ultrafine grained AISI 304 metastable austenite stainless steel

Accepted Manuscript Title: Corrosion properties of phase reversion induced nano/ultrafine grained AISI 304 metastable austenite stainless steel Authors: Yang Lv, Hongyun Luo, Jun Tang, Jingjing Guo, Jinliang Pi, Kanglin Ye PII: DOI: Reference:

S0025-5408(17)33608-5 https://doi.org/10.1016/j.materresbull.2018.04.024 MRB 9962

To appear in:

MRB

Received date: Revised date: Accepted date:

19-9-2017 21-2-2018 14-4-2018

Please cite this article as: Lv Y, Luo H, Tang J, Guo J, Pi J, Ye K, Corrosion properties of phase reversion induced nano/ultrafine grained AISI 304 metastable austenite stainless steel, Materials Research Bulletin (2010), https://doi.org/10.1016/j.materresbull.2018.04.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Corrosion properties of phase reversion induced nano/ultrafine grained AISI 304 metastable austenite stainless steel Yang Lv1,3, Hongyun Luo1,2,3*, Jun Tang1,3, Jingjing Guo1, Jinliang Pi1, Kanglin Ye1 1 Key

Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, China

2

The Collaborative Innovation Center for Advanced Aero-Engine (CICAAE), Beijing University of Aeronautics

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and Astronautics, Beijing, China 3

Beijing Key Laboratory of Advanced Nuclear Materials and Physics, Beijing University of Aeronautics and

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Astronautics, Beijing, China * Corresponding author. Tel.: +86 10 82339905; fax: +86 10 82317108.

E-mail address: [email protected] (H. Luo).

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Graphical abstract

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Highlights

 AISI 304 stainless steel was cold rolled under 3 cryogenic temperatures and room temperature, after annealing bulk 1

nano/ultrafine grained material was obtained.  Microstructure of samples was studied by TEM, -120 ◦C rolled sample had an equiaxed microstructure.  Corrosion resistance of samples was studied by polarization

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curve, EIS, and Mott-Schottky plot, -120 ◦C rolled sample had the best corrosion performance.

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 The distinctive polarization curve of -120 ◦C rolled sample

indicated its passive film had peculiar feature, the M-S plot

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showed its abnormal semiconductor character.

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Abstract: AISI 304 metastable austenite stainless steel was cold rolled under cryogenic temperature and transformed into martensite completely, after reverse transformation by annealing,

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bulk nano/ultrafine grained material was obtained. The material’s microstructure was observed by TEM, and the corrosion resistance was studied by polarization curve, EIS, and Mott-Schottky plot.

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Results showed that samples cold rolled at -120 ◦C had an equiaxed microstructure and displayed the best corrosion performance. The distinctive polarization curve shape indicated that its passive

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film had a peculiar feature, and the M-S plot showed its abnormal semiconductor character. XPS measurements are used to directly characterize the composition of passive film formed on

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specimens. Heterogeneous microstructures might help with the improvement on corrosion resistance of the nano/ultrafine grained stainless steel. Key words: 304 austenite stainless steel; Reverted martensite-austenitic transformation; Passive

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film; Corrosion resistance; Cryogenic temperature. 1 Introduction Stainless steel is a steel alloy with a minimum of 10.5% chromium content by mass. Stainless steel’s resistance to corrosion and staining, low maintenance, and familiar lustre make it an ideal material to be used in cookware, cutlery, surgical instruments, major appliances, industrial 2

equipments, storage tanks and as construction material in buildings. It is known that many austenitic stainless steels are unstable at room temperature so that austenite can be transformed to martensite by deformation [1]. In subsequent annealing, the martensite can be reverted to austenite and lead to noticeable grain refinement. Special nano/ultrafine grain was obtained by controlling phase reversion annealing temperature and amount of cold rolling [1-3], leading to an excellent tensile strength-ductility combination [1,4] and fatigue behavior [5]. However, pitting corrosion

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and stress corrosion crack (SCC) could occur due to destruction of passive film [6, 23]. Previous researches showed that passive films formed on stainless steels exhibited semiconducting behavior

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[8, 9], although the exact correlation between the structure characteristics of the passive films and their stabilities were not fully established. It was revealed that the passive films on many stainless steels contained two layers. For example, the passive film on 316LN stainless steels was

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composed of the inner p-type and the outer n-type semiconductors [9]. Besides, a three-layer

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model was suggested in passive film formed on 304L stainless steels in acidic solutions [10].

