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Passive films on stainless steels*/chemistry, structure and growth

C.-O.A. Olsson *, D. Landolt

Departement des Materiaux, Laboratoire de Metallurgie Chimique, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne EPFL, Switzerland

Abstract

The outstanding corrosion resistance of stainless steels results from the presence of a thin oxide*/passive*/film on the metalsurface, typically 1/3 nm thick. The characterisation of the composition and structure of such thin films and the study of theirinteraction with corrosive environments requires a combination of sophisticated experimental techniques. This paper reviews

progress in the characterisation and understanding of passive films on stainless steels achieved over the past two decades. During

this period, ex situ surface analysis methods have made substantial progress and new in situ methods for the study of passive films

with atomic resolution have been introduced, giving real time information on film chemistry and growth. It has been found that

whereas passive film growth occurs in seconds or minutes, long range film ordering is a considerably slower process that takes

several hours. In situ investigations indicate that at short times, charge transfer at the metal/film or the film/solution interface limits

the rate of film growth on stainless steels. In situ estimates of film composition confirm previous data obtained with ex situ

techniques.

# 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Passive films; Stainless steel; Passivity; X-ray photoelectron spectroscopy; Scanning tunnelling microscope; X-ray absorption near edge

structure; Electrochemical quartz crystal microbalance

1. Introduction

Stainless steels were invented almost a century ago by

Monnartz [1]. Today, their usage shows an average

increase rate of about 5% per annum. They are divided

into four main groups based on their microstructure:

ferritic, austenitic, martensitic and austeno/ferritic(duplex). Among the ferritic steels, one finds a series

of Fe/Cr alloys, e.g. Fe/17Cr (AISI 430), but alsosuperferritics containing high fractions of molybdenum.

Austenitic stainless steels are usually alloyed with Ni, for

example 18Cr/9Ni (AISI 304); superaustentic stainlesssteels containing high fractions of Mo and N can in

certain environments surpass the corrosion resistance of

Ni-base alloys. Stainless steels are subject to continuing

development; some recent inventions are listed in Table

1. Thermodynamic modelling has made it possible to

develop new grades with high contents of molybdenum

and nitrogen (654 SMO), that are extremely resistant to

pitting attacks induced by halides[2]. Another example

of the utility of thermodynamics is the super duplex

stainless steel SAFUREX, that has been developed to

withstand highly oxidative environments [3]. With

thermodynamic models, it is possible to find composi-

tions with high contents of Cr, Ni and N that give the

desired phase balance, normally 50/50 austenite/ferrite.

Extreme strength levels can be obtained by introducing

high fractions of nitrogen and manganese. Further

strength increase can, for austenitic materials, be

obtained by cold deformation. One example is the

high Mn, high N steel PANACEA [4]. Other develop-

ments include surfaces with structures tailored to avoid

bio fouling (UGICLEAN). Copper alloying in combi-

nation with a heat treatment is used to produce a special

line of anti-microbial stainless steels (NSS AM: 1/4).The literature shows that the corrosion resistance of

stainless steels can be drastically improved by suitable

alloying. This is possible because the composition and

properties of the passive films depend on the alloy

composition. Numerous investigations have contributed

to our present understanding of the processes governing

film growth and break down as a function of alloy

composition and environment. Today, it is known that

the passive film adapts to changes in potential or anion* Corresponding author. Fax: /41-21-693-3946.E-mail address: [email protected] (C.-O.A. Olsson).

Electrochimica Acta 48 (2003) 1093/1104

www.elsevier.com/locate/electacta

0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0013-4686(02)00841-1

concentration in the electrolyte. The dynamic properties

of the passive film provide the key to the high resistance

of stainless steels to corrosive attacks. The state of

knowledge of passive films on stainless steels as of 20

years ago was thoroughly reviewed by Fischmeister andRoll [5]. The aim of this paper is to present an overview

of new methods and results reported in the past two

decades. One could say that whereas the keyword of the

70s and early 80s was ex situ surface analysis, recent

research has emphasised the use of different in situ

methods.

2. Experimental methods for the study of passive films

Passive films are formed during an exposure of the

bare metal surface to an oxidising environment. Once a

film is formed, the reaction rate between the metal andthe environment will be several orders of magnitude

lower. The original theory of film formation goes back

to Michael Faraday, who in the 19th century studied

iron surfaces and found them altered. A review of the

early days of passive film research has been written by

Uhlig [6]. A general introduction to the theory of

passivity has been published by Sato [7], whereas the

electronic properties of passive films on differentmaterials have recently been reviewed by Schultze and

Lohrengel [8].

2.1. Ex situ methods

Electron spectroscopy has been used since the early

1970s for the study of the composition and thickness of

the passive films, including the determination of oxida-

tion states and of the depth distribution of the different

constituting elements in the film. X-ray photoelectron

spectroscopy (XPS, also known as ESCA, electron

spectroscopy for chemical analysis) and Auger electron

spectroscopy (AES) have been most widely used, butother methods such as secondary ion mass spectroscopy

(SIMS) and ion scattering spectroscopy (ISS) have also

provided important information.

