Characterization of Native Oxide and Passive Film on Iron and Iron-Chromium Alloy

Home , Passivation (chemistry)

Characterization of native oxide and passive film on iron and iron-chromium alloy

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Master of Science

in Chemical Engineering

in the Department of Biomedical, Chemical and

Environmental Engineering, College of Engineering and

Applied Science

Jingxing Feng, B.S. in Chemical Engineering

July 2014

Committee Chair: Dale W. Schaefer, Ph.D.

Committee members: Anastasios P. Angelopoulos, Ph.D.

Vikram K. Kuppa, Ph.D. Abstract

Passivation refers to the spontaneous formation of a thin protective oxide on a metal.

Passive films determine the corrosion resistance of metals. Through systematic study of both iron and iron-chromium alloy thin films with reflectometry and electrochemistry, the structure and composition of native oxide and electrochemically passivated films are resolved. Analysis of polarized neutron reflectometry of pure iron thin films shows that native oxide is uniform, non-ferromagnetic, dense magnetite with a thickness of 33.7 ±

2.0 Å. Conditions for electrochemical passivation are explored including the passivation potential, passivation kinetics, and effect of solution pH. The optimized conditions to prepare smooth, dense passive oxide are found to be 800 mV vs. SCE, 15 minutes of passivation time in pH = 7 sodium sulfate solution.

iii Acknowledgement

I would like to express my deepest gratitude to my advisor, Professor Dr. Dale Schaefer.

Two years ago, I started my program in Chemical Engineering at the University of

Cincinnati with limited experience in material characterization, poor expression skills in

English and hardly any knowledge in scattering theories. Both teaching and research guidance benefited me a lot and shaped me into a critical thinker. Dr. Schaefer’s mentorship was paramount in leading me step-by-step into the field of small-angle scattering and reflectometry. His rigorous scientific attitude affected me greatly. I am grateful his caring, understanding and patience during the past two years.

I want to thank my thesis committee, Professor Dr. Anastasios Angelopoulos and

Professor Dr. Vikram Kuppa, who enlightened me with valuable discussion and comments. The proposal defense was very helpful for me to realize the weak point of my research and pushed me to think more deeply in the mechanism of electrochemical corrosion. I am grateful that they can attend my defense and bring out hard questions which enables me improve in the hot summer days.

Special thanks Dr. Naiping Hu, who helped me to overcome various problems during the research. Dr. Hu is efficient researcher and is always able to hit the key quickly when starting a project. Discussion with her is beneficial to me making better decisions and planning out the experiments.

I wish to thank Dr. James Browning and Dr. Jaroslaw Majewski. The experiment at national laboratories couldn’t be successfully preceded without the help with them.

iv Thanks to my lab mates Dr. Sandip Angekar, Dr. Yan Zhang, Devesh Shreeram, and

Ahmad Motahari. They offered much encouragement when I had trouble thinking straight during the project.

Finally and more importantly, I am grateful to my parents who supported and encouraged me for the past twenty-two years. Getting away from home and starting a life abroad was a tough trial and my parents were always besides me at every moment

v Table of contents

Chapter 1 Introduction to passivity of Fe and Fe-Cr ...... 1 1.1 Corrosion of metals ...... 1 1.2 Passivation and passive films on metals ...... 1 1.3 Passive film on pure iron ...... 3 1.4 Passive film on Fe-Cr alloy and stainless steel ...... 7 1.5 Summary ...... 9 Chapter 2 Using magnetism to probe native oxide on Fe ...... 11 2.1 Overview ...... 11 2.2 Polarized neutron reflectometry ...... 11 2.3 Experimental Details ...... 14 2.4 Results and discussion ...... 15 2.5 Summary ...... 25 Chapter 3 Electrochemical passivation and reflectivity of passive Fe and Fe-Cr thin films ...... 26 3.1 Overview ...... 26 3.2 Background ...... 26 3.3 Experimental details ...... 28 3.4 Results and discussion ...... 29 3.5 Summary ...... 40 Future plans ...... 42 References ...... 43

vi List of tables and figures

Figure 1.1 Kinetics of passivation (a) without external field and (b) with external field.3 . 3 Figure 1.2 Equilibrium phase diagram for the iron-oxygen system. α, ferrite; γ, austenite.4 ...... 4 Figure 1.3 The change in potential and conductance during cathodic reduction of passivated iron...... 5 Figure 1.4 Chromium compositions in alloy surface (solid line) and oxide film (dash line) 3, on iron-chromium alloys polarized at 100 and 500 mV SCE for 1 hour in 1 N H2SO4. 17 ...... 8 Figure 2.1 Zeeman splitting of neutrons. Spin-up neutrons gain energy and spin-down neutrons lose energy in a superposed of magnetic field. Neutrons in the absence of magnetic field show no energy difference...... 12 Figure 2.2 Scheme of Asterix polarized neutron reflectometer, LANSCE, LANL, Los Alamos, NM ...... 12 Figure 2.3 Transverse geometry for polarized neutron reflection...... 13 Table 2.1 The density and calculated x-ray SLD (Cu Kα, 8.04 keV) and neutron SLDs for

quartz, iron, magnetite (Fe3O4) and maghemite (γ-Fe2O3). For magnetic components the SLD depends on the polarization of the incident beam so two SLDs are possible (nuclear SLD ± magnetic SLD) if the film is magnetized. Densities are calculated from the measured lattice constants. Non-polarized reflectometry measures the average SLD of the two components...... 16 Figure 2.4 X-ray reflectivity data and model fit assuming 2 layers (iron and iron oxide) on a quartz wafer...... 17 Figure 2.5 X-ray reflectivity SLD profile with fitting parameters...... 17 Table 2.2 Summary of fitting parameters and calculated densities for XRR, PNR and NNR. The oxide density is calculated assuming the film is Fe3O4...... 18 Figure 2.6 PNR (points) and fit (solid lines) assuming a non-magnetic oxide for air- passivated, 700-Å iron sample on quartz. Red curve: spin-up (pp) polarization; blue curve: spin-down (mm) polarization. The large difference is due to the contribution of magnetism of the iron film...... 20 Figure 2.7 SLD profiles from the pp and mm reflectivity data in Figure 2.6 The oxide layer is more apparent for the mm data because of the lower SLD of the iron substrate for mm polarization...... 20 Figure 2.8 Best fit (solid lines) assuming magnetic magnetite for air-passivated, 700-Å iron sample on quartz substrate. Red curve: spin-up (pp) polarization; blue curve: spin-down (mm) polarization. The ferromagnetic-oxide model fits well for pp polarization but not for mm...... 20 Figure 2.9 SLD profiles of pp and mm reflectivity data in Figure 6. The average SLD of oxide (8.74/2 +5.26/2 = 7.00) is consistent with non-ferromagnetic magnetite. .... 20

