A11101 41b3

NBSIR 83-2551

Passive Films, Surface Structure

and Stress Corrosion and Crevice

Corrosion Susceptibility

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Inorganic Materials Division

and

Metallurgy Division

Washington, DC 20234

November 1 982

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

issued February 1983

Prepared for

luo

U !>6

NATIONAL BUREAU

OF STANDARDS

LIBRARY MAR 11 1983

NBSIR 83-2551

PASSIVE FILMS, SURFACE STRUCTURE

AND STRESS CORROSION AND CREVICE

CORROSION SUSCEPTIBILITY

J. Kruger, J.J. Ritter, and G.G. Long

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Inorganic Materials Division

and

Metallurgy Division

Washington, DC 20234

November 1 982

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Issued February 1983

Prepared for Office of Naval Research

Department of the Navy

Arlington, VA 2221 7

o

1+

3*

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
I
PART I. Accepted for publication by J. Coatings Technology, December 1982

ELLIPSOMETRIC STUDIES ON THE CATHODIC DELAMINATION OF

ORGANIC COATINGS ON IRON AND STEEL

Joseph J. Ritter

Materials Chemistry Division

National Bureau of Standards

Washington, D.C. 20234

Abstract

A number of mechanisms have been proposed to explain the cathodic

calami nation of organic coatings from iron or steel undergoing corrosion.

El 1 ipsometric studies carried out on these substrates protected by either

collodion, acrylic lacquers, or polybutadiene indicate that dissolution of

the interfacial oxide in the highly alkaline cathodic environment constitutes

the predominant mode of delamination. Moreover, the spatial development of

cathodic areas has been probed both el 1 ipsometrical ly and through subcoating

electrochemical pH measurements.

The oxide dissolution proposition is supported by independent experiments

v/i th uncoated iron in a highly alkaline medium.

Key words: Air-formed oxide films; cathodic delamination; ellipsometry;

organic coatings; pH measurements
'

,
ELLIPSOMETRIC STUDIES ON THE CATHODIC DELAMINATION OF

ORGANIC COATINGS ON IRON AND STEEL

Joseph J. Ritter

Materials Chemistry Division

National Bureau of Standards

Washington, D.C. 20234

Introducti on

The corrosion related failure of organic coatings on iron or steel is

brought about through two destructive processes, the anodic dissolution of

metal and the cathodic delamination of the coating. The development and

effects of these phenomena have been treated in detail by a number of

workers [1-4]. While in a practical sense, the anodic dissolution of metal

is relatively well understood, the mechanisms for coating delamination are

still not resolved. For example, previous authors have proposed various

del ami nation mechanisms ranging from an active role by water molecules in

displacing substrate-polymer bonds to cohesive failure of the polymer

[5-7]. Each of these ideas can be supported by experimental evidence from

specific coating-substrate systems. Thus, perhaps it is most reasonable to

consider a predomi nant mode of delamination in connection with a given

coating-substrate system. The purpose of the study described here is to

present evidence for yet another mechanism of delamination, the dissolution

or alteration of the interfacial air-formed oxide films*.

In an earlier publication from our laboratory we suggested this possibility based on el 1 ipsometric observations of the iron/collodion system

[3]. Workers at Lehigh University have arrived at a similar conclusion for

other systems through independent means [9]. We have shown in previous wor

*For the purpose of this publication, the term "air-formed oxide film"

refers to the oxide present on the metal surface at the end of the surface

preparation regimen.
how qualitative ellipsometry combined with electrochemical pH measurements

and microanalyses can provide valuable information about subcoating phenomena

and chemistry [4,8,10]. While this early work focused on the iron/collodion

system, the results presented in this report are derived from both iron and

steel substrates protected by either collodion, polystyrene, acrylic lacquers,

or polybutadiene. In addition, the use of our newly automated ellipsometer

system [11] has provided detailed insights into the real time evolution of

the phenomena surrounding cathodic delamination. The interpretations are

supported by independent experiments, described here, with uncoated iron in

simulated subcoating environments.

Experimental

All iron specimens were mounted in epoxy and finished on a series of SiC

papers of 320, 400, and 600 grit. Final polishing was done with 6, 1, and

0.05 pm abrasives.

Lew carbon stamping quality steel kindly supplied by Dr. Henry Leidheiser, Jr

Lehigh University, as 0.71 mm sheet, was used with the as-received 600 grit

finish. The sheets were sectioned into 2 cm squares, coated, and cast in

epoxy in a recessed manner, so that only one face of the material was

exposed. Dr. Leidheiser also supplied specimens of this same steel with

20 pm of heat-cured- polybutadiene applied. The coated specimens were

mounted in epoxy in a similar recessed fashion.

With respect to other coating materials, collodion was diluted 1:1

with methanol, while the polystyrene, and two proprietary acrylics, designated

A and 3 were dissolved and/or thinned with toluene. These materials were

applied by dipping, usually resulting in a 25 to 100 pm cured coating after

IS h at room temperature (22 to 25 °C). Heat-cured acrylic coatings were

prepared in the same manner but suDjected to 200 °C for 40 min after the

13 h room temperature cure.
The details of specimen arrangement and the el 1 ipsometric technique

have been described previously [8,10]. In brief, the technique involves

the passage of elliptically polarized light through the transparent coating

and reflection from the substrate surface. In some experiments a miniature

pH probe is used to simul ataneously monitor the environment at the coatingsubstrate interface. Changes in the phase (6/1) and amplitude (o'V) of the

reflected light have been shown to be indicative both of alterations in surface

texture (roughness) and changes in the thickness of the surface oxide film.

Thus, the technique is of value in studying the behavior of the substrate

under the coating as the corrosion process proceeds.

All coated specimens were studied immersed in 0.05 M NaCl at 25 °C.

Uncoated iron specimens were examined in aerated, saturated NaOH at 25 °C.

The corrosion process was initiated by damaging the coating in a single area

with either a 0.07 cm drill point or incising a 1 mm slit with a scalpel.

Preparation of segmented specimens . Segmented specimens were devised

as one means to examine the spatial development of subcoating-cathodes.

This technique permits the initial isolation of remote and proximate (relative

to an anode) cathodes for independent study. A 1 cm diameter x 1 cm long iron

cylinder was divided into two parts, each part fitted with a copper conductor

and cast into a single epoxy slug as shown in figure 1. An opening was

drilled through one of the segments (c), to accommodate a miniature (1.5 mm

dia.) pH probe. The drilled hole was temporarily closed with a polytetraf 1 uoroethylene plug and the entire slug face coated with acrylic lacquer “A" which

was air-cured at 25 °C for 18 h. The specimen was mounted horizontally in an

ellipscmeter cell immersed in 0.05 M NaCl . The cell mount permitted horizontal

translation of the specimen so that either segment a or c could be examined

el 1

": psometri cal ly.
Results and Discussion

General Effects of Coating Cure Procedure

Figures 2 and 3 schematically show the responses of the el 1 ipsometric

phase retardation (A) parameter with time for coated iron and steel specimens.

For the room temperature cured coatings, the curve shapes are similar br ut

the heat-cured coatings show a very different el 1 ipsometric response. The

interpretation of these responses with respect to subcoating phenomena are

given below.

El 1 ipsometri c Responses for Room Temperature Cured Coatings

Figures 4, 5, and 6 show the detailed real time behavior of the ellipsometric parameters A and V for room temperature cured collodion and the two

types of proprietary acrylic coatings. Overall, the curves show similar

behavior on different time scales. Since the coated metal systems are

optically quite complex, the el 1 ipsometric results are examined in a qualitative sense [10].

Within Region I each of the three systems show a rise in A and a concomitant decline in 'F as corrosion sets in. We interpret this type of behavior as

a change in oxide film thickness and the direction of change is indicative of

a film thinning process. The interpretation of the A and V changes is

derived from the behavior of these parameters for uncoated iron in saturated

NaOH (cf. fig. 10). Saturated NaOH represents a reasonable subcoating

environment based on i_n si tu pH measurements [8]. We relate these changes

to the dissolution of the oxide film in the highly alkaline cathodic environment.

Since the coating is bonded to this oxide layer, delamination has occurred.

Within Region II, A and 4* decline sharply in concert. This concerted

response is typical of a change in surface texture (roughening). The

i nterpretati on of the behavior of A and 4> is based on our observations of
uncoated iron in borate buffer solution after potential cycling [8], which,

in effect, gives a smoother surface. In the potential cycling experiments A

and ri se in concert, the opposite effect from that observed for coated metals.

Since phenomenon described in Region I was a thinning of the oxide layer, it is

reasonable to postulate the presence of patches of film-free metal. Th-e high pH

environment promotes direct metal dissolution from these patches and the

surface becomes roughened.