Based on surface analysis, a three-layer model has

been suggested for passive films formed on austenitic

stainless steels in acidic solutions: the outer part of the

film consists of an hydroxide film on top of an oxide

layer. The oxy-hydroxide film is formed on top of a

nickel enriched layer in the metal, the origin of which is

the selective oxidation of Fe and Cr during anodic

polarisation. This picture is obtained essentially from

XPS measurements on passive films, cf. Fig. 1 [9]. The

film is enriched in chromium and the metal closest to the

metal/film interface shows a strong enrichment in nickel.

For an alkaline solution, the solubility is lower for Fe

and higher for Cr; this affects the level of chromium

enrichment in the film. With angular resolved XPS

(ARXPS), it is also possible to obtain information on

the depth distribution of different species and oxidation

states, cf. Fig. 2, which illustrates the angular depen-

dency of different oxidation states of molybdenum in

the passive film after immersion in a ferric chloride

solution [10]. Mo(VI) is enriched in the surface region,

the metal peaks show a buried layer appearance, and the

Mo(IV) oxide and Mo(IV) oxy-hydroxide show an

angular distribution indicating a more homogeneous

distribution through the passive film. The attribution of

Table 1

Examples of recent commercial developments in the field of stainless steel with high impact on the surface behaviour of the material

Product Significance Producer

654 SMO First 7% Mo austenitic stainless steel. Very high resistance to pitting corrosion at elevated temperatures Avesta Polarit

SAFUREX First 29Cr duplex stainless steel. Very high resistance to oxidising acids and localised corrosion, in particular stress

corrosion cracking.

Sandvik Steel

UGICLEAN A special surface structure modified to resist bacterial adhesion Ugine

NSS AM1-4 Cu alloyed stainless steels with special heat treatment giving high resistance to bacterial adhesion Nisshin steel

Zeron 100 Duplex stainless steel alloyed with W Weir Materi-

als

DP3-W W alloyed stainless steel Sumitomo

Fig. 1. XPS results from a passive film formed in the high passive

region in a 0.1 M HCl/0.4 M NaCl solution. The oxide film is foundto be strongly enriched in chromium, whereas the metal layers closest

to the metal/film interface, i.e. the apparent metal concentration, are

strongly enriched in nickel. The passive film itself shows close to no

incorporation of nickel. After Olefjord and Elfstrom [9].

C.-O.A. Olsson, D. Landolt / Electrochimica Acta 48 (2003) 1093/11041094

oxidation states to binding energy has been detailed by

Brox and Olefjord [11].

Superior depth resolution can be obtained using ISS,

which also gives quantitative results. By choosing the

proper primary ions, a resolution sufficient to distin-

guish between the Fe and Cr contributions to the signal

can be obtained. With ISS, Calinski and Strehblow

showed strong concentration gradients within the pas-

sive film on an Fe/Cr alloy [12], cf. Fig. 3. The outerregion of the passive film consists of a layer enriched in

iron, a feature that has also been observed with XPS

sputter depth profiles [13]. In the centre of the passive

film, a strong enrichment of chromium is found,

whereas for the region closest to the metal/film interface,

the composition corresponds to that of the bulk metal.

This implies that the oxidation at the metal/film inter-face closely follows the composition of the bulk metal,

and that the enrichment in chromium is caused by

dissolution of iron. The investigation of passive layers

on different metals with XPS and ISS has been reviewed

by Strehblow [14].

In 1984, Fischmeister pointed out that XPS is

generally better suited for analysis of passive films on

stainless steels than AES [5]. AES has a higher lateralresolution and a higher surface sensitivity for some

elements, e.g. molybdenum. The difficulty in analysing

Cr oxides with AES due to the peak overlap between the

OKLL and CrLMM peaks can be resolved through target

factor analysis (TFA) [15]. It is also possible to facilitate

the analysis of Mo by using krypton as a sputtering gas

instead of argon [15,16]. In spite of these improvements,

Fischmeisters preference is still valid, since the state ofthe art resolution of XPS for spectral information has

been improved to about 5 mm. The reproducibility ofAES and XPS measurements between different labora-

tories has been investigated in a round-robin where two

types of ferritic stainless steels were exposed to acidic

sulphate solutions. In terms of raw data (binding energy

shifts, relative intensities of oxide peaks etc.) the overall

agreement between different laboratories was judgedsatisfactory [17].

A very critical issue with ex situ surface analysis is the

sample transfer from electrolyte to UHV. No matter

how much precaution is taken, the surface will alter

somewhat during the transfer. Different ingenious

methods for sample transfer without contact with

atmosphere have been developed; one example of such

a design is given by Haupt et al. [18]. The level of utilityof an in vacuo transfer system is determined by the

amount of sample oxidation that can be avoided. The

amount of extra oxidation during transfer in air can be

estimated by forming the passive film in a normal

electrolyte, and making the transfer in an atmosphere

enriched in 18O. The amount of 18O in the film will then

be a measure of the amount of oxidation that has

occured in atmosphere. The inverse experiment is alsopossible, i.e. a passive film formed in an electrolyte

enriched in 18O and subsequent transfer in normal

laboratory atmosphere. The different passive films are

then compared using SIMS that can distinguish between

the 16O and 18O isotopes. Graham et al. polarised FeCr

alloys in water enriched in H218O [19/21], and found that

for pure iron, there is a small effect of sample transfer on

the passive film for polarisations in the passive region,whereas a more pronounced thickness change in the film

was found for Fe/26Cr samples; it was suggested thatthis difference could be caused by the higher ability of

