Pickling Behavior of AISI 304 Stainless Steel in Sulfuric and Hydrochloric Acid Solutions

Abstract

Oxide scales as well as a Cr-depleted layer, which grows between the oxide scale and base metal, are formed on AISI 304 stainless steel surface during high temperature processing. Pickling is an important process which includes mechanical and chemical operations to remove oxide scales, Cr-depleted layers, and to recover the surface passivity. The multi-step pickling is commonly used because of its higher efficiency than a single step pickling. In this study, the multi-step pickling of AISI 304 stainless steel in HCl solution was investigated instead of H2SO4 solution for the first step of pickling. HF+HNO3 mixed acid is traditionally used in the second step of pickling. The pickling mechanism of HCl and H2SO4 was discussed based on weight loss and the pickled surface qualities. It was found that the first step pickling efficiency directly affected the surface qualities of the final pickled sample. HCl solution showed much lower pickling efficiency than H2SO4 solution. This resulted in a high concentration of remaining oxide scales and intergranular attack at the Cr-depleted layer, which cannot be completely removed in the second pickling step. Increasing of HCl concentration and electrolytic current did not improve the pickling efficiency. The addition of a small amount of H2O2, which is a strong oxidizing agent, significantly improves the pickling efficiency of HCl. A smooth surface without any oxide scale and free of intergranular attack could be obtained.

Full text

Journal of Metals, Materials and Minerals, Vol.20 No.2 pp.1-6, 2010
Pickling Behavior of AISI 304 Stainless Steel in Sulfuric
and Hydrochloric Acid Solutions
Wanna HOMJABOK1*, Siriwan PERMPOON2 and Gobboon LOTHONGKUM1*
1Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University,
Patumwan, Bangkok 10330, Thailand
2Thainox Stainless Public Company Limited, 324 Moo 8, Highway no. 3191 Road,
Tambol Mabkha, Nikom Pattana, Rayong 21180, Thailand
Abstract
Oxide scales as well as a Cr-depleted layer, which grows between the oxide scale and base metal,
are formed on AISI 304 stainless steel surface during high temperature processing. Pickling is an important
process which includes mechanical and chemical operations to remove oxide scales, Cr-depleted layers, and
to recover the surface passivity. The multi-step pickling is commonly used because of its higher efficiency
than a single step pickling. In this study, the multi-step pickling of AISI 304 stainless steel in HCl solution
was investigated instead of H2SO4 solution for the first step of pickling. HF+HNO3 mixed acid is
traditionally used in the second step of pickling. The pickling mechanism of HCl and H2SO4 was discussed
based on weight loss and the pickled surface qualities. It was found that the first step pickling efficiency
directly affected the surface qualities of the final pickled sample. HCl solution showed much lower pickling
efficiency than H2SO4 solution. This resulted in a high concentration of remaining oxide scales and
intergranular attack at the Cr-depleted layer, which cannot be completely removed in the second pickling
step. Increasing of HCl concentration and electrolytic current did not improve the pickling efficiency. The addition
of a small amount of H2O2, which is a strong oxidizing agent, significantly improves the pickling efficiency
of HCl. A smooth surface without any oxide scale and free of intergranular attack could be obtained.
Key words: Pickling, Hydrochloric acid, Sulfuric acid, Scale, Stainless steel
Introduction
Acid pickling is an important step for the
production of cold rolled stainless steel plates.
Its aim is to remove the oxide scale as well as a
Cr-depleted layer growing between the oxide scale
and the base material. Oxide scale and Cr-depleted
layer are formed during high temperature processing.
Removing oxide scale processes consists of
mechanical descaling and pickling. In mechanical
descaling, scale breaker and shot blasting were
used to break up the oxide scale. This results in
easily penetrating the pickling solution into oxide
scales and enhances the pickling efficiency.(1-5)
Multi-step pickling is used for the pickling process
because it has higher efficiency and better surface
quality than single step.(6-7) In the first step, electricity
was used in order to increase the pickling efficiency.(8)
In this step, the mechanism is that the solution
penetrates into metal Cr-depleted layer and the
oxide scale is undercut and removed.(6) The acid
type and concentration have a strong influence on
the surface finish quality. For the second step,
HF+HNO3 has become widely accepted and used
for removing the remaining oxide scale and
passivation.(8) The sequence at which the pickling
steps are used influences the surface finish
significantly. H2SO4 is a cheap acid and has a good
pickling efficiency, which can be improved by
being used together with electricity, so that
H2SO4 with electricity is generally used for the
first step. However, H2SO4 pickling causes
black smut forming. Even though black smut
can be removed by HF+HNO3 in the next step,
the surface finish has high roughness and
intergranular attack. In this study, the multi-
step pickling behavior of AISI 304 austenitic
stainless steel in HCl solution was experimented
for replacing H2SO4 solution in the first step,
and the HF+HNO3 mixed acid solution was
used traditionally in the second step. HCl
pickling has a uniform dissolution behavior with
no intergranular attack.(8-10) Results were discussed
*Corresponding author E-mails: whomjabok@yahoo.com, Gobboon.L@chula.ac.th

