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Improving the corrosion resistance of PM precision parts by cathodic electrocoating
electrocoating o f f e r s greatly improved consistency of layer thickness.
Improving the Corrosion Resistance of PM Precision Parts by Cathodic Electrocoating
CATHODIC ELECTROCOATING In the a u t o m o t i v e industry, c a t h o d i c electrocoating is a well established process. Virtually all car bodies are coated with primer applied by this technique after a phosphatizing treatment, in addition other components such as car radiators are coated. The bath consists of an aqueous dispersion of mainly inorganic pigments and organic binding agents. The latter must be able to be protonized in order to form cations. The dispersion is stabilized by the positive charges. The part to be coated forms the cathode• If direct current (DC) voltage is applied, the cation primer particles are transported towards the cathode by c o n v e c t i o n , m i g r a t i o n and diffusion. Simultaneously, water is decomposed by electrolysis, setting free hydrogen at the cathode and generating a basic zone there. The - - originally positively charged - - primer particles reach the isoelectrical point and coagulate, forming a film that is already strong enough to be touched. Final hardening is attained by baking at close to 200°C. Since the conductivity of the deposited film decreases with increasing layer thickness, the process is self-regulating with respect to layer thickness, at insufficiently coated places deposition takes place preferentially. Thus, layers with excellent consistency of thickness can be obtained. Generally the coating parameters are adjusted to result in layers approximately 20 I.tm thick. Cathodic electrocoating has already been applied experimentally to sintered iron parts, but with only limited success. This is because the process was copied directly from that common for sheet metal, without considering the peculiarities of porous sintered materials.
H. Danninger, B. Blasch and G. Jangg, (Technische Universit~t Wien, Institut fiir Chemische Technologie anorganischer Stoffe, Vienna, Austria.) B. Kn6fler and R. Otto (Stolllack AG, Guntramsdorf, Austria.) For exterior automotive PM parts, such as sensor rings for anti-lock braking systems, corrosion is a problem. A variety of anti-corrosion treatments exist, but all these processes have drawbacks. By using cathodic electrocoating, a process well known in the automotive industry, PM parts can be made corrosion resistant while retaining dimensional tolerances.
For many powder metallurgy (PM) precision parts corrosion resistance is a stringent requirement, in particular for those employed in automotive applications such as sensor rings for anti-lock braking systems (1). However, corrosion resistance of sintered iron parts is generally poor, often inferior to that of comparable wrought materials, due to the inherent open porosity of all but powder forged parts. Numerous anti-corroslon treatments have therefore been tested (2), but all solutions found so far have their drawbacks. Soaking with oil containing rust inhibitors is simple and cheap but offers only limited protection. The same is true with the well-established technique of steam treatment (3) or soaking with resin (4). A highly effective method is Zn electro-plating, but the open pores require sealing - - and frequently tumbling or abrasive blasting, to remove
superficial resin layers (5) - - prior to electroplating, which increases the cost of the process. Sherardizing or chromizing (6) are possible but offer practical difficulties. Painting of PM products is easy, the porosity enhancing paint adhesion. However, the geometrical precision required is difficult to maintain, since the layer thickness obtained, e.g. by d i p p i n g or s p r a y i n g , varies considerably. Similar problems can be expected with Dacrosealing (7), which is well established for PM products, or epoxy powder coating (2). Compared to these techniques cathodic
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FIGURE 1 Open and closed porosity of sintered iron as a function of the density (Starting powder ASC 100.29). MPR November 1990
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duration
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,
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t 20
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FIGURE 2 Amperage and film formation (= layer weight) as a function of coating time.
0026-0657/90/$3.50 ©, Elsevier Science Publishers Ltd
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(4a): Zn phosphatized sheet material.
FIGURE 3 Penetration of pores by the primer, giving mechanical interlocking.
(4b): Steam treated sintered iron.
FIGURE 4 Surfaces of surface treated iron.
Thus, an endurance of only 72 hours was obtained in the standard salt spray test, before red rust became markedly visible (2). It was, therefore, regarded necessary to adapt the cathodic electrocoating process to porous materials in order to optimize their corrosion resistance.