Fe/Cr films to adapt to different environments. Similar

Fig. 2. Concentration gradients in a passive film for molybdenum,

recorded for a 6Mo superaustenitic stainless steel after immersion in

ferric chloride using XPS. For angles close to grazing, there is a strong

contribution of Mo(VI), whereas metallic Mo dominates for angles

close to perpendicular. The IV-valued oxide and oxy-hydroxide states

show less angular dependence. After Olsson and Hornstrom [10].

Fig. 3. Enrichment and concentration variations for Cr in a passive

film obtained by ISS. Even for low alloying amounts of Cr (5%), there

is an enrichment of Cr in the film, with the exception of the outer part.

After Calinski and Strehblow [12].

C.-O.A. Olsson, D. Landolt / Electrochimica Acta 48 (2003) 1093/1104 1095

experiments have been performed by Courty et al. [22].

A direct estimate of the film change during transfer was

obtained by comparing films formed in H218O and

transfer in 16O2 with films formed in H216O and transfer

in 18O2. For low passive and cathodic potentials, both

laboratories found a significant effect of sample transfer.

Graham et al. reported a transfer influence also for the

high passive region. However, when Courty et al.

transferred samples in 18O2 atmosphere, no uptake of18O in the film was seen, even though enrichment levels

close to those of Grahams were observed for anodisa-

tion in H218O and transfer in 16O2. Thus, the direct

measurement showed an absence of oxygen uptake

during transfer for samples polarised to the high passive

region [22].

The results indicate that caution is needed during the

sample transfer. However, by ensuring that the transfer

is made consistently for all samples, it is possible to draw

valid conclusions from comparisons between series of

measurements. Evidently, the only way to eliminateartefacts associated with sample transfer is to use in situ

techniques.

2.2. In situ techniques

A technique new to the study of passive films is X-ray

absorption near edge structure (XANES), which ac-quires in situ chemical information. It has been applied

to the investigation of passive films on pure iron [23/27], iron/chromium alloys [28] and to the study ofsputter deposited iron/chromium oxides [29]. Thesputter deposited oxide films were used as model

systems for passive films. XANES puts strong demands

on the analysed material. The sample is a thin film of

about 10 nm thickness deposited on top of a goldcontact layer. As substrate, a polymer carrier film that

ensures X-ray transparency is used. Sample character-

isation has been described by Stenberg et al. [30]. The

study of reaction kinetics is somewhat limited, since

acquisition times of the order of one hour are used [28].

A short introduction to XANES applied to passivity

related problems has been written by Schmuki and

Virtanen [31].

A XANES spectrum for Cr and Fe taken from an (Fe,

Cr)2O3 sample is shown in Fig. 4. The height of the

plateau at the high-energy side of the adsorption edge is

proportional to the respective amount of the element in

the sample, and the peak position reflects the oxidation

state. Quantification of XANES spectra is less straight-

forward than for AES and XPS. Davenport et al.

introduced a formalism to separate the oxide and metal

signals from an oxidised sample [26]. This method was

further developed by Oblonsky et al. for the quantifica-

tion of film composition on Fe/Cr alloys [28], and laterNi/Cr [32]. The measurements confirm XPS and AESresults, i.e. for a certain polarisation and bulk composi-

tion, the amount of Cr in the passive film surpasses 50%

of cations [33]. The XANES data also support the

enrichment of Cr in the passive film by selective

dissolution of iron. It has also been possible to

demonstrate the presence of Cr(VI) in the film [34].

In situ analysis of binary alloys can also be performed

by more conventional techniques. By solution analysis

with ICP-AES/MS (inductively coupled plasma atomic

emission spectrometry/mass spectrometry), it is possible

to deduce composition changes in the passive film

resulting from potential changes. This approach was

successfully employed by Castle and Qiu [35] and Hamm

et al. [36]. For an Fe/Cr alloy in the passive region, bothpapers concluded that the dissolved species consisted

almost solely of iron. Similarly, rotating ring-disc

electrode experiments with Fe/Cr alloys showed thatclose to no Cr was emitted from the sample during a

potential change in the passive region [37], in good

agreement with the ICP results. Annergren et al. used

the rotating ring-disc electrode in combination with

electrochemical impedance to study the dissolution and

passivation behaviour of Fe/Cr alloys in sulphuric acid

Fig. 4. XANES spectra (Fe and Cr K edges) of a mixed (Fe, Cr)2O3 sample containing 50% Cr2O3 during anodic potential steps in 0.1 M H2SO4. The

potential was increased in 100 mV steps/15 min, which is illustrated with selected raw spectra. From Schmuki et al. [29].


solution with additions of chloride [38/40]. Theycalculated that ferrous ions were leaving the electrode

with a current efficiency even higher than unity for

intermediate frequencies. The behaviour was explainedby postulating a concurrent chemical dissolution path of

Fe(II) [39].

