NBSIR 79-1904 (Navy)

Passive Films, Surface Structure

and Stress Corrosion and

Crevice Corrosion Susceptibility

J. Kruger, J. J. Carroll, A. J. Melmed

J. J. Ritter, and J. R. Ambrose

Chemical Stability and Corrosion Division

and

Surface Science Division

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

November 1979

Technical Summary Report No. 9

Prepared for Office of Naval Research

Department of the Navy

Arlington, VA 22217

NBSIR 79-1904 (Navy)

PASSIVE FILMS, SURFACE STRUCTURE

AND STRESS CORROSION AND

CREVICE CORROSION SUSCEPTIBILITY

J. Kruger, J. J. Carroll, A. J. Melmed

J. J. Ritter, and J. R. Ambrose

Chemical Stability and Corrosion Division

and

Surface Science Division

National Bureau of Standards

Washington, D.C. 20234

November 1979

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Prepared for Office of Naval Research

Department of the Navy

Arlington, VA 22217

U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

PART I - The Study of the Early Stages of

Hydrogen Embrittlement of Some Metals and Alloys

Using Field Ion Microscopy

J. J. Carroll, A. J. Melmed

Surface Science Division

and

J. Kruger

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D.C. 20234

INTRODUCTION

Field ion microscopy (FIM) is capable of imaging defects such as sub-critical crack formation and growth, and surface rearrangements, and

of detecting new surface phase formations, and other disturbances.

Thus, we have used the technique to characterize various modes of

hydrogen reactions with metal surfaces. (1-3) Previous work from this

laboratory has revealed examples of one mode, the initial crack formation

and growth in the hydrogen-titanium(l ) and hydrogen-zirconium(2) systems.

In each case there were indications of preferential crystallographic

attack producing cracks within the metal matrix approximately 10-15

atomic layers deep in the case of titanium, and 2-5 layers deep in the case of zirconium. We also have shown in the hydrogen-uranium system, (3)

under similar conditions of temperature and pressure, that another mode

of hydrogen attack occurred — surface film formation of a reaction

product. An orthorhombic single crystal of uranium (micrographs obtained

using inert imaging gases) exposed to hydrogen, initially formed a disordered film, and, after prolonged hydrogen interaction, formed

nuclei that eventually coalesced into a small grained (5-30 nm, lateral

dimensions) film of what may likely be uranium hydride. Field evaporation of the overlayer phase revealed a crack-free uranium substrate. An

example of qualitatively less metal attack due to hydrogen was found in

the hydrogen-iron system where FIM studies(3) showed nonuniform surface

damage that did not extend deeply into the iron substrate. The (111)

plane areas were the most sensitive and appeared highly disordered and were probably structurally rearranged, whereas the (100) plane areas remained ordered, and the (110) plane areas showed intermediate damage.

FIM studies of 1080 carbon steel (3) did not reveal any adverse effects

due to hydrogen, even with specimens having grain boundaries intersecting

the surface.

In the present phase of the work, FIM was used to examine microscopic details of the initial stages of hydrogen interactions with some additional pure metals and metal alloys, i.e., niobium, aluminum, 304

stainless steel and iron-24 wt. % chromium steel. The hydrogen-metal

interactions occurred during the FIM imaging process with hydrogen

pressures between 10~ 4 and 10~ 6 torr at metal temperatures between 20

and 80 K, and while the metal was subjected to stresses brought about
by the electric field necessary for imaging the multifaceted, approximately hemispherical metal surface. The method used involved comparison of micrographs obtained during inert gas imaging (neon, helium)

with micrographs obtained in pure hydrogen or mixtures of hydrogen and

inert gases. Usually FIM provides information regarding intragranul ar crack formation except for the relatively rare circumstance of a grain

boundary intersecting the metal surface in the field of view. The

objective of the present work is to explore the different modes of

hydrogen interactions with metals and alloys using FIM in order to

better understand the mechanisms that lead to metal fracture. Materials

chosen for this study are listed in Table 1. Niobium was chosen as an example of a metal that is known to form an embrittling hydride. (4) The

aluminum and 304 stainless steel studies were made to throw some light

on the controversy(5,6) regarding the question of whether these facecenter-cubic metals suffer hydrogen damage — an important factor in

determining the mechanisms of stress corrosion cracking. Finally,

experiments were performed with iron-24% chromium specimens to determine

the structural effects incurred by pretreating the specimens with various

cathodic charging doses of hydrogen. The results demonstrate that FIM

is a powerful method for studying the important metallurgical problem of

hydrogen embrittlement.

