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Corrosion behavior of decorative car parts and outdoor lighting fixtures fabricated from TiN-coated stainless steel
Surface
and Coatings
Technology,
67 ( 1994) 85-93
85
Corrosion behavior of decorative car parts and outdoor lighting fixtures fabricated from TiN-coated stainless steel* K. A. Pischowa, L. Erikssona, A. S. Korhonena, “Laboratory bLaboratory ‘Uniuersity (Received
of Processing and Heat Treatment ofMaterials, of Corrosion and Material Chemistry, Helsinki of Florida. Gainesville. FL3262 1 (USA) December
6, 1993; accepted
in final form January
Helsinki Unioersity
0. ForsCnb, M. Turkiab and E. 0. Ristolainen” Unit>ersity of Technology, Vuorimiehentie 2A. 02150 Espoo (Finland) of Technology, Vuorimiehentie 2A. 02150 Espoo (Finland)
30, 1994)
Abstract The fabrication of TiN-coated decorative car parts and lighting fixtures by deep drawing investigated. Corrosion tests in real and laboratory environments were performed and either before or after forming were compared with uncoated stainless steel hub caps. The were studied, before and after the corrosion tests, using secondary-ion mass spectrometry
1. Introduction TiN is a well-studied standard coating in wearresistant and cutting tool applications. Owing to its golden color it is also well suited for decorative purposes. The porosity of the coating is the biggest drawback, causing hard pitting corrosion attack on the substrate in severe environments. One other disadvantage is the coating process itself, which is fairly expensive and in most cases allows only a limited range of sizes. The possibility of coating a stainless steel strip [ 1,2] continuously and of forming the parts afterwards would markedly cut down the production costs. Our preliminary experiments [ 33 with the formability and corrosion resistance of TiN-coated stainless steel sheet have shown that the adherence of the coating after the forming process is excellent and that the corrosion resistance is not worsened from what is achieved using a similarly formed stainless steel sample. Accordingly, it seems to be possible to use TiN-coated stainless steel parts, which have been formed to the desired design after the coating process, at least for decorative purposes in not too severe environments. The aim of this study was to test decorative car parts in the winter environment in Finland, which includes a lot of salt and sand for melting the ice and increasing the friction on the slippery winter roads. This forms a corrosive and wearing environment for the car parts. *Paper presented Metallurgical Coatings 19-23, 1993.
at the 20th International and Thin Films, San Diego,
Conference CA, USA,
on April
from a TiNcoated stainless steel plate were the behavior of car wheel hub caps coated micro- and surface structures of the hub caps and scanning electron microscopy.
The tested parts were hub caps fabricated from a TiNcoated stainless steel sheet by hydraulic bulging. For applied corrosion and wear tests they were mounted on a test car. Air pollution damages many materials, either directly through atmospheric corrosion, or indirectly through the acidification of soil water. In Scandinavia atmospheric corrosion is primarily due to local emissions, so the damage is greater in urban than in rural areas. Although sulfur dioxide is the chief agent, nitrogen oxides, ozone and acid rain or snow also contribute, which means that synergistic effects are common. Laboratory corrosion tests simulating the atmospheric corrosion due to the acidifying air pollutants [4] on outdoor lighting fixtures were also accomplished. The tested surfaces were inspected using secondary-ion mass spectrometry (SIMS) and scanning electron microscopy (SEM).
2. Experimental
details
The hub cups shown in Fig. 1 were made of an AISI 304 sheet with a thickness of 0.8 mm. The material was factory bright annealed with a surface roughness of R, = 0.05 mm. The first set of samples was coated before, and the second one after, the forming procedure. The forming was performed using a biaxial bulging test apparatus. Two different pressures were used and thus two different dome heights and deformations were produced, as can be seen in Table 1.
Fig. 1. TiN-coated hub cups made of an AISI 304 sheet with a thickness OT 0.8 mm by deep drawing: left. first coated and then deformed; right, first deformed and then coated.
TABLE
1. Sample
preparation
Sample
c + F”
134 134B 170 170B
+
schedule F+Cb
+ + +
Deformation level 134 134 170 170
“TIN coating, then forming. bForming, then TIN coating.
Fig. 2. Two of the test samples Chevrolet van.