2.3. Environmental influence on the film composition

A passive film is in constant exchange of species with

the electrolyte and consequently alters in thickness andcomposition with the environment. Among the factors

that influence the passive film, one finds the potential,

the presence of halides in the electrolyte, the pH, and the

temperature.

2.3.1. Potential

The thickness of passive films on stainless steels grows

linearly with the applied potential. This is illustrated in

Fig. 5 for a Fe15Cr alloy in an acidic sulphate [41] as

well as Fe10Cr and Fe20Cr alloys in a sodium hydroxide

solution [42]. The basic solution gives considerably

thicker films, since dissolution is less pronounced. The

observed increase in film thickness with potential in an

acidic solution is due mainly to the oxide part; thehydroxide part of the film is approximately independent

of potential [43,44]. Development of the passive film

with time has been investigated by, among others,

Maurice et al. [45]. Between 20 min and 20 h of

polarisation, they found only a small change in the

total film thickness; the oxide part of the film was found

to grow at the expense of the hydroxide part.

The composition and chemistry of the film also vary

with potential. For Fe/Cr alloys, enrichment of chro-mium occurs in the film in the low passive region, but in

the high passive region the chromium content decreases,since the stability of iron surpasses that of chromium.

Chromium, in turn, may oxidise to its soluble hexavalent

state. At high passive potentials, Fe has a stabilising

action on the trivalent Cr, making Fe/Cr alloys stableover a wider potential range than pure Cr. Higher

oxidation numbers are also more dominant towards the

higher end of the passive region, as confirmed by

XANES [34].

2.3.2. Chlorides and sulphates

The presence of certain anions adsorbed on the

surface or incorporated in the passive film can be

detrimental to film stability and lead to pit initiation.

For chloride, there is a vast literature attempting to

explain its effects on pitting. Three different models are

frequently quoted: adsorption leading to local film

dissolution, penetration of anions in the film leadingto weakening of the oxide bonds, and film break down

at defects such as cracks and dislocations [46]. Con-

clusive analysis of the chloride content of passive films

has proven difficult, since it is rather low; frequently

quoted values range from 1 to 5%. Due to the very thin

passive film and the possible presence of structural

defects, it is not easy to distinguish adsorbed from

incorporated anionic species. Using AES and XPS, ithas been shown that sulphates are present at or close to

the film surface [47/50]. Chlorides are enriched at thepassive film surface, but the depth distribution is more

homogeneous than for sulphates [51], especially at

elevated temperatures [43]. Mischler et al. have shown

that alloying with molybdenum reduces the surface

enrichment of chlorides and sulphates [52]. Mitrovic-

Scepanovic et al. [53] found a higher tendency forchlorides than sulphates to penetrate the passive film

on Fe/Cr alloys. Chloride penetration into an intactpassive film can be investigated by forming the film in a

non-chloride solution, followed by exposure to chlor-

ides. Such a series of experiments has been performed by

Hubschmid et al. for Fe/25Cr/X alloys. No incorpora-tion was found when chlorides were injected after an

initial hour of film formation in a sulphuric acidsolution [54]. For films formed in chloride electrolytes,

on the other hand, chlorides were found distributed

through the film.

2.3.3. pH

The main effect of an increased pH is a lower

dissolution rate. This leads to a thicker passive film,

and a larger fraction of iron in the film, as iron oxidesare more stable in basic solutions. The pH effect is

illustrated by the data of Schmutz and Landolt, who

compared the response for two FeCr and FeCrMo

Fig. 5. Oxide layer thickness on a stainless steel as a function of

potential for a Fe15Cr alloy in 0.5 M H2SO4, and for Fe10Cr and

Fe20Cr alloys in 1 M NaOH, estimated using XPS. The film growth

region is considerably wider in the basic medium, which also gives

thicker films. After Haupt and Strehblow [41] and Hoppe et al. [42].


alloys to a potential change in basic and acidic solutions

using the electrochemical quartz crystal microbalance

(EQCM) [55]. Increasing the potential in the passive

region lead to a mass loss in the acidic solution (0.1 MH2SO4/0.4 M Na2SO4) and to a mass increase in thebasic solution (0.1 M NaOH). A mass increase corre-

sponds to a net film growth and a low dissolution rate.

The effect of pH on the passive layer of Fe/Cr alloyshave also been studied by Strehblow et al. [41,42]. The

films were found to be thicker in basic solutions. In

addition, there was a marked increase of the amount of

Fe(III) oxide.

2.3.4. Temperature

The effect of temperature on the passive film was

studied by Jin and Atrens using XPS [56]. They found

no differences in composition and thickness between

passive films formed at room temperature and at 90 8Cin a 0.5 M NaCl solution. Mischler et al. quoted slightly

thicker values for films formed on Fe/Cr/Mo alloys at65 8C when compared to room temperature [52]. Thetemperature effect on film thickness for a 6Mo stainless

steel has been quantified by Wegrelius and Olefjord

using XPS [43]. They compared film formation at 22 and

65 8C in an acidic chloride solution. In spite of clearelectrochemical indications of pitting attacks during the

exposure period at the higher temperature, the film was

found to be only 2 A thicker for the higher temperature.

