ESCA investigation of iron-rich protective films on aluminium brass condenser tubes

Corrosion Science, 1976, Vol. 16, pp. 145 to 157. Persamon Press. Printed in Great Britain

ESCA INVESTIGATION OF IRON-RICH PROTECTIVE FILMS ON ALUMINIUM BRASS CONDENSER TUBES* J. E. CASTLE and D. C. EPLER University of Surrey, Guildford, Surrey and

D. B. PEPLOW Fawley Power Station, Fawley, Hampshire Abstract--The object of this investigation was to examine the nature of the protective oxide layers on an aluminium brass condenser tube which had been in service at Fawley under an operating regime in which the cooling water was regularly dosed with ferrous sulphate for protection against pitting corrosion. The protective films have been analysed by means of XPS (ESCA) and found to contain mainly Cu + and Fe 3+ ions. Of more interest were extensive areas in which the iron deposits are nonadherent and non-protective. XPS analyses show that these deposits are separated from the underlying brass by a layer rich in zinc and magnesium but containing no iron. Some indications are given of how this intervening layer could have formed. 1. I N T R O D U C T I O N

THE CONDENSERtubes in a sea-side power station are the interface between corrosive sea water and the extremely pure water required for operation of modern boilers. Failure of the tubes by development of even small leaks usually requires shut-down of the whole generating unit--at a cost of c a . £1,000/h. In common with many other power stations the condensers at Fawley are tubed with aluminium brass and are operated under a regime in which the sea water is regularly dosed with ferrous sulphate as a corrosion inhibitor. 1.2 The broad objective of this investigation was thus to understand the role of iron in the formation of protective oxide on this material. The use of ferrous sulphate as an inhibitor of the corrosion of aluminium brass was first described by Bostwick1 although iron additions had been standard Naval practice during the 1940's.a Bostwick produced detailed statistical evidence relating addition of ferrous sulphate to the number of tube failures by pitting. By way of explanation he envisaged the iron addition acting as compensation for the loss of 'natural' iron which had been available from the water boxes prior to installation of cathodic protection. Ferrous sulphate dosing is now widely practised but there is still no agreement on the mechanism of protection, s Iron may act as a cathodic inhibitor4 or perhaps could be incorporated into a CuaO film, giving it increased passivity by a mechanism similar to that found when iron or nickel are incorporated from cupro-nickel alloys.~,e Other workers~ believe that a relatively thick film of lepidocrocite (hydrated ferric oxide) is formed which reduces the erosive action of sea water and enables a protective film to form on the brass itself. This view is attractive since it implies that the iron film is effective in the commissioning stage of condenser *Manuscript received 10 December 1974; in revised form 2 July 1975. 145

