NBSIR 83-2790

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, D.C. 20234

November 1 983

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Final Report

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

r

-QC —

100

. U56

33-2790

1983

NBSIR 83-2790

L: c

PASSIVE FILMS, SURFACE STRUCTURE

AND STRESS CORROSION AND CREVICE

CORROSION SUSCEPTIBILITY

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

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Inorganic Materials Division

and

Metallurgy Division

Washington, DC 20234

November 1 983

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Final Report

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

z

Q

C

‘'OfAU O*

o

I*

y» o*

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
*
Part I. To be published in the Proceedings fo the 5th International

’ Symposium on Passivity.

STRUCTURAL STUDIES OF PASSIVE FILMS USING SURFACE EXAFS

Jerome Kruger , 1 Gabrielle G. Long , 1 Masao Kuriyama , 1 and Alan I. Goldman 2

Center ?or Materials Science, National Bureau of Standards,

Washington, DC 20234, (U.S.A.)

2Dept. of Physics, State Univ. of N.Y., Stony Brook, N.Y. 11794 (U.S.A.)

ABSTRACT

Iron K-absorption edge spectra were obtained from the passive films on iron for the dried films in air ( ex situ ) and for the films in the passivating solutions ( in situ ). The ex situ results demonstrate that, while

the structures of the films are more disordered than the spinel -like

iron oxides (e.g. 7

-^

303 ), they are nevertheless closely related to these

crystalline oxides. The in situ data shows evidence of a quite different

structure, which may be due to the accommodation of hydrogen containing

species into the structure.

INTRODUCTION

At the Fourth International Symposium on Passivity, Revesz and Kruger

(ref. 1 ) pointed out that a vitreous structure provides a more effective

passive film, and that the incorporation of "glass forming" elements, e.g.,

chromium, promotes the formation of such a desirable structure. This

paper describes studies whose aim was to determine the structure of

passive films on iron, and to explore the effect of chromium on the structure of these films using both near-edge and extended x-ray absorption fine

structure (EXAFS).

Our earlier EXAFS studies (refs. 2 and 3), involving a specially

designed system (ref. 4) to maximize surface sensitivity, were able to produce ex_ sjjtu (but non-vacuum) EXAFS spectra of the passive films formed

on iron in nitrite and chromate passivating solutions. Those studies

found that the EXAFS signatures of the passive films resemble those of

the spinel -like iron oxides, but that the chromate-formed film, which was

found to contain chromium, may be more vitreous than the spinel -type

oxides.

Our approach to the structural study of passive layers, as described

in the present paper, aims to apply x-ray absorption spectroscopy to the

in situ as well as ex situ investigation of the surface films on iron

formed in two passivating solutions, one containing a glass forming

element (a chromate solution), and the other not containing a glass

1
forming element (fi nitrite solution). These studies extend our previous

work In the following ways: 1. a synchrotron source with high resolution

optics was used, enabling us to examine the absorption edge profiles;

2. thinner Iron substrates were used, resulting In film spectra much

less contaminated by the Iron substrate signal; and 3. In situ spectra

were obtained.

Two kinds of valuable Information were extracted from the x-ray absorption

spectra. First, the spectral features within ~ 30 eV of the Iron K-absorptlon edge are sensitive to effects such as excitons and unoccupied final

states of p-character. The positions in energy space of the K-edge of

of the thin films, relative to that for bulk iron, are directly

related to the oxidation state, the coordination, and the degree of

covalency In the bonding. These are Important clues concerning the

electronic structure of the films. The second kind of Information derives

from the fact that the extended x-ray absorption fine structure (from ~ 30

to 1000 eV above the absorption edge) Is sensitive to the geometric

structure in the vicinity of the absorbing atom. EXAFS spectra of model

Iron oxides have been taken to serve as empirical measures of the Ironoxygen and iron-iron phase shifts In these materials. Then, thin films

on Iron, formed In the inorganic passivating solutions, were measured,

yielding structural parameters different from the model structures.

EXPERIMENT

The substrates used in these experiments were ~ 4 nm iron films

deposited on glass slides in an ultra high vacuum system (base pressure ~ lxl0~8 torr). The iron films were immersed immediately after

removal from the vacuum system into either 0.005 M potassium chromate

or 0.1 M sodium nitrite. Passive films formed upon exposures of over

one week to the passivating solutions have been shown (refs. 5 and 6)

to have structure, composition and formation kinetics similar to those

passive layers that form in Fe ++ - free solutions by anodic oxidation.

After immersion in the passivating solutions for more than a week,

the slides were removed, rinsed in distilled water and spectroscopic

grade ethanol, and dried in a stream of pure nitrogen. Near-edge

and EXAFS measurements were carried out on the passivated specimens

either in air ( ex situ ) or in a specially constructed cell (ref. 4)

for in situ measurements. The spectra were taken at the C-l station at the Cornell High Energy Synchrotron Source (CHESS). This station has a double flat crystal Si (111) monochromator with fixed exit

position, suitable for this experiment. C-ata were taken in the

2
fluorescence mode using Mn0

2 filters in front of the detectors to reduce

the signal due to scattering. An array of three detectors was used to

accumulate data simultaneously during each scan. Data near the edges were taken in 0.5 eV steps, while EXAFS data was taken in 3 eV steps.

Delta collection time for each set required more than one CHESS fill

(2-4 hours of running time), and duplicate sets were acquired for each

In situ and ex situ case.

RESULTS

The edge structures we measured are shown in Figs. 1 ( ex situ and in

situ nitrite-formed films) and 2 ( ex situ and in situ chromate -formed

films). The shapes of the in -situ and ex -situ film profiles are very

different from each other. One can examine how different they are by

taking the slope and plotting the derivative spectra. Fig. 3 shows the

derivative spectra of the profiles in Fig. 1, and Fig. 4 shows those for

the profiles in Fig. 2. It is rather striking that the edges from the

two ex situ samples resemble one another much more closely than they

resemble their respective in situ spectra. Furthermore, the ex situ edge

results are also very similar to our edge data for y-Fe 203 , indicating

again a close relationship between these structures. For the in situ

derivative results, we see two peaks for the chromate -formed film and one peak and a diminished second peak for the nitrite-formed film. While

these two results resemble one another more closely than they do the

respective ex situ data, there remain differences between them. Further

analyses of this data will be forthcoming in a future publication (ref.

