7/23/2019 Pitting Corrosion 304
Pitting corrosion of 304ss nanocrystalline thin film
Chen Pan, Li Liu , Ying Li, Fuhui Wang
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Rd., Shenyang 110016, China
a r t i c l e i n f o
Received 5 November 2012
Accepted 25 March 2013
Available online 6 April 2013
A. Sputtered film
C. Pitting corrosion
a b s t r a c t
Pitting corrosion behavior of coarse crystalline (CC) 304ss and its nanocrystalline (NC) thin film have
been investigated by electrochemical measurement and in situ AFM observation in 3.5% NaCl solution.
Results show two effects of nanocrystallization on pitting corrosion behavior: (1) more frequent occur-rence of metastable pits, but with lower probability of transition to stable pits, which is attributable to
differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of NC
thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger
pit cavities, and modifies the morphology of stable pit cavity.
2013 Elsevier Ltd. All rights reserved.
Three hundred and four stainless steels have been extensively
used as good corrosion resistant materials. However, resistance
to pitting corrosion of 304ss in solutions containing Cl is not good
enough and adversely influences the service life and integrity ofstructures made of this material. Hence, there is need to improve
the corrosion resistance of 304ss. Some investigations have shown
that nanocrystallization can significantly enhance the corrosion
resistance of stainless steels. Among several nanocrystalliza-
tion methods, magnetron sputtering technique has attracted con-
siderable attention. Through the magnetron sputtering technique,
a homogeneous thin film, having the same composition as the tar-
get, but with a smaller grain size, can be deposited on a material.
The sputtered thin film has the same chemistry with the substrate,
which ensures good adhesion to the thin film on the substrate.
In addition, the sputtered nanocrystalline thin films have been
found to possess better corrosion resistance than the correspond-
ing conventional coarse crystalline alloysand have been used
successfully in high-temperature applications.
It is well known that the corrosion behavior of 304 stainless
steel mainly includes passive and pitting behavior. A previous
study  has demonstrated that nanocrystallization changed
the nucleation mechanism and the growth structure of the passive
film on rolled coarse crystalline (CC) 304ss and also accelerated the
growth rate of the passive film, thereby enhancing the passivation
ability of the material. On the other hand, the influence of nano-
crystallization on the pitting corrosion mechanism of CC 304ss is
not clearly known; therefore, it is significant to study the influence
of nanocrystallization on the pitting corrosion behavior of CC
Pitting corrosion is a complicated phenomenon, which is largely
dominated by random parameters . The pit generation event
has been widely considered to be stochastic in nature, and pit ini-
tiation processes have been investigated via stochastic analysis[13,14]. It is believed that both pit initiation rate and pit growth
probability influence the pitting corrosion resistance. If a material
exhibits high pit initiation rate, metastable pits would spread over
the surface, and the pit would easily become a larger cavity, pro-
vided the material has a high pit growth rate. This stochastic ap-
proach could indicate the frequency of metastable pit events and
the probability of stable pits being formed, which deeply exposes
the stochastic nature of pitting corrosion.
In this work, a stochastic approach and in situ AFM observation
were employed to study the characteristic features of both the pit
initiation and the pit growth processes of the CC 304ss and its
sputtered thin film in 3.5% NaCl solution. Our goal is to understand
the effect of nanocrystallization on the pitting process.
2.1. Materials preparation
The composition (in wt.%) of CC 304ss was as follows: 8.054%
Ni, 17.10% Cr, 0.091% Mo, 1.280% Mn, 0.277% Cu, 0.003% As,
0.006% Sn, 0.387% Si, 0.045% C, 0.026% P, 0.002% S, and the rest
Fe. The NC thin film was deposited on one side of a glass substrate
using the SBH-5115D DC magnetron sputtering system with CC
304ss as target. A glass substrate, much unlike a stainless steel sub-
strate, will not likely interfere with the electrochemical responses
of the thin film. The magnetron sputtering chamber was evacuated
0010-938X/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.03.022
Corresponding author. Tel./fax: +86 24 2392 5323.
E-mail address:firstname.lastname@example.org(L. Liu).
Corrosion Science 73 (2013) 3243
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7/23/2019 Pitting Corrosion 304
to 5 103 Pa, then filled with Ar, and maintained at 0.2 Pa. The
temperature of the substrate glass was approximately 200C.
The DC power was 1800 W and the sputtering duration was 2 h.
The CC 304ss samples (10 mm 10 mm 10 mm) were ground
to 1000 grit SiC paper and degreased in acetone. The CC 304ss and
NC thin film were either embedded in epoxy resin or paraffin-resin,
leaving an exposed working area.
2.2. Materials characterization
The microstructure of the CC 304ss was characterized by optical
metallography. The grain size of the NC thin film was characterized
by transmission electron microscopy (TEM) (JEM-2000EXII).
Transmission electron microscope, equipped with a high-angle
angular-dark-filed (HAADF) detector and X-ray energy-dispersive
spectrometer (EDS) system, was used for electron diffraction,
HAADF imaging, and composition analysis. The cross-section of
the CC 304 was observed by scanning electron microscopy (SEM)
(XL30FEG). The phases of the two materials were analyzed by
X-ray diffraction (XRD) analysis.
2.3. Electrochemical experiments
All electrochemical measurements were performed using an
Autolab Electrochemical Measurement System (EG&G) in a con-
ventional three-electrode cell, with a large platinum plate as the
counter electrode and a saturated calomel electrode (SCE) as the
reference electrode. All potential values reported in this paper
are with reference to SCE in saturated KCl solution whose potential
value vs. SHE is 0.2438 V. The aggressive medium used in all exper-
iments was 3.5% NaCl solution prepared from reagent grade chem-
icals and distilled water. The test solution was degassed with
nitrogen for 1.5 h before experiment. A water bath was used to
maintain the solutions at 30 1C during testing.
Prior to all electrochemical measurements, the specimens wereinitially reduced potentiostatically at 1 VSCE for 2 min to remove
air-formed oxides on the surface and then kept in solution until a
stable corrosion potential was attained.
For the polarization measurements, the specimens were kept in
the NaCl solution until a stable corrosion potential was attained
and then scanned in 0.333 mV/s. For the induction time measure-
ment, a potentiostatic technique (0.25 VSCE and 0.9 VSCE for CC
304ss and NC thin film, respectively) was used to measure the ano-
dic current trace. The current response to the applied potential was
recorded within a data-sampling interval of 0.2 s. A sudden in-
crease was the result of the pit corrosion, which was confirmed
by morphology observation. The time interval for this sudden cur-
rent increase is defined as the pit induction time.
2.4. Pit diameter and pit depth measurements
For CC 304ss, the radius of pit mouth (a) was determined from
photomicrography by measuring the area of the pit mouth with a
planimeter in the microscope. The estimated error in a is 5%. Pit
depths, h, were measured by applying the Fine Focus Technique
, where the distances required shifting the optical objective be-
tween the focal points on the original surface of sample and on the
bottom of the pit are compared. The estimated error inh is 1 lm.
For the NC thin film, the radius of pit mouth a and pit depth h
was obtained by AFM observation. The AFM resolution is 1.4 nm
inXYscan size and 0.5 nm in Zscan range. The AFM observation
was replicated severally on different samples. The results shown inthe paper are the average sizes.
2.5. In situ AFM measurements
For AFM measurements, the NC thin film was cut into coupons
of dimensions 20 mm 20 mm 2 mm and fi