AlllDO TTMfi3S

NBS

f^^BLICATlONS

NBSIR 81-2396 (ONR)

Passive Films, Surface Structure

and Stress Corrosion and Crevice

Corrosion Susceptibility

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Stability and Corrosion Division

and

Surface Science Division

Washington, DC 20234

August 1 981

Technical Summary Report No. 1

1

Issued November 1981

Prepared for '^^fice of Naval Research

-QC «MHHMipartment of the Navy

2QQ

lington, VA 22217

.U56

81-2396

1981

c. 2

NBSIR 81-2396 (ONR)

PASSIVE FILMS, SURFACE STRUCTURE

AND STRESS CORROSION AND CREVICE

CORROSION SUSCEPTIBILITY

ational bobkau

or standabdb

librart

JAN 8 1982

flat CLC( - (jiAjuD-Cioo

USG

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

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Chemical Stability and Corrosion Division

and

Surface Science Division

Washington, DC 20234

August 1 981

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Issued November 1981

Prepared for Office of Naval Research

Department of the Navy

Arlington, VA 22217

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
UA»tra«t lA^OSTXM

ftfjafAuwATa -HO

^ lilAt'M

tJ \ »

“sHUTDWm^ lomte.VaW^H 3V|^

aaWaflS oSrtAMOsaoafioo WsRTs d«A '-»< - r . ' _m

TOiiaiT'Rao^feua, irtoiaortRpcJ^?

- -

-

I i>wiW<ii|(lft'iii>i| f*!'*'*'* Ai I'l^'^iMinl'i TTllt ^ Hlj '*

-.?3*}‘

i IS%

-J..'

,.\j^'- Z' - ,

• ''j^*' liSWn I

-%'S atii—nsi vl

® ^ '

'Mk-h

yy^ti 4*t<XlO

i hiKnH^PW. . .9 m. m\mj . iiyi — jomfs^fia 30 TMiw I nfmtt.t.0^

W94U* ^HMWH
[Part I - From "Proceedings of the International

Conference on Corrosion Control by

Organic Coatings," Henry Leidheiser, Jr.

ed., NACE, Houston, TX (1981) and from

SP-485, Society of Automotive Engineers,

Warrendale, PA (1981).]

Studies on the Sub-Coating

Enviroment of Coated Iron Using

Qualitative Ellipsometric and

Electrochemical Techniques

J. J. Ritter and J. Kruger

Chemical Stability and Corrosion Div. National Bureau of Standards

ABSTRACT

Transparent organic coatings on iron are used to simulate painted metal surfaces for

simultaneous ellipsometric and electrochemical

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

and in the sub-coating environment during prolonged immersion in dilute Cl media. The

relationship of these changes to phenomena such

as metal passivation, coating delamination, and

inhibitor behavior are addressed.

THE BEHAVIOR OF ORGANIC COATINGS on metals has

been examined with a variety of electrical and

electrochemical measurement techniques including

dc resistance, ac resistance, capacitance,

corrosion potential etc. (1)*. The objectives

of these studies has been both to evaluate

coating efficacy and to probe the processes

which lead to coating failure. We describe in

this work results from the application of a new

technique of combined ellipsometric and electrochemical measurements to a study of the fundamental corrosion processes on metals protected

with organic coatings.

The details of the technique have been

described previously (2). Briefly, painted

metal is simulated using a polished iron specimen

coated with transparent cellulose nitrate (collodion). The specimen is provided with a miniature probe to measure pH under the coating while

visible, elliptically polarized light passes

through the coating and is reflected from the

metal surface. Computer modeling and experiments

with collodion on relatively inert gold surfaces

have indicated that for the ellipsometric conditions chosen (A = 546. 1 nm, 4> = 68°) the ellipsometric parameters A and ip are most sensitive

to changes at the metal-metal oxide surface.

Thus, for example, variations in the ellipsometric parameter A can be related to the growth or dissolution of the oxide film at the surface of

the metal and under the organic coating. At the same time, the pH of the environment to which

*Numbers in parentheses designate References at end

of paper.

the metal is subjected can be assessed.

The technique has been used to study both

coated iron and iron with various inhibitors

available either in or under the coating material.

Due to the complexity of the multilayered system

which must be dealt with in these experiments,

the ellipsometric results are useful only in a qualitative sense. However, cathodic reduction

experiments done on stripped iron specimens in

deaerated borate buffer environments permitted

some semi-quantitative assessment of sub-coating

events

.