2.4. Influence of microstructure and alloying elements on

passive film composition

2.4.1. Microstructure

With classic metallurgical techniques, the composition

range of the bulk material is limited by the precipitation

of intermetallic phases. In addition, inclusions and

segregation cause defects in the passive film. Fordetailed studies of passive films, the amount of inter-

metallic phases and other imperfections can be mini-

mised by utilising thin film materials produced with

PVD (physical vapour deposition). Since PVD is a cold

technique*/a typical deposition temperature is400 8C*/it is possible to obtain nanocrystalline oramorphous material that is virtually free from inter-

metallic phases over a wide composition range.Inturi and Smialowska compared a nanocrystalline

thin film alloy of the same composition with conven-

tional 304 material [57]. The breakdown potential of the

nanocrystalline material in a near neutral NaCl solution

was found to be shifted anodically by about 850 mV, a

behaviour that was attributed to the smaller grain size

and the high purity of the deposited thin film material.

Kraack et al. found a shift in the pitting potential tomore noble values when molybdenum was added to thin

film Fe/Cr alloys in agreement with the well-knownbehaviour of bulk material [58]. A wide range of systems

produced using PVD has been investigated by Hashi-

moto et al. [59/64] who underlined the extremely highcorrosion resistance of amorphous alloys.

A showcase for the importance of microsctructure isthe austeno/ferritic (duplex) stainless steels. Their two-phase structure makes them particularly sensitive to the

formation of intermetallic phases. AES has been used to

investigate the homogeneity of the passive film on the

austeno/ferritic stainless steel 2205 (EN 1.4462) [65]. Atypical phase region for a commercial duplex stainless

steel covers 10/20 mm. It has been demonstrated thatthe passive film composition shows a comparativelyhomogeneous composition, virtually independent of the

underlying phase structure. This homogeneity continues

some Angstroms down in the metal phase where a nickel

and nitrogen enrichment is found for the austenitic and

the ferritic phases [65]. This enrichment would be

sufficient to produce a fully austenitic structure in a

bulk material. Significant differences between phase

regions were found by Hubschmid et al. who studied aFe/25Cr/11Nb model alloy with a considerably coarserdendritic microstructure than for commercial 2205 [66].

It appears the lateral homogeneity of the passive film is

linked to the lateral extension of different phases in the

microstructure.

2.4.2. Fe

In acid solution, anodic polarisation of a Fe/Cr alloyin the passive potential region leads to selective dissolu-

tion of iron, leaving chromium enriched in the passive

film. This behaviour is governed by the difference in

diffusion rates of iron and chromium in the passive film

[13,67]. When performing sputter depth profiles with

AES or XPS, there is normally an enrichment of iron in

the outer region of the passive film. This is possibly a

transfer artefact related to the higher diffusion rate ofiron through the film. At about 0.58 VSHE, iron changes

valence from 2 to 3 [41] for an acidic solution; for a basic

solution, the corresponding potential is about/0.3VSHE [18].

2.4.3. Cr

For moderate anodic polarisations in acidic solutions,the passive film consists essentially of chromium in its

trivalent state. As the potential increases above the

stability limit of chromium III, (about 0.6/0.8 VSHE),the passive film will start to change composition and the

fraction of trivalent iron in the film will increase. For

acidic solutions, the cation fraction of Cr in the passive

film normally amounts to 50/70%. For basic solutions,the solubility of Cr increases, resulting in a higherfraction of iron. The surface chemistry of pure chro-

mium has been studied by Bjornkvist and Olefjord

[68,69]


2.4.4. Ni

Nickel is less readily oxidised than iron and chro-

mium. Consequently, there is an enrichment of Ni in its

metallic state in the metal closest to the oxide/metalinterface. This enrichment could assist in the formation

of a nickel nitride [70]. Nickel could also bring down the

overall dissolution rates of Fe and Cr. Recent scanning

tunnelling microscope (STM) results indicate that the

passive film formed on austenitic stainless steels shows a

higher degree of long-range order within a shorter time

span than the corresponding ferritic alloy [45].

2.4.5. Mn

Recently, addition of Mn to stainless steel has been

used to increase the solubility of nitrogen and molybde-

num, both of which have a strong beneficial influence onthe pitting resistance. Examples include the low nickel

high strength austenitic material PANACEA [4] and the

commercial superaustenitic stainless steel 654 SMO [2].

2.4.6. Mo

Molybdenum is an alloy element with a strong

beneficial influence on the pitting resistance of a

stainless steel. Pure molybdenum does not form a

three-dimensional passive film [71]. It appears that the

passive region of pure molybdenum is associated with a

diffusion controlled oxidation of hydrogen released

from the metal rather than with the formation of apassive oxide film [72]. On the other hand, when

included as an alloying element in a stainless steel,

molybdenum is incorporated into the passive film,

showing a complex oxide chemistry with different states

of oxidation. An example of the concentration gradients

of molybdenum in a passive film on a stainless steel is

given in Fig. 2. Hexavalent Mo is found to be enriched

at the surface, whereas tetravalent states show a morehomogeneous distribution through the film [10]. This

has been shown also for the two phases of a duplex

stainless steel [73]. There are two possible hexavalent

states: MoO3, which is soluble in acidic eletrolytes and

MoO42 which shows a higher stability. Distinguishing

between these two states is difficult even with a high

resolution XPS spectrometer. Attempts have been made

by Lu and Clayton, [70,74/78], who discussed theinteraction between nitrogen and molybdenum. The

passivity of stainless steels was interpreted with a

bipolar model. Another subject of discussion is the

presence of a pentavalent state, cf. Kim et al. [79].

