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Nucleation and formation of zinc phosphate conversion coating on cold-rolled steel
Corrosion Science, Vol. 32, No. 5/6, pp. 635-652, 1991 Printed in Great Britain.
0010-938)(/91 $3.00 + 0.00 © 1991 Pergamon Press plc
N U C L E A T I O N AND F O R M A T I O N OF ZINC P H O S P H A T E " C O N V E R S I O N C O A T I N G ON C O L D - R O L L E D STEEL P.-E. TEGEHALLand N.-G. VANNERBERG Department of InorganicChemistry, Chalmers University of Technologyand University of GOteborg, S-412 96 G6teborg, Sweden Abstract--The basic reactions in the nucleation and formation of zinc phosphate coatings on cold-rolled steel have been studied. The iron oxideson the steel surfacewere found to be crucialfor the formationof a fine-grained phosphate coating. The best result was obtained if the oxide layer consisted mainly of y-Fe203. The temperature of the steel also influences the nucleation of zinc phosphate crystals, whereas accelerators, such as nitrate and nitrite, have little effect on the number of nuclei formed on an oxidized surface. The accelerators increase the growth rate of the phosphate crystals and thereby shorten the time needed for the formation of a coveringphosphate coating. INTRODUCTION ZINC PHOSPHATINGis used in the automotive industries as pretreatment before steel and galvanized steel are painted, to enhance the adhesion of the paint. When electrodeposited paints (ED-paints) were introduced in the 1960s, it was claimed that zinc phosphating was no longer needed. However, it soon became obvious that zinc phosphating pretreatment was more important than ever before and that new demands had to be made on the zinc phosphate coating. 1-6The coating had to be thin and consisted of a large number of crystals. The first ED-paints were deposited on metal objects connected to the anode, but over the last 10--15 years the anodic EDpaints have to a large extent been exchanged for cathodic ED-paints. During the electrodeposition of the cathodic ED-paints, the pH of the solution near the metal surface may increase to values above 12. 7,s It has been shown that the phosphate phases containing iron, nickel and manganese in addition to zinc are less soluble at high pH values and yield better paint adhesion than Zn3(PO4) 2.4H20. 6'9"m Finegrained coatings with a high content of the iron, nickel and manganese containing phases are desirable. The properties of zinc phosphate coatings on steel are affected by the various stages in the phosphating process, as well as by the history of the steel. During the manufacture of the steel many of its alloying elements are enriched on the surface, mainly as oxides but also as sulfides and carbides. 5'1t The surface layer has a great influence on the phosphatability of the steel. The effect of many of the components in the surface layer has been determined, 5'11-1s but of the main constituent, the iron oxides, very little has been written. Cold-rolled steel is annealed to increase its formability. The iron oxides on the steel are formed when the steel is removed from the reducing atmosphere in the annealing furnace. H The temperature at which it is taken out of the furnace determines which oxides are formed. According to Maeda, ~t this temperature is Manuscript received 25 October 1989; in revised form 29 June 1990. 635
636
P.-E. TEGEHALL and N.-G. VANNERBERG
usually below 150°C and the oxide film then has a two-layer structure with F e 3 0 4 next to the metal and 7 - F e 2 0 3 o n top. However, it is not probable that the oxide film consists of two well-defined layers of 7-Fe203 and F e 3 0 4 , respectively. The two phases have the same structure except that one-ninth of the iron atoms in the Fe304 phase are missing in the 7-Fe203 phase. It is likely that there is a gradual transition from 7-Fe203 on the surface to a more iron-rich phase next to the metal. At temperatures above 175°C, the uppermost layer is converted to a - F e 2 0 3.11 The iron oxides are often considered to degrade zinc phosphating 11'19 but Laukonis has found that iron oxides may have a beneficial effect on the result of zinc phosphating. 2° He also reported that the iron oxides survived the zinc phosphating process and could be found under the phosphate layer. The various stages in the zinc phosphating process are (rinses omitted): cleaning, activation, phosphating and passivation. The cleaning is performed using slightly alkaline solutions. Acidic and highly alkaline solutions result in poor phosphatability of the steel. 2a Activation is done to refine the crystal size in the phosphate coating. Colloidal solutions of titanium phosphate are usually used as the chemical activator. The titanium phosphate is often added to the cleaning solution, 22 but the best effect is obtained when it is added to the last rinse before phosphating. 4'21'23 During the activation stage, titanium phosphate particles are adsorbed on the steel surface. The adsorbed particles will then act as nucleation agents in the formation of zinc phosphate crystals in the phosphating stage. The titanium phosphate colloid and the mechanism of activation have been described in previous papers. 24'25 Zinc phosphating is performed either by immersion in the phosphating solution or by spraying the phosphating solution on the steel. 21'22 Accelerators are added to the phosphating solution to shorten the phosphating time and to modify the morphology of the coating. The accelerators are oxidizing agents such as nitrate, nitrite, chlorate and hydrogen peroxide. 21,22Divalent cations of calcium, manganese and nickel are also often added to the solution to refine the crystal size, 5'1°'26-3° but these are usually not referred to as accelerators. For passivation, solutions containing hexavalent chromium have been used to a great extent, but restrictions in the use of chromium(VI) have given rise to a search for new compounds for passivation. Chromium(III), either alone or in combination with chromium(VI), has been reported to have s o m e e f f e c t . 31-33 To understand the reactions in the zinc phosphating process, it is necessary to know what happens in all the stages, and how they interact. The aim of this paper is to explain how the various stages influence the nucleation and formation of zinc phosphate coatings, especially the effect of the iron oxides on the steel surface. Since zinc phosphating is basically a crystallization process, the reactions will be discussed from the point of view of crystallization chemistry. EXPERIMENTAL
METHOD
Preparation of samples T h e composition of the steel used in this investigation was: 0.083% C, 0.48% Mn, 0.04% Si, 0.025% P, 0.023% S, 0.01% Cu, 0.039% AI and 0.10% V. Pieces measuring 2 x 3 x 0.1 cm were wet-ground on one side using silicon carbide paper down to 1000 mesh, washed first in water and then in ethanol, and dried in air. Some of the samples were then oxidized in an oven at temperatures between 125 and 245°C for 20 h. This treatment was performed to remove contaminations on the surface and to form an oxide layer consisting mainly of iron oxides.
