316L不锈钢钝化膜中添加三价铈对电导和防腐蚀性能的影响：角分辨XPS、低能离子散射谱和莫特－肖特基方程的应用

Cerium insertion in 316L passive film：Effect on conductivity and corrosion resistance performances of metallicbipolar plates for PEM fuel cell application

O． Lavigne

C． Alemany－Dumont

B． Normand

P． Delichère

A． Descamps

Abstract

A surface treatment to modify passive filmproperties formed on 316L stainless steel has been designed for bipolar plateapplication in PEMFC systems． This one is based on cerium insertion fromelectrochemical technique． The effect of this element addition on thecomposition of the passive films was evaluated by X－ray photoelectronspectroscopy （XPS） and low energy ion spectroscopy （LEIS）． Potentiodynamic andpotentiostatic curves， and electrochemical impedance spectroscopy （EIS） wereused for the characterisation of the corrosion protection in simulated PEMFCmedia． Interfacial contact resistance （ICR） was also evaluated． Resultshighlight an increase of the polarisation resistance of the cerium treatedsample at ambient temperature， and a good corrosion resistance in anodic andcathodic simulated media． An important diminution of the IRC is obtained （80％with respect to the reference）， which is attributed to the enhancement ofthe passive layer conductivity of the processed sample． Surface analysesshow a great enrichment of cerium in first atomic layers of the films，which explains the better conductivity observed by an increase of chargecarrier density in the passive film．

为什么做

1Passive films show a semiconductive  behaviour． Rare earth elements are often used to dope semiconductor in order  to change their conductive properties

2in the case of stainless steels， it was  shown that modifications of the passive films by cerium implantation or by  immersion in a boiling cerium nitrate solution enhance crevice corrosion resistance．

有时候文章结果好不好不重要，关键思路是否自洽。

制备过程：

Native oxide film formed on the sample  was first removed by applying a cathodic current （−2mAcm−2） for 10 min in an aqueous solution （pH～1．8）  containing 10 g L−1 Na2SO4． The sample was  maintained under polarisation of ＋0．4 V／MSE in a solution containing 10−2M CeO8S2 and 10 g L−1 Na2SO4 （pH 1．8） for 2  h． This applied potential， located in the passive range of the stainless  steel， allows an enhancement of the chromium content in the oxide layer， and  cerium species insertion from reduction mechanism． This latter is possible  regarding the oxido－reduction potential of Ce4＋／Ce3＋ （＋1 V／MSE）．

刚读了一遍，没理解CeO8S2是什么。查了一下，不就是Ce（SO₄）₂么。没理解这里说的是Ce4＋／Ce3＋，原料为什么用二价硫酸高铈。

a saturated mercurous sulfate electrode  （MSE） 汞－硫酸亚汞电极，这里是亚汞不是汞。

Table 1 316L SS chemical composition （wt．  ％）．

Fig． 1． a） O 1s， b） Fe 2p3／2， c） Cr2p3／2  XPS spectra for the 316L passive film formed in air， and d） Ce 3d XPS  spectra．

Ce3d spectra reported in Fig． 1d）  characterise Ce3＋ state．

Table 2a Angle resolved XPS analysis；  elementary chemical compositions of passive films （wt．％）．

才发现XPS还有测量角度的区分——角分辨X射线光电子能谱。小角度表示更靠近表面的成分。

Table 2b

Binding energies （eV） of O 1s， Cr 2p3／2，  Fe 2p3／2 and Ni 2p3／2 XPS peaks and evolution of chemical states of O， Cr， Fe  and Ni with analysis angle．

Fig． 2． LEIS spectra of the cerium  processed sample recorded at a） 30 s， b） 2 min and

c） 8 min．

低能离子散射谱法Low energy ion spectroscopy  （LEIS） is the most sensitive to the very first surface layer of the solid．  This technique has been used to analyse the upper layer and few sub－layers of  the passive film．

Fig． 3． Signal ratio Ce／（Cr＋Fe＋Ni） in  function of time sputtering for the cerium processed sample．

Fig． 4． AFM analysis for the reference  sample （a） and the cerium processed one （b）．

Fig． 5． Potentiodynamic curves for the  reference stainless steel and the cerium processed one in pH 3．5 solution at  ambient temperature．

Fig． 6． Impedance Nyquist plots for the  reference stainless steel and the cerium processed one in pH 3．5 solution at  ambient temperature recorded at OCP． Dashed lines are calculated data from  the electrical equivalent circuit shown in the inset．

Table 3 EIS fit parameters from  measurements on stainless steel reference and processed．

Fig． 7． Potentiostatic curves for 316L  and 316L Ce processed in simulated anodic and cathodic media．

Fig． 8． Mott–Schottky measurements for  the reference stainless steel and the cerium processed one in pH 3．5  solution． f＝500 Hz．

Semiconductive properties of passive  films formed on stainless steel samples can be determined by flat band  potential VFB and doping density ND extracted from Mott–Schottky  plots． Nd指的是载流子浓度

Reference steel behaviour can be  explained from iron contribution principally located in the outer part of the  oxide （n－type oxides）． In this case， charge carriers are electrons  that come from oxygen vacancies （VO 2−） or interstitial cations  （Fe2＋， Fe3＋）．

For the processed sample， passive films  are mainly composed of chromium oxides （p－type oxides）， but some iron  species are still present within the passive layer

The Ce passivated sample shows higher ND  value compared to this latter． It can be explained by the addition of Ce3＋  cations in the first atomic layers of the oxide film which act as dopants．

Table 4 Charge carriers density and flat  band potential for 316L， 316L passivated without

cerium and 316L passivated with cerium．

Table 5 Interfacial contact resistance  （ICR） for the 316L and the 316L cerium processed

（mΩ cm2）．

只是在钝化层中进行三价铈的掺杂接触电阻还是大了一些。

Conclusion

Modification of the passive film formed onthe 316L SS was performed by electrochemical cerium insertion． Thistreatment allows a chromium enrichment of the passive layer． Besides，cerium is found on Ce3＋ form， and is essentially present in the first atomiclayers of the surface． Electrochemical measurements were performed in an aqueoussolution （pH～3．5） containing 5 ppm of F− and 10 g L−1 Na2SO4 at 60 °C bubbled either with H2 or air to simulate PEMFC environmentfor bipolar plate application． It was shown that the cerium passivationtreatment is sustainable and that the measured current densities are much lowerthan recommendations for BP application． The ICR between a carbon paper and themodified passive film displays values clearly enhanced compared to the one ofthe 316L reference （improvement of almost 80％ at 140 N cm−2） due to an enhancement of the oxide layer conductivity （increaseof charge carriers density）．

The positive effect of the cerium on theproperties of the passive film shows the interest of this addition element inthe preparation of a bulk stainless steel which may fit the targeted propertiesfor the bipolar plate application．