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Auger depth profiling and analysis of defects in tinplate surfaces
Materials Science and Engineering, 42 ( 1 9 8 0 ) 321 - 327 © Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
321
Auger Depth Profiling and Analysis of Defects in Tinplate Surfaces*
J. S. J O H A N N E S S E N a n d A. P. G R A N D E
Electronics Research Laboratory (ELAB), University of Trondheim, 7034 Trondheim-NTH (Norway) T. N O T E V A R P
A/S Norsk Jernverk, Blikkvalseverket, 5034 Ytre Laksev~g (Norway)
SUMMARY
A fully automated fine focus Auger microprobe was used to characterize defects in tinplate. Auger microprobe analysis in conjunction with ion sputtering was used to obtain chemical depth profiles o f areas of black staining and wood grain. In this paper we compare profiles through normal tinplate with those o f black staining and wood grain. Our results indicate that extensive dewetting o f electroplated tin may occur in the reflow melting process. In the case o f black staining dewetting and non-wetting is caused by heavy deposits o f carbon in the base metal surface, and in the case o f wood grain dewetring is a result of nearly complete alloying of the deposited tin layer to form an intermetallic compound FeSn 2.
1. I N T R O D U C T I O N
Auger electron spectroscopy (AES) in conjunction with ion sputtering is particularly suited for the chemical analysis of layered structures such as tinplate. We applied a fine focus automated Auger microprobe for characterization of normal tinplate and tinplate with w o o d grain and black staining defects. Normal tinplate may be considered as a multilayer structure consisting of a free surface with a thin passivation layer, beneath it a layer of tin and sandwiched between the tin layer and the base metal (iron) a fairly thick layer of an intermetallic c o m p o u n d FeSn2. The composition of the passivation layer has been thoroughly discussed elsewhere [1]. Our *Presented at the International Chalmers Sympos i u m o n Surface P r o b l e m s in Materials Science and T e c h n o l o g y , G S t e b o r g , S w e d e n , J u n e 11 - 13, 1 9 7 9 .
own results are in general agreement with the published compositional profiles of chromium and Cr203 in the passivation layer [1]. The present paper will mainly be concerned with the appearance of w o o d grain and black staining as seen by Auger analysis and by chemical depth profiling by AES and ion sputtering. The growth of FeSn2 crystallites on polycrystalline iron is well d o c u m e n t e d in the literature [2]. In the normal reflow melting of electroplated tinplate the intermetallic c o m p o u n d rapidly forms a densely interwoven network of fine elongated FeSn2 crystallites. The crystal structure of FeSn2 is tetragonal with lattice constants a = 6.53 A and c = 5.32 A [3]. The FeSn2 crystallites are easily observable in micrographs [2, 4 ] , particularly on the {100) face of iron. Commercial steel strip is of course polycrystalline, and hence the texture and thickness of the intermetallic c o m p o u n d will vary with crystal orientation over the surface of the strip. The most densely knit network of FeSn2 crystallites is found on the higher order iron faces, e.g. (111), (110), (112} etc. [2]. The growth of FeSn2 on single-crystal iron occurs at different rates on the three principal surfaces [2]. Below the melting point of tin the rates are small, typically of the order of 2.6 nm s- i t . However, above the melting point of tin. there is a rapid growth period followed b y a saturation phase in the FeSn2 layer thickness. For extended alloying the growth proceeds linearly with time. The rate of rapid initial growth is of the order of 90 nm s -1 for all the crystal faces of iron. The data referred to here were obtained under idealized laboratory conditions [2]. t l n m = 10 - 9 m = 7.67 × 10 - 7 lb p e r basis b o x ( b b ) ( F e S n 2 ) ; 1 lb b b - 1 = 1 . 3 0 4 / ~ m ( F e S n 2 ) .