146

J.E. CASTLE, D. C. EPLER and D. B. l:~pI.OW

operation and that thereafter iron neither contributes to the passivation action nor is necessary. There may thus be a possibility of reducing or discontinuing iron additions to the cooling water once service conditions are well established. There has until now been little opportunity for the confirmation or rejection of such speculation by the direct analysis of the oxides on condenser tube surfaces since they are too thin for electron microprobe analysis. It was thus impossible to determine the extent to which the oxide was penetrated by the iron additions. Recently the technique known as ESCAa or, more accurately, X-ray photoelectron spectroscopy (XPS), has become available and is being developed for materials investigations.9,1° By use of this technique we are able to analyse the top 2.5 nm of a sampled area measuring some 5 mm in diameter. Further, we can obtain some indication of the concentration of significant elements throughout a thicker oxide by profiling with an argon ion gun. In the expectation that such a technique would enable the distribution of iron in protective oxides to be determined we have collaborated in the removal and examination of tubes at intervals up to 20,000 h when there was an experimental halt in iron additions for 1000 h. The results are informative both in the use of the technique and in understanding the fate of iron in the power station condenser. 2. GENERAL DESCRIPTION OF TUBES All tubes in the station were manufactured to a nominal composition of 76%Cu, 22%Zn, 2%A1, 0-03%As; they are 19.8 m in length with an o.d. of 25 mm and a thickness of 1.2 ram. The condensers are of double tube plate design and contain some 18,000 tubes. There are four condensers to each of the four 500 MW sets in the station. The particular tubes examined in the course of this work were: tube 1, withdrawn from No. 1 unit, 'C' condenser after 7000 h; tube 2, withdrawn from No. 2 unit, 'D' condenser after 17,950 h; tube 3, withdrawn from No. 1 unit, 'A' condenser after 18,960 h, and tube 4, withdrawn from No. 3 unit, 'D' condenser after 20,908 h. The cooling water passing through the tubes had a design velocity of 2 m s-1 and was dosed with 1 ppm FeSO4, by addition of the solid salt, for two 1 h periods each week. However, tubes 3 and 4 were removed after dosing had been discontinued completely for a period of 1000 h. The inner surfaces of all tubes were covered with the same type of oxides: the major part of the tube surfaces were covered by a brown porous deposit which exfoliated in places to reveal an underlying white film (Figs. l, 2, A and B). However, a lesser area of the surface (ca. 20%) was covered only by an adherent yellow-orange film (Fig. 2, C). The brown product was several Fm in thickness and could be analysed by EPMA. It was mainly iron oxide but contained some copper oxide. The white and orange layers were too thin for metallographic sectioning nor could any topographic contrast be found across boundaries between the two layers (Fig. 2, B and C) in the SEM. EPMA analysis gave results which mainly reflected the composition of the underlying brass, ca. 75 % of the total signal being received from copper and zinc (compared with under 20% for the XPS analyses described below). The films were thus in the submicron size range. However, EPMA does show the presence of iron in the orange layer. In places on the tubes rather sharp gradations in the orange colour (Fig. 3A) were found which suggested the presence of layers of the iron rich product in thicknesses intermediate between that of the thick brown layer and the adherent

FIG. 1. Half-section

of a typical tube showing exfoliating layer.

iron oxide overlying a white ~aeing p. 1461

FIG.

2.

Half-section

showing

mixed protective (white-brown).

(orange)

and

non-protective

films

ESCA investigation of iron-rich protective films

147

yellow-orange. These darker orange layers did not exfoliate but could all be stripped with "Sellotape" to reveal a 'brassy' substrate. The yellow-orange layer could not be stripped in this way,, In addition to these major features, parts of the tubes were marked with blue spots. These spots were rich in copper and chloride species and when surface oxides were stripped away showed a ring of deposited copper with a central 'brassy' zone. There was however no evidence of deep pitting nor did any of the spots show 'flow' marks. It seemed most likely that they had formed beneath saline droplets when the condenser was taken out of service. Such areas were therefore avoided from the point of view of obtaining samples representative of operating conditions. 3. E X P E R I M E N T A L 3.1 Photoelectron spectrometry Test pieces measuring 9 × 6 mm were prepared from the white and the orange sectors of the tube surfaces. Samples of exfoliated oxide were mounted separately on gold holders so as to reveal the top or bottom surfaces using "Silver Dag" as an adhesive. Spectra were obtained in Al Ka radiation using the ESCA 2A (Vacuum Generators Ltd., East Griustead, England) spectrometer. The vacuum within the spectrometer was generally in the region of 10-8 Tort during an analysis. The necessity of using such vacuum inevitably means that hydroxides will be destroyed and water lost from the surface during an analysis. However the concentration of other elements should remain unaltered during analysis. More than one hundred spectra have been run of the various tubes and their interfaces. Typically analysis was carried out using a 90 eV analyser energy and a scan rate of 1 eV/s for low resolution sweeps of 1,000 or 500 eV and 0"25 eV/s for high resolution sweeps. The oxides of copper and zinc were identified by the position of the LMM Auger peak as described by Castle and Epler? Concentration profiles across the films were obtained by in situ argon ion etching using the gun supplied by Iontech Ltd. This is a UHV gun with a very uniform etch rate over the entire area from which the spectrum is generated. The gun was run at 3 kV (ion energy 1.5 keV) and at a beam current of 10 ~A (landing current). The calibrated etch rate for brass under these conditions was 0"05 nm/s but was probably no faster than 0.01 nm/s for the oxides. Etch times are included in the figure captions as a guide. XPS is not uniformly sensitive to all elements and the sensitivity factors published by Wagnerit or Jorgensen 1~should be borne in mind when studying the spectra. The values of the principal peak for the elements relative to that for fluorine, are carbon 0"27, oxygen 0"6, chlorine 0"42, sulphur 0.35, aluminium 0'22, copper 3-0, zinc 3"2, sodium 2"0, magnesium 3"0 and iron 1"8. The spectra are, however, particularly free of surface contamination and we are able to comment on the presence of sea water ions with a high degree of confidence. 3.2 Electrochemical investigations The passivity of the various layers was investigated by polarization of prepared surfaces in sea water cells (BDH standard sea water) using a Wenking potentiostat. Samples were cut from representative orange, white or brown sectors of the tube, drilled and tapped for attachment to a brass lead in. The edges, back and point of attachment were blanked off using "Lacomit". All samples were swept in the anodic or cathodic directions at a rate of 3 steps of 2 mV each per min. The cell was magnetically stirred and not deaerated. 4. RESULTS