Within Region III, A and ¥ are again in opposition. The overall

result is indicative of an increase in film thickness. Since the phenomena

in Regions I and II resulted in the dissolution of iron either from the

oxide or from the metal itself, we postulate that soluble iron species*,

like the OH ions, accumulate under the coating. Ultimately, solubility

product constants are exceeded and precipitation occurs. The complexity of

the ellipsometric response in Region III for figures 4 and 5 may arise from

one or both of the following factors: the deposition of metastable films

which partially redissolve and reprecipitate until a stable format is

achieved; or the precipitation of a film which undergoes massive changes in

its refractive index with respect to its immersion medium as it thickens.

Ellipsometric Responses for Heat-Cured Coatings

Figure 3 shows typical ellipsometric responses for steel with either

polybutadiene or an acrylic lacquer which have been heat-cured. Several

factors should be considered initially using a steel specimen with a heat-cured

acrylic coating as an example.

Although the acrylic coating material "A" used was the same as that used

to generate figure 5, the heat treatment process undoubtedly modifies the

*Tne exact nature of these species is under investigation.
final polymer structure. In addition, recent evidence points to a "zone"

of iron ions in that portion of the heat-cured polymer nearest to the oxide

layer [12]. Moreover, the oxide layer under the polymer thickens considerably

during the curing process. Measurements in our laboratory by cathodic

reduction of stripped specimens show typically a 14 nm film while an ordinary

air- formed film will range between 2.5 to 3.5 nm. The utility of cathodic

reduction to estimate oxide film thicknesses has been described previously

[ 3].

In examining the general features of the A and parameters in figure 7,

we note that they move in opposition to each other throughout the experiment.

The suggestion here is that the observed changes are due predominantly to

alterations in the oxide layer and that changes in surface texture (roughening)

are insignificant. This interpretation is supported by lack of evidence

from potential cycling experiments [8] for surface roughening at the conclusion

of these measurements.

A detailed interpretation of this type of curve must remain somewhat

speculative until more evidence can be accumulated. In Region I, the

initial increase in A and decrease in 4* at ^ 1800 min indicate an oxide

film thinning in the high pH environment estimated at ~ 5 nm (total film

~ 14 nm). Coating delamination probably occurs at this point, but the

metal itself is still protected by ^ 9 nm of film. Thus, metal dissolution

leading to surface roughening does not occur.

The events beyond 2000 minutes are not well understood. The sharp rise of

A and decline in f maximizing at ^ 2500 min (Region II) is believed to arise

from changes in the polymer near the interface. The subsequent reversal of

A and y may be related to the precipitation process noted earlier for the room

temperature cured systems. The substantial rise of A and leveling of V along
with the concomitant signal deteriorations in Region III (beyond 5300 min)

may be related to the accumulation of a substantial liquid layer at the

i nterface.

The Dissolution of Air-Formed Oxide Films on Iron

It is well-known that air-formed oxide films on iron can be removed

by cathodic reduction in borate buffer solutions at ~ pH 8.5. Results in our

laboratories have shown that both air- formed and thermally grown films on

iron and steel can also be dissolved in aerated saturated NaOH (pH > 14) at

-1200 mV S.C.E.

Typical ellipsometric evidence for this process on uncoated iron is shown

in figure 10. Note that when the cathodic potential is applied, A and 4* move

in opposite directions as the film is removed. These observations are

consistent with those mentioned earlier for coated specimens undergoing

cathodic delamination, i.e., the presence of a highly alkaline environment

promoting dissolution of the interfaced oxide film. Unfortunately, the

potential at the coati ng/substrate interface is not known so that it is not

possible, at present, to simulate the subcoating conditions exactly. However,

these simulation experiments suggest that oxide film dissolution is a

reasonable process to expect in cathodic regions.

The Development of Cathodic Areas

Since cathodic regions under coatings ultimately become the sites for

coating delamination it is important to understand the development of these

regions. The following experiments were designed to probe, in a preliminary

fashion, the spatial disposition of cathodes in the vicinity of an active

anode.

Polybutadiene on Steel: Figure 8 Shows a study of PBD coated steel

undergoing corrosion. The specimen was arranged to be translated ~ 3 mm to
either side of a central position to observe the progress of subcoating

events at different regions. In this manner, the experiment becomes a study

in triplicate under identical conditions. The coating was damaged at the

specimen center at ~ 1350 min. It will be noted that significant changes in

the ellipsometric A and ¥ parameters occur first at the center "c" position.

The general shape of the curves is similar to that for the heat-cured acrylic

shown in figure 7. As mentioned previously, a detailed analysis of the

ellipsometric responses for the heat-cured coating systems has not yet been

possible. Again we propose that at least a part of the initial rise of A and

decline in H* is due to oxide film thinning. From this effect we infer that

delamination has taken place. The first evidence for film thinning is manifested in the cathodic region immediately surrounding the induced anode.

Further evidence for this activity gradually appears in regions "a" and "b,"

some 3 mm distant, on somewhat different time scales. An asymmetric development of the delamination process is apparent. While, as a first approximation,

it can be proposed that the cathodic regions develop radially outward from the

central anode. The possibility of cathodes developing and propagating remotely

cannot be excluded. Thus, the asymmetry can arise from either a true preferred

propagation of the delamination front from the central holiday towards region "a

or from the progress of a remote cathode in or near region "a." Evidence

for the existence of remote cathodes has been obtained in experiments with

a segmented substrate.

Segmented Fe Specimen with Acrylic Coating "A": In dealing with the

corrosion cf coated metal systems the location of the anodes is usually apparent

However, the location of cathodes which support these anodes is often more diffi

cult to ascertain. Intuitively, one might expect cathodes to develop initially

in the region immediately surrounding an anode. Our experiments with a coated,
segmented iron specimen indicate that the real time development of cathodes can

be a relatively complex process.

Figure 9 shows the phase retardation (A) and pH data for the segmented

iron sample. While the coating is intact during the first 1400 minutes, A

for both segments changes only slightly while the pH is erratic and rarvges

between 5 and 8. The coating on segment "a" is punctured at 1400 minutes,

while that of segment "c" is left intact. If segments "a" and "c" are

coupled electrically by joining the copper conductors attached to each, the

pH on "c" rises to about 13.5 while the el 1 ipsometric A parameter for "c"

is relatively constant. Delta for "a" shows the typical response described

and interpreted earlier for this substrate-coating system (fig. 5) a brief rise

followed by a substantial decline. If the conductors joining "a" and "c"

are uncoupled, the subcoating pH drops to a value near 8 within a few

minutes. Rejoining the conductors results in a subcoating pH reading of

about 13 within 5 minutes. This process is quite reproducible and indicates

that high subcoating levels of OH ions can only be maintained if their

production rate exceeds their loss rate (probably by diffusion). However,

if the segments are electrically coupled, for the next few days, the subcoating

pH on "c" declines. Delta for "a" decreases markedly while A for segment

"c" also declines, but to a much lesser extent.

These responses are interpreted as follows: oxygen and water permeate

the coating in some areas and reach the surfaces of "a" and "c'

; during the

first 1400 minutes. Thus, two of the three components to effect the cathodic

reduction of 0^ are in position under the coating. Moreover, small quantities

of adsorbed 0^ and water were undoubtedly present on the surface before the

coating was applied. When the coating on

n a

1J is damaged, an anode develops

rapidly at the puncture site and supplies the final component, electrons,
for 0^ reduction. For "a" the electron flow is through the metal sector

itself while for "c" the electrons must take the longer path through the

coupled conductors. There is a net accumulation of product OH ions under

the coating in both sectors. Initially the rate of the reduction reactions

is probably limited by the rate of Na

+ ion permeation [13] since these ..ions

serve to transport charge through the coating and maintain electroneutrality

with the subcoating OH ions. However, a second transport mechanism develops

on "a" in the vicinity of the puncture. Oxygen, water, and Na

+ ions start

to migrate laterally and establish proximate cathodes surrounding the

active anode [9]. As these proximate cathodes grow they gradually consume

a larger portion of the anodically generated electrons. Eventually the

remote cathodes on "c" are no longer needed and cease to function. Undoubtedly

the presence of low resistance pathways within "a" and relatively higher

resistive pathways between "a" and "c" contribute to the relative viability

of these cathodic regions.

At the conclusion of the experiment the acrylic coating was stripped

with toluene and each segment individually subjected to cathodic reduction

in borate buffer solution. Segment "a" showed a net oxide film growth of

7.7 nm and evidence of surface roughening. Segment "c" showed a net oxide

film growth of only. 2. 7 nm and little if any evidence of surface roughening.