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MacDonald and Nicic [11] suggested that the anodic passive film on stainless steels contained a high concentration of point defects, such as metal vacancies, electrons and vacancies. They also

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proposed and developed the point defect model (PDM). Lv et al. [12-16] did a lot of research on

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passive films and corrosion properties of stainless steel with nano/ultrafine structures. Ye et al [17]. acquired nano/ultrafine grained AISI 304 metastable austenite stainless steel under rolling at liquid nitrogen temperature. Microstructure and tensile properties were examined,

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but the corrosion resistance of thin passive films formed on phase reversion induced nano/ultrafine grained AISI 304 metastable austenite stainless steel in corrosion solutions was not clear and

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rarely reported. Lv et al. [18] studied the effects of cold rolling temperature on grain size, grain orientation and corrosion resistance of pure iron. The grain refinement obtained by rolling improved the corrosion resistance of iron in sulfuric acid solution, borate buffer solution and

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borate buffer solution with chloride ion. The objective of this work was to evaluate the effect of rolling temperature on the corrosion resistance of passive films formed on nano/ultrafine grained 304 stainless steel in a borate buffer solution. 2 Experimental materials and procedure 2.1. The as-received material 3

Commercial type 304L stainless steel was used in this research and the chemical composition is shown in Table 1. Table 1 Chemical composition (wt.%) of as-received 304L stainless steel. Element

C

Mn

Si

S

P

Cr

Ni

Fe

Wt.%

0.065

1.80

0.08

0.024

0.024

19.08

9.03

Balance

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The as-received commercial type 304L stainless steel was solution annealed at 1050 ◦C for 1 h and quenched into water at room temperature to achieve homogenous microstructure. A Rigaku

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Ultima IV type diffractometer was used to perform X-ray diffraction (XRD) analysis on the stainless steel. The results of as-received and solution annealed materials are shown in Fig. 1.

which illustrated the initial microstructure has some residual martensite, and after the solid

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solution treatment, the material is completely in the austenitic phase.

1000

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γ(111)

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800

γ(200)

400

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CPS

600

γ(220) Solid solution

α'(110)

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200

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0 40

As-received α'(211)

α'(200) 50

60

70

80

90

2

Fig. 1. XRD patterns of as-received and solid solution treated materials.

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2.2 The thermo-mechanical treatment procedure and microstructure observation The metastable 304L stainless steel was cut into rectangular blocks with size of 200 mm long,

15 mm wide and 11.5 mm thick. The calculated Ms and Md30 temperatures are 60 K and 294 K, respectively [17]. The rolling experiments with 87%-92% thickness reduction were conducted at room temperature (RT), -70 ◦C, -120 ◦C and -196 ◦C (liquid nitrogen temperature), more than 20 passes were conducted until the thickness of the sample would not reduce any further. The RT 4

samples were rolled with inter-pass cooling in room temperature water to achieve 100% martensite. The low temperature samples were immersed into liquid nitrogen for 3 minute before each rolling pass. Inter-pass cooling in liquid nitrogen could minimize the effect of temperature increment during the rolling process and reduce dynamic recovery, which benefit in grain size reduction. Phase components and α’-martensite volume fractions of rolled samples were measured by X-ray diffraction (XRD). Samples were then annealed at 1123 K for 180-270 s to complete the

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full reverse transformation from martensite to austenitic. Microstructures before and after annealing were investigated by a JEM-2100F transmission electron microscopy (TEM) operating

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at 200 kV (JEOL Ltd., Tokyo, Japan). Thin foil specimens for TEM were jet-polished in a solution of 90% alcohol and 10% perchloric acid. 2.3 Electrochemical tests

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All the samples were polished with 1000, 2000 and 3000 grit silicon carbide paper and 1.5 lm

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alumina powder. The polished samples were ultrasonically cleaned in acetone and ethanol. Very

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high density graphite and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes, respectively.