EQUIPMENT

The field ion microscope was equipped with a 90 mm diameter microchannel plate image intensifier, and a specimen-quick-change chamber for

rapid specimen insertion and withdrawal. Routinely, images from two

specimens could be obtained within a half an hour's time. This was an important advantage in the studies involving hydrogen interaction with

several types of specimens, such as pure metals and alloys subjected to

a variety of environmental and preimaging treatments to promote hydrogenmetal interactions. Furthermore, the images obtained were generally

unstable, thus requiring image intensification in order to achieve

fractional second, photographic exposure times. The specimen temperatures

were controlled by adjusting the flow of cold gaseous helium from a liquid helium dewar flask over the specimen-mount-feed-through pin.

During field ion microscopy reagent grade imaging gases were continuously

flowed through the imaging chamber. Imaging gas mixtures were made by

simultaneously opening the appropriate leak valves, and mixture compositions

were estimated from pressure gauge measurements.

- 2 -
SPECIMENS

TABLE I

Starting Material and Specimen Preparation Procedures

for Imaging in the Field Ion Microscope

1 .

2 .

3.

4 .

Material Tip Sharpening Procedure

niobium

5 mil diam. wire

aluminum

5 mil diam. wire

304 stainless steel

5 mil diam. wire

a) plastic dish containing cone. H 2 S04 at about 1-1/2" depth followed with about

10 drops of 48°/ HF, leave unmixed, about

7.5 Vdc;

b) wash in water followed by methanol.

a) reduce diam. and taper, 10% HC1

0

4 in methanol, 15-10 Vac;

b) polish to a sharp point, thin liquid

film of 1% HC1

0

4 in glycerin, 30 Vac,

while observing at 100 X;

c) frequent methanol washes during and

after parts a) and b).

a) reduce diam. and polish to a sharp

point in 10% HC1

0

4 in methanol, 50 Vdc

0R_ sharpen tip in 10% HCL, aqueous

thin liquid film while observing at 100 X;

b) frequent methanol washes during and after

parts a) and b).

iron - 24 wt. % chromium

20 mil diam. rods spark

cut from ingot

a) reduce diam. and taper,

10% HC104 in methanol,

12-2 Vdc;

b) polish to a sharp point,

thin liquid film of 1% HC1

0

4 in glycerin, 2 Vdc while observing at 100 X;

c) frequent methanol washes

during and after parts a) and b).

-3 -
NIOBIUM

Niobium images obtained with neon and neon-helium mixtures (Figs,

la, Id) were stable and showed long range bcc crystal order. The field

ion micrograph of Fig. la was obtained from a niobium sample not previously exposed to hydrogen imaging conditions in the microscope. The

micrograph shows many lattice defects including several spiral dislocations. Most likely, these were inherent crystal defects in the sample.

Potentially, the defects provided easy access routes for hydrogen and

rendered the sample more susceptible to attack in subsequent experiments

with hydrogen.

Fig. lb is a hydrogen field ion micrograph obtained from the same specimen as in Fig. la. As opposed to images obtained with inert gases,

the hydrogen images were unstable, with most spots often appearing and

disappearing and with overall image appearance of liquid-like movement.

The formerly long range crystal order was reduced to relatively short

range order. The micrograph of Fig. lb shows many groupings of spots

ranging from 2 to 15-20 spots per group. The groups appear delineated

by relatively dark, non-spotty lines appearing wider than those between

resolved spots within a group. Some groups show ordered spot arrangements. For example, there are areas where spots form arcs that seem to

partially mimic concentric rings. The evidence suggests that hydrogen

was interacting with the niobium surface and formed a surface compound.

Subsequently, the hydrogen imaging gas was removed, and the neon field ion micrograph of Fig. lc was obtained after partially evaporating

the surface layer formed during the previous hydrogen-niobium interaction.

Some underlying substrate crystal planes are in evidence at the upper

and lower left portions of the micrograph along with the relatively

disordered surface layer. The micrograph indicates that the residual

adsorbed film was partially stripped away with no sign of more permanent

bulk defects in niobium brought about by the hydrogen interaction.

Typically, as the electric field was necessarily increased to image with

neon after a hydrogen treatment, the adsorbed layer was observed to

evaporate quickly.

The neon field ion micrograph of Fig. Id was obtained after the

surface layer was completely removed by field evaporation. The micrograph does not show any residual damaging effects to the niobium substrate. The spiral dislocations remain in place.