These results are consistent with the picture of extensive cathodic activity

on "a" and limited cathodic activity on "c" provided by the el 1 ipsometri

c

responses [3].
Summary

1. Iron and steel specimens with selected organic coatings bonded to their

air-formed oxide films have been studied while undergoing corrosion. Ellipsometric evidence on these systems with room temperature cured coatings indicates

that dissolution of the interfacial oxide film is the predominant mode of

coating delamination.

2. Similar studies on systems where the coatings were heat cured, suggest

that a similar delamination mechanism may apply. The complexities of a

thickened interfacial oxide and probable modification of the polymer during

the curing process have rendered the el 1 ipsometric data difficult to fully

i nterpret.

3. Air and thermally formed oxide films on iron and steel can be dissolved

in strong NaOH solution at cathodic potentials. This observation supports

the oxide dissolution mechanism proposed for cathodic del ami nation.

4. El 1 ipsometric experiments combined with subcoating pH measurements and

specimen modifications have been utilized to probe the development of cathodic

areas. The results suggest that remote cathodes are important during the

early stages of corrosion but that proximate cathodes tend to dominate as

corrosion progresses.

Acknow] edaments

The author is grateful to Dr. J. Kruger for many helpful discussions.

This work was partially supported by the Office of Naval Research under

contract No. NAONR 18-69.
References

[1] Henry Leidheiser, Jr., Ind. Eng. Chem. Product Research and Development,

20, 547 (1981).

[2] Henry Leidheiser, Jr., and Martin Kendig, Corrosion, 32, 69 (1976).

[3] W. Funke, Prog. Organic Coatings, 9, 29 (1981).

[4] J. J. Ritter and M. J. Rodriguez, Corrosion, 38, 223 (1982).

[5] J. C. Bolger and A. S. Michaels, Interface Conversion for Polymer

Coatings, Weiss and Cheeves, Eds., Elsevier, New York (1968).

[6] J. S. Hammond, J. W. Holubka, and R. A. Dickie, J. Coatings Technol.,

51, 45 (1979).

[7] E. L. Koehler, Corrosion Control by Organic Coatings, Henry Leideiser, J

Ed., National Association of Corrosion Engineers, Houston, Texas (1981),

p. 87.

[8] J. J. Ritter and J. Kruger, Corrosion Control by Organic Coatings,

Henry Leideiser, Jr., Ed., ibid. , p. 28.

[S] Henry Leidheiser, Jr., Wendy Wang, and Lars Igetoft, Progr. in Organic

Coatings (1982), to be published.

[10] J. J. Ritter and J. Kruger, Surface Science, 96, 364 (1980).

[11] P. K. Schenck and J. J. Ritter, to be published.

[12] J. Castle, private communication.

[13] H. Leidheiser, Jr., and W. Wang, Corrosion Control by Organic Coatings,

Henry Leidheiser, Jr., Ed., NACE, Houston, Texas (1981), p. 74.
Figure Captions

1. Drawing showning the relationships of components for the segmented

specimen, "a" and "c" are sections of iron formed by dividing an iron

cylinder along its altitude and diameter.

2. Plots of phase retardation (A) vs time for three iron/coating systems

undergoing corrosion in a dilute chloride medium. The coatings were

cured in air at room temperatures.

3. Plots of phase retardation (A) vs time for two steel/coating systems

undergoing corrosion in a dilute chloride medium. The coatings were

cured in air at 200 °C.

4. (A) and (HO plotted vs time for the iron/collodion system undergoing

corrosion in 0.05M NaCl.

5. (A) and (HO plotted vs time for the iron/acrylic "A" system undergoing

corrosion in 0.05M NaCl

.

6. (A) and (HO plotted vs time for the iron/acrylic "B" system undergoing

corrosion in 0.05M NaCl.

7. (A) and (HO plotted vs time for the steel /acryl ic "A" system undergoing

corrosion in 0.05M NaCl. Acrylic "A" was heat-cured.

8. Phase retardation (A) and amplitude attenuation (HO plotted vs time for

the steel /polybutadiene system undergoing corrosion in 0.05M NaCl. The

coating was heat-cured. The open ellipses shewn in the inset indicate

the three regions on the specimen face where el 1

i psemetri c measurements

were made. The curves are labeled correspond!' ng "a," "b," and "c."

9. A, HO and pH plotted vs time for the segmented iron/acrylic "A" system

in 0.05M NaCl. Inset shows a facial view of the specimen and open

ellipses indicate regions studied el 1 i psometri cal ly.
10. a, H* , and V plotted vs time for uncoated iron in saturated NaOH. Changes

in A and ^ when potential is lowered to -1200 mV indicate the dissolution

of the air-formed oxide film.
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Figure 10
PART II. To be published in Proceedings of Eighth International Conference

in Organic Coatings Science and Technology, Athens, Greece,

12 July-16 July 1982, in the series Advances in Organic Coatings

Science and Technology.

A NEW TECHNIQUE TO STUDY CORROSION MECHANISMS UNDER ORGANIC COATINGS

J. J. Ritter and J. Kruger

Materials Chemistry Division

National Bureau of Standards

Washington, D.C. 20234 USA

ABSTRACT

Transparent organic coatings on iron are used to simulate painted metal

surfaces for simultaneous el 1 ipsometric and electrochemical measurements.

These studies show that significant changes occur both in the metal oxide film

and in the subcoating environment during prolonged immersion in dilute Cl

media. The relationship of these changes to aspects such as metal passivation,

surface roughening, coating delamination, type of coatings, and inhibitor

behavior are discussed.

(°

Keywords: Corrosion; electrochemistry; el 1 ipsometry ; iron; organic coatings;

oxide" fi 1ms ; passivation; pH
'
A NEW TECHNIQUE TO STUDY CORROSION MECHANISMS UNDER ORGANIC COATINGS

J. J. Ritter and J. Kruger

Materials Chemistry Division

National Bureau of Standards

Washington, D.C. 20234, USA

There exists a great need for in situ techniques that are capable of

studying the growth and dissolution of oxide films and other films that

exist on a painted metal surface exposed to a corrosive aqueous environment.

Also of great value would be a technique that could detect changes in the

organic coating (paint), changes in the environment that develop under the

coating, and finally how such changes lead to the delamination processes,

i.e., the detachment of a paint from an oxide-covered metal surface.

Recent work [1,2] from this laboratory has described such a technique,

a combined ellipsometric-electrochemical technique, that is capable of i_n

si tu studies of the fundamental corrosion processes occurring on a metal

surface under an organic coating. This paper reviews the past work and

extends it to an exploration of the role played by the type of organic

coating in affecting the processes that occur at the metal -coati ng interface.

Description of the Ell ipsometric-El ectrochemi cal Technique

A detailed description of the technique is given elsewhere [1]. In

brief, a coated metal is experimentally modeled by coating' a polished iron

specimen with a transparent organic coating. In the early experiments

cellulose nitrate (collodion) coatings were used because such a coating

permits rapid initiation of the corrosion processes. A transparent coating

is necessary for el 1 ipsometri c measurements to be made. A miniature pH

probe is inserted through a hole in the back of the specimen. The tip of

1
this probe is made flush with the surface of the specimen after the specimen

is coated with a transparent organic coating. Thus, it is possible to make

pH measurements under the coating while also making el 1 ipsometric measurements

by passing el 1 iptical ly polarized light through the coating and reflecting

it from the polished metal surface (see fig. 1). Previous studies [1],

showed that the ell ipsometric parameters A, the relative phase retardation,

and ijj, the relative amplitude reduction, are most sensitive to changes at

the metal-metal oxide interface under an organic coating, all immersed in

an aqueous solution. It is thus feasible to relate, for example, the

parameter A to the growth or dissolution of the oxide film at the surface

of the metal and under the organic coating, while simultaneously measuring

the pH of the micro-environment that develops between the coating and the

metal specimen.

It is possible to apply this technique to study coatings containing

corrosion inhibitory pigments which are incorporated in the organic coating

or placed on the metal surface in a way that does not interfere with the

ell ipsometric measurements. Because of the complexity of the multi-layered

systems involved, the el 1 ipsometric measurements are used in a mainly

qualitative way. This means that times and extents of changes are measured

rather .than exact film thicknesses and film optical parameters. To help in

the interpretation of the optical results, cathodic reduction experiments

are also carried out on iron surfaces from which the organic coating has

been stripped. By this means a semi-quantitative measurement of oxide film

thickness can be made and related to the el 1 ipsometric parameters which are

measured prior to the stripping of the coating on the iron. Specifically,

the extent of oxide film growth under the coatings is estimated at the

conclusion of an experiment by stripping the coating with methanol, and
cathodically reducing the metal surface at -1000 mV S.C.E. in deaerated

borate buffer solutions (pH 8.5) while observing the surface with the

ellipsometer. Oxide film thicknesses are calculated assuming that the

film is essentially anc* eac ^ degree change in A corresponds

tc 0.6 nm of film. The details of the technique are shown schematically

in fig. 2.

