Metallurgical applications of surface analytical techniques

Materials Science and Engineering, 42 ( 1 9 8 0 ) 2 8 9 - 307

289

© 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

Metallurgical Applications o f Surface Analytical Techniques*

V. L E R O Y

Centre de Recherches Mdtallurgiques, Abbaye du Val-Benoft, B 4000-Liege (Belgium)

SUMMARY

Recent techniques for surface analysis such as secondary ion mass spectrometry, Auger spectroscopy and X-ray photoelectron spectroscopy have now made it possible to obtain a good understanding o f the chemistry of real surfaces o f industrial products. The surface chemistry o f steel sheet is o f interest for such industrial operations as cold forming, phosphatizing and painting and tinning or galvanizing. This report describes the surface chemistry o f two important products: black plate and steel sheet for cold forming. For steel coating (phosphatizing or chromate conversion coating) some examples are given o f the improvements that can be achieved with a better knowledge o f the surface chemistry of tinplate and galvanized steel sheet.

1. I N T R O D U C T I O N

In the last few years steel producers and users have given serious consideration to the surface properties of cold-rolled steel sheet. The sheet surface often plays an important part in the subsequent use o f the material. Two main characteristics of sheet surfaces must be considered: the morphology of the free surface, and the chemistry o f the external layers. Recent techniques for surface analysis have led to a greater understanding o f the chemistry of real surfaces, and it has thus become possible to define the surface chemistry of industrial products more precisely [1 - 3]. Such work concerns industries that involve opera*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 6 t e b o r g , S w e d e n , J u n e 11 - 13, 1 9 7 9 .

tions such as forming, tinning, galvanizing or phosphatizing. Unfortunately, however, little information about the real surface of industrial products is available. It was therefore decided to equip a new laboratory at the Centre de Recherches M~tallurgiques (CRM) and to investigate the possibilities of new techniques such as secondary ion mass spectrometry (SIMS) with imaging facilities, Auger spectroscopy and X-ray photoelectron spectroscopy (XPS). The purpose of this paper is to give a general survey of the surface work performed at CRM in the last few years and to give details of some of the quality improvements achieved in such studies. The report is presented in t w o main parts: (1) the surface chemistry of cold-rolled steel sheet (a distinction is made between black plate and steel sheet for cold forming); (2) the passivation treatment of two important products, i.e. tinplate of the type widely used in the packaging industry, and galvanized steel strip.

2. T H E S U R F A C E C H E M I S T R Y ROLLED STEEL SHEET

OF

COLD-

In the cold processing of steel strip the free surface of the sheet is in close contact with many different materials that could affect the composition of the external layers. It can be assumed that the surface chemistry of the final product depends on the contribution of each stage in the cold processing line. For this reason we start our discussion at the end of the pickling line for hot-rolled strip. After pickling in hydrochloric acid or sulphuric acid, oil is applied before coiling to provide a lubricant for the cold reduction step. During cold rolling, a dispersion or an emulsion of oil in water is applied to the strip to aid lubri-

290

cation and to help dissipate the heat generated [4 - 6]. The lubricant formulation depends on the cold reduction rate; in particular the lubricant for black plate, which needs a large reduction in thickness, contains palm oils or blends of tallow with free fatty acids which do not distil off during annealing at high temperature. Before annealing, such residual pollution must be removed b y saponification during electrolytic cleaning in alkaline solutions. Until now this kind of degreasing treatment has not normally been used for the cold forming o f steel sheets, which are cold rolled down to medium gauge with mineral oil emulsion; it is assumed that the residual oil film is removed during annealing.

2.1. Black plate In order to determine the surface chemistry of the product, samples were taken from different stages o f industrial production lines: after descaling, cold rolling, alkaline degreasing, annealing and temper rolling, as shown in Fig. 1. For annealing, two different processes were studied: the batch annealing o f Coils, and continuous annealing in which the free surface o f the steel strip can interact more freely with the protective atmosphere.

Water ~_~ ,

2

91

=I C,

Oil emalsion(T/NOIL)

" u

COIL

(a)

For the samples examined after descaling in hydrochloric acid (with the addition of corrosion inhibitor) and before coating in pickling oil, Auger spectroscopy revealed chlorine and calcium pollution, as shown in Fig. 2(a) where atomic concentrations given b y PHI sensitivity factors are plotted as a function of distance below the free surface. Such pollution may lead to spot rusting on temper-rolled steel sheet. The presence of these residues depends strongly on the rinsing treatment after pickling; the immersion and spraying methods result in some difference. Sulphur and nitrogen were also detected; their presence is due to the inhibitor in the pickling bath which may be, for example, an alkylthiourea. Figure 2 also shows distinct enrichment of copper on the descaled surface, to a degree that is closely related to the bulk copper content of the material. The copper enrichment is induced during hot rolling and coiling of the steel strip. Since copper, in contrast with iron, cannot be oxidized in the prevailing atmosphere, it accumulates below the oxide film and some of it diffuses back into the metallic matrix. After descaling in the presence of the inhibitor it appears on the free surface. Such copper enrichment may affect the required compati-

Hot air t~

HCI tank .,,_._S J_

2.1.1. After descaling in hydrochloric acid

\ pickl,ogoi~l

(b)

( TINOIL)

(c)

(d)

Fig. 1. Schematic view of the cold processing line for black plate: (a) pickling line; (b) cold rolling line; (c) batch annealing (degreasing line 1, sodium silicate); (d) continuous annealing (degreasing line 2, sodium silicate); (e) temper rolling. The arrows indicate the sampling points.