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Before the experiment samples were cathodically polarized at -1.2 VSCE for 300 s to reduce

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the air-formed oxide film. Electrochemical tests were performed using the CHI660b electrochemical workstation provided by Shanghai Huachen. All the experiments were carried out at in pH 9.2 borate buffer solution (0.075 M Na2B4O7-10H2O + 0.05 M H3BO3). A relaxation

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time of 1h was performed to stabilize the stainless steel samples at OCP before potentiodynamic polarization test. The potential period that potential was swept during potentiodynamic

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polarization test was -0.5-1.5V

Electrochemical impedance spectroscopy (EIS) measurements were carried out at 0 VSCE for

1 h using a frequency range of 100 kHz to 10 MHz with 5 mV amplitude of the AC signal.

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2.4. X-ray photoelectron spectroscopy (XPS) measurement The surface compositions of the passive films formed on transformed austenite 304L

stainless steel were measured by XPS. Samples cold rolled under different temperatures were polarized under 0.75V for 1h to form a steady passive film, then sprayed by Ar+ to remove 3nm of materials from the surface. The XPS experiments were performed using PHI Quantera SXM (ULVAC-PHI, INC). 5

Photoelectron emission was excited by monochromatic Al K radiation. The vacuum of the specimen chamber was 6.7 × 10−8Pa. The C 1s peak from adventitious carbon at 284.8 eV was used as a reference to correct the charging shifts. Sputter depth profiles weremeasured and analyzed. XPSPeak4.1 software was used to fit the XPS experiment data. 3 Results and discussion

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3.1 Phase transformation and microstructure characterization The cold rolled samples were cut into 15×15 mm pieces of slice, and annealed at 850 ◦C for

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different temperatures annealed at 850 ◦C are shown in Fig. 2.

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180-270 s to achieve fully austenite structure. The XRD spectrums of samples cold rolled at

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Fig. 2. The XRD spectrum of cold rolled samples before and after annealed at 850 ◦C, (a) RT sample, (b) -70 ◦C sample, (c) -120 ◦C sample, (d) -196 ◦C sample. It is worthwhile to notice that the microstructure of sample cold rolled under room

temperature was fully martensite with random crystallographic orientation of α’(200) and α’(211), while samples cold rolled under cryogenic temperature exhibit a preferred orientation of α’(200) over α’(211), lower rolling temperature produce stronger α’(200) orientation preference. At the 6

rolling temperature of -196 ◦C, more than 90% of the martensite has a α’(200) preference. This may due to the fact that low rolling temperature promoted the activation of the martensite transformation [16], and made the martensite transformation faster at a lower temperature. The early transformed martensite would continue to deform under loading stress with ongoing rolling process, eventually break into a large amount of small sized grains and subgrains, and in the meantime, generates an α’(200) orientation preference. This theory is agreed with our observation

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that the ultimate thickness reductions were 92%, 90%, 89%, 87%, under rolling temperature of RT, -70 ◦C, -120 ◦C, -196 ◦C, respectively. Further rolling would not reduce the thickness any further,

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but start to break the rolling sample. In fact, according to author’s survey, 92% reduction of RT sample could produce 100% α’-martensite, while ~100% α’-martensite was obtained merely after 40% rolling reduction at the temperature of -196 ◦C. The results illustrated that the cryogenic

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temperature significantly promoted α’-martensite transformation during the rolling process.

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After complete reversion, austenite with preferred orientation of γ(220) was obtained. Similar

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trends was observed on the reversed austenite, lower rolling temperature produce stronger γ(220) orientation preference. This observation indicated that the γ(220) orientation austenite may be

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reversed from the α’(200) orientation martensite, which maintained a large amount during low

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temperature rolling. Another fact can be observed from Figure. 2. is that samples rolled under low temperature need more time to fully transform back to austenite, RT rolled sample needs 180 s to reverse transform to 100% austenite, -70 ◦C and -120 ◦C rolled samples need 200s, while -196 ◦C

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rolled sample need 270 s. This proves that low rolling temperature produces much more stable martensite which is difficult to reverse transform back to austenite (mostly with an orientation

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preference of α’(200)). Considering of the possible measuring error on several seconds, -70 ◦C and -120 ◦C rolled samples may actually have different fully transformation time, but the difference is

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not very obvious in this measurement.