vii Figure 2.10 Comparison of spin-up neutrons and non-polarized neutrons as incident beam. Solid purple line represents the best fit to the NNR data with the resulting SLD profile shown in Figure 2.11...... 23 Figure 2.11 SLD profile for non-polarized neutron reflectivity. The high SLD of the iron layer masks the oxide layer...... 23 Table 2.3 The σ of the interfaces (quartz-iron, iron-oxide, oxide-air) for XRR, PNR, and NNR...... 24 Figure 3.1 Illustration of the experimental set up for three-electrode potentiostat. Working electrode (W) is the sample, iron thin film on silicon substrate; counter electrode (C) is a carbon rod; reference electrode (R) is a calomel electrode...... 27 Figure 3.2 Electrochemical cell set up. The sample is thin iron film on 0.5 mm silicon substrate...... 28 Figure 3.3 DCP results for Fe for complete cathodic and anodic polarizations. For pure iron passivation potentials (Ep) of 200, 400, and 800 mV vs. SCE can be identified to be used for the passivation experiment. Dashed line indicates the free corrosion scenario for cathodic branch...... 31 Figure 3.4 Current profiles during 15-min passivation at 200, 400, and 800 mV on the 700 Å wafers...... 32 Figure 3.5 Passivation potential dependence. Comparison of x-ray reflectivity vs. q for air-passivated sample (control), electrochemically passivated sample at different passivation potential...... 33 Figure 3.6 SLD profiles of potential dependence x-ray reflectivity. Four SLD profiles are completely overlapped at 0 to 600 Å from the substrate...... 33 Figure 3.7 Zoom-in SLD profile of Figure 3.6 in the region from 550 Å to 630 Å...... 34 Figure 3.8 Passivation kinetics. Comparison of x-ray reflectivity vs. q for air-passivated sample (control), electrochemically passivated sample for 5 minutes and 15 minutes at 800 mV vs. SCE...... 35 Figure 3.9 SLD profiles of potential dependence x-ray reflectivity. Iron layer is thinner after passivation...... 35 Figure 3.10 Zoom-in SLD profile of Figure 3.9 in the region from 2050 Å to 2350 Å. Arrows are pointed to the SLD of oxide layer...... 35 Figure 3.11 The thickness (left axis) and SLD (right axis) of the oxide layer during passivation. Compared to the native oxide layer, the thickness is unchanged while the SLD increases by 50%. The oxide growth ceases after 5 minutes of passivation...... 37 Figure 3.12 Current profiles during 15 minutes passivation at 800 mV. The current stabilizes after 150 – 200 seconds, suggesting the end of oxide growth at that time...... 37 Figure 3.13 Solution pH dependence. Comparison of x-ray reflectivity vs. q for air- passivated sample (control), electrochemically passivated iron sample for 15 minutes at 800 mV vs. SCE at different solution pH (pH = 3, pH = 5, and pH = 7). ... 39 Figure 3.14 SLD profiles of solution pH dependence x-ray reflectivity on iron sample. Pitting corrosion takes place when solution pH = 5 where SLD of iron layer decreases...... 39

viii Figure 3.15 Solution pH dependence. Comparison of x-ray reflectivity vs. q for air- passivated sample (control), electrochemically passivated iron/chromium alloy sample for 15 minutes at 800 mV vs. SCE at different solution pH (pH = 1, pH = 3, pH = 5, and pH = 7)...... 40 Figure 3.16 SLD profiles of solution pH dependence x-ray reflectivity on iron/chromium alloy sample. Pitting corrosion takes place when solution pH = 3 where SLD of alloy layer decreases...... 40

ix Chapter 1 Introduction to passivity of Fe and Fe-Cr

1.1 Corrosion of metals

Corrosion refers to gradual deterioration of materials due to interaction with their environments.1 Metals, polymers, and ceramics can corrode in every type of environment.

Due to the importance of metals in our daily lives and in industry, control of corrosion of metals is economically important. Unchecked corrosion can also lead to catastrophic failures.

Corrosion depends on both the metal itself and the corrosive media (environment).

Understanding the corrosion mechanism is essential to corrosion prevention or inhibition.

A typical corrosion process consists of three stages: (1) Diffusion of metal from bulk to the metal-media interface; (2) Reaction at the interface; (3) Diffusion of corrosion products from interface to the environment. The overall corrosion rate is dependent on the rates of the three processes.

In most cases, for metal-solution or metal-gas interfaces the reaction rate exceeds the diffusion rate from interface to the media. Thus, metal oxide or hydroxide, which is the product of the corrosion reaction on the interface, is formed as a passive layer.

1.2 Passivation and passive films on metals

Passivation refers to the spontaneous formation of a thin protective oxide on a metal.

Exploration of passivity of metals has continued since passive films were first recognized

1 in 1790 by Kier.2 Despite the low thickness, typically less than 10 nanometers, passive films slow corrosion reactions by many orders of magnitude. Numerous studies have focused on the formation kinetics, structure and composition of passive films.

Thermodynamically speaking, most metals oxidize in the presence of oxygen. The oxidation reaction is limited by the passive film at the interface between metal and environment. The passive film not only slows the diffusion of oxygen from the environment but also decreases the diffusion of metal ions.

The passive film is the product of dynamic equilibrium. Metal oxidization reaction takes place at metal/oxide interface while the oxide is dissolving into the environment.

When the oxidation reaction rate is higher than the diffusion rate, the film thickens; when the oxidation reaction rate is lower than the diffusion rate, the film thins. Passivation reaches equilibrium and the film thickness stabilizes when the corrosion reaction rate is equal to the diffusion rate. Thus, the metal under protection of passive films is still corroding gradually.

Passive film develops differently under the influence of electric field. In the absence of applied field (Figure 1.1a), mass transfer is coupled with electron transfer through the oxide layer.3 In the presence of external field, another mechanism is involved (Figure

1.1b). Electron transfer can take place in bulk metal or the environment in which case, the film can grow without electron transfer through the oxide.

Figure 1.1 Kinetics of passivation (a) without external field and (b) with external field.3

The difference between the two scenarios seems minor but may lead to different passivation products. Comparison of native oxide and electrochemically passivated film is necessary in terms of structure, corrosion protection performance and breakdown.

1.3 Passive film on pure iron

Passivation and passive films on stainless steel draw the most attention among researchers due to the wide application of stainless steel. The anti-corrosion behavior of stainless steel is closely related to the structure and composition of passive film. To fully understand the mechanism of formation and protection of the passive film on stainless steel, it is necessary to first explore the passive oxide on pure iron.

Although the equilibrium iron-oxygen phase diagram predicts a eutectoid mixture of

4 oxygen and Fe3O4 at ambient conditions, a very thin oxide film quickly forms on the surface, preventing further oxidation of iron. The typical thickness of the passive film is

20 to 100 Å. Due to the low thickness and similarity of electron density between iron

3 oxide and the metallic substrate, the ultrathin oxide has eluded unequivocal characterization for over a century.

Figure 1.2 Equilibrium phase diagram for the iron-oxygen system. α, ferrite; γ, austenite.4

Several conflicting models exist regarding the nature of the passive oxide layer.

Nagayama and Cohen,5 for example, proposed a bilayer structure with an inner layer of

6 Fe3O4 and an outer layer of Fe2O3. Other studies support the two-layer model. The two- layer idea originated from the observation of two reduction waves in galvanostatic experiments. The two waves were attributed to reduction reactions of the two different oxide layers.7 Tsuru and Haruyama6a observed similar two reduction waves (three arrests) and recorded change of conductance with a resistometer (Figure 1.3). During the first

1 potential arrest (Ec ), the conductance remained constant and this stage was considered as the reductive dissolution of outer maghemite layer. During the second potential arrest

2 (Ec ), the conductance increased gradually and this stage was considered as the reductive dissolution of inner magnetite layer. Schmuki et al.8 disputed the two-layer idea, claiming

4 the oxide was homogeneous Fe2O3. These authors deposited Fe2O3 on a pure substrate and

2+ yet observed a two-step reduction (from Fe2O3 to Fe3O4 then to Fe (aq)). Although this experiment effectively demonstrated that the two-step reduction is possible with a single layer, it did not preclude the possibility of two layers. Discussion on one-layer or two- layer structure is indispensible in understanding the anti-corrosion mechanism. Besides, it is possible to artificially prepare a passive film of a desired structure with better performance.

Figure 1.3 The change in potential and conductance during cathodic reduction of passivated iron.

Another issue is the composition of the oxide (α-Fe2O3, γ-Fe2O3, Fe3O4, or a mixture).

9 Early electron diffraction seems to rule out the possibility of α-Fe2O3 (hematite). γ-Fe2O3

(maghemite) and Fe3O4 (magnetite), however, share a similar inverse spinel crystal structure with similar lattice constant (8.352 Å vs. 8.396 Å).

All iron cations in maghemite are Fe3+ while in magnetite both Fe2+ and Fe3+ exist.

Maghemite has cation vacancies to maintain charge neutrality. Differentiating the oxide type is important in that the passive film is critical in blocking electron transfer from

5 metal to the environment. From this point of view, because of the existence of vacancies, magnetite might protect the underlying metal better than maghemite because cation vacancies are advantageous to electron transfer. Hence, we are motivated to determine the type of passive film on iron, prepare different passives films, and compare their anti- corrosion performance.