Therefore, these experiments indicate that niobium, although suffering surface damage from hydrogen, does not seem to undergo crystal -

1 ographical ly selective, deep damage as found in titanium(l) and zirconium^) under similarly low temperatures and hydrogen pressures. This

statement is tentative pending further examination of other niobium

specimens. Future studies will proceed in an effort to initiate hydrogen embrittlement by systematically increasing the severity of the

hydrogen-niobium interaction. For example, niobium specimens precharged

with as much as 2 atomic % hydrogen are known to fracture at 277 K.(4)

-4-
ALUMINUM

Aluminum is an example of a material that exhibits extreme instability at neon imaging field conditions in the unbaked field ion microscope with specimens cooled with liquid nitrogen. Aluminum under these

conditions is far more unstable than 1080 steel imaged with hydrogen at room temperature. (3) The neon images obtained from aluminum near liquid

nitrogen temperature are not typical of field ion images showing re- solved surface atoms but, instead, are similar to aluminum field desorption images, (7-9) usually obtained with a double channel plate and

without imaging gas. The image shown in Fig. 2 was obtained from an aluminum specimen neon field evaporating hundreds of layers during the

1/10 s film exposure time. The crystal directions and zone lines appear

relatively dark against an overall gray background. Disparties between

published desorption images(7-9) and those in Fig. 2 occur about the

<100> direction where four relatively bright (instead of dark) lines are symmetrically disposed. This as well as the few bright spots distributed

about the surface do not appear in the normally derived field desorption

patterns(7-9) and are ascribed to artifacts associated with the neon imaging gas.

By cooling from about 80 to 30 K, aluminum was nearly stabilized

(slowly field evaporated) under imaging conditions and became accessible

to neon field ion microscopy. Fig. 3 compares favorably with previously

published neon field ion micrographs of aluminum(10,ll ) in that it shows

overall crystal order. The (111) and (100) plane areas are not normally

imaged fully in neon. (10,11) However, in the (110) and higher index

plane areas (Fig, 3), there appears to be relatively short, dark lines

that interrupt the ordered concentric rings of atoms. These minor line

defects brought about by atomic vacancies are not found in the published

micrographs(10,l 1 ) that were obtained using a baked field ion microscope. The evidence suggests that these defects and aluminum's slight

instability during neon imaging may be due to residual hydrogen in the

background vacuum at low partial pressures.

Hydrogen field ion images of aluminum (Fig. 4a) are composed of

small areas marked by boundaries, sometimes showing ordered square

arrays of spots and partial concentric rings, indicating that hydrogen

may have chemically interacted with aluminum surface atoms to form

another surface phase. Fig. 4b is a neon field ion micrograph of the

previously formed hydrogen-aluminum interactive layer obtained after

removing the hydrogen imaging gas and partially evaporating some of the

interacted layer at the higher neon imaging fields. The long line defects are present in the micrographs of Figs. 4a and 4b but have disappeared in the image of Figure 4d. The micrographs in Fig. 4 occurred

sequentially and were selected from a larger group. They are collectively interpreted as indicating that the observed line defects originate in the aluminum-hydrogen interacted layer and do not extend very

deeply into the aluminum substrate. Furthermore, Fig. 4c shows atomic

resolution in the normally darkened (100) and (111) plane areas. This

indicates that hydrogen adsorbed on aluminum aids in neon imaging of the

(100) and (111) plane areas.
Interesting surface effects were observed by imaging aluminum

specimens with mixtures of hydrogen and neon (Fig. 5). Hydrogen promotes imaging of the aluminum (100) and (111) plane areas but also

promotes formation and field evaporation of the aluminum-hydrogen

interacted layer. The micrographs in Fig. 5 were obtained during field

evaporation. As opposed to imaging with pure hydrogen (Fig. 4a), lower

partial pressures of hydrogen in the imaging gas mixtures prevent

thicker film formations to occur that obscure the symmetry of the

aluminum substrate. Nevertheless, the micrographs (Fig. 5) demonstrate

that there is a multitude of surface defect effects brought about by the

presence of hydrogen.

Aluminum field ion micrographs obtained with hydrogen-helium mixtures (Fig. 6) show more crystalline order than micrographs obtained

with hydrogen-neon mixtures (Fig. 5) indicating that the hydrogenaluminum interacted layer is less extensive. There are more higher

index planes visible with concentric rings and fewer surface defects.

Usually, there are only a few concentric rings about a crystal direction

before the atom spots become more randomly arranged. It is in these areas where surface defects appear and are typical of those found in

hydrogen-neon field ion micrographs of aluminum. Occasionally, long

line defects occur that cut across crystal planes. Future attempts to

image aluminum alloys may be realized best by using hydrogen promoted

field ion microscopy, that is using small traces of hydrogen in an otherwise inert image gas.