291

C/k9F (at.%)~ 7"1

6

q

I 3

2

V~

(a)

0

Pickled

(b)

I

I 1oo

I"'~

I

I

200 ~ eq.F2 °°

Z = 13A A z = 130 A

o[ 0

I

I

I

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250 ~

r~

I

I

200 ,~ eq, Fe300

Z =13A

Cold rolled

L

I

I00

Annealed

Z= 13A A z = 130 A

J ~I

Fig. 2. (a) Auger c o m p o s i t i o n profiles of steel strip (after HCI pickling): - - , Fe (k = 10);~/, C1 (k = 1);~7, S (k = 1); +, N (k = 1); eaC (k = 5); o, O (k = 5);&, Ca (k = 1 ) ; ~ , Cu (k = 1). (b) Copper ion (63Cu ÷) images by SIMS (p(O 2) = 4 x 10 - ~ Tort).

bility b e t w e e n the steel surface and the lubricant during cold rolling. In our experience these enrichments are partially removed during cold rolling, probably b y a wear mechanism, and they disappear more or less completely after batch annealing in an N2-H2 atmosphere as a result of the backdiffusion that occurs under these non-oxidizing conditions, as shown in Fig. 2(b). The ion images shown in the figure were recorded by a SIMS analyser with imaging capability. They show the chemical distribution of selected elements in very thin layers whose thickness and

depth below the initial surface are given in &ngstrSms by the parameters AZ and Z respectively.

2.1.2. After cold rolling and electrodegreasing A general idea o f the variations in surface composition at different stages of the cold processing line can be gained from ion images such as those shown in Fig. 3. For example the carbon image indicates that the residual film of rolling oil is quite homogeneous in the cold-rolled stage. The

z=13A. ~ Z = 6,5 A

SSMn+

Z=13A ~z'= 13A

6Osio~-

Z=26A AZ= 2 6 A

63p0 ~-

Z=26A. AZ= 130A

24C~-

Fig. 3. Ion images by SIMS at different stages o f the cold processing line for black plate: (a) cold rolled; (b) degreased; (c) batch annealed.

Ib)

[a)

~OCa*

'~Z=I30A

Z=2 61~

Z=26A

~D

293 calcium detected on the free surface is due to the hardness of the water used as the basis of the rolling emulsion. Because of the poor burn-cleanability at annealing temperature of rolling oils with a high content of free fatty acid, an orthosilicate electro
2.1.3. After annealing The batch annealing treatment at 660 °C in a protective atmosphere of N2 containing 5% H 2 induces interactions between the silica film and the steel substrate which produce a thicker external layer. At the same time, manganese, phosphorus and chromium segregate strongly to the free surface where t h e y are present as oxides and form a homogeneous film, as shown in Fig. 3. The manganese content in the external layer may be as high as 10% at a rough estimate. The phosphorus distribution is much more complex; phosphate from the degreasing treatment is observed, but phosphorus is also enriched in the grain boundaries o f the microstructure by migration from the bulk to the free surface. Calcium deposited during the degreasing treatment is still present in the as-annealed condition. A comparison between the surface analyses of batch-annealed and continuously annealed steel sheets is shown in Fig. 4. Because of the short annealing time in the continuous process, the film left after degreasing does n o t

interact with the substrate, as shown by the SiO2- and PO 2- ion images. The surface film also seems to be much more amorphous. For the same reason, manganese enrichment is much more limited. Dry temper rolling does not drastically modify the surface chemistry observed after annealing. It may be concluded that the black plate surface in the as-annealed condition is certainly not a clean surface. It is important to know whether such a surface is compatible with the subsequent use of the product, e.g. in a tinning line. The pretreatment in a tinning line should remove the external layers in order to ensure good adherence of the tin deposit and good nucleation of the F e - S n intermetallic compounds formed during the brightening anneal that follows electro-tinning. The ion images reproduced in Fig. 4 clearly show that the degreasing and the sulphuric acid pickle applied in the first stages of the tinning line remove any chemical enrichment present in the external layers except for chromium, which is still present in the grain boundaries, and calcium, which is re-introduced on rinsing in hard water. Experience has shown that the surface chemistry as described here for the as-annealed conditions is quite adequate for tinning. Problems may occur, however, in some specific cases during the batch annealing treatment; the most serious is the formation of graphite on the free surface of batch-annealed steel strip, which can be demonstrated by X-ray diffraction. Because of the degreasing treatment that precedes annealing, this graphite formation cannot be due to carbon pollution from residual rolling oil; instead it is ascribed to cementite destabilization and carbon migration from the bulk to the external surface during heat treatment [8 - 10]. Such graphite formation cannot be removed by any chemical pretreatment; upon tinning it leads to the formation of " f r o s t y " plate. It has been shown that graphite formation can be inhibited by adjusting the steel composition so as to counteract cementite destabilization at high temperature; the carbideforming elements manganese and chromium are particularly effective in this respect [ 11 ]. Residual elements such as copper and nickel which can form stable sulphides on the free surface also reduce graphite formation. It should be recalled that such residual elements

294 SSMn*

z=13A AZ= 13A

6Osio~-

Z=26A AZ= 13A

63p0 ~-

Z=26A ,hZ= 130A

(a)

(~)

(c) Fig. 4. Ion images by SIMS at different stages of the cold processing line for black plate: (a) after continuous annealing; (b) batch annealed; (c) annealed and pickled prior to tinning.