Early studies determined the oxide by calculating the lattice parameter and comparing it to that of maghemite and magnetite. The results of many studies based on this approach are inconclusive, however, because of the similarity of two lattice constants. In recent years, x-ray diffraction,10 Fourier transform infrared spectrophotometry (FTIR),11 and

Mössbauer spectroscopy were applied to identify iron oxides. The FTIR spectra for magnetite and maghemite are different only in finger print area (wave length less than

1000 nm), which makes it very difficult to characterize powder mixtures, let alone oxide films. Mössbauer spectroscopy is considered to be the most suitable technique to index magnetite and maghemite. The magnetite spectrum consists of two discrete sextets whereas the maghemite spectrum consists of only one sextet. Mössbauer spectroscopy is powerful determining pure oxide but not able to qualitatively identify mixtures. For x-ray diffraction, the biggest difference between magnetite and maghemite lies in the position of the (5 1 1) peak. The (5 1 1) peak for pure maghemite is at 57.3° and the peak for pure magnetite is at 56.9°. Although the 0.4° difference is observable for pure oxides, quantitative analysis is inconclusive for a mixture of metal and its oxides.

6 Magnetism of the oxide is also in question. The oxide could be ferromagnetic, paramagnetic, or superparamagnetic. In this thesis, we use “ferromagnetism” to embrace both ferromagnetism and ferrimagnetism. Probing the magnetic properties of the oxide is challenging because of interference from the ferromagnetism of underlying bulk iron.

Polarized neutron reflectometry (PNR), x-ray magnetic circular dichroism (XMCD), and superconducting quantum interference device (SQUID) are the three common methods for elucidating magnetic properties. XMCD has high sensitivity and low requirements regarding the sample quality.12 Determination of magnetic moment using XMCD, however, requires exact number of holes, which is not available without knowing the film thickness.13 SQUID, on the other hand, is sensitive to the average magnetic moment of the whole film and does not distinguish the oxide from the substrate.14 Initial attempts to use PNR also proved unsuccessful,15 due to interference of fringes of oxide and bulk iron in reciprocal space.

1.4 Passive film on Fe-Cr alloy and stainless steel

In 1911, Monnartz discovered that addition of chromium to iron significantly improves the corrosion resistance especially in acidic solutions.16 Iron-chromium alloy became the prototype of stainless steel. In today’s industry, the chromium content in stainless steels varies from 16% to 26% for austenitic type, 10% to 27% for ferritic type, and 12% to 18% for martensitic types. The role of chromium is crucial and the mechanism of corrosion protection after addition of chromium has been debated for decades. Based on surface analytical techniques, the passive film on iron-chromium alloy is enriched with chromium oxide. Ashami et al. studied passive films on iron-chromium alloys with

7 different chromium content.17 No difference in composition of oxide and alloy interface was found when chromium content was less than 12% as shown in Figure 1.4. For iron- chromium alloys that have more than 12% chromium, after one hour passivation in 1 N

H2SO4, chromium was significantly enriched in the oxide film. However, most composition studies comparing chromium content in oxide film and metal were on the basis of x-ray photoelectron spectroscopy (XPS). XPS extracts elemental information averaged over depth from 10 to 100 Å from the surface. As pointed out in the iron passive film part, the passive film is probably more complicated than homogeneous one- layer structured and composition of passive film may vary with depth. Another drawback is that XPS has to be conducted in very low-pressure environment. Even for ambient pressure XPS, the pressure is about 1 kPa to ensure the transmisson of x-rays. It seems impossible for such technique to simulate the exact environment where the sample is passivated.

Figure 1.4 Chromium compositions in alloy surface (solid line) and oxide film (dash line) on iron- 3, 17 chromium alloys polarized at 100 and 500 mV SCE for 1 hour in 1 N H2SO4.

8 In recent years, nickel also proved helpful to generate a stable passive film.18 Nickel is more stable and can reduce the overall dissolution rates of iron and chromium.19 To make the alloy more pitting resistant, molybdenum is added to prevent depassivation by pitting.20 Nitrogen was found to be beneficial and various types of synergism with molybdenum were proposed.21 To enhance the solubility of molybdenum and nitrogen, magnesium was introduced to stainless steel as well.

1.5 Summary

Despite over a century of research on passivity, the structure and composition of passive films remains controversial. In this thesis, native oxide and electrochemical passivated film on iron and iron-chromium are characterized individually and compared.

Chapter 2 discusses the characteristics of native oxide and Chapter 3 focuses on electrochemically passivated film.

In Chapter 2, a novel approach based on polarized neutron reflectometry is proposed and demonstrated for the study of passive films. Polarized neutron reflectometry is an efficient and precise tool to probe the oxide magnetism that exploits the magnetism of the iron substrate. Non-polarized neutron reflectometry is an alternative but insufficient to extract the complicated structure and composition of a magnetic film.

In Chapter 3, iron and iron-chromium thin films are passivated electrochemically. The passivation potential, kinetics, and pH dependence are systematically studied by DC polarization and x-ray reflectometry. X-ray reflectometry proved to be an accurate

9 technique to probe both metal film and oxide film simultaneously. The x-ray reflectivity of air passivated film and electrochemical passivated film is compared.

10 Chapter 2 Using magnetism to probe native oxide on Fe

2.1 Overview

Native oxide on metal is hard to investigate because of its low thickness and similarity with the bulk layer. We present a novel approach to probe the thickness, composition, and magnetic properties of native oxide on iron using polarized neutron reflectometry and x-ray reflectometry. Non-polarized neutron reflectometry is applied as well. The passive film on iron is determined to be dense, non-ferromagnetic magnetite with a thickness of

33.7 ± 2.0 Å.

2.2 Polarized neutron reflectometry

The difficulties regarding investigation of native oxide on iron are (1) extremely low thickness compared to bulk iron and (2) similarity of characteristics between iron and its oxide. However, the difference in magnetic properties may be a path to distinguish the native oxide.

Neutrons have two spins states, spin-up (m = +1/2, where m is the spin projection on the field) and spin-down (m = -1/2). In the absence of environment magnetic field, neutrons show no energy difference. In a magnetic field, spin-up neutrons gain magnetic potential energy and spin-down neutrons lose magnetic potential energy as shown in

Figure 2.1 (Zeeman effect22). In other words, there is a low-energy state and a high- energy state depending on orientation with or against the field. Hence, neutrons can

11 differentiate magnetic and non-magnetic materials because of different behavior of spin- up and spin-down neutrons.

Figure 2.1 Zeeman splitting of neutrons. Spin-up neutrons gain energy and spin-down neutrons lose energy in a superposed of magnetic field. Neutrons in the absence of magnetic field show no energy difference.

Since the first polarized neutron reflectometry (PNR) experiment was reported by

Hughes and his colleagues,23 PNR became a popular tool to study thin film magnetism.

PNR provides the magnetic scattering length density (SLD) profiles of films by inverting the specular reflectivity data for spin-up and spin-down neutrons. Neutrons can penetrate thick substrates and reveal magnetic structures of interfacial films.

Figure 2.2 Scheme of Asterix polarized neutron reflectometer, LANSCE, LANL, Los Alamos, NM

12 Figure 2.2 indicates the configuration of the Asterix polarized neutron reflectometer at

Los Alamos National Laboratory (LANL). The neutron beam is introduced from the right. The polarizer is an iron-silicon super mirror that reflects pure spin-up neutrons.

Spin-flippers change the spin states of neutrons by polarized He3. Electro magnets can polarize sample in any direction.

The geometry for transverse specular reflectometry is shown in Figure 1. Incident neutrons with wave vector ko impinge on the sample at an angle, θ, of less than 2°.

Reflected neutrons are detected at a reflection angle equal to the incident angle. Under such specular conditions, the scatting vector q is perpendicular to the film, making PNR sensitive to the depth dependence of composition and magnetization (projected onto the applied field axis) normal to the surface.

q ko k

θ θ

Figure 2.3 Transverse geometry for polarized neutron reflection.

In transverse geometry an in-plane magnetic field is applied perpendicular to the plane of the incident neutrons. Spin-up neutrons and spin-down neutrons have different reflection behaviors provided that the sample is magnetically polarized. Ferromagnetic

13 components are polarized while paramagnetic films are not because of smaller magnetic susceptibility.