It is clear from these studies with aluminum that significant but

not deep surface defects occur in the presence of hydrogen and the

tensile stress brought about by the imaging electric field. Quantitative determinations of depths to which the defects penetrate the aluminum lattice have yet to be made. (Qualitatively, they appear to be less

deep than those found in the hydrogen-ti tanium(l ) system.) This is a more difficult matter that may require the use of a baked field ion

microscope, in the light of aluminum's sensitivity to hydrogen. Of

course, field-ion-atom-probe-mass-spectrometry(12) would provide very

useful information regarding the chemical identity of the imaged species

in the immediate vicinity of the observed defects.

304 STAINLESS STEEL

Like aluminum, this alloy was suggested as one in which the mechanism of stress corrosion cracking involves hydrogen embrittlement. (6)

It was previously shown(3) that field ion micrographs of 304 stainless

steel exhibited, generally, more disordered imaged areas than found in

1080 high strength carbon steel and iron-24% chromium steel. The better

micrographs(13) showed evidence of grain boundaries and some short-range

order (crystal net plane rings immersed in an array of disordered spots).

When that particular grain was reduced in size by further field evaporation (Fig. 7a), the smaller grain (brighter spots) showed long-range

order, but the larger grain (Figs. 7a and b) showed only short-range

-6 -
order. The particle imaged in Fig. 7a was probably an unmixed alloy

constituent of dimensions approximately 22 x 38 nm. Micrographs intermediate between Figs. 7a and b show it as small as 5 nm in diameter.

It is possible that artifacts may give rise to relatively disordered field ion images of materials that would otherwise appear

ordered. These include (a) premature field evaporation of some alloy

constituents leaving vacancies, (b) significantly different best imaging

fields among the various alloy constituents thereby causing only some to

be imaged, and (c) especially persistent adsorbed films covering the

sought-after substrate.

On the assumption that the latter condition prevailed in the case of 304 stainless steel, the technique of manually pulsed field ion

imaging was attempted and exhaustively explored without successfully

improving image quality. The technique is known to be successful in

removing persistently adsorbed films from iron-24% chromium and uranium

at 30 K. (3) During the high electric field pulse, specimens are dramatically destabilized and several hundred layers are quickly field

evaporated. Following the pulse, the applied voltage is restored to one for best imaging. The specimen is stabilized and, momentarily, is in a relatively film-free state. Unfortunately, no firm conclusions may be

drawn from the above results.

However, more rewarding results were obtained from 304 stainless

steel wires that were vacuum annealed (about 800 to 1000C for four hours

and quickly cooled) before field ion emitters were prepared from them.

Annealing temperatures were high enough so as not to promote sensitization

(precipitation at grain boundaries) and long enough to promote grain

growth without crystal structure changes. The hydrogen ion images in

Fig. 8 demonstrate the improved image quality that results from preannealing the wires. Apparently, in degassed specimens (annealed), a more unobstructed image of the alloy surface is obtained; whereas, in

unannealed specimens, some imaging of dissolved gases occurs and introduces apparent disorder. There are a great many boundaries in evidence

in Fig. 8b and in a subsequently obtained helium-neon ion micrograph of

the same specimen. (Note the regions indicated in Fig. 8c). Their

origin remains unclear, but, nevertheless, the boundaries could indicate

the initial formation of martensite, which a number of workers suggest

is involved in the stress corrosion of 304 stainless steel by a hydrogen

embrittling mechanism. In addition to determining crystal structure of

the regions contained within the boundaries, it would be useful in

future studies to examine 310 stainless steel because, as opposed to

304, it is known(14) not to form martensite in the presence of hydrogen

and stress.

IRON-24 wt. % CHROMIUM

This final segment reveals the kinds of effects brought about by a series of cathodic hydrogen charging experiments on previously field

-7 -
evaporated and characterized iron-24 wt. % chromium surfaces. The

necessary control experiments with alloy specimens not subjected to

cathodic polarization showed minimal effects.

Fig. 9 shows examples of neon field ion micrographs obtained from

liquid nitrogen cooled iron-24 wt. % chromium. Images were slightly

unstable and slowly field evaporated at best image voltages. Overall,

specimens exhibited cubic symmetry with four (111) plane areas about a central (100) plane. Some specimens were vacuum annealed prior to tip

manufacture, and the microscope vacuum system was subjected to varying

degrees of gettering without effecting change in the following two

features of the micrographs. (1) The presence of dark, non-imaging

areas predominantly appearing in the central region of the micrographs — corresponding to the tip apex and indicating preferential attack and

field evaporation in the [110] -[100] zone lines and (110) planes. The

recessed surface atoms may then be field shielded by the outer shell of

imaging atoms and, thereby, not exposed to the high electric fields

necessary for neon imaging. (2) The presence of numerous line defect

boundaries distributed about the imaged surface, not unlike those found

on aluminum specimens imaged in neon-hydrogen mixtures (Fig. 5) and, to

a lesser extent in the neon ion micrographs of aluminum (Fig. 3). The cause of these defects may be low partial pressures of active gases,

such as hydrogen, water vapor and oxygen, in the microscope's base vacuum.