Zinc phosphate conversion coating
637
T h e iron(III) oxides formed in the heat t r e a t m e n t were identified using cathodic reduction of the oxide layer using constant current. 34-37 The reduction was performed in a de-aerated solution of 0.075 M boric acid and 0.0375 M sodium borate (pH 8.4) at a current density of 10 p~A cm 2 The steel samples were taped so that only 3 cm 2 of the polished side was free from tape, as described in Ref. 25. A copper wire was fixed by tape to the unpolished side to obtain an electric connection. The cathodic reduction curves of steel samples oxidized at various temperatures are shown in Fig. 1. Samples oxidized at temperatures below 170°C yielded a plateau at about - 6 0 0 m V (SCE), while those oxidized above 190°C yielded a plateau at about - 8 5 0 mV(SCE). These plateaux at - 6 0 0 and -851) m V ( S C E ) are consistent with values reported for 7-Fe~O3 and a - F ~ O 3 , respectively. 3437 Samples oxidized at 175 and 185°C show two plateaux in the reduction curve implying that the oxide layer on these consists of a mixture of a- and y-Fe203. Unfortunately, using this technique it is not possible to determine whether there is a layer of Fe304 next to the metal. At low potentials hydrogen ions are reduced instead of Fe304. The oven-oxidized steel samples were coloured (interference colours), from very faint yellow when oxidized at 125°C to deep blue at 215°C and faint blue at 245°C. After cathodic reduction, interferencc colours remained only on samples oxidized at 185°C and above. Thus, on these, some iron oxide remained, which is difficult to reduce in the slightly alkaline borate-boric acid solution. It is possible that this oxide is F e 3 0 4. Zinc phosphating
Colloidal titanium phosphate used in the activation stage was prepared by mixing 0.060 tool N a 2 H P O 4 - 2 H 2 0 , 0,020 mol Na4P207 • 10H20 and 0.010 tool KzTiF 6 and then adding 5 ml of water. The mixture was slowly dried in an oven at 100°C. A n activating colloidal solution was then achieved by dissolving 1 g 1 i of the dried mixture in water at room temperature.25 Before the steel samples were activated, they were taped in the same way as for the cathodic reduction measurements. The taped steel samples were activated by dipping them in the colloidal solution for one minute. In some cases the samples were dipped in a 0.1 M Zn(NO3) 2 solution for 5 s after activation. This treatment has been shown to prevent the adsorbed titanium phosphate particles from being desorbed in a water rinse. 2s The samples were then phospbated immediately after the activation. Zinc phosphating was performed by immersion of the samples in a solution containing 160 m M H3PO 4, 58 m M Z n O and in most cases one or two accelerators. W h e n nitrate was used as accelerator H N O 3 was added to a concentration of 70 m M , when nitrite was used N a N O 2 was added to a concentration of 2.2 m M , and when hydrogen peroxide was the accelerator the concentration was chosen
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638
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to be 3.0 mM. The pH of the solution was adjusted to 2.5 at 25°Cusing NaOH, and the temperature was then increased to 60°C. The solution will then be slightly supersaturated with respect to zinc phosphate. Therefore, the solution was kept at 60°C for at least 1 h before any phosphating was done to allow the surplus of zinc phosphate to be precipitated. During the phosphating, the steel samples were moved at about 0.1 m s-1.
Potentialmeasurements The phosphating process was followed by measuring the potential of the steel surface during the phosphating. The potential was measured against a saturated calomel electrode dipped in a phosphating solution at room temperature. This solution was connected to the phosphating bath by a salt bridge. Potential-time measurements have been shown to be a useful tool in evaluation of the phosphating reactions,38-4°but the edges of the samples have to be covered, for example with tape. Otherwise, the measurements will be disturbed. 4° EXPERIMENTAL RESULTS AND DISCUSSION
Basic reactions in zinc phosphating of steel The acidic zinc phosphating solution reacts with the metal surface. In a solution containing only phosphoric acid and zinc ions, the reaction on an oxide-free iron surface is: Fe(s) + 2H + ~ Fe z+ + HE(g). (1) The p H value is increased in the solution adjacent to the metal surface. The phosphating solution is normally saturated with zinc phosphate. As a result of the increased p H value, the solubility product of zinc phosphate will be exceeded and hopeite will be precipitated: 3Zn 2+ + 2H2PO4 + 4 H 2 0 ~ Zn3(PO4)2" 4H20(s) + 4H ÷.
(2)
On steel, phosphophyllite can also be precipitated: Fe 2÷ + 2Zn 2÷ + 2H2PO4 + 4H20--~ FeZn2(PO4)z- 4H20(s) + 4H ÷.
(3)
The most important reaction in the zinc phosphating process is the nucleation of zinc phosphate crystals. The number of nuclei formed determines the properties of the zinc phosphate coating. A large number of nuclei results in a fine-grained coating, whereas a small number of nuclei yields a coarse coating that takes a long time to complete. 4° If nuclei are to be formed, the solution has to be supersaturated with zinc phosphate. The more supersaturated the solution, the more nuclei formed. Hence, if the hydrogen ion consuming reactions on the steel surface can be speeded up, this results in a faster increase o f p H and thus in a more supersaturated solution.
The influence of steel temperature The solution near the metal surface will be cooled if a cold steel object is immersed in the phosphating bath. Since the solubility of zinc phosphate increases with decreasing temperature, the solution will no longer be saturated with zinc phosphate. Furthermore, the rates of the chemical reactions will be slower as a result of the decrease in temperature. Thus, preheating the steel to the temperature of the phosphating bath should be beneficial for the nucleation process. This is indeed the case as can be seen in Fig. 2(a) and (b).
The role of accelerators in the zinc phosphating process The rate of consumption of hydrogen ions at the metal surface can be increased by addition of accelerators to the phosphating bath, for example nitrate, nitrite,
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644
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chlorate and h y d r o g e n peroxide. T h e s e are oxidizing agents that increase the pickling rate of the steel. T h e reaction with nitrate can be written: 4a-43 4Fe(s) + N O B + 10H + ~ 4Fe 2+ + NH~- + 3H2 O.
(4)
Table 1 presents the effect of various accelerators on the pickling rate of steel, and Figs 2 and 3 show the effect on the zinc phosphating. Nitrite and h y d r o g e n peroxide have a m u c h greater effect than nitrate, and the c o m b i n a t i o n of nitrate with nitrite or h y d r o g e n peroxide yields a synergistic effect. H o w e v e r , the strong oxidizing p o w e r of h y d r o g e n peroxide limits the c o n c e n t r a t i o n at which it can be used. T h e fast oxidation of dissolved Fe 2÷ to Fe 3+ by h y d r o g e n peroxide may result in precipitation of a m o r p h o u s i r o n ( I l l ) p h o s p h a t e on the steel surface. In addition to the e n h a n c e m e n t of the nucleation, the higher pickling rate of the steel also results in a faster g r o w t h of the zinc p h o s p h a t e crystals and thereby a
TABLE 1.
T H E INFLUENCE OF ACCELERATORS ON THE PICKLING RATE OF
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Changes in the surface potential during phosphating a sample oxidized at 155°C.
shorter phosphating time. This can be seen in the potential-time curves as a faster increase of the potential (Fig. 3). The appearance of the zinc phosphate coating is also affected by the accelerators (Fig. 2). Hydrogen peroxide yields coatings with an appearance similar to spray phosphated coatings. The reason for this is probably that oxygen from the air is used as accelerator in the spray phosphating process.42 The reactions caused by hydrogen peroxide can be expected to be similar to those caused by oxygen. In the absence of accelerators, very regular crystals are formed, owing to the slow growth rate.
The influence of iron oxides on the result of zinc phosphating A typical potential-time curve for phosphating of an oxidized sample is shown in Fig. 4. As can be seen from the curve, the iron oxide layer is not dissolved until after a certain time has passed, known as the autoreduction t i m e (r). 36 The samples had to be taped, otherwise the oxide layer was immediately dissolved. The autoreduction time for samples oxidized at various temperatures is reported in Table 2. It can be noted that r is shortest for the samples with a- and y-Fe203 mixed
TABLE 2.