The presence of surface roughness is qualitatively detected' by cycling

the specimen potential between -1000 and +200 mV approximately 60 times. In

our experience a significant increase in the A parameter indicates surface

smoothing as a result of electrochemical polishing [2].

Contributions from the coating to changes in A are estimated by

el 1 ipsometrical ly monitoring collodion coated gold specimens in 0.05 N NaCl

and in saturated NaOH (pH ~ 15).

The materials and specimen preparation details for the experiments to

be described are as follows:

The iron specimens were cast in epoxy, polished to a 0.05 pm finish and

dipped in a 1:1 collodion-methanol mixture (1 gm of a proprietary acrylic

in 6 ml toluene and 100 mg polystyrene in 1.5 ml toluene when these coating

materials were studied) to give a coating of from 10 to 30 pm in thickness

over the face of both the metal and the epoxy mount. The prepared specimen

mounted as shown in fig. 1 was placed in a polytetrafl uoroethyl ene lined cell

which was filled with 0.05 N NaCl. Inhibitor materials were either applied

directly to the metal before coating or dispersed as a finely ground powder

in the collodion-methanol mixture. For the case of SrCrO^, the metal specimen

was dip coated, finely powdered SrCrO^ applied to the surface of the semicured coating, and a second coat applied to cover the inhibitor.

3
A phosphate-chromate surface treatment was* accompl i shed as follows:

Solution I: H^PO^ 3.0 g, ZnO 1.5 g, NaNO^ 1.0 g, make up with H^O to 100 ml.

Solution II: K^Cr^Oy 0.7 g, H^PO^ 0.5 g, make up with h^O to 100 ml.

The solutions were used at room temperature. The face of the specimen

was rotated in a horizontal plane at 50 rpm in a mixture comprised of

8 ml of solution I and 2 ml of solution II for a period of six minutes.

This procedure gave a homogeneous surface finish which retained sufficient

specular quality to allow el 1 ipsometric measurements.

Prototype Coating (Collodion) Studies

The el 1 ipsometric measurements on collodion-coated iron specimens showed

changes in both the A and t|j parameters with time (see fig. 3). By far the

largest change was seen in the A parameter which always showed a decrease. It

is known that several factors could contribute to this directional change in

A, among them, metal oxide film growth, surface roughening [3], and possibly

coating deterioration.

At least two stages can be discerned in fig. 2. During the first stage

(generally up to around 1000 mm) a gradual rise in pH was observed, accompanied

, i by an increase of several degrees in A. This increase in A was not always

observed and its origin is not readily explained by the el 1 ipsometric modeling

calculations. The second stage started when the pH approached 11 and the

A values began to decrease significantly. Accompanying the onset of the

second stage was the invariable appearance of small anodic regions (sites

where rust was observed). The overall anodic to cathodic (bright uncorroded)

area ratios ranged from 1/30 to 1/600. The pH remained high during the third

stage, where fig. 2 shows that fluctuations in A were observed.

The most significant change occurs in the A parameter during the second

stage. This change arises from three factors given below along with their

4
estimated contributions to the overall 6A: (a)* metal surface roughening,

55 percent; (b) oxide film growth, ~ 35 percent, and (c) changes in

coating material in a high pH environment, ~ 10 percent. It will be noted

that the second stage changes of A with time appear to approximate a semilogarithmic relationship, i.e., A = k log t + c. This law is frequently

observed in the growth of passive films on metals [4].

Using the stripping-cathodic reduction technique described earlier,

experiments on uncoated iron in solutions simulating the undercoating

environment and microanalytical measurements of the subcoating environment,

a reasonable interpretation of the three stages identified by the ellipsometric measurements can be made as follows:

First stage : During the first stage, oxygen, water, Na

+

, and Cl

permeate the coating. The dilute Cl solution that develops under the

coating promotes the pitting of the pre-existing air-formed oxide film on

the iron surface. The pit sites become anodes where ferrous ions go into

solution, the rest of the iron surface becomes progressively cathodic and

supports the reduction of C^. The production of OH leads to the rise in

pH.

Second stage : When the pH reaches values as high as about 15, this

stage commences with a sharp drop in A-values. Studies of uncoated iron in

saturated sodium hydroxide solutions (pH ~ 15), a solution that simulates

the second stage subcoating environment, suggest that the potential at the

onset of the second stage can vary from -900 to -150 mV SHE (see fig. 4).

The Pourbaix potential pH diagram [5] indicates that at the negative end of

-2 . this potential range iron can go into solution as HFe02 or FeO^ ions.

At the positive end HFeO^ can oxidize forming a solid corrosion product .

film, Fe202- Experiments involving the cathodic reduction of iron surfaces

5
from which the organic coating was stripped during stage two indicated

that both surface roughening (presumably resulting from the dissolution of

-2

the iron to produce HFeC^ and/or FeO^ ) and oxide film growth occur

during this stage and are primarily responsible for the decrease in the

values of A in the manner shown in fig. 4 for the simulated environment.

Thus the high pH environment and concomitant decreases in A-values occur

only when active corrosion is underway somewhere on the surface of the coated

metal. Our technique provides an in situ way to detect the presence of

serious subcoating corrosion and to measure the overall kinetics for some

of the subcoating processes.

What are the implications of the corrosion processes just suggested

for stage two for coating failure mechanisms? Speculating, one might envision

that, while the coating is still intact, the initial formation of small "two

dimensional 11 localized corrosion cells that develop with permeation of the

coating by H^Q, O

2

> Cl , and Na'. This activity will proceed rapidly at

porous or thin portions of the coating. These cells could be the starting

sites for corrosion. Ritter and Rodriguez [5] have recently carried out

mi croanalytical determinations of the concentration build-up of Cl in the

subcoating environment and used that information to propose a picture of

the processes that lead to this delamination. The picture that they have

developed is shown in fig. 5. This view describes how, following the

build up of Cl concentration and the development of separated anodic and

cathodic sites as a result of the chemical reactions shown in fig. 5a, a

"breakthrough" can occur between these separated sites (fig. 5b) during

delamination, causing the mixing of the acidic anodic sites with the alkaline

cathodic sites. Figure 6 from the Ritter and Rodriguez paper shows experimental

evidence for this "breakthrough," the detection of a large drop in pH.
Such a delamination and breakthrough process as a result of the anodic

and cathodic reactions shown in fig. 5a continue to promote the enlargement

of the delaminated regions that are observed during stage two.

Third stage : Experiments with collodion coated gold in a high pH

O 15} solution .indicate organic coating deterioration after prolonged

exposure. This deterioration may be partially responsible for the erratic

fluctuations of A that constitute the third stage.

Effect of Inhibitory Pigments

When a corrosion inhibitory pigment or pigment-like compound such as

l^CrO^ was incorporated into the collodion coating as small "islands"

surrounding the area observed with the ellipsometer, results similar to those

shown in fig. 7 were observed. The first stage was longer, the second stage

involved a much slower decrease in A-values, as compared to a non-i nhibited

coating, and the third stage was absent. Also, the increase in pH was

1

less, usually remaining below pH 10.

Table I lists the rate constants, k, for the decrease in A in the second

stage for coated iron with various applications of different inhibitory

pigments.

With reference to Table I it is readily seen that the rate of ellipsometrically observed subcoating events for coated iron and for coated iron

which had been given a phosphate-chromate pretreatment were quite comparable.

However, when K^CrO^ was present at the metal surface, the rate was slower

by a factor of 10, whereas ZnCrO^ applied in a similar fashion slowed the

rate by only a factor of two. This phenomenon may be partly related to

solubility differences between the two inhibitors. Inhibitors dispersed

throughout the coating gave a similar factor' of two reduction in the rate.

The mechanism by which chromates inhibit corrosion reportedly involved the

7
incorporation of chromium into an existing oxicte film, i.e., a chemical and

structural modification of the existing oxide film which resulted in improved

protection [6]. The rate constants derived from the present work seem to

be related to inhibitor ion mobilities and accessibilities to the substrate

surface. Contributions to the A parameter due to modifications of the. metal

oxide (e.g., upon the incorporation of chromium) which may change its

refractive index, are expected to be small for films < 5 nm in thickness [7].

Thus, the thickness of these modified films could be estimated with reasonable

confidence using parameters for f^O^.