In this chapter, PNR, NNR and XRR were carried out on a 700-Å iron sample on a quartz wafer. The thickness of the iron layer is deliberately much larger the oxide layer so as to present two series of Kiessig fringes in reciprocal space. To minimize the intensity loss due to absorption in the x-ray experiment, a film of 700 Å is considered to be the best candidate. Using a 700-Å iron layer, we avoid the ambiguity experienced by Kruger et al. who used iron films of thickness comparable to the oxide.15

2.3 Experimental Details

A 700-Å iron film was magnetron-sputtered on a quartz wafer by Jon Kevin Baldwin,

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos,

NM. The substrate is 2-inch diameter and 0.4-inch in height quartz wafer with 5-Å roughness (local thickness variation). The composition of the film was verified by x-ray fluorescence spectroscopy to be pure iron.

PNR was carried out at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos

National Laboratory, using the Asterix reflectometer.22, 24 The sample is magnetically saturated in the sample plane with a 5 kOe magnetic field.

PNR causes no damage to samples and samples are kept in a desiccator after being exposed to assure consistency of the passive oxide. The non-polarized neutron

14 reflectometry (NNR) experiment was done at Beamline-4B, Spallation Neutron Source,

Oak Ridge National Laboratory, Oak Ridge, TN. In this case the sample is not in a magnetic field. Specular XRR data were acquired using the PANalytical’s X’Pert PRO

Materials Research Diffractometer at the Advanced Material Characterization Center

(AMCC), University of Cincinnati. In all cases, the incident beam impinges on the sample from the air side.

2.4 Results and discussion

2.4.1 X-ray and neutron scattering length density of magnetic materials

Reflectometry measures the distribution of scattering length density (SLD) normal to the surface. For x-rays, the SLD is the sum of number density times scattering length of each atom.

atoms (2.1) SLDX−ray = ∑ N jbj j=1 where Nj is number density; bj is scattering length of atom j. In this case the SLD is proportional to the electron density with the multiplication factor of classical radius of electron (2.82 × 10-5 Å). By comparing the measure SLD to that calculated for specific compositions and densities one identify the contributions to the SLD profile. Magnetism improves contrast between layers if the neutron beam is magnetically polarized.

For magnetic components the neutron SLD includes both nuclear and magnetic contributions:

15 magnetic atoms atoms SLD = SLDnuclear ± SLDmagnetic = ∑ N jbj ± ∑ Ni pi (2.2) j=1 i=1 where bj is nuclear scattering length; pi is magnetic scattering length.

Table 2.1 The density and calculated x-ray SLD (Cu Kα, 8.04 keV) and neutron SLDs for quartz, iron, magnetite (Fe3O4) and maghemite (γ-Fe2O3). For magnetic components the SLD depends on the polarization of the incident beam so two SLDs are possible (nuclear SLD ± magnetic SLD) if the film is magnetized. Densities are calculated from the measured lattice constants. Non-polarized reflectometry measures the average SLD of the two components. Material Density 106 × x-ray SLD 106 × neutron SLD 106 × neutron SLD s (g/cm3) (Å-2) nuclear (Å-2) magnetic (Å-2)

Quartz 2.65 22.6 + 0.3i 4.19 0.00

Fe 7.87 59.4 + 7.7i 8.00 4.97

Fe3O4 5.36 41.9 + 3.8i 7.19 1.36

γ-Fe2O3 4.97 39.0 + 3.4i 6.80 1.36

Table 2.1 shows the density, magnetic moment, x-ray SLD, and neutron SLD of quartz,

iron, maghemite (γ-Fe2O3) and magnetite (Fe3O4). The observed SLD may be less than these values due to defects in the crystalline structure. The magnetic neutron SLD is comparable to the nuclear SLD for iron and magnetite. Even though magnetite has a larger magnetic moment, the magnetic neutron SLD of magnetite is smaller than iron. For our experiment the neutron SLD of ferromagnetic materials is either the sum or difference of nuclear SLD and magnetic SLD depending on the spin states of incident neutrons,

(8.00 + 4.97) × 10-6 Å-2 for spin-up neutrons or (8.00 - 4.97) × 10-6 Å-2 for spin-down neutrons. Absorption of neutrons is negligible for all three materials.

2.4.2 Structure and porosity determination by x-ray reflectometry (XRR)

16 XRR is sensitive to the composition and porosity of samples. Since the thicknesses of iron and its oxide are on different size scales, XRR shows a series of compressed fringes due to the metal layer and a few gentle bumps due to the oxide layer (Figure 2.4). Using the Parratt analysis method embodied in the Irena analysis package from Argonne

National Laboratory,25 we model the reciprocal-space reflectivity data into a SLD profile in real space (Figure 2.5). The SLD profile is the SLD as a function of surface-normal distance from the quartz substrate. A range where the SLD is constant is regarded as a layer. An SLD gradient represents an interface. The variation of the SLD profile between layers is a measure of the roughness where a sharper change indicates a smoother interface. The analysis package treats roughness as Gaussian-distributed fluctuations in

SLD perpendicular to the surface.

Figure 2.4 X-ray reflectivity data and model fit Figure 2.5 X-ray reflectivity SLD profile with assuming 2 layers (iron and iron oxide) on a fitting parameters. quartz wafer.

From analysis of XRR data, the thickness of the iron film is 703.3 ± 2.0 Å. The sample is extremely smooth with only 7.5-Å rms roughness between the iron and the oxide (1% of the thickness). The thickness of the oxide is 32.0 ± 2.0 Å with similar roughness

(height fluctuations). The SLD of iron matches that of iron in Table 2.1. The oxide SLD

17 lies between Fe3O4 and γ-Fe2O3 implying either a mixture or Fe3O4 with slightly reduced density due to defects and grain boundaries. The measured values are tabulated in Table

2.2. The fitting is much less sensitive to the imaginary parts of the SLD, but fitted values are also close to the calculated values for dense iron and Fe3O4. The errors given are estimated based on how much a parameter can be varied and still give a good visual fit to the data. The estimates are substantially larger than errors estimated from the goodness- of-fit and more closely correspond to systematic variations in fitting of different samples.

Table 2.2 Summary of fitting parameters and calculated densities for XRR, PNR and NNR. The oxide density is calculated assuming the film is Fe3O4. Materials Variables Fitting parameters

Incident X-ray Spin-up Spin-down Unpolarized Average beam 106 × SLD 22.6 4.2 4.2 4.2

Quartz (Å-2) ± 0.3 ± 0.1 ± 0.1 ± 0.1 Substrate Density 2.7 2.7 2.7 2.7 2.7 (g/cm3) ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 Thickness 703.3 702.0 702.0 700.9 702.0 (Å) ± 2.0 ± 2.0 ± 2.0 ± 2.0 ± 2.0 Fe 106 × SLD 56.5 12.4 3.6 12.4

Layer 1 (Å-2) ± 0.3 ± 0.1 ± 0.1 ± 0.1 Density 7.5 7.9 7.9 7.9 7.8 (g/cm3) ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.1 Thickness 32.0 35.0 35.0 32.8 33.7 (Å) ± 2.0 ± 3.0 ± 3.0 ± 3.0 ± 2.0 Oxide 106 × SLD 39.3 7.0 7.0 7.0

Layer 2 (Å-2) ± 0.3 ± 0.5 ± 0.5 ± 0.5 Density 5.0 5.2 5.2 5.2 5.2 (g/cm3) ± 0.1 ± 0.5 ± 0.5 ± 0.5 ± 0.1

Table 2.2 also shows the calculated densities for three techniques. Thickness information for both iron layer and oxide layer is consistent for all three techniques. SLDs for neutron experiments are consistent as well.

2.4.3 Magnetism analysis by PNR

PNR data (Figure 2.6) were collected on the same sample used for XRR (Figures 2.4-

5). The q range is somewhat less because of lower flux and lower SLD contrast with air.

A significant difference is observed between the spin-up and spin-down neutron reflectivities. The spin-up neutrons are better reflected due to the higher contrast with the substrate. In transverse geometry, no spin flip is involved. Thus, the reflected neutrons have the same spin states as the incident neutrons. We define reflectivity of spin-up neutrons as pp polarization and reflectivity of spin-down neutrons as mm polarization.

