NBS

PUBLICATIONS

NBSIR 80-2101 (Navy)

AlllOS 15flOBL

Passive Films, Surface Structure

and Stress Corrosion and

Crevice Corrosion Susceptibility

J. Kruger, J. J. Ritter, J. J. Carroll, and A. J. Melmed

Chemical Stability and Corrosion Division

and

Surface Science Division

National Bureau of Standards

U S. Department of Commerce

Washington, D C. 20234

August 1 980

Technical Summary Report No. 10

Prepared for

% —

^Office of Naval Research

^Department of the Navy

100 Arlington, VA 22217

,U56

80-2101

1980

c. 2

NBSIR 80-2101 (Navy)

PASSIVE FILMS, SURFACE STRUCTURE

AND STRESS CORROSION AND

CREVICE CORROSION SUSCEPTIBILITY

National Bureau ot Standards

library, E-01 Admin. Bldg

OCT 2 4 1980

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no. So-^fol

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J. Kruger, J. J. Ritter, J. J. Carroll, and A. J. Melmed

Chemical Stability and Corrosion Division

and

Surface Science Division

National Bureau of Standards

U.S. Department of Commerce

Washington, D C. 20234

August 1980

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Prepared for Office of Naval Research

Department of the Navy

Arlington, VA 22217

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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[Part I -Accepted for publication in Surface Science]

A Qualitative Ellipsometric-Electrochemical Approach

for the Study of Film Growth Under Organic Coatings

J. J. Ritter and J. Kruger

National Measurement Laboratory

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

Abstract

A feasibility study was made to determine if qualitative ellipsometry

can be used together with electrochemical pH and potential measurements as a

technique for the study of metal substrates protected by transparent organic

coatings. The objective of the work was to gain a deeper understanding of

the mechanisms governing the corrosion protective actions of paints on metal

surfaces. Computer modeling and experiments with collodion coatings on iron

substrates indicated that changes in the ellipsometric parameter A could, for

the most part, be safely interpreted as thickness alterations in the substrate

oxide film. Experiments with the Fe-collodion system in dilute chloride

solutions exhibited three sequential stages of activity, two of which could

be interpreted in terms of corrosion mechanisms using the optical and electrochemical measurements. Chromate ion as a corrosion inhibitor in coatings was

also studied using this technique, and was shown to have significant effects

upon the development of the sub-coating processes.
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A Qualitative Ell ipsometric-Electrochemical Approach

for the Study of Film Growth Under Organic Coatings

J. J. Ritter and J. Kruger

National Measurement Laboratory

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

A recent book [1] Corrosion Control by Coatings gives examples of many

areas where there is still a great lack of understanding of the processes

that control the corrosion protection actions of paints. Of special value

for addressing this need would be a technique or techniques that could carry

out in situ studies of the growth and dissolution of the oxide films and

other films that exist on a metal surface covered by a paint and how changes

in the composition of the paint affects these films. Also of great value

would be a technique that could detect changes in the organic coating that

constitutes the paint, changes in the environment that develops under the

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

result in detachment of the paint from the oxide covered metal surface.

This paper presents a description of the first attempts to evaluate

whether ellipsometry can serve as a tool for examining the processes that

occur under paint films. The approach simulates the painted metal circumstance

by using a transparent organic coating deposited on a metal surface. Particles

of solid chemicals (e.g., chromates) can be placed around the ellipsometer

observation region and under the coatings in simulation of corrosion inhibitors

found in paint formulations. These dissolve slowly and modify the environment

under the organic coating. A pH probe, located under the coating and at the

metal surface, allows monitoring of the sub-coating environment.
Obviously, such a system is too complicated to permit rigorous quantitative el 1 ipsometric determination of film or environmental optical constants

or film thicknesses. Instead, the approach employed in the work described

here is the qualitative one described in the Third International Conference

on Ellipsometry [2]. This approach measures the time dependence of changes

which occur as indicated by significant alterations in A, ^ and correlates

these changes with concomitant electrochemical measurements. Thus, for

example, the rate of change of film thickness (assumed proportional to A) can

be monitored. To help interpret the changes of A and ip observed, and to

judge whether the magnitude and direction are realistic and reasonable, model

calculations using the Drude equations and reasonable optical parameters were

carried out.

An extension of this work will include repassivation rate studies in

simulated sub-coating environments by the triboell ipsometric technique [3].

EXPERIMENTAL

Metallic specimens were cast in epoxy resin and polished on a series of

successively finer abrasives, the final abrasive being 0.05 ym aluminum

oxide. The iron specimens were provided with a Pt wire electrode and an

opening to accomodate a commercial 1.5 mm di a. pH microprobe, as shown in

Fig. 1. During the coating process, the pH probe opening was closed with a

snug fitting polytetrafluoroethylene (PTFE) plug to assure the formation of a

monolithic coating across the entire face of the specimen and mount. In

general, collodion (cellulose dinitrate) and methanol were mixed 1:1 on a

volume basis and the specimen face coated by dipping. The coated specimens

_3 were air dried for about 0.5 h and subjected to a vacuum curing at about 10

torr for 1.5 h. Cured coatings with thickness ranging from 0.01 to 0.03 mm

were obtained in this manner.

- 2 -
For some experiments, a suspension of finely ground K

2 CrOz, in methanol

was placed dropwise directly on the polished iron surface. The evaporation

of solvent left "islands" of I^CrO^ (Cf. Fig. 2) which were then coated

over by dipping the specimen in a 1:1 collodion-methanol mixture. After the

vacuum curing step, the PTFE plug was extracted from the rear of the specimen

mount and the micro pH probe installed so that it just touched the underside

of the coating. The probe lead wire was sealed to the rear of epoxy mount with

a small quantity of vacuum putty.