may be strongly enriched in the external layers during the heating step of the annealing cycle, as demonstrated in Fig. 2(a). Figure 5 shows the dependence of graphite formation induced by batch annealing at 700 °C in an N2-5%H2 atmosphere on the steel composition as expressed by two main parameters, [Mn] + 2 [Cr] and [Cu] × [S], which relate to cementite stabilization and sulphide formation respectively. Graphite formation may also be inhibited by the formation of a new film on the free surface, e.g. a degreasing film. Our work in this field has shown that the SiO2.nH20 film

formed in the rinsing stage of the electrolytic degreasing treatment reduces graphite formation substantially. The thickness of this film depends strongly on the pH of the rinsing water, being greatest after rinsing in neutral water. The addition of sulphur-bearing compounds to the alkaline degreasing solution or to the neutral rinsing bath is also quite effective in inhibiting graphitization [12]. Figure 6(a) shows c o m p o s i t i o n - d e p t h profiles as mea. sured b y Auger spectroscopy after various degreasing treatments, and Fig. 6(b) shows the normalized graphitization index G (based on the intensity of an X-ray diffraction peak

295

of graphite after close-pack annealing) after the same degreasing treatments. Auger and XPS analyses suggest that the addition of sulphur-bearing compounds increases the thickness of the degreasing film. At the same time sulphur is trapped on the free surface as a C-S complex (when the addition is made at the rinsing stage) or as a sulphide {when it is made in the degreasing bath). The last procedure appears to be the most effective and economical way of preventing graphite formation [13].

500

,~OC

300

200

I00

I 200

0

I ~00

I 600

I 800

I tO00

I t200 (Cu].{5)

Fig. 5. T h e d e p e n d e n c e o f g r a p h i t e f o r m a t i o n d u r i n g b a t c h a n n e a l i n g o n steel c o m p o s i t i o n ( a l u m i n i u m stabilized L D steels); ( [ M n ] + 2 [ C r ] ) -- 250 e x p ( - - 2 x l O - 3 [ C u ] x [ S ] ) > 150.

CHjCCI 3

Silicot¢ Th]ourea (rinsing)

Si/icate

2.2. Steel sheet for forming

Silicate ÷ Thiourea (degreasing)

C/k 8 (or'l,) 7

I 6

"\

i

5

S

\

3

! 2

~Xx

q".

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.

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o

~

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I

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~ eq. Fe

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6

~ eq. Fe

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s'o o

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,

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(a) cps/cm2.sec 900 050

ooof

890

D

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155

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:t (b) Fig. 6. T h e i n f l u e n c e o f various degreasing t r e a t m e n t s o n surface g r a p h i t e f o r m a t i o n i n d u c e d b y a b a t c h a n n e a l i n g t r e a t m e n t in a n N 2 - 5 % H 2 a t m o s p h e r e . (a) A u g e r c o m p o s i t i o n profiles f o r f o u r d i f f e r e n t treatments: --, F e (k = 2 0 ) ; - - -, O (k = 5 ) ; - - . - - - - - , Si (k = 2); . . . . . , S (k = 1). ( b ) G r a p h i t i z a t i o n i n d e x G for t h e s a m e t r e a t m e n t s .

,

Steel strips are not usually cleaned before coiling and batch annealing. Consequently the cleanness of the sheets depends on many parameters such as the nature and the drag-out of pickling and rolling oils, the contamination of rolling solutions with tramp oils or metal fines, the extent to which the oils distil at high temperature, the coiling force and the annealing cycle [14 - 16]. Steel producers and users both now recognize the detrimental effect of carbonaceous residues on steel surfaces on the basis of the salt spray test performance of phosphatized and painted parts. There is no d o u b t , however, that the development of new degreasing treatments to remove pollution from the phosphatizing line more effectively and the development of new paint primers insensitive to alkaline undercutting would help to increase the final corrosion resistance of sheet steel products [17]. It is clear that in order to make improvements we need to know the actual surface state of temper-rolled steel sheets more exactly. The easiest test to evaluate sheet cleanness in the batch-annealed condition is the so-called Scotch tape test. Measuring the opacity of the tape by densitometry enables us to quantify the results of this test. Such a test detects all particulate pollution such as carbon black, graphite, iron oxides, metallic particles and salt crystals that might be present on the steel surface and adhere to the tape. More recently the Ford laboratory proposed a new procedure for measuring the total carbon deposit on temper-rolled steel surfaces. After a power wash in an alkaline bath to remove the protective oil, one side o f the steel sheet is mopped with glass fibre filter paper saturated with 50% HC1 solution. After dry-

296 50 FORD TE5T (rng C/m 2)

,(,0

30

/

20

I

10

I

t

I

20

I

I

30

I

,~0 50 COCKERILL TE5T (rag Clrn2)

Fig. 7. The correlation between "total carbon" measurements. I206

'2°°t °xG'NL,

NCN

NCN CARBON

l|

t~ 800

800

o-"/OH" 2,3 2,a

| il/"

,.\~

,ool

i

". // I JH- ~--\~, P"

2~s

i

800~ NCN

IRON

I

53,~

28,$ BE(eV)280 206 NCN

(COOH) \._~,._ I I "~"~ 530 BE (eV)

MANGANESE

¢O6

400

t t

Mn 2*Mn0 I 714

i

F ~ ' F e " l Fe~ i 7tO BE(eV)706

6~6 ~ : BE(e V)

