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Corrosion behaviour of galvannealed steel sheet
Surface & Coatings Technology 200 (2006) 4312 – 4319 www.elsevier.com/locate/surfcoat
Corrosion behaviour of galvannealed steel sheet N. BandyopadhyayT, G. Jha, A.K. Singh, T.K. Rout, Nitu Rani Research and Development Division, Tata Steel, Jamshedpur-831001, India Received 8 December 2004; accepted in revised form 6 February 2005 Available online 25 April 2005
Abstract This paper analyzes the influence of galvannealed coating on the corrosion behaviour of press-formed and painted galvannealed steel sheet (GA) against the painted cold rolled and closed annealed (CRCA) sheet evaluated by electro impedance spectroscopy (EIS), salt spray (SST) and combined cyclic corrosion test (CCT). EIS study showed a significant decrease in the polarization resistance from 6.38 to 0.8 V/ cm2 in case of CRCA painted sheet and from 6.07 to 4.07 V/cm2 in case of GA painted material revealing the superior corrosion resistance of galvannealed sheet. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. Studies on under-paint corrosion from a deep scratch or creep-from-scribe in combined cyclic corrosion tests (CCT), simulating drying and wetting conditions in actual service life, have shown drastic reduction in creep-back for GA material. Salt spray test (SST) exposure of 720 h on painted and cross-scribed samples has also demonstrated the ability of GA material to resist corrosion that inevitably occurs on stone chipping on outer surface of auto body panels. Red rust formation started just after two cycles and propagated from crosscut portion with further test cycles. The initiation of red rust across the crosscut in GA material was not visible even after 65 cycles. The corrosion rates of CRCA painted samples after 30 and 45 cycles were 5.17 and 11.56 mpy, respectively, while in the case of GA painted, it was only 0.412 and 0.732 mpy, respectively. In case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, GA painted sample has shown almost 16 times more corrosion resistance than the painted CRCA material. D 2005 Elsevier B.V. All rights reserved. Keywords: Galvannealed coating; Cosmetic corrosion; Perforation corrosion; Electrochemical impedance spectroscopy; Paint delamination
1. Introduction The use of thinner sheets for automotive application for reducing vehicle weight requires improved material strength as well as higher corrosion protection. Hot dip galvanizing of steel sheets has emerged as a powerful protection technique towards rusting and hence, failures due to corrosion. For the automotive industry, the steel sheets are often galvanized or galvannealed (GA). The galvannealed coating consists of several zinc–iron intermetallic phases, the nature and thickness of which influence the mechanical properties, especially the powdering resistance of the material [1]. The current trend also includes various postgalvannealing treatments to improve corrosion resistance, T Corresponding author. Tel.: +91 657 2422778; fax: +91 657 2271510. E-mail address: [email protected] (N. Bandyopadhyay). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.153
such as paint applications. This dramatically reduces the under-film corrosion deep-scratch or creep-from-scribe [2]. Significant development in the hot dip galvanizing lines, particularly in the areas of consistent thin coating as well as elimination of surface defects, has led to large scale replacement of electrogalvanized (EG) products by hot dip galvanized material. The highest quality standards are currently being demanded by the automobile industries, followed by the white goods manufacturers. Among the various coated products, galvannealed steel sheets are now being widely used by the auto makers because they exhibit excellent corrosion resistance after paint application. The surface appearance of galvannealed steel is grey, matted, uniform and without spangle because it is reheated (500–550 8C) just before solidifying. This product features excellent paint adherence and corrosion resistance after it is painted. The secondary advantage of galvannealing is a
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
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Table 1 Chemical composition of substrate, wt.%
Table 3 Coating weight and iron content of galvannealed coating
Steel ID
Substrate chemistry C
Mn
S
P
Si
Nb
Ti
Serial no.
Coating wt. (gm/m2)
0.003 0.003
0.13 0.12
0.006 0.008
0.013 0.012
0.011 0.008
0.014 0.012
0.058 0.053
Coating thickness (Am)
% Fe in coating
CRCA GA
Sheet thickness (mm)
1 2 3
0.80 0.80 0.80
112.00 108.23 110.8
8.00 7.76 7.83
10.10 9.75 9.51
harder coating compared to cold rolled and annealed steel sheet, resulting in higher resistance to scratching. It also shows relatively good weldability for spot and seam welding. Therefore, GA coating, in combination with paint, provides a synergistic effect against corrosion and hence, is used for aesthetically demanding applications such as automobile exposed body panels [3]. In India, there exists a large consumer market for auto and white goods. The users are not aware of the advantages of galvannealed coated sheet for corrosion protection. They are still going for cold rolled and annealed sheet due to less cost. However, both galvannealed as well as cold rolled sheets are given a post-painting treatment on the automotive paint line. The galvannealed sheet after the painting gives a synergistic effect in terms of corrosion resistance and becomes economical in the long run. In order to establish the superiority of galvanneaed sheet over conventional cold rolled sheet, the present study was undertaken to evaluate the corrosion behaviour of press-formed and painted component of GA sheet. For comparison, the same component formed with cold rolled sheet and painted was taken.