COATING TECHNIQUE The sintered iron samples used for the tests were pressed from water atomized iron powder ASC 100.29. Various pressures were applied (200, 400, 600 and 800 MPa). The samples were then sintered in hydrogen at 1120°C. Total and open porosity were measured by
conventional displacement and infiltration techniques, they are depicted as a function of the density in Figure 1. The primer used was a standard quality product supplied by Stolllack AG (Stollaquid G 1083) used at a bath temperature of 28-30°C and an applied DC voltage of 270 V. Coating time was 3 minutes although it was found that virtually all of the paint was deposited within the first few seconds, as indicated by the effective current (Figure 2). The samples were then rinsed with water and baked in a revolving air furnace at 180°C for 20 minutes. Testing comprised layer thickness, layer weight, depth of infiltration of the pores (metallographically), corrosion resistance in a salt spray cabin (Standard SS DIN 50 021) and adhesion after corrosion testing. The latter was
done by rubbing an adhesive tape against two crossing cuttings applied before salt spray testing and then jerkily tearing the tape off.
RESULTS The experiments showed that cathodic electrocoating yields protective layers that are dense and adhere well. It was found that pre-treatments common with sheet material, such as phosphatizing, are unsuitable here, since the salts contained by the solutions tend to remain in the pores and cause corrosion. Coating of the as-sintered (if necessary, degreased) parts proved to be successful, since the porosity seems to give sufficient adhesion by mechanical interlocking (Figure 3) - - to render phosphatizing unnecessary. At low porosity levels, with the samples compacted at 800 MPa, the adhesion of the paint on as s i n t e r e d s a m p l e s was s o m e w h a t less satisfactory, some paint near the crossing cuttings was torn off after salt spray testing. Here, steam treatment was found to be an ideal solution, since the steam oxidation results in a similar, though finer, surface structure as phosphatizing (Figures 4a and 4b), giving excellent adhesion. The layer thickness was shown to be a function of the porosity of the samples. Higher porosity resulted in thinner layers, while the layer weight - - which is primarily a function of the voltage applied - - was unchanged or even higher (Figure 5). This discrepancy between layer thickness and weight is apparently due to infiltration of the pores by the primer, during coating and especially during baking. In the baking process the viscosity is initially low, to give a smooth surface finish. Lower layer thickness results in lower corrosion resistance; however, the p o r o s i t y effect becomes -
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FIGURE 5 Layer thickness plotted against layer weightfor samples of different porosity.
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FIGURE 6 Extreme case of primer pulling off from the edges of sheet material.
p r o n o u n c e d only at p o r o s i t y levels approaching 20%. In this case, steam treatment, which reduces the diameter of the pore channels, proved to be useful. Sealing of the pores prior to coating proved unnecessary; it was found that the pores are infiltrated by the liquid in the bath to only a limited extent, since the primer rapidly chokes the pores, preventing further intrusion of the liquid. As the bath consists solely of volatile components, the liquid contained by the pores is completely removed during baking. With high porosity samples, removal of the bath liquid from the pores caused blistering during baking in some cases. A pre-drying step at
100-120°C before baking completely solved this problem. Corrosion resistance of the coated samples was generally excellent provided sufficient layer thickness was obtained. It was found that the layers were thinner at the edges, since during baking the primer tends to pull away from there (Figure 6), and an average layer thickness of 15 I.tm was required to attain sufficient thickness - - and thus adequate protection - at the edges. During salt spray testing corrosive attack invariably started at the edges, however, well coated samples easily surpassed 300 hours of salt spray testing without showing any red rust. Figure 7 compares the corrosion resistance of electrocoated sintered parts with that of plain sintered iron, of steam treated material and of Zn electroplated iron; the superior performance of the electrocoated parts is evident. In addition to laboratory samples, actual automotive parts of varying density, such as camshaft belt pulleys and sensor rings, were coated and tested, in all cases excellent corrosion resistance was measured (Figure 8). In production, electrocoating requires contact with every single part, since the deposited layer is non-conducting. Coating in rotating drums, as frequently done with Zn electroplating, is therefore impossible. This means considerable handling of the parts, but automation in PM plants is so well advanced today that this handling could be done automatically. Principally, electrocoating is a well-established process in the automotive industry, the primers used are carefully optimized, and also cheap materials and considerable know-how with material and plant design is available; thus, the introduction of electrocoating for surface protection of PM