AUTOREDUCTION TIME FOR SAMPLES OXIDIZED AT VARIOUS TEMPERATURES
O x i d a t i o n t e m p e r a t u r e (°C) A u t o r e d u e t i o n t i m e (s)
125 -70
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185 -60
215
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Zinc phosphate conversion coating
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in the oxide layer. It was observed that for samples with only v-Fe203 in the oxide layer, r varied with the humidity of the air during the preparation of the samples. The higher the humidity, the longer the time needed for the autoreduction. A possible explanation is that in the presence of water molecules in the air, HFe508 is formed, 44 but that is beyond the scope of this investigation and is not discussed further. The iron oxides can be forced to dissolve by changing the surface potential in the cathodic direction. This is most easily done by connecting the sample to a polished iron surface which is immersed in the phosphating bath directly before the connection is made. The properties of the phosphate coating varied with the distance from the contact point if the connection was made on the surface of the sample. This could be avoided by making the connection via the copper wire taped to the backside of the sample. If the connection is made 1 min after the sample is immersed in the phosphating bath, both a- and 7-Fe203 are immediately dissolved at a very high rate. A connection time of fractions of a second is enough to start an electrochemical dissolution of the oxide layer. The surface potential then drops from about + 150 mV to below - 6 0 0 mV(SCE) in <1 s when the surface layer consists of ~-Fe203 or a mixture of a- and v-Fe203, and in a few seconds when the surface layer consists of cz-Fe203. If the connection is made before the steel sample has been heated to the temperature of the phosphating solution, the oxide layer is considerably more difficult to dissolve, owing to the lower temperature of the surface. In this case a connection time of up to 5-10 s is needed to make the electrochemical dissolution continue by itself when the connection is broken. The result is the same, independent of the composition of the oxide layer. These results show that the chemical dissolution of the iron oxides is very slow. The oxide layer is not uniform. At some points the oxide layer is dissolved more quickly, and free iron surfaces are exposed. These points can be inclusions of other iron oxides, easily dissolved oxides of other elements or scratches in the oxide layer. The free iron surfaces exposed result in an electrochemical dissolution of the rest of the oxide layer: Fe(s) + Fe203 + 6H + -+ 3Fe 2+ + 3H20.
(5)
This reaction consumes hydrogen ions and, thus, also yields a supersaturation of the solution. To achieve a quick increase in p H value, the dissolution of the oxide layer should be as fast as possible. The rate-limiting reaction when the electrochemical dissolution starts is the anodic reaction (oxidation of iron) since the anode area is very small, as compared with the cathode area. Hence, the nucleation can be expected to be better on a surface with mixed oxides in the oxide layer, i.e. an oxide layer with many weak points. Figure 5 shows that this is indeed the case. The most nuclei are formed on the samples with a- and v-Fe203 mixed in the oxide layer. If the oxide layer was forced to dissolve by connection to a polished iron surface, the anodic reaction was no longer rate limiting. The largest number of nuclei was then obtained for the samples with only }'-Fe20 3 in the oxide layer (Fig. 6). This implies that the electrochemical dissolution of y-Fe203 is faster than that of a-Fe203. If they are dissolved at the same rate, the samples with a-Fe203 should give a larger increase of pH since the oxide layer is thicker on these than on samples with v-Fe20 3. If the edges of the sample are not covered, they will be excellent anodic areas when the oxide layer dissolves, and the composition of the phosphate coating will
648
P.-E. TEGEHALLand N.-G. VANNERBERG
vary with the distance from the edges. Therefore, it is important to protect the edges from being in contact with the phosphating solution when phosphating experiments are performed. Both the forced dissolution and the autoreduction of the oxide layers consisting of v-Fe203, yielded larger numbers of nuclei the thicker the oxide layer was. This is expected, since a dissolution of a thicker oxide layer should yield a larger increase of pI-L As before, if the steel sample was cold when it was immersed in the phosphating bath, far fewer nuclei were formed if it was not allowed to be heated to the bath temperature before the oxide layer was forced to dissolve (Fig. 6h). Since the oxide layer is reduced during the dissolution, oxidizing agents should not increase the rate of the dissolution. Hence, if the nuclei are formed during the period of the dissolution of the oxide layer, the number of nuclei formed should not be less if the oxidizing agents were excluded from the phosphating solution. Figure 7 presents the result of phosphating with no accelerators. A comparison with Fig. 6 shows that the number of crystals is about the same. However, the rate of growth of the crystals is slower in the absence of accelerators. Therefore, it takes a longer time to form a completely covering phosphate coating (Fig. 8). After being phosphated, the phosphate coatings on the samples were dissolved in 5% CrO3 .1 There were no signs of residual oxide on any of the samples.
Activation with colloidal titanium phosphate The most effective way to facilitate nucleation of crystals is to add nucleation agents. This is done in the phosphating process by activation with colloidal titanium phosphate. The colloidal particles are adsorbed on the metal surface during the activation and then act as crystal nuclei in the phosphating s t a g e y Without activation, very few zinc phosphate crystals are formed. 5'25'29 The amount of titanium phosphate remaining on the steel surface after activation in the colloidal solution, immersion in 0.1 M Zn(NO3)2 and then rinsing in water, was too low to detect titanium using ESCA (electron spectroscopy for chemical analysis). On samples covered with oxide layers with different compositions, the number of titanium phosphate particles adsorbed on the surface may have varied. The investigations of the influence of the steel temperature and the effect of addition of accelerators were performed using samples treated in the same way before being phosphated, i.e. the amount of titanium phosphate adsorbed on the surface was the same on the samples. The result of phosphating these samples clearly shows that the rate of increase of pH is crucial for the number of nuclei formed (Fig. 2). Furthermore, a comparison of the number of crystals formed on samples covered with y-Fe203, which have been oxidized at the same temperature, shows that forced dissolution of the oxide layer yields a more fine-grained coating than dissolution by autoreduction (Figs 5 and 6). Thus, a fast increase in pH value is necessary to utilize the adsorbed titanium phosphate particles as nucleation agents. Figure 9 shows the change in surface potential after the oxide layer has been forced to dissolve on samples oxidized at 155°C. For the sample without activation it takes a few seconds before the minimum is reached, whereas for the sample with activation the minimum is reached within 1 s. Furthermore, the potential decreases to a lower value before it increases for the sample with activation and the rate of increase is then faster. A possible explanation for the difference could be that without activation an amorphous phosphate layer is first formed at the steel surface.
Zinc phosphate conversion coating
649
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650
P.-E. TEGEHALLand N.-G. VANNERBERG
After a few seconds, zinc phosphate crystals start to form and the potential then increases. If the surface has been activated, zinc phosphate crystals are formed immediately and no amorphous phosphate will be formed near the crystal nuclei. Thus, the potential will decrease to a lower value. It will also start to rise more quickly since phosphate crystals are formed earlier in the process and, as a consequence of the larger number of crystals, the rate of increase will be greater.