2 HOMJABOK, W. et al.
based on weight loss and surface finish of the
pickled samples.
Materials and Experimental Procedures
Material
AISI 304 austenitic stainless steel strips
were hot-rolled down to a thickness of 3 mm. The
chemical composition of this material is listed in
Table 1. After mechanical descaling process, test
samples of 25x50x3 mm were cut. Subsequently,
only the unexposed area was painted with EPIGEN
XD005 (acid-resistant at high temperature), and
cleaned with acetone and ethanol. The test samples
were finally dried with air and kept in a desiccator
before experiment.
Table 1. Chemical composition (wt%) of AISI 304
stainless steel used in this study, analyzed
by Optical Emission Spectrometer (OES).
Pickling
To prepare the HCl, H2SO4, HF and HNO3
electrolytes, analytical grade was used. A purity of
50% of H2O2 was used in this study. During
pickling, temperature was controlled constantly in
a water bath with constant stirring. After pickling,
the samples were rinsed with tap water and brushed
for removing any reaction products. The first step
of pickling conditions was 4.0, 4.5, 5.0, 6.0-M HCl
at 85°C or 4.0-M HCl with electricity at 85°C or
4.0-M H2SO4 with or without electrolytic at 85°C
or 4.0-M HCl + 0, 5, 10, 15 g/L H2O2 at 60°C
depending on the purposed tests. After the first step
of pickling, the 1.0-M HF+1.0-M HNO3 mixed
acid solution at 45°C was traditionally used in the
second step.
Characterization
The surface finish was characterized with
roughness profiler (Telescan 150) for surface
roughness. Optical microscope (OM) at 200X and
scanning electron microscopy (SEM) at 3000X
were used for analysis of the remaining oxide
level. Figure 1 shows the evaluation of the
remaining oxide on the sample surface after the in-
house standard.
Figure 1. Remaining oxide evaluations, after the in-house
standard, on 6-area observations on test
sample surface at 200X.
Results and Discussion
HCl solution was investigated instead of
H2SO4 solution for the first step of pickling.
HF+HNO3 mixed acid solution was still traditionally
used in the second step. The traditional and studied
conditions for this experiment are shown in
Figure 2. The total weight loss resulting from those
multi-step pickling conditions is shown in Figure 3.
The weight loss of H2SO4 condition was high, but
some oxide scales remained on the pickled surface
in level 2 as shown in Figure 4(a). Pickling by
H2SO4 solution with electricity followed by
HF+HNO3 solution increased the weight loss and
allowed achieving a surface finish free of any oxide
scale as shown in Figures 3 and 4(b), respectively.
Figure 2. Multi-step pickling of AISI 304 stainless steel
between the traditional and studied conditions.
Figure 3. Total weight loss of multi-step pickling of
AISI 304 stainless steel in H2SO4 at 85°C or
HCl at 85°C followed by HF+HNO3 at 45°C.
Element C Cr Ni Mn Si P S Fe
Content
(wt%) 0.04 18.1 8.03 1.076 0.342 0.029 0.001 Balanced

