corrosion of iron by a perfluorpolyalkylether identified by mössbauer spectroscopy

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applied surface science

ELSEVIER Applied Surface Science 93 (1996) 329-337

Corrosion of iron by a perfluorpolyalkylether identified by Miissbauer spectroscopy

George John av * , Jeffrey S. Zabinski b, Vijay K. Gupta ’ a Department of Engineering Physics, AFIT/ENP, Air Force Institute of Technology, 2950 P Street, Wright-Patterson AFB, OH

45433-7765, USA

b WL / MLBT, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, OH 45433, USA ’ Department of Chenaist~, Central State University, Wilbetforce, OH 45387, USA

Received 18 July 1995; accepted 28 September 1995

Abstract

Miissbauer spectroscopy was used to identify the corrosion products produced when various samples of pure iron were heated in a branched peffluoropolyalkylether (PFPAE) at temperatures between 232°C and 343°C (450°F and 650°F). The samples that were treated are: (1) a 2 mm thick piece of 99.99% pure natural iron, (2) a 567 A film of iron enriched in “Fe evaporated onto 1018 steel, and (3) powdered iron. The predominant products produced for treatments of 24 to 50 h at 288°C 316°C and 343°C and with minimal oxygen present, are FeFz, and Fe,O, modified by the substitution of fluorine

for oxygen at the Fe” site (site B). A treatment at 232°C for 25 h did not produce any FeFz but did show some sites in the spectrum that may be fluorides or oxyfluorides in addition to the modified Fe,O,. For the seven day treatment at 232°C FeF, is observed. When powdered iron was treated at 343°C in a vessel where oxygen was introduced, FeF,, FeFz , and the Fe3+ site (site A) of Fe,O, with little or no site B were observed. These results suggest that the formation of FeF3, which catalyzes the decomposition of PFPAE fluids, involves the production and fluorination of Fe,O,.

1. Introduction

The studies described in this article were conducted in support of a program to develop fluids and lubricants for use as hydraulic fluids, gyro-flotation

fluids, high temperature turbine engine oils, and greases at the environmental extremes encountered in aerospace applications. The types of fluids and the details of their structure, properties, development, and potential have been reviewed in Refs. [l-3] and

l Corresponding author. Fax: + 1 513 255 2921; e-mail:

[email protected].

the references contained therein. One class of these

fluids, based on the family of perfluo- ropolyalkylethers (PFPAE), have promising properties for operation over a broad temperature range and are available commercially in either a linear or branched form. Unfortunately, their use has been

impeded because they decompose and cause corrosion of both ferrous and non-ferrous metals and alloys under various operating conditions.

Insight to the processes that degrade linear and branched PPPAE fluids and cause corrosion of metals has been gained from the results of many studies conducted under diverse thermal and tribological conditions [4- 141. These studies, which examined

0169-4332/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved

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330 G. John et al./Applied Surface Science 93 (1996) 329-337

reactions of PFPAEs with ferrous and non-ferrous are: (1) spectra are less sensitive to adsorbed fluids, metals and alloys, as well as with some oxides (2) reactions of fluid adsorbed on metal surfaces are (cr-Al&),, cw-Fe,Ox) and halides (AlCl,, AIF,, not initiated by the radiation used to induce the FeF,), have led researchers to propose a two-step Mijssbauer effect, and (3) many compounds have mechanism for the decomposition of the fluids [4- unique spectra that fixes their identification. As with 6,10,12]. Although the decomposition paths for lin- all analytical probes, there are factors that limit ear and branched PFPAEs differ [5], there is general abilities for unique identifications. Among the phe- agreement that the first step in the degradation pro- nomena that confound identification are: non- cess is a reaction that converts the metal’s surface or stoichiometric compounds, complex mixtures with its oxide coating to a fluoride. The metal fluoride, small differences in identifying parameters, particle MF,, which is a strong Lewis acid, then catalyzes size effects, and effects arising from polarization of the rapid degradation of the PFPAE. electric and magnetic fields in source and absorber.

