Electrochemical Corrosion Behavior of AISI 409L Stainless Steel Aluminized
by Hot-Dip Coating Method in Automotive Exhaust Gas Solution
Min-Jun Kim, Seok-Ho Woo and Jung-Gu Kim
+Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, South Korea
This paper focuses on the pitting resistance of hot-dip aluminized (HDA) stainless steel after the depletion of the coating layer formed in the exhaust condensed solution. HDA stainless steel is composed of a coating layer, an interdiffusion layer, and a substrate. In the interdiffusion layer which is mainly composed of Al7(Fe,Cr)2Si and (Fe,Cr)(Al,Si)3, microcracks were observed in the (Fe,Cr)(Al,Si)3 layer. From electrochemical tests, the corrosion potential of the Al-10Si coating-removed 409L is higher than that of the interdiffusion layer-removed 409L. The pit depth of the substrate (anode) exposed by microcracks of the interdiffusion layer (cathode) is drastically increased when the coating layer is extinct. [doi:10.2320/matertrans.M2015134]
(Received March 31, 2015; Accepted June 24, 2015; Published July 31, 2015)
Keywords: stainless steel, hot-dip aluminizing, pitting corrosion, interdiffusion layer, galvanic current
1. Introduction
Ferritic stainless steel is mostly used as materials for automotive exhaust system because it has good corrosion resistance and heat resistance. However, the materials indicating better corrosion resistance have been required because the regulation for exhaust gas has been tightened up recently and the service-life requirements for automobile have been increased. Especially, at the internal site of cold-end exhaust system, condensate water which is mixture of moisture and exhaust gas is generated and it can cause severe internal corrosion of muffler because the condensate water includes aggressive ions such as acetate (CH3COO¹), nitrate
(NO3¹), sulfate (SO42¹) and chloride (Cl¹) ions.13)
Metallic coating is one of the methods to improve the corrosion resistance of the substrate by cathodic protection. Especially, hot dip aluminizing process has been used in various applications because of low manufacturing cost compared with other coating techniques and good brightness of the coating surface. In the case of applications such as muffler, heat exchange tube and boiler, Si is added to the Al melt in order to increase the heat resistance.4,5)In addition, it
was reported that the addition of Si forms thinner and more homogeneous interdiffusion layer.69)Moreover, aluminizing
time and temperature also remarkably affect the thickness and phases of the interdiffusion layer.912) Although the
inter-metallic compound layer can have an effect on the corrosion behavior of coated material, most of research has focused on the sacrificial anode effect of Al coating. Especially, it was reported that the corrosion resistance or high temperature oxidation resistance of aluminized materials were higher than uncoated materials in corrosive environments.13,14)However, the corrosion resistance of substrate after the consumption of coated material is also important when total life of material is considered.
Al indicates localized corrosion in neutral solution because aluminum oxide forms protective passive film. However, because the pH of condensate in automotive muffler can be
lowered down to 2, uniform corrosion can be occurred in acid solution due to the unstable passive oxide on the surface. In that case, Al coating layer can be consumed rapidly due to the high dissolution rate and sacrificial anode effect.
Recent study reported that the steel substrate was only protected by the Al layer and not by the interdiffusion layers in NaCl solution through the measurement of galvanic and Volta potentials.15) This means that the interdiffusion layer
could have a negative effect on the corrosion resistance of the steel substrate when the coated Al is depleted. The research on the corrosion mechanism of aluminized stainless steels has not been fully investigated although the materials were already developed and produced. In addition, it was reported that the accumulation of voids at the interface between the aluminized layer and the steel substrate or the formation of cracks in brittle interdiffusion layer could have an adverse effect on the corrosion resistance.16,17)
In this paper, the electrochemical behavior for aluminized stainless steel in condensed solution after dissolution of coating layer was studied by electrochemical tests, immersion test and surface analysis.
2. Experimental Procedures
2.1 Specimens and solution
Uncoated and Al-10 mass%Si coated AISI 409L stainless steel with thickness of 1 mm were used as test specimens. Table 1 lists the chemical composition of 409L stainless steel. The surface of the uncoated stainless steel was ground with 600-grit silicon carbide paper.
[image:1.595.303.550.755.786.2]Al-Si coating layer was attached to the cold-rolled 409L stainless steel sheet by passing through the molten Al-Si bath above 600°C after heat treatment in continuous galvanizing line. During hot dip aluminizing process, H2gas was injected
Table 1 Chemical composition of 409L stainless steel (in mass%).