EXPERIMENTAL

Specimen preparation details have been

published previously (2). For the present purposes it suffices to say that the iron specimens were cast in epoxy, polished to a 0.05 pm finish

and dipped in a 1:1 collodion-methanol mixture to give a coating of from 10 - 30 pm in thickness

over the face of both the metal and the epoxy

mount. The prepared specimen was inserted in an

improved mount (see Fig. 1) and set in a polytetraf luoroethlene lined cell which was filled with

0.05 N NaCl. Inhibitor materials were either

applied directly to the metal before coating or dispersed as a finely ground powder in the collodion-methanol mixture. For the case of SrGrOi,,

the metal specimen was dip coated, finely powdered

SrCr04 applied to the surface of the semi-cured

coating and a second coat applied to cover the

inhibitor

.

The phosphate-chromate surface treatment was accomplished as follows: Solution I : H3PO4 3.0 g, ZnO 1.5 g, NaN03 1.0 g, make up with H2O to 100 ml.

Solution II : K2Cr207 0.7 g, H3PO4 0.5 g, make up

with H2O to 100 ml.

The solutions were used at room temperature.

The face of the specimen was rotated in a horizontal plane at 50 rpm in a mixture comprised of 8 ml of solution I and 2 ml of solution II for a period of six minutes. This procedure gave an homogeneous surface finish which retained sufficient specular quality to allow ellipsometric

measurements.

The ellipsometric measurements on coated

iron specimens show changes in both the A and

34

0 parameters with time (see Figs. 2 and 3). By

far, the largest change is seen in the A parameter

which always shows a decrease. It is known that

several factors could contribute to this directional change in A, among them, metal oxide film

growth, surface roughening (3) and possibly

coating deterioration. These effects and their

relative contributions to the systems under study were investigated as described below.

The extent of oxide film growth under the

coc' ings was estimated at the conclusion of an experiment by stripping the coating with methanol,

and cathodically reducing the metal surface at -1000 mV S.C.E. in deaerated borate buffer solutions (pH 8.5) while observing the surface with

the ellipsometer . Oxide film thicknesses were calculated assuming that the film was essentially

Fe 203 and that each degree change in A corresponded to 0.6 nm of film.

The presence of surface roughness was qualitatively detected by cycling the specimen potential

between -1000 and +200 mV approximately 60 times. A significant Increase in the A parameter indicated surface smoothing as a result of electrochemical polishing.

Contributions from the coating to changes in

A were estimated by elllpsometrically monitoring

collodion coated gold specimens in 0.05 N NaCl

and in saturated NaOH (pH vl5) .

RESULTS AND DISCUSSION

The examples of experimental results shown

in Figs. 2 and 3 indicate at least two stages of

activity. In the first stage, (generally up to

''-1000 min) the pH tends to rise gradually while

the elllpsometric parameter A tends to increase

several degrees*. As the pH approaches 11, the

A values undergo a significant decrease which

signifies the beginning of the second stage.

Invariably, small anodic regions become visible

during the second stage with final overall anodic

to cathodic area ratios ranging from 1/30 to 1/600.

In some experiments, particularly in those

wV 3re no inhibitor is present and the pH remains

high, a third stage is observed where A fluctuates.

This behavior can be seen in Fig. 2. The most significant change occurs in the A parameter during the second stage. In cases where no Inhibitor is present this change arises

from three factors given below along with their

estimated contributions to the overall 6A: (1)

metal surface roughening, ''-55 percent; (2) oxide

film growth, ''-35 percent, and (3) changes in

coating material in a high pH environment ''-10 percent. It will be noted that the second stage

changes of A with time appear to approximate a semi logarithmic relationship, i.e., dA = k log t + c. This law is frequently observed in

the growth of passive films on metals (4) . The

rate constants, k, for coated iron with various

applications of inhibitors are summarized in

Table 1.

*In some experiments, the Avalues tend to decrease

several degrees over the first stage. The con- tributing factors to this portion of the curve

are not fully known.