Elucidation of the two-step decomposition model from analyses of wear debris and deposits in wear tracks observed in tribological studies has been aided by a variety of analytical techniques. Several of the techniques reported in published studies of corrosion products produced by PFPAE fluids are: X-ray pho- toelectron spectroscopy (XPS) [4,7, lo- 131, sec- ondary ion mass spectrometry (SIMS) [l I- 131, and Auger electron spectroscopy (AES) [ 131. Additional analytical probes reported in studies with other lubricants are: scanning electron microscopy @EM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDX) [15]. Using XPS and/or SIMS, several researchers have inferred the presence of fluorides [7,10-131 and oxides [ 101 in the wear products generated by PFPAE fluids. Supplemental studies have shown that at elevated temperatures linear and branched PFPAE fluids are decomposed by halides such as AlCl,, AIF,, FeF, [5,12] and oxides such as a-A1203, cu-Fe,O, [4,6]. In the studies with the oxides, it was shown that the oxides were converted to fluorides before catalytic decomposition began.

The current exploratory study has produced results that provide insight to the corrosion process and suggests the direction for additional studies. Both oxides and fluorides of iron have been identified as corrosion products produced when pure iron is heated in a branched PFPAE for various lengths of time. The effects of temperature and the presence of oxygen on corrosion chemistry are evaluated.

2. Experimental

2.1. Sample preparation

The goal of this study is to assess the utility of using MSssbauer spectrometry to identify the iron compounds produced when pure iron is heated in a branched PFPAE lubricant at temperatures between 232°C and 343°C. The PFPAE fluid used is the branched form of perfluoropolypropylene oxide, such as a commercial fluid krytox, for which the general- ized formula is, R,-0-(CF(CF&F,O>,-R, where R, may be CF,, C,F,, or C,F,. Mijssbauer spectrometry complements other surface probes, and has several advantages over some of the other analytical methods used for probing surface deposits. These

Samples of natural iron and iron enriched in the Miissbauer isotope 57Fe were heated in the branched PFPAE fluid at selected temperatures between 232°C and 343°C. In most instances the samples were treated in a covered Pyrex container that was not hermetically sealed from ambient laboratory air. In a few cases specimens were heated in a sealed Pyrex tube in PFPAE purged with nitrogen. Two forms of polycrystalline, 99.99% pure natural iron were used as samples: a 2 mm thick rectangular slab, and powder. The iron sample, enriched to 96% in “Fe, was prepared by vacuum evaporation onto 1018 steel.

Except as noted otherwise, the procedure for treat- ing the samples is as follows. Samples are heated in 10 ml Pyrex vessels immersed in a fluidized sandbath maintained with a temperature controller to + 2°C of the selected temperature. The Pyrex vessels are equipped with an outside ground joint and an inside ground Pyrex lid. Each sample is placed in the vessel, covered with 3 to 4 ml of cold PFPAE, and then capped without use of sealant on the ground

G. John et al./Applied Surface Science 93 (1996) 329-337 331

joint. The cold vessel is then lowered into the heated fluid&d sandbath until the lip of the lid is just at the surface of the sand. When the sample consists of powdered iron, the vessel is agitated after five to ten minutes of heating to disperse the powder in the PFPAE.

At the end of the heating period, the vessel is removed from the sandbath and the samples are prepared for analysis by MZjssbauer spectrometry. Metallic disks, after rinsing with trichlorotrifluo- roethane (TCTFE) to free the corrosion film of ex- cess PFPAE fluid, are immediately ready for analysis by CEMS (conversion electron Miissbauer spectrometry). Powdered samples are filtered while the slurry of powder and PFPAE is warm and consequently less viscous. After rinsing the powder with TCTFE, a sample that is thin enough for analysis by TMS (transmission Miissbauer spectrometry) is prepared. To accomplish this, a portion of the powder paste is placed on scotch tape that covers a 1.27 cm diameter hole in an aluminum plate that is 0.079 cm thick. The paste is then covered with scotch tape to form a thin disk.

2.2. Sample characterization

Three characteristics of Miissbauer spectra that identify chemical compounds are, the isomer shift, electric quadrupole splitting, and magnetic hyperfine splitting. Electrostatic interactions of the nuclear charge with surrounding electrons and ions produce the isomer shift and quadrupole splitting while the magnetic field produced by the crystal at the nucleus causes the Zeeman splitting. Since the electromag- netic environments do differ for different iron compounds and crystal lattices, unique spectra are possible.

Mbssbauer spectra were obtained with a constant acceleration electromechanical transducer operated in fly-back mode. Spectra were obtained at room temperature with a 57Co source in a rhodium matrix. A gas flow proportional counter operated with a mixture of 3% methane in helium was used for CEMS. Transmission spectra were obtained with a commercial, sealed proportional counter filled with krypton plus 10% CO,. Various high purity iron compounds were used to obtain reference spectra.