7).

Fig. 1. K-edge profiles versus energy

for the nitrite-passivated ex situ (--)

and in situ ( —) films.

3

Fig. 2. K-edge profiles versus energy for the chromate-passi vated

ex situ ( —) and in situ ( —) films.
The magnitudes of the Fourier transforms of the In situ and ex situ

nitrite data and chromate data are shown In Figs. 5(a) and (b). As was

shown earlier (refs. 2 and 3) the ex situ results for the passive films

formed In the two different passivating solutions are quite similar.

Since there Is less bulk Iron signal here than In refs. 2 and 3, the

resemblance between these passive film results and the data for the

crystalline Y-Fe2^3 1s much stronger. Both films appear to be more disordered than the model y-Fe 203 , with the chromate -formed film

showing the less order of the two. The In situ results, shown In Fig.

5(b), suggest that these structures, again while related to each other,

are different from the ex situ structures. Also, It appears that the In situ structures differ from one another more than the ex situ structures

do. All of this Is consistent with the observed changes In the

Fe K-absorptlon edges and the derivative spectra shown In Figs. 1-4.

The details of the analysis will be presented elsewhere (ref. 7) since

the present purpose Is to give an overview of these results.

Fig. 3. Derivative spectra of the In situ (triangles)

and of the ex situ (dots) nitrite edge spectra.

Fig. 4. Derivative spectra of the in situ (triangles)

and of the ex situ (dots) chromate edge spectra.

4
Fig. 5. Magnitudes of the Fourier transforms for the (a) ex situ and

(b) in situ passive films. The nitrite film results are given by the dashed

lines and the chromate film results by the solid lines.

DISCUSSION AND SUMMARY

The use of thinner iron substrates in this work yielded a clearer picture

than before of the close relationship between the ex situ structures of

the passive films on iron. Nevertheless, the chromate -formed structures

were still found to be less well ordered than the nitrite-formed structures,

and this result is interpreted as being due to the incorporation of

chromium into the chromate-formed film, promoting a more glassy structure.

When the passive films were immersed in their respective passivating

solutions, large differences in both the near-edge and the EXAFS spectra were

observed. Both the bonding distances and the coordination are seen to change.

The in situ structures, while again they are related to each other, show

greater differences between their respective structures than do the pair

of ex situ structures. This suggests that hydrogen containing species are

part of the in situ structures, and that the influence of chromium incorporation may be greater for the in situ case. The hydrogen containing species

have been suggested to appear in the structure as protons (ref. 8), water

(ref. 9) or hydroxyl ions (ref. 9), through experiments involving Mossbauer

Spectroscopy (ref. 10), SIMS (ref. 11) and radiotracer methods (ref. 8).

The structures that produce these x-ray absorption spectra reflect the

profound influence that glass formers such as chromium and additives such

as hydrogen may have on the passive film on iron.

5
' ACKNOWLEDGMENT

The contribution of J. Kruger was supported by the Office of Naval Research under contract NAONR 18-69 NRO 36-082.

REFERENCES

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

and J. Kruger, Eds., Electrochem Soc., PrTnceton, NJ (1978) 137 pp.

2 G. G. Long, J. Kruger, D. R. Black, and M. Kurlyama, J. Electrochem.

Soc. 130 (1983) 240 pp.

3 G. G. Long, J. Kruger, D. R. Black, and M.. Kuriyama, J. Interfacial and

Electroanal. Chem. , in press.

4 G. G. Long and M. Kuriyama, Proc. V Int'l Symposium on Passivity.

5 C. L. Foley, J. Kruger and C. J. Bechtoldt, J. Electrochem. Soc. 114

(1967) 994 pp.

6 J. Kruger, J. Electrochem. Soc. 110 [1963) 654 pp.

7 G. G. Long, J. Kruger, M. Kuriyama, A. I. Goldman, D. R. Black, E. Farabaugh and D. Sanders, to be published.

8 H. T. Yolken, J. Kruger, and J. P. Calvert, Corrosion Science 8(1968) 103 c 9 T. Noda, K. Kudo and N. Sato, Z. Phys. Chem. N. F. , 98 (1975) 271 pp.

10 W. F. O'Grady, J. Electrochem Soc., 127 (1980) 555 pp.

11 D. J. Murphy, J. 0'M. Bockris, T. E. Pou, D. L. Cocke, and G. Sparrow,

J. Electrochem. Soc. 129 (1 982) 2149 pp.

6
Part II. To be published in Proceedings 5th International Symposium

on Passivity.

EX-SITU AND IN-SITU SAMPLE-AND-DETECTOR CHAMBERS FOR THE STUDY OF

PASSIVE FILMS USING' SURFACE EXAFS

G. G. LONG, J. KRUGER and M. KURIYAMA

Center for Materials Science, National Bureau of Standards, Washington, DC 20234

ABSTRACT

Two sample-and-detector chambers for the study of surface films on metals

using x-ray absorption spectroscopy are described. Results have been obtained

using both a high intensity rotating anode x-ray generator and using the Cornell

High Energy Synchrotron Source (CHESS).

INTRODUCTION

The structural study of surface passive films on metals requires a high

sensitivity experiment which examines the film in an environment as near as possible to that in which the real films are used. An extended x-ray absorption

fine structure (EXAFS) experiment can be used to probe the near neighbor

distribution in the film, and the near edge spectral features are sensitive to

the electronic structure in the film. The high sensitivity to the surface

signal is achieved here by having an ultrathin substrate layer and by using an innovative detection system. For the in solution ( in situ ) experiment, the x- ray fluorescence product of the K-absorption event is detected, while for the

ex situ (but non-vacuum) experiment the total yield electron product is detected.