Zinc phosphating of annealed cold-rolled steel Pickling in acid before phosphating is known to decrease the number of zinc phosphate nuclei formed on the surface. 19,21,45 The reason for this is that the oxide film is dissolved during the pickling. Re-oxidation at 155°C before phosphating restores the phosphatability of the steel. Thus, the original oxide film on annealed steel is important for the formation of a fine-grained coating. During the annealing of cold-rolled steel, manganese oxide is enriched in grains on the surface. 15,40,46,47 According to Maeda, 5'4s the manganese oxide shortens the autoreduction time of the oxide film in a neutral borate-chloric acid buffer solution. Hence, the enriched manganese oxide is obviously beneficial for fast dissolution of the oxide film of y-Fe20 3. The addition of divalent manganese cations, as well as calcium and nickel cations, to the phosphating bath is known to refine the crystal size in the phosphate coating. 5'1°'26-3° The divalent cations are included in the crystal structure. It is likely that the manganese ions from the oxide layer are included in the phosphate layer and, thereby, affect the crystal size. From zinc phosphating of zinc surfaces, it is known that some ferrous ions have to be added to the bath to obtain good phosphating, 4'23 implying that ferrous ions may also have a refining effect. Passivation with chromium containing solutions Restrictions in the use of hexavalent chromium have caused a decrease in the use of chromic acid for passivation after phosphating. Instead, solutions containing a mixture of chromium(VI) and chromium(III) or chromium(III) only have been used. 31-33 The chromate solutions have been believed to cause a passivation of the metal surfaces between the phosphate crystals. Solutions of chromium(III) cannot have this effect. It is doubtful that the beneficial effect is caused by a passivation of the free metal surfaces. Since the chromium-containing solutions are acidic, one effect of the passivation could be a dissolution of the parts of the phosphate coating which are easiest to dissolve, for example protruding crystals. Less zinc phosphate is then dissolved during the electrodeposition of the paint. Cheever has shown that chromium atoms can be found on the passivated surface, and that they are evenly distributed over the surface and not only between the phosphate crystals. 49 According to van Ooij, 9 chromium(III) can be found on the surface of the phosphate crystals. To study if the zinc ions in hopeite can be ion exchanged with chromium(III) the following experiment was performed. Hopeite crystals, precipitated from a phosphating solution, were added to a 0.1 M solution of Cr(NO3) 3 while stirring. The pH value of the solution increased from 2.6 to 3.6 during the first minute and then slowly fell to 3.3. After a few minutes the crystals had a faint violet colour, and after 1 h only Cr(PO4) •6H20 could be detected in an X-ray diffractogram of the crystals. Thus, the hopeite crystals were completely converted to chromium phosphate in less than 1 h. The Cr(NO3)3 solution contains the violet hydrated complex [Cr(H20)6] 3+. The
Zinc phosphate conversioncoating
651
violet colour of the chromium phosphate formed indicates that the chromium ion is still hydrated with six water ligands in the solid phase. At 150°C the chromium phosphate was dehydrated and a green phase was formed. If phosphoric acid was added to a 0.1 M Cr(NO3) 3 solution to make a 1 M solution of H3PO 4 and the pH was adjusted to 3.0, the colour of the solution slowly turned green. The change of colour implies that a chromium phosphate complex was formed. The slow formation of the complex is due to the very slow exchange of iigands on chromium(Ill). Addition of hopeite crystals to this green solution also resulted in an ion exchange of zinc ions, but in this case a green amorphous chromium phosphate was formed. Several days were needed for the transformation of the hopeite crystals to chromium phosphate. The violet chromium phosphate dissolved immediately if it was added to a 1 M N a O H solution. If it was added to pure water and the pH was then slowly increased, it was converted to a green phase. This phase did not dissolve in 1 M N a O H , neither did the green phase formed by ion exchange on hopeite crystals. CONCLUSIONS The reactions on cold-rolled steel in the zinc phosphating solution can be split into four major reactions: (i) (ii) (iii) (iv)
Dissolution of the oxide film on the metal surface Pickling of the metal surface in the acidic solutions Nucleation of zinc phosphate crystals on the surface Growth of the phosphate crystals.
The most important reaction in controlling the thickness of the phosphate coating is the nucleation of phosphate crystals. To obtain a large number of nuclei, and thereby a fine-grained coating, the increase in p H value at the metal surface should be as fast as possible during the first seconds of the formation of the phosphate coating. Besides activation, the nucleation of phosphate crystals is controlled by the dissolution rate of the oxide layer. The best result is obtained if the oxide layer consists of ~-Fe20 3 mixed with an easily dissolved oxide such as manganese oxide. The iron oxide is then electrochemically dissolved at a very fast rate. The number of nuclei formed on an oxidized surface is much higher than on an oxide free surface. The steel has to be preheated to the temperature of the phosphating solution to completely utilize the beneficial effect of the fast dissolution of the oxide film. Accelerators added to the phosphating solution increase the pickling rate of the steel. This yields a faster growth of the zinc phosphate crystals and, thereby, a shorter time is necessary for the formation of a covering phosphate coating. It also enhances nucleation on areas where the oxide layer has been removed before phosphating. The zinc ions in hopeite can be ion exchanged with chromium(III). The composition of the chromium(III) phosphate formed depends on the ligands on the chromium(III) ion in the water solution. REFERENCES
]. 2. 3. 4.
S. SPRING,Metal Finish. 63(5), 61 (1965). J. I. MAURERand R. M. LACY,Adv. Chem. Set. 119, 7 (1973). B. A. COOKEand N. M. NESS,Trans. Inst. Metal Finish, 57,155 (1979). K. WooDsand S. SPRING,Metal Finish. 77(3), 24 and (4) 56 (1979).
652 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49.
P.-E. TEGEHALL and N.-G. VANNERBERG S. MAEDA, J. Coat. Technol. 55(707), 43 (1983). T. YOSHIHARAand H. OIOTA, Trans. Iron Steellnst. Jap. 23,984 (1983). S. MAEDA, T. ASAI and H. Or,ADA, Corros. Engng 31,268 (1982). R. D. WYV1LL,Metal Finish. 80(1), 21 (1982). W. J. VAN OoIJ and T. H. VISSER,Spectrochim. Acta 39B, 1541 (1984). R. D. WYVILLand T. CAPE,Products Finish. 52(1), 68 (1987). S. MAEDA, Prog. Org. Coat. 11, 1 (1983). S. SHIMADAand S. MAEDA,Trans. Iron Steel Inst. Jap. 15, 95 (1975). S. SHIMADA,S. MAEDA and T. EGAWA, Trans. Iron SteelInst. Jap. 17, 11 (1977). J, J. WOJTKOWIAKand H. S. BENDER,J. Coat. Technol. 50(642), 86 (1978). S. WAKANO, N. Usur3, M. NISHmARA and N. FUJINO, Tetsu-to-Hagan~ 66,945 (1980). R. A. IEZZI and H. LEIDHEISER, JR, Corrosion 37, 28 (1981). S. MAEDA, T. ASAI, S. ARAI and K. SuzuKi, Tetsu-to-Hagand 68, 2497 (1982). J. A. KARGOLand D. L. JORDAN, Corrosion 38,201 (1982). W. MACHU, Werkst. Korros. 14, 566 (1963). J.V. LAUKONIS,Interface Conversion for Polymer Coatings (eds P. WEISS and C. D. CrlEEVER),p. 182. Elsevier, New York (1968). D. B. FREEMAN,Phosphating and Metal Pre-Treatment. Industrial Press Inc., New York (1986). W. RAUSCH, Die Phosphatierung yon Metallen, 2. Auflage, Chaps 3 and 4. Leuze, Saulgau/Wfirtt. (1988). G. H. PIMBLEY,Metal Prog. 80(2), 103 (1961). P.-E. TEGEHALL, Colloid Surfaces 42, 155 (1989). P.-E. TEGEHALL, Colloid Surfaces 49,373 (1990). J. CURRANand W. MACHU,Metal Finish. 70(10), 54 (1972), M. KITAYAMA,H. ODASHIMA,Z. SAITOand Y. HIRANO, Tetsu-to-Hagand 71, 1343 (1985). N. SATOrI, Surf. Coat. Technol. 30, 171 (1987). N. SATO~Iand T. MINAMI, Surf. Coat. Technol. 34, 331 (1988). G. N. BI4AR, N. C. DEBNATHand S. RoY, Surf. Coat. Technol. 35, 171 (1988). D. B. FREEMAN,Finish. Industr. 2(8), 25 (1978). J. W. DAVIS, SAE Tech. Paper Series, No. 820336 (1982). W. J. VAN OOH, Surf. Interface Anal. 9, 367 (1986). H. G. OSWIN and M. COHEN, J. electrochem. Soc. 104, 9 (1957). K. H. BUOB, A. F. BECK and M. COI~EN,J. electrochem. Soc. 105, 74 (1958). C, D. STOCKaRIDGE, P. B. SEWELLand M. COHEN, J. electrochem. Soc. 108, 928 (1961). M. NAGAYAMAand M. COHEN, J. electrochem. Soc. 109, 78l (1962). E. L. GHALI and R. J. A. POTVIN, Corros. Sci. 12, 583 (1972). N. HELEIWELL, D. R. GABE and M. O. W. RICHARDSON, Trans. Inst. Metal Finish. 54, 185 (1976). P.-E. TEGEHALL and P.-E. AUGUSTSSON, Proc. 9th Scandinavian Corrosion Congress, p. 315, Copenhagen (1983). L. O. GILBERT, Tech. Proc. a Cony. Am. Electropl. Soc. 44, 73 (1957). W. JAENICKEand B. LORENZ, Werkst. Korros. 10,681 (1959). R, V. MOSHTEV,E. B. BUDEVSKYand N. I. CHRISTOVA,Corros. Sci. 3, 125 (1963). M. C. BLOOMand L. GOLDENBERG, Corros. Sci. 5,623 (1965). W. Mgclau, Interface Conversion for Polymer Coatings (eds P. WEISS and C. D. CHEEVER),p. 128. Elsevier, New York (1968). P.-E. AUGUSTSSON,I. OEEFJORDand Y. OLEFJORD,Werkst. Korros. 34, 563 (1983). P.-E. AUGUSTSSON,Thesis, Gothenburg (1983). S. MAEDA, T. ASAI, T. HAYASHI, K.-I. SuzugI and Y. YANAGISAWA,Abstract from Presentations to the 100th ISIJ Meeting, Trans. Iron Steel Inst. Jap. 21, B-94 (1981). G. D. CHEEVER, J. Paint TechnoL 39(504), 1 (1967).