2. Experimental Six coils (each of 20 tons) were cold rolled in cold rolling mill complex. Out of six cold rolled coils, two were sent to continuous galvanizing line for GA coating while remaining for coils were processed through batch annealing route as uncoated CRCA material. The chemical composition of steel substrate for CRCA and GA are shown in Table 1. The composition of molten zinc bath used to produce GA coating is shown in Table 2. The samples from zinc bath were collected at three regular intervals, i.e., at start, in between and at the end of the zinc coating process. Samples of galvannealed sheets were collected for evaluation of mechanical properties, substrate chemistry, coating and corrosion behaviour. The relative phase thickness of each layer was measured from the corresponding cross-sectional SEM micrograph. The coating chemistry across the coating
thickness was measured by energy dispersive X-ray spectrometer (EDS). The coating weight and iron content of three samples taken from GA sheets were determined by wet stripping and titration methods, respectively in order to check the homogeneity and consistency of the Zn–Fe alloy coating. The coated sheet was immersed in 10% sulphuric acid to dissolve the coating. The solution containing the dissolved coating was titrated with 0.01 N potassium permanganate solution. The volume of the titrant needed to equilibrate the solution was measured along with the weight difference of the coated sample before and after the coating removal to determine the total weight percent iron present in the coating. The results are shown in Table 3. Similarly the mechanical properties of GA sheets were also evaluated. The blanks from GA and CRCA were press-formed into center flap panels used in front of the radiator of the large commercial vehicles and painted at automotive paint line. A schematic diagram of the various layers of the paint system on GA material is shown in Fig. 1. The corrosion studies on bare GA as well as press-formed and painted samples of GA and CRCA materials were carried out by salt spray test (SST), cyclic corrosion test (CCT), electrochemical test using DC polarization and electrochemical impedance spectroscopy (EIS) study and the coating degradation was studied with the help of an image analyzer. To evaluate the cosmetic corrosion performance of the galvannealed material, coupons of 150 100 mm2 were prepared and a diagonal scribe (0.5 mm width) was made with a carbide lath tool to scratch through the paint in order to expose the bare uncoated substrate. Combined cyclic corrosion tests (CCT) were performed as per J2334 specification, developed by the American Society for Automobile Engineers. Samples were removed at intervals of 30, 45 and 65 cycles. The scribe-creep and delamination of paint next to the scribe were evaluated by scrapping and
Topcoat
(63 – 73 µm) (45 – 47 µm)
Table 2 Chemical composition of galvanizing bath
E-coat
Bath
% Al
% Fe
% Sb
% Pb
% Sn
Initial Intermediate Finished
0.14 0.155 0.137
0.056 0.063 0.059
0.019 0.023 0.021
0.007 0.009 0.005
Not traceable 0.002 b0.002
Zinc coating
Primer
(19– 24 µm) (7.83 µm)
phosphating (1 µm)
Steel
Fig. 1. Schematic diagram of the GA painted sample.
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The initial phase growth was dominated by the outburst, and various intermetallic phases were identified by their iron concentration, as shown in Fig. 2. The analysis showed that the bulk of the structure was delta phase with small columnar zeta crystals at the edges as shown in Fig. 2. 3.2. Mechanical properties of GA sheet
X 4000
AVERAGE COATING THICKNESS : 7.83 µm γ PHASE THICKNESS :1.91 µm δ PHASE THICKNESS : 5.89 µm ξ PHASE THICKNESS : 0.03 µm Fig. 2. SEM photograph of intermetallic phases in the GA coating.
tapping of the exposed samples to remove the loose paint. Total delamination was measured at 10 points at 1 cm interval along the scribe and averaged. The coupons were cleaned and weighed for mass loss. To study the effect of craters on delamination of paint, painted samples were put in 3.5% salt solution for 30 days and then subsequently evaluated under the image analyzer.
3. Results and discussion 3.1. Coating characterization As shown in Table 3, uniform coating weight of 120 g/ m2 and an optimum iron content of 8–10% were observed in the GA coating, which were within the desired specifications. However, iron content alone cannot determine the degree of alloying or predict the formability performance of the coating. The adhesion of hot-dip galvannealed coating is known to be inversely proportional to their iron content and thickness. Therefore, for optimum coating adhesion, an alloy layer with high iron content should be minimized by careful control of galvanneal temperature. In addition, coating mass should be controlled as low as practically possible within the specification range [4]. Effective aluminum (Al) content was strictly maintained below 0.13% to facilitate the inter-diffusion of iron in the coating during galvannealing operation.
Mechanical properties of the IF GA sheet conducted on three samples are shown in Table 4. The quality of the GA coating is dependent on several parameters such as incoming material properties, consistent zinc bath chemistry, uniform coating thickness across and along the strip and good control of heating and soaking temperatures. In general, there is a decrease in the r-value (deep-draw ability) after galvannealing. This is due to the transformation of Zn-coating into a brittle Zn–Fe intermetallic coating which provides greater resistance to any flow along the sheet plane. Therefore, interstitial-free steels with good deep-drawing properties even after galvannealing treatment are preferred for good formability during press forming operation. Since grain boundaries are known to serve as nucleation sites for zinc–iron phases, the development of morphology can be enhanced by the presence of more thermodynamically active grain boundaries which are essentially carbon-free, specifically in the case of Ti and Nb stabilized IF steel. It may also be seen that good strength/elongation balance and low yield ratio (yield stress/tensile strength) for deep drawing were achieved in case of IF steel, as shown in Table 4. Surface roughness was also observed to be very suitable for paint application. 3.3. Evaluation of corrosion behaviour 3.3.1. Bare material The results of corrosion tests conducted on bare GA and CRCA samples by anodic polarization (variation in current density with potential) studies are shown in Fig. 3. The open circuit potential (E Corr) of GA is more negative as Zn is placed in negative to iron in EMF series. The potential dependent dissolution current represented the transfer of metal ions across film/solution interface i.e., the rate of dissolution of the oxide film. As evident from the potential vs. current plot, there is a kink in GA material at 0.018 A/ cm2 current density revealing the full dissolution of Zn. From this study, it was found that GA as well as CRCA material got corroded continuously with time exposure. This
Table 4 Mechanical properties of galvannealed material Serial no.
