Surface characterization of iron and steel prior to coating

Progress in Organic Coatings,

SURFACE COATING

15 (1987)

CHARACTERIZATION

125 - 148

125

OF IRON AND STEEL PRIOR TO

G. REINHARD Corrosion Protection Group, Department of Chemistry, Technology, DDR-8027 Dresden (G.D.R.)

Dresden

University

of

Contents Summary .................................................. 1 Introduction. ............................................. 2 Physical structure of the surface of ferrous materials. ................... 2.1 Surface structure ........................................ 2.2 Influence of alloy elements and surface pollutants .................. 2.3 Surface roughness ....................................... 3 Chemical structure of the surface of ferrous materials. .................. 3.1 Primary oxide layer ...................................... 3.2 Interactions of primary oxide layers with water. ................... 3.3 Interactions of the primary oxide layer with sulphur dioxide impurities. ............................................ 4 Conversion layers on ferrous materials ............................. 4.1 Phosphate layers and their properties. .......................... 4.2 Oxide layers and their properties. ............................. 5 Conclusions .............................................. Acknowledgements ........................................... References .................................................

and other

125 126 126 126 128 131 132 132 136 138 140 140 143 144 145 145

This report presents the latest knowledge on surface properties by which the mechanism and intensity of the adhesion of organic coatings can be influenced on iron materials. By this, it is to be made clear above all that these surface properties depend on a large number of single factors. Since, however, their complex combined action has not been quite clear, it is not yet possible to reconcile the partly different conceptions of the most favourable condition of steel surfaces for a subsequent coating [ 1 - 141. On the basis of experimental results there are given reasons for the conclusion that greater attention than has been the case so far should be paid to the hydroxyl group-containing primary oxides being formed spontaneously on steel surfaces following the mechanical or chemical pretreatment during contact with air, when coating materials and techniques are selected. If such a continuous, homogeneous primary oxide layer exists, its amphoteric surface hydroxyl groups can be utilized for the chemical adhesion of suitable organic film-formers. On steels whose surface is heterogeneous due to the 0033-0655/87/$8.90

0 Elsevier Sequoia/Printed

in The Netherlands

126

island-shaped distribution of primary oxides, non-metallic deposits and pollutants, homogenization can be achieved by producing a conversion layer. Usually, phosphate and oxide layers have amphoteric surface hydroxyl groups, too, whose isoelectric point (IEPS) varies with the composition, so that it is possible to select thermodynamically favourable conditions for the chemical adhesion of organic coatings.

1 Introduction Unalloyed and low-alloyed steels occupy a preferential position as materials. Since they are subject to corrosion in the atmosphere as well as in the majority of technical media, their practical use always requires systemspecific measures of protection against corrosion [15 - 171. In this respect, the application of organic coatings has been of the greatest importance [17 - 211. As is known, the efficiency of corrosion protection by organic coatings depends on a large number of factors, with the strength of the adhesion of the respective polymer film to the metal substrate having a decisive influence [l, 7, 9, 11,14, 21 - 271. By this process of adhesion all the active spots on the metal surface at which anodic or cathodic partial processes of metal dissolution may take place under the influence of a corrosive medium are effectively blocked. This desired situation does not always coincide with a high bond energy between the polymer film and the substrate, as has frequently been demonstrated by pull-off tests described in various publications [ll, 23, 25, 27]. Such a measured quantity is always determined by the respective adhesion mechanism prevailing [ 14,281. Furthermore, the value obtained may only reflect conditions arising from a relatively few bonded spots, in which case the polymer film acts only as a barrier against diffusion and provides only a temporary protection against corrosion [14, 25,27, 281. In order to optimize the adhesion process and hence increase corrosion protection, both for already known coating techniques as well as for the development of novel coating materials, a detailed knowledge of the initial state of the given ferrous material to be coated is required. In the following discussion, the results obtained to date are classified and discussed in terms of the application of various in situ techniques. Since very little has been established on the influence of various alloying elements in steels on the nature of the resulting surface, the results obtained on superpure iron as a model system are included, if only to illustrate differences. 2 Physical structure of the surface of ferrous materials 2.1 Surface strut ture To date, no results are available from investigations of the microscopic and submicroscopic surface structures of ferrous materials on the influence