Fig. 8. XPS analysis of various as-annealed sheet steel surfaces; ~ , grade 1 ; - - - - , grade 2; . . . . --, grade 3. ing, the total carbon c o n t e n t of the paper is measured in a combustion test and is expressed in milligrams per square metre o f sheet. The specimen area of 900 cm 2 should be r e m o t e from the edge o f the strip and close to the eye o f the annealing coil {inner wraps} [ 1 8 ] . We also used the procedure developed by Renard and Lemaire [19] for measuring the total a m o u n t o f carbon on the free surface. In

this "Cockerill t e s t " the carbon present on the free surface is oxidized in a cold glow discharge produced by an r.f. power supply. The carbon oxide is measured using an IR detector. The equipment can be calibrated to show a mean value o f the total carbon c o n t e n t in milligrams per square metre of sheet surface. As shown in Fig. 7, the correlation between the t w o techniques is quite good except at low carbon contents for which the Ford procedure seems to give measurements in excess. Surface carbon and ot her elements can be analysed semiquantitatively by XPS analysis on an area of 5 m m 2 with a depth resolution of 50 A. Figure 8 shows such an analysis o f the main elements detectable on a batchannealed steel surface for three strips graded differently by the opacity A U of the Scotch tape test. These results* show t hat the surface chemistry is quite complex. We observe the following. (a) The carbon peak o f energy corresponding to C-C bonds increases in intensity with theA U parameter. (b} Oxygen is detected as two separate peaks corresponding to a thermal oxide (O 2-} and a hydroxi de. The intensity o f the 0 2peak decreases with the A U parameter. (c) Manganese is enriched on the free surface in the form o f an oxide [ 2 0 ] . Pollution by chlorine or calcium due to the pickling bath or to the hardness o f the water used in the different stages o f the cold processing line was also observed but is not shown in this figure. Zinc, phosphorus and lead were also detected in some cases, indicating the presence o f tramp oil in the emulsion or of ext rem e pressure additives. This study led us to conclude that an uncont am i nat ed surface consists of an i r o n - m a n ganese oxide layer form ed in the N2-5%H2 atmosphere of the batch anneal; during storage in air an external h y d r o x i d e layer is formed on this layer. As a first approach we tried to correlate the carbon pollution as measured in three different tests: the Ford test, the Scotch tape test and XPS analysis of the free surface. The carbon c o n t e n t given by XPS is expressed in terms o f the normalized area under the peak *The photoelectron intensities NCN (normalized count number} are expressed in terms of counts per second per electronvolt and are normalized with respect to CN = 31300 for Au 4f7/2.

297 after background subtraction and JSrgensen correction [21]. Figure 9 shows the correlation between the three tests. The large scatter bands may be partially explained as follows. (a) The Scotch tape test included not only the amorphous carbon and some of the graphite that might be present in the free surface but also metallic particles and salt crystals, as indicated above. Degreasing treatments applied just before tape testing reduced the tape opacity A U without changing the carbon deposit shown by XPS analysis. This is seen most clearly for degreasing treatments that include some mechanical action (e.g. spraying or ultrasonic dipping) to reduce the number of particles such as iron fines or salt crystals detected on the tape by scanning electron microscopy (cf. Fig. 10). With XPS we observed a strong carbon peak with contributions from different carbon bonds. More precisely, the depth profiles in Fig. 11 show that the position and shape of the carbon peak change with depth below the free surface. On the external surface we detected a carbon peak with a binding energy equal to 284.3 eV, corresponding to a C-C bond as in a carbonaceous deposit or in graphite. In the next sublayer the carbon peak was shifted to a higher binding energy corresponding to a C - H bond, which may indicate the presence o f some residual hydrocarbon C~HyO~ n o t removed by distillation during the recrystallization treatment. Still further below the free surface the carbon peak was shifted to a lower binding energy corresponding to a C - F e bond such as is formed in cementite. At this depth all oxide had been removed, as is shown by the iron peaks reproduced in the lower part of Fig. 11. This analysis shows that some cementite formation t o o k place in a sublayer of about 500 A thickness. Such observations may account for some or all of the scatter between the three different tests (Fig. 9). In particular it is not clear whether the etching test in the HC1 solution can dissolve the cementite enrichment; the same remark may be made concerning the adhesion of partially cracked hydrocarbons to the Scotch tape. Turning now to the problem of the cleanness o f the steel sheet, the presence of three different carbon species in the external layers of the batch-annealed steel sheet suggests that

the incomplete cracking of the non~listillable residual hydrocarbon CxHyOz may be represented by TO CxHyOz ~ nCO + mCH 4 + rH 2 + Csoot (1) the soot formation increasing with the C/O and C/H ratios o f the residual hydrocarbon [22]. In actual production some of the carbon pollution is removed by the formation of volatile compounds which escape from the coil to some extent. However, secondary reactions must be considered for the remaining gaseous components: 2CO .

" CO2 + (C)

(2)

CH4

" 2H2 + (C)

(3)

The dissociate adsorption of carbon monoxide on pure iron above 300 K has been reported by Rhodin and Brucker [23]. Such reactions may produce carburization of the steel but they are very slow in comparison with the reaction between carbon monoxide and hydrogen CO+H2 .