were mounted
on the rear wheels of a
2.1. Applied corrosion and wear tests
Two of the test samples were mounted on the rear wheels of a Chevrolet van, as shown in Fig. 2. Two additional samples were bolted inside the rear fenders, where the major flux of sand and salt from the wheels hits. The testing time was 6 months. 2.2. Laboratory
corrosion tests
To select the best materials for in-service use, a fast and reliable laboratory test is needed to minimize the in situ testing. This laboratory test time-consuming should be based on the vehicle “microclimates” in which the materials are exposed. As a simulation of these “microclimates”, several standard accelerated laboratory tests are in use, e.g. the neutral salt spray test (ASTM standard B-117). Unfortunately, there is in general a lack of correlation between salt spray test results and corrosion behavior in service [ 51. 2.2.1. Accelerated corrosion
testing
Electrochemical measurements were used to evaluate the porosity and general corrosion behavior of coldformed titanium-nitride-coated AISI 304 stainless steels. The actual salt concentration on the surface of running cars is not known in practice. Therefore the road envi-
ronment pollution (REP) solution was used as an electrolyte in the accelerated corrosion tests (Table 2). The REP should represent polluted (more real) road environments with a variety of occurring pollutants [ 61. The pH of the REP solution was 4.7 and the pH of authentic rain water in the Espoo area (southern part of Finland) was 4.4. The pH of drinking water is 6.5-8.0 [ 71. Redox potential us. a Pt calomel electrode was - 358 mV for the REP solution and -330 mV for authentic rain water, which means that in the pH range 4-5 TIN is stable and Fe dissolves according to Pourbaix diagrams. TABLE
2. The contents
Compound
Sodium Calcium Sodium Sodium Sodium Sodium Deonized
chloride chloride nitrate sulfate sulfite nitride water
of 1 I REP solution Formula
Content (mg I-‘)
NaCl CaCl, NaNO, Na,SO, Na,SO, NaNO,
30 20 0.3 0.3 0.5 0.25 Balance
H,O
The experiments were performed at ambient temperature. All potentials were measured 1)s. the saturated calomel electrode (SCE) and platinum plate was used as a counterelectrode. The surface areas of the test specimens were 0.79 cm’. After the tests each specimen was examined by optical microscopy. Short-term electrochemical measurements were made with a computer-controlled measurement system developed at the Helsinki University of Technology [8,9]. The system uses an NF2000 potentiostat and an NF5050 frequency response analyzer. The measurement software is written in BASIC and data processing is carried out with commercial software packages. The short-term measurement procedure for coated samples consisted of two diverse measurements: long cyclic anodic polarization (LCAP) and potentiostatic exertion (PE). Anodic polarization curves were produced by the potentiostatic polarization method using 10 mV steps at 6 s intervals. The polarization scan started at -500 mV us. SCE, and at 500 mV LIS.SCE the polarization changed direction towards the starting point, unless the current density first reached the threshold current density 5 mA cm-‘, when the polarization scan direction was automatically reversed. In the short-term potentiostatic measurement using (using PE) the determination of i us. r curves was made using a polarization of 200 mV vs. SCE for 3600 s. The potential used in PE was selected on the basis of anodic polarization measurements. 2.3. SIMS analysis Depth profiles were produced with SIMS PHI 6600 using an O+ gun with 5 keV primary high voltage and 1 uA current. The analyzed area was 300 x 300 urn’. For the point analysis a Ga+ gun with 25 keV primary high voltage and 0.1 nA current was used.
3. Results 3.1. Applied corrosion and wear test The problems caused by adding a noble coating such as TIN to a corrosion system can be seen in Fig. 2 (bottom part). After remounting the hub caps from the test car they were washed in water and alcohol and dried in hot air. Figure 3 shows two hub cups from the rear wheels: the hub cap on the left was coated before the deep drawing and the one on the right after. No decohesion of the coating nor visible pit corrosion attack can be visually detected. Figure 4 shows the two hub cups which were mounted inside the rear fender. Even here no decohesion nor corrosion pits can be seen. Furthermore, there do not
Fig. 3. Two hub cups from the rear wheels; the left-hand one is coated before the deep drawing and the right-hand one after. No decohesion of the coating nor visible pit corrosion attack can be detected.
seem to be any hit marks of the sand.
of the stones nor wear marks
3.1 .I. Optical microscopy The microscopic inspection with an optical microscope confirmed the above-mentioned observations. No signs of decohesion were found; however, some pinholes were clearly visible in samples first deformed and then coated and some broadening of the cracks was visible on sample 170B which had the higher deformation level. 3.1.2. Scanning electroil microscopy SEM analysis of the applied corrosion and wear test samples that were first TIN coated and then formed (C + F) is shown in Fig. 5. The typical surface structure of a sample being formed to the higher deformation level can be seen in Fig. 5(a). Some of the cracks are very broad in this area. However, energy-dispersive X-ray (EDX) analysis showed high titanium and also iron and chromium contents, which indicates that the surface is not in all places completely covered by the TIN coating. The same sample after the applied corrosion and wear test, on the back ring, in Fig. 5(b) does
3.1.3. SIMS analysis
Figure 7 shows the depth profiles of samples 134B and 134 (Figs. 7(a) and 7(b)) and samples 170B and 170 (Figs. 7(c) and 7(d)) after the applied corrosion tests. In samples 134B and 170B only titanium and nitrogen were detected. However, in samples 134 and 170 there are high iron, chromium, silicon and carbon contents in addition to titanium and nitrogen throughout the whole coating thickness. It should also be pointed out that a peak in the carbon profile occurs in all the samples at the coating-substrate interface. The point analysis from the surface of sample 134 after the applied corrosion and wear tests showed the following compounds: titanium oxides, chromium oxides, iron oxides, sodium sulfides and iron chlorides. 3.2. Laboratory
Fig. 4. The two hub cups which were mounted inside the back fender. Even here no decohesion no corrosion pits can be seen. Furthermore, there do not seem to be any hit marks of the stones nor wear marks of the sand.
not markedly differ from the untested one. Figure 5(c) shows broad cracks on which Fe and Cr were also found, indicating a crevice corrosion attack. However, even here the upper part of the crack is still partly covered with TIN. In samples formed to the lower deformation level the structure is the same as before but the width of the cracks appears to be smaller, as expected. In Fig. 5(d) a tested surface from inside the rear fender is shown. In the middle of the figure there might be a stone hit, and in this pit EDX analysis revealed iron and chromium. SEM analysis of the applied corrosion and wear test samples that were formed and then TIN coated (F+C) is shown in Fig. 6. A typical surface structure of an untested sample can be seen in Fig. 6(a). A tested sample from the left-hand rear wheel is presented in Fig. 6(b) and in greater detail in Fig. 6(c), showing the increased size of the pinholes caused by pit corrosion. Figure 6(d) shows a scratch-like feature on the surface, from which iron and chromium were analyzed using EDX, indicating a coating failure.