YS (Mpa)
UTS (Mpa)
% El
Yield ratio
g value
Hardness (HRB)
Roughness, Ra (micron)
1 2 3
149 144 143
293 301 299
51 46 51
0.51 0.50 0.48
0.27 0.25 0.27
31.2 33.5 32.1
0.83 0.93 0.87
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1.000
Potential (V) vs Eref
0.500
0.000 CRCA -0.500
-1.000 GA -1.500 -10.0
-9.0
-8.0
-7.0
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
Log Current Density (A/cm2) Fig. 3. DC polarization test on bare CRCA and GA sheets.
study suggested that GA sheet without painting is as good as the CRCA sheet in 5% NaCl solution. The higher corrosion resistance of the GA material is due to the sacrificial protection by zinc, which has a lower electrochemical potential compared to steel and hence, leads to zinc dissolution in atmospheric condition only [5]. 3.3.2. Press-formed and painted material In general, the drawn parts show more damage than flat part, and the corrosion in drawn parts tends to proceed rapidly. In the case of painted GA material, the deterioration in corrosion resistance in the drawn part is negligible
25 mm
PAINTED CRCA
25 mm
PAINTED GA
BEFORE TEST START
25 mm
PAINTED CRCA
because the coating film exfoliation is limited. The low degree of paint film exfoliation is due to the good paint ability of the GA coating and the sacrificial protective effect of the base metal coating. The results of the salt spray test (SST), as per ASTM B117, are shown in Fig. 4. It is evident from the photographs that the red rust formation starts only after 48 h in painted CRCA, whereas in the case of painted GA even after 720 h there was no visible sign of red rust formation. These tests demonstrate the ability of the GA product to resist corrosion around the inevitable areas of damage that occurs on stone chipping on the outer surface of auto body panels [6].
25 mm
25 mm
PAINTED CRCA
PAINTED GA
AFTER 2 DAYS SALT SPRAY TEST
25 mm
PAINTED GA
AFTER 14 DAYS SALT SPRAY TEST
25 mm
PAINTED CRCA
25 mm
PAINTED GA
AFTER 30 DAYS SALT SPRAY TEST
Fig. 4. Salt spray test on painted sheet.
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After salt exposure of 30 days, the width of the paint delamination along the scribe in painted CRCA went up to 3.5–4 mm whereas the painted GA was intact. Swelling of paint across the crosscut was also observed on the painted CRCA sample. On the other hand, painted GA sample even after 30 days salt exposure, the surface appearance was similar to fresh one. The results emphatically highlighted the superior paint adhesion, as well as higher corrosion resistance of the painted GA material. The coating layer of GA is mainly consisted of delta phase (FeZn7; Fe concentration of about 10%) and is covered with inactive oxide films thicker than the galvanized or CRCA sheets. For these reasons, GA material presents excellent paint corrosion resistance with all types of paints. In aqueous corrosion tests conducted in salt spray, GA auto body panels effectively prevent any occurrence of film blisters and scabs at damaged parts. As drying and wetting are repeated in actual service environment, combined cyclic corrosion test (CCT) was considered to be close to service condition. The cycles for this test were followed as per SAE J2334. All automobiles use their own cyclic corrosion test procedures. However, most of the laboratory tests are cyclic in nature and consists of repeated cycles of intermittent exposure to salt solution, elevated humidity temperature and drying. The cosmetic corrosion performance of painted cold rolled steel versus GA product is presented in Fig. 5. The tests results clearly demonstrate that galvannealed coating
25 mm
25 mm
PAINTED CRCA
PAINTED GA
BEFORE TEST START
25 mm
PAINTED CRCA
drastically reduce under-paint corrosion from a deep scratch or creep-from-scribe. Since this blemish can be very much visible on the car, even when there is little structural damage, improvement in this corrosion performance significantly enhances long-term appearance and the manufacturer’s ability to provide a long-term warranty. The difference in corrosion performance between the GA and CRCA can be discerned by examining the relationship between creep-from-scribe against the number of cycles as shown in Fig. 6. It is worthwhile to mention here that undercut film corrosion widened up to 4.3 mm within 30 cycles in CRCA painted material which is equal to one and a quarter year of vehicle test in aggressive condition. Red rust formation started just after two cycles and propagated from the crosscut portion with further test cycles. The initiation of red rust across the crosscut in the GA material was not visible even after 65 cycles. It can be seen from the photo that development of red rust from the crosscut portion is suppressed by the sacrificial protective effect of the intermetallic layer of the GA coating. The corrosion rates of CRCA and galvannealed painted samples measured by weight loss method revealed that after 30 and 45 cycles in combine cyclic corrosion test, the corrosion rates were 5.17 and 11.56 mpy, respectively for CRCA, while in the case of GA painted, it was only 0.412 and 0.73 mpy, respectively. In the case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, the GA painted sample has shown almost
25 mm
PAINTED CRCA
25 mm
PAINTED GA
AFTER 30 DAYS CC TEST
25 mm
PAINTED GA
AFTER 45 DAYS CC TEST (CORROSION PRODUCT REMOVED)
25 mm
25 mm
PAINTED CRCA
PAINTED GA
AFTER 65 DAYS CC TEST (CORROSION PRODUCT REMOVED)
Fig. 5. Cyclic corrosion test on painted sheet.
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
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8
Scribe Creep Back,mm
7 6
Painted CRCA
5 4 3 2 1
Painted GA
0 0
10
20
30
40
50
60
70
No. of Cycles Fig. 6. Cosmetic corrosion of painted GA and CRCA sheets.
16 times more corrosion resistance than the painted CRCA material. The investigation clearly demonstrated the corrosion resistance from salt environment on the surface for interior exposure to retard inside-out perforation and good under-film corrosion resistance of GA coating on the exterior surface [7]. 3.3.3. Electrochemical impedance spectroscopy study In general, DC corrosion test methods, such as potentiodynamic scanning, are not used for coated material because of the high electrical resistance of the coating. In addition,
polarizing a coating with DC current can also damage the coating, thereby causing errors in the result [8]. Electrochemical impedance spectroscopy (EIS) has been found to be a powerful tool for the study of coating performance and undercoating metallic corrosion. Corrosion parameters such as charge transfer resistance (Rct) and coating parameters such as coating capacitance (Cc) are derived from EIS data. The Bode plots from AC impedance studies for CRCA painted and GA painted samples are shown in Figs. 7 and 8, respectively. The coating capacitance (Cc) and polarization resistance (Rp) obtained from the Bode plot are shown in
Fig. 7. Bode plot for painted CRCA sample in 5% NaCl solution.