H20+(C)

(4)

for which the reaction rate is 10 - 100 times higher [24]. It has been established that in some cases, during the direct reduction of iron for example, soot formation may be retarded by the addition of small quantities of water vapour or sulphur-bearing gases (H2S or SO2) [25]. It should be mentioned that Hudson and Stragand [26] have detected CO, CO2, CH4 and H 2 in the gas phase present between the turns of the coil; during certain periods of a box annealing treatment in an N2-5%H 2 atmosphere the proportions of CO and CO2 rise as high as 10 and 18 vol.% respectively. It is evident that if reaction (4) is to proceed to cementite formation the water vapour must be removed from the system, e.g. by external or internal oxidation o f some alloying elements. However, the resulting oxide layer might help to prevent graphite formation on the free surface during cooling by destabilization o f the cementite present in the sublayers. Such graphite formation occurs only on a clean and non-oxidized iron surface. To summarize this section on carbon pollution we repeat that four different types of carbon may be detected (amorphous carbon,

298 FORD TEST (mgC/m2) 5 0 - - 50

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3000 NCN

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(b) Fig. 10. Influence o f various degreasing treatments on the surface chemistry o f as-annealed sheet steel: (a) XPS spectra; (b) analysis by scanning electron microscopy.

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Z = 13 A AZ = 0,6 A

SSMn +

Z = 14 A AZ = 6.5 A

J.

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9 ~

~

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~'ig. 12. SIMS of specimens in the as-annealed condition: (a) cut from the centre; (b) cut from the edges.

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a)

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Z = 13 A AZ= 130A

4780 +

250p

rl

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300 residual hydrocarbon, cementite and in some cases graphite) depending on the annealing cycle and other parameters such as the a m o u n t and the nature of the residual oil present on the cold-rolled surface, the coiling force and the position in the coil. With regard to processing practice, the surface chemistry of the as-annealed steel sheet does not only indicate carbon pollution. As an example of additional factors, Fig. 12 shows the chemical distributions of some alloying elements or residuals such as manganese, chromium and phosphorus in the external layers of specimens cut from (a) the centre and (b} the edges of batch-annealed steel strip. At the centre we detected a strong migration of manganese to the surface where it forms an external (Fe,Mn)O oxide. At the edges chromium forms an external homogeneous film whereas manganese and phosphorus are now detected in the grain boundaries of the microstructure where they are present as pure oxides such as MnO in a sublayer of about 1000 A thickness. This situation corresponds more or less to a free exchange reaction as we observed in the case of an open-coil annealing cycle in a protective N2-5%H2 atmosphere of very low dew point. In the final stage of our work we tried to find to what extent temper rolling can modify the surface chemistry o f the industrial product. Samples were taken after " w e t " temper rolling. The temper rolling oil is quite complex because it serves both as a lubricant and as a protective agent. Its main components are ethyl and methyl radicals, amine and sodium benzoate and nitrite. Figure 13 gives a comparison of as-annealed and as-temper-rolled steel sheets after ultrasonic degreasing in a chlorothene bath. The XPS spectra show how much the rolling lubricant has changed the surface chemistry of the product. It was not possible to remove this additional film with the degreasing treatment used here. In conclusion, then, the surface chemistry of the steel strip is quite complex and can strongly affect the subsequent use of the product.

2.3. Influence o f surface composition on corrosion resistance Parts made by deep drawing steel sheet are generally used in the phosphatized and painted

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I t63

NITROGEN (Is)

BE$V)~04

Fig. 13. XPS analysis of steel sheets: ~ , annealed; -- -- --, temper rolled (wet).

~00

396

batch

condition. It seems evident that in the complex steel/phosphate/paint system the surface chemistry of the steel is an important factor in determining the corrosion resistance of coated products. Hosparaduk et al. [17] have reported previously that the corrosion resistance of phosphatized and painted parts depends strongly on the carbon pollution on the free surface. In order to check this assessment the surface chemistry o f 30 steel sheets was investigated by XPS. Figure 14(a) shows (in arbitrary units) the dependence of the C, 0 2 - and OH- peaks on the intensity of the Fe 3÷ peak which can be considered a "cleanness index" of the surface. We observed that the C peak decreases in intensity as the Fe 3+ peak intensity increases, i.e. as the surface cleanness increases. On the abscissa of this diagram, which is the "cleanness axis", we have marked the positions of three different grades G1, G2 and Ga as measured in industrial practice by the Scotch tape test. Specimens corresponding to two steels A

301 Steel B

Steel A

After 288

),

hrs

After 192 hrs

E

P

L

(b)

Ic, Io-; IoH. :c:o (A U. ) 80

-\

76

Steel A 66

56 11"

%

°

3C

.-:£2

-

s,.,

B

2(,

16

-.

I

(a)

I 10

?3

I 20

G2

I

G1

I 30

~ IF,(AU.)

I ,f,O

Fig. 14. XPS analysis for various as-annealed LD steels: (a) the dependence of the carbon and oxygen intensities on iron intensity chosen as a "cleanness index" (o, C ls; o, O l s ; +, OH+ = C = O ) ; (b) salt spray test according to ASTM Bl17.