0010-938)(/91 $3.00 + 0.00 © 1991 Pergamon Press plc
N U C L E A T I O N AND F O R M A T I O N OF ZINC P H O S P H A T E " C O N V E R S I O N C O A T I N G ON C O L D - R O L L E D STEEL P.-E. TEGEHALLand N.-G. VANNERBERG Department of InorganicChemistry, Chalmers University of Technologyand University of GOteborg, S-412 96 G6teborg, Sweden Abstract--The basic reactions in the nucleation and formation of zinc phosphate coatings on cold-rolled steel have been studied. The iron oxideson the steel surfacewere found to be crucialfor the formationof a fine-grained phosphate coating. The best result was obtained if the oxide layer consisted mainly of y-Fe203. The temperature of the steel also influences the nucleation of zinc phosphate crystals, whereas accelerators, such as nitrate and nitrite, have little effect on the number of nuclei formed on an oxidized surface. The accelerators increase the growth rate of the phosphate crystals and thereby shorten the time needed for the formation of a coveringphosphate coating. INTRODUCTION ZINC PHOSPHATINGis used in the automotive industries as pretreatment before steel and galvanized steel are painted, to enhance the adhesion of the paint. When electrodeposited paints (ED-paints) were introduced in the 1960s, it was claimed that zinc phosphating was no longer needed. However, it soon became obvious that zinc phosphating pretreatment was more important than ever before and that new demands had to be made on the zinc phosphate coating. 1-6The coating had to be thin and consisted of a large number of crystals. The first ED-paints were deposited on metal objects connected to the anode, but over the last 10--15 years the anodic EDpaints have to a large extent been exchanged for cathodic ED-paints. During the electrodeposition of the cathodic ED-paints, the pH of the solution near the metal surface may increase to values above 12. 7,s It has been shown that the phosphate phases containing iron, nickel and manganese in addition to zinc are less soluble at high pH values and yield better paint adhesion than Zn3(PO4) 2.4H20. 6'9"m Finegrained coatings with a high content of the iron, nickel and manganese containing phases are desirable. The properties of zinc phosphate coatings on steel are affected by the various stages in the phosphating process, as well as by the history of the steel. During the manufacture of the steel many of its alloying elements are enriched on the surface, mainly as oxides but also as sulfides and carbides. 5'1t The surface layer has a great influence on the phosphatability of the steel. The effect of many of the components in the surface layer has been determined, 5'11-1s but of the main constituent, the iron oxides, very little has been written. Cold-rolled steel is annealed to increase its formability. The iron oxides on the steel are formed when the steel is removed from the reducing atmosphere in the annealing furnace. H The temperature at which it is taken out of the furnace determines which oxides are formed. According to Maeda, ~t this temperature is Manuscript received 25 October 1989; in revised form 29 June 1990. 635
636
P.-E. TEGEHALL and N.-G. VANNERBERG
usually below 150°C and the oxide film then has a two-layer structure with F e 3 0 4 next to the metal and 7 - F e 2 0 3 o n top. However, it is not probable that the oxide film consists of two well-defined layers of 7-Fe203 and F e 3 0 4 , respectively. The two phases have the same structure except that one-ninth of the iron atoms in the Fe304 phase are missing in the 7-Fe203 phase. It is likely that there is a gradual transition from 7-Fe203 on the surface to a more iron-rich phase next to the metal. At temperatures above 175°C, the uppermost layer is converted to a - F e 2 0 3.11 The iron oxides are often considered to degrade zinc phosphating 11'19 but Laukonis has found that iron oxides may have a beneficial effect on the result of zinc phosphating. 2° He also reported that the iron oxides survived the zinc phosphating process and could be found under the phosphate layer. The various stages in the zinc phosphating process are (rinses omitted): cleaning, activation, phosphating and passivation. The cleaning is performed using slightly alkaline solutions. Acidic and highly alkaline solutions result in poor phosphatability of the steel. 2a Activation is done to refine the crystal size in the phosphate coating. Colloidal solutions of titanium phosphate are usually used as the chemical activator. The titanium phosphate is often added to the cleaning solution, 22 but the best effect is obtained when it is added to the last rinse before phosphating. 4'21'23 During the activation stage, titanium phosphate particles are adsorbed on the steel surface. The adsorbed particles will then act as nucleation agents in the formation of zinc phosphate crystals in the phosphating stage. The titanium phosphate colloid and the mechanism of activation have been described in previous papers. 24'25 Zinc phosphating is performed either by immersion in the phosphating solution or by spraying the phosphating solution on the steel. 21'22 Accelerators are added to the phosphating solution to shorten the phosphating time and to modify the morphology of the coating. The accelerators are oxidizing agents such as nitrate, nitrite, chlorate and hydrogen peroxide. 21,22Divalent cations of calcium, manganese and nickel are also often added to the solution to refine the crystal size, 5'1°'26-3° but these are usually not referred to as accelerators. For passivation, solutions containing hexavalent chromium have been used to a great extent, but restrictions in the use of chromium(VI) have given rise to a search for new compounds for passivation. Chromium(III), either alone or in combination with chromium(VI), has been reported to have s o m e e f f e c t . 31-33 To understand the reactions in the zinc phosphating process, it is necessary to know what happens in all the stages, and how they interact. The aim of this paper is to explain how the various stages influence the nucleation and formation of zinc phosphate coatings, especially the effect of the iron oxides on the steel surface. Since zinc phosphating is basically a crystallization process, the reactions will be discussed from the point of view of crystallization chemistry. EXPERIMENTAL
METHOD
Preparation of samples T h e composition of the steel used in this investigation was: 0.083% C, 0.48% Mn, 0.04% Si, 0.025% P, 0.023% S, 0.01% Cu, 0.039% AI and 0.10% V. Pieces measuring 2 x 3 x 0.1 cm were wet-ground on one side using silicon carbide paper down to 1000 mesh, washed first in water and then in ethanol, and dried in air. Some of the samples were then oxidized in an oven at temperatures between 125 and 245°C for 20 h. This treatment was performed to remove contaminations on the surface and to form an oxide layer consisting mainly of iron oxides.