The positions of the critical edges and momentum transfer (qc) are determined by SLD of the metal film. The SLDs of iron for pp and mm polarizations are calculated from critical edge, qc, where the beam first penetrates the sample:

2 1 ⎛ qc ⎞ SLD = ⎜ ⎟ , (2.3) π ⎝ 4 ⎠ which gives (3.6 ± 0.1) ×10-6 Å-2 and (12.4 ± 0.1) ×10-6 Å-2 for spin-down and spin-up neutrons respectively. The average of these numbers, (8.0 ± 0.2) ×10-6 Å-2 is consistent with fully dense iron, but SLD of the magnetic contribution, (4.4 ± 0.2) ×10-6 Å-2, is 11% less than the theoretical contribution of the bulk iron.

Figure 2.7 is the SLD profile obtained by Parratt modeling of the R-q data assuming the oxide is non-ferromagnetic. The quartz substrate surface is at position 0. The total thickness for the iron is 702.0 ± 2.0 Å and the thickness for the oxide is 35.0 ± 3.0 Å.

Figure 2.6 PNR (points) and fit (solid lines) Figure 2.7 SLD profiles from the pp and mm assuming a non-magnetic oxide for air- reflectivity data in Figure 2.6 The oxide layer is passivated, 700-Å iron sample on quartz. Red more apparent for the mm data because of the curve: spin-up (pp) polarization; blue curve: lower SLD of the iron substrate for mm spin-down (mm) polarization. The large polarization. difference is due to the contribution of magnetism of the iron film.

Figure 2.8 Best fit (solid lines) assuming magnetic Figure 2.9 SLD profiles of pp and mm reflectivity magnetite for air-passivated, 700-Å iron sample on data in Figure 6. The average SLD of oxide (8.74/2 quartz substrate. Red curve: spin-up (pp) +5.26/2 = 7.00) is consistent with non- polarization; blue curve: spin-down (mm) ferromagnetic magnetite. polarization. The ferromagnetic-oxide model fits well for pp polarization but not for mm.

To examine the possibility of ferromagnetism in the oxide we compare fits assuming

both ferromagnetic oxide and non-ferromagnetic oxide. In ferromagnetic case, the SLD

20 varies for spin-up and spin-down neutrons. By comparison of the best fits for both models, we judge whether ferromagnetism in the oxide is observable.

1) Non-ferromagnetic-oxide model

In non-ferromagnetic model, as shown in Figure 4 and Figure 5, the SLDs of oxide for spin-up and spin-down neutrons coincide at 7.0 × 10-6 Å-2.

-6 -2 pp SLDoxide = mm SLDoxide = 7.0 ± 0.3 × 10 Å (2.4)

The non-ferromagnetic model yields a good fit for both pp and mm reflectivities

(Figures 2.6 and 2.7). The value of the SLD is consistent with bulk magnetite (7.19 × 10-

6 Å-2).

2) Ferromagnetic-oxide model

Assuming the oxide is ferromagnetic, the reflectivity and fits are shown in Figure 2.8.

While the model yields a decent fit for spin-up neutrons it does not fit the spin-down data. The contribution of oxide to reflected neutrons is seriously underestimated.

Therefore, we conclude that the film must be non-ferromagnetic. Apparently the film is thin enough to suppress ferromagnetism, which is expected for both bulk magnetite and bulk maghemite. Whether the film is superparamagnetic or paramagnetic cannot be determined.

Although there are no data on magnetism in thin iron oxide films, pure iron does show a thickness-dependent superparamagnetic-ferromagnetic transition with increasing thickness. Thomassen et al.,26 for example, studied iron ultrathin films on copper (100)

21 using surface magneto optical Kerr effect (SMOKE). Copper substrate was used because the lattice mismatch favors non-ferromagnetic fcc iron at low coverage. Below 5 monolayers, iron does not form a continuous film. Between 5 and 11 monolayers the iron is weakly magnetic fcc with magnetic moment normal to the substrate. Above 11 monolayers, the entire film switches to ferromagnetic bcc with the magnetic moment in the plane. Hayashi et al.27 studied iron films on rhodium (001). The iron film shows in- plane ferromagnetic and the magnetic moment is the same as bulk bcc iron above 6 monolayers. Based on these studies on iron, the transition from two-dimensional to three- dimensional behavior occurs at around 10 monolayers. The radius of an atomic iron is

1.26 Å. The distance of two atoms in α-iron is 2.87 Å, so for films over 30 Å, iron can be regarded as ferromagnetic bulk iron with a bcc structure.

Although data on magnetism on thin oxides is lacking, the magnetism of colloidal maghemite is known to depend on size and shape. The change from ferromagnetic to superparamagnetic for spherical maghemite takes place at 200 Å in diameter at room temperature.28 Whether its value is relevant to two-dimensional maghemite films is not known. Our results, however, show that the critical thickness to achieve ferromagnetism is larger than 30 Å.

2.4.4 Comparison between polarized and non-polarized neutron reflectometry on magnetic sample

After the ferromagnetic sample was removed from polarized neutron reflectometer, magnetism of the sample weakens but still exists. Figure 2.10 compares NNR and PNR.

22 The non-polarized neutron beam consists of half spin-up neutrons and half spin-down neutrons. Hence, the total intensity is the sum of the two reflectivities. This procedure assumes that magnetic domains are larger than the coherency of the neutron beam (~

µm). As a result, two critical edges are observed (0.0145 ± 0.001 Å-1 and 0.0250 ± 0.001

Å-1). The corresponding SLDs are 4.2 ± 0.1 × 10-6 Å-2 and 12.4 ± 0.1 × 10-6 Å-2.

NNR covers a wider q range making it more sensitive to short-scale structure.

Similarity of polarized data and non-polarized data is observed for the position of critical edge and the fringes. The SLD of iron for spin-up neutrons can be determined from the critical edge. Thickness information is derived from the fringes. However, this information is less reliable for spin-down neutrons due to the lack of distinct fringes.

Figure 2.10 Comparison of spin-up neutrons and Figure 2.11 SLD profile for non-polarized non-polarized neutrons as incident beam. Solid neutron reflectivity. The high SLD of the iron purple line represents the best fit to the NNR data layer masks the oxide layer. with the resulting SLD profile shown in Figure 2.11.

Figure 2.11 shows the SLD profile obtained from the non-polarized data. The oxide layer is barely recognizable. Nevertheless these data confirm the thickness, magnetism

23 and composition of the iron as determined by XRR and PNR. The oxide SLD also agrees with that from PNR (Table 2.2).

The primary difference in the calculated profile for non-polarized neutrons is the breadth of the interfaces, as described by the rms roughness, σ. Table 2.3 compares σ for the three techniques. The artificial broadening of the non-polarized profile is caused by enhanced reflection of spin up neutrons compared to spin down. Thus, although non- polarized neutrons can be used to determine the layer thicknesses, the interface widths values are compromised.

Table 2.3 The σ of the interfaces (quartz-iron, iron-oxide, oxide-air) for XRR, PNR, and NNR. Quartz-iron Iron-oxide Oxide-air Techniques (Å) (Å) (Å) XRR 3.8 7.5 7.7 PNR 1.0 8.5 4.0 NNR 23.0 25.2 9.0

From the discussion above, a one-layer, non-ferromagnetic-oxide model is consistent with all three reflectometry approaches. There is no evidence of a bilayer oxide structure.

No doubt, introducing additional layers in the reflectivity model can yield a better fit.

However, added complexity is not demanded by the data. We conclude that oxide is a homogeneous single-layer non-ferromagnetic oxide. The mean SLD for the oxide is just

0.4% below that of crystalline magnetite.

24 2.5 Summary

Through the combined analysis of PNR, NNR and XRR, we conclude the studied sample consists of a homogeneous, ferromagnetic, 702.0 ± 2.0 Å, iron layer with a non- ferromagnetic, homogeneous, 33.7 ± 2.0 Å oxide. The average neutron SLD of the oxide

-6 -2 is 7.0 ± 0.5 × 10 Å , which is very close to that of Fe3O4. Therefore we conclude that the native oxide is non-ferromagnetic magnetite film. The mean density calculated from all four measurements is 5.2 ± 0.1 g/cm3, which is within 2% of that expected for magnetite.