With reference to Table 1 it is readily seen

that the rate of elllpsometrically observed subcoating events for coated iron and for coated

iron which had been given a phosphate-chromate

pretreatment are quite comparable. However, when

K2 Cr04 is present at the metal surface, the rate

is slower by a factor of 10, whereas ZnCrOu

applied in a similar fashion slows the rate by

only a factor of two. This phenomenon may be

partly related to solubility differences between

the two inhibitors. Inhibitors dispersed throughout the coating give a similar factor of two reduction in the rate. The mechanism by which

chromates inhibit corrosion reportedly involves

the incorporation of chromium into an existing

oxide film, i.e. a chemical and structural modification of the existing oxide film which results

in improved protection (5). The rate constants

derived from the present work seem to be related

to inhibitor ion mobilities and accessibilities

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

(e.g. upon the incorporation of chromium) which

may change its refractive index, are expected to

TABLE I

Stage II Rate Constants for Elllpsometrically

Measured Sub-coating Events on Collodion-Coated

Iron in 0.05 N NaCl

Summary of Elllpsometric Rate Constants for

Stage II Events

Semi-Logarithmic Behavior Assumed

dA = k log t + c

Specimen Immersion Medium k(deg)

Uncoated Fe Sat'd NaOH 76

Coated Fe 0.05 M NaCl 83

Coated Fe with

deliberate holiday

tl 73

Coated Fe with

phosphate-chromate

surface pretreatment

ft 76

Coated Fe with

K-CrO, "islands"

2 4

If 8

Coated Fe with

ZnCrO, "islands"

4

II 35

Coated Fe with

ZTOC* dispersed in

coating

It 58

Coated Te with

K

2 CrO^ dispersed in

coating

It 42

Coated Fe with II 56

SrCrO, located between

4 two layers of collodion

*Zinc tetraoxychromate

[Zn(OH)

2

] 2^

• ZnCrO^
<j' •?&

*

_Xi ii ' .1*1 f.itV

j *» #k.j

#J;wr , I ' (fcrfqeo^? p;

’

Tj(#« . -iV ^fTt>'‘> 'V-..|:» »3U^) »: j fhf • , j'fti • fitlt* iftW ^4', t*^^i''^'

.•>wp<^?5 .: <ii tfi' . >» » *>a^ * t<f'

'3 »*•' ,

«! i"

.•jr

'ii. . 'T

J

t S

fcijl-

>d #»9t'©^nafio’ »lHT , iXO; :: a<iTl A

,t> i-tr^Xft.-

7

Tii BflO’S':' ^ i i.a ^o J +HI I i ;''-A}i?.-olXi-0; e3{«i’v?l 4(0^- • fl"'' '17I=«»N

' ^ .^v ne>7l

'mj. Kiiift'UO«a J&3t7s 56 ^ltsaett>

' ’ tr a*'

hv^Wi-aA -3 >.y#2

4-'

) gAA,.f- • it- : >;

i'fi'

. 3W'V»

6^ j „„

-l$S2Si ^ 4o? w

irf'i-V'-M b®24^*C>'

n*feta(»J»i

'*

^ dJlvjrt |i(|Joo3

5* Ak 4xh# <t b^URoD*

111 J>uflr77K|«jb *'^>Qti'

ItllTlDi

i

<' =5^ ftT b i'ioo^ lb

' j- ' ^

a.

ririTiii.- •3|b>f4f6fl:t ‘‘'^-

5q> ioflitaU 6 ;if4;«6.‘i (fj jmv

^ X*> )S'M(<6ft5% ?|TT. .•»>*$' iif3 nb

> no^«9<{%.jr» i^jDrit<f«i ijVrt otrfa' «6 icynX tt6jti?--‘7H.'f^,30;3^r*vti»ft6o*Vb .^3 ^ iA>iioawift..ii -4^. I ^rnJU.

dsjidw wJ VJ «;it'i-iiM4 :u>44i to'

*

. i3Wf»^^^r^ft>a tj.'ai 6”d't' .^3^0 1^-

'.4»?r'-X0'i 'td^Od W'?0w/^(ir«V97n *>X^^ ti*v>v4S>.

sr6>r«- Uian.tih irf-';

oj 'Roj3«qSi.' ftVfj’'jat-X'ax •’ X| UjUf.s fni ts^ae X ’

: I

j^Lkma^ ,

*1 r - ._

J*: rfyK 1*1 bbfoo^-

*u,j*v*>>»4<f ^653n£i

jsn-*i(:J.r. f*<5„o fi- >1 ' A>rijpW‘ » »c(d

• /' «*]|ij>* vwij! # W6l<a 7,:^;

' mi. ^C} <ni>T»i®B|< -» 4kl. s‘ii6 , voi3lii»h'«3%^^vf6>3Voo

xf»yl<i *9jbw« mrtt * 3i.(i/t':;l/yti^ oi?fc3«N

ba;:vvj|i j4tiV(4'‘ «T4W

»f?6 6ebrt6'4>«vi,* «|.U«»btxOi^1iii’A7M -'X» m ^0 'Sif.tftMi ji#-:^'.,, wv

y>: -yfo* - 1 , l*“I w» ' tiO0 ) -• I?tiv *345 ^ ya) art4X-» i'fnv iMi :,brt|.