Prepared specimens were mounted vertically and optically aligned in

a PTFE- lined cell. Reference electrodes for the electrochemical measurements

were connected to the system through Luagin capillary salt bridges set in

the bulk solution. Zero time for the experiments was taken at the moment

of specimen immersion in 0.05 N NaCl solution.

Infrared spectra of the coatings on the specimens before and after

immersion were obtained by reflectance techniques using a commercial double

beam infrared spectrophotometer.

Specimens were maintained at open circuit potential which was measured

with respect to a saturated calomel electrode and reported with respect to

a standard electrode.

Computer modeling was done on the basis of a three film system with

the estimated parameters shown in Fig. 3. The multiple film portion of a

computer program for ellipsometry was utilized for the computations [4].

RESULTS AND DISCUSSION

Computer Modeling

The results of computer modeling as illustrated in Figs. 4 and 5 allow

the determination of both the direction and the relative orders of magnitude

- 3 -
of the changes in A and \J> to be expected for the three film system. It is

readily seen from a comparison of |^- and ^ values that A is a factor of ten

more responsive to changes in the oxide film thickness (d

3

) than to changes

in the liquid film thickness (d

2

). Moreover, an increase in either d

2 or d3

results in a decrease in both A and

Collodion Coating on Gold

Ellipsometric measurements were made on polished gold to determine

whether significant optical changes can be expected when it it immersed in

aqueous 0.05 N NaCl . Random fluctuations of A and ip of the order of only +

0.2° over a period of 1400 min were observed and indicated that minimal

contributions to changes in A and \J> would arise from the gold surface. The

same specimen coated with collodion showed a decrease in A values H°) over

a 1800 min period, while ij> remained essentially constant as illustrated in

Fig. 6. The reflectance infrared spectra of the collodion before and after

the extended immersion in the chloride medium were essentially identical.

The observed decrease in A is probably due to the development of a liquid

film under the coating and is consistent with the computer modeling results.

The thickness of this liquid film, d

2

, is estimated at about 3 nm on the

basis of computer modeling.

Thus, the work with the relatively inert gold surfaces in dilute Cl

media shows the magnitude of the ellipsometric changes which arise from the

collodion and the formation of a liquid film under the collodion. These

changes, as is apparent from Fig. 6, are small and can be readily accounted

for in experiments with iron substrates.

Collodion Coating on Iron

Fig. 7 indicates that on the basis of changes in A, the Fe-collodion

1 / system proceeds through three distinct stages. During the first 1000 min..

- 4 -
A rises gradually, decreases as does The pH increases during 0

p

g

.

this period. The next stage covering from 1000 to 6000 min. shows a considerable decline in A (^25°) concomitant with a continued decrease in ij>, 0

F

. The pH tends to higher and higher values. In the third stage, A underooes a

series of reversals, while i|> and pH increase. By 5400 min. an anodic area

was well developed at the upper corner of the specimen, and remote fron the

pH probe and ellipsometer observation regions.

On the basis of computer modeling, the first stage of the experiment

where A increases and ifj decreases is difficult to reconcile with reasonable

events thought to occur under the organic coating. One set of conditions

which does predict the divergence of A and \p as observed is the formation

of a liquid film combined with an increase of the liquid film index (n

2

) and

A

a radical decrease in n^ for the oxide film. This would suggest a substantive

change in either the chemical composition or the molecular and/or atomic

arrangement of the existing oxide film. The possibilities here will be

explored using the derivative plot in n, k space approach developed at MBS by

Melmed and Carroll [5].

The second stage with a significant decrease in A and i|> probably represents

the growth of an oxide film on the substrate of the order of 15 nm in thickness.

The interpretation of events in this stage is quite reasonable in view of the

sub-coating pH and specimen potential which comprise very favorable conditions

for the formation of Fe

2 0g.

Several events may be occurring in the final stage. The pH and potential

at this point are favorable for the dissolution of Fe and its stabilization

_2

as the FeO^ ion. The first of these would result in surface roughening.

- 5-
while the second would lead to an optically absorbing liquid film since the

ion is colored. Additional experiments and more sophisticated computer

modeling are required to correlate the observed changes in A and \p to events

in this stage.

Collodion Coating on Iron with I^CrO^ Islands

Fig. 8 shows the first stage of the experiment lasts about 1200 min. and

indicates both A and \p decreasing. The change in A is reminiscent of the

changes seen for the gold-collodion system and may simply represent the

accretion of liquid under the coating. The concomitant decrease in is

consistent with this interpretation, but the magnitude of the change is not

as readily explained and must await additional experiments.

The second stage of the process is notably different from previous

experiments in that the decrease in A is of the order of only 8° as compared

_2

to the 25° change seen with the previous system. It appears that the CrO^

greatly inhibits film growth in this stage.

Evidence for a third stage is absent, suggesting an inhibition of the

metal dissolution process.

During the experiment, the pH rises to about 10, levels off and finally

fluctuates between 9.3 and 10. This signifies some attenuation of the

corrosion process and is consistent with the expectations for the effect of

chromate inhibitor. After 9800 min. immersion, the chromate islands were

nearly obliterated, and three separate and very small anodic regions

remote from the pH probe and optical region were noted on the surface.

In general, the presence of the chromate "islands" markedly alters both

the first and second stages of the sub-coating-processes while eliminating

the third stage altogether within the time frame examined.

- 6 -
CONCLUDING COMMENTS

The main objective of this work was to explore the feasibility of using

a qualitative ellipsometric approach as a tool for obtaining a better understanding of the corrosion processes occurring under organic coatings. The

results described in the prior section indicate that even in its rudimentary

stage, an ellipsometric examination of corrosion processes under coatings can

provide useful information hitherto not available from previously tried

methods of study. It has been shown using this technique that:

1) It is possible to identify three different stages occurring during

4 the first 10 minutes with the collodion coating system used in

this study. Moreover, one can begin to correlate these optically

measured stages with time, pH and potential measurements.