Because of the similarities in x-ray and neutron scattering properties of the competing oxides, pure iron is the worst-case scenario for definitive determination of their compositions. However, even in this case, we achieve substantial contrast using the magnetic properties of the iron film below to be able to address the structure of the thin passivating oxide. Contrast can be further enhanced using alloys such Fe-Ni, or Fe-Cr especially for particular isotopes of these alloying elements.

25 Chapter 3 Electrochemical passivation and reflectivity of

passive Fe and Fe-Cr thin films

3.1 Overview

In this chapter, iron and iron-chromium alloy films are electrochemically passivated using a three-electrode potentiostat. X-ray reflectometry is conducted on the native and freshly passivated iron. Reflectivity of native oxide and oxide on electrochemically passivated film is compared.

3.2 Background

3.2.1 Two-electrode and three-electrode cell system

A potentiostat is capable of running most electroanalytical experiments. Two-electrode, three-electrode, and four-electrode cells are typical configurations. With the potentiostat, either a known current, or potential is applied between the working and auxiliary (counter) electrodes and the other variable is measured.

Two-electrode cell system is the simplest configuration for potentiostat, consisting of working electrode and counter electrode. The current is measured by an ammeter while the potential data are collected by a voltmeter. The resistance of the ammeter is not zero and the resistance of the voltmeter is not infinite, which can generate systematic errors.

To avoid such errors, three-electrode potentiostat is used for most electrochemical experiments.

Three-electrode system has an additional reference electrode. Figure 3.1 shows the schematic view of three-electrode potentiostat. The introduction of reference electrode makes it possible to measure the potential by calculating the difference between working electrode and reference electrode, which eliminates the error caused by voltmeter.

Figure 3.1 Illustration of the experimental set up for three-electrode potentiostat. Working electrode (W) is the sample, iron thin film on silicon substrate; counter electrode (C) is a carbon rod; reference electrode (R) is a calomel electrode.

In our experiment, due to the fact that the sample is not bulk metal, but stainless steel coating on silicon or quartz wafers, full immersion in solution may cause severe problems such as bad connections and large errors or inconsistency. As a result, the set-up is changed by mounting the sample, i.e. the working electrode, horizontally. A cylinder filled with solution is clipped on to the sample (Figure 3.2). The auxiliary carbon-rod electrode and calomel, reference electrode is immersed in the solution and are kept 1 cm away from the working electrode. This configuration prevents problems caused by immersion of the whole electrode and offers a good control of the area corroded. In short, this revised setup is beneficial to study the passivation of stainless steel thin films.

Potentiostat

Carbon rod Calomel

1 cm

Sample (0.5 mm Si substrate)

Figure 3.2 Electrochemical cell set up. The sample is thin iron film on 0.5 mm silicon substrate.

3.3 Experimental details

3.3.1 Sample description

Iron films and iron-chromium films with a thickness of 700 Å were magnetron sputtered on silicon substrates by Jon Kevin Baldwin, Center for Integrated

Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM. The substrate is 4 inches in diameter and 0.4 inches in height with 5-Å roughness (local thickness variation).

The composition of the film was verified by x-ray fluorescence spectroscopy to be pure iron.

28 3.3.2 Electrochemical testing and passivation

Electrochemical passivation of iron and iron-chromium alloy thin films is conducted using a Gamry Electrochemical Station. The sample to be tested or passivated is the working electrode with a carbon rod as the counter electrode. The potential is determined by the difference of potential between the sample and the reference electrode. The current is recorded by the circuit between reference electrode and counter electrode. The applied potential can vary from -5 V vs. SCE (saturated calomel electrode) to +5 V vs. SCE which meets the need of electrochemical testing and passivation.

Electrochemical passivation is a three-step process. Step 1: Open circuit potential

(OCP) determination. For iron sample, OCP is around -500 mV vs. SCE. Step 2:

Stripping. The native oxide should be removed right before growth of the electrochemically passivated film by stripping at – 1.5 V vs. SCE (1 V below OCP) on the iron sample. The stripping process takes 15 seconds. Step 3: Passivation. Immediately after stripping, a positive potential is applied to grow a passive film electrochemically.

The passivation potential is determined by a direct current potential (DCP) experiment.

After passivation, the sample is air-dried and kept in a desiccator.

3.4 Results and discussion

3.4.1 Passivation potential and its effect on passive films

Direct current polarization (DCP) is used to evaluate the corrosion behavior by polarizing gradually from a low potential to a high potential while recording the current.

DCP is capable of quick determination of OCP, which exists when the current is

29 infinitely close to zero. For materials without passivation (free corrosion) a typical DCP curve should follow the Tafel equation for both cathodic branch and anodic branch, shown as the dashed line in the Figure 3.3. For passive metals, the current should drop in the passivation region due to suppression of the corrosion reaction by the oxide film. One

can apply DCP to determine the passivation region and pick Ep, the passivating potential.

A DCP experiment is conducted on a pure iron sample with a thickness of 3000 Å. As is shown in Figure 3.3, a large passivation region exists for iron (-200 mV vs. SCE to

1000 mV vs. SCE). Four potentials (200 mV vs. SCE, 400 mV vs. SCE, 800 mV vs. SCE and 1.2 V vs. SCE) were picked to passivate pure iron samples for 15 minutes. Pictures were taken after 15 minutes of passivation at different potentials. The area surrounded by the circle is the exposed region. The dark spots in the pictures are silicon wafers and if they are observed on the wafer, it is an indication that the metal at the some areas on the sample is fully corroded (stripped). As can be seen, pitting takes place on the film passivated at 1.2 V vs. SCE while for other three polarization potentials, the metal still looks shiny and is protected by the passive film. This result coincides with the DC polarization experiment in that 1.2 V vs. SCE is not in passivation region. Pitting takes place because the sample is in transpassive state. Hence, 1.2 V vs. SCE is ruled out for preparation of passive film. Choice of passivation potential restricted to the other three potentials.

30 1.2 V vs. SCE Pitting

800 mV vs. SCE Passivation

400 mV vs. SCE

Passivation

200 mV vs. SCE Passivation

Figure 3.3 DCP results for Fe for complete cathodic and anodic polarizations. For pure iron passivation potentials (Ep) of 200, 400, and 800 mV vs. SCE can be identified to be used for the passivation experiment. Dashed line indicates the free corrosion scenario for cathodic branch.

During the passivation experiment, current data over passivation time is collected and shown in Figure 3.4. It is apparent that 800 mV vs. SCE current data is a lot higher than the others. The current is a direct indication of the corrosion rate. Hence, the passive film is generated faster and dissolves faster than at 400 mV and 200 mV vs. SCE. The change of slope of log-linear scale current profile is an indication of equilibrium. The slope decreases during the passivation process and reaches zero for infinite long time. The current for 800-mV sample stabilizes at 40 µA before 15 minutes; while for 200 mV and

400 mV, the current is still decreasing during the 15th minute. Thus the equilibrium is reached faster for higher passivation potential. Despite the small changes in current, a stable passive film is created at all three passivation potentials after 15 minutes of polarization.

Figure 3.4 Current profiles during 15-min passivation at 200, 400, and 800 mV on the 700 Å wafers.

The x-ray reflectivity and SLD profiles are compared in Figure 3.5-6 to analyze whether there is a difference in oxide structure in the film. The iron samples are coated on silicon wafers so that an amorphous silicon oxide layer exists between the metal and silicon substrate. The amorphous silicon oxide has slightly larger SLD, which is represented in Figure 3.6 by an increase at distance 0. As compared in the previous chapter, iron oxide has a lower SLD than iron. Thus, the sample is three-layer structure with a 20-Å silicon oxide layer, a 700-Å iron layer, and a 30-Å iron oxide layer among which iron has the highest SLD. The SLD of oxide formed at 200 mV vs. SCE is significantly less than SLD of oxide of other three samples. Lower SLD is an indication of higher porosity. The passive film passivated at 200 mV vs. SCE is the most porous film among the four samples.

Figure 3.5 Passivation potential dependence. Figure 3.6 SLD profiles of potential dependence Comparison of x-ray reflectivity vs. q for air- x-ray reflectivity. Four SLD profiles are passivated sample (control), electrochemically completely overlapped at 0 to 600 Å from the passivated sample at different passivation substrate. potential.