.

if.ytf!^

' •iVoa.A rr •>

' 4;(P',o i 'bii'

s/*;

«r.jt»f«f^i «d>b^T><i||?gy’V.* • *f --Jitt. = ^'1%'!

.; . -^_... ._ .-.-^ 1^ ‘Up. i '

'4ir <

f *4 3 1'iSTe.ir

j -^aM ^ ^0.6 .ftly«ii A *1^7 A Kw

fci i^/tir4% r?6

-I I

;w p5i«“

,

Cdi Jits’/:

‘

: ..; .. .

=

. .'

.'.Srvrtfl ' *^1'*', - -; : >7

^t 't tg aftiT 03- y !*0*J *(l* ^1/4 .-:<i:. iiJ

I

'-i’ '»4ri4j -i^ t.M-tij^fj:

li^j. f^jj. » r . C j|

^ ^(Ea«54?A(>s-4*i ,4 6il34j' IM. »i^3;..-'f •

-fy L I tb'ocu', iHTfiiij Tfi-^ ^,--‘is gi‘i

^ i’Oij^.rert

' ilborfi^

tt8<3*42liFi4 f^jy*.^‘>'?Ja»j aMtSFpT

Mflt l|4* tinxi sJ Virj

J. ^ ® ^ -d*d

e iS*i ^i, -«4i* atfl _

r4«,io «V ;f^4 ;v^ 6^ nutfrtsimiy^c;

e.«^l'i3» if^pwt>i’’/p4^i VaaiE#56 :.'.iMX ** tl»43 xM>v

’ ^ 'tSA aim ftivii

CI> 46b' ila3»vr’ *''^S rt* •ri'oIi^4i7rti-6'

fttitea ii) i i% 3 v« j* 4it;tiC^

t,/}fi “,'3fl753iifl dCv--,v.^>tj-oV

ec-/ .^6 4;^^! a'4rt*i*if3o^^ *o465a6^‘' -^ j •loin ndi 'OsiXi^lV ii, • .

tt 03 -^31^ A w 'ilii^ll^' • *ib ,<jt<fwol:3aX»x ttXirt'33r*0|«^^

al 4^w*iafc*J4 xr7fivup4>3 «tliK'' 4:'

* ;* ,-(*> ii-»a4*N4i no wJit A

hualttv 4>lv <*#tl tf*J£i!il3 ‘m 4ii t4A<5i#j4!iS^

Hi b»ai7aWWMg *30

5^3 S

n1

©6 6rt*3.o*M^o,v'3 *r«i#i fii-*' iKfO i'*'^ :kff» *» i»»il li'iii; 73V&, w»'7.*(4b

*iin,**> M* lo *r»Ofti*V |nilOrfl‘*3
35

Fig. 1 - Schematic drawing showing details of the

specimen mount

Fig. 3 - A, pH and <j)p^ versus time for Fe with

collodion and K-CrO, islands in 0.05 N NaCl

2 4

Fig. 2 - A, relative phase retardation, \p relative

amplitude reduction, pH and potential of the

iron surface versus standard hydrogen electrode

(S.H.E.) versus time for Fe with collodion in 0.05 N NaCl
36be small for films <5nm in thickness (6). Thus

the thickness of these modified films can be

estimated with reasonable confidence using parameters for Fe 203 . Preliminary results from uncoated iron in a simulated sub-coating environment of pH 15 (sat'd

NaOH) , 0.05 M K2 Cr04 and 0.05 M NaCl show a change in A of which approximately 1/3 is due to oxide film growth and the remainder ascribed to surface roughening. The oxide film growth on uncoated Fe even in this highly alkaline environment was only 2.4 nm as compared to the typical 6 nm growth measured for coated iron in the absence

of inhibitor. Additional simulated environment

experiments with other inhibitors under a variety

of conditions are currently in progress and will

be the subject of a future publication. ^

During the first stage, oxygen, water, Na

and Cl permeate the coating. The dilute Cl

medium thus generated promotes localized breakdown of the existing air-formed oxide film originally present on the metal surface. Active

metal sites are exposed and iron oxidation is

promoted in these regions. Less active portions

of the surface serve as cathodic regions for the

reduction of O2 to OH . Hence the gradual rise

of pH during the first stage.