and B were degreased in hot alkaline solution, phosphatized using a zinc phosphate spray system and painted with an acrylic resin. These specimens were scribed with a cross cut through to the steel surface prior to salt spray testing according to ASTM specification Bl17. Figure 14(b) shows the undercutting detected by the tape test after 288 h of salt spray testing for the two different surface conditions. These results clearly confirm the influence of carbon pollution on the corrosion resistance

of phosphatized and painted parts. However, it appears that in some cases low carbon pollution alone is not sufficient to ensure good resistance to undermining corrosion. For example, Fig. 15 shows the undermining corrosion observed after a salt spray test for specimens taken from different locations in a sheet with low carbon pollution. The specimens were degreased, phosphatized and painted under the same conditions. XPS of the temper-rolled specimens showed no significant differences for carbon and oxygen. In the iron spectra of specimens taken from the edges of the strip there was a minute signal from metallic iron as well as the dominant but unvarying oxide peak. By using imaging SIMS manganese was found to be segregated to the external surfaces but at the edge positions it was located mainly in the grain boundaries of the microstructure where it formed an oxide network down to about 1000 £ depth. This type of enrichment would be difficult to remove by the phosphoric acid treatment that occurs in the first stage of phosphatizing. Following Shimada et al. [27] it may be suggested that such enrichment in the grain boundaries provides local cathodes in the phosphatizing treatment, which is a multi-electrode process. At the same time, manganese depletion in the metallic matrix next to the grain boundaries reduces the beneficial depolarizing effect of this element during the phosphatizing treatment.

712

285

~.

708

283

528

c)

~)

a)

14/~

L

,.

i

f2,",~

•

25o~

t

.-

,I

i

o ==..=

F

~c~

Z~Z-6 5,~ After192hrs

>l;

-. ~ r

Az=6,5~ z-~ooo/~

Fig. 15. Influence o f the surface chemistry o n the corrosion resistance in the salt spray test for p h o s p h a t i z e d and painted specimens: . . . . , (b), c e n t r e ; - • --, (c), left edge.

eV

'RON

287 eV

716

532

CARBON f

536 eV

OXYGEN . A

55Mn +

, (a), right edge;

After288hrs

303 information can be obtained in the case of two important sheet products.

C/k (c ~)s/scan)

3.1. Tinplate

o

2o

~o

6o

~

8'0

(a) C/k (cps /scan)

0

20

~0

60

80

100

200

~

300

(b)

Fig. 16. Composition profiles by XPS of passivated tinplate : (a) dip passivation; (b) electrolytic passivation; e, C (k = 100); 4, Cr3+ (k = 2000); A, Cr0 (k = 400); V, Sn°x (k = 2000);V, Sn° (k = 3000); +, O (k = 2000).

3. T H E P A S S I V A T I O N T R E A T M E N T O F TINPLATE AND GALVANIZED STEEL SHEET

The main effect of the passivation treatment is to stabilize the surface against corrosion or oxidation by converting the natural oxide formed in the processing line. This treatment develops a passivating layer whose thickness is normally in the range of a few monolayers on the external surface. New physical techniques for surface analysis are thus particularly helpful in studying such layers. We show what

After the brightening anneal of the tin deposit, electrolytic tinplate is usually passivated on the production line in a solution containing 20 - 30 g l-1 sodium dichromate at temperatures in the range 60 - 85 °C. The treatment can be applied by brief immersion or by electrolysis with the tinplate as cathode at a current density of about 10 - 20 A dm -2. The purpose of this treatment is to stabilize the surface against oxidation during baking for the curing of lacquers or against staining by the sulphur compounds present in some foods, whilst providing a good basis for lacquer adhesion [28 - 30]. For example, Fig. 16 gives the composition profiles of two tinplates passivated on industrial lines (a) by dipping and (b) by electrolysis in dichromate solution. In this figure the relative intensities of the photoelectron peaks are plotted in arbitrary units as a function of the distance below the free surface. Since sputtering rates vary with composition, the sputtering depth is only an estimate based on measured rates of sputter erosion for metallic tin. For films of high chromium content the distances are likely to be overestimated since the sputtering rate for tin is 2.4 times that for metallic chromium and 4 times that for chromium oxide [31]. The composition profiles show that chromium oxide and tin oxide are present in the dip passivation film. For the electrolytic passivation treatment the film is much thicker and its trivalent chromium content much greater; also, metallic chromium is clearly detected in the passive film. The sublayer of metallic chromium is in contact with the tin substrate and seems to have an irregular thickness, penetrating into the tin at some points. At the original surface of both specimens a film of dioctylsebacate (DOS) protective oil a few ~ingstrSms thick is indicated by the high but narrow C l s peak. The ratio of the content of metallic chromium to the total chromium content can be deduced from the ratio of the areas below the composition profile curves. If we allow for the fact that the sputtering rate for the metal is 1.7 times that for the oxide, this ratio is equal to 0.17 for the electrolytic passiva-

304

5÷" t ./

3A (a)

(b)

(c)

Fig. 17. XPS analysis of the rupture face in a lacquered tinplate: (a) lacquer; (b) rupture face; (c) tinplate;e, C (k = 100);A, Cr 3~" (k =01000); A, Cr 0 (k = 1000); T, Sn °x (k = 1000); £7, Sn (k = 2000); +, O (h =

1ooo).