corrosion tests
The base material AISI 304 was in the active stage during LCAP in REP solution and the area of the hysteresis loop was negative. Undeformed TiN/AISI 304 had a short passive area between 40 and 200 mV and the area of the hysteresis loop was also negative in REP solution. After the test, no damage could be observed by optical microscopy on the base material nor on the undeformed TiN/AISI 304 sample. All deformed samples showed large positive hysteresis during cyclic anodic polarization. In Fig. 8 are shown six different LCAP curves measured in REP solution and for the sake of clarity only the forward scan is drawn. The polarization curves are divided into three classes: the class of reference materials AISI 304 and TiN/AISI 304, the class of first coated and then deformed (C + F) samples and the class of first deformed and then coated (F + C) samples. The differences between the classes are more clearly seen in the cathodic part of the LCAP curves, where the lowest cathodic current densities were measured for an undeformed but coated TiN/AISI 304 sample and the highest cathodic current densities were measured for the base material and the first coated and then highly deformed sample (170). The cathodic current densities of the first coated and then deformed samples, 170B and 134B, were between the upper and lower limit. In the anodic part of the LCAP curves the highest positive current densities were measured for the first coated and then highly deformed sample 170. Deformation increased the anodic positive current densities by one order of magnitude compared with the anodic current densities of the TiNcoated undeformed reference sample. According to the optical microscopy the corrosion mechanism was clearly different between the class of C+F samples and the class of F +C samples. The corrosion mechanism of the F+C class was pitting. A clear crevice corrosion effect was seen in the samples first coated and then formed.
K. A. Pischow
et d.
/ Corrosion
hehiorrr
of‘decorcuior
cur
Fig. 5. SEM analysis of the applied corrosion and wear test samples that were first TiN coated and an area where some of the cracks are very broad; however, EDX analysis showed high titanium indicates that the surface is not in al) places completely covered by the TiN coating. (b) The same test, on the back ring, does not markedly differ from the untested sample. (c) SEM scan showing found indicating a crevice corrosion attack. (d) SEM scan showing a tested surface from inside the
Figure 9 shows the results of PE measurements for five different corrosion systems measured in REP solution. The first and the fourth curves are for samples from the C+ F class. The second and the fifth are for samples from the F + C class. The third system was AISI 304, the base material. Galvanic corrosion between the base material and the inert TiN film plays a leading role in the general corrosion behavior of TiN-coated materials. At a potential of 200 mV us. SCE, dissolution of the base material AISI 304 is rapid. After 5 mm the current densities settled down to a constant value of 17 uA cm-‘. After the PE, strong crevice corrosion and a large pit were observed on the stainless steel.
puts
then deformed (C+F). (a) SEM scan showing and also iron and chromium contents which sample after the applied corrosion and wear broad cracks on which Fe and Cr were also rear fender.
The current densities of the first coated and then formatted sample reached the current densities of the base material after 50 min and mild crevice corrosion was observed on the sample. Current densities of some samples were even higher than the corrosion densities of the base material because of the formation of galvanic couples between the anodic base material and the cathodic TIN coating. After the PE test, these samples were clearly damaged by crevice attack, but the degree of damage was different depending on whether the sample was first coated and then formatted or vice versa. The degree of damage was higher for samples of the C + F class. The best survival was in a dense sample from the
Fig. 6. SEM analysis of the applied corrosion and wear test samples that were first formed and then TIN coated (F+ C). (a) A tested sample. (b) A tested sample from the left-hand rear wheel. (c) A higher magnification scan showing the increased size of the pinholes caused by pit corrosion. (d) A scratch-like feature on the surface from which iron and chromium were analyzed by EDX, indicating a coating failure.
F+C class. The current densities were three orders of magnitude lower than the current densities of the base material; the constant value was 1.4 uA cmm2 for the first 38 min. After that the current densities started to increase but still stayed one order of magnitude lower than the current densities of the AISI 304 base material. A slight initial crevice corrosion attack was observed by optical microscopy on the surface of the best sample.
4. Discussion Visually, the surface quality of all the tested hub cups was good and no changes in the appearance after the test period can be detected. However, the testing time is short even if it covers the most corrosive time of the
year. The wear protection is also good because no decohesion of the coating was detected. The wear properties of the formed TiN coating must, however, be confirmed with laboratory wear tests at a later stage. In optical microscopy after the applied corrosion and wear tests the pinholes appear more distinct than they were on the untested samples and this could be an indication of the beginning of pit corrosion attack. Likewise, in sample 170B, which was formed after the TIN deposition and had the higher deformation level, some broadening of the cracks is visible, pointing to a crevice corrosion attack. SEM scans and EDX analysis of the samples coated and afterwards deformed to the higher deformation value showed that, in addition to titanium, iron and chromium were also present in the broader cracks,
K. A. Pischow
loZ’ (4
t4
et al.
a
/ Corrosion
’
’
behaviour
’
’
ojdecorative
car
parts
91
7
aQD mcl
rang
Sputter limo I*1 UF-SIMS
Fig. 7. SIMS depth profiles of samples (a) 134B and (b) 134 and samples (c) 170B and (d) 170 after the applied corrosion tests.
indicating that no protection of the substrate occurred on those areas. After the applied corrosion tests on those areas some broadening of the cracks was visible and in some areas only iron and chromium were detected. The corrosion mechanism was crevice corrosion because the corrosion attack seemed to be directed to the broad cracks which were not completely protected by the coating. In this case there has not been a severe corrosion attack through the pinholes, which are thus hardly visible in the SEM pictures. The EDX analysis of the cracks in Figs. 6(b) and 6(d) shows that even the broader cracks are covered by titanium, and still after the corrosion attack the remnants of a thin Ti layer can be detected by EDX and seen on the upper part of the Fig. 6(d). This behavior indicates that the Ti interlayer is able to deform plastically between the substrate and TiN coating, thus maintaining the adhesion of the broken TiN fragments and covering
the crack bottoms. The results presented by Ignat et al. [lo] reporting interfacial cracking only after an important deformation are also indicative of this. In the samples first deformed and then coated the corrosion mechanism during the applied corrosion test seems to be different. The corrosion attack occurs through the pinholes, which appear to be markedly increased in diameter and can be clearly distinguished also at lower magnifications, giving a clear indication of pitting corrosion. The SIMS analysis confirmed the aforementioned. However, the difference in favor of the first formed and then coated samples is bigger than expected. This may be due to the very local nature of the pit corrosion. The most interesting feature in the SIMS depth profiles is the behavior of the carbon. The high carbon content close to the substrate surface clearly indicates a carbide precipitation, which is quite unexpected at the
K. A.
Pischow
et al.