As the pH reaches high values (>13) other

events can proceed depending upon the potential

of the metal in its subcoating environment. For

example, experiments with uncoated iron in saturar<^d sodium hydroxide solution (pH 'vl5) suggest

that this potential may vary over the range

from -900 to -150 mV S.H.E. Under these conditions

iron can be brought into solution through the

formation of either the HFe02 or FeOu^ ions and

oxide film can grow, the oxidation of HFe02

being one source of Fe 203 . The experimental

evidence from cathodic reductions of stripped

iron specimens and from deliberate electrochemical

metal smoothing indicate that both surface roughening and oxide film growth phenonena occur under

the coating and are primarily responsible for the

large decreases observed in the A parameter

during the second stage. Thus the high pH environment and concomitant decreases in the A parameter

occur only when active corrosion is undetway

somewhere on the surface of the coated metal.

The new technique serves to detect the presence

of serious subcoating corrosion and provides a measure of the overall kinetics for some of the

sub-coating processes.

The implications of the observed surface

roughening and oxide film growth phenomena for

coating failure mechanisms remains somewhat

speculative at this point. For the case of an intact coating, one might envision the initial

formation of small, "two dimensional" localized_

corrosion cells as a result of O2 , water and Cl

permeation through the coating. The development

of these cells may entail local delaminations of

the coating. Anodic and cathodic regions develop

within the cells, the anodic/cathodic surface

area ratio being very small due to a paucity of

active sites.* Thus much of the local cell area

*The limitation of active sites may be related to the number of susceptible crystal planes at the

surface. For example, the iron {110} plane shows

a higher tendency toward passive film breakdown

and pitting than do other crystal faces.

develops an alkaline environment which is con- ducive to surface roughening and oxide film

growth. Topographical and possible compositional

changes of the substrate surface promote more extensive coating delamination and lead to the

coalescence of local cells into larger units.

These events probably occur during the first

stage noted in the experiments.

The anodic and cathodic reactions continue

to promote the enlargement of the delaminated

regions by the processes already described and

the active area gradually comes within the

sensitivity boundary of the elllpsometer. These

events are then detected as the second stage.

It is believed that by the time the second stage

is observed ellipsometrically , the sub-coating

corrosion processes are widespread over the

specimen face.

As corrosion products build up in anodic areas they contribute to the stresses which

finally cause coating rupture. With relatively

free access to the electrolyte at the rupture

site, the anodic activity accelerates and the

electrolyte pH in cathodic regions may exceed

14. Experiments with collodion coated Au in

NaOH at pH 15 indicate coating deterioration

after prolonged exposure. This deterioration

may be partially responsible for the erratic

behavior of A noted during the third stage.

The effects noted in the presence of K2 Cr04

inhibitor are a generally lower sub-coating pH

and comparatively less surface roughening and

oxide film growth. This is consistent with the

reported behavior of Cr04 ^ in supressing anodic

dissolution of iron through the formation of an effective passive film. Under these conditions,

the cathodic reduction of O2 must also be supressed and the sub-coating environment becomes

only mildly alkaline. Therefore surface roughening and oxide film growth are also attenuated.

Presumably this minimization of substrate topographical and constitutional changes translates

into reduced coating delamination and hence

longer coating life.

SUMMARY AND CONCLUSIONS

(1) The combined ellipsometric-electrochemical technique provides an effective, non destructive, in situ approach to the study of corrosion

processes under transparent coatings.

(2) The studies reveal the growth of oxide

film and surface roughening on those portions of

the substrate under the coating which are subjected to a highly alkaline environment.

(3) It is proposed that the Initial changes

in topography and possibly in the constitution

of the substrate surface may contribute to coating delamination.

(4) The presence of Inhibitors such as K2 Cr04 attenuates active dissolution of iron and

thus suppresses OH buildup. The resultant

attenuated film growth and surface roughening

can be monitored ellipsometrically, thus suggesting that the new technique may be a valuable

tool for the evaluation of inhibitor performance.

ACKNOWLEDGEMENT

This paper is reproduced from "Proceedings

of the International Conference on Corrosion
Control by Organic Coatings", Henry Leldhelser,

Jr., ed.. National Association of Corrosion

Engineers, Houston TX, (1981), with the kind

permission of the editor.

REFERENCES

1. Henry Leldhelser, Jr. "Progress in Organic

Coatings," (1979)" 79.