Liquid nitrogen cooled iron-24 wt. I chromium specimens imaged with

hydrogen (Fig. 10a) were stable and did not field evaporate at imaging

voltages. Overall, the (111) planes appear to chemically interact the

least with hydrogen imaging gas, as evidenced by the preservation of

their concentric ring appearance. The centrally located [ 1 1 0] - [ 1 00] zone

lines and (110) planes show non-ordered imaging spots. This specimen

previously imaged with neon showed no imaging points in these same areas. Possibly, under hydrogen imaging conditions, the supposedly

recessed [ 1 1 0] -[ 1 00] zone lines and (110) planes became visible via a hydrogen adsorbed layer. In cases where iron-24% chromium was reimaged

with neon following hydrogen ion microscopy (Fig. 10b), it was necessary

to remove several tens of layers of (111) planes before zone line darkening would begin to reappear (Fig. 10c). Although it might seem that

the prior hydrogen interactions caused extensive damage in these areas,

we prefer, instead, to interpret persistent zone line and (110) plane

imaging after a hydrogen exposure as arising from residual hydrogen

promoting neon imaging of these areas. Similar zone line imaging occurred

with controlled mixtures of hydrogen and neon.

In another approach to the hydrogen-iron-24% chromium interaction,

additional experiments were carried out that explored the effects of

cathodically charging the alloy lattice with hydrogen. Typically,

liquid nitrogen cooled specimens were initially characterized (field

evaporatored ) in neon and/or hydrogen. The field evaporated tip was

- 8 -
removed from the microscope, dipped and withdrawn from a 10% aqueous

perchloric acid solution under an applied voltage at room temperature,

and subsequently reimaged near 80 K in the field ion microscope. A

platinum counter-electrode was used, and the applied voltage and immersion

time were varied. Following cathodic polarization, micrographs were obtained with hydrogen since all specimens catastrophically fractured

near neon imaging fields. As summarized in Table II, the results obtained

varied from little to no detectable alloy surface damage at 10 Vdc, 4s,

to film formation without significant substrate damage at 10 Vdc, 4

min., and significant substrate damage without film formation at 20 Vdc,

4 min.

- 9 -
TABLE II

Annotations from FIM Studies of Cathodically Charged

Iron-24 wt. % Chromium Specimens in Aqueous Solutions of

10% Perchloric Acid Solution Versus a Platinum Counter-Electrode.

Series Appl ied

Vol ts

(dc)

Exposure

Time

Remarks

a 10 4 s no significant surface damage (Fig. 11)

b 10 1 min no significant surface damage

c 10 2 min considerable tip blunting with surface damage

(Fig. 12)

d 10 4 min film formation without detectable substrate

damage

e 10 8 min film formation without significant substrate

damage (Fig. 13)

f 20 4 min great alloy surface damage without film formation

(Fig. 14)

Figs. 11a and b show hydrogen and neon ion micrographs of an iron24% chromium specimen obtained prior to cathodic polarization in series

a. Figs. 11c through f are hydrogen ion micrographs obtained at progressively

higher voltages directly after cathodic polarization. The images of

Figs. 11c and d were unstable and typical of those showing fleeting

image points from a desorbing accumulation. The image of Fig. lie

showed the first signs of surface order and increased stability of

imaging points. The left crescent shaped area of spots came from an area of residual accumulation that was not field evaporated. Some spots

in this area appear as large doublets and quadruplets and are similar in

appearance to those in Figs. 11c and d. Note that the transient image

spots and the crescent shaped area in Fig lie are not present in Fig. 11

F. These accumulations are usually short lived in the field ion microscope

and are distinguished from more permanent film formations (probably

thicker) that occurred in series d and e. A fully field evaporated

hydrogen ion image of the alloy surface in Fig. Ilf shows evidence of a disturbed (111) plane cut by a possible grain boundary.

The sequence of hydrogen ion micrographs in Fig. 12 was obtained

from series c with micrograph 12a obtained before cathodic polarization.

Figs. 12c and d show evidence of surface damage in the dark lined areas. However, the origin of these defects is inconclusive since a tear-off

- 10-
event occurred between Figs. 12b and c. A tear-off event is defined as an uncontrolled field evaporation of a material during imaging. This

was the only series of the 10 V set in which a tear-off event occurred

as well as detectable alloy surface damage and a relatively large increase

in average specimen radius after cathodic polarization.