/ Corrosion
behaviour
of decorative
car
parts
100
10
o0
oc
c-
_/a--_.--
-
/?AIS1;04
1
C + mild F) v 3 4 E
0.1
0.01
0.001
-w--e-
0.0001 -600
-400
-200
0 E , m”
“I1.
200
400
600
SCE
0
500
1000
1500
2000
Thld*
Fig. 8. LCAP curves measured in REP solution (for clarity only the forward scan is drawn).
Fig. 9. PE test results for five different corrosion systems measured in REP solution.
coating temperature of about 500°C. It can also be seen in the SIMS depth profiles that the cold forming accelerates the precipitation process and the higher deformation level leads to increased precipitation. This precipitation sensitizes the stainless steel substrate and deteriorates the corrosion properties of the coating substrate system. The SIMS analyses from the sample surface after the applied corrosion and wear tests clearly showed that the effective corrosive elements were chlorides and sulfites. In spite of extensive work over more than 70 years on atmospheric corrosion, a number of questions remain. In particular, no satisfactory method for predicting the performance of materials in the atmosphere exists either as an on-site method or laboratory test. Long-term exposure methods require much time to determine the behavior of materials in atmospheric conditions. When the outdoor atmospheres are characterized by periodic wetting of the metallic surfaces from water condensation, relative humidities greater than 80%, fast temperature variations and precipitation, it is obvious that accelerated laboratory corrosion tests can give only a slight idea about what is going to happen in the real
environment. The constant wet environment of the accelerated tests differs significantly from the real situation, which is estimated in southern Scandinavia [ 111 to be 2750-4100 h wet in 1 year. Laboratory corrosion tests confirmed the corrosion mechanisms detected in the applied tests. The LCAP curves gave qualitative information about the general electrochemical behavior of the corrosion systems. Evaluating the corrosion resistance of the coated sample, it is essential to take account of the corrosion behavior of the base material, affected through pores in the coating. The rate at which the base material reacts at the bottom of the pinholes obviously depends on the number and size of these defects. Thus when the defects are long and narrow they actually decrease the reaction rates by reducing the mass transport between the bottom of a pinhole and the corrosive environment. Specimens of the C+ F class clearly suffer from crevice corrosion in REP solution, unlike specimens of the F+C class, which had plain pitting. The fact that the corrosion mechanism was converted depending on the order of the coating process and forming was clearly seen in the cathodic part of the LCAP curves.
K. A. Pischorv
et a/. / Corrosiorl
5. Conclusions
In the applied corrosion and wear tests both the coated and deformed hub caps and deformed and coated hub caps showed no indication of wear. However, the 6 months testing time is still short compared with the expected lifetime of the product. Likewise, on a macroscopic level no sign of corrosion was detected, although the microscopic investigation showed the beginning of local corrosion attack in all the samples and it was severest, as expected, in the sample that was first TIN coated and then deformed to the higher deformation level. Two different corrosion mechanisms were discovered: firstly, a crevice corrosion, in which the corrosion attack on the samples deformed after the coating procedure was directed to the broad cracks and there was no indication of pitting corrosion through the pinholes; secondly, a localized pitting corrosion in which, on the samples coated after the deformation process, a heavy corrosion attack through the pinholes was observed. This indicates that the lifetime of the samples after both fabrication processes does not differ much and that the behavior of the substrate is the critical factor in determining the lifetime of the product. The SIMS analysis clearly showed that a precipitation process had occurred on the stainless steel substrate surface during the coating process. The cold forming process prior to the coating accelerates the process and the higher deformation level leads to increased precipitation. This precipitation process sensitizes the stainless steel substrate and deteriorates the corrosion properties of the coating substrate system. The performance of the coated and then deformed parts and also of the deformed and then coated parts can be improved if a more passive and better stabilized stainless steel substrate is used. Some evidence was found in the SEM analysis that the Ti interlayer deforms plastically during the forming process of TiN-coated stainless steel sheet, thus main-
behariour
y/decorative
car
93
parts
taining the adhesion of the fragmented TIN coating and the substrate.
Acknowledgements
We would like to thank Risto Suominen from IMST, Helsinki University of Technology, for taking the SEM scans.