127

of such structures on the adhesion of organic coatings. In addition, detailed information is even lacking on differences in the surface conditions of individual grades of unalloyed and low-alloyed steels caused by the type of casting process employed [29, 301. Iron is known to possess pronounced polymorphism, with crystal forms being more stable at higher temperatures (e.g. ferrite, austenite). These forms are frozen by rapid cooling, so that they are also capable of existing at room temperature. Hence, it is possible that ferrous surfaces scheduled for organic coating may differ regarding the nature of their structure, with the result that the kinetics of surface processes such as adsorption and adhesion are affected. Observation of such differences is not possible experimentally however, since a variety of factors influence the physical state of real metal surfaces, and these cannot be distinguished separately because of the limitations of the techniques available. Thus, a real metal surface because of its polycrystalline nature exhibits various defects [29 - 311. Dislocations exist which involve small-angle and large-angle grain boundaries. Screw dislocations and dislocations having a screw component perpendicular to the surface give rise to further steps and semi-crystalline regions, so that overall a random surface structure results with terraces and grooves, and hence possessing a variety of surface elements with different surface energies. Atoms located at the edges and corners of crystalline faces have a lower bonding energy than those present within a face or inside the crystal. For this reason, they constitute sites with a higher surface ‘energy and are especially favoured in all processes leading to a reduction in the specific surface energy. Thus, lattice destruction during metal dissolution, as well as the adsorption of species of the adjacent medium, takes place preferentially at the semi-crystalline positions [ 16,29, 301. Such behaviour suggests that these sites also play an important function in the adhesion of organic coatings, provided that they have not been blocked by other processes (e.g. by the transposition of metal ions from the lattice to the adsorption layer with the subsequent formation of a metal oxide [16]). It should be remembered, of course, that generally the surfaces of engineering materials are treated mechanically and/or chemically prior to coating. In particular, as a result of mechanical processing, such as sawing, bending or rolling as well as grinding, polishing or cold forming, the physical and chemical conditions of the surface may exhibit considerable local differences. Depending upon the technique and on the hardness of the material, a working layer (Beilby layer) with a finite thickness exists which differs from the bulk of the material in respect of its crystallinity, higher dislocation density or bulk stresses, and by the existence of impurities acquired from the tool employed or from the atmosphere. In addition, as a result of the frictional heat generated during machining, iron surfaces may also take up nitrogen, oxidize, recrystallize, undergo phase transformations in the solid state or solidify with a different structure after limited local melting. With steels, decarbonizing processes should also be taken into account [29, 301, as well as the fact that steels cast in an unkilled state

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will have a different carbon concentration at their surface from that existing in bulk. To date, no information has appeared in the literature on possible differences arising in the adhesion of applied organic coatings between steels cast in a killed state and in an unkilled state. It is probable that such differences can be neglected, however, since on phosphatizing steel surfaces the formation, crystallinity and porosity of the resulting phosphate layer has proved to be independent of the kind of processing employed, being determined solely by the type of mechanical deformation, surface roughness and impurity content of the steel surface concerned [32,33]. 2.2 Influence of alloy elements and surface pollutants Unalloyed standard steels contain at a minimum the natural companions of iron (Mn, Cr and Si) as well as slight proportions of sulphur, phosphorus and carbon. Low-alloyed steels may be modified by the addition of other metals, and provided the conditions employed correspond to complete mixed-crystal formation, the respective components are present in a homogeneously dissolved state. However, inside the individual basic crystallites and preferentially at grain boundaries, heterogeneous segregation also occurs which gives rise to the existence of additional defects at the metal surface [16,29, 301. The inclusion of manganese sulphide at grain boundaries has been recognized as a very corrosion-promoting process [33 - 361. However, only a few comments have been made in the literature on the effect of small proportions of metal-alloying elements on the surface topography of ferrous materials. One exception is the proof that the austenitic structure is stabilized by nickel, for example [ 301. In the presence of an electrolyte, such heterogeneous segregation leads principally to the formation of corrosion elements, with the less noble elements being subject to the greatest dissolution [ 15 - 171. If there are regions containing such noble elements in the metal substrate beneath an organic coating, and if these regions are not involved in the adhesion process, they will constitute the start points for undercutting and under-rustingafter permeation of water and oxygen across the polymer film [9,11,14,23, 27, 371. The inclusion of manganese sulphide on steel surfaces also ranks as a source of under-rusting as a result of oxidation which can occur according to the overall equation [ 361: 2MnS + 4.50, + 2Hz0 -

MnzOs + 4H+ + 2SOa2-

(1)

Such oxidation leads to both acidification of the wet film on the metal surface and to the formation of soluble ions, a process which in addition to the corrosion of the metal substrate also leads to the osmotic transport of water through the organic coating. No detailed results of investigations of such processes have appeared, however. There is still controversy in the literature [12, 38 - 431 on the function of carbon regarding the properties of steel surfaces. This is probably due to the fact that previous conventional analytical methods were unable to

129

determine the precise form in which carbon was present and the origin of that carbon. By combining AES and SIMS, however, it is now possible to distinguish between graphitic carbon dissolved in steel and carbon exist on the surface either as hydrocarbons or having derived from such compounds as a result of thermal decomposition [40]. Iron has a limited temperaturedependent solubility towards carbon, and in steels this carbon may be present as the element or in the form of the intermetallic phase Fe& [15, 29, 301. Homogeneous solution should only result in a change in the size of the mixed crystals, but have no essential effect on adsorption or adhesion processes occurring at the steel surface. As heterogeneous segregations, graphite and carbide particles will act preferentially in a cathodic manner on contact with oxygen-containing solutions. Hydroxide ions, arising from the reduction of such oxygen according to the equation: 0, + 2H20 + 4e- -

