retardants despite they might produce a corrosion phenomena on various equipments during extinction operationsnd storage. Moreover, some aluminium components present a blue layer on the surface after use of some of the fire retardants which alarmed there operators.The aim of this paper is to evaluate the corrosion behaviour of aluminium and steel alloys in contact with Fire Trol® 931 and Fire Foam® 103

sing electrochemical measures. Complementary results about morphology are obtained with microscopy before and after corrosion tests. Structure

nd composition of the corrosion products on the electrode surface are also analysed. In these solutions the corrosion rate of aluminium is low,hereas steel alloy shows a moderate corrosion. The anodic reaction of corrosion phenomenon favours the deposit of a Prussian blue layer on theetallic samples surface. This layer reduces the corrosion phenomenon on both aluminium and steel alloys. 2007 Elsevier B.V. All rights reserved.

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eywords: Corrosion; Electrochemical techniques

. Introduction

Fire retardants and foams are rapidly gaining acceptance asffective and efficient tools by fire management agencies [1,2].

Two types of chemicals are basically used in wildland fireontrol: long-term and non-foam fire retardant chemicals andhort-term foam fire suppressant chemicals [3].

Long-term retardants are the red liquids dropped from the air-ankers, often seen in media coverage of wildland fire-fightingctivities. These products are supplied as either wet or dry con-entrates, and are mixed with water to ensure uniform dispersalefore they are dropped on the target area. When the water hasompletely evaporated, the remaining chemical residue forms arebreak that retards vegetation or other materials from igniting4]. Long-term fire retardants are effective fertilizers, composed

f ammonium phosphate and ammonium sulphate salts [2].everal research papers are dedicated to highlight the fire extin-uishing capability of LTR and substantiated the effectiveness

∗ Corresponding author. Tel.: +39 0461 882442; fax: +39 0461 881977.E-mail address: Stefano.Rossi@ing.unitn.it (S. Rossi).

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f long-term ammonia salt retardants [3,5–9]. In fact, the ammo-ium salts chemically combine with cellulose upon heating, thusemoving the fuel [2] and altering the way the fire burns. Theesult is a decrease in both the fire intensity and the rate of spread,ven after the water originally contained in the mixture has evap-rated. Retardants can be supplied as wet or dry concentrates toe prepared at a tanker base to produce mixed retardant that isnthickened or low, medium, or high viscosity, gum-thickenedhen mixed with water for use [4]. Long-term retardants, mixed

or delivery to the fire, contain about 85% water, 10% fertilizer,nd 5% other ingredients, such as thickeners made of natu-al gum and attapulgite clay thickener (hydrated magnesiumilicate), corrosion inhibitors, such as sodium dichromate orodium fluorosilicate and bactericides [3,5,10]. Small quanti-ies of colouring agents, such as iron oxide are added to markhe location of retardant drops [3].

The second type of chemical products used in the fire con-rol is foam. Foams are supplied as liquid concentrates that are

ixed with water. They contain foaming agents, which affectow the product clings to surfaces and how quickly the mixedater drains out of the foam and wetting agents. They also raise

he ability of the drained water to penetrate fuels increasing

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ater efficiency and reducing the ability of the fuels to ignite2,4,10,11]. The use of foam suppressants in fire fighting isecoming more prevalent because the amount of water requiredn fire fighting can be reduced by over 60% [3].

Considering the wide use during fire-fighting operations inarticular in natural reserves with high environmental value its very important that these substances show no toxic effect onhe ecosystems. Several reviews are focused on the study ofhe toxicity and environmental stability of fire retardants con-idering vegetation, terrestrial fauna, fish, aquatics invertebratesnd algae [1–3,9,10,12–16]. Other important aspect is that thesehemicals could have minimal effects on people’s health whoight be exposed to them (firemen and person employed to use

he fire retardant). For this reason, many researches focus on thisspect [3,6,7,9,11,17,18].

Other papers focus on the working action of these substances3,9,11,19,20].