2. J. J. Ritter and J. Kruger, "Surface Science,"

1980, in press.

3. J. Kruger, "Corrosion," (1966), 88.

A. F. P. Fehlner and N. F. Mott, "Oxidation of

Metals," (1970), 59.

5. J. E. 0. Mayne and M. J. Pryor, "J. Chem.

Soc." (19A9) 1831.

6. J. Kruger, "Advances in Electrochemistry and

Electrochemical Engineering," R. H. Muller,

ed., Vol 9, (1973), J. Wiley and Sons., p 250.

7. J. Kruger, "J. Electrochem Soc." (1959), 763.

8. J. Kruger, "J. Electrochem Soc." (1963), 65A.

9. C. L. Foley, J. Kruger and C. J. Bechtholt,

"J. Electrochem Soc.," (1967) 994.
Fig. 1 - Schematic drawing showing details of the

specimen mount

Fig. 3 - A, pH and versus time for Fe with

collodion and K-CrO, islands in 0,05 N NaCl

2 4

Fig. 2 - A, relative phase retardation, \p relative

amplitude reduction, pH and potential of the

iron surface versus standard hydrogen electrode

(S.H.E.) versus time for Fe with collodion in 0.05

N NaCl
[Part II - To be submitted to CORROSION (1981).]

Abstract

The corrosion of metals protected by organic coatings is an incompletely

understood phenomenon. Recent investigations by an in situ ell ipsometricelectrochemical technique combined with microanalytical studies have

provided new insights into the destructive processes. Events such as

the roughening of the metal surface, chloride ion accumulation and the

thickening of the surface oxide have been detected under the coating and

are interpreted in terms of the chemistry which develops between the

microenvironment and the substrate.
r 'JO

tp.0/yf

'i?<f S'r J.

3(^

'

\f#Sj^Ci.<T<|' l^.-l I

3«5 ii

Mi few psi|»!pS|6f'i^r toi-f s(!S,^^.’«-j^«|e^ S'.'uis:’. »

w

6n»i gnHfop, Elf}' tEfeni! s#

9(tJ. >1S^3W^9d;: ZcVfsO-jb ;t>fiSK'Vr>Sfea9rt»;*^

T

'“w. ?

ci

>>aik

5 ; ji

^ S?;

^

“" ' "^:: - ..-v-l

‘C

(dt

. -a

cte-''

?^:-

S3 '

‘ifr, **t1 A

>ij

•E- »S»i

*: i ‘•''^.^'V -l^iT

I-

' .3

,M’*, ? f"*.

fe: ^' •.. ^

i

. J^." _i . ’f;.i»
AN EXAMINATION OF SOME OF THE

ASPECTS OF THE CORROSION OF IRON

PROTECTED BY A CELLULOSE NITRATE COATING

J. J. Ritter and M. J. Rodriguez

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D. C. 20234

Introduction :

The corrosion of metals protected by organic coatings represents an

extensive practical problem whose fundamentals are poorly understood.

Studies in these laboratories have been designed to probe some of the

basic aspects of this problem and elucidate some of the events which

occur at the coating-substrate interface.

A combined ellipsometric-electrochemical technique, developed at

NBS, has been used for the nondestructive, iji situ detection of corrosion

events occurring at the surface of an iron specimen protected by a

transparent cellulose nitrate coating.* For this system, the predominant

phenomena detected include the development of a highly alkaline subcoating

environment, a roughening of the metallic surface, an increase in the

*The technique involves the passage of elliptically polarized light

through the transparent coating and reflection from the substrate surface

while a miniature pH probe simultaneously monitors the environment at

the coating-substrate interface. Changes in the phase (6a) and amplitude

of the reflected light have been shown to be indicative of alterations

in surface texture (roughness) and changes in the thickness of the

surface oxide film. The optical system in its present configuration is

relatively insensitive to changes in the cellulose nitrate coating and

to the development of the microenvironment. Thus, the technique is of

value in studying the behavior of the substrate under the coating as the

corrosion process proceeds.
2

thickness of the metal surface "oxide" film, and an accumulation of Cl

ions under the coating. The results of these investigations affirm that

the subcoating microenvironment, i.e., the thin liquid layer which

gradually forms at the coating-substrate interface, takes an important

role in the subcoating corrosion process. It is the purpose of this

note to examine the subcoating phenomena in terms of the chemistry which'

ensues between this microenvironment and the substrate.