Zinc phosphate conversion coating
637
T h e iron(III) oxides formed in the heat t r e a t m e n t were identified using cathodic reduction of the oxide layer using constant current. 34-37 The reduction was performed in a de-aerated solution of 0.075 M boric acid and 0.0375 M sodium borate (pH 8.4) at a current density of 10 p~A cm 2 The steel samples were taped so that only 3 cm 2 of the polished side was free from tape, as described in Ref. 25. A copper wire was fixed by tape to the unpolished side to obtain an electric connection. The cathodic reduction curves of steel samples oxidized at various temperatures are shown in Fig. 1. Samples oxidized at temperatures below 170°C yielded a plateau at about - 6 0 0 m V (SCE), while those oxidized above 190°C yielded a plateau at about - 8 5 0 mV(SCE). These plateaux at - 6 0 0 and -851) m V ( S C E ) are consistent with values reported for 7-Fe~O3 and a - F ~ O 3 , respectively. 3437 Samples oxidized at 175 and 185°C show two plateaux in the reduction curve implying that the oxide layer on these consists of a mixture of a- and y-Fe203. Unfortunately, using this technique it is not possible to determine whether there is a layer of Fe304 next to the metal. At low potentials hydrogen ions are reduced instead of Fe304. The oven-oxidized steel samples were coloured (interference colours), from very faint yellow when oxidized at 125°C to deep blue at 215°C and faint blue at 245°C. After cathodic reduction, interferencc colours remained only on samples oxidized at 185°C and above. Thus, on these, some iron oxide remained, which is difficult to reduce in the slightly alkaline borate-boric acid solution. It is possible that this oxide is F e 3 0 4. Zinc phosphating
Colloidal titanium phosphate used in the activation stage was prepared by mixing 0.060 tool N a 2 H P O 4 - 2 H 2 0 , 0,020 mol Na4P207 • 10H20 and 0.010 tool KzTiF 6 and then adding 5 ml of water. The mixture was slowly dried in an oven at 100°C. A n activating colloidal solution was then achieved by dissolving 1 g 1 i of the dried mixture in water at room temperature.25 Before the steel samples were activated, they were taped in the same way as for the cathodic reduction measurements. The taped steel samples were activated by dipping them in the colloidal solution for one minute. In some cases the samples were dipped in a 0.1 M Zn(NO3) 2 solution for 5 s after activation. This treatment has been shown to prevent the adsorbed titanium phosphate particles from being desorbed in a water rinse. 2s The samples were then phospbated immediately after the activation. Zinc phosphating was performed by immersion of the samples in a solution containing 160 m M H3PO 4, 58 m M Z n O and in most cases one or two accelerators. W h e n nitrate was used as accelerator H N O 3 was added to a concentration of 70 m M , when nitrite was used N a N O 2 was added to a concentration of 2.2 m M , and when hydrogen peroxide was the accelerator the concentration was chosen
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638
P.-E. TEGEHALLand N.-G. VANNERBERG
to be 3.0 mM. The pH of the solution was adjusted to 2.5 at 25°Cusing NaOH, and the temperature was then increased to 60°C. The solution will then be slightly supersaturated with respect to zinc phosphate. Therefore, the solution was kept at 60°C for at least 1 h before any phosphating was done to allow the surplus of zinc phosphate to be precipitated. During the phosphating, the steel samples were moved at about 0.1 m s-1.
Potentialmeasurements The phosphating process was followed by measuring the potential of the steel surface during the phosphating. The potential was measured against a saturated calomel electrode dipped in a phosphating solution at room temperature. This solution was connected to the phosphating bath by a salt bridge. Potential-time measurements have been shown to be a useful tool in evaluation of the phosphating reactions,38-4°but the edges of the samples have to be covered, for example with tape. Otherwise, the measurements will be disturbed. 4° EXPERIMENTAL RESULTS AND DISCUSSION
Basic reactions in zinc phosphating of steel The acidic zinc phosphating solution reacts with the metal surface. In a solution containing only phosphoric acid and zinc ions, the reaction on an oxide-free iron surface is: Fe(s) + 2H + ~ Fe z+ + HE(g). (1) The p H value is increased in the solution adjacent to the metal surface. The phosphating solution is normally saturated with zinc phosphate. As a result of the increased p H value, the solubility product of zinc phosphate will be exceeded and hopeite will be precipitated: 3Zn 2+ + 2H2PO4 + 4 H 2 0 ~ Zn3(PO4)2" 4H20(s) + 4H ÷.
(2)
On steel, phosphophyllite can also be precipitated: Fe 2÷ + 2Zn 2÷ + 2H2PO4 + 4H20--~ FeZn2(PO4)z- 4H20(s) + 4H ÷.
(3)
The most important reaction in the zinc phosphating process is the nucleation of zinc phosphate crystals. The number of nuclei formed determines the properties of the zinc phosphate coating. A large number of nuclei results in a fine-grained coating, whereas a small number of nuclei yields a coarse coating that takes a long time to complete. 4° If nuclei are to be formed, the solution has to be supersaturated with zinc phosphate. The more supersaturated the solution, the more nuclei formed. Hence, if the hydrogen ion consuming reactions on the steel surface can be speeded up, this results in a faster increase o f p H and thus in a more supersaturated solution.
The influence of steel temperature The solution near the metal surface will be cooled if a cold steel object is immersed in the phosphating bath. Since the solubility of zinc phosphate increases with decreasing temperature, the solution will no longer be saturated with zinc phosphate. Furthermore, the rates of the chemical reactions will be slower as a result of the decrease in temperature. Thus, preheating the steel to the temperature of the phosphating bath should be beneficial for the nucleation process. This is indeed the case as can be seen in Fig. 2(a) and (b).
The role of accelerators in the zinc phosphating process The rate of consumption of hydrogen ions at the metal surface can be increased by addition of accelerators to the phosphating bath, for example nitrate, nitrite,
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FIG. 2. Scanning electron micrographs of zinc phosphated steel samples. Samples (a)-(e) were polished, activated, dipped in 0.1 M Zn(NO3) 2 for 5 s, preheated in a water bath at 60°C for 30 s and were then phosphated for 3 min, except for sample (a) which was cooled in a water bath at 25°C for 30 s after the preheating but before phosphating. The zinc phosphating was accelerated using: (a) and (b) nitrate and nitrite; (c) nitrate; (d) no accelerator; and (e) nitrate and hydrogen peroxide. Sample (f) is a spray phosphated sample using nitrate and nitrite as accelerators. 639
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FI~. 6. Scanning electron micrographs of samples phosphated for 1 min using nitrate and nitrite as accelerators. The samples had been oxidized at: (a) 125°C; (b) 155°C; (c) 165°C; (d) 175°C; (e) 185°C; (f) 215°C; (g) 245°C; and (h) 155°C. The oxide layer was forced to dissolve after 1 min in the phosphating bath, except in case (h) where it was dissolved as soon as the sample was immersed in the bath.
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F:G. 7. Scanning electron micrographs of samples phosphated for 1 min without accelerators. The samples had been oxidized at: (a) 125°C; (b) 155°C; (c) 185°C and (d) 215°C. After activation, the samples were dipped in 0.1 M Zn(NO3)2 for 5 s, preheated in a water bath at 60°C for 30 s and the oxide layer was then forced to dissolve as soon as the samples were immersed in the bath.
644
Zinc phosphate conversion coating
645
chlorate and h y d r o g e n peroxide. T h e s e are oxidizing agents that increase the pickling rate of the steel. T h e reaction with nitrate can be written: 4a-43 4Fe(s) + N O B + 10H + ~ 4Fe 2+ + NH~- + 3H2 O.
(4)
Table 1 presents the effect of various accelerators on the pickling rate of steel, and Figs 2 and 3 show the effect on the zinc phosphating. Nitrite and h y d r o g e n peroxide have a m u c h greater effect than nitrate, and the c o m b i n a t i o n of nitrate with nitrite or h y d r o g e n peroxide yields a synergistic effect. H o w e v e r , the strong oxidizing p o w e r of h y d r o g e n peroxide limits the c o n c e n t r a t i o n at which it can be used. T h e fast oxidation of dissolved Fe 2÷ to Fe 3+ by h y d r o g e n peroxide may result in precipitation of a m o r p h o u s i r o n ( I l l ) p h o s p h a t e on the steel surface. In addition to the e n h a n c e m e n t of the nucleation, the higher pickling rate of the steel also results in a faster g r o w t h of the zinc p h o s p h a t e crystals and thereby a
TABLE 1.