Finally, the corrosive effect of fire retardant is not studiedore extensively [9]. Concentrated fire retardant solutions are

ighly corrosive, and often contain corrosion inhibitors [3].n fact in some cases the fire-retardant substances may causexcessive corrosion on the materials of plant tanks, air-tankersnd helicopters [6]. A more complete study about corrosionroblems is made by USDA (United States Department of Agri-ulture Forest Service) [21]. In these documents corrosion testsere performed on several wildland fire chemicals. However

hey present only the execution of corrosion test for uniformnd intergranular corrosion for the used alloys in aircraft andanks using immersion test (total and partial) for 90 days inhe aggressive solutions following optical observations. John-on and George evaluated the corrosivity of the most frequentlysed long-term fire retardants, short-term fire retardants andoam under different conditions studying the damage causedo the alloy by uniform and intergranular corrosion. The alloyssed in these studies were representative of those exposed in air-ankers and helicopters and in use at retardant storage facilities:024 aluminium, AISI 4130 steel, yellow brass and Az31-Bagnesium [9,21]. Another more exhaustive paper about the

revention of corrosion problem due to fire retardant is writteny Gehring and George [22]. The authors confirmed the possibil-ty to observe corrosion phenomena in the use of fire retardant.he analysis is wider with regard to the previous paper [21]:ore metal alloys and more conditions of fire-retardants expo-

ures are considered. Also more corrosion typologies are presentncluding uniform corrosion and several types of localized cor-osion. The paper represents a guide to control the corrosionhenomena using fire retardants.

One of the most used LTR and foam fire suppressant areire Trol® 931 and Fire Foam® 103 produced by Chemonicsndustries (Canada) and in Europe by Biogema (France).

Fire Trol® 931 is a liquid concentrate fire retardant designedo be diluted with water and used for aerial application to controlildfire [4,23]. It is a concentrate mixture of ammonium phos-

hate (80–95%), sodium hexacyanoferrate (0.1–3%) iron oxide0.1–3%, which provides the colour) and a minor amount of alay thickener like attapulgite to suspend the colour and enhanceisibility. Actual amounts of each ingredient may vary from

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d Physics 105 (2007) 260–267 261

ime to time [4,6,23]. Mixing is accomplished through simpleroportioning of the retardant concentrate and water.

Fire Foam® 103 is a mixture of foaming and wetting agents innon-flammable solvent. The active ingredients are commonlysed in hair shampoo and dish washing liquid formulas. Theroduct contains hexylene Glycol 25 ppm [4,6,23]. Only fewesults concerning these products are in the literature.

Fire Trol® 931 and Fire Foam® 103 have been tested by theSDA Forest Service according to 90 days weight loss corrosion

equirements for Fire Chemicals Specification 5100-304a.Moreover, these tests only considered the mass loss and the

orphology of the corrosion attacks without considering theeal corrosion rate of the involved metals in the fire retardants.he experimental procedures present a very long test time [21]nd for this reason a more effective and useful methodology isesirable.

Due to these aspects and induced by the fact that some alu-inium components present, after use of some fire retardants,

n the surface a blue layer which alarmed the fire operators, newests to evaluate the corrosion behaviour of aircraft materials inre-retardant substances are carried out. The aim of this paper

s then to assess the corrosion behaviour of aluminium alloysnd steel in contact with Fire Trol® 931 and Fire Foam® 103sing electrochemical measures. In fact, electrochemical mea-urements allow to obtain accurate information regarding theorrosion rate of the materials within a short period of time.hese tests offer a high reproducibility and for these reasons

hey are still more frequently used both in research studies andn industrial laboratories.

. Materials and methods

Discs of 10 mm of section (0.7 cm2) made of aluminium alloy 2024 (4% Cu,.6% Mg, 0.7% Mn, 0.7% Fe, 0.5% Si, Al bal.) and carbon steel (0.1% C, Feal.) are used. They are, respectively, referred as to Al and Fe. The samplingurfaces are polished with an emery paper (800 mesh) and then degreased withcetone using ultrasonic bath before testing.

LTR Fire Trol® 931 and Fire Foam® 103 manufactured by Biogema (France)re tested. The electrochemical solutions, normally used in the fire control, ares followed:

Solution A: 20% of Fire Trol® 931 in tap water.Solution B: 15% of Fire Trol® 931 with 0.3% of Fire Foam® 103 in tap water.