Preliminary results from uncoated iron in a simulated subcoating environment cf pH 15 (sat'd NaOH), 0.05 M I^CrO^ and 0.05 M NaCl showed a change in

A of which approximately one-third was due to oxide film growth and the

remainder ascribed to surface roughening. The oxide film growth on uncoated

Fe even in this highly alkaline environment was only 2.4 nm as compared

to the typical 6 nm growth measured for coated iron in the absence of

i nhibi tor.

Effect of Type of Organic Coating

All of the experiments thus far described were carried out using a

cellulose nitrate (collodion) coating. In this section, experiments will be

described that show that the el 1 ipsometric measurements can identify different

processes leading to coating failure that depend on the type of organic

coating used. Figure 8 shows three different types of el 1 ipsometric results

that are characteristic of different organic coatings. Figure 8a shows the

behavior previously described for a room temperature air-cured collodion

coating, an identical behavior for a room temperature air-cured polystyrene

coating. If the collodion was replaced by a' propri etary acrylic coating

also room-temperature air-cured, the behavior observed was similar except for

8
a small difference observed during the first stage where no change in A was

observed until the acrylic coating was purposely punctured. Shortly thereafter

A rose sharply indicating perhaps a dissolution of the native oxide film under

the coating followed by stage two where the surface roughening and oxide film

regrowth, described earlier for the collodion coatings, was observed.

Rather pronounced differences in behavior were found for acrylic on

iron or polybutadiene coatings on steel that were air-cured at a temperature

considerably above room temperature (200 °C). This behavior can be seen in

fig. 8c. First, the A values in stage one were considerably lower than the

air-cured cases. This was due to the existence of a 10 to 14 nm oxide film

that formed under the organic coating during a 40-minute 200 °C curing

process. Separate experiments holding an uncoated steel surface at the

curing conditions verified the corresponding decrease in A-values and the

formation of the 10 to 14 nm oxide film. As fig. -8c shows, the low A-values

remained constant until the cured coatings were punctured, at which time the

A-values increased to a much greater extent than that observed during stage

one for the acrylic (fig. 8b). This total increase in A could not be

accounted for by assuming it to be only the result of the dissolution of

the thick oxide film formed during the curing process. It was estimated

that only about one-half of the changes in A-values were possibly caused by

oxide dissolution. The origin of the rest of the increase in A is not

known. However, calculations aimed at relating the changes in the el 1 ipsometric

parameters, A and i|i to changes in the optical constants of the polymeric

coatings suggest that it is possible to model the optical changes observed

if one assumes a decrease in refractive index in a 50 to 100 nm region of

the coatings next to the metal oxide surface: Such a decrease in refractive

9
index (or density) of the polymer next to the rmetal could come about as a

result of the dissolution of the oxide causing a destruction of the polymeroxide bonds followed by as yet unexplained events responsible for the reduction of density in the interface region. This suggestion is a preliminary one

and highly speculative. More work is needed to put such an interpretation of

stage one events on firmer ground. Stage two appears to follow the same

behavior as that observed for the coatings described in figs. 8a and 8b.

Summary and Conclusions

(1) The ell ipsometric-electrochemical technique has shown that during

the failure of an organic coating in an aqueous chloride environment oxide

film and surface roughening take place on those areas of metal surface

which are subjected to a highly alkaline environment.

(2) The results obtained suggest that the initial changes in the

topography and possibly in the composition of the metal surface may be, at

least, partially responsible for coating delamination.

(3) Inhibitory pigments retard active dissolution of iron and thereby

suppress OH build-up which slows down oxide film growth and metal surface

roughening. Use of the el 1 ipsometri c-el ectrochemical technique to follow

such changes may be a valuable way to evaluate inhibitory pigment performance.

(4) The ell ipsometri c-el ectrochemi cal technique can detect differences

in organic coating behavior; for example, high temperature cured coatings

show much larger increases in A-values during stage one as compared to room

temperature coatings.

Acknowledgment

We are grateful to the Office of Naval Research which supported this

work under NAONR 18-69 NRO 36-082.

10
References

[1] J. J. Ritter and J. Kruger, Surf. Sci., 96, 364 (1980).

[2] J. J. Ritter and J. Kruger, in H. Leidheiser (Ed.), Corrosion Control

by Organic Coatings, N.A.C.E., Houston, 1981, p. 28.

[3] J. Kruger, Corrosion 22, 88 (1966).

[4] F. P. Fehlner and N. F. Mott, Oxid. Met., 2, 59 (1970).

[5] J. J. Ritter and M. J. Rodriguez, Corrosion, in press.

[6] J. E. 0. Mayne and M. J. Pryor, J. Chem. Soc. , 1949 , 1831 (1949).

[7] J. Kruger, in R. H. Muller (Ed.), Advances in Electrochemistry and

Electrochemical Engineering, Vol. 9, J. Wiley and Sons, New York, 1973,

p. 250.
Figure Captions

Fig. 1. Details of the mounted iron specimen.

Fig. 2. A, relative phase retardation, i{i relative amplitude reduction, pH

and <j)p , potential of the iron surface versus standard hydrogen

electrode (SHE) versus time, for Fe with collodion in 0.05 N NaCl.

Fig. 3. Schematic drawing showing changes in A values during three steps

in the film stripping-cathodic reduction procedure for measuring

the film growth and surface roughening that occurs on iron surfaces

under organic coatings after exposure to aqueous chloride environments.

Fig. 4. Changes in A, ip , and <t>p (potential of iron surface vs SHE) of an

iron surface in a saturated NaOH solution (pH 15).

Fig. 5. (a) A summary of the equations proposed by Ritter and Rodriguez [5]

for subcoating events leading to surface roughening, oxide film

thickening, and the building up of Cl concentration.

(b) Schematic diagram showing Cl ion entry into the hollow cone of

anodic corrosion products in the presence of a remote cathode.

(c) Schematic diagram suggesting one mode of "breakthrough" between

anodic and cathodic areas. From Ref. 5.

Fig. 6. Experimental curve showing changes in A and pH as the corrosion

process under an organic coating proceeds. The steady decrease in

pH after 1200 min indicates a "breakthrough" between anodic and

cathodic areas in the vicinity of the micro pH electrode. From

Ref. 5.

Fig. 7. A, ijj, pH, and versus time for Fe with collodion and K^CrO^ islands

in 0.05 N NaCl

.
Fig. 8. A comparison of el 1 ipsometric (A vs log time) results for iron

coated with (a) collodion or polystyrene, room temperature air cure,

(b) acrylic, room temperature air cure, and (c) steel coated with

polybutadiene or acrylic cured at 200 °C for 40 min in air.
Table I. Stage II Rate Constants for El 1 ipsometrical ly Measured Subcoating

Events on Collodion-Coated Iron in 0.05 N NaCl

Summary of El 1 ipsometric Rate Constants for Stage II Events

Semi-Logarithmic Behavior Assumed

dA = k log t + c

Specimen Immersion Medium k(deq)

Uncoated Fe Sat'd NaOH 76

Coated Fe 0.05 M NaCl 83

Coated Fe with

deliberate holiday " 73

Coated Fe with " 76

phosphate-chromate

surface pretreatment

Coated Fe with " 8

K^CrO^ "islands"

Coated Fe with " 35

ZnCrO^ "islands"

Coated Fe with 11 58

ZTOC* dispersed in

coati ng

Coated Fe with " 42

K

2

Cr0

4 dispersed in coating

Coated Fe with SrCrO^ " 56

located between two

layers of col lodion

*Zinc tetraoxychromate

[Zn(0H)

2

] 4

• ZnCr0

4
Vt

auxiliary

o

4*V

u

Figure
14

Figure 2
140

I

o O O O o o

ro CM — o CD 00

<

STEP

I

STEP

STEP

III
32

3 I 30

29

28

27

26 , 25

I I 2

I 10

108

106

104

102

100

98

96

Fe (6 ) in air Sat'd. NaOH sol'n

mV S.H.E.

Figure 4
CATHODIC

REGION]

<03 s fT3 s I ® +_

.2 °ro ^ X 2 +

(3LU

CCooo o

o'

a>

CVJ

000> t

CD U_

Xo

a> itt

00

X

00+ c ° o °

0)

k— oa

a> oooowooEi Xo

ooto

oo X+

00 O

-| co

+• a>

00

•2 § o — 2 2 o •- w .2- o o

.S a> E 1 S ® « ? 0 +

oo o X oo

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u_XXo

ro

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00

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Figure 5
A

( deg

)

Fe coated with collodion

14

13

12

10

9

8

7

Figure 6
Figure 7
Fe with acrylic

Fe with collodion room temp, air cure

or polystyrene, room

Figure 8
PART III. Accepted for publication as an accelerated brief communication by

J. Electrochemical Society.

\

EXAFS Study of the Passive Film on Iron

*

Gabrielle G. Long, Jerome Kruger, David R. Black , and Masao Kuriyama

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

*Member of the Electrochemical Society

irk Also at the Department of Physics, Colorado State University, Fort Collins,

Recent Mossbauer studies (1,2) of the

passive film on iron have emphasized the

important role of water in the structure of

the in situ thin film. In these experiments,

the "di sorder" in the structure of the film

was shown to be very similar to that of

"amorphous" iron (III) oxide or hydroxide.