4.1 Photoelectron spectrometry Typical analyses, o b t a i n e d by coverting peak heights to a t o m i c per cent using Jorgensen's factors, for tubes 1-3 a n d for the 'brassy' surface u n d e r n e a t h deposits o f intermediate thickness (Fig. 3B, D - F ) o n tube 2, are given in T a b l e 1. There are only m i n o r differences in the analyses indicating that neither operating time (10,000 h between 1C a n d 2D) n o r ferrous sulphate dosing (3D o b t a i n e d after cessation o f dosing) has m u c h influence o n the composition. The analysis o b t a i n e d f r o m the 'brassy' surface was equally rich in m a g n e s i u m showing that the t r a n s l u c e n t film

148

J.E. CASTLe,D. C. EPLEKand D. B. PEPLOW

TABLE1. SURFACECOMPOSITIONOFKEPRESENTATIVEWHITEREG1ONSONTUBESREMOVEDFROMSERVICE Composition, at. Yoexcluding C Tube unit, cond.

Service time (h)

Mg

Na

Zn

Cu

Fe

AI*

Cl

O

1, C

7 000

3"3

0

13-4

10"2

0

<11

0

61"5

2, D

17,950

8"5

0

15"6

5"6

0

< 7.5

5"3

57.4

3, D

20,908

8"0

0

11"1

7.5

0

<12

3"6

57

2, D (Brassy)

17"950

2"8

3"5

13"0

8"7

1"5

<14

5"4

52

*Aluminium is convoluted with 3p and 3s peaks of copper. covering the brass in these regions is of similar character to the white layer. The analyses obtained from the underside o f deposits were also close to those of the corresponding white layers indicating that the fracture path was within the white substance (Fig. 4). Ion bombardment of a typical white zone on tube 2 (Fig. 5) showed that its

--\ tgls

Cu 2p

~0

I

1000 Binding E n e r g y

'

' 1300 (ev)

i 7 o

t lOOO

I

I 1300

Binding Energy ( ev )

Fie. 4. Spectra obtained from the outer surface and fracture interfaces of: A, the thick brown deposit; and B, deposits of intermediate thickness. A key is included to show the relationship of the spectra. spectrum remained substantially unchanged up to the point of breakthrough to the underlying metal; there was no intervening layer. The composition derived from the peaks in these spectra after 780 s ion etching, i.e. just before breakthrough, is oxygen 70 at.%, zinc (Zn 2+) 14.1 at.%, magnesium (Mg z+) 7.5 at.%, copper (Cu +) 4.6 at.% and chlorine 3-4 at. % which is not substantially different from that obtained before etching. The absence o f iron from these layers (as is particularly evident in Fig. 4) which underlie the thickest iron deposits in the tubes is remarkable, so too is the relative enrichment of zinc and magnesium when compared to the compositions of

ESCA investigation of iron-rich protective films

~,

149

A, (c~)

r L~

Z280S ~1011

30 •

J I 886

I 1086 ~ 986 Binding E n e r g y (ev)

1300

FzG. 5. Photoelectron spectra 900-1300 eV. The splitting in the copper peaks at 0s indicates the presence of cupric compounds which have decomposed in the ion beam. The splitting in the copper peaks after 1680s is caused by differential charging between the oxide and the exposed metal. The inset shows the positions of the aluminium peaks.