40H-

(2)

in the pores of organic coatings, may then lead to the de-adhesion of the coating [l, 7,21,22,44 - 461. The carbon which is present on steel surfaces in the form of hydrocarbons usually comes from lubricating oils or agents added for temporary corrosion protection [ 12,40,42,43]. If the active surface regions necessary for the adhesion process associated with the organic bonding agent are blocked by such carbon, the resulting coatings will be characterized by a poor adhesive strength and will be especially prone to underfilm corrosion [18 - 201. Very high demands relative to the cleansing of steel surfaces prior to coating must be made if the subsequent conversion layer is to be produced by phosphatizing [12, 40, 42, 47, 481 or if water-dilutable paints [49,50], electrodeposition coating technologies [32, 33, 51 - 531 or powder coatings are to be employed [ 51. For such intensive cleaning, aqueous alkaline media are now preferred rather than organic solvents [48], since such media are also capable of removing graphite deposits caused by the thermal decomposition of oils (e.g. during mechanical deformation) [40,43]. Deposits of this kind have been characterized as catalysts in the underrusting of organic coatings [32, 42,431. However, with the use of alkaline media for cleaning purposes, it must be remembered that some grades of steel undergo structural changes in hot alkaline solution. Thus, for example, grain boundary segregation of manganese may occur, making the steel prone to intergranular corrosion [54]. To reduce the amount of carbon and its compounds on steel surfaces to <6.4 mg m-2, as is required for phosphatizing and the cathodic electrodeposition of coatings [12, 40, 421, cleaning by means of electrolytic processes carried out at room temperature is said to be particularly suitable [ 421. Because such methods may be applied in neutral or alkaline media, the hydrogen loading of the steel also remains negligibly low whereas in acidic solutions or during flash pickling of steel surfaces it may become considerable [ 15 - 17,48, 551, leading both to structural

130

changes in the metallic lattice [56, 571 and to a reduction in the adhesive strength of any coatings subsequently applied [48]. Frequently, mechanical techniques such as blast cleaning, grinding or polishing are also used for treating steel surfaces prior to the application of an organic coating [5,18 - 20,481. From the use of such methods, excellent results have been reported with regard to the removal of rust and grease residues [ 5,481, but usually remnants of the abrasives applied by bombardment or friction remain on the metal surface, and these are difficult to remove even by very careful post-cleaning. This is illustrated by the examples depicted in Figs. 1 and 2, where the surfaces of cold-rolled steel and platinum sheets are shown after a particularly careful cleaning process of a type not generally applied for all practical coating techniques. Initially, these sheets were ground with waterproof emery papers whose grain size gradually reduced to 800. This was followed by vibration polishing with an alumina suspension (using a Metrapolan instrument supplied by VEB Rathenower Optische Werke, G.D.R.), then ultrasonic washing with n-heptane, methanol and ethanol, and drying in uucuo at room temperature for 0.5 h. Despite such treatment, these sheets still exhibited abrasive residues capable of detection by SEMQ, the quantity present on Pt as the material with the lower hardness (Fig. 2) being considerably greater than that on the grade of steel investigated (Fig. 1). Such non-metallic impurities could change the corrosion tendencies of steel surfaces and their ability to adhere organic coatings quite considerably [ 10,12, 58, 591, so that relevant investigation

Fig. 1. SEMQ photograph of the surface of a superpure iron specimen after vibration polishing and ultrasonic treatment.

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Fig. 2. SEMQ photograph of the surface of a platinum sheet after vibration ‘polishing and ultrasonic treatment (Kevex@-ray 5100 ARL X-ray energy spectrometer).

on mechanically pretreated steels prior to coating is always advisable to record the type and quantity of abrasive retained as measured by SEMQ and Mijssbauer spectroscopy, for example [lo, 591. 2.3 Surface roughness Real steel surfaces are never ideally smooth but possess a certain degree of roughness associated with crystalline defects and machining marks. The use of the term ‘surface roughness’ indicates that with respect to interactions with an adjacent phase the real surface available is larger than the macroscopic surface of the material. Since a great degree of surface roughness is equivalent to a high specific surface energy, and since therefore a very rough metal surface has many active centres, most theories of adhesion regard a substrate surface in such a state as being very favourable towards achieving a high adhesive strength for a given organic coating [3, 5,lS - 20,23,25,47,48]. However, it should be remembered that machining marks on steel surfaces may be so rich in energy that they give rise to regions possessing a particular ability to assist the transition of atoms from the metal surface lattice into the adsorption layer as a primary step in the process of metal dissolution [16, 601. Moreover, a particularly pronounced surface roughness may cause a heterogeneous energy profile even in the microscopic regions of the steel surface. This could lead to local differences in the adhesion of organic coatings and also

132

to an increase in underfilm corrosion due to the formation of corrosive elements [ 371. This suggests that for each metallic material there exists an optimum surface roughness to achieve the adhesion of organic coatings, which should be dependent on the type of coating material (e.g. solvent-based or waterthinnable coatings, powder coatings or coatings achieved by electrodeposition). Systematic investigation of this factor has not been undertaken to date, possibly due to the fact that the dependence of surface roughness on pretreatment is very difficult to characterize. However, the roughness factor rf (which is equal to the ratio between the microscopic and macroscopic areas) or the mean peak-to-valley height rh (in pm) can be determined in most cases from the physical or chemical adsorption of gases or from the adsorption of a given component from a mixed liquid phase. In addition, other commonly used techniques include microscopy, X-ray diffraction or light absorption as well as various mechanical methods [61 - 631. The data available from such measurements, however, are only reliable and representative for larger areas when rf > 10 [ 61. For metal surfaces with smaller rf values, electrochemical methods are especially suitable. Thus, measurements of the double-layer capacity Cdl, the conductance R,-i (reciprocal polarization resistance), the impedance, the adsorption under an applied potential (under-potential adsorption), and of the exchange current for redox systems as well as chronopotentiometry have been used to determine rf [ 63 - 701. In addition, it has been demonstrated and explained theoretically recently that the extent of the frequency dispersion occurring during impedance measurements at metal electrodes during the initial immersion stage is mainly determined by the surface roughness. This is characterized by the depression angle 6, which may lie in the range 0” < 4 < 45” and is proportional to rf. As shown in Fig. 3 which applies to cold-rolled steel electrodes, the respective r&value is also influenced by the particular surface state generated, either mechanically by grinding with waterproof emery paper of different grain sizes, or electrochemically (EP) or by mechanical polishing with an alumina suspension (MP) (see also ref. 71). In principle, using the methods mentioned, it is possible to obtain the value of rf or rh for ferrous materials over a wide range, allowing a determination of the influence of the surface roughness on the adhesion of selected organic coatings.