Figure 3.7 is a zoom-in of SLD profile at the region 550 Å to 630 Å, which shows the

SLD change at metal-oxide interface. With the increase of corrosion potential, the thickness of the iron layer decreases, which coincides with the current profile. As can be observed in the current profile, the current for 800 mV vs. SCE is the highest and the sample corrodes fastest. The iron layer is thinner for sample passivated at higher potential, an indication of dynamic equilibrium of passivation. Based on the analysis of air passivated film and electrochemically polarized film at different potentials, the films do not show much difference in terms of oxide structure. All three potentials are suitable for passivation experiments and here we choose 800 mV vs. SCE.

Figure 3.7 Zoom-in SLD profile of Figure 3.6 in the region from 550 Å to 630 Å.

3.4.2 Passivation kinetics

Passivation time for the previous research is 15 minutes. 15 minutes can be regarded as a safe operation condition. But it is still necessary to discuss the change of the film dependence on passivation time. From Figure 3.4 (current profile vs. time), the passivation process starts to stabilize at around 200 seconds as indicated by leveling-off of the current. To further study the kinetics, samples with a 2000-Å iron coating are passivated for 5 minutes and 15 minutes. We intentionally apply thicker samples for kinetic research because when the iron thickness is larger than 2000 Å, the Kiessig fringes for the iron can hardly be detected. The x-ray reflectivity data and the SLD profiles for the iron samples plotted in Figure 3.8-9 reveal the oxide growth kinetics. The fringes for 2000 Å iron are invisible but the bumps representing the oxide still exist. The thick film idea to characterize the oxide is feasible and effective to study the oxide alone.

Figure 3.8 Passivation kinetics. Comparison of x- Figure 3.9 SLD profiles of potential dependence ray reflectivity vs. q for air-passivated sample x-ray reflectivity. Iron layer is thinner after (control), electrochemically passivated sample for passivation. 5 minutes and 15 minutes at 800 mV vs. SCE.

Figure 3.10 Zoom-in SLD profile of Figure 3.9 in the region from 2050 Å to 2350 Å. Arrows are pointed to the SLD of oxide layer.

Reflectivity of air-passivated film (black line) is lower than reflectivity of solution passivated films (red and blue lines), which is an indication of higher roughness. The fringe (peak) in the reflectivity data (at q = 0.15 Å-1) is caused by a thin layer that has a

35 thickness of about 30 Å consistent with an oxide layer. The comparison of SLD profiles confirms that during the passivation process, the iron thins and the roughness of the iron decreases as well. The original oxide first dissolves; the metal surface is electrochemically polished; and then a fresh film is generated. The roughness of the sample is better controlled a for 700-Å coating than 2000-Å coating, which leads to a rougher virgin metal film. Though it is a better idea to achieve the oxide information with a thicker film, the quality for air-passivated film can not be assured. We decided to stick to 700 Å for further study so we can better compare the air passivated oxides with the solution passivated ones.

Thickness and SLD information for the three passive films (native oxide sample, 5 minutes sample, and 15 minutes) are summarized in Figure 3.11. The difference between native oxide and electrochemically-passivated oxide is seen in the SLD. The SLD of the oxide almost doubles after it is electrochemically passivated. Higher SLD is an indication of higher density. Thus, we assume that the native oxide is less dense than the freshly passivated oxide. The density change is related to the roughness of the metal layer as well. For oxide film on a rougher interface, chances for losing metal atoms are larger because of the unevenness. As is discussed above, the solution plays a role of polishing the metal coating and enables the growth of a passive layer that is denser and more stable.

36 40 40

30 30 10 6 x SLD (Å

20 20 -2 ) Thickness (Å) 10 10

0 0 0 4 8 12 16 Passivation time (min.)

Figure 3.11 The thickness (left axis) and SLD (right axis) of the oxide layer during passivation. Compared to the native oxide layer, the thickness is unchanged while the SLD increases by 50%. The oxide growth ceases after 5 minutes of passivation.

Figure 3.12 Current profiles during 15 minutes passivation at 800 mV. The current stabilizes after 150 – 200 seconds, suggesting the end of oxide growth at that time.

The current profile (Figure 3.12) confirms that oxidation stabilizes after 5 minutes. In fact, the current drops to 100 µA at the 220th second. It is reasonable to infer that corrosion is at steady state and the passive layer is unchanged between 5 minutes to 15

37 minutes because no significant difference has been observed in terms of current, thickness, and SLD.

3.4.4 Effect of solution pH on passivation

The solution in the electrochemical passivation experiment is 1 N sodium sulphate

(Na2SO4). The pH for 1 N Na2SO4 solution is 7. By the addition of sulphuric acid

(H2SO4), the solution pH is decreased to pH = 1, 3, 5. With x-ray reflectometry, we may gain understanding as to whether the passive films prepared at different pHs show different structure and composition. The effect of solution pH is studied on both iron

(Figure 3.13-14) and iron/chromium alloy samples (Figure 3.15-16). The passivation potential and time are consistent for both groups. All samples are passivated at 800 mV vs. SCE for 15 minutes.

For pure iron, as is discussed previously, passivation at neutral pH generates an oxide with similar thickness but higher SLD than the native oxide. At pH = 5, the reflectivity drops faster than film prepared with increasing q at pH = 7. The decrease of reflectivity is a sign of oxide roughness. However, the fringes at higher q are still present so the iron layer is still smooth. The acidic solution seems to polish the metal surface. In acidic solution, the passive film becomes thicker because driving force is larger. When the sample is removed from solution, top of the thick passive oxide dissolves into the solution causing the roughness increase. The decrease of SLD at solution pH = 5 is caused by pitting corrosion. As shown in SLD profile (Figure 3.14), the SLD of iron layer decrease from 58.8 to 52.1. The SLD drop is a direct indication of density drop by 11.4%.

38 The metal layer transforms from a dense film to a porous film. Compared to the thickness drop, the decrease of SLD is more significant. At pH = 3, the metal is almost stripped during 15 minutes and the oxide layer is not able to protect the film. With the loss of iron layer, the fringes are also gone in reflectivity vs. q plot.

Figure 3.13 Solution pH dependence. Figure 3.14 SLD profiles of solution pH Comparison of x-ray reflectivity vs. q for air- dependence x-ray reflectivity on iron sample. passivated sample (control), electrochemically Pitting corrosion takes place when solution pH = passivated iron sample for 15 minutes at 800 mV 5 where SLD of iron layer decreases. vs. SCE at different solution pH (pH = 3, pH = 5, and pH = 7).

The iron/chromium alloy has 17 wt.% chromium, Due to the corrosion resistance for iron/chromium samples, a solution with pH = 3 does not destroy the film in 15 minutes and fringes still exist judging by the reflectivity vs. q plot (Figure 3.13). But the top surface of sample is too rough after exposure in pH = 3 solution to be explained by ordinary reflectivity models. Local thickness variation is significantly increased and fringes in reflectivity vs. q are barely detectable. Similar to iron sample passivated in pH

= 5 solution, pitting corrosion takes place on alloy sample passivated at pH = 3 solution.

39 The SLD drops from 55.0 to 35.3 which points to a 35.8% density decrease. Comparing fits with the data, the fringes are more obvious which indicates that the fits overestimate the smoothness of the film. At pH = 1, the metal is totally washed off from the substrate and the residue is oxide. For pH = 1 sample, the diffuse reflection of the rough surface makes x-ray reflectometry difficult to evaluate.

Figure 3.15 Solution pH dependence. Figure 3.16 SLD profiles of solution pH Comparison of x-ray reflectivity vs. q for air- dependence x-ray reflectivity on iron/chromium passivated sample (control), electrochemically alloy sample. Pitting corrosion takes place when passivated iron/chromium alloy sample for 15 solution pH = 3 where SLD of alloy layer minutes at 800 mV vs. SCE at different solution decreases. pH (pH = 1, pH = 3, pH = 5, and pH = 7).

3.5 Summary

In this chapter, passivation potential, passivation kinetics, and solution pH are determined by DC polarization and reflectometry. The passive region is first determined by direct current potential (DCP) experiment. Reflectivity of iron samples passivated at

200 mV, 400 mV, and 800 mV vs. SCE are compared. 800 mV vs. SCE is selected as the passivation potential and applied on the later experiments. Kinetics study reveals that the

40 passivation process reaches dynamic equilibrium (steady state corrosion) before 200 seconds. Passivation time is set to be 15 minutes to assure the film is fully passivated and stable. The comparison of reflectivity shows similarity of both SLD and thickness for the oxide layer.