322
However, the melting times and temperature curves were chosen to be as close to the real production parameters as possible; the temperature of 300 °C was reached in about 12 s [2]. The FeSn2 saturation thickness is 330 nm for the (100) face and 65 nm for the (110) face, while there is only a small kink in the growth curve for the (111) face of iron at 100 nm [4]. On the basis of this information we can expect to find FeSn2 layers of the order of 130 - 200 nm thick in commercial tinplate. The growth of FeSn 2 is an important factor in the formation of wood grain in tinplate since dewetting will occur if the intermetallic c o m p o u n d consumes a substantial part of the tin layer. Hence the tinplate surface will lose its specular reflectivity. Sometimes a wood grain pattern occurs. If the steel strip is heavily contaminated by carbon (graphite) the formation of FeSn2 is prevented and dewetting will occur in the reflow melting process. The surface becomes spotted and dull. This surface defect is called black staining. Wood grain is n o t detrimental to the properties of tinplate but black staining gives a low corrosion resistance and poor soldering properties.
Auger maps are plots of the Auger peak height (background subtracted) over a selected area of the sample. We shall make use of both depth profiles and maps in the discussion of black staining in a subsequent section of the paper. All the samples referred to in this paper were taken from the electrolytic tinning line of A/S Norsk Jernverk, Blikkvalseverket. The thickness of the tin deposit was made to specifications. Unless otherwise specified the total tin layer thickness was in the range 385 - 1540 nm. The sputtering time of the chemical depth profiles can be converted to depth into the sample by applying known sputtering rates. We used Ar + ions of 3 keV energy and ion current densities of 10 - 100 /aA cm -2, with near normal incidence of the beam. The ion beam diameter was 2.5 mm full width at half maximum. Theoretical values of the sputtering rates of tin, iron and graphite are 0.61, 0.13 and 0.03 nm min -1 (pA cm-2) -1 respectively. These values are based on recent theoretical calculations of sputtering yields for a large number of elements [8].
3. EXPERIMENTAL RESULTS 2. EXPERIMENTALTECHNIQUE
3.1. Optical imaging In the present work we made use of a fine focus automated Auger microprobe (AAM by Varian) with a spatial resolution of less than 0.5/am and with a spectroscopic energy resolution of 0.25%. The principles of Auger analysis and chemical depth profiling by ion sputtering are described elsewhere [5, 6]. In the AAM the cylindrical mirror analyser o u t p u t is digitized and stored in a m e m o r y during analysis. After the analysis is completed the data are transferred to a permanent magnetic memory (floppy disk) for further data handling and presentation on the computer terminal screen or on paper. The spectra described here are in the N(E) form as stored on the floppy disk. The chemical depth profiles are plots of Auger peak area against sputtering time. The Auger peak area can be converted to atomic percentage by applying empirical multiplication factors and a suitable normalization procedure [7].
Wood grain and black staining can easily be detected by the naked eye after reflow melting during the production of electrolytic tinplate. In Fig. 1 we show optical micrographs of (a) normal tinplate, (b) a wood grain pattern and (c) a black staining area. The magnification is 20× reduced in reproduction to 66%. At this magnification the surface of normal tinplate has an orange peel appearance with clearly visible striations along the length of the strip. Wood grain consists of parallel patterns of bright and darker lines (perpendicular to the length of the strip). Figure l(b) shows a section of a bright wood grain line. The black staining shown in Fig. 1(c) consists of bright spots surrounded by dull dark-spotted regions.
3.2. Secondary electron imaging At higher magnification normal tinplate has a uniform specular reflectance. In wood grain, in contrast, droplets of tin on a fine-grained background of FeSn2 are typical (Fig. 2(a))
323
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(b) Fig. 2. Electron micrographs (secondary electron images) of (a) a wood grain sample (magnification, 970× )and (b) a black staining sample (magnification, 260× ).