ell tl. "13
parts should not offer too many problems. Parts to be treated might comprise those at the exterior of car bodies, such as sensor tings for anti-lock brake systems, which are directly exposed to corrosive attack from the road. SUMMARY Cathodic electrocoating of PM precision parts is a simple and effective technique for improving their corrosion resistance while maintaining dimensional tolerances. In contrast to sheet material, pre-treatment such as phosphatizing is unnecessary and even detrimental. As-sintered or steam treated samples of varying porosity coated with sufficiently thick (15 I.tm) layers showed excellent corrosion resistance, easily surviving 300 hours of salt spray testing without rusting. Minor problems such as insufficient adhesion after corrosion testing or blistering during baking can easily be solved. The wide experience available with electrocoating, together with the excellent consistency of the layer thickness and good corrosion resistance obtained, should make it a promising technique for producing corrosion resistant precision parts. REFERENCES (1). M. Hanada et al, Metal Powder Rep, 44 (10) (1989), pp. 695• (2). M. J. Nash, PowderMet. 33 (1990), pp. 22. (3). J. H. Eggleston and R. D. Fisher, Progress in Powder Met. 38 (1982), pp. 613. (4). K. Mulcahy, Proc. Conf. Productivity in Powder Met., Frankfurt, MPR Publishing Services eds., Shrewsbury, UK, 1982, pp. 123. (5). H. E. Boyer, Metals Handbook Vol. 7 Powder Metallurgy, 9th Ed., ASM, Metals Park OH (1984), pp. 460. (6). S. Audisio et al, Powder Met. 27 (1984), pp. 147. (7). M. J. Narusch and G. E Pauline, Progress in Powder Met. 42 (1986), 659.
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MPR November 1990
FIGURE 8 Electrocoated camshaft belt pulley after 528 hours salt spray testing.
749
Improving the Corrosion Resistance of PM Precision Parts by Cathodic Electrocoating
CATHODIC ELECTROCOATING In the a u t o m o t i v e industry, c a t h o d i c electrocoating is a well established process. Virtually all car bodies are coated with primer applied by this technique after a phosphatizing treatment, in addition other components such as car radiators are coated. The bath consists of an aqueous dispersion of mainly inorganic pigments and organic binding agents. The latter must be able to be protonized in order to form cations. The dispersion is stabilized by the positive charges. The part to be coated forms the cathode• If direct current (DC) voltage is applied, the cation primer particles are transported towards the cathode by c o n v e c t i o n , m i g r a t i o n and diffusion. Simultaneously, water is decomposed by electrolysis, setting free hydrogen at the cathode and generating a basic zone there. The - - originally positively charged - - primer particles reach the isoelectrical point and coagulate, forming a film that is already strong enough to be touched. Final hardening is attained by baking at close to 200°C. Since the conductivity of the deposited film decreases with increasing layer thickness, the process is self-regulating with respect to layer thickness, at insufficiently coated places deposition takes place preferentially. Thus, layers with excellent consistency of thickness can be obtained. Generally the coating parameters are adjusted to result in layers approximately 20 I.tm thick. Cathodic electrocoating has already been applied experimentally to sintered iron parts, but with only limited success. This is because the process was copied directly from that common for sheet metal, without considering the peculiarities of porous sintered materials.
H. Danninger, B. Blasch and G. Jangg, (Technische Universit~t Wien, Institut fiir Chemische Technologie anorganischer Stoffe, Vienna, Austria.) B. Kn6fler and R. Otto (Stolllack AG, Guntramsdorf, Austria.) For exterior automotive PM parts, such as sensor rings for anti-lock braking systems, corrosion is a problem. A variety of anti-corrosion treatments exist, but all these processes have drawbacks. By using cathodic electrocoating, a process well known in the automotive industry, PM parts can be made corrosion resistant while retaining dimensional tolerances.