THE EXAFS EXPERIMENT

The x-ray absorption experiment requires that a monochromatic (i.e. AE of

the order of 1-10 eV) x-ray beam be continuously tuned through energies from

several eV below a K- or L-absorption edge to about 1000 eV on the high energy

side of the edge. The basic x-ray spectrometers - to be used in EXAFS optics

as "monochromators" - are divided into two classes: flat-crystal instruments

and bent-crystal instruments. The flat-crystal instruments with two or more crystals generally deliver the higher resolution. The bent-crystal instrument

collects and diffracts photons within an energy band pass of the incident beam

and focuses them to a position on the Rowland circle. This method of

monochromatization reconcentrates the beam at the exit slit of the instrument

and is very efficient (ref. 1).

7
EX SITU X-RAY ABSORPTION DETECTOR

A high-pass photocathode x-ray ionization chamber, especially designed for

surface EXAFS (refs. 2 and 3) was adapted for structural studies of passive

films on metals. The sample, of which less than 10% may be the substrate metal

or alloy, serves as the conducting cathode plate Inside the helium filled

detector. A filter assembly in front of the detector limits the energy band

pass of the detector to the region between the absorption edge of the photocathode and that of the filter. This means, for example, that an iron K-edge

absorption experiment would make use of a cobalt filter, so that the high

bandpass of the detector would be between 7.1 and 7.7 keV.

A schematic of the sample-and-detector chamber is shown in Fig. 1. The x- rays enter through an aluminized ii\ylar window and impinge on the sample (which

is also the photocathode) at grazing incidence. The photocathode plate absorbs

the x-rays and emits electrons, which are amplified through field intensified

ionization in the helium gas. The current passing through the detector is

measured by an electrometer connected to the plate. The result is an experiment

similar to total yield spectroscopy in which the electrons are collected as a function of the energy of the incident x-ray beam.

Anode

Fig. 1 The ex situ EXAFS detector and sample chamber.

Four kinds of electrons emerge from the photocathode; photoelectrons,

secondaries, inelastical ly scattered electrons, and Auger electrons. The

photoelectrons are excited simultaneously with the absorption of the x-rays,

but the kinetic energy of these electrons near the absorption edge is very low,

and thus they will not be able to produce much detector current. The secondary

electrons that have been excited by collisions, or the inelastic electrons that

are photo- or Auger electrons that have suffered inelastic collisions, have a wide spectrum of energies. The Auger electrons are emitted by the decay of the core hole states created by the photoabsorption. These electrons have large

kinetic energies. The subsequent decay of the L shell holes is usually
dominated by Auger emission so that it is possible that nearly one electron ray

be excited for each photon absorbed.

The electrons generated near the surface of the cathode undergo field

intensified ionization, increasing the current by a multiplying factor. Further

sensitivity is achieved in that this detector collects over nearly the entire

2 it steradian hemisphere. Using a 12 kW laboratory x-ray generator, focusing

optics (ref. 1) and the present detector, the thin film EXAFS spectra of passive

films shown in Fig. 2 were each obtained in the approximately

ning. The magnitudes of the Fourier transforms of these data

3.

4 hours of scanare given in Fig.

Fig. 2 EXAFS spectra of the films

formed in each of two

passivating solutions.

Fig. 3

r(Z)

Magnitude of the Fourier

transforms for the data

shown in Fig. 2.

_IN SITU X-RAY ABSORPTION CELL

The in situ EXAFS measurement requires that the x-ray absorption spectra be

taken while the thin film is immersed in an aqueous solution where electrochemical studies are performed. Fortunately, water becomes transparent again in

the hard x-ray region of the electromagnetic spectrum. In this case, the x-ray

absorption event is followed by measuring the photon fluorescence product of

the decay of the core hole. Data collection is somewhat hampered by a large

scattering component from the solution. This is brought under control through

the use of an x-ray filter assembly in front of each x-ray detector, so that

the signal is dominated by the fluorescence photons.

9
A schematic of the cell is shown in Fig. 4. All of the elements of the cell are embedded in an inert epoxy so that the surface of the film under

investigation can be brought nearly into contact with the x-ray window. The

reference electrode is connected to a Luggin capillary embedded in the epoxy

with its opening in the plane of the specimen. The Pt counter electrode also

lies in the same plane of the specimen and is connected through the epoxy to

the rear of the cell. The metal sample is deposited on the surface of a glass

slide and connection is made through a plug of bulk metal through the glass.

Fig. 4 The j_n_ situ EXAFS cell.

It is desirable to do the electrochemistry in a rather larger volume of

solution, so the entire cell is mounted in a piston arrangement in a teflon

outer case. This permits us to reduce the "as received" sample and grow the

film on a bare metal surface in a large volume of solution. Once the film has

formed, the current is very small, and the sample can safely be brought to

within a small fraction of a mm of the x-ray window for measurement while under

potentiostatic control.

The cell was tested to demonstrate that a passive film could be formed and

maintained under anodic polarization. The cell was also tested to demonstrate

that spectra could be obtained through the x-ray window and the aqueous

electrolyte. In situ and ex situ data were taken using the C-l monochromator

at CHESS. Fig. 5 shows the magnitude of the Fourier transform for the (a) ex situ and (b) in situ data, both taken in high resolution geometry. Fig. 5(aT

is to be compared with Fig. 3 which was derived from laboratory data using the

ex situ detector. Data collection times were similar.

10
Fig. 5 Magnitudes of the Fourier transforms for the (a) ex situ and (b) in

situ passive films. The nitrite-formed film results are given by the

dashed lines and the chromate-formed film results by the solid lines.

REFERENCES

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

51 (1980) 273 pp.