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Fig. 3. Microstructures of samples annealed at 850◦C: (a) RT cold rolled sample, (b) -70 ◦C cold

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rolled sample, (c) -120 ◦C cold rolled sample and (d) -196 ◦C cold rolled sample. Fig. 3 shows TEM observation of samples after cold rolling at different temperature and

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annealed at 850 ◦C. The grain size was uneven in every picture except the -120 ◦C sample, which shows a nearly equiaxed grain structure. Each picture shows large grains with diameter of several

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micrometers, and smaller grains with size of 100-1000 nm, and the smallest grain reached nano-scale (<100 nm). Deformation twin exist in both large and small grains in every sample, RT

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and -196 ◦C cold rolled samples have more twins than the other two temperature. A large number of stacking faults were observed at grain boundaries. -196 ◦C cold rolled sample has most severely crushed microstructure, with more twisted and ambiguous GBs, indicating its sub-grain

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structure produced by cold rolling was not fully recovered during the heating process, although it took the longest anneal time. Average grain size was measured from several TEM figures of every rolling temperature, As can be seen in Fig. 4, the grain sizes decrease with the rolling temperature, and samples cold rolled at room temperature have a bimodal distribution, they have a large number of large grains(>1 μm), and also have huge amount of nano-size grains, both have proportions of ~25%. The -196 ◦C cold 8

rolled samples have a well distributed grain size in the 100-800 nm range, and every size takes about 10% of the total number, which matches the TEM observation of its severely crushed microstructure. The -70 ◦C cold rolled sample has grain sizes mostly distribute from 200 to 400 nm, nearly 70% of all the grains lay in this range and well distributed. The -120 ◦C cold rolled samples have a similar grain size range to the -70 ◦C sample, but with a much more concentrated distribution. More than one third of all the grains have the size of 300nm, that gave -120 ◦C cold

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rolled samples more equalized shape, which can also be observed from TEM result of Fig3 (c). As discussed earlier, lower rolling temperature produces faster martensite transformation progress,

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leading to a longer grain crushing progress afterwards, thus lower rolling temperature produce

microstructure with smaller grain size. The -120 ◦C cold rolled sample may receive an appropriate amount of deformation and achieved a nearly equalixed shape, while -196 ◦C cold rolled sample

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may go through more severe strain and stress loadings during a longer deformation process,

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resulting a finer microstructure filled with crushed grains, twins, and sub-grain structures.

Fig. 4. Average grain size of samples cold rolled at different temperature and annealed at 850 ◦C. 3.2 Electrochemical results

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RT -70 -120 -196

1.5

4 3 Ⅲ

0.5

2 Ⅱ

1

0.0

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Potential/V

1.0

Ⅰ

-10

-9

-8

-7

-6

-5

-4

Current/A

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-0.5 -3

-2

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Fig. 5. Potentiodynamic curves of samples cold rolled at different temperatures.

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Fig. 5 shows the polarization curves of samples cold rolled under different temperatures in

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borate buffer solution. From the polarization curves, it is obviously seen that -120 ◦C cold rolled

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sample has a different shape of the polarization curve compared to other three types of samples, and the polarization curves of samples cold rolled at RT, -70 ◦C,-120 ◦C are almost the same. This

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implies that the formation, dissolution, and the content of passive film may undergo different changes at samples cold rolled at -120 ◦C cold rolled temperatures.