The sequence of hydrogen ion micrographs in Fig. 13 was obtained

from series e. An interesting feature in this set is the presence of a boundary, bisecting the image in Fig. 13a — obtained before cathodic

polarization, showing apparently dissimilar coverages in Figs. 13b and c — consecutively obtained after cathodic polarization, and persisting

after film removal. The sequence of micrographs was obtained at progressively higher voltages. Fig. 13d shows a stage in which the adsorbed film (Figs. 13b and c) receded from the central area of the

micrograph to reveal the underlying substrate.

Fig. 14 shows a sequence of hydrogen ion micrographs obtained from

series f. In this case there was no evidence of an adsorbed film since

the images obtained after cathodic polarization immediately showed

ordered substrate areas. The micrographs clearly show great surface

damage, far in excess to that observed in series a through e. Several

tens and possibly hundreds of layers were evaporated without successfully removing some of the more deeply damaged areas.

In summary, it was demonstrated that FIM detected a spectrum of

surface effects brought about by a broad brush variation of cathodic

polarization conditions in iron-24% chromium. It is desirable, however,

to remove hydrogen from all post-cathodic polarization imaging because

a) hydrogen is the agent promoting hydrogen embrittlement, and b) neon

ion micrographs show more clearly the line defects inherent in the

material. Since all cathodically polarized specimens failed at the

higher stresses associated with the larger imaging fields necessary for neon imaging, it suggests that smaller hydrogen charging doses are necessary to avoid failure at neon imaging fields. To this end, future

FIM experiments will be directed.

From the preceding work it is clear that FIM is capable of characterizing different modes of hydrogen interactions with metals under

stress. Although present studies were limited to a relatively low

temperature regime, it should be possible in future work to stressinteract at room temperature with intermittent low temperature microscopy at least with materials more refractory than aluminum.

-ll-
REFERENCES

(1) J. J. Carroll, J. Smit, and A. J. Melmed, "Field Ion Imaging of

Titanium and Effects Due to Hydrogen", Surface Sci. 45 (1974) 640-8.

(2) J. J. Carroll and A. J. Melmed, "Field Ion Microscopy of Zirconium",

Surface Sci. 5J3 (1976) 601-4.

(3) J. J. Carroll, A. J. Melmed, and J. Kruger, "Field Ion Microscopy

Studies of the Interaction of Hydrogen with Selected Metals and Alloys",

i n : The Role o f Passive Film Growth Kinetics and Properties in Stress

Corrosion and Crevice Corrosion Susceptibility, NBSIR 78-1429, Part I,

(May 1978) 1-16.

(4) M. L. Grossbeck, P. Williams, C. A. Evans, Jr., and H. K. Birnbaum,

"Application of Ion Probe Analysis to Studies of Hydrogen Behavior in

Solids", Phys. Stat. Sol. (a) 34 (1976) K97-9.

(5) R. J. Gest and A. R. Troiano, "Stress Corrosion and Hydrogen Embrittlement

in an Aluminum Alloy", Corrosion 30 (1974) 274-9.

(6) R. W. Staehle, "Stress Corrosion Cracking of the Fe-Cr-Ni Alloy System",

i n : The Th eo ry of Stress Corrosion Cracking in Alloys , J . C . Scu 1 1 y

,

Editor, TNTA.T.O. , Brussels, 1971), 221-286.

(7) E. D. Boyes, A. R. Waugh, P. J. Turner, P. F. Mills, and M. J. Southon,

in: Developments i n Electron Microscopy and Analysis, 1977 , (EMAG 77),

D. L. Misell, Editor^ Inst. Phys. Conf. Ser. No. 36 (1977) Chapter 9,

343-6, (Fig. 4).

(8) A. R. Waugh, E. D. Boyes, and M. J. Southon, "Investigations of Field

Evaporation with a Field-desorption Microscope", Surface Sci. 6]_ (1976)

109-142, (Figs. 3 and 6).

(9) M. J. Southon, E. D. Boyes, P. J. Turner, and A. R. Waugh, "Some Applications

of Field-ionization and Field-evaporation Techniques in the Study of

Surfaces", Surface Sci. 53 (1975) 554-580, (Fig. 11).

(10) Ref. 9, (Fig. 4)

(11) W. Zingg and H. Warlimont, "On Imaging of Aluminum by Field Ion

Microscopy", Phys. Stat. Sol. (a) 45 (1978) 117-122, (Fig. 2).

II (12) E. W. Muller and T. T. Tsong, Field Ion Microscopy; Principles and

Appl icati ons (Elsevier, New York, 1969), pp. 130-4.