Al-brass and sea water respectively. The copper in the white layers, and in the brown layers, and in the brown layers, was in the cupric state but decomposed on ion etching. The aluminium peaks, unfortunately, coincide with minor peaks of copper but we have included their probable positions in Fig. 5. By contrast, the orange layers (Figs. 2, C and 3A) are rich in iron (Fig. 6). As the insert to this figure shows it was in the ferric state (binding energy 710.29). The composition of this layer (at. %) after a comparable etching time (1200 s) is oxygen 69, chlorine, zero, iron (Fe 3+) 11.5, sodium (Na +) 7.4, copper (Cu +) 6.2, magnesium (Mg ~+) 2.5, zinc (Zn ~+) 2.5. It is noticeable that copper, zinc and sodium and magnesium have more appropriate relative concentrations in these orange layers. Nevertheless the lack of chlorine peaks shows that the distinctive sodium peaks of these layers do not arise from surface contamination with sea water. Unlike the other layers the copper in the orange layers was in the cuprous state. The above composition was retained (Fig. 6) throughout the layer: even a final attempt to remove the residual iron with "Sellotape" was unsuccessful. 4.2 Electrochemical results The current-voltage plots obtained by stepwise polarization of the white and orange zones are quite different in both the anodic (Fig. 7) and cathodic (Fig. 8) regions. The regions covered with white layers show a sharp increase in anodic current

150

J.E. CASTLE,D. C. EPLER and D. B. PEPLOW

I 700

t 720

Cu2p

I I

Zn2p

Nals

I II u

~,e, SeUo~o, Extraction

tJ~q~.

6,030 s

I "/00

I 800

I 886

I 986 BindLng E n e r g y ( o r )

I 1086

FIG. 6. Spectra obtained from the orange layer during ion etching. Note the high concentration of iron w i t l ~ the entire tl~cl~ess of this layer. The inset shows this to be in, the ferric state.

/

/

AI-Brass White

/

/

Orange Brown Brown Orange 120

240 380 Potenttat ~SIIE ) n l V .

480

FIG. 7. Step potential/current plots showing the anodic behaviour of the orange and white regions. For clarity the curves are each displaced vertically by O. 5 mA.

rising to a peak at ca. 240 inV. The current peak in the white regions mirrors tl~at found on freshly abraded aluminium brass. The current rise occurs over the potential region in which Picketing and Byme is found simultaneous dissolution of copper and zinc on an 80/20 ~-brass. Neither the white layer nor the brown layer had any effect on this process. The orange layer on the other hand suppressed the current rise for

ESCA investigation of iron-rich protective films

A

151

la

White Layers I

Brown Layers

I °~ -100

-200 . -600 -700 (SHE) inV.

-800

PotenUal

F/G. 8. Step potential/current plots for the cathodic region. For clarity curves are displaced verticallyby 50 t~A. potentials of up to 100 mV more anodic. It can be rubbed vigorously with chamois leather or even tightly abraded with 600 grit without losing its ability to protect. In the cathodic regions two current peaks A and B are readily distinguished (Fig. 8). Since we did not use chromic acid for the cathodic polarization as suggested by North and Pryor, 6 the peaks are probably associated with reduction of compounds in the brown layer A, and in the white layer B. Neither of these peaks is found on the cathodic curve for the orange layer which reduces cathodic activity to a remarkable extent. 4.3 X-ray diffraction X-ray diffraction patterns were obtained from powdered samples of the white and brown layers. Both contained peaks for paratacamite, Cu2(OH)aCI; in addition hydrotalcite, Mge Ala (COs) (OH)x6 was present in the white layer: no other patterns were strong enough to identify. The brown layer fluoresced strongly in copper radiation but was run successfully in cobalt radiation: the peaks from lepidocrocite could not be found. 4.4 Electronprobe microanalysis As we have indicated above, the important layers on these tubes were too thin for metallographic sectioning. However, some of the surfaces were examined, in plan, by microprobe analysis using a beam energy of 20 kV. The results obtainedfrom thethree regions, A, B, C (Fig. 2) on tube 1C are given in Table 2. It will be appreciated that the analysis depth is greater than the thickness of a single layer [with the possible exception of the brown layer (A)] so that the interpretation of these figures is not easy. For example the high copper and zinc values for the (C) and (]3) layers are derived from the underlying metal. However the results are informative in connection with the distribution of aluminium. Aluminium peaks are strongest where they are in association with other sea water solids such as silicon or calcium. The relationship is seen most clearly in the analyses of a grey deposit found to overlie both the brown and the orange layers, but persists in the analyses of these two layers themselves. We ascribe the aluminium here to material of aluminosilicate character deposited from the sea rather than derived from the aluminium brass. However the white layer which was

152

J . E . CASTLE,D. C. EPLF_,Rand D. B. PEPLOW TABLE 2.