These two solutions are used to simulate typical application of fire retardantollowing the indication of European LTR producer. The use of foam gives aore wrapping mixture that allows the concentration of LTR to decrease from

0 to 15%.The tap water coming from the aqueduct of Marseille has the following

omposition: 9.3 mg l−1 dissolved oxygen, 236 mg l−1 total dissolved salts,60 mg l−1 bicarbonate, 54 mg l−1 calcium, 5.9 mg l−1 magnesium, 17 mg l−1

odium, 1.5 mg l−1 potassium, 27.8 mg l−1 chlorides, 27 mg l−1 sulphate,.2 mg l−1 silica, 164 mg l−1 calcium carbonate, pH 8.1, 350 �S cm−1 conduc-ivity.

Electrochemical tests [24–28] are carried out to obtain information abouthe corrosion behaviour of these metals and about the aggressiveness of the fireetardant liquids.

Corrosion potential is measured as a function of immersion time.Then electrochemical impedance spectroscopy (EIS) measures are carriedout with a signal amplitude of 10 mV, in the frequency range of 100 kHz and10 mHz at free corrosion potential, to obtain the polarization resistance of

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the samples and to evaluate the behaviour of samples during immersion time[29,30].After that, potentiodynamic polarization tests are performed using a polar-ization rate of 0.5 mV s−1, starting from cathodic potentials.These tests are sometimes continued by new corrosion potential in functionof time or impedance experiments.

he three electrodes electrochemical cell is used with an SCE (+242 mV SHE)eference electrode and a platinum grid with a large area (3 cm2) as counterlectrode. The counter electrode and the working electrode are set in front ofach other. An EG&G 273 potentiostat connected to a FRA Solartron 1260 andPC is used. The impedance data are fitted using Zplot software to obtain the

alues of parameters of electrical equivalent circuit.After 5 days of immersion in solutions or after the potentiodynamic tests the

amples surface presents a thick, dense and hard blue deposit. Several analysesre made on this corrosion product:

Morphological observations using optical and environmental scanning elec-tron microscopes TMP ESEM FEI (secondary electron image obtained in lowvacuum mode).Chemical composition by energy dispersive X-ray spectroscopy (EDXS) anal-ysis.Structure by X-ray diffraction XRD (Siemens D5000).Chemical bonds correlated to the structure of the compound by Fourier Trans-form Infra Red (FTIR) analysis, carried out using Biorad FTS 165. The analysisrange is of 4000–500 cm−1.

. Results and discussion

.1. Corrosion potential trend before polarization

Fig. 1 shows the trend of the corrosion potentials of Al and Feamples in the two solutions. Considering the aluminium alloysn the both solutions it is possible to observe at immersion auick decrease of the potential to values about −980 mV SCE,ndicating the dissolution of a film present on the surface. Thisotential is typical of the aluminium in the active state. The steelhows just at the beginning a very stable value about −750 mVCE, indicating also in this case an active condition.

.2. Electrochemical impedance tests before polarization

This electrochemical impedance (EIS) test is often used tovaluate the corrosion behaviour of different metals with min-

ig. 1. Free corrosion potentials of aluminium alloys and steel in A and Bolutions.

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nd Physics 105 (2007) 260–267

mal perturbations [29,30]. In this way it is possible to studyhe evolution of corrosion phenomena during immersion time.

oreover, the formation of layers on the surface is easilyighlighted. The impedance data are fitted using an electricalquivalent circuit which simulates the different contributes of theamples. The extrapolation at low frequency of the impedancelot and the intersection with the Z real impedance axis allowo obtain the total resistance (Rtot). In the first approximationhe total resistance is inversely proportional to the corrosion rate29,30].

This value, together with the anodic and cathodic Tafel slopesba and bc, can be obtained from polarization plots), allowing toalculate the corrosion rate of the metal in the solution [26,27]ith the following equation.

cor = ba × bc

2.3(ba + bc)× 1

Rtot= B

Rtot(1)

is called the Stern–Geary coefficient, varying between 13 and6 mV dec−1. A mean value of 17.5 mV dec−1 is often used forhe calculations. We also take this value.