Mijssbauer spectra were analyzed with a least

squares program that could fit theoretical models for up to eight iron sites. A Monte Carlo simulation routine was used to provide a non-linear error analysis based on fluctuations predicted by counting statis- tics. The uncertainties reported with the Mijssbauer parameters in the tables are two standard deviations of the Monte Carlo distributions. They do not in- clude systematic errors. Systematic errors arising from non-linearities of the spectrometer and in values of the line positions for a-Fe used to calibrate the spectrometer are estimated to be less than 0.02 mm/s.

3. Results and analyses

3.1. Pure Fe slab

Two Miissbauer spectra were obtained of the corrosion products from exposure of the 2 mm thick

Fc304@3)’

Fe304(Bl)*

Fig. 1. CEMS of corrosion produced by heating pure iron for

seven days in PFPAE at 232°C in a sealed tube. * See text.

332 G. John et al./Applied Surface Science 93 (1996) 329-337

specimen of pure iron to PFPAE at 232°C: the first, after treatment for seven days, and the second, after cleaning the specimen and exposing it for 25 h. Samples were sealed in borosilicate glass tubes with air in the void volume. The spectra, obtained by CEMS, are shown in Fig. 2 and Fig. 3.

The spectrum in Fig. 1 from the corrosion products for the seven day treatment, is dominated by a quadrupole doublet that is identified in Table 1 as anhydrous FeF,. As cited in the last column of Table 1, this doublet constitutes about 30% of the spectral area. Additional information is needed from depth- selective CEMS and the recoil-free fraction of the FeF, deposit to make a more quantitative assessment of the FeF, present in the film.

The remaining hyperfine sites in Fig. 1 are princi- pally sites A and B of non-stoichiometric Fe,O, with fluorine replacing some of the oxygen, particularly at the Fe in site B. This assessment is supported by the observed broadening and asymmetry of the peak normally attributed to site B, and by similar results reported by Franke et al. [16,17]. In their studies of Fe,O,_,F, (with 0 <x 5 0.25) they observed that the Mijssbauer lines associated with the Fe3’(A) site of magnetite remain practically unaffected by substitution of fluorine into the magnetite, and that the B

site was broadened as the fluorine substitution was increased. They concluded that a random distribution of fluorine ions particularly around Fe in the B-sub- lattice of Fe30, gives rise to three hyperfine fields. The improvement in the fit to the spectrum in Fig. 1 by the use of three hyperfine spectra, labeled Fe,O&B 11, Fe,O,(B2) and Fe,O&B3), is consistent with their results. For comparison, parameters are provided in Table 1 for site B and for anhydrous FeF, which might be produced if all of the oxygen in the B site was replaced by fluorine.

A low intensity iron hyperfine spectrum from the unreacted substrate is present, suggesting that the thickness of the corrosion layer is less than N 3000 A. Most of the conversion electrons from the iron substrate could not penetrate the thick layer of corrosion products that had developed on the iron surface. The low signal-to-noise is caused by the low activity of the 57Co source used to obtain the spectrum.

The same piece of iron was cleaned and exposed again to PFPAE at 232°C for just 2.5 h in a sealed tube. The results of the analysis are presented in Fig. 2 and Table 2. The best fit to the spectrum is obtained with three hyperfine sites and two quadrupole sites. Two of the hyperfine sites are identified as site A and a modified and diminished

Table 1

Mossbatter parameters obtained for corrosion products of natural iron disk heated in PFPAE at 232°C for seven days; comparison with

reference values

Compound Isomer shift a

(mm s-‘)

Quadrupole splitting

(mm s- ‘)

Internal magnetic

field (T) Relative area

FeF,

Fe,O,(A)

Fe,O&B 1)

Fe,O&B2)

Fe,O,(B3)

FeF, a-Fe

Exp.

Ref. Exp.

Ref.

Exp.

Ref.

Ref

ExP Ref

1.341 * 0.006 2.78 f 0.01 _ 0.29 f 0.01 1.361 * 0.001 0.28 f 0.03

0.300 f 0.001

0.62 * 0.03

0.655 + 0.002

0.63 + 0.06

0.61 f 0.15

0.485 * 0.001 0

0

2.752 f 0.002 0 48.6 i 0.3 0.23 f 0.04 b

49.79 + 0.01 c d

45.9 f 0.5 0.21 rt 0.1 b 46.24 f 0.02 ’ d

44.1 f 0.7 0.1 f 0.1

41.2 f 1.4 0.09 0.08 f 39.47 f 0.01 _

33.0 * 0.4 0.09 f 0.04 33.19 * 0.01

a All isomer shifts in the report are relative to iron.

b Area A/(Bl + B2 + B3) = 0.58.