3
Pickling Behavior of AISI 304 Stainless Steel in Sulfuric and Hydrochloric Acid Solutions
The case of HCl pickling instead of H2SO4
pickling showed that HCl had lower pickling efficiency
than H2SO4 (Figure 3) and many oxide scales remained
(Figure 4(c)). Increasing HCl concentration and HCl
pickling with electricity did not result in increasing
the pickling efficiency to be higher than H2SO4
pickling efficiency. The surface finish of HCl pickling
had a rougher surface and more intergranular attack
than H2SO4 pickling as shown in Figure 4(d).
The result was not the same as reported by Li et al.
(2008) who stated that uniform dissolution
and no intergranular attack were observed by
HCl pickling.
Figure 4. SEM surface characterization of AISI 304
stainless steel after multi-step pickling.
To understand the pickling mechanism by
both HCl and H2SO4 in the first pickling step,
which has a significant effect on the final surface
finish after HF+HNO3 pickling, weight loss was
analyzed step by step as shown in Figure 5, and the
surface was characterized by SEM as shown in
Figure 6. HCl pickling had much lower weight loss
than H2SO4 pickling, and the surfaces of both samples
were covered with oxide scale (Figure 6(a) and 6(b)).
HCl pickling had smooth surface compared
with H2SO4 pickling. After the second pickling
step with HF+HNO3, HCl pickling had higher
weight loss than H2SO4 pickling and the intergranular
attack became more pronounced on surface finish
(Figure 6(c) and 6(d)).
Figure 5. Step by step weight loss of AISI 304 stainless steel
after pickling in 4.0 M H2SO4 at 85°C or 4.0 M
HCl at 85°C followed by HF+HNO3 at 45°C.
(a)
4.0 M H2SO4; 85°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 3.29 μm
Remaining oxide level 2
(c)
4.0 M HCl; 85°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 3.34 μm
Remainin
g
oxide level 3
(b)
4.0 M H2SO4 (Electricity); 85°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 3.30 μm
Remaining oxide level 0
4.0 M HCl (Electricity); 85°C
followed by HF+HNO3; 45°C
(d)
Roughness (Rq) = 3.51 μm
Remainin
g
oxide level 3
Roughness (Rq) = 3.81 μm
Remaining oxide level 3
(a)
4.0 M H2SO4; 85°C

4 HOMJABOK, W. et al.
Figure 6. SEM surface characterization of AISI 304
stainless steel after multi-step pickling under
the same conditions as in Figure 5.
According to the previous results (Figures
5 and 6) and discussion, the evolution of surface
during multi-step pickling in H2SO4 and HCl
solutions followed by HF+HNO3 can be described
as in Figure 7(a) and 7(b), respectively. The
original metal surface consists of oxide scale,
Cr-depleted layer and base metal. On H2SO4
pickling in the first step, H2SO4 transports into
oxide scale. Then, the Cr-depleted layer is attacked
or dissolved. Finally, the oxide scale is removed by
undercutting. Most oxide scales, but only some
Cr-depleted layers, are removed. The surface
is rough because H2SO4 pickling behavior is
non-uniform dissolution. In the next step of the
pickling process by the selective dissolution of
HF+HNO3, an intergranular attack appears. The
remaining oxide scale and Cr-depleted layer are
almost removed. The final surface finish is completely
free of oxide scales. The evolution of the surface
finish after pickling in HCl followed by a pickling
in HF+HNO3 is shown in Figure 7(b). The same
mechanism as H2SO4 is obtained. However, HCl
has lower pickling efficiency than H2SO4. Most of
the oxide scales and Cr-depleted layers still remain.
The observed surface is smooth because HCl
pickling behavior is uniform dissolution. By
HF+HNO3 pickling in the second step, intergranular
attack appears because of a selective dissolution on
the remaining Cr-depleted layer.
Figure 7. The multi-step pickling mechanism models of
intergranular attack.
According to the mechanism, the most
important finding is that the surface finish obtained
from multi-step pickling is greatly affected by the
pickling efficiency of the first step. Multi-step
pickling will successively allow achieving a
smooth surface finish free of any oxide scale when
a high enough pickling efficiency with uniform
dissolution in the first step is available. The result
illustrates that increasing of HCl concentration and
electrolytic currents were not enough to improve
its pickling efficiency to be more than the H2SO4
efficiency. The addition of H2O2, which is a strong
Roughness (Rq) = 3.15 μm
Remaining oxide level 3
(b)
4.0 M HCl; 85°C
Roughness (Rq) = 3.29
μ
m
Remainin
g
oxide level
3
(c)
4.0 M H2SO4; 85°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 3.34
μ
m
Remaining oxide level
3
4.0 M HCl; 85°C
followed by HF+HNO3; 45°C
(d)