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

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

11
Part III. To be submitted to Journal of Coatings Technology.

El 1 1 psometric Studies of Chelating Inhibitor Effects on the

Cathodic Delamination of an Organic Coating on Iron

J. J. Ritter

Inorganic Materials Division

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

Qualitative ellipsometry has been used to study the effects of

chelating inhibitors on the cathodic delamination of an acrylic coating

from an iron surface. The chelating inhibitors 8-hydroxyqui noline (8-

HQ) and 2,5 dimercapto 1,3,4 thiadiazole (DMTDA), when dispersed in the

coating were observed to delay the onset of delamination. Catechol was

found to be an ineffective inhibitor. A similar beneficial effect was

noted with a two-layer system employing a zinc chromate primer. However,

when 8-HQ and DMTDA were applied by an anodic pretreatment procedure

they were relatively ineffective, whereas 4-methyl catechol similarly

applied, exhibited impressive inhibition.
Ellipsometric Studies of Chelating Inhibitor Effects on the

Cathodic Delamination of an Organic Coating on Iron

J. J. Ritter

Inorganic Materials Division

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

INTRODUCTION

Previous publications from our laboratories have shown the effectiveness of qualitative ellipsometry in the study of interfacial cathodic events during the corrosion of painted metal surfaces Since

coatings bond to the metal oxide films, and since many corrosion inhibitors seem to function at or within this interfacial metal oxide

film, it seemed a natural extension of our previous work to examine the

effects of corrosion inhibiting species on cathodic delamination. While

many paint systems in service presently rely heavily on inorganic

species to achieve inhibition, an active search for substitute materials

goes on to obtain more effective inhibition, reduce costs, meet environmental standards, and minimize the consumption of strategic metals.

We selected a small group of organic molecules with a potential for

forming coordination complexes with iron. The effectiveness of these

types of chelating molecules in corrosion inhibition has been described

5-6

previously by other investigators.

The present work will describe the cathodic delamination behavior

of acrylic coated iron where chelating inhibitors have been supplied in

two different ways. In the first method, the inhibitor is homogeneously

dispersed within the coating; while, in the second method, the inhibitor

is applied in a two step anodization process, as recently described by

13
Leidheiser et al.^ An acrylic coating is applied over this pretreated

surface.

EXPERIMENTAL

Iron specimens were mounted in epoxy and finished on a series of

SiC papers of 320, 400, and 600 grit. Final polishing was accomplished

with 6.0, 1.0, and 0.05 ym abrasives.

A clear, proprietary acrylic coating material was dissolved in

toluene and specimens coated by dipping. The chelating inhibitors

employed and their structural formulae are given in Fig. 1. Chelating

inhibitors were dispersed in the coating material in the 1 to 2 percent

range by first dissolving the inhibitor in toluene and then using this

toluene as the solvent to prepare the coating medium. The two layer,

nine percent zinc chromate primer system shown in Fig. 2 was prepared by

pressing a 6.0 mm dia. polytetrafluoroethylene cylinder to the center of

the metal surface and applying the proprietary primer by brushing.

After the primer had cured, the PTFE mask was removed, and the clear

acrylic topcoat was applied by dipping. The 6.0 mm view of the metal

surface through the clear acrylic provided the necessary observation

"window" for the el 1

i psometric experiments on this specimen.

The two step anodization procedure for the applications of chelating inhibitors was adapted from the method described by Leidheiser

et al.^ The method is summarized as follows. An uncoated, mounted iron

specimen is aligned in the ellipsometer cell and submerged in 170 ml

pH 8.4 borate buffer solution. The solution is deaerated with N

2 and

the specimen is observed el 1 ipsometrical ly under open circuit conditions

for a, 1.0 h. An anodic film is grown on the metal surface with an

impressed voltage of 1.2 V S.C.E. After •v 10 min of anodization, 50 ml

14
of borate buffer containing a quantity of chelating material calculated

-4

to give ^10 M chelate concentration in the total solution is added.

Rapid mixing is effected by the continuing N

2

flow, and the potential

was held at 1.2 V S.C.E. until the inhibitor layer is well developed.

The layered structure is shown schematically in Fig. 3. The specimen is

removed from the cell, rinsed with water, dried with an air jet, and

coated with acrylic by dipping. Typical coating cure times were 24 h at

25 °C.

The corrosion of all coated specimens was conducted in 0.05 M NaCl

at 25 °C. Typically, the coated specimens were observed for 400 to

500 min. before the corrosion process was initiated by puncturing the

coating ^ 0.5 cm from the ellipsometric observation region, as shown in

Fig. 2. Ellipsometric data were acquired at a rate of one set of points

per minute, under computer control.

Interpretation of Ellipsometric Behavior for the Iron/Acrylic System

For comparison purposes, a typical ellipsometric response curve for

acrylic coated iron undergoing corrosion in 0.05 M NaCl is shown in

Fig. 4. A detailed interpretation of this type of response has been

4 given previously, but a brief summary is provided here. Previous

observations on uncoated iron systems under potential control in either

borate buffer solutions or in saturated NaOH solutions indicate qualitatively

that simultaneous changes in the A and ^ parameters in opposite directions

is indicative of a changes in the metal oxide film thickness. A simultaneous

change of a and i|> in the same direction is indicative of a surface

texture change; i.e., surface roughening or smoothing. The coating on

the system shown in Fig. 4 was punctured at t = 0. At about 500 min,

the A signal rises while the \p signal simultaneously declines. These

15
changes are interpreted as a thinning of the interfacial oxide film on

the basis of our ellipsometer observations on uncoated iron in a controlled

environment. Cathodic regions under coatings are known to become hiqhly

alkaline. 1 3 In earlier work/ we have shown that native oxide films on

iron will dissolve (i.e., become thinner) in hiqhly alkaline media.

Since the puncture site is the focus of anodic activity, cathodic sites

develop elsewhere under the coating, eventually reaching the reqion

being observed el 1 ipsometrical ly. Thus, the observed mode of delamination

for the system shown in Fig. 4 is the chemical dissolution of the interfacial

oxide film. Subsequent el 1

i psometric changes show events such as surface

roughening and reprecipitation of dissolved iron species as indicated on

Fig. 4. Experience gained from experiments stopped soon after surface

roughening is detected ell ipsometrically indicates almost invariably

that the coating can be readily detached with forceps. This evidence

leads us to assert that surface roughening is a post-delamination event.

Thus, the el 1

i psometrical ly detected onset of surface rougheninq can be

used as an indicator for the failure of the coating in the el 1 i psometrical ly

monitored region of the specimen.