2) It is possible to develop reasonable and consistent (consistency

between optical, pH and potential measurements) interpretations

of the processes occurring in the second and third stages but not

for the first.

3) It is possible to interpret the effect of the addition of

inhibitory chemicals on the processes observed and to consider them

with respect to measurements of repassivation kinetics.

, Repassivation kinetic studies of iron in simulated sub- coating

i 4 environments using the triboell ipsometric technique [3] are in

progress and will be the subject of a future publication.

Thus, while much remains to be done to refine both the experimental and

interpretive approaches (for example, the paper by Theeten, et a]_ [6] suggest

the possibility that variation of X may help), this feasibility study has

shown that the application of qualitative ellipsometry to the study of the

corrosion processes under organic coatings shows considerable promise.

- 7 -
ACKNOWLEDGEMENTS

We are most grateful to the Office of Naval Research, which supported

this work under Contract NAONR 18-89 NRO 36-082.

- 8-
FIGURE CAPTIONS

1. Details of the mounted iron specimen.

2. Mounted iron specimen showing location of chromate islands.

3. Three film system and parameters for computer modeling.

4. Results of computer modeling of rate of change of A and ip for fixed

oxide film (d^ = 3 nm) and variable liquid film, d

2

.

5. Results of computer modeling of rate of change of A and \p for a

fixed liquid film (d

2 = 5 nm) and a variable oxide film, d

3

.

6. A and i|> vs. time for Au with collodion in 0.05 N NaCl

7. A, ij>, pH and 0 Fg

vs. time for Fe with collodion in 0.05 N NaCl

p

8. A, 4», pH and 0pe vs. time for Fe with collodion and CrO^"

0.05 N NaCl

islands in
.. . *

.. ; i
REFERENCES

[1] Corrosion Control by Coatings, H. Leidheiser, Jr., ed.. Science

Press, Princeton, 1979.

[2] J. Kruger and J. R. Ambrose, Surf. Sci., 56, (1976), 394.

[3] J. R. Ambrose and J. Kruger, Corrosion, 28, (1971), 30.

[4] F. L. McCrackin, "A Fortran Program for Analysis of Ellipsometer

Measurements", NBS Technical Note 479 (1969).

[5] A. J. Melmed and J. J. Carroll, this Conference.

[6] J. B. Theeten, D. E. Aspenes and R. R. H. Chang, J. Appl . Phys.,

49 (1978), 6097.
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PART- I

I

Hydrogen Interactions with Stressed Titanium-Palladium Alloys

and Stressed Vanadium Explored with Field Ion Microscopy

J. J. Carroll and A. J. Melmed

Surface Science Division

and

J. Kruger

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

Three titanium-palladium alloys, nominally containing one, two, and

three at. % palladium, were prepared by vacuum annealing titanium wires

that were previously electroplated with palladium thin films. Field ion

micrographs obtained from these materials at temperatures less than 30 K

indicated significant alloy inhomogeneities. In addition, the micrographs

indicated that the alloys offered no greater resistance to hydrogen/stress

induced crack formation and development than unalloyed titanium under

similar conditions.

Field ion micrographs of 99.9+ percent pure vanadium showed surfaces

having significant short-range disorder, an effect that might be due to

imageable impurities. Experiments demonstrated that improved surface

order is gained in the top-most layers by exposing specimens to hydrogen

imaging conditions at temperatures less than 30 K. This hydrogen

treatment leaves the ordered vanadium surfaces free of defects.
. T -
Hydrogen Interactions with Stressed Titanium-Palladium Alloys

and Stressed Vanadium Explored with Field Ion Microscopy

J. J. Carroll and A. J. Melmed

Surface Science Division

and

J. Kruger

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D. C. 20234

1 . FORWARD

Field ion microscopy presents an atomic view of a multifaceted,

metal surface. Images are obtained while the specimen, in contact with

imaging gas, is subjected to appreciable stress brought about by the

necessary electric field. The advantage of the technique is that it

permits observation of a metal surface before, during and after exposure

to the potentially corrosive gas, hydrogen, or to a more inert gas,

neon, while the specimen is under stress. Thus, the occurrence of

hydrogen/stress induced metal defects is determined readily. The

following is a description of exploratory experiments with titaniumpalladium alloys and vanadium carried out with a conventional field ion

microscope, previously described [1].

2. TITANIUM-PALLADIUM ALLOYS

2.1. Introduction

It was learned more than twenty years ago [2] that one may improve

the corrosion resistance of titanium in oxidizing and reducing acid

media by several hundred-fold by alloying with small amounts of palladium.

Improved corrosion resistance is accompanied by sharply increasing

material hardness with increasing palladium contents of the alloy. The

solubility of palladium in alpha titanium is slightly less than 3 at.%

at room temperature [3], so relatively small amounts of the palladium
- 2 -

constituent have relatively large effects. In view of the previous

success with field ion microscopy in observing the initial crack formation in the presence of stress and hydrogen with pure titanium [4],

it is of interest to pursue the effects on titanium-palladium alloys, to

evaluate their susceptibility to stress corrosion cracking due to hydrogen

embrittlement with the field ion microscope, and to compare the results

with those previously obtained with pure titanium [4].