Black staining viewed by secondary electron imaging is shown in Fig. 2(b). The densely distributed black spots are carbon-rich areas partially covered by tin. The situation is typical for non-wetting conditions. (c) Fig. 1. Optical micrographs showing (a) a normal tinplate surface, (b) a wood grain surface and (c) a black staining surface. (Magnification, 13× .)
as seen by secondary electron imaging. Figure 2(a) is characteristic for the dewetting of tin on FeSn 2.
3.3. Auger analysis Figure 3(a) is an Auger spectrum of the free surface of passivated normal tinplate. The sample surface consists of tin and chromium oxides contaminated by carbon, chlorine and sulphur. Figure 3(b) is an Auger spectrum of the base metal from an angle lap near the inter-
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Fig. 3. An Auger spectrum of (a) a passivated normal tinplate surface and (b) an angle lap section of the base metal near the intermetallic interface.
Fig. 4. Chemical c o m p o s i t i o n a l profiles through (a) a n o r m a l t i n p l a t e sample and ( b ) a d e w e t t e d region o f a
metallic layer. The surface has been sputter cleaned. The base metal iron seems to contain some nickel, cobalt and carbon. The tin, oxygen, nitrogen and argon are contaminants from the lapping and sputtering processes. In the chemical profiles to be discussed we used the areas of the Fe(700 eV), O(510 eV), Sn(426 eV) and C(267 eV) Auger peaks. Whenever chromium occurred we used the Cr(532 eV) Auger peak.
3.4. Chemical profiles of wood grain The chemical composition in atomic per cent of a normal reflow-melted tinplate is shown in Fig. 4(a). The "range" gives the minimum and maximum values throughout the profile. The small negative value is due to chemical shifts of the Auger peaks not accounted for in the integration process. One minute of sputtering time corresponds to the
wood grain sample. The sputtering rates for tin, iron and carbon are 160, 36.5 and 8.5 nm rain -1 respectively.
removal on average of 60 nm of material. Notice the wide interface layer w h e r e t i n gradually decreases to zero and iron increases towards 100%. An estimate of the interface width yields 300 nm. The graded interface is due to the fine needle-shaped FeSn 2 crystallites extending into the tin coating. Estimated sputtering rates for tin, iron and carbon are 160, 36.5 and 8.5 nm min -1 respectively. Figure 4(b) shows a chemical profile of an area between tin droplets in Fig. 2(a). On comparing Figs. 4(a) and 4(b) we immediately observe a difference in the iron profiles. Close to the surface (t = 0) there is in both cases a thin (~5 nm) surface passivation layer. In Fig. 4(b) there seems to be a reasonably uniform FeSn2 phase (33 at.% Fe and 67 at.% Sn) through the entire layer.
325
in Fig. 5(a) is similar to that of normal tinplate shown in Fig. 4(a). However, the profile in Fig. 5(b) shows a large carbon concentration, between 20 and 25 at.% C throughout the layer, and a very small amount of tin at the surface. The outer layer is most probably cementite F e 3 C . The first 10 min of the profiles of Fig. 5(b) is expanded in Fig. 5(d). For clarity the iron profile is left out. The first 0.6 min of the profiles are associated with a surface contamination layer. However, between 0.6 and 4 - 5 min the carbon concentration is constant at about 25 at.%. Beyond about 5 min of sputtering the carbon concentration drops to 20 at.% and it finally disappears after about 40 min of sputtering. The conversion to atomic percentage is subject to errors in the sensitivity factors used.
It is quite evident that w o o d grain is a case of extensive alloying where almost all the tin is consumed in the alloying process. The surface tension of the remaining molten tin is then sufficient to form droplets during freezing. Dewetting has occurred.