References I. Itoh, Y. Oikawa, M. Hashimoto. S. Saita. T. Komori, M. Onoyama, S. Itoh and T. Murat, Nippon Steel Tech. Rep. 43, October 1989, p. 8. I. Itoh, Y. Oikawa, M. Hashimoto, T. Komari, M. Onoyama and M. Ueshima, Nippot Steel Tech. Rep. 46, July 1990, p. 8. L. Eriksson, E. Harju, A. S. Korhonen and K. Pischow, Surf. Con?. Techno/., 53 ( 1992) 1533 160. V. Kucera, A. T. Coote, J. Henrikscn, D. Knotkova, C. Lcygraf and I J. Reinhardt. Iwmrwticw nu(l Terhaology Transfcrjor Corrosion Cil~~rifl. frrrr, I /t/r Int. Corrr~siotl Coffgress, ha/J: 1990, Vol. 2, pp. 2.177 2.w F. Blehkcnho~~. E. Nagel Soepenberg, M. Roelofsen and J. P. Schoen, Proc. 9th Eur. Congr. on Corrosion, Utrecht, October 2-4, 1989, p. TR-188. 6 E. Mattsson, Accelerated corrosion testing of automotive radiators of copper materials-a critical survey, Society of Automotive Engineers Int. Congr. and Exposition, Detroit, MI, 1992, Paper 920181. 7 CRC Handbook qf Chemistry and Physics, 57th edn., CRC Press, Boca Raton, FL, 1976-1977, p. D-135. 8 J. Aromaa, K. Knuutila, 0. ForsCn and S. Ylasaari, Proc. 5th Asian-Pacific
Corrosion
Australasian Corrosion 9 M. Tavi, 0. For& Electrochemical
Methods
Conf.,
Melbourne,
November
Association, Parkville, 1987. and J. Aromaa, in B. Elsner in Corrosion
Research
Symp.,
1987,
(ed.), July,
Proc. 1988,
Trans Tech, Zurich, 1989, pp. 15-27. 10 M. Ignat, A. Armann, L. Moberg and F. Sidieude, Surf. Coat. Techno/., 49 (1991) 514-518. 11 V. Kucera, S. Haagenrud, L. Atteraas and J. Gullman, Corrosion of steel and zinc in Scandinavia with respect to the classification of the corrosivity of atmospheres, Am. Sot. Test. Mater., Spec. Tech. Pub/., 965 ( 1988) 264.
and Coatings
Technology,
67 ( 1994) 85-93
85
Corrosion behavior of decorative car parts and outdoor lighting fixtures fabricated from TiN-coated stainless steel* K. A. Pischowa, L. Erikssona, A. S. Korhonena, “Laboratory bLaboratory ‘Uniuersity (Received
of Processing and Heat Treatment ofMaterials, of Corrosion and Material Chemistry, Helsinki of Florida. Gainesville. FL3262 1 (USA) December
6, 1993; accepted
in final form January
Helsinki Unioersity
0. ForsCnb, M. Turkiab and E. 0. Ristolainen” Unit>ersity of Technology, Vuorimiehentie 2A. 02150 Espoo (Finland) of Technology, Vuorimiehentie 2A. 02150 Espoo (Finland)
30, 1994)
Abstract The fabrication of TiN-coated decorative car parts and lighting fixtures by deep drawing investigated. Corrosion tests in real and laboratory environments were performed and either before or after forming were compared with uncoated stainless steel hub caps. The were studied, before and after the corrosion tests, using secondary-ion mass spectrometry
1. Introduction TiN is a well-studied standard coating in wearresistant and cutting tool applications. Owing to its golden color it is also well suited for decorative purposes. The porosity of the coating is the biggest drawback, causing hard pitting corrosion attack on the substrate in severe environments. One other disadvantage is the coating process itself, which is fairly expensive and in most cases allows only a limited range of sizes. The possibility of coating a stainless steel strip [ 1,2] continuously and of forming the parts afterwards would markedly cut down the production costs. Our preliminary experiments [ 33 with the formability and corrosion resistance of TiN-coated stainless steel sheet have shown that the adherence of the coating after the forming process is excellent and that the corrosion resistance is not worsened from what is achieved using a similarly formed stainless steel sample. Accordingly, it seems to be possible to use TiN-coated stainless steel parts, which have been formed to the desired design after the coating process, at least for decorative purposes in not too severe environments. The aim of this study was to test decorative car parts in the winter environment in Finland, which includes a lot of salt and sand for melting the ice and increasing the friction on the slippery winter roads. This forms a corrosive and wearing environment for the car parts. *Paper presented Metallurgical Coatings 19-23, 1993.
at the 20th International and Thin Films, San Diego,
Conference CA, USA,
on April
from a TiNcoated stainless steel plate were the behavior of car wheel hub caps coated micro- and surface structures of the hub caps and scanning electron microscopy.
The tested parts were hub caps fabricated from a TiNcoated stainless steel sheet by hydraulic bulging. For applied corrosion and wear tests they were mounted on a test car. Air pollution damages many materials, either directly through atmospheric corrosion, or indirectly through the acidification of soil water. In Scandinavia atmospheric corrosion is primarily due to local emissions, so the damage is greater in urban than in rural areas. Although sulfur dioxide is the chief agent, nitrogen oxides, ozone and acid rain or snow also contribute, which means that synergistic effects are common. Laboratory corrosion tests simulating the atmospheric corrosion due to the acidifying air pollutants [4] on outdoor lighting fixtures were also accomplished. The tested surfaces were inspected using secondary-ion mass spectrometry (SIMS) and scanning electron microscopy (SEM).
2. Experimental
details
The hub cups shown in Fig. 1 were made of an AISI 304 sheet with a thickness of 0.8 mm. The material was factory bright annealed with a surface roughness of R, = 0.05 mm. The first set of samples was coated before, and the second one after, the forming procedure. The forming was performed using a biaxial bulging test apparatus. Two different pressures were used and thus two different dome heights and deformations were produced, as can be seen in Table 1.
Fig. 1. TiN-coated hub cups made of an AISI 304 sheet with a thickness OT 0.8 mm by deep drawing: left. first coated and then deformed; right, first deformed and then coated.
TABLE
1. Sample
preparation
Sample
c + F”
134 134B 170 170B
+
schedule F+Cb
+ + +
Deformation level 134 134 170 170
“TIN coating, then forming. bForming, then TIN coating.