Using icor it is possible to calculate the corrosion rate rcorsing the following equation:

cor = l

t= Aicor

nFρ(2)

here l is the thickness (cm); t the time (s); A the molar massg mol−1); icor the current density (A cm−2); n the number ofxchanged electrons; F is the Faraday constant = 96,500 C andis the density (g cm−3)Fig. 2 shows the EIS diagrams (Nyquist representation) of the

lloys in both electrolytes. The aluminium alloy shows clearlyigher impedance values. There is a first loop correlated to thelectrochemical reactions on the metal surface. The presence ofstarting part of a second loop, at lower frequencies, could be

orrelated to the adsorption phenomenon with the formation of alm. The impedance data (limited to first loop) are fitted using aimple equivalent electrical circuit, made of one resistance andne capacity in parallel; the resistance values are reported inable 1 with the corresponding corrosion rate. The aluminiumlloy spectrum obtained in solution B is very similar. Consid-ring the total resistance, it is possible to observe that the alloyhows a lower corrosion rate in solution B.

In solution A, the steel sample shows an impedance diagramith one time constant. The fitting of data produces a valuef 460 � cm2, lower respect of those of aluminium indicatinghigher corrosion rate. In the solution B steel shows a lower

able 1alues obtained by fitting the impedance data and corrosion rate obtained usingq. (2)

amples Total resistance(� cm2)

Corrosion rate(mm year−1)

luminium alloy in solution A 2081 0.09luminium alloy in solution B 2759 0.07teel in solution A 460 0.44teel in solution B 165 1.23

S. Rossi et al. / Materials Chemistry and Physics 105 (2007) 260–267 263

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ca(see Eq. (2)) [24,27] reported in Table 2. It is possible to note thataluminium alloys present a weak corrosion rate in both solutions.On the contrary, the steel samples highlight a higher corrosioncurrent with a corrosion risk, despite the formation of a pro-

Table 2Corrosion current of the samples in the two solutions (�A cm−2)

ig. 2. (a) Impedance diagrams of steel and aluminium alloy in A and B solu-ions. (b) A magnification of impedance diagrams of steel in A and B solutions.

otal resistance value indicating a higher corrosion rate in thisnvironment.

Considering the corrosion rate it is possible to affirm that theluminium shows a low corrosion rate in both solutions. On theontrary, the steel shows a higher corrosion rate, in particular inhe solution B. Nevertheless these values remain low.

After the test immersion time (about 2 h) the Fe sample doesot show the blue layer, which is observed after polarization. Onhe contrary, on Al, a slight blue deposit is present.

.3. Potentiodynamic curves

The polarization curves of Al and Fe are reported in Fig. 3an solution A and Fig. 3b in solution B.

Whatever the solution used, Al samples present always theame trend. In solution A, aluminium alloy shows a very lownodic current density, lower than 4 × 10−4 A cm−2, indicat-

ng a low reactivity of this material. The passive behaviour isot observed while the anodic current increases with the poten-ial applied. Similar behaviour is observed in the solution Bontaining also the Fire Foam®.

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ig. 3. Potentiodynamic curves of aluminium alloys and steel: (a) in solution; (b) in solution B.

Steel samples show a different behaviour of the aluminiumlloys. In solution A the steel shows more noble values ofotential than those of aluminium alloy confirming the trendf the corrosion potential. The formation of a protective layer isbtained after an attack of the surface, while the anodic currentrst increases before decreasing. The rising in current at poten-

ial higher than 100 mV SCE indicates the beginning of this layerestruction. In solution B, the passive behaviour is observed too.onsidering this behaviour it is possible to affirm that the passivehenomenon is not direct, but implies the passage by a corrosionf the surface.

From the polarization curves it is possible to calculate theorrosion current (intersection between the corrosion potentialnd the Tafel tangents) which is correlated to the corrosion rate

amples Solution A Solution B

luminium alloys 8 9teel 50–200a 330a

a In active condition.

264 S. Rossi et al. / Materials Chemistry and Physics 105 (2007) 260–267

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Samples Total resistance(� cm2)

Corrosion rate(mm year−1)

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ig. 4. Impedance diagrams of aluminium alloy in solution A after polarizationxperiments.

ective layer. Nevertheless, the passive form is obtained aftersurface dissolution, for a potential higher than the corrosion

otential, while icor is obtained at this corrosion potential.After the polarization test on all surfaces a blue film is present.

his film is not very compact.

.4. Electrochemical experiments after polarization

To investigate the protective nature of the blue layer obtainedn the surface after polarization, corrosion potential trend as aunction of time and EIS experiments are carried out in solution.The potential trend presents as a function of time a nobler

otential on steel and on aluminium. Considering steel, the initialotential at −750 mV reaches a value of 250 mV, and on Al itaries from −980 to −375 mV. These potential rises show therotective character of the blue layer observed on the surface.