3.5. Depth profiling of black staining Figures 5(a) and 5(b) are chemical profiles through one of the bright spots and the dull stained area of Fig. l(c) respectively. The comparatively large negative "range" values are due to chemical shifts in the tin spectrum in going from oxidized tin to metallic tin at the surface. Chemical shifts in Auger spectra are well known for other elements [9, 10] and will n o t be discussed further in this paper. It took between 30 and 40 min of sputtering to reach the base metal. The profile RANGE- 18.78Ee TO
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326
We feel that the carbon concentration of 20 - 25 at.% underneath the surface contamination layer (0 - 0.6 min) strongly suggests a fairly thick cementite layer. In order to explore the lateral distribution of carbon in the regions of black staining we performed simultaneous profiling at two locations 40 pm apart. Location 1 was chosen on a tin-rich area and location 2 on a black spot. The chemical profiles are shown in Fig. 6(a) (stained area) and Fig. 6(b) (tin-rich area). Auger maps of the carbon distribution {Fig. 6(c)) and the tin distribution (Fig. 6(d)) after 4 min sputtering show quite clearly that I~RNGE - 1 . 0 2 9 2 TO
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there is a non-uniform deposit of carbon in the base metal surface. The origin of the carbon in black staining cannot be deduced from these experimental results. It may have been formed by surface segregation of carbon from the base metal during batch annealing [ 11]. Another possibility is graphitization from roller oil residues due to incomplete degreasing. We consider that surface segregation ought to form a more uniform carbon distribution at least over distances of 40 pm. Oil residues are more likely to occur in a patchwork manner on the base metal surface. However, at the
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327 present state of our investigations the experimental results are n o t conclusive in this respect.
4. CONCLUSIONS We have characterized normal tinplate, tinplate with wo o d grain and tinplate with black staining by means of fine focus Auger analysis, chemical profiles and Auger maps. The characteristic feature of w ood grain is extensive intermetallic alloying and subsequent dewetting o f the remaining tin deposit. The f o r m a t i o n o f a wood-grain-like pattern is imposed by external parameters such as the a.c. heating (100 Hz pow er dissipation) and the speed o f the strip [ 1 2 ] . Wood grain is n o t observed in thick (>1 ~m) tin coatings. It m os t f r eq u en tly occurs in thinner coatings ( ~ 0 . 2 5 p m) [ 1 2 ] . Black staining is characterized by Fe3C and heavy carbon cont am i na t i on (~ 20 at.%) in the base metal surface. However, the localized elliptical to circular spots that occur in the coating are similar to those in normal tinplate. The size of the normal spots is of the order of millimetres to several centimetres in diameter. Contaminants ot he r than carbon have n o t been observed in the bulk of the stain. Dewetting and non-wetting of tin on Fe3C and in the carbon-contaminated strip surface are characteristic features of black staining.
We consider that fine focus Auger analysis is a powerful tool in tinplate research. Our research programme as outlined in the present paper will continue with emphasis on surface problems related to passivation and surface c o n t a m i n a t i o n of tinplate.
REFERENCES 1 V. Leroy, J. P. Servais, L. Habraken, L. Renard and J. Lempereur, Proc. 1st Int. Tinplate Conference, London, 1976, Publ. no. 530, International Tin Research Institute, London, 1976, p. 399. 2 H. E. Biber and W. T. Hartler, J. Electrochem. Soc., 113 (1966) 828. 3 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 2nd edn., 1958. 4 J. V. Castell-Evans, Trans. Inst. Met. Finish., 47 (1969) 71. 5 C. C. Chang, in P. F. Kane and G. B. Larrabee (eds.), Characterization o f Solid Surfaces, Plenum, New York, 1974, Chap. 20. 6 J. S. Johannessen, W. E. Spicer and Y. E. Strausset, J. Appl. Phys., 47 (1976) 3028. 7 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook o f Auger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, Minnesota, 1976. 8 H. H. Andersen, Appl. Phys., 18 (1979) 131. 9 C. D. Wagner, Anal. Chem., 47 (1975) 1201. 10 J. S. Johannessen, W. E. Spicer and Y. E. Strausser, Appl. Phys. Lett, 27 (1975) 452. 11 V. Leroy, personal communication, 1979. 12 W. E. Hoare, E. S. Hedges and B. T. K. Barry, The Technology o f Tinplate, Edward Arnold, London, 1965, Chap. 9, p. 284.