Fig. 2. Two of the test samples Chevrolet van.
were mounted
on the rear wheels of a
2.1. Applied corrosion and wear tests
Two of the test samples were mounted on the rear wheels of a Chevrolet van, as shown in Fig. 2. Two additional samples were bolted inside the rear fenders, where the major flux of sand and salt from the wheels hits. The testing time was 6 months. 2.2. Laboratory
corrosion tests
To select the best materials for in-service use, a fast and reliable laboratory test is needed to minimize the in situ testing. This laboratory test time-consuming should be based on the vehicle “microclimates” in which the materials are exposed. As a simulation of these “microclimates”, several standard accelerated laboratory tests are in use, e.g. the neutral salt spray test (ASTM standard B-117). Unfortunately, there is in general a lack of correlation between salt spray test results and corrosion behavior in service [ 51. 2.2.1. Accelerated corrosion
testing
Electrochemical measurements were used to evaluate the porosity and general corrosion behavior of coldformed titanium-nitride-coated AISI 304 stainless steels. The actual salt concentration on the surface of running cars is not known in practice. Therefore the road envi-
ronment pollution (REP) solution was used as an electrolyte in the accelerated corrosion tests (Table 2). The REP should represent polluted (more real) road environments with a variety of occurring pollutants [ 61. The pH of the REP solution was 4.7 and the pH of authentic rain water in the Espoo area (southern part of Finland) was 4.4. The pH of drinking water is 6.5-8.0 [ 71. Redox potential us. a Pt calomel electrode was - 358 mV for the REP solution and -330 mV for authentic rain water, which means that in the pH range 4-5 TIN is stable and Fe dissolves according to Pourbaix diagrams. TABLE
2. The contents
Compound
Sodium Calcium Sodium Sodium Sodium Sodium Deonized
chloride chloride nitrate sulfate sulfite nitride water
of 1 I REP solution Formula
Content (mg I-‘)
NaCl CaCl, NaNO, Na,SO, Na,SO, NaNO,
30 20 0.3 0.3 0.5 0.25 Balance
H,O
The experiments were performed at ambient temperature. All potentials were measured 1)s. the saturated calomel electrode (SCE) and platinum plate was used as a counterelectrode. The surface areas of the test specimens were 0.79 cm’. After the tests each specimen was examined by optical microscopy. Short-term electrochemical measurements were made with a computer-controlled measurement system developed at the Helsinki University of Technology [8,9]. The system uses an NF2000 potentiostat and an NF5050 frequency response analyzer. The measurement software is written in BASIC and data processing is carried out with commercial software packages. The short-term measurement procedure for coated samples consisted of two diverse measurements: long cyclic anodic polarization (LCAP) and potentiostatic exertion (PE). Anodic polarization curves were produced by the potentiostatic polarization method using 10 mV steps at 6 s intervals. The polarization scan started at -500 mV us. SCE, and at 500 mV LIS.SCE the polarization changed direction towards the starting point, unless the current density first reached the threshold current density 5 mA cm-‘, when the polarization scan direction was automatically reversed. In the short-term potentiostatic measurement using (using PE) the determination of i us. r curves was made using a polarization of 200 mV vs. SCE for 3600 s. The potential used in PE was selected on the basis of anodic polarization measurements. 2.3. SIMS analysis Depth profiles were produced with SIMS PHI 6600 using an O+ gun with 5 keV primary high voltage and 1 uA current. The analyzed area was 300 x 300 urn’. For the point analysis a Ga+ gun with 25 keV primary high voltage and 0.1 nA current was used.
3. Results 3.1. Applied corrosion and wear test The problems caused by adding a noble coating such as TIN to a corrosion system can be seen in Fig. 2 (bottom part). After remounting the hub caps from the test car they were washed in water and alcohol and dried in hot air. Figure 3 shows two hub cups from the rear wheels: the hub cap on the left was coated before the deep drawing and the one on the right after. No decohesion of the coating nor visible pit corrosion attack can be visually detected. Figure 4 shows the two hub cups which were mounted inside the rear fender. Even here no decohesion nor corrosion pits can be seen. Furthermore, there do not
Fig. 3. Two hub cups from the rear wheels; the left-hand one is coated before the deep drawing and the right-hand one after. No decohesion of the coating nor visible pit corrosion attack can be detected.
seem to be any hit marks of the sand.
of the stones nor wear marks
3.1 .I. Optical microscopy The microscopic inspection with an optical microscope confirmed the above-mentioned observations. No signs of decohesion were found; however, some pinholes were clearly visible in samples first deformed and then coated and some broadening of the cracks was visible on sample 170B which had the higher deformation level. 3.1.2. Scanning electroil microscopy SEM analysis of the applied corrosion and wear test samples that were first TIN coated and then formed (C + F) is shown in Fig. 5. The typical surface structure of a sample being formed to the higher deformation level can be seen in Fig. 5(a). Some of the cracks are very broad in this area. However, energy-dispersive X-ray (EDX) analysis showed high titanium and also iron and chromium contents, which indicates that the surface is not in all places completely covered by the TIN coating. The same sample after the applied corrosion and wear test, on the back ring, in Fig. 5(b) does
3.1.3. SIMS analysis
Figure 7 shows the depth profiles of samples 134B and 134 (Figs. 7(a) and 7(b)) and samples 170B and 170 (Figs. 7(c) and 7(d)) after the applied corrosion tests. In samples 134B and 170B only titanium and nitrogen were detected. However, in samples 134 and 170 there are high iron, chromium, silicon and carbon contents in addition to titanium and nitrogen throughout the whole coating thickness. It should also be pointed out that a peak in the carbon profile occurs in all the samples at the coating-substrate interface. The point analysis from the surface of sample 134 after the applied corrosion and wear tests showed the following compounds: titanium oxides, chromium oxides, iron oxides, sodium sulfides and iron chlorides. 3.2. Laboratory
Fig. 4. The two hub cups which were mounted inside the back fender. Even here no decohesion no corrosion pits can be seen. Furthermore, there do not seem to be any hit marks of the stones nor wear marks of the sand.