Cr Si Mn Ti Ni C N Fe
409L 11.20 0.55 0.25 0.20 0.12 0.006 0.008 Bal.
+Corresponding author, E-mail: kimjg@skku.ac.kr
into the molten bath to prevent oxidation of stainless steel surface. In order to expose the interdiffusion layer, Al-Si coating layer was selectively removed by dipping aluminized stainless steel in 1 mol L¹1NaOH solution.18)For the removal of interdiffusion layer, 60 vol%nitric acid solution was used to expose the substrate of aluminized stainless steel.19)
The synthetic condensed solution for automotive muffler specified in JASO M 611 (Japanese Automotive Standard) was used as the test solution and Table 2 lists the concentration of ions in the test solution and its pH. The temperature of the test solution was 80°C and the solution was aerated with air.
2.2 Surface and composition analyses
The chemical compositions of the coating and the interdiffusion layer for aluminized stainless steel were investigated by scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS) Model S-3000H (Hitachi, Japan) and X-ray diffraction (XRD) Model D8 Discover (Bruker, Germany) with a scan rate of 4 degree min¹1. Electron probe microanalysis (EPMA) Model
JXA-8900R (JEOL, Japan) was performed to analyze the composition and distribution of the elements in the cross section of the specimen. After immersion test of the aluminized stainless steel, SEM was used to measure the pit depth of the specimens through cross-sectional image.
2.3 Anodic polarization test and galvanic corrosion test
Anodic polarization test for aluminized stainless steel was performed by using a 3-electrode system: the specimen with an area of 1 cm©1 cm as the working electrode (WE), graphite rod as the counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode (RE). After open-circuit potential (OCP) was stablilized, anodic polarization potential was swept at a scan rate of 10 mV min¹1from the OCP to 1.2 V vs. SCE.
Anodic polarization tests for Al-Si coating layer, inter-diffusion layer and stainless steel were separately performed in the synthetic condensed solution at 80°C. All anodic polarization tests were run at least twice using potentiostat Model PARSTAT 2263 (Princeton Applied Research, USA) and the data were obtained clearly and reliably.
Zero resistance ammeter (ZRA) test was performed to measure the direction and quantitative values of the galvanic currents. The galvanic currents of three different galvanic couples (Coating layer (WE)/interdiffusion layer (CE), coat-ing layer (WE)/substrate (CE), and substrate (WE)/ inter-diffusion layer (CE)) were measured periodically during 5 days in the synthetic condensed solution. The stable galvanic currents were measured once a day. The exposed area of the specimens was controlled to 1 cm©1 cm. ZRA test was performed using Model VMP2 multichannel potentiostat/ galvanostat (Bio-Logic Science Instruments SAS, France).
2.4 Pit depth measurement and electrochemical impe-dance spectroscopy (EIS)
The pit depth of aluminized 409L stainless steel was measured periodically through the cross section of corrosion coupons immersed in the condensed solution using SEM. The open-circuit potential and electrochemical impedance were also periodically measured to observe the corrosion
resistances of the specimens. EIS measurement was
performed with an amplitude of «10 mV from 100 kHz to 10 mHz using Model VMP2 multichannel potentiostat/ galvanostat. The polarization resistance was calculated by
fitting the Nyquist plots obtained by EIS.
3. Results and Discussion
3.1 Structure and composition of aluminized stainless steel
[image:2.595.308.546.69.429.2]Figure 1 shows the EPMA line profiles for the cross section of the aluminized 409L stainless steel. Three layers were observed, which represent the coating layer, interdiffu-sion layer and substrate, respectively. The EPMA line profiles indicated that Al, Fe, Si and Cr elements were included in the interdiffusion layer (patterned area in Fig. 1(b)). Table 3 lists the compositions at the positions marked in Fig. 1(a) measured by EDS analysis. During the hot-dip aluminizing Table 2 Concentration of ions in the synthetic condensed solution and pH
(in ppm).
SO42¹ SO32¹ NO3¹ Cl¹ CH3COO¹ pH
Concentration 600 600 20 100 800 4 (non dim.)