Ex situ measurements using electron diffraction (3) have generally concluded that the

main protective film on iron was crystalline

Fe 304 at the metal -oxide interface and

yFe 203 at the oxide-air interface. These

results taken together can be understood

if the vacuum environment of the ex situ ' experiments caused water to be removed from

the films, causing their crystallization.

Laboratory-EXAFS (extended x-ray absorption fine structure) measurements (4) were

undertaken in an attempt to determine the

structure of the passive film using a direct

method that avoids the need for a vacuum environment. (Background references on EXAFS

are given in (4)). These exploratory measurements, while not in situ , did not require

exposure of the fi ims to a vacuum environment,

and can be considered as intermediate, non- equilibrium results with respect to possible

water in the film. These are the first

results for the short range order parameters

in the passive film, derived from measurements in a non-vacuum environment. Such

information cannot be obtained directly

from Mossbauer measurements.

The samples were 5 nm thick iron films

vapor deposited on glass. They were passivated by immersing them in either a potassium chromate (0.005M) or a sodium nitrite

(0.1M) passivating solution. Films formed

this way have been shown (3,5) to have the

structure, composition, and formation kinetics of the passive layers that form in Fe ++ - free solutions by anodic oxidation.

The passive film experiment, which is a fluorescence-EXAFS measurement, utilizes a photocathode x-ray ionization chamber described earlier (6,7). The detector collects

electrons emitted from the sample and amplifies them through field-intensified ionization in helium gas.

Transmi ssion-EXAFS measurements were performed on (greater than 98% pure) finely

powdered Fe 304 , ancl YfeO(OH),

and on (99.998% pure) bulk iron. These were then used as model compounds for the

determination of phase shifts in the analysis of the thin films.

The i ron-to-oxygen bond distances which were derived for the passive films are 1 . 91

0.05 and 1.99±0.05 A for for the nitritepassivated and the chromate-passivated films

respecti vely. These values are in the

middle of the range of Fe-0 and Fe-OH bond

Figure 1. EXAFS signals (k%(k)) for yFe 203 , ^ 6304 , yFeO(OH) and the passive films. Note

that only yFeo(OH) has two oscillations over

this region in k-space.
lengths derived for the model compounds. The

first iron-to-iron distances derived for the

passivated films are shorter than those for

Fe 304 and yFe^, and similar to that for

yFeO(OH). It is useful to examine the

EXAFS signal directly. Since Fe 304 and yFe 203 are both cubic spinels, where the latter is a defect version of the former, the EXAFS signals from these compounds should be quite

similar. The FeO(OH) compounds are orthorhombic with a tetramolecular unit cell, and

thus their EXAFS signals should be different

from the spinel-like iron oxides. The EXAFS

signal. k%(k), over the range from k=3 to

k=7 A"1 for the two sets of passive films

are compared to those for the model compounds

in Figure 1. The EXAFS signatures of the thin

films are different from that of YFeO(OH),

whereas they bear some resemblance to the

signatures of the cubic spinels.

The magnitudes of the inverse Fourier

transforms are shown for the crystalline iron

oxides and iron hydroxide in Figure 2, and

those for the passive films are shown in

Figure 3. As indicated above, the positions

of the first peaks (related to the Fe-0

distance) in the passive films are very

similar to positions for the crystalline

compounds, whereas the positions of the second

peaks (Fe-Fe distance) are different. Further,

the amplitude ratios of the first to the

second peak for the passive films are unlike

r (A)

Figure 2. The magnitude of the inverse

Fourier transform as a function of the radial

distance, r, from the iron atom for the model

crystalline compounds.

r ( A)

Figure 3. The magnitude of the inverse

Fourier transform as a function of the

radial distance, r, from the iron atom

for the two passive films. The origin

of the third peak is explained in Ref. 10.

those for the crystalline compounds. Since

the data range in k-space used was as nearly

identical as possible, this alteration in am- plitude ratio may indicate a relative change

in the number of nearest neighbors as well.

This demonstrates that the passive film on iron is either different from these model

compounds or a random mixture of many

crystalline forms.

Although it has been shown that the

structures of the nitrite-passi vated and the

chromate-passivated films are quite similar,

the first two peaks in r-space are better re- solved for the ni tri te-formed film. This

suggests that there may be a greater range of

structures in the chromate-formed film, indicating greater disorder for this case.

If the two films had the same structure in an aqueous environment, then the evident

disorder in the chromate-passi vated film

could be the result of the improved stability

of that film in the helium environment due

to the inclusion of chromium (8). It was

found, using a comparison of the fluorescence

intensities at the chromium and the iron

K-absorption edges, that more than 12% of
the cations in the film were chromium.

Furthermore, the absorption edge shift,

as measured from the position of the K-edge of

pure iron, was greatest for yFe 203 . If the

passive films consist only of iron (III) compounds, the smaller edge shifts measured for

the films would be evidence of a greater

degree of covalency in the bonding (9). Another interpretation is that there may be both

Fe ++ and Fe +++ oxidation states present in the

passive film, with the possibility of greater

covalency as well. The smallest edge shift was found for the chromate-passivated film. This

result taken together with the detection of Cr

in this film suggests that it is the more

vitreous of the two (8).

In conclusion, the EXAFS signatures of

the passive films measured here resemble those

of the cubic spinel ferric oxides. The sharpness of the peaks, however, is different, and

at least the chromate-formed film may be more

vitreous than the model crystalline compounds.

A complete account of the experiment and

data analysis is to be published (10).

ACKNOWLEDGMENT

The contribution of J. Kruger was supported by the Office of Naval Research under

MAHMD V.OHWI uwu i u twmv 1Q^£Q MPfP£_nQ9

REFERENCES

1. W. E. O'Grady, This Journal 1 27 , 555 (1980).

2. J. Eldridge, M. E. Kordesch and R. W. Hoffman, J. Vac. Sci. Tech. _20, 934 (1 982).

3. C. L. Foley, J. Kruger and C. J. Bechtoldt,

This Journal VI4, 994 (1 967).

4. G. G. Cohen, D. A. Fischer, J. Colbert and

N. J. Shevchik, Rev. Sci. Inst. j51_, 273

(1980).

5. J. Kruger, This Journal 110 , 654 (1963).

6. N. J. Shevchik and D. A. Fischer, Rev.

Sci. Inst. 50, 577 (1979).

7. D. A. Fischer, G. G. Cohen and N. J. Shevchik, J. Phys. F J_0, L139 (1 980).

8. A. G. Revesz and J. Kruger, Passivity of

Metal

s

, R. P. Frankenthal and J. Kruger,

Eds., The Electrochemical Society,

Princeton, NJ, 137 (1978).

9. I. A. Ovsyannikova, S. S. Batsanov, L. I. Nasanova, L. R. Batsanova, and E. A. Nekrasova, Bull. Acad. Sci. USSR, Phys.

Ser. (English translation) _31_, 936 (1 967).

10. Proceedings of the " Internati onal Conference on the Electronic and Molecular

Structure Of the Electrode-Electrolyte

Interface, held in Logan, Utah, July 25-30,

1982. To be published in J. Electroanal.

Chem.
'
PART IV. To be published in the Proceedings of the International Conference on Electronic and Molecular Structure of Electrode-Electrolyte Interfaces,

Logan, Utah, 25-30 July 1982, in J. of Interfacial and Electronal. Chem.

Structure of Passive Films on Iron Using a New Surface-EXAFS Technique

by

G.G. Long, J. Kruger, D.R. Black* and M. Kuriyana

Metallurgy Division

Center for Materials Science

National Bureau of Standards

Washington, DC 20234

ABSTRACT

There exists considerable controversy about the structure, the bonding,

and the composition of the passive films that form on iron surfaces in

aqueous electrolytes. A major problem is that most of the surface analytical

techniques used to characterize the passive film require exposure to vacua,

which can alter the structure of the passive film. This study seeks to overcome

this problem through the application of a new surface-EXAFS (extended x-ray

absorption fine structure) technique that is both extremely sensitive to structural and bonding changes in the 2 to 3 nm passive film and does not require

the use of a vacuum environment.