’ Reported values for Mossbatter hyperfme parameters of magnetite vary over a wide range; H(site A): 49.8 to 47.5 T, H(site B): 46.4 to

43.5 T [ 191. d For stoichiometric Fe,O, one expects 66% of the iron to be at the octahedral site (site B), and 33% to be at the tetrahedral sites (site. A),

which should produce a ratio of Miissbauer spectral area of A/B = 0.5; Sawatzky et al. [ 181 reported an experimental value of 0.54.


,‘l__/-\\ _________________---’ I , ----_____________

(Fe%)? r\ ,; /III : ’ : \ _______________-__---’ ----_--_________-____

l *Fe3+

h ‘\ ,I! :_: \

Fe304(A)

Fig. 2. CEMS of corrosion produced by heating pure iron for 25 h

in PFPAE at 232°C in a sealed tube. * See Table 2.

site B of Fe,O,. The other hyperfine site is that of the Fe substrate. Because of the shorter exposure to PFPAE, the oxidation layer is thin enough in this

case to permit a significant contribution from the iron substrate. Two overlapping quadrupole sites are used to produce a good fit to the data because of a noticeable broadening of the left peak of the doublet. Although the parameters for the dominant quadrupole site do not match any of the fluorides, oxides or oxyfluorides reported in the literature, their isomer shifts fall within the range 0.3 to 0. 5 mm/s cited for various iron compounds in the 3 + state [21]. The oxidation state for the minor quadrupole site is un- certain since its isomer shift lies between that for the 3 + state and the 2 + state for which the isomer shift is generally between 1.2 and 1.4 mm/s. Al- though the fit to the spectrum is measurably im- proved by using the three quadrupole contributions, it is possible that they arise from superparamagnetic relaxation of magnetite modified by substitution of fluorine for oxygen. Additional studies at low temperatures or with an external magnetic field applied to the samples are needed to determine whether superparamagnetic effects are occurring.

The identification of the corrosion products of the 25 h treatment is less certain than for the seven day treatment in which FeF, was uniquely identified. The presence of spectra with isomer shifts and quadrupole splittings intermediate to those of Fe2+ and Fe3+ that are produced in the 25 h treatment suggest that these may be transitional states to production of FeF,. Further studies are needed to discern the role of Fe,O, in the corrosion process. It is clear from the spectra that fluorine from the PFPAE or one of its decomposition products does substitute for oxygen in the B site of magnetite.

Table 2 M&batter parameters obtained after second exposure of natural iron disk to PFPAE at 232°C for 25 h in a sealed tube

Compound Isomer shifts Quadrupole splitting Internal magnetic Relative area

(mm s-‘I (mm s- ‘) field (T)

Fe2+? a 0.9 f 0.1 2.5 L- 0.2 0.12 * 0.02 Fe’+ a 0.38 f 0.01 0.85 f 0.06 0.12 * 0.04 Fe3+ a 0.31 f 0.03 1.4 * 0.01 0.062 &- 0.04 a-Fe 0 0 33.22 f 0.04 0.18 + 0.01 Fe,O,(B) b 0.52 k 0.04 0 45.3 * 0.4 0.17 * 0.04 c Fe,O,(A) 0.31 z!z 0.01 0 49.47 * 0.01 0.35 + 0.0 c

a These three sites do not match any published values of stoichiometric iron compounds, but could result from supcrparamagnetic

relaxation of small particles of Fe,O, or Fe(O,F,) [20].

b Diminished B site of Fe,O, possibly modified by random substitution of fluorine for oxygen.

’ Area A/B = 2.1 compared with 1 in the reference Fe,O,.

334 G. John et al./Applied Surface Science 93 CI9%) 329-337

3.2. Enriched iron on 1018

This sample was prepared by evaporating a 567 A film of iron enriched to 96% 57Fe onto a disk of 1018 steel, 1.27 cm in diameter and - 0.3 cm in thickness. The enrichment of 57Fe in the sample by a factor of - 45 over its abundance in natural iron increases the sensitivity of the Miissbauer effect proportionately. After heating in PFPAE for 24 h at 288 + 2°C the disk was cooled and cleaned with TCTFE before obtaining the Miissbauer spectrum presented in Fig. 3.