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Zhicao Feng [19] reported that the turning points of potentiodynamic polarization correlate to the thickness and composition change of passive films, and also the n-type and p-type properties

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alter at different applied potential ranges, using Mott-Schottky, AAS, and XPS method. Their polarization curve results have a similar shape to the -120 ◦C cold rolled sample in our present research. According to their research, turning points on the polarization curve may reflect the

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component and structure change of the passive film. At the forming phase, the passive film mainly consists of iron-oxide, Cr(OH)3 and little Fe2(MoO4)3 (correlate to the curve between turning points 1 and 2); then the passive film is mostly made of mixed iron-oxide, NiO and Cr2O3(correlate to the curve between turning points 2 and 3); in the third phase, the formation of CrO3 occurs(correlate to the curve between turning points 3 and 4) [19]. In later part we will characterize the composition of passive film by ourselves. 10

In our present research, the -120 ◦C cold rolled sample has similar shape to the polarization curve of Zhicao Feng [19], indicating a similar passive film construction, while samples cold rolled at -70 ◦C,-196 ◦C, and room temperature has a different shapes in polarization curves. Compared with the -120 ◦C cold rolled sample, these three curves have 3 turning points instead of 4, the shape between turning points Ⅰand Ⅱis similar to the shape between turning points 2 and 3 displayed from the -120 ◦C sample, and the shape between turning points Ⅱand Ⅲis similar to the

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shape between turning points 3 and 4 from -120 ◦C sample. Instead of the phase between turning

points 1 and 2 from -120 ◦C sample, these three curves have a small but obvious fluctuate,

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indicating they may have an unstable period at the beginning stage of passive film. Table 2 Parameters calculated from potentiodynamic curves. ip/Acm-2

RT

-0.223094343

0.696168

6.364501

-70℃

-0.242413887

0.797078

-120℃

-0.2884063

0.618134

-196℃

-0.183538078

0.843531

Pitting Potential /V

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Icorr /Acm-2

Passive Area

0.637

6.441953

0.9093

0.6822

6.694364

0.905

0.615

6.307786

0.9088

0.608

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0.9131

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Ecorr/V

/V

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Table 2 shows parameters calculated from potentiodynamic curves, where Ecorr is the corrosion potential, Icorr is the corrosion current, Ip stands for maintaining passivity current density, respectively. It can be seen that passivation current densities and pitting potentials are

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similar, but the corrosion current of -120 ◦C rolled sample is the lowest among 4 temperatures, implying the best corrosion resistance. The RT sample is the second best, then comes to the -70 ◦C,

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and the -196 ◦C is the worst.

EIS plots of cold rolled stainless steel as a function of rolling temperature are shown in Fig. 6.

The Nyquist diagrams in Fig. 6a show capacitance loops, where the diameter of the semicircle of

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Nyquist plots indicates corrosion resistance. The equivalent circuit of the specimen/solution interface is given in Fig. 6a, where Rct represents the measurement of the electron transfer across the metal-electrolyte surface and could be regarded as the corrosion resistance of the steel in this system. The values of Rct calculated are given in Fig. 7. The charge transfer resistances appear in exact order as corrosion current indicated from potentiodynamic curve, where the -120 ◦C is the 11

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best, then RT and -70 ◦C, and -196 ◦C is the worst.

Fig. 6. EIS data of samples cold rolled at different temperatures. (a) Nyquist plots, (b) and (c) Bode plots. 12

Fig. 6b and c shows the Bode plots of samples cold rolled at different temperatures. High angles at low frequencies indicate typical passivaty characteristic of the material and broadening plateau in the middle frequency region reveals that stainless steel possesses an intact passive film in these samples, the -120 ◦C cold rolled sample showed the highest phase degree, indicating its

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passive film is more capacitive, thus has a more condense and intact film.

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Rct (kΩ)

600

400

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200

-70℃

-120℃

-196℃

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RT

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0

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Fig. 7. The charge transfer resistance (Rct) of the passive film.

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Semiconductor characteristic of the passive films formed on the surface of 304L stainless steel can be investigated by the Mott-Schottky plot analysis. Based on Mott-Schottky theory, the

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space charge capacitance of the n-type and p-type semi-conductors of the passive films can be given by Eqs. (1) and (2), respectively, 2



2

( E  E

CC E C

2



kT



2

 0 e N D

(E  E

fb



(1)

)

e

A

kT

(2)

)

e

A

C

 0 e N

fb

where ε0 is the vacuum permittivity(8.854×10-12 Fm-1), ε is the dielectric constant of the sample, e is the electron charge, k is the Boltzmann constant(1.38×10-23JK-1), ND and NA are the donor and acceptor concentrations, respectively. T is the absolute temperature and Efb is the flatband potential, respectively. The donor ND concentration and acceptor concentration NA can be determined from slopes of Mott-Schottky plots. 13