3 Chemical structure of the surface of ferrous materials 3.1 Primary oxide layer All ferrous materials possess the common property that a thin primary oxide layer forms spontaneously on their surfaces when the latter are in contact with oxygen, and that this layer then protects the metal from further corrosion provided that the conditions necessary for the formation of the layer are maintained [6, 13,16, 36, 721. Although such layers were

133

'150

Fig. 3. Results of impedance measurements on iron electrodes after exposure for 2 min to HzS04, pH = 1 at room temperature relative to the kind and intensity of surface pretreatment. Complex plane plots were determined on the basis of a series circuit consisting of resistance R, and capacity C, for every frequency f; (R, - RE) is the real part of the impedance corrected by the electrolyte resistance R,; -(wC,)-~ is the capacitive component of the impedance (imaginary part) with o = 2nf (angular frequency). Numbers on the curves indicate the grain size of emery paper, MP corresponds to mechanically polished and EP to electrochemically polished (see also ref. 71).

first detected some time ago [73 - 751, their existence has not been taken sufficiently into account to date neither for elucidating the primary steps of atmospheric corrosion [15,17, 76 - 781 nor for elucidating various problems such as the adhesion of organic films on to metal surfaces [l, 3,20,23,24, 271. In recent publications such layers have been included in the interpretation of paint adhesion failure, undercutting and the underrusting of organic coatings [7,12 - 14,21,37,79, SO], but little is known of how the structure and composition of this layer varies for various grades of steel, and the influence of atmospheric oxygen and surface pretreatment has hardly been explored. According to optical [72,74,75], electron optical [81 - 831, chemical [84,85] and electrochemical [86 - 881 studies, the primary oxide layer developed on polycrystalline superpure iron in dry air at 20 “C has an equilibrium thickness d of 3.5 - 5 nm depending on the surface roughness. Generally, it possesses the cubic structure characteristic of an inverse spinel, it grows epitaxially to the metal surface and its composition is largely similar to the passive layer on an iron anode. The composition of this layer is similar to Fes04 adjacent to the metal. Toward the surface it becomes depleted progressively of Fe’+ ions and acquires the limiting composition

134

Fe2.6704 (-r-Fez03), with the cubically dense packing of 02- ions being maintained [6,73,88]. Important information regarding the properties of such primary oxide layers has been obtained from the application of ellipsometry [13, 36, 89 - 921. Since this method is based on changes in reflection at highly polished reflecting surfaces, for the interpretation of the measured data to be completely free of doubt it is essential that the peak-to-valley height rh of the specimen surface to be characterized is of the same order of magnitude as the wavelength of the light used. The cleaning procdure described in Section 2.2 above, which leads to the final state depicted in Fig. 1, meets this particular condition with rh values of 0.5 pm when monochromatic light of 547.1 nm wavelength is employed in the ellipsometry measurements [92]. For this reason, we have repeated our previous measurements [ 891 on polished iron and mild steel surfaces. Sheets from an St-38u2-type coldrolled steel and a KTS 45/2-type weathering steel were chosen as the steel grades. Generally, the ellipsometry led to a pair of optical parameters, the relative phase retardation A and the relative amplitude reduction tan $, between the parallel and perpendicular to the incident-plane oriented polarization component of the light, caused by reflection on the substrate surface. These ellipsometric pairs of values A; ti, recorded for each surface at cpo= 60” (incidence angle between the incident beam and the normal to the specimen’s surface) were evaluated on the basis of a single-layer model with metal substrate( 2)-primary oxide layer( 1)-environment( 0) and specified expectation ranges for the complex refractive index N1 = ni - ih 1 (see also refs. 89 - 92). The expectation ranges were defined from a statistical evaluation of published data and comparative measurements on the respective iron oxides. For the base metal, the relationship Nz(Fe) = 3.35 - 3.84i was uniformly characteristic [ 13,91,92] and for the environment the relationship no = 1.00 (air) was established. Typical results are listed in Table 1. For all the ferrous materials investigated with a given pretreatment, the average thickness di of the primary oxide layer (pox) was within the range 4.0 6.0 nm and the real part (nl) of the complex refractive index N1 was within

TABLE 1 Typical ellipsometric results for primary oxide layers on different iron materials in dry air at 298 K (mean values from 19 measurements [91]) Fe(99.99) @psometric

A

JI

parameters (145.13 + 1.54)” (34.27 f 0.16)’

Layer parameters (single-layer 2.34 - 0.24i N, 5.5 nm dl

St,-38u2

KTS 45/2

(144.84 ? 2.56)” (34.30 f 0.18)”

(145.87 f 0.82)” (34.32 f 0.21)”

2.310.21i 5.5 nm

2.34 - 0.24i 5.0 nm

model)

135

‘J-

3.p

3.0.