RESULTS AND DISCUSSION

Iron/Acrylic System with Coating Dispersed Inhibitors

Catechol

Figure 5 shows the el 1

i psometric responses for the iron/acrylic/

catechol system. The vertical arrow (+) in this figure and in subsequent

figures marks the time at which the coating was punctured. Note that

the el 1

i psometric activity prior to puncture suqgests possible subcoatinq

development of an inhibitor film (A declines, ^ rises). However, within

16
about 100 min after puncture, surface roughening is indicated (a and ^

decline in concert). Obviously, catechol provides no inhibition; and,

in fact, when compared to the post-puncture time scale in Fig. 4, may

actually accelerate the delamination process. This result is not

surprising, since catechol reportedly offers little protection to iron

5 systems.

2,5 dimercapto-1 ,3,4 thiadiazole (DMTDA)

Figure 6 shows the el 1 ipsometric responses for the iron/acrylic

system with DMTDA dispersed in the coating. The pre-puncture signals

suggest changes in surface texture (probably surface smoothing) as the

inhibitor interacts in the interfacial region. After the coating is

punctured, the onset of delamination is delayed nearly 1800 min, whereas, in the uninhibited system (Fig. 4), delamination is typically seen

within 500 minutes. Note also, that A and \[> initially decline in concert, indicating that delamination has occurred just previously, and

surface roughening has set in. In comparing this behavior to that seen

in Fig. 4, the lack of initial rise in A is interpreted as delamination

having occurred by a mechanism other than oxide film dissolution. The

presence of the inhibitor has not only delayed the onset of delamination, but has probably also altered the mode of delamination.

8-hydroxyguinol ine (8-HQ)

Figure 7 shows the case for inhibition by 8-hydroxyquinol ine. Note

very little pre-puncture activity but that A and \p converge very slowly

after the coating is damaged. This convergence is interpreted as the

development of an interfacial chelate layer. Finally, after 'v 5000 minutes, delamination sets in with a probable initial chemical destruction

of the chelate/oxide interface, followed by the expected surface rough17
ening phenomenon. This inhibitor delays the onset of delamination but

probably does not alter the mechanism.

Zinc Chromate Primer/ Acryl ic System

For comparison purposes, the zinc chromate primer/acrylic topcoat

system was studied under comparable conditions. Figure 8 shows a decline in A and a very slight rise in \p, suggesting a possible ingress

of chromate into the interfacial region. Delamination is delayed by

about 5000 min; a performance rather comparable to that of 8-hydroxyquinoline. Note that when the large el 1 ipsometric changes occur, A and

V move in concert, signifying the onset of surface roughening without

preceeding oxide dissolution; i.e., A and \p do not show an initial

divergence, as in Fig. 4. Thus, in this system, delamination is delayed

and probably occurs by some mechanism other than that of oxide dissolution.

Inhibitor Application During Anodization

Leidheiser and co-workers at Lehigh University have recently developed a two step anodization process for the application of chelating

inhibitors to uncoated surfaces.^ This work has prompted us to examine

this procedure ell ipsometrically and also study the corrosion performance of coated samples which had been subjected to this method of

inhi bi tor appl ication.

El 1 ipsometric Changes During the Two Step Anodization

Figure 9 shows, initially, the stable el 1 ipsometric response

for uncoated iron at open circuit potential in borate buffer solution.

At t ^ 50 min, a potential of +1.2 V S.C.E. was applied to the specimen.

Delta declines sharply, ip rises slightly but measurably. These changes

signify the growth of an anodic film on top of the existing native oxide

18
film. At t ^ 65 min, the potential being held at +1.2 V S.C.E., 8-

hydroxyquinol ine was added to the solution. Delta and \p changes indicate the growth of about 10 nm of additional film, presumably incorporating the chelating inhibitor.

Figure 10 shows the somewhat more complex behavior of DMTDA application with this procedure. The initial anodization at t % 120 min

proceeds as noted earlier. However, upon the addition of DMTDA, film

growth is followed by surface smoothing and then by surface roughening

before a stable situation is achieved at t ^ 150 min. The final disposition of A and ip values beyond 150 min suggest that some chelate film

is probably present, although the preceeding surface texture changes

(i.e., the smoothing and roughening) render the estimation of its thickness difficult.

A third case of inhibitor application by two step anodization is

shown in Fig. 11. The early behavior of the specimen at open circuit

and anodization is similar to the preceeding two examples. However, the

addition of 4-methyl catechol produces only a small gradual change in A

and a very noticeable jump in These changes are interpreted to arise

from the growth of a rather thin (one monolayer or less) chelate film.

Thus, in the application of different inhibitors by the two step

anodization process, we have observed very diverse modes of surface

modification, including the formation of thick films, surface texture

changes, and the growth of thin films.

The Effects of Two Step Anodization Pretreatment on the Corrosion of

Coated Iron

Figures 12 and 13 show the respective performances of the acrylic

coating over 8-HQ and DMTDA pretreated surfaces. In each case, delamination is seen within a few hundred minutes after the coatings are

19
damaged, as evidenced by the early large changes in A and ^ . In the

case of the 8-HQ treated surface (Fig. 12), the delamination appears to

be due to some dissolution of the anodic film, since A and ^ initially

move in opposing directions. This detail is not seen in the case for

DMTDA (Fig. 13) and suggests that a different delamination mechanism may

be operative. The case for the specimen anodized in the presence of

4-MC (Fig. 14) is particularly interesting when compared to the other

inhibitors studied. Note that, from t = 0 to t ^ 4800 min, A and show

a concerted decline of only a few degrees, suggesting rather minor

surface texture changes. At about 4800 minutes, A begins to increase

slowly, while ^ continues to decline, suggesting a very gradual dissolution

of the anodized layer and a concomitant gradual delamination. Indeed,

at the termination of this experiment at 9000 minutes, the coating was

found to be rather well adhered to the specimen surface. In contrast,

the coatings on the 8-HQ and DMTDA pretreated systems could be easily

detached from the substrate after only 6000 minutes.