Solution pH is varied for both iron and iron/chromium alloy thin films. Iron films are resistant at pH = 5 solution but destroyed at pH = 3 solution. Iron/chromium films are resistant at pH = 3 solution but are destroyed at pH = 1. At low solution pH (pH = 5 for iron and pH = 3 for iron-chromium alloy), the decrease of density of metal layer is caused by pitting corrosion in acidic environment. To process a iron or iron/chromium alloy film with a smooth metal layer and smooth oxide, the passivation conditions should be 800 mV vs. SCE, 15 minutes of passivation in pH = 7 sodium sulfate solution.

41 Future plans

In the future, this project can be expanded in the following aspects. In the first place, polarized neutron reflectometry of electrochemically passivated films will be effective to reveal the passive oxide structure. Comparison of polarized neutron reflectivity of air- passivated and solution-passivated film will possibly lead to some interesting results. In the second place, non-polarized neutron reflectometry can be further developed to study magnetic thin films using liquid cell. Non-polarized neutron reflectometer is more available to users and costs 80% less time per sample than polarized neutron reflectometer. Ideally, non-polarized neutron reflectometry can yield reflectivity of neutrons of one spin-state. Combined with x-ray reflectometry, film structure and composition can be determined. In the third place, the corrosion protection performance can be further evaluated by electrochemical impedance spectroscopy. We have concluded that the SLD and thickness of oxide on air passivated films and on electrochemically passivated films are similar. Electrochemical impedance spectroscopy can be applied to evaluate the performance of the films individually. Last but not the least, the idea of studying passivation of iron and iron/chromium alloys with reflectometry can be adapted to other metals such as nickel and nickel alloys.

42 References

1. Stansbury, E. E.; Buchanan, R. A., Fundamentals of Electrochemical Corrosion. A S M International: 2000. 2. Kier, J., Passivity of iron. Phil Trans 1790, 80, 359. 3. Schmuki, P., From Bacon to barriers: a review on the passivity of metals and alloys. J Solid State Electr 2002, 6 (3), 145-164. 4. Talbot, D. E. J.; Talbot, J. D. R., Corrosion of Iron and Steels. In Corrosion Science and Technology, CRC Press: 1997; p Ch 7. 5. Nagayama, M.; Cohen, M., The Anodic Oxidation of Iron in a Neutral Solution: I . The Nature and Composition of the Passive Film. J Electrochem Soc 1962, 109 (9), 781- 790. 6. (a) Tsuru, T.; Haruyama, S., Resistometric Study of Passive Film on an Evaporated Iron Electrode. Corros Sci 1976, 16 (9), 623-635; (b) Rahner, D.; Forker, W.; Kohl, S., Studies on Chemical Passivation of Iron by Means of Nitrogen in Neutral Media. Z Chem 1978, 18 (3), 115-115.

7. Cohen, M.; Hashimot, K., Cathodic Reduction of Gamma-FeOOH, Gamma-Fe2O3, and Oxide-Films on Iron. J Electrochem Soc 1974, 121 (1), 42-45. 8. Schmuki, P.; Virtanen, S.; Davenport, A. J.; Vitus, C. M., In situ x-ray absorption near-edge spectroscopic study of the cathodic reduction of artificial iron oxide passive films. J Electrochem Soc 1996, 143 (2), 574-582. 9. Cohen, M., An Electron Diffraction Study of Films Formed by Sodium Nitrite Solution on Iron. The Journal of Physical Chemistry 1952, 56 (4), 451-453. 10. (a) Davenport, A. J.; Oblonsky, L. J.; Ryan, M. P.; Toney, M. F., The structure of the passive film that forms on iron in aqueous environments. J Electrochem Soc 2000, 147 (6), 2162-2173; (b) Kim, W.; Suh, C. Y.; Cho, S. W.; Roh, K. M.; Kwon, H.; Song, K.; Shon, I. J., A new method for the identification and quantification of magnetite- maghemite mixture using conventional X-ray diffraction technique. Talanta 2012, 94, 348-352. 11. Namduri, H.; Nasrazadani, S., Quantitative analysis of iron oxides using Fourier transform infrared spectrophotometry. Corros Sci 2008, 50 (9), 2493-2497. 12. (a) Stohr, J., X-ray magnetic circular dichroism spectroscopy of transition metal thin films. J Electron Spectrosc 1995, 75, 253-272; (b) Stohr, J., Exploring the microscopic origin of magnetic anisotropies with X-ray magnetic circular dichroism (XMCD) spectroscopy. J Magn Magn Mater 1999, 200 (1-3), 470-497. 13. Dhesi, S. S.; Durr, H. A.; van der Laan, G.; Dudzik, E.; Brookes, N. B., Electronic and magnetic structure of thin Ni films on Co/Cu(001). Phys Rev B 1999, 60 (18), 12852- 12860. 14. Bland, J. A. C.; Vaz, C. A. F., Polarised neutron reflection studies of thin magnetic films. In Ultrathin Magnetic Structures III: Fundamentals of Nanomagnetism, Bland, J. A. C.; Heinrich, B., Eds. 2007; Vol. 3. 15. Kruger, J.; Krebs, L. A.; Long, G. G.; Anker, J. F.; Majkrzak, C. F., Passivity of metals and alloys and its breakdown - New results from new non-electrochemical techniques. Mater Sci Forum 1995, 185-, 367-376.

43 16. Monnartz, P., The study of ironchromium alloys with special consideration of their resistance to acids. Metallurgy 1911, 161. 17. Asami, K.; Hashimoto, K.; Shimodaira, S., An XPS study of the passivity of a series of iron—chromium alloys in sulphuric acid. Corros Sci 1978, 18 (2), 151-160. 18. Brookes, H. C.; Bayles, J. W.; Graham, F. J., Nucleation and Growth of Anodic Films on Stainless-Steel Alloys .1. Influence of Minor Alloying Elements and Applied Potential on Passive Film Growth. J Appl Electrochem 1990, 20 (2), 223-230. 19. Olsson, C. O. A.; Landolt, D., Passive films on stainless steels - chemistry, structure and growth. Electrochim Acta 2003, 48 (9), 1093-1104. 20. Scarberr, R. C.; Graver, D. L.; Stephens, C. D., Alloying for Corrosion Control - Properties and Benefits of Alloy Materials. Mater Prot 1967, 6 (6), 54-&. 21. Lu, Y. C.; Ives, M. B.; Clayton, C. R., Synergism of Alloying Elements and Pitting Corrosion-Resistance of Stainless-Steels. Corros Sci 1993, 35 (1-4), 89-96. 22. Felcher, G. P.; Adenwalla, S.; Dehaan, V. O.; Vanwell, A. A., Zeeman Splitting of Surface-Scattered Neutrons. Nature 1995, 377 (6548), 409-410. 23. Hughes, D. J.; Burgy, M. T., Reflection of Neutrons from Magnetized Mirrors. Physical Review 1951, 81 (4), 498-506. 24. (a) Majkrzak, C. F., Polarized Neutron Reflectometry. Physica B 1991, 173 (1-2), 75-88; (b) Zhu, Y., Modern techniques for characterizing magnetic materials. Springer: 2005. 25. Ilavsky, J.; Jemian, P. R., Irena: tool suite for modeling and analysis of small- angle scattering. J Appl Crystallogr 2009, 42, 347-353. 26. Thomassen, J.; May, F.; Feldmann, B.; Wuttig, M.; Ibach, H., Magnetic Live Surface-Layers in Fe/Cu(100). Phys Rev Lett 1992, 69 (26), 3831-3834. 27. Hayashi, K.; Sawada, M.; Harasawa, A.; Kimura, A.; Kakizaki, A., Structure and magnetism of Fe thin films grown on Rh(001) studied by photoelectron spectroscopy. Phys Rev B 2001, 64 (5). 28. Serna, C.; Morales, M., Maghemite (γ-Fe2O3): A versatile magnetic colloidal material. In Surface and Colloid Science, Springer: 2004; pp 27-81.