321
Auger Depth Profiling and Analysis of Defects in Tinplate Surfaces*
J. S. J O H A N N E S S E N a n d A. P. G R A N D E
Electronics Research Laboratory (ELAB), University of Trondheim, 7034 Trondheim-NTH (Norway) T. N O T E V A R P
A/S Norsk Jernverk, Blikkvalseverket, 5034 Ytre Laksev~g (Norway)
SUMMARY
A fully automated fine focus Auger microprobe was used to characterize defects in tinplate. Auger microprobe analysis in conjunction with ion sputtering was used to obtain chemical depth profiles o f areas of black staining and wood grain. In this paper we compare profiles through normal tinplate with those o f black staining and wood grain. Our results indicate that extensive dewetting o f electroplated tin may occur in the reflow melting process. In the case o f black staining dewetting and non-wetting is caused by heavy deposits o f carbon in the base metal surface, and in the case o f wood grain dewetring is a result of nearly complete alloying of the deposited tin layer to form an intermetallic compound FeSn 2.
1. I N T R O D U C T I O N
Auger electron spectroscopy (AES) in conjunction with ion sputtering is particularly suited for the chemical analysis of layered structures such as tinplate. We applied a fine focus automated Auger microprobe for characterization of normal tinplate and tinplate with w o o d grain and black staining defects. Normal tinplate may be considered as a multilayer structure consisting of a free surface with a thin passivation layer, beneath it a layer of tin and sandwiched between the tin layer and the base metal (iron) a fairly thick layer of an intermetallic c o m p o u n d FeSn2. The composition of the passivation layer has been thoroughly discussed elsewhere [1]. Our *Presented at the International Chalmers Sympos i u m o n Surface P r o b l e m s in Materials Science and T e c h n o l o g y , G S t e b o r g , S w e d e n , J u n e 11 - 13, 1 9 7 9 .
own results are in general agreement with the published compositional profiles of chromium and Cr203 in the passivation layer [1]. The present paper will mainly be concerned with the appearance of w o o d grain and black staining as seen by Auger analysis and by chemical depth profiling by AES and ion sputtering. The growth of FeSn2 crystallites on polycrystalline iron is well d o c u m e n t e d in the literature [2]. In the normal reflow melting of electroplated tinplate the intermetallic c o m p o u n d rapidly forms a densely interwoven network of fine elongated FeSn2 crystallites. The crystal structure of FeSn2 is tetragonal with lattice constants a = 6.53 A and c = 5.32 A [3]. The FeSn2 crystallites are easily observable in micrographs [2, 4 ] , particularly on the {100) face of iron. Commercial steel strip is of course polycrystalline, and hence the texture and thickness of the intermetallic c o m p o u n d will vary with crystal orientation over the surface of the strip. The most densely knit network of FeSn2 crystallites is found on the higher order iron faces, e.g. (111), (110), (112} etc. [2]. The growth of FeSn2 on single-crystal iron occurs at different rates on the three principal surfaces [2]. Below the melting point of tin the rates are small, typically of the order of 2.6 nm s- i t . However, above the melting point of tin. there is a rapid growth period followed b y a saturation phase in the FeSn2 layer thickness. For extended alloying the growth proceeds linearly with time. The rate of rapid initial growth is of the order of 90 nm s -1 for all the crystal faces of iron. The data referred to here were obtained under idealized laboratory conditions [2]. t l n m = 10 - 9 m = 7.67 × 10 - 7 lb p e r basis b o x ( b b ) ( F e S n 2 ) ; 1 lb b b - 1 = 1 . 3 0 4 / ~ m ( F e S n 2 ) .