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Fig. 8. Bode plot for GA painted sample in 5% NaCl solution.
Table 5. It was observed that the Log IzI i.e. log modulus impedance at lower frequency values decreased at the elapse of time in both painted CRCA and painted GA. It indicated that the paint/coating was permeable to water and ions. It was also observed that coating capacitance is constant in both cases. This indicated that water and ions were dispersed throughout the coating and not located at the coating–metal interface as free electrolytes. Accumulation of free water and ions at the coating–metal interface is necessary for metallic corrosion to occur [9]. The charge transfer resistance could not be measured and did not decrease during the course of studies. The corrosion and coating delamination of the GA painted sheet may not occur Table 5 Components of bode plot Log IzI, V
Rp V/cm2
Cc AF / cm2
CRCA painted 15 days 30 days
10.12 5.35
6.38 0.80
9.40 9.38
GA painted 15 days 30 days
10.79 6.79
6.07 4.07
9.41 9.37
in the marine atmosphere with this type of capacitive dielectric behaviour. Moreover, there was a significant decrease in the polarization resistance from 6.38 to 0.8 V/ cm2 in the case of CRCA painted sheet and from 6.07 to 4.07 V/cm2 in the case of GA painted material. As the polarization resistance is inversely proportional to the corrosion rate, painted GA has shown superior corrosion resistance. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. 3.4. Image analysis of painted CRCA and GA sheet The paint ability of coated sheet would normally be defined by its compatibility with the existing plant paint system assembly and its appearance after the final topcoat. The quality of the final paint finish is highly dependent on the condition of the coated steel surface. The surface finish should be within a specified range of roughness and peak density in order to yield a paint finish with an acceptable DOI (distinctness of image). Normal phosphating bath was used on the galvannealed surface to produce a satisfactory coating. Application of the cataphoretic primer coating (E-
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
CRCA sheet
4319
GA sheet
Fig. 9. Image analysis study on GA and CRCA painted after 30 days exposure in 3.5% salt solution.
coat) on the GA surface did not present any problem in the painting line. Anionic electro-deposition primer has now been adopted by the auto makers for excellent corrosion resistance. During the painting process, pinhole like defects called craters (about 0.2–0.3 mm in diameter) may occur in some cases. Such craters can be observed in GA coating layers and they are mainly of intermetallic phases of Zn and iron [10]. In order to study the effect of these catering defects in cationic electro-deposition, painted CRCA and painted GA samples were put in 3.5% salt solution for 30 days to find out any degradation of paint on the surface. Under the image analyzer, it was observed that the pore radius on painted CRCA significantly increased, whereas there was only a marginal change on painted GA material. This is shown in Fig. 9. The study indicates that the water molecules diffused through the pores simultaneously promoted the corrosion reaction across the substrate and coating interface. Our EIS study has also confirmed that the paint on the CRCA material failed completely whereas in the case of the GA material, there was absolutely no change.
4. Conclusions 1. Salt spray exposure clearly demonstrated the effective prevention of any occurrence of film blisters and scabs at damaged parts for GA coating. 2. The results from the CCT tests also clearly demonstrated that the galvannealed coating drastically reduced the under-paint corrosion from a deep scratch or creep-fromscribe. This improvement in this corrosion performance signifies the durable performance of the painted and galvannealed sheets.
3. After 45 cycles of testing, the GA painted sample has shown almost 17 times more corrosion resistance than the painted CRCA material. 4. EIS study has shown that the paint on CRCA material failed completely whereas in the case of the GA material, there was absolutely no change.
Acknowledgement The authors would like to thank the management of Tata Steel for allowing us to publish this paper.
References [1] W. Warnecke, R. Bode, Rand Kothe, L. Meyer, Thyssen Technische Berichte (Germany) 23 (1) (1991) 75. [2] M. Espenhahn, W. Dechene, R. Bode, W. Warnecke, Zinc and Zinc– Alloy Coated Steel—What Is the Best Coating for Automotive Application? Verlag Stahleisen Mgh, Germany, 1992, p. 273. [3] N. Kino, M. Yamada, Y. Tokunaga, H. Tsuchiya, Production of Nb-Ti Added Ultra-Low Carbon Steel for Galvannealed Application, The Minerals, Metals and Materials Society, 1990, p. 197. [4] T.C. Simpson, J.D. Hoffman, SAE Transaction: Journal of Materials and Manufacturing, vol. 104, Society of Automotive Engineers, USA, 1965, p. 246. [5] N.D. Kunde, G.M. Michal, J.H. Payer, 40th Mechanical Working and Steel Processing Conference Proceedings, Iron and Steel Society/ AIME, vol. 36, Iron & Steel Society, Warrendale, PA, USA, 1998 (Oct.), p. 157. [6] W.C. Sievert, L.L. Franks, A Galvanized-Galvannealed Sheet Steel for Automotive Body Panels, American Society for Metals, 1981, p. 165. [7] Y. Miyoshi, Boshoku Gijutsu (Corrosion Engineering) 35 (1) (1986) 38. [8] W. Tait Stephen, Journal of Coating Technology 61 (768) (1989) 57. [9] H. Leidheiser Jr., Corrosion 39 (5) (1983) 189. [10] A. Yasuda, H. Koumura, K. Yamato, K. Onizawa, H. Ota, ASM International (1988) 45.