For many powder metallurgy (PM) precision parts corrosion resistance is a stringent requirement, in particular for those employed in automotive applications such as sensor rings for anti-lock braking systems (1). However, corrosion resistance of sintered iron parts is generally poor, often inferior to that of comparable wrought materials, due to the inherent open porosity of all but powder forged parts. Numerous anti-corroslon treatments have therefore been tested (2), but all solutions found so far have their drawbacks. Soaking with oil containing rust inhibitors is simple and cheap but offers only limited protection. The same is true with the well-established technique of steam treatment (3) or soaking with resin (4). A highly effective method is Zn electro-plating, but the open pores require sealing - - and frequently tumbling or abrasive blasting, to remove
superficial resin layers (5) - - prior to electroplating, which increases the cost of the process. Sherardizing or chromizing (6) are possible but offer practical difficulties. Painting of PM products is easy, the porosity enhancing paint adhesion. However, the geometrical precision required is difficult to maintain, since the layer thickness obtained, e.g. by d i p p i n g or s p r a y i n g , varies considerably. Similar problems can be expected with Dacrosealing (7), which is well established for PM products, or epoxy powder coating (2). Compared to these techniques cathodic
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t,o
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6/*
6.8
Density
72 ~Mglm 3
0,0
, O
FIGURE 1 Open and closed porosity of sintered iron as a function of the density (Starting powder ASC 100.29). MPR November 1990
,
I 10
duration
,
,
of electrocoating
t 20
30 [s)
FIGURE 2 Amperage and film formation (= layer weight) as a function of coating time.
0026-0657/90/$3.50 ©, Elsevier Science Publishers Ltd
747
L "! g)'¢ t X~ j ,
." I
. . . . .x
I
I
lO0gm
(4a): Zn phosphatized sheet material.
FIGURE 3 Penetration of pores by the primer, giving mechanical interlocking.
(4b): Steam treated sintered iron.
FIGURE 4 Surfaces of surface treated iron.
Thus, an endurance of only 72 hours was obtained in the standard salt spray test, before red rust became markedly visible (2). It was, therefore, regarded necessary to adapt the cathodic electrocoating process to porous materials in order to optimize their corrosion resistance.
COATING TECHNIQUE The sintered iron samples used for the tests were pressed from water atomized iron powder ASC 100.29. Various pressures were applied (200, 400, 600 and 800 MPa). The samples were then sintered in hydrogen at 1120°C. Total and open porosity were measured by
conventional displacement and infiltration techniques, they are depicted as a function of the density in Figure 1. The primer used was a standard quality product supplied by Stolllack AG (Stollaquid G 1083) used at a bath temperature of 28-30°C and an applied DC voltage of 270 V. Coating time was 3 minutes although it was found that virtually all of the paint was deposited within the first few seconds, as indicated by the effective current (Figure 2). The samples were then rinsed with water and baked in a revolving air furnace at 180°C for 20 minutes. Testing comprised layer thickness, layer weight, depth of infiltration of the pores (metallographically), corrosion resistance in a salt spray cabin (Standard SS DIN 50 021) and adhesion after corrosion testing. The latter was
done by rubbing an adhesive tape against two crossing cuttings applied before salt spray testing and then jerkily tearing the tape off.
RESULTS The experiments showed that cathodic electrocoating yields protective layers that are dense and adhere well. It was found that pre-treatments common with sheet material, such as phosphatizing, are unsuitable here, since the salts contained by the solutions tend to remain in the pores and cause corrosion. Coating of the as-sintered (if necessary, degreased) parts proved to be successful, since the porosity seems to give sufficient adhesion by mechanical interlocking (Figure 3) - - to render phosphatizing unnecessary. At low porosity levels, with the samples compacted at 800 MPa, the adhesion of the paint on as s i n t e r e d s a m p l e s was s o m e w h a t less satisfactory, some paint near the crossing cuttings was torn off after salt spray testing. Here, steam treatment was found to be an ideal solution, since the steam oxidation results in a similar, though finer, surface structure as phosphatizing (Figures 4a and 4b), giving excellent adhesion. The layer thickness was shown to be a function of the porosity of the samples. Higher porosity resulted in thinner layers, while the layer weight - - which is primarily a function of the voltage applied - - was unchanged or even higher (Figure 5). This discrepancy between layer thickness and weight is apparently due to infiltration of the pores by the primer, during coating and especially during baking. In the baking process the viscosity is initially low, to give a smooth surface finish. Lower layer thickness results in lower corrosion resistance; however, the p o r o s i t y effect becomes -
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.,
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,
,
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lager Nelght [ng/r~')
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,
I
10
FIGURE 5 Layer thickness plotted against layer weightfor samples of different porosity.