322
However, the melting times and temperature curves were chosen to be as close to the real production parameters as possible; the temperature of 300 °C was reached in about 12 s [2]. The FeSn2 saturation thickness is 330 nm for the (100) face and 65 nm for the (110) face, while there is only a small kink in the growth curve for the (111) face of iron at 100 nm [4]. On the basis of this information we can expect to find FeSn2 layers of the order of 130 - 200 nm thick in commercial tinplate. The growth of FeSn 2 is an important factor in the formation of wood grain in tinplate since dewetting will occur if the intermetallic c o m p o u n d consumes a substantial part of the tin layer. Hence the tinplate surface will lose its specular reflectivity. Sometimes a wood grain pattern occurs. If the steel strip is heavily contaminated by carbon (graphite) the formation of FeSn2 is prevented and dewetting will occur in the reflow melting process. The surface becomes spotted and dull. This surface defect is called black staining. Wood grain is n o t detrimental to the properties of tinplate but black staining gives a low corrosion resistance and poor soldering properties.
Auger maps are plots of the Auger peak height (background subtracted) over a selected area of the sample. We shall make use of both depth profiles and maps in the discussion of black staining in a subsequent section of the paper. All the samples referred to in this paper were taken from the electrolytic tinning line of A/S Norsk Jernverk, Blikkvalseverket. The thickness of the tin deposit was made to specifications. Unless otherwise specified the total tin layer thickness was in the range 385 - 1540 nm. The sputtering time of the chemical depth profiles can be converted to depth into the sample by applying known sputtering rates. We used Ar + ions of 3 keV energy and ion current densities of 10 - 100 /aA cm -2, with near normal incidence of the beam. The ion beam diameter was 2.5 mm full width at half maximum. Theoretical values of the sputtering rates of tin, iron and graphite are 0.61, 0.13 and 0.03 nm min -1 (pA cm-2) -1 respectively. These values are based on recent theoretical calculations of sputtering yields for a large number of elements [8].
3. EXPERIMENTAL RESULTS 2. EXPERIMENTALTECHNIQUE
3.1. Optical imaging In the present work we made use of a fine focus automated Auger microprobe (AAM by Varian) with a spatial resolution of less than 0.5/am and with a spectroscopic energy resolution of 0.25%. The principles of Auger analysis and chemical depth profiling by ion sputtering are described elsewhere [5, 6]. In the AAM the cylindrical mirror analyser o u t p u t is digitized and stored in a m e m o r y during analysis. After the analysis is completed the data are transferred to a permanent magnetic memory (floppy disk) for further data handling and presentation on the computer terminal screen or on paper. The spectra described here are in the N(E) form as stored on the floppy disk. The chemical depth profiles are plots of Auger peak area against sputtering time. The Auger peak area can be converted to atomic percentage by applying empirical multiplication factors and a suitable normalization procedure [7].
Wood grain and black staining can easily be detected by the naked eye after reflow melting during the production of electrolytic tinplate. In Fig. 1 we show optical micrographs of (a) normal tinplate, (b) a wood grain pattern and (c) a black staining area. The magnification is 20× reduced in reproduction to 66%. At this magnification the surface of normal tinplate has an orange peel appearance with clearly visible striations along the length of the strip. Wood grain consists of parallel patterns of bright and darker lines (perpendicular to the length of the strip). Figure l(b) shows a section of a bright wood grain line. The black staining shown in Fig. 1(c) consists of bright spots surrounded by dull dark-spotted regions.
3.2. Secondary electron imaging At higher magnification normal tinplate has a uniform specular reflectance. In wood grain, in contrast, droplets of tin on a fine-grained background of FeSn2 are typical (Fig. 2(a))
323
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(b)
(b) Fig. 2. Electron micrographs (secondary electron images) of (a) a wood grain sample (magnification, 970× )and (b) a black staining sample (magnification, 260× ).
Black staining viewed by secondary electron imaging is shown in Fig. 2(b). The densely distributed black spots are carbon-rich areas partially covered by tin. The situation is typical for non-wetting conditions. (c) Fig. 1. Optical micrographs showing (a) a normal tinplate surface, (b) a wood grain surface and (c) a black staining surface. (Magnification, 13× .)
as seen by secondary electron imaging. Figure 2(a) is characteristic for the dewetting of tin on FeSn 2.