INTENSmES OF X-RAY LINES

Element

Brown (A)

Zn Cu Fe M.n Ti Ca K CI S P Si Al Mg Na

2.2 9.2 49.6 0 0 6.8 3.4 3-1 4.0 4-5 9.6 4.0 1.9 1-5

Orange (C)

(ka) DETECTED

IN EMISSION FROM FIVE INTERFACES

(Intensities as Yo of total count) White Grey (13) (A)*

18.91" 47-2t 8.8 0 0 3.2 0 3.4 3.7 2.6 2.4 4.8 0 4'4

22-9"f 52"6t 0 0 0 0 0 4.0 4.6 0 0 6.7 2.0 7.0

1.0 (1.2) 3.1 (3.8) 16.5 (0) 2"4 (2.9) 1.6 (2.0) 5.3 (6.4) 6.5 (7-8) 2.0 (2.3) 2.5 (3.0) 0 (0) 40.4 (47.9) 18-3 (21.9) 0 (0) 0 (0)

Grey (C) 1.6 5.0 0 0 2.4 8.8 8.8 3.8 3.3 0 46.6 17.4 0 0

*The bracketed figures exclude the value for Fe to facilitate comparison with grey (C) figures. TThese figures are from the underlying metal.

shielded from direct deposition by overlying brown oxides does yield a strong signal for aluminium in the complete absence of a silicon peak. This is thus in accord with the diffraction evidenoe for hydrotalcite. This particular relationship of aluminium with the white layer is also found in the set of analyses obtained by EPMA in a similar way, from the colours included in the colour code of Fig. 3B (tube 3D). The data are presented in Fig. 9 andarranged to show Fe

700-

-- (Cux|)

600-

500 E,

"~ 400

"~

300 -

-~-(Zn)

...../_ A

B

G

•

:

C

F

,A,, E

D

FIG. 9. Chart illustrating the intensities of X-ray lines obtained by EPMA. The intensities are plotted according to the colour code adopted in Fig. 3(B). Intensities for clean Al-brass are given on the right.

ESCA investigationof iron-rich protectivefilms

153

the iron level relative to the underlying copper and zinc in the initial brown deposit, the white underlying deposit, the 'brassy' surface, G, and then the sequence of colours, D, E, F, formed by successive periods of iron deposition on to this substrate. The relationship of iron content to the colour has proved useful in discussing the distribution of iron on the tube surfaces. In this data we have provided also the intensities ot copper, zinc and aluminium obtained from freshly abraded aluminium brass: against these the enrichment of aluminium is seen to follow that of zinc and is again strongest in the white layer. 5. DISCUSSION 5.1 The orange layer

The two key zones on the surface of these tubes are those coloured orange and white respectively: the orange because it clearly shows some protective ability and the white because of its role in separating the iron compounds from the metal over large areas of the tube. Curiously these layers have the same large oxygen content, presumably indicative of hydroxy compounds, but their cation mixes are very different. The mixed ferric and cuprous ion content of the orange layer is in accord with the structure of layers expected from the ideas of North and Pryor e but not with those of Gasparini et aL 7 who envisaged lepidocrocite to overlie cuprous oxide. However, the sodium content of this layer has not been reported by earlier authors (e.g. Shone3). Sodium is a normal contaminant of laboratory produced specimens but is invariably accompanied by a prominent chlorine peak. The absence of a chlorine (or carbon) peak in the field samples suggests that the sodium is incorporated into the compound of copper and iron. The approximate empirical formula NaCuFe~Ox0 was retained throughout the layer but attempts to identify the compound byX-ray diffraction(using the multiple reflection diffractometer) were unsuccessful because of its small thickness. We have been unable to duplicate this film in the laboratory by iron addition to sea water cells. Its protective character perhaps results from its mixed cation content as discussed by Pryor for the case of the cupro-nickel alloys. 5.2 The white layer The composition of the white layer is equally striking. Whilst zinc oxide or hydroxy compounds would be the expected con osion product at potentials below that at which simultaneous dissolution of copper and zinc commences,18 its selective enrichment in the surface product could be reproduced in the laboratory only with horizontal specimens in stagnant sea water: there was no potential range in which the zinc concentration exceeded the copper concentration within the oxide layers on vertical test pieces in well stirred sea water3 4 The selective retention of magnesium from sea water and in the laboratory by the zinc rich white layer was a feature common to all samples examined in ESCA. Shone lists the magnesium compound MgeFe~COs(OH)xe, pyroaurite, as being that most frequently detected on sea water surfaces. With our method of analysis we have never found magnesium and iron to be mixed in single layers although this level of iron would have been readily detectable. X-ray analyses of hydroxides co-precipitated from solutions containing Zn 2+ and AI a+ ions were indistinguishable from natural hydrotalcite. It appears that Zn 2+ can substitute for all or part of the Mg ~+ component with ease. s° There were lines attributable to para-