Different molybdenum oxides have been thoroughly

characterised by Brox et al. [11,71], who found the XPS

peak of the MoO2 compound to be considerably less

shifted than expected for an ionic bond. This effect wasnot seen for a tetravalent molybdenum oxy-hydroxide

and was explained by core-level screening. The literature

on the effects of Mo on the corrosion/pitting resistance

has been reviewed by Jargelius-Pettersson and Pound

[80].

2.4.7. W

Tungsten is fairly recent as a major alloying element

in commercial stainless steels. It has been attributed

properties similar to those of molybdenum [81]. The

surface properties of ferritic alloys containing Mo and/

or W have been thoroughly investigated by Landolt et

al. [52,54,82/84]. An important difference betweentungsten and molybdenum oxides lies in the different

stability of their oxides in acid solution. While hexava-lent molybdenum oxide dissolves at potentials well

below oxygen evolution, the stability of hexavalent

tungsten oxide extends to anodic potentials of several

tens of volts [85].

2.4.8. N

Nitrogen is the element attributed the strongest

beneficial influence on localised corrosion in the pitting

resistance equivalent formula (PREN). Different typesof synergism with molybdenum have been proposed, as

listed in [77]. As for molybdenum, nitrogen also shows

strong concentration gradients in the passive film. It has

been suggested that ammonia or ammonium ions react

with free chlorine to form combined chlorine species

that are less effective oxidants, thereby inhibiting

chlorination enhanced localised corrosion [86]. Another

possibility is the formation of a nitride at the metal/filminterface which brings down the dissolution rates for the

individual elements in the alloy [70,87]. Nitride has also

been put forward as a possible mechanism for synergy

between nitrogen and molybdenum [70].

2.4.9. Others

Copper is added to highly corrosion resistant auste-

nites (904L, 254SMO) to further boost the corrosionresistance. The influence of copper in Fe/Cr alloys hasbeen studied by Postrach et al. [88,89]. Copper additions

are also used to give the surface anti-bacterial proper-

ties, cf. Table 1. Another class of alloying elements with

intriguing electronic properties are the rare-earth metals

(REMs). They are used to enhanced the high tempera-

ture corrosion resistance by improving the adhesion of

oxide scales.

3. Atomic arrangement

Besides chemistry and elemental composition of the

passive films, their structure is also a vital parameter

that influences the corrosion resistance. The first in-

vestigation on the crystal structure of passive films onFe/Cr alloys was performed by McBee and Kruger in1972 [90] using electron diffraction in a transmission

electron microscope. They found that the films formed


on alloys with 24% Cr were amorphous, whereas 0/10%Cr in the bulk resulted in a spinel-like structure. With

the invention of the STM in the early 1980s, it became

possible to study the crystal structure of the passive filmin situ with atomic resolution.

3.1. Scanning tunnelling microscopy

One of the first studies on the atomic arrangement of

active and passive stainless steel surfaces was performed

by Ryan et al. [91,92]. By studying iron in a borate

buffer solution and Fe/Cr alloys in sulphuric acidsolutions, they confirmed the conclusion of McBee

and Krueger [90] that the passive film is amorphousfor Cr concentrations above 12/19%. Marcus et al. havemade detailed investigations on a series of single crystal

surfaces: Ni [93], Cr [94], Fe/Cr [48] and Fe/Cr/Ni[45]. The passive films were found to grow close to

epitaxially (Cr (110)) in small (3 nm diam) crystalline

regions on iron/chromium alloys. Aging under polar-isation led to growth of the crystallised regions, i.e. an

enhanced long range order. The ordering process wasobserved to be slower for austenitics, which also showed

a lower production rate of Cr2O3 on the surface. It was

suggested that these effects were caused by the enrich-

ment of nickel at the metal/film interface which limited

the access of chromium. An example of an STM image

is given in Fig. 6, which shows the atomic arrangement

of a passive film on an austenitic Fe/Cr/Ni alloy [45].The STM can also be used to obtain depth informa-

tion. This was demonstrated by Zuili et al. on Cr

surfaces [94]. By varying the voltage of the scanning

tip, it is possible to control the depth from which the

major part of the response is acquired. The structure

found was not a flat hydroxide layer, but resembled a

buried island structure, much like the one suggested by

Wegrelius and Olefjord based on XPS results [95]. STMstudies of passive films on different materials have

recently been reviewed by Marcus [96].