The Mott-Schottky plots in Fig. 8 reveal that the passive films formed on 304l stainless steels exhibit n-type and p-type semiconducting characteristics. Positive and negative slopes in Mott-Schottky plots indicate that the dual layers of the passive films contain iron oxides and chromium oxides. Samples show negative slope at range of -1.2 V to -0.4 V, indicating p-type semiconductor character, which is caused by the Cr2O3 inner layer of passive films. Samples at range of -0.4 V to 0.3 V shows n-type semiconductor character, indicating an Fe2O3 and Fe(OH)3

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outer layer of passive films. Sample cold rolled at -120 ◦C firstly starts to change at potential of

about 0.25 V, from an n-type semiconductor to a p-type semiconductor, and changes back to

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n-type semiconductor at potential of about 0.5 V, indicating the composition and structure change of the passive film, where other 3 cold rolled samples haven’t experienced.

RT -70℃ -120℃ -196℃

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Fe2O3, Fe(OH)3, n-type Cr2O3, p-type

Film Structure change

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10

0

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C-2109/F-2 cm4

20

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-1.0

-0.5

0.0

0.5

Potential/V

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Fig. 8. Mott-Schottky plot of passive films formed on samples cold rolled at different temperatures.

The point defect model (PDM) depicted that the cation vacancies were formed at the

film/solution interface, while cation interstitials and oxygen vacancies occurred at metal/film

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interface. The donor and the acceptor concentrations in passive films formed on the surface of 304 stainless steels in borate buffer solution are shown in table 3. Low donor and acceptor concentrations increase the corrosion resistance of the passive films, and the -120 ◦C rolled sample shows similar donor and acceptor concentrations to -70 ◦C rolled sample, which both are slightly bigger than RT cold rolled sample and smaller than -196 ◦C rolled sample. The outstanding corrosion resistance of -120 ◦C rolled sample may be caused by the abnormal film semiconductor 14

character change at potential of 0.5 V. Table 3 Parameters calculated from Mott-Schottky plot NA

positive slopes

NA

RT

-2.29E+10

5.14717E+20

1.56E+10

7.54312E+20

-70℃

-1.27E+10

9.23065E+20

1.46E+10

8.08258E+20

-120℃

-1.40E+10

8.39033E+20

1.3866E+10

-196℃

-9.71E+09

1.21132E+21

1.21E+10

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negative slopes

8.481E+20

9.71987E+20

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Combining the corrosion resistance test results with the TEM observations of surface structure, the -120 ◦C rolled sample has the best corrosion performance, and has the most homogeneous microstructure. Intergranular corrosion is often induced by impurity segregation and

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precipitation at grain boundaries. Defects such as dislocations and grain boundaries have also

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intrinsic susceptibility to local attack, and reactivity of these defects increases with increasing

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extra free energy associated with intrinsic structural disorder [20, 21]. The -196 ◦C rolled sample

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has a large amount of defects due to the most severe plastic deformation, thus has the worst corrosion resistance. Beaunier et al. [22, 23] proposed an atomic scale model which described the

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penetration rate at grain boundaries, using kinetic parameters such as activation energy for the formation of the active site and the dissolution. According to the model, the dissolution rate of the surface intersecting with the grain boundaries is higher than that in the grain interiors, and the

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ratio of both the dissolution rates is related to the difference in the activation energy of the formation of active sites. Thus, larger area of grain boundaries due to grain refinement lead to

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higher dissolution, and the -70 ◦C and -120 ◦C rolled sample have more grain boundaries than the RT rolled sample, which should have worse corrosion resistance, but -120 ◦C rolled sample showed the best corrosion resistance. Miyamoto et al. [24] made a similar observation to this and

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found that the more homogeneous structure showed better corrosion resistance. They discussed that Beaunier’s model was based on the assumption that the entire specimen was under anodic polarization. In heterogeneous structures, however, the defects, such as dislocations and grain boundaries, have higher free energy, and have therefore, lower half-cell electrode potential whereas the grain interiors having relatively low energy have higher half-cell electrode potential. 15

Thus, there is a spatial distribution of the potentials on the surface. They found that the corrosion damage of homogeneous structure is macroscopically rather uniform whereas an obviously preferential grain boundary degradation and selective corrosion of some grain interiors was observed in less flawed but heterogeneous structures. -120 °C sample has relatively homogeneous grain size, and less stacking faults than the -70°C and -196 ◦C samples, which effectively reduced the generation of oxygen vacancies. This may illustrate the abnormal film structure and

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outstanding corrosion resistance of -120 ◦C rolled sample.