Experimental: The details of the el 1 ipsometric-electrochemical

measurement technique have been presented previously.^ In brief, the

coated iron sample is monitored ell ipsometrical ly and electrochemical ly

while immersed in a 0.05M NaCl solution. After approximately one week,

the iron sample is removed from the ellipsometer cell, and it is transferred

to a 100 percent humidity chamber. It is carefully rinsed with chloridefree distilled water, and the excess of external liquid is removed with

absorbent filter paper. The organic coating is incised with a sharp

scalpel, and an aliquot is removed from the blister by means of a micropipet.

The aliquot (usually from one to eight microliters) is transferred to a -12

volumetric flask, and it is diluted with low chloride content (10 M)

distilled water.

2

This chloride solution is analyzed potentiometrical ly using the

3 method adapted from Cunningham, Kirk and Brooks . An aliquot is treated

with dilute sulfuric acid, until the final acid concentration is (approximately)

0.3 to 0.4 N, then it is titrated with standard silver nitrate. The end

point is determined potentiometrical ly using a silver-silver oxide and a

silver-mercury amalgam electrode pair. The error in the analysis is of

the order of ±0.5 percent for 0.5 yg of chloride.
3

Results and Pi scussion

I . The Concentration of Cl in the Subcoatinq Microenvi ronment .

It is generally held that a viable subcoating microenvironment can

exist in a water layer as thin as 3nm. In the collodion-coated iron

systems this microenvironment will often achieve macroscopic dimensions

after about one week so that from 1 to 8 yl of liquid can be withdrawn

by micro techniques and subjected to analyses. Preliminary results of

Cl analyses on these microenvironment liquids indicate a Cl' ion concentration

three or four times that of the bulk immersion medium. Evidently there

exists an effective driving .force to promote the migration of Cl’ ions.

This driving force may be similar to that which promotes the concentration

of Cl’ ions in pits and crevices. With this idea in mind, we may explore

the development of corrosion on an organically coated iron specimen

through the following steps:

(1) The initiation of corrosion due to any number of causes such

as a defect in the coating, mechanical rupture of the coating,

the presence of a subcoating soluble surface impurity which

promotes coating rupture through the generation of osmotic

pressure, etc.

(2) In the early stages, anodes and cathodes develop in proximity

and as iron goes into solution it is rapidly precipitated out

as a hydroxide due to appreciable concentrations of OH’ ions

(from the cathodic reduction of O

2

) nearby. The hydroxide

corrosion products form into a hollow cone which ultimately

becomes a barrier to the free transport of neutral species

such as O^.
4

(3) Lack of oxygen to support the cathodic reaction would ordinarily

stop the corrosion process. However, elsewhere on the specimen

surface sufficient oxygen, water and some ions have penetrated

the coating and these regions now become cathodic sites. The

system continues to corrode with anodes and cathodes physically

isolated from each other.

I o (4) In the anodic regions, the Fe ions initially formed undergo

hydrolysis to produce additional solid corrosion products plus

ions. The ions require negative counter ions, and

for the coated metal system, sources are limited. For example,

the cathodically produced 0H~ ions are maintained captive near

their production sites by the organic coating. However,

chloride ions are abundant in the bulk solution and tend to

migrate into the hollow cone of corrosion products to maintain

charge neutrality (Fig. 1). This process is identical to that

4 proposed in crevice or pitting corrosion.

(5) The migration of Cl" into the anodic regions would leave an

excess of positively charged Na^ in the bulk except that these

ions will readily penetrate the coating at the cathodic sites

and act as counter ions to the OH" species. Watts and Castle

have verified the presence of significant amounts of Na in

areas of cathodic delamination through the use of surface

5 analysis techniques. The migration of the ions accomplishes

the charge transport necessary to sustain the corrosion reactions.

(6) At this point there are distinct regions on the specimen with

very different chemical compositions. The anodes will be
5

quite acidic and hold large quantities of Cl while the cathodes

will be very basic and rich in Na^. Thus it is quite possible

that the superimposed processes of Fe dissolution and surface

film thickening observed in the highly alkaline regions under

the coating occur in a low Cl" environment.

(7) As the coating system deteriorates, a breakthrough between

anodic and alkaline areas may occur with concomitant neutralization

reactions between and OH’ as well as dilution of the Cl’

ion concentration. Since the liquid volumes involved in such

a "breakthrough" are likely to be small, it is very reasonable

to expect that the resultant Cl’ concentration would still be

higher than that of the bulk solution. Experimental evidence

for such a breakthrough is shown by the pH behavior in Fig 2 and

is schematically represented in Fig. 1 c.