,

A :

:

/

Fe,O,,

B’r-Fe,O,

N-2.5

-0.45i

N=2.55-0.351

d, Inm Fig. 4. Real part n1 of the complex refractive index (interpreted on the basis of the single-layer model with 12, = 0) for primary oxide layers on ferrous materials and different iron compounds relative to the layer thickness [ 911.

the range 2.15 - 2.35, with the imaginary part hl varying between 0.10 and 0.25. The values npox (nr at k, = 0) and d,,, (d, at k I = 0) are illustrated in Fig. 4. From this it is obvious that for the various types of materials investigated no significant differences occur as far as their layer parameters are concerned, and hence the structure and composition of the primary oxide layers formed largely agree with each other provided that the surface pretreatment is always of the same kind. This leads to the conclusion that the formation of a primary oxide layer conceals the influence of structure and alloying elements on the surface properties, at least for the grades of steel investigated in this work. Using ellipsometry, however, it is only possible to establish the situation over cu. 1.5 mm* of the specimen surface, and thus all workers are forced to choose a part of the surface where especially reproducible results are obtained. For this reason, the data derived from such work are only representative of the particular point-shaped range selected. When larger sheets of steels pretreated in the same way were exposed to aqueous media, it was found that their primary oxide layers were much less stable relative to superpure iron [13,36]. The selective participation of other elements incorporated in the steel in accordance with eqn. (l), as well as the limited island-shaped appearance of the various primary oxide layers, was demonstrated by the rapid activation of these steel sheets during immersion in COP-free distilled water with a simultaneous reduction in the pH value by almost two units [13,36,90].

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3.2 Interactions

of primary

oxide

layers with water

The formation of a primary oxide layer on ferrous materials usually occurs in an atmosphere with a measurable relative air humidity (RH). For this reason, water must have an influence on this process. As confirmed by many experiments, however, at or about room temperature the surface reaction of iron with atmospheric oxygen proceeds much faster than the corresponding conversion of the metal in the presence of air-borne water vapour [6,15 - 17, 36, $81. Hence, such water reactions mainly involve interactions with the primary oxide layer already formed on the surface. The literature contains a large number of assumptions regarding the changes occurring on oxide surfaces during their reaction with water [6,13, 31,36,72,88,93 - 991. Thus, IR spectroscopic investigations of a-FezOs have shown that when in contact with steam this oxide exhibits typical bands corresponding to OH groups and molecular water [ 94,991. These OH groups were not removed even when the samples were stored in vacua (0.133 Pa) at 298 K for 1 d. They were only removed by heating the oxide to 758 K, but reappeared again when the temperature was lowered in the presence of steam. These observations were confirmed in principle by dielectric, immersion calorimetric and gas adsorption measurements, and led to the suggestion that a dissociative mechanism applies to the chemisorption of water on iron oxide surfaces [88,95 961. Hydroxyl groups attached to the surface can adsorb another layer of molecular water through the participation of hydrogen bonding, thus obtaining a directional orientation of the adsorbed layer [6,88,95,96,100,101]. Subsequent layers of molecular water are only physically adsorbed however, and can be readily removed simply by isothermal treatment in uacuo [6, 72, 88, 95, 961. With Fe”’ oxide hydroxides, the interactions with steam depend very much on the particular modification of solid employed. Thus BET and dielectric constant measurements have shown that two energetically different types of OH group are generated on the surface of a-FeOOH during its interaction with water, whereas those of y-FeOOH are energetically equivalent [6,98]. The results obtained with iron oxide and hydroxide powders suggest the existence of surface hydroxyl groups on primary oxide layers of ferrous materials even after storage in humid air, and this has been confirmed experimentally [6,13,72,88 - 92,100 - 1021. Thus, in Fig. 4 the ellipsometric results for the primary oxide layer on various grades of steel have been compared with the data obtained on pure, dried or hydrous iron oxides. From this it follows that the components involved in the build up of the primary oxide layer are not solely FesO, and y-FezOs, as suggested, for example, by results obtained from the application of galvanostatic cathodic reduction methods [6, 87, 881. Optically less dense, low-absorbing species are also assumed to be present, and these lead to the lower N, values and higher d, values observed in measurements of the total layer. Such species may be OH groups, residual water in an adsorbed form derived from the cleaning process or intercalary ‘hydrate water’, which are not desorbed

137

0

100

200

tlmin

tlmin -

300

-

Fig. 5. Water layer thickness dH,O on different steels depending on the exposure time and relative humidity at 298 K [91].

$! 60. KTS 4512 A

i? g 40.