Corrosion behaviour of galvannealed steel sheet N. BandyopadhyayT, G. Jha, A.K. Singh, T.K. Rout, Nitu Rani Research and Development Division, Tata Steel, Jamshedpur-831001, India Received 8 December 2004; accepted in revised form 6 February 2005 Available online 25 April 2005
Abstract This paper analyzes the influence of galvannealed coating on the corrosion behaviour of press-formed and painted galvannealed steel sheet (GA) against the painted cold rolled and closed annealed (CRCA) sheet evaluated by electro impedance spectroscopy (EIS), salt spray (SST) and combined cyclic corrosion test (CCT). EIS study showed a significant decrease in the polarization resistance from 6.38 to 0.8 V/ cm2 in case of CRCA painted sheet and from 6.07 to 4.07 V/cm2 in case of GA painted material revealing the superior corrosion resistance of galvannealed sheet. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. Studies on under-paint corrosion from a deep scratch or creep-from-scribe in combined cyclic corrosion tests (CCT), simulating drying and wetting conditions in actual service life, have shown drastic reduction in creep-back for GA material. Salt spray test (SST) exposure of 720 h on painted and cross-scribed samples has also demonstrated the ability of GA material to resist corrosion that inevitably occurs on stone chipping on outer surface of auto body panels. Red rust formation started just after two cycles and propagated from crosscut portion with further test cycles. The initiation of red rust across the crosscut in GA material was not visible even after 65 cycles. The corrosion rates of CRCA painted samples after 30 and 45 cycles were 5.17 and 11.56 mpy, respectively, while in the case of GA painted, it was only 0.412 and 0.732 mpy, respectively. In case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, GA painted sample has shown almost 16 times more corrosion resistance than the painted CRCA material. D 2005 Elsevier B.V. All rights reserved. Keywords: Galvannealed coating; Cosmetic corrosion; Perforation corrosion; Electrochemical impedance spectroscopy; Paint delamination
1. Introduction The use of thinner sheets for automotive application for reducing vehicle weight requires improved material strength as well as higher corrosion protection. Hot dip galvanizing of steel sheets has emerged as a powerful protection technique towards rusting and hence, failures due to corrosion. For the automotive industry, the steel sheets are often galvanized or galvannealed (GA). The galvannealed coating consists of several zinc–iron intermetallic phases, the nature and thickness of which influence the mechanical properties, especially the powdering resistance of the material [1]. The current trend also includes various postgalvannealing treatments to improve corrosion resistance, T Corresponding author. Tel.: +91 657 2422778; fax: +91 657 2271510. E-mail address: [email protected] (N. Bandyopadhyay). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.02.153
such as paint applications. This dramatically reduces the under-film corrosion deep-scratch or creep-from-scribe [2]. Significant development in the hot dip galvanizing lines, particularly in the areas of consistent thin coating as well as elimination of surface defects, has led to large scale replacement of electrogalvanized (EG) products by hot dip galvanized material. The highest quality standards are currently being demanded by the automobile industries, followed by the white goods manufacturers. Among the various coated products, galvannealed steel sheets are now being widely used by the auto makers because they exhibit excellent corrosion resistance after paint application. The surface appearance of galvannealed steel is grey, matted, uniform and without spangle because it is reheated (500–550 8C) just before solidifying. This product features excellent paint adherence and corrosion resistance after it is painted. The secondary advantage of galvannealing is a
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
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Table 1 Chemical composition of substrate, wt.%
Table 3 Coating weight and iron content of galvannealed coating
Steel ID
Substrate chemistry C
Mn
S
P
Si
Nb
Ti
Serial no.
Coating wt. (gm/m2)
0.003 0.003
0.13 0.12
0.006 0.008
0.013 0.012
0.011 0.008
0.014 0.012
0.058 0.053
Coating thickness (Am)
% Fe in coating
CRCA GA
Sheet thickness (mm)
1 2 3
0.80 0.80 0.80
112.00 108.23 110.8
8.00 7.76 7.83
10.10 9.75 9.51
harder coating compared to cold rolled and annealed steel sheet, resulting in higher resistance to scratching. It also shows relatively good weldability for spot and seam welding. Therefore, GA coating, in combination with paint, provides a synergistic effect against corrosion and hence, is used for aesthetically demanding applications such as automobile exposed body panels [3]. In India, there exists a large consumer market for auto and white goods. The users are not aware of the advantages of galvannealed coated sheet for corrosion protection. They are still going for cold rolled and annealed sheet due to less cost. However, both galvannealed as well as cold rolled sheets are given a post-painting treatment on the automotive paint line. The galvannealed sheet after the painting gives a synergistic effect in terms of corrosion resistance and becomes economical in the long run. In order to establish the superiority of galvanneaed sheet over conventional cold rolled sheet, the present study was undertaken to evaluate the corrosion behaviour of press-formed and painted component of GA sheet. For comparison, the same component formed with cold rolled sheet and painted was taken.