T H E INFLUENCE OF ACCELERATORS ON THE PICKLING RATE OF
COLD-ROLLED STEEL AT 6 0 ° C IN A SOLUTION CONTAINING 1 6 0 m M H 3 P O 4 . THE
pH WAS
ADJUSTED TO 2 . 5 AT 2 5 ° C BEFORE THE EXPERIMENTS
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FIG. 3. Potential-time curves recorded during phosphating samples (b)-(d) in Fig. 2. The ripples in the curves are caused by the movement of the samples in the phosphating bath.
646
P.-E. TEGEHALL and N.-G. VANNERBERG 2OO
0
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Changes in the surface potential during phosphating a sample oxidized at 155°C.
shorter phosphating time. This can be seen in the potential-time curves as a faster increase of the potential (Fig. 3). The appearance of the zinc phosphate coating is also affected by the accelerators (Fig. 2). Hydrogen peroxide yields coatings with an appearance similar to spray phosphated coatings. The reason for this is probably that oxygen from the air is used as accelerator in the spray phosphating process.42 The reactions caused by hydrogen peroxide can be expected to be similar to those caused by oxygen. In the absence of accelerators, very regular crystals are formed, owing to the slow growth rate.
The influence of iron oxides on the result of zinc phosphating A typical potential-time curve for phosphating of an oxidized sample is shown in Fig. 4. As can be seen from the curve, the iron oxide layer is not dissolved until after a certain time has passed, known as the autoreduction t i m e (r). 36 The samples had to be taped, otherwise the oxide layer was immediately dissolved. The autoreduction time for samples oxidized at various temperatures is reported in Table 2. It can be noted that r is shortest for the samples with a- and y-Fe203 mixed
TABLE 2.
AUTOREDUCTION TIME FOR SAMPLES OXIDIZED AT VARIOUS TEMPERATURES
O x i d a t i o n t e m p e r a t u r e (°C) A u t o r e d u e t i o n t i m e (s)
125 -70
155
165
-120
-180
175 -70
185 -60
215
245
-120
-450
Zinc phosphate conversion coating
647
in the oxide layer. It was observed that for samples with only v-Fe203 in the oxide layer, r varied with the humidity of the air during the preparation of the samples. The higher the humidity, the longer the time needed for the autoreduction. A possible explanation is that in the presence of water molecules in the air, HFe508 is formed, 44 but that is beyond the scope of this investigation and is not discussed further. The iron oxides can be forced to dissolve by changing the surface potential in the cathodic direction. This is most easily done by connecting the sample to a polished iron surface which is immersed in the phosphating bath directly before the connection is made. The properties of the phosphate coating varied with the distance from the contact point if the connection was made on the surface of the sample. This could be avoided by making the connection via the copper wire taped to the backside of the sample. If the connection is made 1 min after the sample is immersed in the phosphating bath, both a- and 7-Fe203 are immediately dissolved at a very high rate. A connection time of fractions of a second is enough to start an electrochemical dissolution of the oxide layer. The surface potential then drops from about + 150 mV to below - 6 0 0 mV(SCE) in <1 s when the surface layer consists of ~-Fe203 or a mixture of a- and v-Fe203, and in a few seconds when the surface layer consists of cz-Fe203. If the connection is made before the steel sample has been heated to the temperature of the phosphating solution, the oxide layer is considerably more difficult to dissolve, owing to the lower temperature of the surface. In this case a connection time of up to 5-10 s is needed to make the electrochemical dissolution continue by itself when the connection is broken. The result is the same, independent of the composition of the oxide layer. These results show that the chemical dissolution of the iron oxides is very slow. The oxide layer is not uniform. At some points the oxide layer is dissolved more quickly, and free iron surfaces are exposed. These points can be inclusions of other iron oxides, easily dissolved oxides of other elements or scratches in the oxide layer. The free iron surfaces exposed result in an electrochemical dissolution of the rest of the oxide layer: Fe(s) + Fe203 + 6H + -+ 3Fe 2+ + 3H20.
(5)
This reaction consumes hydrogen ions and, thus, also yields a supersaturation of the solution. To achieve a quick increase in p H value, the dissolution of the oxide layer should be as fast as possible. The rate-limiting reaction when the electrochemical dissolution starts is the anodic reaction (oxidation of iron) since the anode area is very small, as compared with the cathode area. Hence, the nucleation can be expected to be better on a surface with mixed oxides in the oxide layer, i.e. an oxide layer with many weak points. Figure 5 shows that this is indeed the case. The most nuclei are formed on the samples with a- and v-Fe203 mixed in the oxide layer. If the oxide layer was forced to dissolve by connection to a polished iron surface, the anodic reaction was no longer rate limiting. The largest number of nuclei was then obtained for the samples with only }'-Fe20 3 in the oxide layer (Fig. 6). This implies that the electrochemical dissolution of y-Fe203 is faster than that of a-Fe203. If they are dissolved at the same rate, the samples with a-Fe203 should give a larger increase of pH since the oxide layer is thicker on these than on samples with v-Fe20 3. If the edges of the sample are not covered, they will be excellent anodic areas when the oxide layer dissolves, and the composition of the phosphate coating will
648
P.-E. TEGEHALLand N.-G. VANNERBERG
vary with the distance from the edges. Therefore, it is important to protect the edges from being in contact with the phosphating solution when phosphating experiments are performed. Both the forced dissolution and the autoreduction of the oxide layers consisting of v-Fe203, yielded larger numbers of nuclei the thicker the oxide layer was. This is expected, since a dissolution of a thicker oxide layer should yield a larger increase of pI-L As before, if the steel sample was cold when it was immersed in the phosphating bath, far fewer nuclei were formed if it was not allowed to be heated to the bath temperature before the oxide layer was forced to dissolve (Fig. 6h). Since the oxide layer is reduced during the dissolution, oxidizing agents should not increase the rate of the dissolution. Hence, if the nuclei are formed during the period of the dissolution of the oxide layer, the number of nuclei formed should not be less if the oxidizing agents were excluded from the phosphating solution. Figure 7 presents the result of phosphating with no accelerators. A comparison with Fig. 6 shows that the number of crystals is about the same. However, the rate of growth of the crystals is slower in the absence of accelerators. Therefore, it takes a longer time to form a completely covering phosphate coating (Fig. 8). After being phosphated, the phosphate coatings on the samples were dissolved in 5% CrO3 .1 There were no signs of residual oxide on any of the samples.