Fe& x CL-Fe203 +

20 i I 1.0

2.0

dH2,1nm _

3.0

CO

Fig. 6. Water adsorption isotherms for different iron oxides and primary oxide layers of ferrous materials (300 min; 298 K) [91].

even after exposure of the test material in the ellipsometric measuring cell to a relative humidity (RH) of 0% and a temperature of 298 K for several hours. When samples with this initial state were exposed to a positive relative humidity in clean air at 298 K, ellipsometric measurements on the basis of the two-layer model indicated that water adsorption occurred on the primary oxide layer of a given ferrous material, with equilibrium being attained after ca. 300 min (see Fig. 5). At RH < 80%, on the basis of a molecular diameter for water of 0.31 nm [36, 931, calculations show that only one to two molecular layers of water are present on the specimen surface. At a certain ‘critical’ value of RH, i.e. (RH),,it, the value of dHzO increases very rapidly, and at RH = 100% (and independent of the type of material studied) it attains an average value which suggests the development of ca. 10 to 12 molecular layers of water (see Fig. 6). From this it may be assumed that at RH > (RH),,, because of the protolytic cooperation of the amphoteric surface hydroxyl groups on the respective primary oxide, i.e. according to

138

+ OH- e

-Fe(OH)OH2+

-Fe(OH)OH

+ Hz0 G

-Fe(OH)O-

+ H,O+ (3)

the dissociative equilibrium of water is changed in such a way that more water molecules are present in the adsorbed film than in a simple monomolecular adsorption layer, and that the continued attachment of additional Hz0 molecules through hydrogen bonding finally leads to a film exhibiting the properties of liquid water. This film may then act as an electrolyte for electrochemical reactions as is required for atmospheric corrosion [ 36,88 - 921. Presumably, beneath organic coatings connected to the primary oxide layer of a steel substrate, autoreduction of Fe”’ oxides only initiates underfilm corrosion [7, 121 in the non-bonded regions, i.e. by means of the following reaction: Fe -

Fe2+ + 2e-

(4)

Fe,O, + 3H,O + 2e- -

2Fe2+ + 6OH-

(5)

and these only start after water has permeated across the polymer film and is present at the free oxide surface in amounts larger than a monolayer. If the surface hydroxyl groups of the primary oxide layer of ferrous materials are to be utilized solely for reactions involving the chemical adhesion of organic coatings, as has already been discussed [4,6 - 8, 13, 14, 25, 26, 79 88,103 - 1051 it would appear necessary to apply the respective coating material at RH < (RH),,,, since the tendency of bonding reactions of the type: -Fe(OH)O-

+ HOOC-R

-Fe(OH)OH2+

ti

+ NR 3 e

-Fe(OH)200C-R

(6)

-Fe(OH)?HNR3

(7)

occurring decreases if there are large amounts of adsorbed water associated with the surface hydroxyl groups. 3.3 Interactions of the primary oxide layer with sulphur dioxide and other impurities With bonding reactions such as those depicted in eqns. (6) and (7), certain components of the atmosphere either inhibit the process by being adsorbed instead of water or are capable of totally removing the primary oxide layer through initial local attack on the surface hydroxyl groups, according to the reactions: -Fe(OH),

+ 2H” + e- -+

-Fe(OH),

+ 20H- -

Fe’+ + 2H20

(see ref. 88)

(8)

Fe(OH),

(9)

(see refs. 6, 7,13, 46, 79, 80,101,106,107) -Fe(OH)2 2 -Fe(OH),

+ qLy--

FeL,(OH)21-Yq

+ SO2 + H+ +

(see ref. 88)

2Fe2+ + HS04

+ 2H20

(16) (see ref. 36)

(11)

139

To date, further detailed investigations on these reactions have not been published, but for greater exposure times one can assume that similar processes occur to those known for the primary reactions involved in atmospheric corrosion (see, for example, refs. 36, 77, 88 - 92,108,109). We have tried to determine the influence of SO2 (a known pollutant in an industrial atmosphere) ellipsometrically. As a comparison of Fig. 7 with Fig. 5 shows, when cso, < 6 mg m -3 the thickness of the adsorbed layer of water on the ferrous materials investigated in humid air is not affected. If SO2 is added after formation of the adsorbed water film, a further increase in d is observed, which becomes more pronounced the higher the value of Ry(see Fig. 8). The initial decrease in the dH o equilibrium values found at RH = 100% when SO2 was added at higher conkentrations may be attributed to the ‘compression’ of the water structure in the adsorbed layer by the inclusion of polar SO, molecules and the formation of clathrates of the type SO, - 7H,O [91,110]. As SO2 is taken up by the adsorbed water film associated with the primary oxide layer, conditions are generated which lead to the destruction of the passive state and the initiation of atmospheric corrosion [36, 1101. However, it is not possible to observe this type of destruction ellipsometrically. Despite this difficulty, the results depicted in Figs. 7

KTS n,,,=2.37.d,,,=5.0nm Fe ‘n ,,,=2.63.d,,,=5.6nm St

.n ,,=2.75.d,,,=L.5nm

(....,,...,...., 0

100

200

tlmin

300

-

Fig. 7. Dependence of the thickness of the adsorbed water layer dH,O on the exposure time of different ferrous materials in humid air containing SOa at 298 K (RH = 90%; cso, = 3.0 ?r 0.4 mg mP3) [91].

tlmin

-

tlmin -

Fig. 8. Dependence of the thickness of the adsorbed water layer dH,O on the exposure time and RH value as the amount of SOz is increased (4) in humid air at 298 K [91].