(13) Ref. 3, (Fig. 7).

(14) M. L. Holzworth, "Hydrogen Embrittlement of Type 304L Stainless Steel",

Corrosion 25 (1 969)“ 107-115.

- 12-
I

(c) (d)

Fig. 1. Consecutive field ion micrographs obtained from the same niobium

specimen, liquid nitrogen cooled. (a) neon image, 14,500 volts;

(b) hydrogen image, 8,800 volts; (c) neon image, 13,300 volts;

and (d) neon image, 14,200 volts.

- 13 -
Fig. 2. Neon image obtained from liquid nitrogen

cooled aluminum, about 13,000 volts.

Fig. 3. Neon field ion micrograph obtained from

aluminum, 30 K, 12,000 volts.

- 14-
Fig. 4. Consecutive

specimen of

(b) and (c)

volts

.

field ion micrographs obtained from the same aluminum, 30 K. (a) hydrogen image, 7000 volts;

neon image, 10,700 volts; (d) neon image, 12,000

-15 -
(c)

Fig. 5. Aluminum field ion micrographs obtained with neon-hydrogen

gas mixtures, 30 K. (a) 4600 volts; (b) 4700 volts; (c)

7850 volts.

- 16 -
(b)

Fig. 6. Aluminum field ion micrographs obtained with heliumhydrogen gas mixtures, 30 K. (a) 7300 volts; (b)

13,000 volts.

- 17 -
I

(b)

Fig. 7. Consecutive field ion micrographs [(a), (b)]

obtained from an unannealed 304 stainless steel

specimen imaged with a neon-helium gas mixture,

30 K, 12,500 volts.

-18-
(b) (c)

Fig. 8. 304 stainless steel field ion micrographs (a) unannealed specimen

imaged with hydrogen, 30 K, 6900 volts; (b) annealed specimen imaged

with hydrogen, 30 K, 4700 volts; (c) annealed specimen imaged with

a neon-helium gas mixture, 30 K, 7800 volts.

- 19 -
(b)

Fig. 9. Neon field ion micrographs obtained from iron-24% chromium,

liquid nitrogen cooled. (a) 8800 volts; (b) 14,500 volts.

- 20-
Fig. 10. Consecutive field ion micrographs obtained from the same specimen

of iron-24% chromium, liquid nitrogen cooled. (a) hydrogen image,

10,300 volts; (b) neon image, 14,800 volts; (c) neon image,

15,200 volts.

-21 -
(b)
( e

)

(f)

Fig. 11. Consecutive field ion micrographs obtained from the same iron-24% chromium specimen, liquid nitrogen cooled.

(a) hydrogen image, 8000 volts; (b) neon image, 12,400

volts. Hydrogen field ion micrographs (c) through (f)

obtained after cathodic polarization, series a, Table II

(c) 4900 volts; (d) 5900 volts; (e) 6800 volts; (f) 8100

volts

.

- 23 -
Fig. 12. Consecutive field ion micrographs obtained from

chromium specimen, liquid nitrogen cooled. (a)

6500 volts. Hydrogen field ion micrographs (b)

obtained after cathodic polarization, series c, 7000 volts; (c) and (d) 7700 volts.

the same iron-24%

hydrogen image,

through ( d

)

Table II. (b)

-24-
(a) (b)

Fig. 13. Consecutive field ion micrographs obtained from

chromium specimen, liquid nitrogen cooled. (a)

7800 volts. Hydrogen field ion micrographs (b)

obtained after cathodic polarization, series e, 6700 volts; (c) 7100 volts; (d) 8000 volts.

the same iron-24%

hydrogen image

,

through (d)

Table II. (b)

- 25 -
(a) (b)

(c) (d)

Fig. 14. Consecutive field ion micrographs obtained from the same iron-24%

chromium specimen, liquid nitrogen cooled. (a) hydrogen image,

8800 volts. Hydrogen field ion micrographs (b) through (d) obtained

after cathodic polarization, series f, Table II. (b) 7600 volts;

(c) 11,500 volts; (d) 12,900 volts.

- 26 -
PART II An Improved Triboel 1

i psometric Apparatus

J. J. Ritter and J. R. Ambrose

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington D. C. 20234

The triboelli psometric apparatus herein described is similar to

that used by Laser et. al . (1) The unit is specifically designed to

effectively remove, by mechanical polishing, a corrosion or passive film

which may have formed on a specimen in a simulated service environment.

El 1

i psometric and electrochemical measurements on the abraded specimen

can be initiated within 1ms of the cessation of polishing after rapid

displacement of the polishing arbor away from the specimen surface.