not markedly differ from the untested one. Figure 5(c) shows broad cracks on which Fe and Cr were also found, indicating a crevice corrosion attack. However, even here the upper part of the crack is still partly covered with TIN. In samples formed to the lower deformation level the structure is the same as before but the width of the cracks appears to be smaller, as expected. In Fig. 5(d) a tested surface from inside the rear fender is shown. In the middle of the figure there might be a stone hit, and in this pit EDX analysis revealed iron and chromium. SEM analysis of the applied corrosion and wear test samples that were formed and then TIN coated (F+C) is shown in Fig. 6. A typical surface structure of an untested sample can be seen in Fig. 6(a). A tested sample from the left-hand rear wheel is presented in Fig. 6(b) and in greater detail in Fig. 6(c), showing the increased size of the pinholes caused by pit corrosion. Figure 6(d) shows a scratch-like feature on the surface, from which iron and chromium were analyzed using EDX, indicating a coating failure.
corrosion tests
The base material AISI 304 was in the active stage during LCAP in REP solution and the area of the hysteresis loop was negative. Undeformed TiN/AISI 304 had a short passive area between 40 and 200 mV and the area of the hysteresis loop was also negative in REP solution. After the test, no damage could be observed by optical microscopy on the base material nor on the undeformed TiN/AISI 304 sample. All deformed samples showed large positive hysteresis during cyclic anodic polarization. In Fig. 8 are shown six different LCAP curves measured in REP solution and for the sake of clarity only the forward scan is drawn. The polarization curves are divided into three classes: the class of reference materials AISI 304 and TiN/AISI 304, the class of first coated and then deformed (C + F) samples and the class of first deformed and then coated (F + C) samples. The differences between the classes are more clearly seen in the cathodic part of the LCAP curves, where the lowest cathodic current densities were measured for an undeformed but coated TiN/AISI 304 sample and the highest cathodic current densities were measured for the base material and the first coated and then highly deformed sample (170). The cathodic current densities of the first coated and then deformed samples, 170B and 134B, were between the upper and lower limit. In the anodic part of the LCAP curves the highest positive current densities were measured for the first coated and then highly deformed sample 170. Deformation increased the anodic positive current densities by one order of magnitude compared with the anodic current densities of the TiNcoated undeformed reference sample. According to the optical microscopy the corrosion mechanism was clearly different between the class of C+F samples and the class of F +C samples. The corrosion mechanism of the F+C class was pitting. A clear crevice corrosion effect was seen in the samples first coated and then formed.
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Fig. 5. SEM analysis of the applied corrosion and wear test samples that were first TiN coated and an area where some of the cracks are very broad; however, EDX analysis showed high titanium indicates that the surface is not in al) places completely covered by the TiN coating. (b) The same test, on the back ring, does not markedly differ from the untested sample. (c) SEM scan showing found indicating a crevice corrosion attack. (d) SEM scan showing a tested surface from inside the
Figure 9 shows the results of PE measurements for five different corrosion systems measured in REP solution. The first and the fourth curves are for samples from the C+ F class. The second and the fifth are for samples from the F + C class. The third system was AISI 304, the base material. Galvanic corrosion between the base material and the inert TiN film plays a leading role in the general corrosion behavior of TiN-coated materials. At a potential of 200 mV us. SCE, dissolution of the base material AISI 304 is rapid. After 5 mm the current densities settled down to a constant value of 17 uA cm-‘. After the PE, strong crevice corrosion and a large pit were observed on the stainless steel.
puts
then deformed (C+F). (a) SEM scan showing and also iron and chromium contents which sample after the applied corrosion and wear broad cracks on which Fe and Cr were also rear fender.
The current densities of the first coated and then formatted sample reached the current densities of the base material after 50 min and mild crevice corrosion was observed on the sample. Current densities of some samples were even higher than the corrosion densities of the base material because of the formation of galvanic couples between the anodic base material and the cathodic TIN coating. After the PE test, these samples were clearly damaged by crevice attack, but the degree of damage was different depending on whether the sample was first coated and then formatted or vice versa. The degree of damage was higher for samples of the C + F class. The best survival was in a dense sample from the
Fig. 6. SEM analysis of the applied corrosion and wear test samples that were first formed and then TIN coated (F+ C). (a) A tested sample. (b) A tested sample from the left-hand rear wheel. (c) A higher magnification scan showing the increased size of the pinholes caused by pit corrosion. (d) A scratch-like feature on the surface from which iron and chromium were analyzed by EDX, indicating a coating failure.
F+C class. The current densities were three orders of magnitude lower than the current densities of the base material; the constant value was 1.4 uA cmm2 for the first 38 min. After that the current densities started to increase but still stayed one order of magnitude lower than the current densities of the AISI 304 base material. A slight initial crevice corrosion attack was observed by optical microscopy on the surface of the best sample.