tion treatment. By chemical analysis the ratio was found to be 0.27. However, consideration of the efficiency of the extraction treatments used in the chemical method suggests explanations for the discrepancy [32 - 34]. XPS has also been applied to the problem of lacquer adhesion. In our experience different lacquering conditions can affect the adhesion of the lacquer film to a given tinplate. XPS has been used to study unpolluted fracture surfaces which reveal the weakest layer of the tin/passivation film/lacquer system in a tensile test. In such a test a disc of tinplate lacquered on one side is sealed with adhesive to two coaxial cylinders (diameters 16 and 23 mm), the smaller being fixed to the lacquered face. The adhesive used must be such as to avoid any chemical reaction with the lacquer. For high tensile rates (+ 100 mm s-1) fracture seems to occur between the lacquer and the tinplate with a rupture stress in the region of 500 kN m - 2 for an electrolytic passivation treatment. The fracture can be localized more specifically b y XPS analysis, It is found that the

fracture occurs not, as might be expected, at the lacquer-film interface but at the film-tin interface. Figure 17 shows composition profiles for both surfaces from the same rupture test performed on a lacquered tinplate which showed poor adhesion properties in service. The profiles clearly show that fracture occurred at the film-tin interface. The whole passive layer remained fixed to the lacquer film. Furthermore a much more substantial layer of tin oxide than would be expected in a normal passivation film was apparent on the lacquer film. This additional tin oxide film may have been formed during baking o f the lacquer. It formed a sublayer of poor mechanical properties. It must n o t be inferred, however, that the presence of tin oxide is always detrimental to lacquer adhesion. Surfaces passivated b y immersion treatments carry much more tin oxide than cathodically passivated surfaces, and it is well known that they can show considerably better lacquer adhesion. The reasons for the variations in the effects of tin oxide probably lie in its origins and in the relative proportions o f SnO and SnO2 present. It is known that the composition and properties of tin oxide differ according to whether it is produced during storage or baking, as in the example of poor adhesion cited, or at high temperature during fusion in the process o f flow brightening. 3.2. Galvanized steel sheet The purpose of the passivation treatment usually applied on-line to hot
305

C/k (cps/scan)

C/k (cps/scan) 6

0

100

200 ,~ 300

(a) Z=0A

0

(b) ~Z=0,2A

t00

200

300

~00

500,~ 600

271~d+

Z=58.5A

~Z=I,3A

Z=117A

.~Z=I,3A

(c) Fig. 18. (a), (b) C o m p o s i t i o n profiles by XPS o f galvanized steel sheet (a) w i t h o u t and (b) with a c h r o m a t e conversion coating: e, C ( k = 200); o, Cr 3+ (k = 2000); +, Zn 2+ (k = 1000); V, Zn 0 (k = 1 0 0 0 0 ) ; O(k = 2000); A, V, Al 3+ (k = 100); &, Pb 0 (k = 1000). (c) SIMS o f a galvanized steel sheet w i t h o u t a c h r o m a t e conversion coating.

chromates in combination with silicates, phosphates, fluorides etc. [35]. Recent developments have shown that certain organic compounds such as esters or polyesters of thioglycolic acid [36] or tannic acid in the presence of thiourea [37] may improve the corrosion resistance of galvanized steel sheet. Complex solutions o f zinc, manganese, iron or lead phosphates in the presence of an organic reducing agent such as an aldehyde, a resin which is not a reducing agent and chromic

acid which produces Cr 3÷ by oxidation of the reducing agent have also been proposed for the same purpose [38]. The object of our work in this field was to provide a m e t h o d of improving the performance o f conventional passivation treatments using chromic acid solution. As a first step we tried to define the surface chemistry of the industrial product just before passivating. This work clearly shows that substantial enrichment of aluminium and lead occurs on the free

306 surface during freezing of the zinc coating. XPS analysis confirms this and shows that aluminium is present as an oxide whereas lead is not oxidized. The extent of this enrichment during solidification of the liquid coating depends on such parameters as the composition of the zinc bath, the thickness of the steel strip and the line speed. Figure 18 gives XPS composition profiles obtained from an industrial product before and after the passivation treatment; they clearly show that the zone o f aluminium and lead segregation is a b o u t 100 A thick as expressed in terms of sputter-equivalent metallic zinc. The ion images obtained b y SIMS show that alumina is present as a more or less homogeneous film on the free surface. It is also apparent from these results that the chromic acid solution cannot remove the alumina film; consequently the conversion treatment cannot develop a uniform passivating film, and poor corrosion resistance results. It was proposed that we should solve this problem by decreasing the pH o f the solution to 0.8 - 2.2 and by using some fluoride additions [ 39 ] or b y applying a cathodic electrolytic pretreatment of the surface in a dilute aqueous solution of a stable neutral salt [ 4 0 ] . Because o f its nature we decided to dissolve the segregation film in a hot alkaline solution to form aluminate ion (A102) 2- ; in this way we can readily produce zinc surfaces compatible with chromate conversion by immersion or by spraying treatments applied on-line. Our experiments showed that such a pretreatment of a few seconds in soda solution decreases the alumina content from 8 to 1 mg m-2 [41]. SIMS analysis confirmed that the soda treatment removes the alumina film from the externai surface quite effectively; the zinc oxide is also mostly dissolved, allowing chromic acid to react more effectively with the zinc surface. The corrosion resistances of the passivated product with and without the soda pretreatment were compared in salt spray tests according to ASTM specification B l 1 7 . Figure 19 shows the percentage of surface corroded by white rust as a function of exposure time for materials of comparable chromium deposit from the chromate conversion after different pretreatments. It is clear that the soda pretreatment decreases the corrosion rate very effectively during the first 40 h in the salt

Corroded area tO0

V

(%)

50

zOI--

ol

•

I

+++

i

20}-

~

•

•

f I 1"

0

w

0

o

10

20

30

~0

50

60

70

80

90 I00 Time (hrs)

Fig. 19. Corrosion resistance in the saltspray test for untreated and soda-treated galvanized steel sheets: o, 15 rng Cr m-2;A, 22 rng Cr m-2; +, 19 m g Cr m -2.

spray test; consequently this pretreatment may help to prevent white rust formation during storage easily and cheaply.