Near edge and extended x-ray absorption fine structure spectra from passive

films on iron were measured and compared with those from pure iron and a polycrystalline iron oxide of known structure. The EXAFS data provide a measure of

the relative disorder in the passive films, and they were used to derive bond

lengths for the i ron-to-oxygen and the iron-to-iron coordination shells. It

was found that there was greater disorder in a chromate formed film than in a

nitrite formed film. The i ron-to-oxygen bond lengths were within the usual

range for the crystalline iron oxides but the iron-to-iron distances corresponded

to none of the known structures. The near-edge data reveal differences in the

electronic configurations of passive film samples formed in different aqueous

electrolytes.

*Also at the Department of Physics, Colorado State University,

Fort Collins, Colorado
INTRODUCTION

Passivity is a crucial factor in determining the capability of a metal

to resist environmental attack. (See [ 1 ] for a comprehensive rev iew.) The

nature and structure of the passive film that is responsible for this

phenomenon has been a key focus of efforts aimed at developing an understanding of passivity. There have been a number of attempts to determine

the nature and structure of the passive film, especially on iron [ 1 ].

For example, electron diffraction experiments [ 2 ] found that the passive

film on iron was crystalline y-Fe203. Other surface analytical approaches

have also been used, such as Auger electron spectroscopy [ 3 ] and photoelectron spectroscopy [ 4 ]. It is often concluded that the main protective film

on iron is cubic Fe304 at the metal -to-oxi de interface, and y-Fe203 and

hydrated oxides at the filn-to-air interface.

These conclusions have been criticized by O'Grady [ 5 ], who .suggested

that previous attempts used techniques that required ex situ examination

of the passive film on iron in a vacuum, which could remove the water or

hydrogen-containing species from the films and cause their crystall ization.

O'Grady, using in situ Mossbauer spectroscopy, found that the passive

layers on iron were iron (III) oxides arranged in polymeric chains. When

these films were dried by exposure to a vacuum, they appeared to be composed

of polycrystalline y-Fe203.

The work described here attempts to exploit, for the first time, the

opportunities provided by a new surface-EXAFS (extended x-ray absorption

fine structure) technique [ 6 , 7 ] to extract information about the passive

film on iron in a non-vacuum environment. The technique is capable of

supplementing O'Grady 's findings by providing parameters on the specific

structure and composition of the film.

2
The bonding flexibility, which arises from the type of bonding in

the film, is another important factor in the quality of passivation. The

role of partially ionic and partically covalent bonds in the formation of

stable glassy structures has been discussed by Myuller [8]. In this

work, the degree of covalency of the iron to oxygen bonds in the passive

film is investigated through a study of the K-absorption edge threshold

and the effective "coordination charge" of the thin films.

EXPERIMENT AND DISCUSSION

The samples were 5 nm thick iron films vapor-deposi ted on glass.

They were passivated by immersing them in either a chromate (0.005 M) or

a nitrite (0.1 M) passivating solution. Near edge and extended x-ray

absorption fine structure spectra were measured on these films, as well

as on foils of pure iron and on purified Fe 304 powder.

The K-absorption spectrum for pure iron shows a rounded absorption

edge, as is usual for a transition metal, .and that for Fe 304 is sharply

peaked. These two spectra, shown in Figure 1, were measured in the

transmission EXAFS geometry, using a tunable laboratory EXAFS system

described elsewhere [9]. The positions of the K-edqes were determined

from the known dispersion of the monochromator and from Fe K3i 5 3

and Ni Ka, Kct i lines which were present due to impurities in

the rotating anode x-ray target. The latter pair of lines effectively

ended our EXAFS data set about 360 eV above the Fe edge.

Transmission geometry EXAFS is not generally sufficiently sensitive

to the surface layers to be useful for a surface-EXAFS measurement. A

f 1 uorescence-EXAFS technique is required. It is desirable to collect

only those photons or electrons that originate in the surface region.

The passive film experiment makes use of a photocathode x-ray ionization

3
chamber. This detector was especially developed for surface-EXAFS [6], and

was demonstrated to be sensitive to the oxidation processes in metallic

copper [7], The detector collects electrons emitted from the sample

(which serves as the cathode plate) and amplifies them through field

intensified ionization in helium gas. Since the electrons emerging from

the sample must create electron-ion pairs in the gas, and the ionization

potential of helium is ~30 eV, the detector weights the higher energy

electrons more strongly than the lower energy ones. Near the absorption

threshold, the photoelectron energy is low and not able to yield much

detector current. The Auger electron, created by the decay of the core

hole, is substantially more energetic and able to produce a much larger

signal. The glass substrate absorbs xrays passing into it far from the

photocathode overlayer, and so contributes little to the current.

The absorption spectra for the chromate- and the nitrite-passivated

films are shown in Figure 2. The raw data for the two sets of films are

quite similar with the important exceptions of the positions of the

aDsorption threshold and the near-edge fine structures. The nitrite formed

film shows evidence of an enhancement in the near-edge absorption coefficient that is suppressed in the chromate-formed film. This is evidence

of a difference in the density distribution of empty or parti ally-empty

molecular orbitals in the two films. This near-edge enhancement [10] in

the K-absorption spectrum of the nitrite-formed film indicates that it

has an increased density of available final states of p-cha^acter.

Further evidence of differences in the electronic structure of the two

films comes from the measured positions of the inflection points of their

iron K-edges. The positions and energy shifts measured in this work for

pure Fe, Fe 30a, and the two Dassivated films are given in Table 1, along

d
with energy shifts for FeO and Fe 203 measured by Chen [11]. The observed

energy shifts reflect changes in the charge distribution of the valence

band, the degree of ionicity, and the coordination number. This effect

can be described by the effective "coordination charge," which is given by

n = m - [

n kCk

K

where m is the oxidation plate, C« is the degree of covalence of a bond

(Ck = 1 - i k where i« is the ionicity), and n|< is the number of such bonds.

There is a nearly linear relationship between AE (= E conpouncj Emetal),

the energy shift, and the coordination charge of the compound [13,14].

The effective coordination charge for Fe 3C>4 was calculated using the

ionicities for FeO and Fe 203 derived from Levine [14] and the structure

as determined by Shull et al . [15] using neutron diffraction. (One third

of the iron cations are tri valent and tetrahedral ly coordinated, one

third are divalent and octahedrally coordinated, and the remainder are

trivalent and octahedrally coordinated.) Our result for Fe 304 , and Chen's

[11] for FeO and Fe 203 , are shown in Table 1. Assuming a linear relationship between AE and n, the effective coordination charges for the passive

films can be derived. There is a significantly lower coordination charge

for the chromate-passivated film, indicating greater covalency in the

bonding around the Fe in this film. This lends support to the idea that

the chromate-passi vati ng solution, which should introauce chromium into

the passive film, can thereby, according to Revesz and Kruger [16], promote

the formation of non-crystalline (glassy) passive films. A comparison of the

edge step at the chromium and the iron edges for this film indicated that at

least 12% of the cations in the film were chromium.

Up to this point, we have been considering the region in the absorption

5
spectrum from 0 to 30 eV above the edge. In this portion of the spectrum,

the features are due to many body interactions and band effects. Recently,

this region has been interpreted [17] also by a multiple scattering approach,

which should be sensitive to bond angles, and as should prove to be important

in the study of covalently bonded systems (e.g. glasses). Further work will

attempt to exploit this new, still developing technique.

We next look at the EXAFS region, which in this work commences at ~35

eV and continues to ~360 eV above threshold. The oscillatory structure

in this portion of the spectrum can be interpreted using the assumption that

single scattering contributions dominate. This leads, in a straight forward

way, to the determination of structural parameters [18]. The EXAFS data can

be analyzed by taking a Fourier transform of the spectrum, properly weighted

to allow for the backscatteri ng amplitude and phase variations. Since we are

interested in obtaining the Fe-0 and the Fe-Fe bond lengths, this procedure

would require taking two transforms, where the one would display peaks at the

bond lengths rj for Fe-0 and the other would be used to find the rj for Fe-Fe.

Each would also show additional peaks, at incorrect positions, for the other

type of bond. It was decided instead to calculate the Fourier transforms of

the pure Fe and the Fe 304 data, and use their known structures to determine

emperically the required phase factors. Since iron and oxygen are far apart

in the periodic table, the backscatterinq amplitudes attributed to them are

very different. These are shown in Figure 3, calculated from Teo and Lee [19].