2. Experimental Six coils (each of 20 tons) were cold rolled in cold rolling mill complex. Out of six cold rolled coils, two were sent to continuous galvanizing line for GA coating while remaining for coils were processed through batch annealing route as uncoated CRCA material. The chemical composition of steel substrate for CRCA and GA are shown in Table 1. The composition of molten zinc bath used to produce GA coating is shown in Table 2. The samples from zinc bath were collected at three regular intervals, i.e., at start, in between and at the end of the zinc coating process. Samples of galvannealed sheets were collected for evaluation of mechanical properties, substrate chemistry, coating and corrosion behaviour. The relative phase thickness of each layer was measured from the corresponding cross-sectional SEM micrograph. The coating chemistry across the coating
thickness was measured by energy dispersive X-ray spectrometer (EDS). The coating weight and iron content of three samples taken from GA sheets were determined by wet stripping and titration methods, respectively in order to check the homogeneity and consistency of the Zn–Fe alloy coating. The coated sheet was immersed in 10% sulphuric acid to dissolve the coating. The solution containing the dissolved coating was titrated with 0.01 N potassium permanganate solution. The volume of the titrant needed to equilibrate the solution was measured along with the weight difference of the coated sample before and after the coating removal to determine the total weight percent iron present in the coating. The results are shown in Table 3. Similarly the mechanical properties of GA sheets were also evaluated. The blanks from GA and CRCA were press-formed into center flap panels used in front of the radiator of the large commercial vehicles and painted at automotive paint line. A schematic diagram of the various layers of the paint system on GA material is shown in Fig. 1. The corrosion studies on bare GA as well as press-formed and painted samples of GA and CRCA materials were carried out by salt spray test (SST), cyclic corrosion test (CCT), electrochemical test using DC polarization and electrochemical impedance spectroscopy (EIS) study and the coating degradation was studied with the help of an image analyzer. To evaluate the cosmetic corrosion performance of the galvannealed material, coupons of 150 100 mm2 were prepared and a diagonal scribe (0.5 mm width) was made with a carbide lath tool to scratch through the paint in order to expose the bare uncoated substrate. Combined cyclic corrosion tests (CCT) were performed as per J2334 specification, developed by the American Society for Automobile Engineers. Samples were removed at intervals of 30, 45 and 65 cycles. The scribe-creep and delamination of paint next to the scribe were evaluated by scrapping and
Topcoat
(63 – 73 µm) (45 – 47 µm)
Table 2 Chemical composition of galvanizing bath
E-coat
Bath
% Al
% Fe
% Sb
% Pb
% Sn
Initial Intermediate Finished
0.14 0.155 0.137
0.056 0.063 0.059
0.019 0.023 0.021
0.007 0.009 0.005
Not traceable 0.002 b0.002
Zinc coating
Primer
(19– 24 µm) (7.83 µm)
phosphating (1 µm)
Steel
Fig. 1. Schematic diagram of the GA painted sample.
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The initial phase growth was dominated by the outburst, and various intermetallic phases were identified by their iron concentration, as shown in Fig. 2. The analysis showed that the bulk of the structure was delta phase with small columnar zeta crystals at the edges as shown in Fig. 2. 3.2. Mechanical properties of GA sheet
X 4000
AVERAGE COATING THICKNESS : 7.83 µm γ PHASE THICKNESS :1.91 µm δ PHASE THICKNESS : 5.89 µm ξ PHASE THICKNESS : 0.03 µm Fig. 2. SEM photograph of intermetallic phases in the GA coating.
tapping of the exposed samples to remove the loose paint. Total delamination was measured at 10 points at 1 cm interval along the scribe and averaged. The coupons were cleaned and weighed for mass loss. To study the effect of craters on delamination of paint, painted samples were put in 3.5% salt solution for 30 days and then subsequently evaluated under the image analyzer.
3. Results and discussion 3.1. Coating characterization As shown in Table 3, uniform coating weight of 120 g/ m2 and an optimum iron content of 8–10% were observed in the GA coating, which were within the desired specifications. However, iron content alone cannot determine the degree of alloying or predict the formability performance of the coating. The adhesion of hot-dip galvannealed coating is known to be inversely proportional to their iron content and thickness. Therefore, for optimum coating adhesion, an alloy layer with high iron content should be minimized by careful control of galvanneal temperature. In addition, coating mass should be controlled as low as practically possible within the specification range [4]. Effective aluminum (Al) content was strictly maintained below 0.13% to facilitate the inter-diffusion of iron in the coating during galvannealing operation.
Mechanical properties of the IF GA sheet conducted on three samples are shown in Table 4. The quality of the GA coating is dependent on several parameters such as incoming material properties, consistent zinc bath chemistry, uniform coating thickness across and along the strip and good control of heating and soaking temperatures. In general, there is a decrease in the r-value (deep-draw ability) after galvannealing. This is due to the transformation of Zn-coating into a brittle Zn–Fe intermetallic coating which provides greater resistance to any flow along the sheet plane. Therefore, interstitial-free steels with good deep-drawing properties even after galvannealing treatment are preferred for good formability during press forming operation. Since grain boundaries are known to serve as nucleation sites for zinc–iron phases, the development of morphology can be enhanced by the presence of more thermodynamically active grain boundaries which are essentially carbon-free, specifically in the case of Ti and Nb stabilized IF steel. It may also be seen that good strength/elongation balance and low yield ratio (yield stress/tensile strength) for deep drawing were achieved in case of IF steel, as shown in Table 4. Surface roughness was also observed to be very suitable for paint application. 3.3. Evaluation of corrosion behaviour 3.3.1. Bare material The results of corrosion tests conducted on bare GA and CRCA samples by anodic polarization (variation in current density with potential) studies are shown in Fig. 3. The open circuit potential (E Corr) of GA is more negative as Zn is placed in negative to iron in EMF series. The potential dependent dissolution current represented the transfer of metal ions across film/solution interface i.e., the rate of dissolution of the oxide film. As evident from the potential vs. current plot, there is a kink in GA material at 0.018 A/ cm2 current density revealing the full dissolution of Zn. From this study, it was found that GA as well as CRCA material got corroded continuously with time exposure. This
Table 4 Mechanical properties of galvannealed material Serial no.
