Pitting Corrosion 304

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    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

    Article history:

    Received 5 November 2012

    Accepted 25 March 2013

    Available online 6 April 2013

    Keywords:

    A. Sputtered film

    B. AFM

    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.

    1. Introduction

    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[13]. 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[4].

    In addition, the sputtered nanocrystalline thin films have been

    found to possess better corrosion resistance than the correspond-

    ing conventional coarse crystalline alloys[57]and have been used

    successfully in high-temperature applications[810].

    It is well known that the corrosion behavior of 304 stainless

    steel mainly includes passive and pitting behavior. A previous

    study [11] 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

    304ss.

    Pitting corrosion is a complicated phenomenon, which is largely

    dominated by random parameters [12]. 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[14].

    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. Experimental

    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:liliu@imr.ac.cn(L. Liu).

    Corrosion Science 73 (2013) 3243

    Contents lists available at SciVerse ScienceDirect

    Corrosion Science

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i

    http://dx.doi.org/10.1016/j.corsci.2013.03.022mailto:liliu@imr.ac.cnhttp://dx.doi.org/10.1016/j.corsci.2013.03.022http://www.sciencedirect.com/science/journal/0010938Xhttp://www.elsevier.com/locate/corscihttp://www.elsevier.com/locate/corscihttp://www.sciencedirect.com/science/journal/0010938Xhttp://dx.doi.org/10.1016/j.corsci.2013.03.022mailto:liliu@imr.ac.cnhttp://dx.doi.org/10.1016/j.corsci.2013.03.022
  • 7/23/2019 Pitting Corrosion 304

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    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

    [15], 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