Activation with colloidal titanium phosphate The most effective way to facilitate nucleation of crystals is to add nucleation agents. This is done in the phosphating process by activation with colloidal titanium phosphate. The colloidal particles are adsorbed on the metal surface during the activation and then act as crystal nuclei in the phosphating s t a g e y Without activation, very few zinc phosphate crystals are formed. 5'25'29 The amount of titanium phosphate remaining on the steel surface after activation in the colloidal solution, immersion in 0.1 M Zn(NO3)2 and then rinsing in water, was too low to detect titanium using ESCA (electron spectroscopy for chemical analysis). On samples covered with oxide layers with different compositions, the number of titanium phosphate particles adsorbed on the surface may have varied. The investigations of the influence of the steel temperature and the effect of addition of accelerators were performed using samples treated in the same way before being phosphated, i.e. the amount of titanium phosphate adsorbed on the surface was the same on the samples. The result of phosphating these samples clearly shows that the rate of increase of pH is crucial for the number of nuclei formed (Fig. 2). Furthermore, a comparison of the number of crystals formed on samples covered with y-Fe203, which have been oxidized at the same temperature, shows that forced dissolution of the oxide layer yields a more fine-grained coating than dissolution by autoreduction (Figs 5 and 6). Thus, a fast increase in pH value is necessary to utilize the adsorbed titanium phosphate particles as nucleation agents. Figure 9 shows the change in surface potential after the oxide layer has been forced to dissolve on samples oxidized at 155°C. For the sample without activation it takes a few seconds before the minimum is reached, whereas for the sample with activation the minimum is reached within 1 s. Furthermore, the potential decreases to a lower value before it increases for the sample with activation and the rate of increase is then faster. A possible explanation for the difference could be that without activation an amorphous phosphate layer is first formed at the steel surface.
Zinc phosphate conversion coating
649
-300
- 4OO
- 500
b
-6O0
I
I
1
I
0
I
2
3
t
(rnin)
Fro. 8. Potential-time curves recorded for samples oxidized at 155°C and phosphated (a) with nitrate and nitrite as accelerators and (b) without accelerators. The pretreatment of the samples was as described in Fig. 7.
-3O0
-4O0 0
> E lad - - 5 0 0
-6o(]
I 0
1
I
I
I0
20
30
(s)
Fro. 9. Potential-time curves showing the change in surface potential during the first 30 s of phosphating after the oxide layer has been forced to dissolve on samples oxidized at 155°C: (a) with and (b) without activation.
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P.-E. TEGEHALLand N.-G. VANNERBERG
After a few seconds, zinc phosphate crystals start to form and the potential then increases. If the surface has been activated, zinc phosphate crystals are formed immediately and no amorphous phosphate will be formed near the crystal nuclei. Thus, the potential will decrease to a lower value. It will also start to rise more quickly since phosphate crystals are formed earlier in the process and, as a consequence of the larger number of crystals, the rate of increase will be greater.
Zinc phosphating of annealed cold-rolled steel Pickling in acid before phosphating is known to decrease the number of zinc phosphate nuclei formed on the surface. 19,21,45 The reason for this is that the oxide film is dissolved during the pickling. Re-oxidation at 155°C before phosphating restores the phosphatability of the steel. Thus, the original oxide film on annealed steel is important for the formation of a fine-grained coating. During the annealing of cold-rolled steel, manganese oxide is enriched in grains on the surface. 15,40,46,47 According to Maeda, 5'4s the manganese oxide shortens the autoreduction time of the oxide film in a neutral borate-chloric acid buffer solution. Hence, the enriched manganese oxide is obviously beneficial for fast dissolution of the oxide film of y-Fe20 3. The addition of divalent manganese cations, as well as calcium and nickel cations, to the phosphating bath is known to refine the crystal size in the phosphate coating. 5'1°'26-3° The divalent cations are included in the crystal structure. It is likely that the manganese ions from the oxide layer are included in the phosphate layer and, thereby, affect the crystal size. From zinc phosphating of zinc surfaces, it is known that some ferrous ions have to be added to the bath to obtain good phosphating, 4'23 implying that ferrous ions may also have a refining effect. Passivation with chromium containing solutions Restrictions in the use of hexavalent chromium have caused a decrease in the use of chromic acid for passivation after phosphating. Instead, solutions containing a mixture of chromium(VI) and chromium(III) or chromium(III) only have been used. 31-33 The chromate solutions have been believed to cause a passivation of the metal surfaces between the phosphate crystals. Solutions of chromium(III) cannot have this effect. It is doubtful that the beneficial effect is caused by a passivation of the free metal surfaces. Since the chromium-containing solutions are acidic, one effect of the passivation could be a dissolution of the parts of the phosphate coating which are easiest to dissolve, for example protruding crystals. Less zinc phosphate is then dissolved during the electrodeposition of the paint. Cheever has shown that chromium atoms can be found on the passivated surface, and that they are evenly distributed over the surface and not only between the phosphate crystals. 49 According to van Ooij, 9 chromium(III) can be found on the surface of the phosphate crystals. To study if the zinc ions in hopeite can be ion exchanged with chromium(III) the following experiment was performed. Hopeite crystals, precipitated from a phosphating solution, were added to a 0.1 M solution of Cr(NO3) 3 while stirring. The pH value of the solution increased from 2.6 to 3.6 during the first minute and then slowly fell to 3.3. After a few minutes the crystals had a faint violet colour, and after 1 h only Cr(PO4) •6H20 could be detected in an X-ray diffractogram of the crystals. Thus, the hopeite crystals were completely converted to chromium phosphate in less than 1 h. The Cr(NO3)3 solution contains the violet hydrated complex [Cr(H20)6] 3+. The
Zinc phosphate conversioncoating
651
violet colour of the chromium phosphate formed indicates that the chromium ion is still hydrated with six water ligands in the solid phase. At 150°C the chromium phosphate was dehydrated and a green phase was formed. If phosphoric acid was added to a 0.1 M Cr(NO3) 3 solution to make a 1 M solution of H3PO 4 and the pH was adjusted to 3.0, the colour of the solution slowly turned green. The change of colour implies that a chromium phosphate complex was formed. The slow formation of the complex is due to the very slow exchange of iigands on chromium(Ill). Addition of hopeite crystals to this green solution also resulted in an ion exchange of zinc ions, but in this case a green amorphous chromium phosphate was formed. Several days were needed for the transformation of the hopeite crystals to chromium phosphate. The violet chromium phosphate dissolved immediately if it was added to a 1 M N a O H solution. If it was added to pure water and the pH was then slowly increased, it was converted to a green phase. This phase did not dissolve in 1 M N a O H , neither did the green phase formed by ion exchange on hopeite crystals. CONCLUSIONS The reactions on cold-rolled steel in the zinc phosphating solution can be split into four major reactions: (i) (ii) (iii) (iv)
Dissolution of the oxide film on the metal surface Pickling of the metal surface in the acidic solutions Nucleation of zinc phosphate crystals on the surface Growth of the phosphate crystals.
The most important reaction in controlling the thickness of the phosphate coating is the nucleation of phosphate crystals. To obtain a large number of nuclei, and thereby a fine-grained coating, the increase in p H value at the metal surface should be as fast as possible during the first seconds of the formation of the phosphate coating. Besides activation, the nucleation of phosphate crystals is controlled by the dissolution rate of the oxide layer. The best result is obtained if the oxide layer consists of ~-Fe20 3 mixed with an easily dissolved oxide such as manganese oxide. The iron oxide is then electrochemically dissolved at a very fast rate. The number of nuclei formed on an oxidized surface is much higher than on an oxide free surface. The steel has to be preheated to the temperature of the phosphating solution to completely utilize the beneficial effect of the fast dissolution of the oxide film. Accelerators added to the phosphating solution increase the pickling rate of the steel. This yields a faster growth of the zinc phosphate crystals and, thereby, a shorter time is necessary for the formation of a covering phosphate coating. It also enhances nucleation on areas where the oxide layer has been removed before phosphating. The zinc ions in hopeite can be ion exchanged with chromium(III). The composition of the chromium(III) phosphate formed depends on the ligands on the chromium(III) ion in the water solution. REFERENCES
]. 2. 3. 4.
S. SPRING,Metal Finish. 63(5), 61 (1965). J. I. MAURERand R. M. LACY,Adv. Chem. Set. 119, 7 (1973). B. A. COOKEand N. M. NESS,Trans. Inst. Metal Finish, 57,155 (1979). K. WooDsand S. SPRING,Metal Finish. 77(3), 24 and (4) 56 (1979).
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