YS (Mpa)
UTS (Mpa)
% El
Yield ratio
g value
Hardness (HRB)
Roughness, Ra (micron)
1 2 3
149 144 143
293 301 299
51 46 51
0.51 0.50 0.48
0.27 0.25 0.27
31.2 33.5 32.1
0.83 0.93 0.87
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1.000
Potential (V) vs Eref
0.500
0.000 CRCA -0.500
-1.000 GA -1.500 -10.0
-9.0
-8.0
-7.0
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
Log Current Density (A/cm2) Fig. 3. DC polarization test on bare CRCA and GA sheets.
study suggested that GA sheet without painting is as good as the CRCA sheet in 5% NaCl solution. The higher corrosion resistance of the GA material is due to the sacrificial protection by zinc, which has a lower electrochemical potential compared to steel and hence, leads to zinc dissolution in atmospheric condition only [5]. 3.3.2. Press-formed and painted material In general, the drawn parts show more damage than flat part, and the corrosion in drawn parts tends to proceed rapidly. In the case of painted GA material, the deterioration in corrosion resistance in the drawn part is negligible
25 mm
PAINTED CRCA
25 mm
PAINTED GA
BEFORE TEST START
25 mm
PAINTED CRCA
because the coating film exfoliation is limited. The low degree of paint film exfoliation is due to the good paint ability of the GA coating and the sacrificial protective effect of the base metal coating. The results of the salt spray test (SST), as per ASTM B117, are shown in Fig. 4. It is evident from the photographs that the red rust formation starts only after 48 h in painted CRCA, whereas in the case of painted GA even after 720 h there was no visible sign of red rust formation. These tests demonstrate the ability of the GA product to resist corrosion around the inevitable areas of damage that occurs on stone chipping on the outer surface of auto body panels [6].
25 mm
25 mm
PAINTED CRCA
PAINTED GA
AFTER 2 DAYS SALT SPRAY TEST
25 mm
PAINTED GA
AFTER 14 DAYS SALT SPRAY TEST
25 mm
PAINTED CRCA
25 mm
PAINTED GA
AFTER 30 DAYS SALT SPRAY TEST
Fig. 4. Salt spray test on painted sheet.
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After salt exposure of 30 days, the width of the paint delamination along the scribe in painted CRCA went up to 3.5–4 mm whereas the painted GA was intact. Swelling of paint across the crosscut was also observed on the painted CRCA sample. On the other hand, painted GA sample even after 30 days salt exposure, the surface appearance was similar to fresh one. The results emphatically highlighted the superior paint adhesion, as well as higher corrosion resistance of the painted GA material. The coating layer of GA is mainly consisted of delta phase (FeZn7; Fe concentration of about 10%) and is covered with inactive oxide films thicker than the galvanized or CRCA sheets. For these reasons, GA material presents excellent paint corrosion resistance with all types of paints. In aqueous corrosion tests conducted in salt spray, GA auto body panels effectively prevent any occurrence of film blisters and scabs at damaged parts. As drying and wetting are repeated in actual service environment, combined cyclic corrosion test (CCT) was considered to be close to service condition. The cycles for this test were followed as per SAE J2334. All automobiles use their own cyclic corrosion test procedures. However, most of the laboratory tests are cyclic in nature and consists of repeated cycles of intermittent exposure to salt solution, elevated humidity temperature and drying. The cosmetic corrosion performance of painted cold rolled steel versus GA product is presented in Fig. 5. The tests results clearly demonstrate that galvannealed coating
25 mm
25 mm
PAINTED CRCA
PAINTED GA
BEFORE TEST START
25 mm
PAINTED CRCA
drastically reduce under-paint corrosion from a deep scratch or creep-from-scribe. Since this blemish can be very much visible on the car, even when there is little structural damage, improvement in this corrosion performance significantly enhances long-term appearance and the manufacturer’s ability to provide a long-term warranty. The difference in corrosion performance between the GA and CRCA can be discerned by examining the relationship between creep-from-scribe against the number of cycles as shown in Fig. 6. It is worthwhile to mention here that undercut film corrosion widened up to 4.3 mm within 30 cycles in CRCA painted material which is equal to one and a quarter year of vehicle test in aggressive condition. Red rust formation started just after two cycles and propagated from the crosscut portion with further test cycles. The initiation of red rust across the crosscut in the GA material was not visible even after 65 cycles. It can be seen from the photo that development of red rust from the crosscut portion is suppressed by the sacrificial protective effect of the intermetallic layer of the GA coating. The corrosion rates of CRCA and galvannealed painted samples measured by weight loss method revealed that after 30 and 45 cycles in combine cyclic corrosion test, the corrosion rates were 5.17 and 11.56 mpy, respectively for CRCA, while in the case of GA painted, it was only 0.412 and 0.73 mpy, respectively. In the case of CRCA, the corrosion rate increased with the passage of time. After 45 cycles of testing, the GA painted sample has shown almost
25 mm
PAINTED CRCA
25 mm
PAINTED GA
AFTER 30 DAYS CC TEST
25 mm
PAINTED GA
AFTER 45 DAYS CC TEST (CORROSION PRODUCT REMOVED)
25 mm
25 mm
PAINTED CRCA
PAINTED GA
AFTER 65 DAYS CC TEST (CORROSION PRODUCT REMOVED)
Fig. 5. Cyclic corrosion test on painted sheet.
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
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8
Scribe Creep Back,mm
7 6
Painted CRCA
5 4 3 2 1
Painted GA
0 0
10
20
30
40
50
60
70
No. of Cycles Fig. 6. Cosmetic corrosion of painted GA and CRCA sheets.
16 times more corrosion resistance than the painted CRCA material. The investigation clearly demonstrated the corrosion resistance from salt environment on the surface for interior exposure to retard inside-out perforation and good under-film corrosion resistance of GA coating on the exterior surface [7]. 3.3.3. Electrochemical impedance spectroscopy study In general, DC corrosion test methods, such as potentiodynamic scanning, are not used for coated material because of the high electrical resistance of the coating. In addition,
polarizing a coating with DC current can also damage the coating, thereby causing errors in the result [8]. Electrochemical impedance spectroscopy (EIS) has been found to be a powerful tool for the study of coating performance and undercoating metallic corrosion. Corrosion parameters such as charge transfer resistance (Rct) and coating parameters such as coating capacitance (Cc) are derived from EIS data. The Bode plots from AC impedance studies for CRCA painted and GA painted samples are shown in Figs. 7 and 8, respectively. The coating capacitance (Cc) and polarization resistance (Rp) obtained from the Bode plot are shown in
Fig. 7. Bode plot for painted CRCA sample in 5% NaCl solution.