2.2. Specimen Preparation

Three titanium-palladium alloys containing nominally 1,2, and 3

at.% palladium were made by electroplating approximately 270, 530 and

800 nm thick palladium films, respectively, onto 0.013 cm diameter

titanium wires, previously strain annealed in vacuum at approximately

1000 °C, and subsequently annealing the composite at 1000 °C for two

hours and at 520 °C for about 17 hours. The titanium wire was originally

characterized as 99.99+ percent pure, and was used in the previous field

ion microscopy study of titanium [4], where it was found that strain

annealing titanium wire markedly increased the average lifetime of

specimens in the field ion microscope. The palladium electroplating

bath contained 200 g PdCl

2

, about 100 g NH^Cl , 3.8 £ H

2

0 and enough HC1

to bring the pH in the 0.1-0. 5 range. The bath was held at about 40 °C

2 and agitated. The electroplating was carried out at about 75 amps/m

using a stainless steel counter electrode. The electroplating bath was

designed to electroplate palladium on metal more noble than titanium,

and, as a consequence, titanium wires were cleaned prior to electroplating

by a chemical dip into a bath containing 11 parts Ni Cl

2

, six parts HC1

(cone.), and one part NH^F. Ultimately, the alloy wires were electrolytically

sharpened at about 50 V (dc) vs. a carbon rod cathode in a methanol
- 3

-

solution containing one percent HCIO^ and held at -50 °C.

2.3. Results

Within the limitations of conventional field ion microscopy, where

assessments of chemical identification are derived sometimes from qualitative differences in appearance among micrographs, it is concluded that

the alloying procedure was not completely successful, i.e., although

evidence from some specimens indicated the presence of titaniumpalladium alloy, other specimens showed no evidence of alloy formation.

Each of the three alloys were sampled by field ion microscopy several

times. Field ion micrographs from all samples of titanium-1 at.% palladium alloy were indistinguishable in appearance from those obtained

from pure titanium, showing characteristic cracking behavior and the

usual crack depths of 10-15 layers. It is assumed that these samples

corresponded to unalloyed titanium. Field ion micrographs obtained from

samples of titanium-2 at.% and -3 at.% palladium alloys indicated that

these preparations were of nonuniform composition in that micrographs

from some samples showed clearly different pattern appearance than

micrographs from other samples, which were indistinguishable from those

obtained from pure titanium.

Micrographs obtained from the titanium-1 at.% palladium alloy

preparation showed neon and hydrogen field ion patterns typical of those

obtained from pure titanium [4]. Patterns obtained using neon imaging

gas [Fig. 1(a)] have uniform spot intensity and contrast, overall, with

the (1011) type planes showing the most order, typical of neon field ion

micrographs from titanium. Patterns obtained using hydrogen imaging gas

[Fig. 1(b)] show several defects, crack- like in appearance, predominately in the <101 0> -< 1 1 20> zone with the (1011) type planes remaining
- 4 -

relatively defect free, again typical of hydrogen field ion micrographs

from pure titanium. A lingering question remains, and is as follows.

Are the micrographs of Figs. l(a,b) from alloyed or unalloyed titanium?

In the absence of atom-probe field ion microscopy [5] evidence the

question can't be answered definitively. If the alloying procedure

worked perfectly and produced the sought after titanium-1 at.% palladium

alloy, then only one pattern spot in a hundred would be due to palladium. Perhaps the electronic structure of the alloy is not sufficiently

different from pure titanium to alter its imaging characteristics from

those of titanium. Perhaps the plating procedure deposited less than

the desired quantity due to the less noble character of the titanium

substrate than the more usual gold substrate. Such an alloy might not

be detectable with the presently used qualitative field ion microscopy.

Without some positive indication that the alloying procedure was successful, then we assume that it was not.

Field ion microscopy of titanium-2 at.% and -3 at.% palladium

alloys showed clearly that these preparations were not uniform in composition. In addition to the usual looking neon and hydrogen field ion

micrographs obtained from some samples of titanium-2 at.% and -3 at.%

palladium, shown respectively in Figs. 2(a,b) and Figs. 3(a,b), other

samples from each alloy showed marked differences in pattern contrast

characteristics, and. In the titanium- 3 at.% palladium alloy, both

contrast and structural differences from ordinary titanium patterns.

Specifically, a titanium-2 at.% palladium alloy sample showed evidence

of material inhomogeneities in the early stages of imaging. Figure 4(a)

shows a dimly imaged, ordered material having several crystal planes and

j- a brightly imaged, disordered area of another material. Field evaporation
of the tip surface leaves only a few remaining atoms of the brightly

imaged inclusion and the development of another brightly imaged area in

the lower right region [Fig. 4(b)], With further field evaporation, the

brightly imaged material occupies a greater proportion of the tip's

imaging surface [Figs. 4(c,d)] until, eventually, the entire tip area is

comprised of the brightly imaged material [Fig. 4(e)]. There were no

further changes in composition evident during the lifetime of this tip.

The sequence of micrographs in Fig. 4 are related to a schematic diagram

in Fig. 5 that indicates approximately a possible configuration of the

two imaged phases. It is assumed that the more ordered material imaged

in Fig. 4(a) is pure titanium and that the less ordered, more brightly

imaged material, imaged partially in Fig. 4(a) and exclusively in

Fiq. 4(e), is the desired titanium-2 at.% palladium alloy.

Hydrogen ion microscopy revealed that the apparent titanium-2 at.%

palladium alloy [Fig. 4(e)] has no greater resistance to cracking caused

by the combination of hydrogen and stress than does pure titanium. The

hydrogen field ion micrograph shown in Fig. 6(a) indicates severe crack

development in the <1 1 20>-< 1 01 0> zone along with the less effected

(1011) type plane areas, a pattern typical of pure titanium under the

same imaging conditions. When the hydrogen imaging gas was removed from

the microscope prior to imaging and field evaporating in neon, the crack

development ceased and the number and severity of the imaged defects

decreased upon continued field evaporation in the inert imaging gas.

Figs. 6(b,c) are neon field ion micrographs of the previously attacked

specimen [Fig. 6(a)] obtained after about three and twelve (1120)

planes field evaporated, respectively. Some defects remain [arrows.