154

J . E . CASTLe,D. C. EPLER and D. B. PEPLOW

tacamite Cu~(OH)aCI in both the white and brown layers. This is consistent with ESCA analysis which showed the copper compound to be in the cupric state in these layers (although it broke down into cuprous compounds under argon ion bombardment). The thick brown layer, although very rich in iron, was not lepidocrocite, indeed no lines attributable to iron compounds could be found. Again however X-ray analysis demonstrated the ease with which iron is incorporated in the paratacamite structure ~ when co-precipitated from chloride solutions containing copper. There seems no doubt that it is this type of mixed hydroxychloride which forms the non-protective brown layer. The corrosion product structure which emerges from these investigations is depicted in Fig. 10. This represents the quasi-stable layers found over, in some cases, entire Cooling Water

q 4

•

• ~/~---~0

Precipiatlon

v~

Copper Products

\

/

\ 1 Cu~"

Ct~Xl

• a"~

~

/~gf~gg~'.~'~g°ogq.'~'~

Acretinnof A

C~ides

/., ~s/ ] O - 0 o ~ g ~ ; o ' ~ D ~ O',og~cOls_iaq~ M

c~oo O D u m O O Q ~ " 0 o •

Brown Deposit

;~n~..... ", ...............: : :., ".::.~:'~,.-.:,~,:.-~~:.q::~-- Growth el White Layer ~-* • . ...." :::. "'~,,>.,~,:~:~.>.~-,~,.-'.,~: byPreetpitatlo o!

~--

Brass

Flo. 10. Diagrammatic reconstruction of the formation of the duplex film.

tubes and invariably over more than 50% of the tube surfaces during periods of regular iron dosing. Setting aside the problem of whether such a layer structure is harmful it is clearly not beneficial to heat transfer nor, as the electrochemical results show, to corrosion resistance. The inner layer contains no iron, and in its presence there is no possibility of forming the kind of cuprous oxide/iron oxide layer (the orange layer) which appears to have protective properties. Zinc corrosion products are normally considered soluble or gelatinous and poorly adherent and would not be expected to hinder incorporation of iron with copper compounds at the metal surface but are here stabilized by the presence of A1a+ and Mg 2+ ions. It may be that a period of exposure of the tubes to sea water followed by drying out prior to iron additions is necessary to produce the interfering zinc-rich layer. Such a situation has been simulated in the laboratory and relevant XPS spectra are given in Fig. l l. Spectrum I shows the presence of zinc 'oxide' and magnesium 'oxide' on the surface after a periodin seawater (16.2 C at -- 200 mV) and spectrum II taken after a further period in sea water (0.8 C at -- 200 mV) containing 100 ppm of ferrous sulphate, shows that this is covered completely by subsequent deposition of iron. Such layers are however nonadherent as well as non-protective in the corrosion sense. They may be stripped with "Sellotape" to reveal the zinc/magnesium oxide layers. A duplex structure once established would grow (a) at the outer interface with the cooling water by accretion of iron products and (b) within the white layer by trapping of zinc corrosion product by co-precipitation with magnesium. Copper

ESCA investigation of iron-rich protective films

155

~e

~ I

' 3~0

I

,Joo' ,~o 5(~0 I 7~0 I Binding Energ'J (ev)