3.2. Geometrical considerations

Conjectures based on the geometrical arrangement of

atoms in or near the passive film have been developed as

a possible explanation for some aspects of passivity. The

simplest geometrical approach is to consider a bcc-lattice where every atom has eight neighbours. For

iron/chromium, this means that if 1/8 (12.5%) of theatoms are Cr, at least 50% of the Cr atoms will have at

least one Cr as their nearest neighbour. This corre-

sponds to the onset of a continuous network of

chromium atoms in the bulk material. The fraction 1/8

also equals the lower limit of chromimum needed to

make a steel stainless. Percolation theory is related tothe 1/8 concept. It has previously been used in materials

science to treat different phenomena in amorphous

materials, e.g. phase transformations. A general treat-

ment of percolation has been presented by Zallen [97].

Percolation is used to describe network connectivity.

For a one-dimensional system (e.g. a string with node

points), the percolation limit is trivial, since the first

node cut will cause a break in connectivity between theend points. For a 2D structure, it is possible to find an

exact percolation limit which will vary with the number

of connections of each node (atom or molecule) in the

network. For a 3D structure, the issue is more complex

and percolation limits become less well defined, since the

number of possible connections are much higher.

The connectivity of Fe/Cr matrices was studied for aseries of different compositions by Newman et al. Fortheir Monte Carlo simulations, they initially considered

a square two/dimensional lattice in which they ran-domly placed iron and chromium atoms [98,99]. By

assuming a high dissolution rate for iron and zero

dissolution for chromium, they derived percolation

limits of 12% for onset and 17% for completed passivity.

These percentages are in good agreement with practical

experience.Later, the percolation model was modified to incor-

porate pit initiation [100]. By assuming a low dissolution

rate for Cr, it was shown that an intermittent dissolution

of iron clusters at discrete times could be obtained. If

large enough, these clusters could serve as initiation sites

for pitting. Recently, the percolation results have been

compared to experimental data for Fe/Cr PVD filmsexposed to acidic chloride media [101,102]. The percola-tion approach has also been modified by assuming

higher dissolution rates at defect sites on the surface, e.g.

kinks, or if one alloy component is restrained to the

Fig. 6. Topographic STM image of a passive film on a (100) Fe/18Cr/13Ni substrate after passivation in 0.5M H2SO4 at 500 mVSHEfor 2 h. An almost hexagonal lattice is superimposed to show the

directions of the close-packed rows. From Maurice et al. [45]


surface by thermodynamic limitations. These aspects

have been reviewed by Heusler [103]. The percolation

model can be used to investigate how different dissolu-

tion rates of iron and chromium affect the surface

roughness in the nano range [99].

Newman et al. considered the surface to be passive

when a hydroxide or oxide chain can bridge the gap and

form a link between chromium atoms at the surface

[104]. Another approach is to consider the connectivity

between cations within the oxide film. This concept has

been applied to Fe/Cr [105] and other binary Cr alloys[106] by McCafferty, who considered a series of binary

systems following a formalism developed by Megirditch-

ian [107] and Randic [108]. For a certain ratio of Fe/Cr

atoms in the oxide matrix, a connectivity between the

chromium atoms is obtained. Thus, for a certain

amount of iron, the continuous chromium network

through the oxide will be lost. The connectivity para-

meter was successfully compared with parameters such

as passivation currents, passivation potentials or experi-

mental data on passive film compositions [105].

4. Film growth kinetics

Passive films are constantly changing and adapting to

the environment. Thus, understanding film growth and

dissolution is essential to the understanding of passive

film stability in different and changing environments.

High reaction rates and small dimensions of the film (1/3 nm thickness) make it difficult to obtain unique results

for passive films on stainless steels. Some valve metals

show thicker films and are thus more easily studied.

Numerous attempts to model film growth based on

different concepts have been made; a recent review has

been written by Macdonald [109]. More details on

earlier models for film growth dynamics have been

given by Chao et al. [110].

For a passive film covering the entire surface, two

types of rate-limiting processes can be distinguished:

high field assisted ionic transport through the film or

charge transfer at either the metal/film or film/electro-

lyte interface. These models will be referred to as high

field (HFM) and interface models (IFM), respectively.

Most models presented in the literature give growth

equations that reduce to either the IFM or the HFM

type.

If the electric field is assumed to increase upon a

change in potential, the film growth will be limited by

high field ion conduction through the oxide (HFM). An

early formalisation of this concept was made by Verwey

[111]. The HFM is in widespread use for the modelling

of passive currents, and can be respresented by an

equation of the form

@d

@trgk

hf1 e

BU=d (1)

where d is the film thickness, rg is the growth fraction,

k1hf a high field model constant containing the specific

volume and valence, B is an oxide-specific constant, and

U is the applied potential. For the HFM, the thicknessof the passive film is found in the denominator of the

exponential.