The EIS data confirm this result, because in all cases a veryigh increase of impedance, driving to a growing up of Rtot anddecrease of icor and rcor according the Eqs. (1) and (2). An

xample is shown in Fig. 4, obtained on aluminium in solutionbefore and after polarization. The beginning of a second loop

ppears at lower frequencies, due to the presence of a blue theayer on the surface. The results obtained from fitting of EISata are summarized in Table 3.

.5. Long time immersion test

To evaluate if the formation of a blue layer needs time to

row, impedance measurements were carried out in time, leavingmmersed the samples in the solutions.

In this condition the alloys show theirs corrosion behaviourike in service life. Considering the trend of potential it observes

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luminium alloy after polarization 4875 0.04teel after polarization 2052 0.1

or aluminium alloy a general continuous increase of valueshich reached after 18 h values in the range of −500 mV SCE,

tarting from about −1000 mV SCE. This aspect indicates againtendency to passivate. Also the EIS spectra change during

mmersion. At immersion, only one loop is present. Duringmmersion a second loop appears at lower frequencies, indicat-ng the formation of a layer on the surface. The total resistance,tarting from about 2080 � cm2, increases to 60,000 � cm2,ndicating a drastic decrease of corrosion rate. In the same timen the surface the formation of a blue deposit is observed.

Like aluminium alloy the corrosion potential of steelncreases with immersion time. It starts from −730 to reach0–90 mV SCE after 18 h. In the same time the total resistancerows from initial values of 460 to 10,400 � cm2, indicatingn increase of corrosion resistance. Respect to the case ofluminium alloy the resistance values results lower. The blueeposit is also obtained on steel but it appears weaker.

We could conclude that the blue layer is formed during anodicolarization or more slowly during free immersion in fire retar-ant solution. This layer presents a protective character againstorrosion. In case of free immersion the deposit is more consis-ent on aluminium alloy surface that steel one.

.6. Analysis of the blue deposits

To understand the nature of these deposits and the mechanismf their formations during polarization and during 5 days freemmersion, the blue layers are analysed by ESEM, XRD (X-rayiffraction) and FTIR.

Fig. 5 shows the blue deposit obtained during polarization onluminium alloy surface. It appears jagged and not compacted.ifferent structures are present: one with globular morphology

nd the second one with a stretched form.The EDXS analysis, reported in Fig. 6, shows the presence of

e, C, N, P related to the presence of phosphates and compoundade by iron and cyanic groups. The aluminium and copper

ignals are related to the substrate alloys.Considering the deposits formed during free immersion in

TR (Fig. 7) solution, the layer appears less uniform than in therevious case. There are areas very well compacted and othersith dispersed deposit. The globular and big crystal like struc-

ures as previously observed are present. On the globular depositn intense signal of C, N, and Fe is performed by EDXS anal-sis, indicating probably compound of iron and cyanic group.

he EDXS analysis on the regular structure indicates essentiallyn intense signal of P probably due to the presence of phosphate.

The deposits formed on steel substrate after immersionFig. 8) are not homogeneous too with larger and regular crys-

S. Rossi et al. / Materials Chemistry and Physics 105 (2007) 260–267 265

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dewout also on LTR solution. In this case the peaks at 2066 and at593 cm−1 are not present. Considering these experimental dataand the typical wave numbers correlated to Prussian blue it is

ig. 5. ESEM micrographies of blue deposit formed on aluminium alloy.

allites than on Al. On these large crystal structures, the signalf the P is more intense due to the probable presence of phos-hate. The signals of C and N and Fe are less intense than thosebtained on Al, relatively to the important P signal.

XRD patterns are also performed on the blue layers obtainedn Al and steel after polarization. Fig. 9 presents the diffrac-ograms obtained on the two substrates compared with theowder standard JCPDS (vertical line on the figure) of the cor-

esponding compounds suspected: iron (III) hexacyanoferrateII), (Fe4[Fe(CN)6]3) also called Prussian blue. This confirmshat the nature of the blue layer is the same either on Al or onteel.

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Fig. 6. EDXS spectrum of blue deposits formed on aluminium alloy.