Fig. 6(c)] but, nevertheless, the appearance of the < 1120>-<1010>
- 6 -

zone has improved. The reproducibility of this experiment was established

by reimaging the surface, previously neon field evaporated [Fig. 6(c)],

in hydrogen. The hydrogen field ion micrograph of Fig. 6(d) shows the

formation of crack- like defects, and the subsequent neon field ion

micrographs of Figs. 6(e,f) show the alloy surface after six and sixteen

(1011) planes have field evaporated. Figure 6(e) indicates that the

crack-like features have disappeared. The brightly imaged cross area of

the <11 20> -<101 0> and <11 20> -< 1 1 22> zones clearly make the neon field

ion micrographs from this alloy distinguishable from those of pure

titanium [4]. The above evidence indicates that crystallographically

specific attack of the titanium-2 at .% palladium alloy surface occurred

by the corroding agent, hydrogen, in combination with metal tensile

stress (17x10 N/m ) and lead to stress corrosion cracking. The crack

depths formed in this alloy were not significantly different from those

formed in pure titanium [4]. Crack depths were estimated from a count

of the number of nearby (1011) planes that were required to field evaporate in neon in order to cause the disappearance of defects that originated during previous hydrogen imaging.

Hydrogen field ion micrographs [Fig. 7(a)] obtained from an atypical specimen or titanium- 3 at.% palladium alloy indicated that the

material failed to exhibit the expected crack-like defects, and possibly

might have a cubic crystal structure. The defects present in Fig. 7(a)

appear to be less disruptive of the surface order than in other hydrogen

field ion micrographs [Figs. 1(b), 2(b), 3(b), 6(a,d)]. Neon field ion

micrographs [Fig. 7(b)] from this specimen show uniform contrast instead

of local variations in contrast, a feature found in a titanium-2 at.%

palladium alloy specimen. Assuming that this contrast feature would
- 7

-

prevail in the titanium-3 at.% palladium alloy then one may conclude

that the imaged material, in this case, is either unalloyed palladium or

titanium with a grain or twin boundary. The latter interpretation

involves the identification of the three large planes in Fig. 7(a) as

(1011) type planes and one of the defects as a grain or twin boundary.

If this is the case, then the especially effected <1 01 0> -< 1 1 20> zone is

not imaged.

It is recognized that the above survey of these titanium-palladium

alloys using field ion microscopy was limited by not assuring that the

specimens were uniform in composition and homogeneous. There are significant uncertainties, as a result, regarding the chemical composition

of the imaged material, i.e., titanium- pa Uadi urn alloy, titanium, or

palladium. In one important case of a titanium-2 at.% palladium specimen the results of hydrogen field ion microscopy were attributed

clearly to a titanium-palladium alloy without knowledge of the alloy's

precise composition.

3. VANADIUM

3.1. Introduction

Vanadium absorbs substantial quantities of hydrogen at elevated

temperatures in hydrogen atmospheres and retains as much as 42 at.%

hydrogen at room temperature [6], leading to hydrogen embrittlement [7].

Hydrogen absorption occurs at room temperature but may be inhibited by a

monolayer of oxide [8]. In some cases [8,9] it is found that oxides and

other surface impurities are difficult to remove, requiring heat treatments to within a few degrees of vanadium's melting point. Finally,

there is atom- probe evidence [10] for the formation of vanadium-hydride

ions in 10 ® Pa residual hydrogen at 21 K, indicating that a strong
- 8 -

vanadium- hydrogen interaction occurs under field ion microscopy conditions. In the following, experiments will be described that probe this

interaction and its resultant effect on vanadium.

3.2. Sample Preparation

The vanadium source material was 99.9+ percent pure, zone refined

-4

{100} oriented single crystal rod from which several 5x10 m rods were

spark cut with the {100} plane approximately perpendicular to the rod's

axis. Two electropolishing steps were used to make sharp tips suitable

for field ion microscopy. (1) The diameter of a rod was significantly

reduced and the surface made shiny bright by electropolishing at 20 V (dc)

in a 1:1 or 1:2 solution of perchloric acid (70 percent) and methanol.

(2) The final polishing step, performed while observing the process at

150 magnification, used a 1:2 solution of HC1 (cone.) and water at

2 V (ac). The electrolyte was contained in a loop of 0.13 mm platinum

wire. An alternative step (2) procedure used a 1:3 solution of HNO^ (cone.)

and HF at 2 V (ac) without the aid of an optical microscope. Steps (1.)

and (2.) were performed usually with the electrolyte near room temperature.

Image quality was not effected, however, by using electrolyte (a) cooled

from -30 to -50 °C. Methanol was used to rinse specimens at appropriate

stages during tip manufacture.

3.3. Results

The appearance of neon field ion micrographs obtained from vanadium

is dependent upon the specimen's recent history of exposure to hydrogen

in the microscope, i.e., patterns obtained from a specimen previously

exposed to hydrogen imaging conditions or to moderate hydrogen partial

pressures are more ordered and show more crystal planes than patterns

: j- obtained with purer neon from a specimen not recently imaged with
- 9 -

hydrogen.

Directing attention to the appearance properties of the latter case

first, the neon field ion micrographs shown in Fig. 8 were obtained from

a specimen not previously imaged in hydrogen and with the microscope

-8

vacuum system especially gettered to reduce the base pressure to 10

Pa. The principle planes imaged are the central (100) and the four

large {110} planes. The {111} planes were not imaged in this study.