5
Pickling Behavior of AISI 304 Stainless Steel in Sulfuric and Hydrochloric Acid Solutions
oxidizing agent, possibly improved the pickling
efficiency of HCl. The temperature for this study
must be fixed at 60°C because H2O2 decomposes at
temperatures higher than 60°C.
Figure 8. Step by step weight loss of AISI 304 stainless
steel by pickling with H2SO4 (Electricity) at
85°C or HCl at 60°C or HCl+H2O2 at 60°C
followed by HF+HNO3 at 45°C.
Addition of H2O2 to improve the pickling
efficiency of HCl in the first step resulted in
increasing weight loss and effected on the second
step pickling by HF+HNO3 by decreasing weight
loss, as shown in Figure 8. It also reduced intergranular
attack and delivered a smooth surface finish as shown
in Figure 9(c) and 9(d). Multi-step pickling was
successive at 10g/L H2O2 added to HCl solution. It
allowed achieving a higher pickling efficiency than
H2SO4 efficiency, a smooth surface finish free of
oxide scales, and no intergranular attack.
Figure 9. SEM surface characterization of AISI 304
stainless steel after pickling in H2SO4, HCl,
HCl + H2O2 solutions followed by HF+HNO3
at 45°C.
Conclusions
The multi-step pickling of AISI 304 stainless
steel in HCl solution as the first step followed by
HF+HNO3 as the second step was investigated.
Mechanism models of pickling by HCl or H2SO4 in
the first step were proposed. The following
conclusions can be drawn from this study:
1. HCl solution has lower pickling efficiency
than H2SO4 solution.
2. HCl solution can not completely remove
Cr-depleted layer and oxide scale.
3. H2O2 addition can improve pickling efficiency
of the HCl solution. The addition of 10g/L H2O2 is
enough to deliver a smooth surface without any
oxide scales and free of intergranular attack after
HF+HNO3 pickling.
Acknowledgements
The authors would like to thank the
Research and Development Center of Thainox
Stainless Public Company Limited for test samples,
Roughness (Rq) = 3.30 μm
Remaining oxide level 0
(a)
4.0 M H2SO4 (Electricity); 85°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 3.24 μm
Remainin
g
oxide level 3
(b)
4.0 M HCl; 60°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 2.92 μm
Remaining oxide level 0
(c)
4.0 M HCl+10g/L H2O2; 60°C
followed by HF+HNO3; 45°C
Roughness (Rq) = 2.95 μm
Remaining oxide level 0
4.0 M HCl+15g/L H2O2; 60°C
followed by HF+HNO3; 45°C
(d)

6 HOMJABOK, W. et al.
discussion and analysis equipment. We would also
like to express our gratitude to the Thailand
Research Fund (TRF) and the Office of Small and
Medium Enterprises Promotion (OSMEP) for the
research fund, content number MRG-OSMET
505034.
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