WC

0-h

Results of the analysis of this spectrum, presented in Table 3, clearly identify the principal corrosion products as FeF,, non-stoichiometric Fe30,, and a small amount of Fe,O,. There is also a broadened quadrupole doublet, labeled “Fe3+?” in Fig. 3, with parameters that do not match any referenced iron compounds. Again the parameters of the oxides devi- ate from the reference values because fluorine can substitute for oxygen. As was noted previously, this broadens the spectrum for site B of Fe,O, so that more than one hyperfine site is needed to fit the broadened spectrum. These results and the unam- biguous identification of FeFZ again suggest that the modified oxide and unidentified component may be transitional to the production of FeF,.

Fig. 3. CEMS of corrosion products from 567 A enriched Fe on

1018 steel heated in PFPAE at 288°C for 24 h.

Two additional sites are identified in this spectrum as compared with the spectrum obtained with the natural iron specimen, one for Fe,O, and the other for Fe,C. This is not surprising since the natural iron was heated in a sealed tube at a lower temperature, whereas the enriched iron film was heated in a capped Pyrex vessel in the fluidized

sandbath. Although the cap is a ground glass joint, it is not impermeable to diffusion of some ambient air into the vessel during treatment. The presence of Fe,C is plausible since carbon is a likely contami- nant of enriched iron both in its production and during evaporation.

Table 3

MBssbauer parameters obtained for corrosion products of enriched iron film on 1018 steel exposed to PFPAE at 288°C for 24 h in a capped pyrex vessel

Compound Isomer shift Quadrupole splitting Internal magnetic Relative area (mm s-‘) (mm s- ‘1 field (T)

FeF, 1.347 * 0.002 2.793 f 0.006 0.093 f 0.004

Fe3+? 0.30 * 0.02 1.30 0.04 * 0.088 0.006 f

Fe,C 0.185 It 0.006 0.77 * 0.04 20.47 f 0.05 0.191 f 0.008

cy-Fe 0.0 0.0 33.23 0.02 f 0.071 i- 0.02

Fe,O,(A) 0.284 f 0.004 0.0 48.40 f 0.03 0.11 * 0.01

Fe,O.,(B I ) 0.574 f 0.006 0.0 45.01 f 0.04 0.21 f 0.01

Fe,O,(B2) 0.63 + 0.22 - 0.29 f 0.06 39.3 f 0.1 0.16 f 0.01

a-Fe,O, 0.359 jz 0.006 -0.33 * 0.12 50.83 * 0.05 0.083 f 0.004


0.

2.

4.

6

a.

10

Fe304B ” ”

Fe304A” ”

0.c

2.c

4.c

6.C

8.C

101

Fig. 4. TMS of corrosion products from powdered iron heated in PFPAE at 343°C for - 50 h. Fig. 5. TMS of powdered iron heated in PFPAF at 343°C for 66.5

h; vessel cracked during run.

3.3. Powdered iron

Three samples of 99.99% pure Fe powder were exposed to PFPAE$, one at 316°C and two at 343°C. Two treatments were done at 343°C because in the first run the Pyrex vessel developed a crack some time during the run. As a consequence, oxygen entered the vessel and markedly affected the corrosion products produced. The two comparable runs at 316°C and 343°C were treated in capped Pyrex vessels heated in the fluidized sandbath for about 50 h.

The results of the comparable runs at 316°C and

343°C are presented in Fig. 4 and Table 4. Since the spectra are similar, only the spectrum for the sample treated at 343°C in the intact Pyrex vessel is shown in Fig. 4. The six-line hyperfme spectrum of iron metal dominates the spectrum, which is as expected for transmission Miissbauer since the corrosion products are a minor constituent on the surface of the iron grains. In Table 4, the relative intensities of the corrosion product peaks are presented for 3 16°C and 343°C with the MGssbauer parameters that identify the products as Fe,O, and FeF,. As the relative

Table 4 Miissbauer parameters for corrosion products from powdered iron heated in an intact pymx vessel in PFPAE for - 50 h at 343°C with relative peak areas for products produced for exposure at 316°C for - 50 h

Fe+3?