Other crystal plane areas appeared disordered. The {112} planes, for

example, were expected to be moderately prominent in Fig. 8(a). Subsequent micrographs from this specimen indicated the presence of a thin

plate of material, perhaps vanadium of different orientation, as shown

in the fuzzy area in Fig. 8(b), and at substantially lower imaging

voltage in Fig. 8(c). The plate boundaries are clearly defined. The

specimen returned to its former appearance [Fig. 9(d)] following field

evaporation sufficient to remove the plate imperfection. The micrographs

of Figs. 8(b,c) are used to support the following conjecture concerning

the origin of the fuzzy appearance of some atom spots, particularly pronounced in Fig. 8(a). Most imaging atoms of the plate appear fuzzy and

unresolved in Fig. 8(b) because the plate may have slipped in the stress

of the applied field and protruded sufficiently from the remaining

hemispherical ly shaped cap surface. The resultant larger fields local

to the protruding plate allow neon ionization to occur further away from

the surface atoms, thus reducing the atomic resolution of the plate.

Analogously, similar actions attributed to single atoms or groups of

atoms--possibly impurities, within the vanadium matrix would account for

the appearance of the occasional, lesser resolved spot, a characteristic

j- found in the foregoing neon field ion micrographs from this material.
- 10 -

Exposing vanadium to hydrogen imaging conditions dramatically

effects the appearance of neon field ion micrographs obtained subsequently,

as demonstrated in Fig. 9. The hydrogen field ion micrograph shown in

Fig. 9(a) was obtained with vanadium in an unstable, field evaporating

condition, whereas. Fig. 9(b) was obtained under stable imaging conditions

at lower fields. Notice that two of the {112} planes are visible. The

remaining micrographs in Fig. 9 were obtained with neon imaging gas

after the hydrogen was pumped away while maintaining the field. The

procedure minimizes surface contamination during imaging gas changes.

Figures 9(c,d,e,f) were obtained after field evaporating less than a

monolayer, one layer, three and five layers, respectively, from the

central (100) plane. The {112}, {103}, and {115} planes are fully

resolved most clearly in Fig. 9(f). The vanadium field ion micrograph

of Fig. 10 was obtained in a helium and neon gas mixture from another

specimen preimaged in hydrogen. Helium tends to highlight the well

ordered, high index plane areas near the central (100) plane.

It is interesting and informative to compare helium-neon field ion

micrographs obtained from ordered vanadium [Fig. 10] and disordered

vanadium [Fig. 11]. The specimen shown in Fig. 11 was not imaged previously in hydrogen. The brighter spots are scattered randomly across

the imaged surface and the higher index areas near the central (100)

plane are not highlighted as they are in Fig. 10. It is clear that an

explanation involves the imageability of impurity atoms, present in Fig.

11, and not in Fig. 10, obscuring the overall crystal order. Hydrogen

imaging appears to clean the imageable impurities from surface layers of

vanadium. The conventional picture of the image process includes the

:

* J* thermal accommodation of imaging gas onto the surface through a series
- 11 -

of bounces. In addition to the other events of the imaging process,

hydrogen may be absorbed by an active metal such as vanadium.

There is one question that is answered by the following experiment

and it concerns whether hydrogen effects bulk cleaning or whether its

effect is confined to a few surface layers. Ordered inert gas images

obtained following hydrogen imaging conditions should be sustained with

continued field evaporation if bulk cleaning is effected, or should be

changed to disordered inert gas images if the effect is confined to a

finite depth. Figures 12(a,b) correspond to images obtained using

helium-neon and neon, respectively, from a specimen not previously

exposed to hydrogen imaging conditions. They each show a lack of

ordered {112}, {103}, and {105} planes, which is characteristic of

contaminated vanadium. Figure 12(a) shows scattered, non-ordered,

highlighted areas and Fig. 12(b) shows the occasional fuzzy spot.

Figure 12(c) shows a hydrogen image of the specimen and Fig. 12(d) shows

the first subsequently obtained neon image, corresponding to the surface

undergoing less than a monolayer of field evaporation following the

hydrogen treatment. Notice that the four, square formated, darker lines

connecting the {110} planes in Fig. 12(c) are apparent in Fig. 12(d) but

not in Fig. 12(e), obtained after field evaporating one (100) plane

layer. They correspond to recessed areas formed by preferential field

evaporation in hydrogen, about a layer deep in this case. Upon further

field evaporation the micrographs of Figs. 12(f,g) were obtained after

eleven and sixteen (100) plane layers, respectively. Notice that a

deterioration of surface order has occurred after sixteen layers, but

not a total obliteration. Compare Figs. 12(f,g). Imaging with a

helium- neon mixture shows a more disordered surface [Fig. 12(h)] than
12

that from a probably purer vanadium specimen [Fig. 10], more order than

from another vanadium specimen not exposed to hydrogen imaging conditions [Fig. 11], and approximately the same order as the starting

conditions for the present specimen [Fig. 12(a)]. The effective penetration depth for hydrogen in this vanadium specimen, about sixteen

layers, is in moderate agreement with depths found for hydrogen induced

cracks in titanium [4] and pall adium-titani urn alloys. In this case,

however, the interaction leaves the vanadium matrix free from crack

defects.

As a side note, the normal imaging process was interrupted occasionally by momentary bursts in the rate of field evaporation with no

apparent change in image quality. These events were observed using

hydrogen, neon or helium-neon gas mixtures at liquid helium cooled

temperatures and with neon at liquid nitrogen, a wide variety of imaging

conditions. We suspect that during normal imaging and field evaporation

there is an adsorbed film, probably involving hydrogen, that tends to

stabilize the surface. We associate the momentary bursts in field

evaporation rates with a corresponding removal of the adsorbed film

which causes the surface to destabilize until the film is reformed.