4. CONCLUSION

The purpose of the examples cited here was to demonstrate the potential of new surface analytical techniques and to illustrate the improvements that can be achieved with a better knowledge of the surface chemistry. In our experience in this field, in many cases the surface chemistry of the product is critical in the subsequent use o f the material. It is encouraging that such techniques can already yield information concerning the real surface of industrial products that is otherwise unattainable. The development and application of such techniques will no d o u b t result in further improvements in the quality o f industrial sheet products.

307 ACKNOWLEDGMENTS

The author wishes to express his thanks to M M . L. Renard, Head of Department at S.A. Cockerill, and M. V. Polard, Engineer at Phenix Works, for discussion and help in providing samples and industrial observations. The research was carried out with the financialsupport of IRSIA and CECA.

REFERENCES 1 A. J. Socha, Surf. Sci., 25 (1971) 147. 2 V. Leroy, J. P. Servais and L. Habraken, MetaU. Rep. CRM, 35 (1973) 69. 3 R. Castaing and G. Slodzian, J. Microsc. (Paris), 1 (1965) 395. 4 H. J. Drake, Iron Steel Eng. (1965) 40. 5 J. R. Ludwig, Blast Furn. Steel Plant (1969) 641. 6 T. J. Bishop, Iron Steel Eng., Yearbook (1968) 747. 7 P. Pascal, Traitd de Chimie Mindrale, Vol. 8, No. 2, Masson, Paris, 1959, p. 476. 8 Y. Inokuti, J. Iron Steel Jpn, 15 (1975) 314. 9 L. J. Brown, Plating Surf. Finish. (1975) 587. 10 V. Leroy, J. Richelmi and H. Graas, Metall. Rep. CRM, 49 (1976) 49. 11 R. F. Hunter and E. G. Baird-Kerr, A E S Continuous Plating Seminar, Pittsburgh, Pennsylvania, 1974. 12 U.S. Patent 3 , 7 5 6 , 9 2 6 (1973), to Penwalt Co. 13 Belgian Patent 8 6 4 , 7 2 4 (1978), to Centre de Recherches M~tallurgiques. 14 Y. Tamai and M. Sumitomo, J. A m . Soc. Lubr. Eng., 31 (2) (1975) 81. 15 R. H. Mollet and R. J. Morris, BHP Tech. Bull., 31 (2) 19 (1975) 1. 16 M. Shamaiengar, Iron Steel Eng., 44 (5) (1967) 135. 17 V. Hospadaruk, J. Huff, R. W. Zurilla and H. T. Greenwood, Soc. A u t o m o t i v e Eng. Congr., Detroit, Michigan, 1978, Techn. Paper Series 78 0186, Society of Automotive Engineers, 1978.

18 Engineering Specification Materials, Ford Laboratory, 1977. 19 L. Renard and J. P. Lemaire, European Coil Coating Association 1979, Brussels, to be published. 20 R. M. Hudson, H. E. Biber, E. J. Oles and C. J. Warning, Metall. Trans. A, 7 (1976) 1857. 21 H. Berthou and C. J~brgensen, Anal. Chem., 47 (1975) 3. 22 M. Bost, Bourdil, Cadilhac, Chaussin, El Haik, Goutel, Guimier and Marty, Traitement Thermique, 105 (1976) 55. 23 T. N. Rhodin and C. F. Brucker, Solid State Commun., 23 (1977) 275. 24 R. Collins, S. Gunnarson and D. Thu]in, J. Iron Steel Inst. (1972) 777. 25 R . J . Frueman, Metall. Trans., 4 (1973) 212. 26 R. M. Hudson and G. L. Stragand, Trans. A m . Soc. Met., 52 (1960) 135. 27 S. Shimada, S. Maeda and T. Egawa, Tetsu To Hagane, 61 (1975) 2639. 28 R. P. Carter, J. Electrochem. Soc., 8 (1961) 782. 29 S. C. Britton and J. M. Hancox, Sheet Met. Ind., 1 (1965) 15. 30 S. E. Rauch and R. N. Steinbicker, J. Electrochem. Soc., 120 (6) (1973) 735. 31 V. Leroy, J. P. Servais, L. Habraken, L. Renard and J. Lempereur, 1st Int. Tin Plate Conf., London, 1976, International Tin Research Institute, London, 1976, p. 399. 32 J. P. Servais, J. Lempereur, L. Renard and V. Leroy, Br. Corros. J., 14 (3) (1979) 126. 33 S. C. Britton, Br. Corros., J. (1965) 91. 34 Ph. Aubrun and G. A. Pennera, 1st Int. Tin Plate Conf., London, 1976, International Tin Research Institute, London, 1976, p. 295. 35 W. C. Glassman and C. V. Gladysz, 2nd Annu. Int. Lead Zinc Res. Org. Galvanizing Seminar, St. Louis, Missouri, 1976, Ilzro, 1976, p. 1. 36 U.S. Patent 4,093,780 (1978), to Noranda. 37 French Patent 2,363,640 (1976), to Cont. Parker. 38 Brit. Patent 1,482,457 (1973), to Brugarolas. 39 French Patent 2,102,374 (1970), to Amchem. 40 U.S. Patent 3,629,078 (1967), to Nippon Steel Co. 41 J. P. Servais and V. Leroy, unpublished work, 1979.