Thus, surface film regrowth kinetics may be effectively examined at a much earlier stage than has been heretofore possible. (2)

Mechanical and Electrical systems: Figure 1 shows the disposition

of mechanical and electrical components. The components are mounted on

a massive frame and are physically separate from the ellipsometer base

upon which the PTFE-lined speciman cell is mounted. This arrangement

minimizes the transmission of vibration to the specimen. The spindle

(g) operates at 300 rpm and is coupled to the drive shaft (m) by means

of a spindle connector (e). The spindle connector is supported on the

drive shaft by four rows of ball bearings as shown in Section AA. The

rows of ball bearings are physically locked to the spindle connector and

one of the rows engages a longitudinal slot in the drive shaft. Thus,

while being rotated, the drive shaft is free to move longitudinally to

compress the spindle pressure spring. A similar "ball bushing" device

is utilized to connect the drive pulley (d) to the drive shaft and

allows both rotational and longitudinal movement of the drive shaft.

The drive pulley is coupled to the electric motor (n) by means of a toothed belt and speed reduction pulley. The polishing arbor is fitted

with a sponge rubber pad and nylon lap cloth impregnated with 3ym

diamond paste.

The electrical circuitry is shown in Figure 2. The unit features a control for varying the abrasion time between 5 and 15 seconds and an externally available output pulse which can be used to trigger the

storage oscilloscope used for recording.

Operational Details: With the polishing spindle in the fully

retracted position, the specimen is aligned for maximum reflection of

incident light into a photomultiplier tube (PMT). The input light is

modulated at 1kHz and the PMT signal synchronously processed with a lock-in-ampl ifier.

An electric motor (n) is activated and a preselected pulse of from

5-15 sec. is applied to solenoid (a). This causes the driveshaft (m) to

- 27 -
move forward and the polishing arbor (k) to contact the specimen (h).

The amount of force applied to the speciman can be varied by altering

the compression of the spring within (e). This, in turn, is accom- plished by adjusting the position of, and thus the plunger stroke of

solenoid (a). When the preset polishing time has elapsed, solenoid (a)

is deenergized and a 200 ms pulse applied to solenoid (f). A triggering

signal is simultaneously transmitted to the recording oscilloscope. The

armature within (f) is accelerated to the rear and ultimately strikes

the spindle connector (e). The consequent momentum transfer results in

a rapid spindle retract.

The oscilloscope trace shown in Figure 3 indicates the sequence of

events when solenoid (f) is energized. The upper left horizontal portion

of the trace represents the travel time required before armature (o)

strikes connector (e). Since the light path is blocked by the polishing

arbor, no PMT signal is observed. As the arbor is retracted the PMT

intensity increases and reaches a maximum within %0.75ms. Thus, film

regrowth studies can be initiated ^]ms after polishing ceases.

- 28-
References

1. D. Laser, M. Yaniv and S. Gottesfeld, J. Electrochemical Soc.

125 , 358 (1978).

2. J. R. Ambrose and J. Kruger, Corrosion, 28, 30 (1971).

- 29 -
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NBS-114A (REV. 9-76)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS IR 79-19042. Gov’t 3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Passive Films, Surface Structure and Stress Corrosion and

Crevice Corrosion Susceptibility

5. Publication Date November 1979

$. Performing Organization Code

7. AUTHOR(S) J. Kruger, J. J. Carroll, A. J. Melmed, J. J. Ritter, and

J. R. Ambrose

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 5610448

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No. NAONR 18-69, NR036-082

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP) Office of Naval Research

Department of the Navy

Arlington, VA 22217

13. Type of Report & Period Covered

interim 12/77-12/78

15. SUPPLEMENTARY NOTES

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

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

literature survey, mention it here.) Field ion microscopy studies

niobium, aluminum, 304 stainless

Nb and Al exposed to hydrogen at

surface disarray, but the effects

The 304 stainless steel show some attack. Cathodically charged Feof the effects of hydrogen interactions with

steel and Fe-24% Cr steel were carried out. Both

80 K and 30 K respectively showed significant

were not deep or crystallographically selective,

indications of crystallographically selective

24% Cr showed a broad spectrum of effects.

A new triboellipsometric apparatus which is capable of measuring repassivation

events occurring in times of the order of 0.1 ms is described.

17. KEY WORDS ( six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Aluminum; field ion microscopy; iron—chromium alloys; niobium; repassivation kinetics;

stainless steel; triboellipsometry

18. AVAILABILITY pq Unlimited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-

|~xl Order From National Technical Information Service (NTIS), Springfield, VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

34

22. Price

$4.50

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