140

and 8 suggest that chemisorption of atmospheric SO* by steel surfaces covered with a primary oxide layer occurs mainly when a particular RH value is exceeded. Since the incorporation of SOz in the adsorbed water film enables the latter to take up further water from the environment, a ‘critical’ layer thickness is attained and the film acquires the properties of a surface electrolytic solution already at a lower RH value than that existing in clean air. As soon as the adsorbed water film reaches this condition, the atmospheric components taken up should be capable of removing the primary oxide layer (at least locally), a situation which corresponds to the present state of knowledge regarding the initial processes involved in the pitting corrosion of ferrous materials [15 - 17,36, 88,111 - 1151. All oil and grease residues left on steel surfaces act adversely towards the chemical adhesion of organic coatings. This conforms to the requirements of eqns. (6) and (7) above, since such residues inhibit the formation of surface hydroxyl groups or interact with them. The extent to which other organic compounds may also be included in this category depends mainly on the type of organic coating material under discussion. In this connection, however, it should be remembered that for waterdilutable coating materials, in particular, types of substances exist which react with the surface hydroxyl groups of the primary oxide layer of metals as well as with the functional groups of the respective bonding agents, thus increasing the adhesion process and acting as so-called adhesion promotors for organic coatings [4, 8, 9, 20, 25,26,116 - 1191. 4 Conversion layers on ferrous materials 4.1 Phosphate layers and their properties Phosphatization of ferrous surfaces is the most common technique currently employed to produce a conversion layer (see, for example, refs. 5, 10,12,13,18 - 20,32,33,47 - 49,51,120,121). Such a layer provides the uniform surface conditions necessary for the subsequent application of an organic coating. Additional demands arise from the type of coating material and coating technique chosen [lo, 12,20,33,48,51,120,121]. Since phosphatization is at present particularly important as a surface pretreatment for the cathodic electrodeposition of coatings, the following statements refer mainly to such coating techniques. For the cathodic electrodeposition of coatings, of the large number of phosphatization techniques available only those which lead to an extremely thin, regularly shaped, microcrystalline coating with a mass per unit area of <3 g m-‘, and in which the zinc phosphate is present in the form of hopeite Zn,(PO&*4H,O and/or phosphophyllite FeZn,(PO,),*4H,O, are to be preferred [lo, 12,33,42,122 - 1251. As is known, such phosphate layers may be obtained in the absence of an external current by partial dissolution of the metal substrate in an acidic phosphatizing bath. Since the generation of hydrogen by means of a partial cathodic reaction has an unfavourable

141

influence on the layer-forming process, modern technical phosphatizing agents also contain other reducible species such as chlorate, nitrate, nitrite or hydrogen peroxide [lo, 12, 33, 48, 511. Their reduction in the vicinity of the phase boundary also leads to a very steep increase in the pH value of the system, i.e. through the reaction: CIOS- + 3H+ + 6e- -

Cl- + 30H-

(12)

which results in the solubility products of the secondary and tertiary phosphates of iron and zinc being exceeded rapidly, so that the latter commence to precipitate. If the submicroscopic surface steps (which are etched during metal dissolution) act as crystallization centres, these phosphates intergrow epitaxially with the substrate. During the continued growth of the primary microcrystallites in the tangential and vertical directions, the uncoated metal surface gradually decreases in area so that finally the pore area of the phosphate layers amount to
We3P0&W@h10

+

[Me3(P~4)2(H~~)~(~H)lo~+ ho+

(13)

[Me3PWBb%10

+

PWW04)2(H~0)31o+ + OH-

(14)

[Me3(Po4)2(H20)2(oH)lo- + [HNW” +

[M~~(PO~)~(H~O)~...NR~IO (15)

PWW’0MWMo+

+ [OOC-RI-

+

]Mes(PO&(H20)3

l

* * HOOC-R]

o

(16)

142

On this basis adhesion should depend on the isoelectric point (IEPS) of the respective phosphate layer on the surface of the substrate. The notation employed above for these reactions, uiz. [ lo, relates to the respective species involved in the conversion layer. For this reason great importance must be attached to the characterization of the protolytic properties of the various phosphate layers produced by commercial agents. It has already been shown [13,26,107,127,128] that the drift in pH values observed when single phosphated steel specimens are exposed to a relatively small volume of a 0.01 M oxygen-free KNOs solution of specified initial pH may be determined potentiometrically (drift method). The initial pH value, which remains constant over a period of at least 1 h without any drift, may be regarded as the IEPS of the phosphate layer investigated. The justification for this assumption is provided by determinations of the rate of dissolution of 32P-labelled phosphate layers in buffer solutions with different pH values. From the IEPS obtained prior to drift, it is also possible to obtain the minimum rate of dissolution of the respective phosphate layer. Selected values are listed in Table 2. TABLE 2 Isoelectric points (IEPS) of phosphate layer surfaces produced with different commercial phosphatizing agents on cold-rolled steel sheets Phosphatizing agenta

IfiA(k?mw2)

Main layer components

IEPS

Phosphorsal Phosphatol Phosphorsal Phosphorsal

4.7 5.6 3.2 1.9

scholcite, hopeite different iron phosphates hopeite hopeite, phosphophyllite

5.9 5.8 5.6 5.3

P 150 R 60 H 70 X 635

+ + + k

0.5 1.5 0.2 0.2

+ 0.1 f 0.3 If-0.1 + 0.1

aPhosphatizing agents supplied by Haertol, Magdeburg (G.D.R.).