The alternative to high field rate control is to consider

a rate limiting reaction at either the metal/film or the

film/electrolyte interface. A rate limiting reaction at the

metal/film interface is the base for the point defect

model (PDM) that was introduced in the early 80s. The

PDM concept has since then been developed anddiscussed extensively in a series of publications by

Macdonald et al. [110,112/117]. A growth rate limitingreaction at the film/electrolyte interface has been pro-

posed by Vetter and Gorn [118]. It can be shown that

the form of the rate limiting reaction is independent of

which interface is considered [119]. Thus, the film

growth equation for the interface type models (IFM:s)

takes the form

@d

@tkif1 e

k2 [DUE0 Dd] (2)

where k1if and k2 are constants with different meanings

depending on which model is represented [119]. For the

IFM, the electric field E0 is usually assumed constantduring the applied external potential change. The film

will show a thickness change Dd until the potentialapplied across the film is balanced. For the IFM, the

thickness change is found in the nominator and the

growth rate is not explicitly dependent on the absolute

film thickness.

An attempt to unify the IFM and HFM conjectures

was made by Kirchheim in 1987 [120], focussing ongalvanostatic experiments on iron in acid environments.

Kirchheim divided the current into a growth and a

corrosion part: itot/icorr/igrowth. The growth currentwas assumed to be controlled by an interface equili-

brium, as previously suggested by Vetter and Gorn

[118,121], and the total current was assumed to be

limited by the HFM. The corrosion part can be

estimated by solution analysis. Kirchheim showed thatneither the interface equilibrium nor the high field

limitation for the total current on its own could

successfully explain all experimental observations [120].

To understand steady-state dissolution in the passive

region, Heusler introduced different diffusion rates for

the iron and chromium cations in the film. He found an

agreement with experimental data for diffusion rates of

iron eight times higher than for those of chromium [67].A similar approach was argued by Kirchheim et al., who

adopted different transport rates for cations in combi-

nation with the concepts developed for iron [120].


Measurements of the passive current [122] as well as

estimates of the amount of dissolved material [123] were

both interpreted as a lower mobility for chromium in the

passive film. Further investigations also included ellip-

sometry [124], that was used to estimate the film growth

current igrowth. Another possibility is to use solution

analysis to measure the dissolution current icorr. This

approach was adopted successfully by Castle and Qiu

[35], who showed that the dissolved material was heavily

dominated by iron.

A combination of the above ideas was introduced in

an attempt to create a general theory for passive film

growth-the mixed conduction model (MCM), which has

been applied to pure Cr [125] as well as Fe/Cr [126/128], Ni/Cr [129], Fe/Mo [130], and Fe/Cr/Mo alloys[131,132]. The MCM includes a rate limiting step at an

interface as well as transport through the film. It starts

out from a diffusion equation, which for potential

sweeps in the anodic direction essentially reduces to

the HFM. The MCM has been successful in fitting

impedance as well as passive film conductivity data.

Recently, the electrochemical quartz crystal micro-

balance (EQCM) has been used to investigate passivefilm growth [55,133,134]. It gives in situ information

with a time resolution sufficient to provide real time

growth curves of the passive film. This has been

illustrated by Olsson et al., who used the EQCM to

study passive films on Cr [119], Fe/Cr [135], and Fe/Cr/Ni/(Mo/W) alloys [136]. The film growth during apotential change in the anodic direction for a Fe/Cr/Ni/Mo stainless steel PVD alloy is illustrated in Fig. 7a[136]. The fit parameters for the IFM and HFM were

fixed for a 304L type PVD material and it was assumed

that the addition of Mo would essentially affect the

ability of the film to withstand an increased electric field,

a hypothesis supported by the difference in thickness

change between the alloys. As can be seen, the IFM

produces a satisfactory fit to the growth curve, whereas

less correspondence is found for the HFM. This experi-ment was repeated for alloys with additions of Cr and

W. The electric fields in the film derived from the HFM

and IFM fits were then compared to that calculated

directly from dividing the applied potential change by

the experimental EQCM film thickness change, cf. Fig.

7b. The IFM fit values of the electric field gave a good

correlation with the direct estimates, whereas an oppo-

site trend was found for the HFM. In addition, it wasfound that alloying with Mo and W gave considerably

higher mass losses than alloying with Cr. This indicates

that their presence renders the film more permeable to

ion transport, which would allow the film to reach an

equilibrium composition faster.

5. Concluding remarks

There is no such thing as a static passive film. The

passive film on a given stainless steel changes with the

environment. It can grow or dissolve, and may adsorb

or incorporate anions. A parameter critical to the

stability of a passive film is the time necessary to

respond to an environmental change. With the intro-

duction of in situ techniques, different time dependent

parameters have become accessible.The passive film thickness changes within a couple of

seconds in response to a potential change. Structural

ordering, on the other hand, follows considerably slower

kinetics. Both these parameters are important factors in

the growth and break down of passive films.

Acknowledgements

We are indebted to Mats Liljas at Avesta Polarit

R&D, and Marie-Gilles Verge for valuable comments to

Fig. 7. (a) Experimental EQCM growth curve for a passive film on a

Fe/Cr/Ni/Mo alloy undergoing a potential change in the anodicdirection within the passive region. This curve is compared to growth

curves simulated using Eqs. (1) and (2) for the HFM and IFM. (b)

Values of the electric field yielding a best fit to experimental data is

shown for different alloys for the IFM and HFM:s. The IFM fit values

compare better with the electric fields calculated directly from the

EQCM thickness change. From Olsson et al. [136].


the script. Financial support was provided by Fonds

Nationale Suisse.

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