Fe304A

Product Isomer shift Quadrupole splitting Internal magnetic Relative area Relative area (mm s-‘1 (mm s- ‘1 field (T) 343°C 316°C

FeF, 1.38 f 0.07 2.75 f 0.04 0.025 f 0.001 0.006 f. 0.002 a-Fe 0 0 33.25 f 0.01 1.00 1.00 Fe,OXA) 0.30 f 0.01 0 49.1 f 0.1 0.033 f 0.002 0.030 f 0.003 Fe,O,@) 0.64 f 0.01 0 46.16 f 0.06 0.036 f 0.002 0.058 f 0.007

336 G. John er ol./Applied Surface Science 93 (1996) 329-337

Table 5

Mossbatter parameters obtained for corrosion products from powdered iron heated in a cracked pyre.x vessel in PFPAJZ at 343 C” for 66.5 h

Product Isomer shift Quadrupole splitting Internal magnetic Relative area (mm s-‘) (mm s- ‘) field (T)

Fe3+? a 0.11 * 0.01 1.47 f 0.03 0.065 f 0.005

FeF, 1.363 f 0.001 2.786 f 0.002 0.5 10 f 0.005

a-Fe 0 0 33.20 f 0.01 1 .oo

FeF, 0.484 f 0.004 41.35 * 0.02 0.62 f 0.01

Fe&),(A) 0.30 * 0.01 0 49.28 f 0.06 0.195 f 0.008

a No published parameters for iron compounds match this set.

intensities show, the higher temperature produces more FeFz for the same treatment time.

The influence of oxygen on the reactions of PF- PAE with iron are manifest in the run at 343°C for 66 h in a vessel that inadvertently cracked during the run. When the vessel was removed from the sandbath, it was noted that the inner surface of the vessel was etched and that the sample was reddish brown. As seen from the Miissbauer spectrum in Fig. 5 and the parameters in Table 5, the principal products are FeF, and FeF, and the A-site (Fe3’> for Fe,O,. The absence of the B-site of Fe,O, suggests that the B-site has been almost completely converted to FeF3. In addition, a minor, unidentified corrosion product was also produced.

In the runs at 316°C and 343°C in the intact vessels, the ground glass cap limited the oxygen to that which was present in the free space above the PFPAE (about 10 ml) and to the small amount that might diffuse into the vessel through the ungreased joint. Neither vessel used in these runs appeared etched and the products, Fe,O, and FeF,, appeared gray to black.

4. Summary and discussion

The use of Miissbauer spectrometry to identify corrosion products produced by the reactions of pure iron immersed in a heated branched PFPAE has shed some light on the corrosion process. Although the corrosion mechanism has not been determined unam- biguously, some parts of the process have been illuminated.

Several general observations can be made. Two corrosion products are produced in most of the ex-

periments: FeF,, and Fe30, modified by the substitution of fluorine for oxygen. There is also evidence that the corrosion process and the products formed are affected by the temperature and the presence of oxygen. The two treatments of the natural iron slab at 232”C, one for 24 h and the other for 7 days, show that FeFz is formed only for the long exposure to the PFPAE fluid. At the shorter exposure, the over- lapped quadrupole doublets as well as the modified Fe,O, suggest that these are fluorides and oxyfluorides which may be intermediary compounds to the formation of FeF,. The other experiment with enriched iron conducted at 288°C for 24 h, and the powdered iron treatments at 316°C and 343°C for - 50 h, show that FeFz production increases with increased temperature.

The dramatic effect of oxygen became evident with the inadvertent cracking of the Pyrex vessel containing powdered iron and PFPAE heated at 343°C. In this instance the predominant products were anhydrous FeFz and FeF, with a small amount of site A of Fe,O, and no site B. It is noted that FeF, is aggressive forward promoting decomposition of PFPAEs. This result combined with the observations that the parameters for site A of Fe,O, are marginally affected whereas site B is always broadened and skewed towards lower magnetic splitting suggest that corrosion begins with the formation of Fe,O, followed by the substitution of fluorine for oxygen via site B. This process could lead to the formation of both FeFz and FeF3.

Additional support for Fe,O, as an intermediate product is seen in the comparison of spectra obtained for short and long treatment times of the natural iron slab. The short-term treatment shows some intermediate compounds that may be oxyfluorides or the


results of particle size and superparatnagnetic relaxation, but no FeF,, whereas the long-term treatment shows mainly FeF,.

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corrosion of iron by a perfluorpolyalkylether identified by mössbauer spectroscopy

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