0 1000 2000 3000
Co
un
ts
Distance, r/μm
Fe (b)
(a)
0 100 200 300 400
Counts Si
0 100 200 300 400
Counts Cr
0 1000 2000 3000 4000 5000
Counts Al
0 5 10 15 20 25 30 35 40 45 50
0 5 10 15 20 25 30 35 40 45 50
0 5 10 15 20 25 30 35 40 45 50
0 5 10 15 20 25 30 35 40 45 50
0 5 10 15 20 25 30 35 40 45 50
0 50 100 150
Interdiffusion layer Substrate
Counts
O Coating layer
[image:2.595.46.292.94.122.2]process, Al is diffused inward from the coating and Cr is diffused outward from the substrate. It was confirmed that the substrate below the interdiffusion layer (position 1 and 2) also included considerable Al content. From this result, it turns out that the exposed surface of the aluminized stainless steel substrate could be different from the surface of uncoated stainless steel. Thus, this composition variation can influence the corrosion resistance of the steel.
In the case of aluminized steel with Al bath except for Si, it was reported that Fe-Al interdiffusion compound layers consist of two sublayers which are usually formed by Fe3Al
and Fe2Al5 compounds.4,6,7,2022) In this study, the average
ratio of Fe:Al:Si in the interdiffusion layer indicated ca. 7.0 : 1.8 : 1.2 from EDS analysis. There were many reports of the composition of the intermetallic compound consisting of Al, Si and Fe. El-Mahallawy8)reported that the
Al20Fe7Si, Al19Fe8Si, Al7Fe2Si and Al3Fe2Si phases are
identified in the interdiffusion layer of Al-Si hot dipping steel. Most of others2327)also reported that the interdiffusion
layer between coating material and substrate is usually ¸5
-Al7Fe2Si phase when Si is added in the Al bath at 700°C. It
was reported that Cr diffused from the stainless steel substrate could be substituted for iron in the Fe-Al-Si intermetallic phase.27)J. L. Songet al.7)also found that Si atoms substitute Al atoms of FeAl3-ordered structure and Cr atoms from the
substrate substitute Fe atoms in interdiffusion layer. Figure 2 shows XRD analysis of coating layer and interdiffusion layer on 409L stainless steel. The peaks of Al and Si were detected in the coating layer, while Al7(Fe,Cr)2Si and (Fe,Cr)(Al,Si)3
intermetallic phases were detected in the interdiffusion layer. Figure 3 shows the interdiffusion layer of aluminized stainless steel after the removal of Al coating top layer using NaOH solution. It was found that the shape of the upper interdiffusion layer indicated complex dendritic structure and another thin layer with thickness of 1 µm was observed below the upper interdiffusion layer. It is suggested that the upper layer is mainly consisted of Al7(Fe,Cr)2Si intermetallic phase
and the inner layer is manily (Fe,Cr)(Al,Si)3 intermetallic
phase.
The thin layer had some microcracks as shown in Fig. 3(b). It is also reported that the cracks may be present at the last interdiffusion layer after the forming process in Al-Si-Fe coated steel.28)It was confirmed that some microcracks
were formed in the inner interdiffusion layer during the aluminizing process and these sites may befilled with coating
material before the coated layer is removed. It is suggested that the substrate can be exposed to the solution with interdiffusion layer because some cracks are formed in the inner interdiffusion layer as soon as the coating material in the defects of the interdiffusion layer is dissolved to the solution.
3.2 Anodic polarization test and ZRA test
[image:3.595.311.538.71.238.2]Figure 4 shows the anodic polarization curves of uncoated 409L stainless steel and three kinds of surface-modified 409L stainless steels with aluminizing process (Al-10Si coated Table 3 Composition of the positions marked with black dots in Fig. 1(a)
(in at%).
Position Al Si Fe Cr
1 19.7 70.3 10.0
2 45.5 49.0 5.5
3 71.9 11.7 16.4
4 70.3 12.0 17.8
5 73.7 13.0 13.4
6 75.7 16.3 8.0
7 73.3 26.7
8 65.8 34.2
20 25 30 35 40 45 50 55 60 65 70 75 80
(Fe,Cr)(Al,Si)3 Al7(Fe,Cr)2Si
Interdiffusion layer
In
tensit
y
Diffraction angle, 2θ (degree) Al Si Al-Si coated layer
Int
ensity
Fig. 2 XRD analysis of Al-Si coated layer and interdiffusion layer.
(a)
(b)
[image:3.595.45.292.92.206.2] [image:3.595.320.533.276.592.2]409L, Al-10Si coating-removed 409L, and interdiffusion layer-removed 409L) in the synthetic condensed solution at 80°C and the polarization data are summarized in Table 4. Al-10Si coated 409L indicates active corrosion behavior and this is due to the low pH of the solution which is difficult to form the passivefilm of Al oxide. In addition, Al-10Si coated 409L indicates the lowest corrosion potential (Ecorr) among
the layers. Therefore, the coating layer protects the substrate and interdiffusion layer when the coating layer and the local sites of the substrate or interdiffusion layer are exposed to the solution simultaneously.