These values indicate that the IEPS depends on the main components present in the phosphate layers. It is quite remarkable that no corrosion occurred with any of the phosphated steel specimens characterized by the pH drift method, whereas the St-38u2 steel of the same grade was activated at a rapid rate in the unphosphatized state in aqueous solution due to its unstable primary oxide layer which contained manganese sulphide inclusions (see also refs. 13,36,90). Thus uniform surface conditions were achieved in all cases by phosphatization. This result was not unexpected since it had been shown previously that MnS inclusion, as well as Ni deposition, inside the primary oxide layer on steels promotes reductive degradation of such layers, thereby creating additional crystal nuclei and contributing to the increase in the pH value required for the deposition of tertiary phosphates by OH- ions generated during the cathodic reaction (see eqn. (5)) [12]. To date, confirmation of the postulated adhesion mechanism for the cathodic electrodeposition of coatings has been difficult, since only the phosphate layers generated via

143

the phosphatizing agent X 635 remain sufficiently stable during cathodic charging. When Luhydran 8515 (pH = 8.1; 2 min; 300 V) was used the rate of degradation determined radiochemically was always
2Fe2+ + 30H-

(17)

To prevent this process, which impairs both the quality of the deposited polymer film and the stability of the electrodeposition bath, phosphated steel surfaces have been successfully treated with solutions containing CrV’ species prior to electrodeposition [32,33,123,128,132]. Various formulations are available for this, which lead to the formation of Cr”‘-containing mixed spinels such as Fe(Fe, Cr),04 in the pores of the phosphate layer. These are not reduced during the cathodic electrodeposition of coatings, so that coating systems with enhanced corrosion protection properties are generated. However, in practice, the use of chromate solutions requires expensive safety precautions due to the relatively high toxicity of these materials.

144

Through the use of aqueous solutions with >O.Ol M KMn04 (which, in addition, contain a buffer system for securing a constant pH value over the neutral range), it is possible to obtain an oxidic conversion layer on cleaned steel surfaces with remarkable corrosion protection properties. The qualitative composition Fe/FesO,, y-FezOs, cx-Mn,03 [as (Mn,_,Fe,)20,], MnOz has been proposed for such layers on the basis of IR spectroscopic [ 1331 and electrochemical investigations [ 1341. Using the drift method mentioned above in Section 4.1 and through radiochemical investigations with 54Mnlabelled MnOz, it has been shown that this conversion layer exhibits isoelectric behaviour over the range 3.5 Q pH =G4.5. For waterdilutable polyacrylate dispersions with pH values in the weakly acid pH range, this layer has proved to be particularly advantageous relative to the phosphate layers listed in Table 2 [13, 261. Furthermore, since for the individual phosphate layers listed the electrochemically assessed adhesion relative to a given polyacrylate dispersion increased with increasing difference between the IEPS of the conversion layer and the pH value of the respective dispersion paint, this may be regarded as confirmation of the existence of a chemical adhesion mechanism similar to that depicted in eqns. (6) and (7) [13, 26, 1071. Investigations of the suitability of this MnOz-containing conversion layer for the cathodic electrodeposition of coatings have yet to be carried out. 5 Conclusions If an organic coating is to be anchored at all the reactive spots on the surface of a ferrous material and protect the latter from corrosion, this surface must be very homogeneous down to the submicroscopic range. The physical and chemical surface conditions of ferrous materials are established via a large number of individual factors. However, it has not been sufficiently clarified to what extent these various factors interact. For this reason, only suppositions have been advanced in this report. Thus, both structural dislocations and semicrystal positions of real metal surfaces can be detected successfully through the use of electron spectroscopy, and indeed there are many methods available for assessing roughness. In contrast, detailed investigations have yet to be carried out to ascertain the surface characteristics of the primary oxides formed on steel surfaces during contact with air and how these oxides lead to chemical and electrochemical properties observed for such surfaces. The experimental results presented in this report demonstrate how difficult it is even with modern, highly sensitive measuring methods to establish principles which are relevant to practical applications, since such methods only study point-like areas of metal surfaces. However, it can be regarded as completely confirmed that mechanically or chemically cleaned surfaces of ferrous materials always possess primary oxide coatings associated with OH- groups arising from contact with the air prior to coating.

145

These may be present as a continuous layer or in the form of island structures, with their properties being also influenced by the participation of alloying elements and surface pollutants. Electrochemical measurements have indicated that a continuous primary oxide layer with homogeneous properties only exists on superpure iron. Such layers may serve as the basis for the chemical adhesion of organic binders. In contrast, with unalloyed and low-alloyed steels, it is more expedient to generate a surface conversion layer which tends towards protolytic interactions prior to the application of a coating material. As a result it is possible to homogenize a surface successfully to a high degree, and by selection of a suitable layer-forming agent (e.g. phosphatizing or oxidizing agent) an IEPS can be achieved which is favourable for chemical adhesion.

Acknowledgements I would like to thank Dr. Ursula Rammelt for her care in performing the impedance measurements described in the original article, and for her many helpful suggestions. Moreover, I thank Dr. K.J. Eichhorn and Dipl.-Chem. U. Kuenzelmann for the accurate ellipsometric results published elsewhere.

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