I

FIG. l 1. CurveI, laboratory sample prior to deposition of iron. Curve II, sample after exposure, to sea water containing 100 ppm FeSO4, at a potential of -- 200 inV. Note the disappearance of zinc. corrosion product, probably Cu2(OH)aCI, is found throughout both layers at a relatively low concentration and must be derived from corrosion of the underlying brass: it is this which stabilises the outer deposit of iron. The more interesting transport cycle however is that of zinc which is closely analogous to that proposed by Vermilyea for aluminium, 15and magnesium, le In his model the presence of precipitated forms of aluminium or magnesium oxides, close to the metal surface, are shown to promote the continuing dissolution of these metals and prevent the formation of passive layers. 17 If the deposition of zinc hydroxides were occurring beneath the iron deposits in the present case it would both prevent iron uptake and also represent an occluded corrosion cell of the type described by Brown. is The metal-sea water contact is made beneath the thick deposit a.nd is insulated from the benefit of the high flow rate established in the tube. Under such circumstances it would not be surprising if pitting occurred, and indeed 'rashes' of small pits are found within the white layers. Examination of tubes particularly after long periods of running does show that the thick iron deposit flakes off to reveal the white inner layer, Fig. 1. Iron is then accreted on to a new iron-free surface and since flaking appears to occur only at infrequent intervals such as periods of major overhaul when the tubes dry out it is not surprising that the colours (Fig. 3) on the surface involve relatively few tints. The same colours reoccur on all the tubes with the protective layer readily distinguishable as the palest orange of the set. EPMA shows that the colours are related to the total amount of iron (Fig. 9). Such a set of points gives an indication of the probable time sequence by which the surface colourations are produced but little idea of the complex interface chemistry revealed by XPS. We have included the points for layer C, the protective layer, since it adds weight to the correlation between the weight of deposited iron and signal attenuation from the underlying brass. However, as we have stated earlier we believe this layer to be quite different in character and not part of the time sequence described above. 5.3 Aluminium It would be inappropriate to conclude such a detailed examination of the behaviour

156

J . E . CASTLE,D. C. EPLER and D. B. PEPLOW

of aluminium brass without commenting on the distribution of aluminium. Although aluminium has a low sensitivity there is no problem in determining small coverages (sub-monolayer) of alumina on aluminium. However the aluminium peaks lie particularly close to the 3 p and 3 s peaks for copper (AIa+2 p 76, Cu+3 p 76-5, Ala+2 s 120, Cu+3 s - 120). The presence of some aluminium can only be inferred from the shape and size of these peaks in relation to the behaviour of the copper 2 p peaks for an ion-etch sequence. It is difficult to place a level of detection for aluminium under these conditions but a mono-layer of oxide would have given a stronger signal than those here observed. We note however that whereas the indication, given by EPMA, of zinc enrichment in the white oxide is confirmed and emphasized by XPS the similarly strong indication for aluminium is not confirmed to be present as a distinct aluminium oxide layer by this technique. The amount of aluminium, relative to zinc, detected by EPMA would represent a 10-fold enrichment which is difficult to explain in the absence of massive general corrosion. However, aluminium-containing minerals have also been detected by X-ray diffraction. The compound hydrotalcite can vary in composition over quite wide limits including CI- in place of OH- and other divalent ions in place of Mg~+.19 Further study of this compound may reveal something of the local chemistry, e.g. Pritchard has commented on its buffering action. 1° However it is not likely to afford complete protection and aluminium compounds are absent from the orange layer. We conclude that the good behaviour of Al-brass is not associated with a discrete alumina layer. 6. C O N C L U S I O N S

The technique of XPS (or ESCA) is well suited to the examination of field samples drawn from a situation of aqueous corrosion. In particular the method is useful for elucidating the interface structure of complex and exfoliating layers. By its use we have been able to distinguish protective and non-protective layers on aluminium brass in sea water and to comment on their mechanism of formation. Acknowledgement It is a pleasure to thank Mr. W. Wright, Station Superintendent at Fawley, for his advice and encouragement in this work, Professor M. B. Waldron for help and provision of departmental facilities, the University of Surrey A.V.A. Unit for accurate photographic recording of the surface colours throughout this work, the C.E.G.B. SW Regional Scientific Services Department for helpful discussions, and the S.R.C. for help in the purchase of the XPS spectrometer. We thank Mr. Wright, C.E.G.B., SW Region, and Professor Waldron for permission to publish the work.

1. 2. 3. 4. 5. 6. 7. 8.

9. 10.

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