3.3. Auger analysis Figure 3(a) is an Auger spectrum of the free surface of passivated normal tinplate. The sample surface consists of tin and chromium oxides contaminated by carbon, chlorine and sulphur. Figure 3(b) is an Auger spectrum of the base metal from an angle lap near the inter-
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Fig. 3. An Auger spectrum of (a) a passivated normal tinplate surface and (b) an angle lap section of the base metal near the intermetallic interface.
Fig. 4. Chemical c o m p o s i t i o n a l profiles through (a) a n o r m a l t i n p l a t e sample and ( b ) a d e w e t t e d region o f a
metallic layer. The surface has been sputter cleaned. The base metal iron seems to contain some nickel, cobalt and carbon. The tin, oxygen, nitrogen and argon are contaminants from the lapping and sputtering processes. In the chemical profiles to be discussed we used the areas of the Fe(700 eV), O(510 eV), Sn(426 eV) and C(267 eV) Auger peaks. Whenever chromium occurred we used the Cr(532 eV) Auger peak.
3.4. Chemical profiles of wood grain The chemical composition in atomic per cent of a normal reflow-melted tinplate is shown in Fig. 4(a). The "range" gives the minimum and maximum values throughout the profile. The small negative value is due to chemical shifts of the Auger peaks not accounted for in the integration process. One minute of sputtering time corresponds to the
wood grain sample. The sputtering rates for tin, iron and carbon are 160, 36.5 and 8.5 nm rain -1 respectively.
removal on average of 60 nm of material. Notice the wide interface layer w h e r e t i n gradually decreases to zero and iron increases towards 100%. An estimate of the interface width yields 300 nm. The graded interface is due to the fine needle-shaped FeSn 2 crystallites extending into the tin coating. Estimated sputtering rates for tin, iron and carbon are 160, 36.5 and 8.5 nm min -1 respectively. Figure 4(b) shows a chemical profile of an area between tin droplets in Fig. 2(a). On comparing Figs. 4(a) and 4(b) we immediately observe a difference in the iron profiles. Close to the surface (t = 0) there is in both cases a thin (~5 nm) surface passivation layer. In Fig. 4(b) there seems to be a reasonably uniform FeSn2 phase (33 at.% Fe and 67 at.% Sn) through the entire layer.
325
in Fig. 5(a) is similar to that of normal tinplate shown in Fig. 4(a). However, the profile in Fig. 5(b) shows a large carbon concentration, between 20 and 25 at.% C throughout the layer, and a very small amount of tin at the surface. The outer layer is most probably cementite F e 3 C . The first 10 min of the profiles of Fig. 5(b) is expanded in Fig. 5(d). For clarity the iron profile is left out. The first 0.6 min of the profiles are associated with a surface contamination layer. However, between 0.6 and 4 - 5 min the carbon concentration is constant at about 25 at.%. Beyond about 5 min of sputtering the carbon concentration drops to 20 at.% and it finally disappears after about 40 min of sputtering. The conversion to atomic percentage is subject to errors in the sensitivity factors used.
It is quite evident that w o o d grain is a case of extensive alloying where almost all the tin is consumed in the alloying process. The surface tension of the remaining molten tin is then sufficient to form droplets during freezing. Dewetting has occurred.