It would be useful to carry out further atom-probe investigations

directed toward chemically characterizing the above observations, and to

perform vanadium field ion microscopy in a baked vacuum chamber in order

to gain further insights into the hydrogen- vanadi urn interaction. If the

conclusion that hydrogen exposures at low temperatures tend to clean the

top few surface layers of vanadium is supported by the atom-probe, then

the hydrogen exposure technique could be adapted to larger, single

crystal vanadium studies where the attainment of a clean vanadium surface
- 13 -

proved to be a formidable achievement [8,9,11].

4. SUMMARY

Hydrogen field ion microscopy of titanium-palladium alloys at

temperatures less than 30 K indicated that the alloys offer no greater

resistance to the development of surface cracks in the <11 20> -<101 0>

zone and the (1120) and (1010) plane areas, induced by the combination

of hydrogen and stress, than does unalloyed titanium.

Field ion micrographs of 99.9+ percent pure vanadium showed surfaces having significant short-range disorder, an effect which was

attributed to imageable impurities. Experiments demonstrated that

improved surface order is gained in the top-most layers by exposing

specimens to hydrogen imaging conditions at temperatures less than 30 K.

This hydrogen treatment leaves the vanadium surfaces ordered and free of

defects.

5. ACKNOWLEDGEMENTS

We recognize the assistance of John P. Young of the Chemical

Stability and Corrosion Division who palladium plated titanium wire

specimens, and of Francis S. Biancaniello of the Metal Science and

Standards Division who vacuum annealed the palladium plated titanium

specimens.
- 14 -

REFERENCES

[1] J. J. Carroll, A. J. Melmed and J. Kruger, "The study of the early

stages of hydrogen embrittlement of some metals and alloys using

field ion microscopy", in: Passive Films , Surface Structure and

Stress Corrosion and Crevice Corrosion Susceptibility , NBSIR 79-

1904 (Navy), Part I, (November, 1979) pp. 1-26.

[2] M. Stern and H. Wissenberg, "The influence of noble metal alloy

additions on the electrochemical and corrosion behavior of titanium", J. Electrochem. Soc., 106 (1958) 759-764; L. F. Plock,

"Titanium alloys for crevice corrosion resistance", RMI Report R&D

505, Reactive Metals, Inc., Niles, Ohio (1967).

[3] H. Nisimura and T. Hirama Tsu, J. Japan Inst. Metals, 22 (1968) 88.

[4] J. J. Carroll, J. Smit and A. J. Melmed, "Field ion imaqing of

titanium and effects due to hydrogen", Surface Sci . 45 (1974) 640-

648. ~

[5] E. W. Muller and T. T. Tsong, Field Ion Microscopy ; Principles and

Applications (Elsevier, New York, 1969) , pp. 130-134.

[6] William Rostoker, The Metallurgy of Vanadium (John Wiley, New York,

1958), p. 50.

[7] ibid, p. 136.

[8] A. Benninghoven, K. H. Muller, C. Plog, M. Schemmer and P. Steffens,

"SIMS, EID and flash-filament investigation of 0

2

, H

2

, (0

2 + H2

) and H

2 0 interaction with vanadium". Surface Sci. 63 (1977) 403-416.

[9] T. W. Haas, A. G. Jackson and M. D. Hooker, "Adsorption on niobium

(110), tantalum (110), and vanadium (110) surfaces, J. Chem. Phys.

46 (1967) 3025-3033.

[10] E. W. Muller and S. V. Krishnaswamy , "Atom-probe FIM studies of

vanadium, rhenium, and a copper-beryllium alloy", Phys. Stat. Sol.

(A) 3 (1970) 27-31.

[11] K. K. Vijai and P. F. Packman, "Gas adsorption studies on the (100)

plane of vanadium by low-energy electron diffraction", J. Chem.

Phys. 50 (1969) 1343-1349.
- 15

Figure 1 Field ion micrographs obtained from titanium-1 at.%

palladium.

(a) neon, 24 kV, 30 K; (b) hydrogen, 16 kV, 30 K.
16

Figure 2 Field ion micrographs obtained from titanium-2 at .%

palladium.

(a) neon, 21.5 kV, 30 K; (b) hydrogen 15.1 kV, 30 K.
- 17 -

Figure 3 Field ion micrographs obtained from titanium- 3 at. %

palladi urn.

(a) neon, 15 kV, 30 K; (b) hydrogen, 8.6 kV, 30 K.
- 18 -

Figure 4 Neon field ion micrographs obtained from a titanium-2 at.% palladium.

specimen, near 30 K, that show evidence of material inhomogeneities,

(a) and (b) 5.8 kV; (c) 6.5 kV ; (d) 6.8 kV; (e) 7.3 kV.
- 19 -

Figure 5

4(a)

4(b)

4(c,d)

4(e)

A cross-sectional schematic diagram of the two-phase specimen

imaged in Figure 4.
- 20 -

Figure 6 Consecutive field ion micrographs obtained from a titanium-2 at. %

palladium specimen that show evidence of hydrogen/stress induced

crack formation, (a) hydrogen, 6.7 kV, 30 K; (b) neon, 8.2 kV, 30 K;

(c) neon, 8.6 kV, 30 K; (d) hydrogen, 9.8 kV, 30 K;
- 21

Figure 6 (e) neon, 12.5 k V, 30 K; (f) neon, 13.2 kV, 30 K.

Figure 7 Field ion micrographs obtained from an atypical specimen of

titanium- 3 at. % palladium alloy, (a) hydrogen, 11 kV, 30 K;

(b) neon, 16 kV, 30 K.
- 22

Figure 8 Neon field ion micrographs obtained from vanadium, near 30 K,

not imaged previously with hydrogen, (a) 14 kV; (b) 18.2 kV

;

(c) 16 kV; (d) 18.5 kV.
- 23 -

Figure 9 Consecutive field ion micrographs obtained from vanadium showing

the effect that preimaging with hydrogen has on neon field

micrographs obtained subsequently, (a) hydrogen, 7.2k V, 30K;

(b) hydrogen, 6.3kV, 30K; (c) neon, 11.5kV, 30K; (d) neon,

1 2kV , 30 K;
- 24 -

Figure 9 (e) and (f) neon, 12 kV, 30 K.
- 25 -

Figure 10 A helium-neon field ion micrograph obtained from vanadium

preimaged with hydrogen, 12 kV, 30 K.