Fig. 10 shows the FTIR diagrams of the deposits formeduring immersion on steel and aluminium substrates. Fromxperimental data characteristic peaks are observed at followingave numbers: 2066 and 593 cm−1. FTIR analysis was carried

ig. 7. ESEM micrographies of the deposit formed on aluminium alloy duringree immersion.

266 S. Rossi et al. / Materials Chemistry a

Fig. 8. ESEM micrographies of the deposit formed on steel substrate duringfree immersion.

Fig. 9. X rays diffractograms obtained on the deposit formed on steel and alu-mFi

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inium substrates after polarization, compared with the powder standards Al,e, Prussian blue JCPDS (Al, characteristic peaks of aluminium; Fe, character-

stic peaks of steel; PB, characteristic peaks of Fe4[Fe(CN)6]3).

ery probable that these peaks are related to Fe CN bond and totretch of CN, corresponding to iron (III) hexacyanoferrate (II),alled normally the Prussian blue compound [31,32].

The FTIR analysis shows that the spontaneous deposits

ormed on Al and Fe during immersion are constituted by Prus-ian blue, like the layer obtained under polarization. The signaln deposits produced during polarization shows a higher inten-ity, probably due to the higher thickness of the deposits.

ig. 10. FTIR spectra of the deposits formed on steel and aluminium alloy duringree immersion.

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nd Physics 105 (2007) 260–267

After the characterization of blue layers the important pointsre following. Blue deposits are observed on aluminium andron alloys components during service life using solution A (Firerol®) and B (Fire Trol® with Fire Foam®). The same depositsre formed during anodic polarization or during free immersionn these solutions. The deposits formed during polarization resultn being more compact allowing a higher thickness. Consideringhe deposits obtained during free immersion, the layer present onluminium appears more compact and with more quantity withespect of that formed on steel substrate. The EDSX, XRD andTIR analysis show that the deposits are constituted by iron (III)exacyanoferrate (II) (Prussian blue), which is not present in theTR solutions essentially composed by ammonium phosphate.

The Prussian blue is formed during electrochemical reactionsn open circuit or under polarization in the two solutions. LTRontains a red pigment Fe2O3 and sodium hexacyanoferrate asorrosion inhibitor. In fact, the oxidation of metal to ion duringnodic dissolution reaction permits an acidification of the elec-rolyte. For example the oxidation of Al in Al(OH)3 leads to aH− consumption:

l = Al3+ + 3e−

l3+ + 3OH− = Al(OH)3

n the other hand Fe2O3 in solution is in equilibrium with Fe3+

ollowing:

e2O3 + 3H2O = 2Fe3+ + 6OH−

decrease of the pH value leads to a rise of [Fe3+] in solutionhat reacts with the corrosion inhibitor to form the blue verytable complex Fe(III)-CN-Fe(II) [32]:

Fe3+ + 3[Fe(CN)6]4− = Fe4[Fe(CN)6]3 ks ≈ 1040

hen the pH is decreasing, Fe3+ is following the reaction: theeduction reaction from (Fe(III)-CN-Fe(III) to form the blueFe(III)-CN-Fe(II)) very stable complexes on the metallic sur-ace [32].

. Conclusion

Water solutions with Fire Trol® 931 and with the mix of FireTrol® 931 and Fire Foam® 103 (solutions A and B) simulat-ing the actual service liquids for fire retardants, showed littlecorrosivity for aluminium alloy, showing a mean corrosionrate of 0.07–0.09 mm year−1. The formation of a blue layeron the surface allows a reduction of the corrosion rate. Theformation mechanism of the Prussian blue on the aluminiumsurface has been proposed.In the same solutions, steel is less resistant to corrosionthan aluminium, with a corrosion rate ranging from 0.44 to1.23 mm year−1.The anodic reaction of corrosion phenomenon favours the

deposition of a Prussian blue layer on the metallic samplessurface.This layer reduces the corrosion phenomenon on both alu-minium alloy and steel. After the blue deposit formation the

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corrosion rate is about halved in the case of aluminium; areduction of four times is observed with steel.The blue layer is formed on both metals during polarizationand during free immersion. In the latter, the layer is clearlyvisible after two hours on aluminium surface and after about10 h on steel one.

cknowledgment

The authors are grateful to Biogema (France) for the materialsnd for interesting discussions.

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