SUMMARY

For the iron/acrylic system studied, chelating inhibitors homogeneously dispersed in the coating affect the coating failure process by

delaying the onset of delamination and/or altering the mechanism of

delamination. DMTDA and 8-HQ are effective in delaying coating failure,

whereas catechol is not. Comparable results for inhibition have been

obtained with a zinc chromate primer/acryl ic topcoat system. Our ellipsometric observations on the deposition of different chelating inhibitors during a two step anodization have shown a wide range of behaviors,

including the formation of relatively thick deposits, metal dissolution,

and the deposition of near-monolayer films. DMTDA and 8-HQ are not

20
effective inhibitors when anodically applied, whereas good results were

obtained with 4-MC.

Thus, our results indicate that, for a given coating system, both

the method of chelating inhibitor introduction and its chemical nature

can affect its performance in retarding coating failure.

ACKNOWLEDGEMENTS

The author would like to thank Dr. J. Kruqer, NBS, for encouragement and many useful suggestions. This work was partially funded by the

Office of Naval Research, under contract number NAONR 18-69 NRO 36-082.

21
REFERENCES

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

2. Ritter, J. J. and Kruger, J., "Corrosion Control by Organic Coatings", Henry Leidheiser, Jr., ed., Ntl . Assoc. Corr. Enq., Houston,

TX (1981) p. 28.

3. Ritter, J. J. and Rodriguez, M. J., Corrosion, 38^, 223 (1982).

4. Ritter, J. J., J. Coatings Tech., 54_, 51 (1982).

5. Weisstuch, A.; Carter, D. A.; and Nathan, C. C., Matls. Protection

& Performance, U), 11 (1971).

6. Riggs; Olen, L., Jr., "Corrosion Inhibitors", C. C. Nathan, ed.,

Ntl. Assoc. Corr. Eng., Houston, TX (1973).

7. Leidheiser, H., Jr. and Konno, H., J. Electrochem. Soc. (1983).

22
FIGURE CAPTIONS

1. Structural formulae for the chelatinq inhibitors.

2. Proposed layered structure for specimen pretreated with an inhibitor in a two-step anodization process.

3. Cutaway view of the specimen arrangement used for the zinc chromate

primer/acrylic toDCoat system.

4. Typical ell ipsometric response curve for an acrylic-coated iron

specimen undergoing corrosion in dilute NaCl solution.

5. Ell ipsometric responses for corrosion in the iron/acrylic system

with 1.4 percent catechol dispersed in the acrylic coating.

6. Ell ipsometric responses for corrosion in the iron/acrylic system

with DMTDA dispersed in the acrylic coating.

7. Ell ipsometric responses for corrosion in the iron/acrylic system

with 1.7 percent 8-hydroxyquinol ine dispersed in the acrylic coating.

8. El 1 ipsometric responses for corrosion in the iron/zinc chromate

primer/acryl ic topcoat system.

9. Typical ell ipsometric responses for the application of 8-hydroxyquinoline in a two-step anodization procedure.

10. Typical el 1 ipsometric responses for the application of DMTDA in a

two-step anodization procedure.

11. Typical ell ipsometric responses for the application of 4-methyl

catechol in a two-step anodization procedure.

12. Ell ipsometric responses for the iron/acrylic system undergoing

corrosion where the iron surface had been pretreated with 8-HQ in a

two-step anodization process.

23
13. Ellipsometric responses for the iron/acrylic system undergoinq

corrosion where the iron surface had been pretreated with DMTDA in

a two-step anodization process.

14. Ellipsometric responses for the iron/acrylic system undergoinq

corrosion where the iron surface had been pretreated with 4-MC in a

two-step anodization process.

24
CDO

M

4-methylcatechol

catechol
ell

iptically

polnrized o

VO £o

ino

<?

VO

CVJ
comp 1 ex/ inhibitor

layer

-anodic film

native oxide

27
140

(6ep) isd OOlCONUJtfjyfON-OOlOONIfiin^lONr-O yK)K)IOK)«H)K)»OIOlONNNNNNNNNN

M I I'j I I ! I

|

I lO O

K> rO

i

j

i m f-tH +-) n h | n n |i in o m o in

(N <N — O

H-+

o

o

I

I

I I I t o

in o

o> cn (6ep) vrod

500

tooo

1500

2000

2500

TIME

(min)
140

(6»P) ISd

m o

to lO

m o

Cs| CM

(6ep) V113Q

500

1000

1500

2000

2500

3000

3500

4000

4500

TIME

(min)
130

(6ep) ISd

o o o

cs o

(6©p) V113Q

90

-h

whi

n|iim)iii|nniim|miiiiii|iiiimi

i|umuu|tMMmt

|

nmn

ii|

mmu»|

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

TIME

(min)
13U

(6ep) ISd

o in

K)

o in

K> CM o

(6©p) vn3Q

TIME

(min)
150

(

6e P) ISd

K) CM o> oo

momomomo ^^KJKTCNCS-r-*-

(6ep) vn3<3

1000

2000

3000

4000

5000

6000

7000

8000

9000

TIME

(min)
105

(Sep) ISd

o m o in ^ m m cs

(Sep) vroa

TIME

(min)
(6ep) V113Q

100

120

140

160

ISO

200

220

TIME

(min)
115

o

K}

(6ep) ISd

cn

(N

oo

<N

C

£

ui

LDCO

m

o

o

o

m

cn ocn (6©p) Yii3a
140

(6ep) ISd

o in o

rO IO

(6ep) V113Q

1000

2000

3000

4000

5000

6000

TIME

(min)
130

mom(6ep) |Sd

K> K) Cvi

(6#p) V113Q

1000

2000

3000

4000

5000

TIME

(min)
134

ifi'tffOCMt- (6sp) ISd

K) hO fO fO tO

(6ap) V113Q

1000

2000

3000

4000

5000

6000

7000

8000

9000

TIME

(min)
Part IV To be published in the Proceedings of International Conference

on Ellipsometry and other Optical Methods for the Analysis of

Surfaces and their Films.

Further Development of the Qualitative El 1 ipsometri c Technique for the

Study of Corrosion Processes under Organic Coatings.