The corrosion potential of the Al-10Si coating-removed 409L (interdiffusion layer) is higher than that of the interdiffusion layer-removed 409L (substrate) and the passive current of the Al-10Si coating-removed 409L is two orders of magnitude lower than that of the interdiffusion layer-removed 409L. The pitting potential (Epit) of Al-10Si coating-removed
409L is also higher than that of the interdiffusion layer-removed 409L. From the polarization curves, the interdiffu-sion layer has excellent corrointerdiffu-sion resistance in the condensed solution and it is cathodically protected by the substrate when all the coating materials are dissolved. The galvanic coupling between the interdiffusion layer and the substrate is formed. I. D. Graeve15) also reported similar results that the interdiffusion layer indicated more noble potential than substrate for pure Al and Al-Si coated steels.
The uncoated 409L indicates a higher corrosion potential and a lower passive current density than the interdiffusion layer-removed 409L. When stainless steel is hot-dip alumi-nized, the protective property of passive film formed on the substrate in the solution is declined compared with the film
formed on uncoated stainless steel. In the case of the substrate of aluminized stainless steel, it could not form protective passive film like that on uncoated stainless steel due to the decrease of Cr and the increase of Al content by diffusion in the substrate adjacent to the interdiffusion layer as shown in Table 3. Therefore, the intermetallic compounds consisting of interdiffusion layer decrease the corrosion resistance of the substrate and can accelerate the corrosion rate of the substrate of aluminized stainless steel.
ZRA test was performed for the clear distinction of galvanic effects among the layers consisting of different materials. Figure 5 shows the galvanic current between two materials connected by a lead wire. All of the directions of the galvanic currents among three pairs of electrodes obtained from ZRA test were identical to the directions of the galvanic current implied by the corrosion potentials obtained from the anodic polarization test in Fig. 4. The initial galvanic current between Al-10Si coated 409L and interdiffusion layer-removed 409L (30³40 µA·cm¹2) was
higher than that between Al-10Si coated 409L and Al-10Si coating-removed 409L (10³25 µA·cm¹2). When the coating
layer was depleted, the galvanic current can flow from the substrate to the interdiffusion layer, which is enough to damage the substrate. Therefore, it is critical to the corrosion resistance of the substrate when coating layer is extinct in the condensed solution.
3.3 Pit depth, OCP, and EIS measurements and cross-sectional analysis after immersion test
Figure 6 shows the maximum pit depth for aluminized 409L stainless steel as a function of immersion time in the condensed solution. The maximum pit depth of aluminized 409L stainless steel was less than about 5 µm until 56-day of immersion because the substrate was protected by Al-Si coating layer.
However, after 84-day of immersion, the maximum pit depth of aluminized 409L stainless steel increased drastically and the maximum pit depth during 110 days is about 105 µm for aluminized stainless steel. During the same immersion time, OCP and EIS measurements were performed to observe the variation of electrochemical characteristics. Figure 7 shows the OCP and polarization resistance of aluminized 409L stainless steel as a function of immersion time in the condensed solution. It was confirmed that the OCP of the specimen was stable during about 50-day of immersion and it was the potential of aluminized coating layer. After 50 days, the OCP of the specimen increased steadily with immersion time. At that time, the polarization resistance which is inversely proportional to the corrosion current also increased with increasing OCP. The increased polarization resistance
10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
P
o
te
nt
ial,
E
/ V v
s. SC
E
Current density, i/ A⋅cm-2 Al-10Si coated 409L
Al-10Si coating-removed 409L Interdiffusion layer-removed 409L Uncoated 409L
[image:4.595.51.549.83.161.2]Fig. 4 Anodic polarization curves of uncoated 409L stainless steel and aluminized 409L stainless steel in the condensed solution at 80°C.
Table 4 Anodic polarization test results of uncoated and aluminized 409L stainless steels in the synthetic condensed solution.
Specimen Corrosion behavior Ecorr/V
vs. SCE ipass/µA·cm
¹2 Epit/V vs. SCE
Aluminized 409L
Al-10Si coated 409L Active ¹0.46 -
-Al-10Si coating-removed 409L Passive ¹0.11 0.05 0.32
Interdiffusion-removed 409L Passive ¹0.32 2.20 0.31
[image:4.595.50.287.125.362.2]value after 50 days is not the resistance by the coating layer but by the interdiffusion layer.