4. Discussion Visually, the surface quality of all the tested hub cups was good and no changes in the appearance after the test period can be detected. However, the testing time is short even if it covers the most corrosive time of the
year. The wear protection is also good because no decohesion of the coating was detected. The wear properties of the formed TiN coating must, however, be confirmed with laboratory wear tests at a later stage. In optical microscopy after the applied corrosion and wear tests the pinholes appear more distinct than they were on the untested samples and this could be an indication of the beginning of pit corrosion attack. Likewise, in sample 170B, which was formed after the TIN deposition and had the higher deformation level, some broadening of the cracks is visible, pointing to a crevice corrosion attack. SEM scans and EDX analysis of the samples coated and afterwards deformed to the higher deformation value showed that, in addition to titanium, iron and chromium were also present in the broader cracks,
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Fig. 7. SIMS depth profiles of samples (a) 134B and (b) 134 and samples (c) 170B and (d) 170 after the applied corrosion tests.
indicating that no protection of the substrate occurred on those areas. After the applied corrosion tests on those areas some broadening of the cracks was visible and in some areas only iron and chromium were detected. The corrosion mechanism was crevice corrosion because the corrosion attack seemed to be directed to the broad cracks which were not completely protected by the coating. In this case there has not been a severe corrosion attack through the pinholes, which are thus hardly visible in the SEM pictures. The EDX analysis of the cracks in Figs. 6(b) and 6(d) shows that even the broader cracks are covered by titanium, and still after the corrosion attack the remnants of a thin Ti layer can be detected by EDX and seen on the upper part of the Fig. 6(d). This behavior indicates that the Ti interlayer is able to deform plastically between the substrate and TiN coating, thus maintaining the adhesion of the broken TiN fragments and covering
the crack bottoms. The results presented by Ignat et al. [lo] reporting interfacial cracking only after an important deformation are also indicative of this. In the samples first deformed and then coated the corrosion mechanism during the applied corrosion test seems to be different. The corrosion attack occurs through the pinholes, which appear to be markedly increased in diameter and can be clearly distinguished also at lower magnifications, giving a clear indication of pitting corrosion. The SIMS analysis confirmed the aforementioned. However, the difference in favor of the first formed and then coated samples is bigger than expected. This may be due to the very local nature of the pit corrosion. The most interesting feature in the SIMS depth profiles is the behavior of the carbon. The high carbon content close to the substrate surface clearly indicates a carbide precipitation, which is quite unexpected at the
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Fig. 9. PE test results for five different corrosion systems measured in REP solution.
coating temperature of about 500°C. It can also be seen in the SIMS depth profiles that the cold forming accelerates the precipitation process and the higher deformation level leads to increased precipitation. This precipitation sensitizes the stainless steel substrate and deteriorates the corrosion properties of the coating substrate system. The SIMS analyses from the sample surface after the applied corrosion and wear tests clearly showed that the effective corrosive elements were chlorides and sulfites. In spite of extensive work over more than 70 years on atmospheric corrosion, a number of questions remain. In particular, no satisfactory method for predicting the performance of materials in the atmosphere exists either as an on-site method or laboratory test. Long-term exposure methods require much time to determine the behavior of materials in atmospheric conditions. When the outdoor atmospheres are characterized by periodic wetting of the metallic surfaces from water condensation, relative humidities greater than 80%, fast temperature variations and precipitation, it is obvious that accelerated laboratory corrosion tests can give only a slight idea about what is going to happen in the real
environment. The constant wet environment of the accelerated tests differs significantly from the real situation, which is estimated in southern Scandinavia [ 111 to be 2750-4100 h wet in 1 year. Laboratory corrosion tests confirmed the corrosion mechanisms detected in the applied tests. The LCAP curves gave qualitative information about the general electrochemical behavior of the corrosion systems. Evaluating the corrosion resistance of the coated sample, it is essential to take account of the corrosion behavior of the base material, affected through pores in the coating. The rate at which the base material reacts at the bottom of the pinholes obviously depends on the number and size of these defects. Thus when the defects are long and narrow they actually decrease the reaction rates by reducing the mass transport between the bottom of a pinhole and the corrosive environment. Specimens of the C+ F class clearly suffer from crevice corrosion in REP solution, unlike specimens of the F+C class, which had plain pitting. The fact that the corrosion mechanism was converted depending on the order of the coating process and forming was clearly seen in the cathodic part of the LCAP curves.
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5. Conclusions
In the applied corrosion and wear tests both the coated and deformed hub caps and deformed and coated hub caps showed no indication of wear. However, the 6 months testing time is still short compared with the expected lifetime of the product. Likewise, on a macroscopic level no sign of corrosion was detected, although the microscopic investigation showed the beginning of local corrosion attack in all the samples and it was severest, as expected, in the sample that was first TIN coated and then deformed to the higher deformation level. Two different corrosion mechanisms were discovered: firstly, a crevice corrosion, in which the corrosion attack on the samples deformed after the coating procedure was directed to the broad cracks and there was no indication of pitting corrosion through the pinholes; secondly, a localized pitting corrosion in which, on the samples coated after the deformation process, a heavy corrosion attack through the pinholes was observed. This indicates that the lifetime of the samples after both fabrication processes does not differ much and that the behavior of the substrate is the critical factor in determining the lifetime of the product. The SIMS analysis clearly showed that a precipitation process had occurred on the stainless steel substrate surface during the coating process. The cold forming process prior to the coating accelerates the process and the higher deformation level leads to increased precipitation. This precipitation process sensitizes the stainless steel substrate and deteriorates the corrosion properties of the coating substrate system. The performance of the coated and then deformed parts and also of the deformed and then coated parts can be improved if a more passive and better stabilized stainless steel substrate is used. Some evidence was found in the SEM analysis that the Ti interlayer deforms plastically during the forming process of TiN-coated stainless steel sheet, thus main-
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taining the adhesion of the fragmented TIN coating and the substrate.
Acknowledgements
We would like to thank Risto Suominen from IMST, Helsinki University of Technology, for taking the SEM scans.
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