It is evident that oxygen backscatteri ng dominates at low energy (k) and iron

at high energy. The Fourier transforms of both sets of passive film data were

dominated by the pure iron substrate when the transform was taken over a high k

bandwidth, a low k bandwidth, however, yielded information dominated by the

oxide film, with only the wourth shell Fe-Fe visible. Fourier frequencies

6
due to the lower shells in bulk Fe are not seen since the high k region, where

they dominate, was omitted from the low k bandwidth.

The results for the magnitudes of the Fourier transforms for the

passive film data are shown in Figure 4, and the Fe-0 and Fe-Fe distances,

corrected for phase shift, are given in Table 2. Uncertainties in these

results are estimated to be ± 0.05 A. As could be expected from the

raw data, the geometric arrangement of the two films is quite similar.

It is noteworthy, however, that using nearly identical Fourier resolution

(i.e. the same range in k space, and the same sampling rate) for the two

data sets, the first two peaks in r space are well resolved for the nitritepassivated film and much less well-resolved for the chromatepassi vated film.

This suggests that, although the coordination for the two films is similar,

there is a greater range of structural parameters (greater disorder) in

the chromate-formed film, indicating greater disorder in this film. This

result supports the suggestions of Revesz and Kruger [16] that the introduction of chromium, a glass former, into the passive film results in a

more non-crystalline structure with bonds having a higher percent of

covalency than films containing only iron, oxygen and hydrogen. The Fe-0

distances for the thin films are in the range of the ususal Fe-0 and

Fe-OH distances found in the iron oxides. The Fe-Fe distances determined

here are significantly smaller than those found in Fe 304 or y-Fe 203 , and

they are larger than those of ct- and y-FeOOH. Thus, the structure of

the passive film does not really resemble these categories of crystalline

oxides. This is supportive of the conclusions of O'Grady [5] that the

structure of the passive film on iron resembles a polymeric film of hydrated

iron oxide. But these results go one step further in that they add numerical

parameters and place structural limitations on the suggested model.

7
CONCLUSIONS

The usefulness of a new surface-EXAFS technique to examine 2-3 nm

passive films on iron for composition, structure, and bonding characteri sti cs

has been demonstrated. Passive films formed on iron in nitrite and in chromate solutions show Fe-Fe distances different from those observed for the

known iron oxides and oxy-hydroxides. The passive film formed on iron in a

chromate solution shows bonding of a higher percentage of covalency than

that formed by a nitrite solution. Furthermore, the chromate-formed film shows

evidence of being more non-crystal 1 ine than the nitrite-formed film, and it

was found that more than 12% of the cations in the chromate-formed film were

chromi urn.

ACKNOWLEDGEMENTS

We are very grateful to Drs. E. N. Farabaugh and D. M. Sanders for providing us with the thin film iron specimens. We also thank the Office of Naval

Research for partial support under contract NAONR 1 8-69NR036-082.

8
REFERENCES

1. R.P. Frankenthal and J. Kruger, Editors, Passivity of Metals, The

Electrochemical Society, Princeton, 1978.

2. C.L. Foley, J. Kruger, and C.J. Bechtoldt, J. Electrochen. Soc., 114

(1 967) 994.

3. M. Seo, J.B. Lumsden, and R.W. Staehle, Surf. Sci., 50 (1975) 541.

4. G. Okamote, Corros. Sci., 13 (1973) 471.

5. W.F. O'Grady, J. Electrochen. Soc., 127 (1980) 555.

6. N.J. Shevchik and D.A. Fischer, Rev. Sci. Instrum. 50 (1979) 577.

7. D.A. Fischer, G.G. Cohen, and N.J. Shevchik, J. Phys. F, 10 (1980) L139.

8. R.L. Myuller, Solid State Chemistry, O.U. Borisova, Editor, (translated

from Russian), Consultants Bureau, New York, 1966, p. 1.

9. G.G. Cohen, D.A. Fischer, J. Colbert and N.J. Shevchik, Rev. Sci. Instrum.,

51 (1 980) 273.

10. M. Brown, R.E. Peieris, and E.A. Stern, Phys. Rev. B 1 5 (1 977) 738.

11. H. Chen, J. Phys. Chem. Solids, 41 (1980) 641.

12. I. A. Ovsyannikova, S.S. Batsanov, L.I. Nasonova, L.R. Batsanova, and

E.A. Nekrasova, Bull. Acad. Sci., USSR Phys. Ser. (English translation)

31 (1967) 936.

13. S.P. Cramer, T.K. Eccles, F. Kutzler, K.0. Hodgson, and L.E. Mortenson,

J. Am. Chem. Soc. 98 (1976) 1285.

14. B.F. Levine, J. Chem. Phys. 59 (1973) 1463.

15. C.G. Shull, E.0. Wollan and W.C. Koehler, Phys. Rev. 84 (1951) 912.

16. A.G. Revesz and J. Kruger, Passivity of Metals, R.P. Frankenthal and

J. Kruger, Editors, The Electrochen. Soc., Princeton, 1978, p. 137.

17. P.J. Durham, J.B. Pendry and C.H. Hodges, Sol. St. Conmun. , 38 (1981)

159 .

9
18 .

19.

E.A. Stern, Phys. Rev. B, 1 0 (1 974) 3027 F.W.

and E.A. Stern, Phys. Rev. B 11 (1975) 4825.

B.K. Teo and P.A. Lee, J. Am. Chem. Soc. , 101

Lytle, D.E. Sayers,

(1 979) 281 5.

10 -
TABLE I

Positions of the Fe K-edges and the calculated effective coordination charges

Sample

K-edge

(eV)

AE

(eV) n

pure Fe 7112 0

Fe

. 905°

a 5

a

1 . 238a

Fe 304 7121 9 1.920

YFe 2°3 11 2.0031

nitrite passivated

fi 1m

7120 8 1.670

chromate passivated

film

7117 5 1.238

aFrorn reference [11].

11
Table 2

Bond type versus bond distance in ftngstroms

Bond type Fe3°4

a SFe00H b nitrite chromate

Fe - 0 1.82 1.94 2.02 2.09

2.10 2.13

Fe - OH 2.05

Fe - Fe 2.97 2.88 3.27 3.18

3.46

3.63

Calculated using the space group 0^ (Fd3m) with u = 0.379 and a = 8.396 $.

R. W. G. Wyckoff, Crystal Structures, 2

nd Edition, Vol . 3, J. Wiley,

New York* 1965.

b R. W. G. Wyckoff, Crystal Structures, 2 nd Edition, Vol. 1 , J. Wiley,

New York, 1 963.
FIGURE CAPTIONS

Figure 1.

Figure 2.

Figure 3.

Figure 4.

X-ray absorption spectra for iron and Fe 304 .

X-ray absorption spectra for the thin films, showing an edge

enhancement in the nitrite-formed film.

Backscattering amplitudes for iron and oxygen, as a function of k.

The magnitude of the Fourier transform versus or for the nitriteand chromate-passi vated films. Both were calculated from the low

k region of the data.

13

^8 Fe 3 ^4

—

ENERGY

FIG 1
CURRENT

CHROMATE - TREATED NITRITE -TREATED

—vENERGY
FIG 3

i
r( A)

FIG 4
NBS-114A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 83-2551 February 1983

4. TITLE AND SUBTITLE

PASSIVE FILMS, SURFACE STRUCTURE, AND STRESS CORROSION AND CREVICE

CORROSION SUSCEPTIBILITY

5. AUTHOR(S)

J. Kruger, J. J. Ritter, and G. G. Long

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions) 7. Contract/Grant No. national bureau of standards NAONR 18-69 NRO 36-082

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Office of Naval Research

Arlington, Virginia 22217

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

Parts I and II: Transparent organic coatings on iron and steel are used to simulate

painted metal surfaces for simultaneous ellipsometric and electrochemical measurements. The studies have revealed that significant changes in the interfacial oxide

layer occur as corrosion proceeds. These changes have been, in part, attributed to

a dissolution of the interfacial oxide and this phenomenon is identified as one mechanism of cathodic coating delamination. Additional phenomena such as surface

roughening, oxide film regrowth, and spatial development of cathodes are discussed.

The effects of coating cure and the presence of inhibitors are also presented.

Parts III and IV: The bonding, structure, and composition of passive films are still

controversial subjects. A new surface EXAFS technique, capable of probing a passive

film on iron in its native aqueous environment has been applied to this problem. The

studies show that Fe-Fe distances for passive films formed in nitrite and chromate

solutions are different from those of known iron oxides and oxyhydroxides . Moreover,

differences in the degree of covalency of iron and in the crystallinity of these

films have also been detected.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Chromates; corrosion; electrochemistry; el 1 ipsometry ; EXAFS, Fe-Fe bond lengths;

iron; nitrites; organic coatings; oxide films; passivation

13.

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