3.5. Depth profiling of black staining Figures 5(a) and 5(b) are chemical profiles through one of the bright spots and the dull stained area of Fig. l(c) respectively. The comparatively large negative "range" values are due to chemical shifts in the tin spectrum in going from oxidized tin to metallic tin at the surface. Chemical shifts in Auger spectra are well known for other elements [9, 10] and will n o t be discussed further in this paper. It took between 30 and 40 min of sputtering to reach the base metal. The profile RANGE- 18.78Ee TO
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326
We feel that the carbon concentration of 20 - 25 at.% underneath the surface contamination layer (0 - 0.6 min) strongly suggests a fairly thick cementite layer. In order to explore the lateral distribution of carbon in the regions of black staining we performed simultaneous profiling at two locations 40 pm apart. Location 1 was chosen on a tin-rich area and location 2 on a black spot. The chemical profiles are shown in Fig. 6(a) (stained area) and Fig. 6(b) (tin-rich area). Auger maps of the carbon distribution {Fig. 6(c)) and the tin distribution (Fig. 6(d)) after 4 min sputtering show quite clearly that I~RNGE - 1 . 0 2 9 2 TO
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there is a non-uniform deposit of carbon in the base metal surface. The origin of the carbon in black staining cannot be deduced from these experimental results. It may have been formed by surface segregation of carbon from the base metal during batch annealing [ 11]. Another possibility is graphitization from roller oil residues due to incomplete degreasing. We consider that surface segregation ought to form a more uniform carbon distribution at least over distances of 40 pm. Oil residues are more likely to occur in a patchwork manner on the base metal surface. However, at the
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327 present state of our investigations the experimental results are n o t conclusive in this respect.
4. CONCLUSIONS We have characterized normal tinplate, tinplate with wo o d grain and tinplate with black staining by means of fine focus Auger analysis, chemical profiles and Auger maps. The characteristic feature of w ood grain is extensive intermetallic alloying and subsequent dewetting o f the remaining tin deposit. The f o r m a t i o n o f a wood-grain-like pattern is imposed by external parameters such as the a.c. heating (100 Hz pow er dissipation) and the speed o f the strip [ 1 2 ] . Wood grain is n o t observed in thick (>1 ~m) tin coatings. It m os t f r eq u en tly occurs in thinner coatings ( ~ 0 . 2 5 p m) [ 1 2 ] . Black staining is characterized by Fe3C and heavy carbon cont am i na t i on (~ 20 at.%) in the base metal surface. However, the localized elliptical to circular spots that occur in the coating are similar to those in normal tinplate. The size of the normal spots is of the order of millimetres to several centimetres in diameter. Contaminants ot he r than carbon have n o t been observed in the bulk of the stain. Dewetting and non-wetting of tin on Fe3C and in the carbon-contaminated strip surface are characteristic features of black staining.
We consider that fine focus Auger analysis is a powerful tool in tinplate research. Our research programme as outlined in the present paper will continue with emphasis on surface problems related to passivation and surface c o n t a m i n a t i o n of tinplate.
REFERENCES 1 V. Leroy, J. P. Servais, L. Habraken, L. Renard and J. Lempereur, Proc. 1st Int. Tinplate Conference, London, 1976, Publ. no. 530, International Tin Research Institute, London, 1976, p. 399. 2 H. E. Biber and W. T. Hartler, J. Electrochem. Soc., 113 (1966) 828. 3 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 2nd edn., 1958. 4 J. V. Castell-Evans, Trans. Inst. Met. Finish., 47 (1969) 71. 5 C. C. Chang, in P. F. Kane and G. B. Larrabee (eds.), Characterization o f Solid Surfaces, Plenum, New York, 1974, Chap. 20. 6 J. S. Johannessen, W. E. Spicer and Y. E. Strausset, J. Appl. Phys., 47 (1976) 3028. 7 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook o f Auger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, Minnesota, 1976. 8 H. H. Andersen, Appl. Phys., 18 (1979) 131. 9 C. D. Wagner, Anal. Chem., 47 (1975) 1201. 10 J. S. Johannessen, W. E. Spicer and Y. E. Strausser, Appl. Phys. Lett, 27 (1975) 452. 11 V. Leroy, personal communication, 1979. 12 W. E. Hoare, E. S. Hedges and B. T. K. Barry, The Technology o f Tinplate, Edward Arnold, London, 1965, Chap. 9, p. 284.