Figure 11 A helium-neon field ion micrograph obtained from vanadium

not imaged previously with hydrogen, 19 kV, 30 K.
- 26 -

Figure 12 Consecutive field ion micrographs obtained from vanadium,

(a) helium-neon, 12 kV, 30 K; (b) neon, 12 kV , 30 K;

(c) hydrogen, 8.2 kV, 30 K; (d) neon, 11 kV, 30 K;
- 27 -

Figure 12 (e) neon,

(g) neon.

12.5 kV, 30 K; (f) neon, 13.2 kV, 30 K;

14 kV, 30 K; (h) helium-neon, 14 kV, 30 K.

NBS-H4A I w e v. 2 -aci u.s. OEPT. OF COMM.

*

U.s. Of

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 80-2101 (Navy

2. Performing Organ. Report No

) 561

3. Publication Date

August 1980

4. TITLE ANO SUBTITLE .

' Passive Films, Surface Structure and Stress Corrosion and Crevice Corrosion

Susceptibility

5. AUTHOR(S)

J. Kruger, J. J. Ritter, J. J. Carroll, and A. J. Melmed

6. PERFORMING ORGANIZATION (If joint or other than N8 S. see in structian s) 7. Contract/Grant No. national BUREAU OF STANDARDS

NR036-082/1-7-69

DEPARTMENT OF COMMERCE

* WASHINGTON, D.C. 20234

8. Type of Report £. Period Covered

Annual Report

11/79-8/80

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

Office of Naval Research

Arlington, VA 22217

-

10. SUPPLEMENTARY NOTES •

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

11.

ABSTRACT (A 200-word or less fcctual summary of most significant in formation, if document includes a significant

bibliography or literature survey, mention it here)

PART I: A feasibility study has shown that qualitative-ellipsometry combined with

electrochemical-pH and potential measurements comprise valuable techniques for the

study of the corrosion of iron protected by an organic coating. A simulated painted

metal system using cellulose nitrate over iron and immersed in dilute Cl medium showed

a concomitant rise of sub-coating pH to high values (v 14) and a thickening of the

metal oxide film. These processes could be modified by the presence of a chromate

inhibitor under the coating.

Part II: Three titanium-palladium alloys, nominally containing 1, 2, and 3 at. %

palladium were prepared and imaged in the field ion microscope at temperature less

than 30 K. Although significant alloy inhomogeneities occurred in the wire preparations, the results indicated that the alloys offered no greater resistance to hydrogen/

stress induced crack formation and development than unalloyed titanium under similar

conditions. €Field ion micrographs of 99.9+ percent pure vanadium showed surfaces having

significant short-range disorder, an effect that might be due to imageable impurities.

Experiments demonstrated that. improved surface order is gained in the top-most layers

by exposing specimens to hydrogen imaging conditions at temperatures less than 30 K.

This hydrogen treatment leaves the ordered vanadrum surfaces free of defects.

12.

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

Chromate; electrochemistry; ellipsometry ; field ion microscopy; hydrogen; inhibitors;

iron; organic coatings; titanium-palladium alloys; vanadium

13.

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4 . TITLE (and Subtitle) • . Passive Films, Surface Structure and Stress

Corrosion and Crevice Corrosion Susceptibility

5. TYPE OF REPORT 6 PERIOD COVERED

Annual Report

11/79-8/80

6. PERFORMING ORG. REPORT NUMBER

7. AUTHOR!- *.) J. Kruger, J. J. Ritter, J. J. Carroll, and

A. J. Melmed

8. CONTRACT OR GRANT NUMBERpJ

NR036-082/1-7-69

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, DC 20234

10. PROGRAM ELEMENT. PROJECT, TASK

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18.

SUPPLEMENTARY NOTES

19.

KEY WORDS (Continue on reverse tide II necessary and Identify by block number)

Titanium-palladium alloys; field ion microscopy; vanadium, hydrogen, iron,

organic coatings, ellipsometry , electrochemistry, inhibitors, chromate

20.

ABSTRACT (Continue on reverse side It necessary and Identity by block numbsrj

Part I: A feasibility study has shown that qualitative-ellipsometry combined

with electrochemical-pH and potential measurements comprise valuable techniques

for the . study of the corrosion of iron protected by an organic coating. A

simulated^painted metal system using cellulose nitrate over iron and immersed in

dilute Cl medium showed a concomitant rise of sub-coating pH to high values

(v 14) and a thickening of the metal oxide film. These processes could be

modified by the presence of a chromate inhibitor under the coating.

( cont ' d .

)

1473 EDITION OF I NOV 65 is OBSOLETE Unclassified

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Part II: Three titanium-palladium alloys, nominally containing 1, 2, and 3 at. % palladium were prepared and imaged in the field ion microscope at

temperature less than 30 K. Although significant alloy inhomogeneities

occurred in the wire preparations, the results indicated that the alloys

offered no greater resistance to hydrogen/stress induced crack formation and

development than unalloyed titanium under similar conditions. j

Field ion micrographs of 99.9+ percent pure vanadium showed surfaces having

significant short-range disorder, an effect that might be due to imageable

impurities. Experiments demonstrated that improved surface order is gained

in the top-most layers by exposing specimens to hydrogen imaging conditions

at temperatures less than 30 K. This hydrogen treatment leaves the ordered

vanadium surfaces free of defects.

unclassified

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