J. J. Ritter and J. Kruger

Center for Materials Science

National Bureau of Standards

Washington, DC, USA

39
Resume

Le developpement d'une technique ell i psometrique utilisee pour etudier les

processus de corrosion sous revetements organiques est decrite, Ce

developpement comprend la capacite' de detecter 1' influence de differents

revetements organiques, de distinguer entre 1* apparition de rugosite'

et la croissance d'un film ou la dissolution, enfin d'etudier le processus

de corrosion se produisant sur des sites anodiques et cathodiques separes.

Abstract - We describe the further development of an ellipsometric

technique used to study corrosion processes under organic coatings.

These developments include the ability to (1) detect the effect of

different organic coating, (2) distinguish between surface roughening and

film growth or dissolution, and (3) study corrosion processes occurring on separate anodic and cathodic sites.

The qualitative ellipsometric technique developed for the study of

corrosion processes under organic coatings described at the International

Conference on Ellipsometry held in Berkeley, California has been

refined and extended. These new developments which will be described

in this paper are the following. (1) the demonstration that the

ellipsometer is capable of revealing the effect of using different

organic coatings; (2) the development of an ellipsometric method to

distinguish between surface roughening and film growth or dissolution

processes occurring under organic coatings and (3) the development of

a segmented specimen technique that allows an ellipsometric study

of the separate anodic and cathodic areas that form under an organic

coating.

Studying the Effect of Organic Coating Type

Previous studies (1,2) applying ellipsometry to the investigation of

corrosion processes under organic coating have utilized air-cured

collodion (cellulose dinitrate), assuming that the nature of the

transparent organic coating would not have a significant effect on the change of the ellipsometric parameters, A and during the

early stages of the corrosion processes occurring under the coatings.

As Figs, la and b show, this is not so. Indeed, both the nature of

the polymer constituting the coating and the temperature of the cure

(heat treatment) strongly influence the changes in A with time when

the coated metal is immersed in an aqueous electrolyte. The interpretation of Fig. la (the air cured room temperature coatings) is given

in Fig. 2 and discussed in the next section. When the coatings are cured in air at an elevated temperature (200°C), it is more difficult

to give a detailed interpretation. The large rise in A at the beginning

of the corrosion process is probably due to the dissolution of a rather

thick oxide film (~ 14 n m) formed during the curing process of 200°C.

The events beyond the initial sharp rise in A are more difficult to

interpret. A more detailed discussion is given by Ritter /3/.

40
Fi g„ la - Variation of A with time

for three iron/coating systems

undergoing corrosion in an aqueous

chloride solution. The coatings

shown were cured in air at room temperature.

Fig. lb - Same as Fig. la

but the coatings were cured

in air at 200°C

Distinguishing between Roughening and Film Growth or Dissolution

Fig. 2 shows A and plots vs. time obtained using an automatic

ellipsometer for an iron surface coated with a room temperature

air-cured acrylic undergoing corrosion in 0.05M NaCl solution.

Three regions can be delineated. The significance of these regions

are described in Fig. 2. It can be seen in Fig. 2 that when dA/dt

and d ip/dt have the same sign surface roughening or surface

smoothing the coated iron to taking place; when dA/dt and d\p/dt

have opposite signs, film growth or dissolution occurring.

This interpretation of the behavior of A and '4) is based on our observations of uncoated iron in a buffered solution after carrying out potential

cycling, a process which produces a smoother surface. More details are given in an earlier paper /2/.

Observing Corrosion Processes at Anodic and Cathodic Sites

Fig. 3a shows the arrangement developed by Ritter /3/ for studying the

spatial development of subcoating cathodes and anodes using the

ellipsometer. By puncturing one segment of the segmented specimen, one

can promote the immediate formation of an anode on that specimen. This

procedure then allows one to follow the real time development of cathodes

on the unpunctured segment.

Fig. 3b gives the variation of A and pH (an important parameter in

subcoating corrosion) with time for the anodic ("a") and cathodic ("c")

segments. During the first 1400 min before the cpating is .punctured, r there is little change in A for both segments. The pH varies between b

and 8. When the segment "a" is punctured at 1400 min, thereby making it

41
Fig. 2 - A and ^ plots vs. time made using an automatic ellipsometer

for an iron/acrylic coating system undergoing corrosion in 0.05M NaCl

solution.

the anode, a significant rise in A is observed. If the segments "a" and

"c" are connected by joining the copper wire conductors attached to each

(shown in Fig. 3a), the pH on "c" rises to about 13.5 and the A measured

for "c" by the ellipsometer remains relatively constant. The change in

A found for "a" however, is similar to that shown in Regions I and II in

Fig. 2. When the conducting wires to "a" and "c" are disconnected, the

subcoating pH drops to 8 in a few minutes. Reconnecting brings the pH

back to 13 within 5 min. If the anodic and cathodic segments are left

coupled for a few days, the subcoating pH on "c" declines, A for "a"

decreases markedly while A for "c" also goes down but not as much.

A detailed discussion of the meaning of these results in terms of corrosion

processes under coatings is given by Ritter in an earlier publication /3/.

Summary

1. It has been established that the ellipsometer is capable of

revealing the effect of different organic coatings on the subcoating

corrosion proceses.

2. Using an automated ellipsometer, it has been demonstrated for

the iron systems studied that when dA/dt and d^/dt have opposite

signs, corrosion product films are growing or dissolving and when they

have the same sign, surface is becomming smoother or rougher.

Thus, surface roughening surface smoothing and film growth processes or dissolution can be identified.

42
Fig. 3a - The segmented specimen

that allows the el 1 ipsometric

examination of the development

of anodic ("a") and cathodic

("c") sites on coated iron.

Fig. 3b - A, \\> and pH

plotted vs time for a segmented iron /acrylic

system in 0.05M NaCl

solution. Inset shows the

arrangement on the surface

of the specimen, the ellipses

shown being the regions

studied el 1

i psometrical ly

3. A segmented specimen has been developed that allows el 1 ipsometri

c

study of separate anodic and cathodic areas that form under an organic

coating.

Acknowledgement

The authors are grateful to the Office of Naval Research who partially

supported this work under contract No. NAONR 18-69. NRO 36-082

References

/l/ RITTER, J.J. and KRUGER, J., Surf. Sci . 96^ (1980) 364.