748
10pro
-
MPR November 1990
FIGURE 6 Extreme case of primer pulling off from the edges of sheet material.
p r o n o u n c e d only at p o r o s i t y levels approaching 20%. In this case, steam treatment, which reduces the diameter of the pore channels, proved to be useful. Sealing of the pores prior to coating proved unnecessary; it was found that the pores are infiltrated by the liquid in the bath to only a limited extent, since the primer rapidly chokes the pores, preventing further intrusion of the liquid. As the bath consists solely of volatile components, the liquid contained by the pores is completely removed during baking. With high porosity samples, removal of the bath liquid from the pores caused blistering during baking in some cases. A pre-drying step at
100-120°C before baking completely solved this problem. Corrosion resistance of the coated samples was generally excellent provided sufficient layer thickness was obtained. It was found that the layers were thinner at the edges, since during baking the primer tends to pull away from there (Figure 6), and an average layer thickness of 15 I.tm was required to attain sufficient thickness - - and thus adequate protection - at the edges. During salt spray testing corrosive attack invariably started at the edges, however, well coated samples easily surpassed 300 hours of salt spray testing without showing any red rust. Figure 7 compares the corrosion resistance of electrocoated sintered parts with that of plain sintered iron, of steam treated material and of Zn electroplated iron; the superior performance of the electrocoated parts is evident. In addition to laboratory samples, actual automotive parts of varying density, such as camshaft belt pulleys and sensor rings, were coated and tested, in all cases excellent corrosion resistance was measured (Figure 8). In production, electrocoating requires contact with every single part, since the deposited layer is non-conducting. Coating in rotating drums, as frequently done with Zn electroplating, is therefore impossible. This means considerable handling of the parts, but automation in PM plants is so well advanced today that this handling could be done automatically. Principally, electrocoating is a well-established process in the automotive industry, the primers used are carefully optimized, and also cheap materials and considerable know-how with material and plant design is available; thus, the introduction of electrocoating for surface protection of PM
ell tl. "13
parts should not offer too many problems. Parts to be treated might comprise those at the exterior of car bodies, such as sensor tings for anti-lock brake systems, which are directly exposed to corrosive attack from the road. SUMMARY Cathodic electrocoating of PM precision parts is a simple and effective technique for improving their corrosion resistance while maintaining dimensional tolerances. In contrast to sheet material, pre-treatment such as phosphatizing is unnecessary and even detrimental. As-sintered or steam treated samples of varying porosity coated with sufficiently thick (15 I.tm) layers showed excellent corrosion resistance, easily surviving 300 hours of salt spray testing without rusting. Minor problems such as insufficient adhesion after corrosion testing or blistering during baking can easily be solved. The wide experience available with electrocoating, together with the excellent consistency of the layer thickness and good corrosion resistance obtained, should make it a promising technique for producing corrosion resistant precision parts. REFERENCES (1). M. Hanada et al, Metal Powder Rep, 44 (10) (1989), pp. 695• (2). M. J. Nash, PowderMet. 33 (1990), pp. 22. (3). J. H. Eggleston and R. D. Fisher, Progress in Powder Met. 38 (1982), pp. 613. (4). K. Mulcahy, Proc. Conf. Productivity in Powder Met., Frankfurt, MPR Publishing Services eds., Shrewsbury, UK, 1982, pp. 123. (5). H. E. Boyer, Metals Handbook Vol. 7 Powder Metallurgy, 9th Ed., ASM, Metals Park OH (1984), pp. 460. (6). S. Audisio et al, Powder Met. 27 (1984), pp. 147. (7). M. J. Narusch and G. E Pauline, Progress in Powder Met. 42 (1986), 659.
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a &. LJ at
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0 L,I Q ~m ql cJ
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.
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50
.
.
.
.
I
t
100
5O0
.
.
.
.
1001)
duration of salt spraR test [hi FIGURE 7 Corrosion during salt spray testing of differently treated PM iron parts.
MPR November 1990
FIGURE 8 Electrocoated camshaft belt pulley after 528 hours salt spray testing.
749