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Fig. 8. Bode plot for GA painted sample in 5% NaCl solution.
Table 5. It was observed that the Log IzI i.e. log modulus impedance at lower frequency values decreased at the elapse of time in both painted CRCA and painted GA. It indicated that the paint/coating was permeable to water and ions. It was also observed that coating capacitance is constant in both cases. This indicated that water and ions were dispersed throughout the coating and not located at the coating–metal interface as free electrolytes. Accumulation of free water and ions at the coating–metal interface is necessary for metallic corrosion to occur [9]. The charge transfer resistance could not be measured and did not decrease during the course of studies. The corrosion and coating delamination of the GA painted sheet may not occur Table 5 Components of bode plot Log IzI, V
Rp V/cm2
Cc AF / cm2
CRCA painted 15 days 30 days
10.12 5.35
6.38 0.80
9.40 9.38
GA painted 15 days 30 days
10.79 6.79
6.07 4.07
9.41 9.37
in the marine atmosphere with this type of capacitive dielectric behaviour. Moreover, there was a significant decrease in the polarization resistance from 6.38 to 0.8 V/ cm2 in the case of CRCA painted sheet and from 6.07 to 4.07 V/cm2 in the case of GA painted material. As the polarization resistance is inversely proportional to the corrosion rate, painted GA has shown superior corrosion resistance. This was also confirmed from the scribed samples exposed to salt spray test. The corrosion product was also very low in GA painted compared to CRCA at scribed areas. 3.4. Image analysis of painted CRCA and GA sheet The paint ability of coated sheet would normally be defined by its compatibility with the existing plant paint system assembly and its appearance after the final topcoat. The quality of the final paint finish is highly dependent on the condition of the coated steel surface. The surface finish should be within a specified range of roughness and peak density in order to yield a paint finish with an acceptable DOI (distinctness of image). Normal phosphating bath was used on the galvannealed surface to produce a satisfactory coating. Application of the cataphoretic primer coating (E-
N. Bandyopadhyay et al. / Surface & Coatings Technology 200 (2006) 4312–4319
CRCA sheet
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GA sheet
Fig. 9. Image analysis study on GA and CRCA painted after 30 days exposure in 3.5% salt solution.
coat) on the GA surface did not present any problem in the painting line. Anionic electro-deposition primer has now been adopted by the auto makers for excellent corrosion resistance. During the painting process, pinhole like defects called craters (about 0.2–0.3 mm in diameter) may occur in some cases. Such craters can be observed in GA coating layers and they are mainly of intermetallic phases of Zn and iron [10]. In order to study the effect of these catering defects in cationic electro-deposition, painted CRCA and painted GA samples were put in 3.5% salt solution for 30 days to find out any degradation of paint on the surface. Under the image analyzer, it was observed that the pore radius on painted CRCA significantly increased, whereas there was only a marginal change on painted GA material. This is shown in Fig. 9. The study indicates that the water molecules diffused through the pores simultaneously promoted the corrosion reaction across the substrate and coating interface. Our EIS study has also confirmed that the paint on the CRCA material failed completely whereas in the case of the GA material, there was absolutely no change.
4. Conclusions 1. Salt spray exposure clearly demonstrated the effective prevention of any occurrence of film blisters and scabs at damaged parts for GA coating. 2. The results from the CCT tests also clearly demonstrated that the galvannealed coating drastically reduced the under-paint corrosion from a deep scratch or creep-fromscribe. This improvement in this corrosion performance signifies the durable performance of the painted and galvannealed sheets.
3. After 45 cycles of testing, the GA painted sample has shown almost 17 times more corrosion resistance than the painted CRCA material. 4. EIS study has shown that the paint on CRCA material failed completely whereas in the case of the GA material, there was absolutely no change.
Acknowledgement The authors would like to thank the management of Tata Steel for allowing us to publish this paper.
References [1] W. Warnecke, R. Bode, Rand Kothe, L. Meyer, Thyssen Technische Berichte (Germany) 23 (1) (1991) 75. [2] M. Espenhahn, W. Dechene, R. Bode, W. Warnecke, Zinc and Zinc– Alloy Coated Steel—What Is the Best Coating for Automotive Application? Verlag Stahleisen Mgh, Germany, 1992, p. 273. [3] N. Kino, M. Yamada, Y. Tokunaga, H. Tsuchiya, Production of Nb-Ti Added Ultra-Low Carbon Steel for Galvannealed Application, The Minerals, Metals and Materials Society, 1990, p. 197. [4] T.C. Simpson, J.D. Hoffman, SAE Transaction: Journal of Materials and Manufacturing, vol. 104, Society of Automotive Engineers, USA, 1965, p. 246. [5] N.D. Kunde, G.M. Michal, J.H. Payer, 40th Mechanical Working and Steel Processing Conference Proceedings, Iron and Steel Society/ AIME, vol. 36, Iron & Steel Society, Warrendale, PA, USA, 1998 (Oct.), p. 157. [6] W.C. Sievert, L.L. Franks, A Galvanized-Galvannealed Sheet Steel for Automotive Body Panels, American Society for Metals, 1981, p. 165. [7] Y. Miyoshi, Boshoku Gijutsu (Corrosion Engineering) 35 (1) (1986) 38. [8] W. Tait Stephen, Journal of Coating Technology 61 (768) (1989) 57. [9] H. Leidheiser Jr., Corrosion 39 (5) (1983) 189. [10] A. Yasuda, H. Koumura, K. Yamato, K. Onizawa, H. Ota, ASM International (1988) 45.