/2/ RITTER, J.J. and KRUGER, J., Corrosion Contral by Organic Coatings,

H. Leidheiser, Jr., Ed., (National Association of Corrosion

Engineers, Houston, TX) 1981, p. 28.

/3/ RITTER, J.J., J. Coatings Technology 54 (1982) 51.

43

SECURITY CLASSIFICATION OF THIS PAGE (Vfhen Data Bnlarad) REPORT DOCUMENTATION PAGE READ INSTRUCTIONS

BEFORE COMPLETING FORM

T7 REPORT NUMBER

NBSIR 83-2790

2. GOVT ACCESSION NO. 3. RECIPIENT’S CATALOG NUMBER

Passive FiTms Surface Structure and Stress

Corrosion and Crevice Corrosion Susceptibility

5. TYPE OF REPORT 4 PERIOD COVERED

Final Report

10/82 - 9/83

6. PERFORMING ORG. REPORT NUMBER

7. AUTHORf*)

J. Kruger, J. J. Ritter, G. G. Long, M. Kuriyama

and A. I. Goldman

8. CONTRACT OR GRANT NUMBERfs)

NA 0NR 18-69 NR0 36-082

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Inorganic Materials Div. aNd Metallurgy Div.

National Bureau of Standards

Washington, DC 20234

10. PROGRAM ELEMENT. PROJECT, TASK

AREA 4 WORK UNIT NUMBERS

5610448

H. CONTROLLING OFFICE NAME AND ADDRESS

Office of Naval Research

Arlington, VA 22217

12. REPORT DATE

Nov. 1983

13. NUMBER OF PAGES

14. MONITORING AGENCY NAME 4 ADDRESSf// different from Controlling Office)

Same as 1 1

.

15. SECURITY CLASS, (of thla report) Unclassified

15*. DECLASSIFICATION/DOWNGRADING

SCHEDULE

16. DISTRIBUTION STATEMENT (of thla Report)

Uni i mi ted

17. DISTRIBUTION STATEMENT (of the abstract entered In Block 20, If different from Report)

Unlimited

18. SUPPLEMENTARY NOTES

19. KEY, WORDS. (Continue on reverse side If necessary and Identity by block number)

Passive films, organic coatings, chelating inhibitors, ellipsometry, EXAFS,

iron oxide films, electrochemistry, cathodic delamination.

20. ABSTRACT (Continue on reverse side If necessary and Identify by block number)

I. Iron K-absorption edge spectra were obtained from the passive films on iron for the dired films in air (ex situ) and for the films in the passivating solution (in situ). The ex situ resutls demonstrate that., while

the structures of the films are more disordered than the spinel -like

iron oxides (e.g. Y-Fe-Oj , they are nevertheless closely related to these

crystalline oxides. The in situ data shows evidence of a quite different

structure, which may be due to the accommodation of hydrogen containing (Cont)

DD 1 JAN^73 1473 EDITION OF 1 NOV 65 IS OBSOLETE

S/N 01 02-LF-01 4-6601 SECURITY CLASSIFICATION OF THIS PAGE (Whan Data Bntarad)
SECURITY CLASSIFICATION OF THIS PAOE fWhMi Data SntaradQ

Block 20 Cont'd

species into the structure.

1 1. Two sample-and-detector chambers for the study of surface films on metals using x-ray absorption spectroscopy are described. Results have

been obtained using both a high intensity rotating anode x-ray generator

and using the Cornell High Energy Synchrotron Source (CHESS).

III. The effects of chelating inhibitors on the cathodic delamination

of an acrylic lacquer on iron have been studied by qualitative

el 1 ipsometry. The experiments indicate that both the chemical nature

of the inhibitor and the mode of Inhibitor intorduction affect the rate

of coating failure.

IV. The development of an elllpsometer technique to study corrosion

under organic coatings has been advanced to 1) detect the effect of

oxide film growth or dissolution, 2) distinguish between surface

roughening and film growth, and 3) study corrosion on separate anodic

and cathodic sites.

SECURITY CLASSIFICATION OF THIS PAGE(TWi«n Data Bn(arad)
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 83-2790 November 1983

4. TITLE AND SUBTITLE

PASSIVE FILMS SURFACE STRUCTURE AND STRESS

CORROSION AND CREVICE CORROSION SUSCEPTIBILITY

5. AUTHOR(S)

J. Kruger, J. J. Ritter, G. G. Long, M. Kuriyama, and A. I. Goldman

6. PERFORMING ORGANIZATION (If joint or other than N BS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

S. Type of Report & Period Covered

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

Inorganic Materials Div. and Metallurgy Div.

National Bureau of Standards

Washington, DC 20234

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) I. Iron K-absorption edge spectra were obtained from the passive films on iron

for the dried films in air ( ex situ ) and for the films in the passivating

solution (in situ) . The ex situ results demonstrate that, while the structures

of the films are more disordered than the spinel -like iron oxides (e.g. y-FeoOo)*

they are nevertheless closely related to these crystalline oxioes. The i_n_ sTto

data shows evidence of a quite different structure, which may be due to the accommoda

tion of hydrogen containing species into the structure.

II. Two sample -and-detector chambers for the study of surface films on metals

using x-ray absorption spectroscopy are described. Results have been obtained

using both a high intensity rotating anode x-ray generator and using the Cornell

High Energy Synchrotron Source (CHESS).

III. The effects of chelating inhibitors on the cathodic delamination of an acrylic lacquer on iron have been studied by qualitative ellipsometry. The

experiments indicate that both the chemical nature of the inhibitor and the mode

of inhibitor introduction affect the rate of coating failure.

IV. Ell ipsometric study of corrosion under paints advanced to 1) detect film dissolution, 2) separate surface roughening and film growth effects, 3)study anodes & catlrode.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cathodic delamination; chelating inhibitors; electrochemistry; electrochemistry;

ellipsometry; EXAFS; iron oxide films; organic coatings; passive films

13. AVAILABILITY

fcunl imited

j ] For Official Distribution. Do Not Release to NTIS

~~l Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[X^] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

46

15. Price

$8.50

U S COMM- D C 6043-P80

V