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3.3 XPS measurements

Fig. 9. The detailed XPS spectra of Fe 2p3/2 for the passive films formed on (a) RT, (b) -70°C, (c) -120°C, and (d) -196°C cold rolled sample under 0.6 VSCE for 1 h in borate buffer solution,

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respectively.

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Fig. 10. The detailed XPS spectra of Cr 2p3/2 for the passive films formed on (a) RT, (b) -70°C, (c)

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-120°C, and (d) -196°C cold rolled sample under 0.6 VSCE for 1 h in borate buffer solution,

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respectively.

Fig. 9 and 10 show the surface Fe 2p3/2 and Cr 2p3/2 XPS spectra for passivated (a) RT, (b) -70°C, (c) -120°C, and (d) -196°C cold rolled sample, respectively. The possible species in passive

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films were characterized according to standards from the Handbook of X-ray photoelectron spectroscopy [25]. It is suggested that the passive film formed on 304 SS after polarization at 0.75

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V is mainly made of Fe2O3, FeO, Fe3O4，FeOOH, Cr2O3, Cr(OH)3, and CrO3. Based on the XPS spectra of Fe 2p3/2, iron oxides like Fe2O3, FeO, Fe3O4, FeOOH make a

complex composition of outer layer structure of passive film. This may cause looser outer layers

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than single component outer layer passive films. Thus, the chromium oxides from inner layer of passive films make major influence on the passivity, especially Cr2O3. As shown in Fig. 10, only -120°C sample has zero Cr(0) detected, which means it has thicker or more condense chromium oxides constructed inner layer. Compared with RT and -70°C samples, -120°C sample has lower level of Cr(OH)3, which improves corrosion resistance of the passive film of stainless steel in borate buffer solution. Although -196°C sample has a high level of Cr2O3, according to the 17

Mott-Schottky analysis, the high acceptor concentration NA, representing a loose or thin film structure, deteriorates its corrosion resistance. Besides, -196°C sample has high level of Fe(0), indicating a loose or thin outer layer, Thus -196°C sample has a poor corrosion resistance. The XPS analysis provides some extra evidences to support the discussions of Mott-Schottky, and to clarify the relationship between compositions of passive film and corrosion properties.

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4 Conclusions The present study shows the effect of nano/ultrafine grain obtained by martensite phase transformation and its reversion on passive properties. The main conclusions are as follows.

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(1) The average grain size of martensite transformation induced and reversed austenitic 304 SS decreases with cold rolling temperatures. Samples cold rolled at -120 ◦C have a nearly homogeneous grain size distribution.

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(2) The -120 ◦C cold rolled samples have the best passive properties, then comes RT and -70

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◦C samples, and the -196 ◦C sample is the worst. Defects like dislocations and grain boundaries

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deteriorate the corrosion resistance, and the heterogeneous structures may help improve the

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corrosion resistance of phase reversion induced nano/ultrafine grained stainless steel. (3) From the Mott-Schottky plot, an abnormal film semiconductor character change of the

corrosion resistance.

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-120 ◦C rolled sample at potential of 0.25 V was found. This may result in its outstanding

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(4) XPS spectra of Fe 2p3/2 and Cr 2p3/2 show a complex composition of iron oxides in the outer layer, and the differences between the chromium oxides from the inner layer make major

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influence on the passivity of the passive films. Acknowledgment

This work was financially supported by National Key Technology R&D Program of China

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(2015BAG20B04 and 2015BAF06B01-3), the National Key Research and Development Program of China (2017YFF0210002, 2016YFC0801903 and 2016YFF0203301) and National Natural Science Foundation of China (No. 51175023 and No. U1537212). References 18

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