It might be explained by the extinction of all aluminized coating layer by dissolution and the exposure of interdiffu-sion layer and/or substrate. From Figs. 6 and 7, it could suggest that the pit depth of the substrate for aluminized stainless steel drastically increased after the coating layer completely disappeared. It must be pointed out that the passive film of aluminized stainless steel substrate is not protective due to the limited Cr-content in the passive film.
Figure 8 shows the SEM images of the pits of the aluminized stainless steel after immersion test. It was
confirmed that the interdiffusion layer was remained on the substrate, although the pits of the substrate were grown. This means that the interdiffusion layer was protected by the substrate.
As shown in Fig. 8(c), the presence of interdiffusion layer after the pit growth into the substrate is related to the increase of the pit growth rate after the extinction of coating layer. When the interdiffusion layer is exposed to the solution, the localized area of the substrate is also exposed because the inner intermetallic compound (Fe,Cr)(Al,Si)3 has
micro-cracks. In addition, these cracks can act as a kind of crevice. Because the solution within the crevice is stagnant, the oxygen depleted can not be replenished29)and it additionally
decreases the corrosion resistance of the substrate. Since the substrate (small anode area) and the interdiffusion layer (large cathode area) form micro-galvanic coupling, the pit propagation rate of the substrate is drastically increased.
In brief, the diffusion of elements during hot-dip aluminizing and subsequent galvanic corrosion between the interdiffusion layer and the substrate are the main cause of the increased pit growth rate of the stainless steel substrate in the exhaust condensed solution.
4. Conclusions
In this study, the corrosion resistance of hot-dip aluminized stainless steel in the exhaust condensed solution after the
10 15 20 25 30 35 40 45 50 Galva nic current , j / μ A
Immersion time, t /day
Galvanic current between Al-10Si coated 409L and Interdiffusion layer-removed 409L
(a) 10 15 20 25 30 35 40 45 50 (b) Galvanic current , j / μ A
Immersion time, t / day
Galvanic current between Al-10Si coated 409L and Al-10Si coating-removed 409L
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 (c) Galva nic current , j / μ A
Immersion time, t / day
Galvanic current between Al-10Si coated 409L and interdiffusion layer-removed 409L
Fig. 5 Galvanic currents between (a) Al-10Si coated 409L (WE) and interdiffusion layer-removed 409L (CE), (b) Al-10Si coated 409L (WE) and Al-10Si coating-removed 409L (CE), and (c) interdiffusion layer-removed 409L (WE) and Al-10Si coating-layer-removed 409L (CE).
0 10 20 30 40 50 60 70 80 90 100 110
0 10 20 30 40 50 60 70 80 90 100 110 Maximu m pit dep th , d / μ m
Immersion time, t / day Aluminized 409L
Fig. 6 Maximum pit depths for aluminized 409L stainless steel as a function of immersion time in the condensed solution at 80°C.
0 10 20 30 40 50 60 70 80 90 100 110 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0 5 10 15 20 25 30 35 P o la ri zat ion r e si st ance, Rp /k Ω ⋅ cm 2 P o te n tia l, E / V v s. S C E
Immersion time, t / day
Open circuit potential Polarization resistance
[image:5.595.62.277.66.576.2] [image:5.595.319.533.71.232.2] [image:5.595.312.540.271.417.2]dissolution of coating layer was investigated and the conclusions are as follows:
(1) In the hot-dip aluminized 409L stainless steel, Fe, Cr, Al and Si elements were included in the interdiffusion layer and the layer was composed of two sublayers. From XRD analysis, Al7(Fe,Cr)2Si and (Fe,Cr)(Al,Si)3
peaks were found in the interdiffusion layer.
(2) From anodic polarization curves for aluminized stain-less steel in the condensed solution, the corrosion potential and pitting potential of the interdiffusion layer were higher than those of the substrate of aluminized stainless steel.
(3) Immersion test and electrochemical measurements indicated that the pit depth of aluminized stainless steel
was increased after the coating layer was extinct. (4) The interdiffusion layer and the substrate can be
exposed simultaneously by the defects in the inner (Fe,Cr)(Al,Si)3 layer and micro-galvanic coupling
between the interdiffusion layer (cathode) and the substrate (anode) is formed. As a result, the pit growth rate of the substrate can be accelerated.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (Ministry of Education and Science Technol-ogy) (No. NRF-2012R1A2A2A03046671).
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(a)
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[image:6.595.62.277.69.562.2]