II. Surface Roughening and Surface Film Thickening .

A preliminary analysis of the ellipsometrically observed subcoating

phenomena of surface roughening and surface film thickening presents an

enigma. The typical regions of a specimen under el 1 ipsometric examination

are the focal points of the cathodic reduction of oxygen, yet in these

regions we observe surface roughening; i.e., the uneven dissolution of

metal, which is definitely an anodic process. This disparity can be

resolved by postulating the movement of cathodic activity from one local

area to another.

The processes are summarized in the following steps:

(1) The arrival of sufficient oxygen and water in a local subcoating

region to support the cathodic reaction.
6

(2) Product OH" ions from the oxygen reduction reaction are kept

captive in the local microenvironment by the organic coating.

(3) At high pH (14 or 15) the cathodic activity in this local area

ceases, and iron dissolves. Thus the metal surface roughens.

(4) The corrosion activity continues with reductive support from

new cathodic sites which begin to emerge under the coating on

other areas of the specimen surface.

(5) The soluble iron species are held captive in the high pH

region by the organic coating. When a critical species concentration

is achieved, precipitation occurs and the surface film thickens.

(6) Each emerging cathodic site progresses through steps 1, 2, 3

and 5 and the ellipsometer integrates these processes over a

finite region of the specimen surface.

While the exact species and mechanisms by which these processes

occur have not yet been identified, the chemistry of the dihypoferrite

iron^, (HFe 02 "), offers a thermodynamically reasonable explanation.

Fe + 30H” — HFe02~ + H

2

O + 2e (iron dissolution/surface roughening)

2HFe0p" —^ + H^O + 2e (oxidation, precipitation/surface film

thickening)

Ellipsometrically, the behavior of uncoated iron in high pH environments

is very similar to that of coated iron undergoing active corrosion.

This similarity is illustrated in Fig. 3. Surface roughening is a

regular feature of uncoated iron immersed in saturated NaOH both with
7

and without Cl”. However, surface film thickening is not typically

observed in these simulated environments. This difference in behavior

between coated iron undergoing corrosion and uncoated iron reacting in a

simulated subcoating medium is taken as support for the dissolutionprecipitation mechanism presented earlier. Metal dissolution in alkaline

media takes place in both cases, but the dissolved iron species are held

captive by the organic coating in the one case while in the second case

these species simply diffuse away so that adequate concentrations for

precipitation do not develop.

Summary

In this note we have developed some preliminary views to encompass

the results of our ellipsometric-electrochemical and microanalytical

measurements on a coated iron system. It has become apparent during

this work that the nature of the subcoating microenvironment is an

important factor to the course of corrosion. The ellipsometer provides

a nonintrusive means of studying surface events at the molecular level

and probes the results of the chemistry which occurs between the microenvironment

and the substrate. It is this chemistry which is responsible for surface

roughening and oxide film thickening.

The superimposed process of Cl” concentration follows directly from

the tendency of mobile ionic systems to achieve electroneutrality. This

part of the subcoating phenomena is identical to that which prevails

during crevice or pitting corrosion. The feature which differentiates

the coated metal system from a pit or crevice system is the presence of

the organic coating which can act as a semipermeable barrier to the

migration of materials.
8

While the present studies deal primarily with substrate changes as

a result of interaction with the microenvironment, the sequence of these

changes with respect to the rupture of polymer-substrate bonds is not

yet clearly defined. This problem is directly related to the question

of whether changes in surface texture or oxide properties constitute a

mechanism for coating-substrate bond failure. We are currently implementing

modifications to the ellipsometric-electrochemical technique which may

give further insights to this important question.

Acknowledgement

We are grateful to the Office of Naval Research which supported

this work under O.N.R. contract 1889 NRO 36082.
9

References

1. J. J. Ritter and J. Kruger, Surface Science, p. 364 (1980).

2. Paul Kirk, Quantitative Ultramicroanalysis, John Wiley & Sons,

Inc., New York, NY (1950).

3. B. Cunningham, P. L. Kirk, and S. C. Brooks, J. Biol. Chem., 139 ,

p. 11 (1941).

4. M. G. Fontana and N. D. Greene, Corrosion Engineering, McGraw-Hill,

New York, pp. 41, 51 (1978).

5. J. E. Castle and J. F. Watts, Proceedings of the International

Conference on Corrosion Control by Organic Coatings, Henry Leidheiser,

Jr., (ed.). National Association of Corrosion Engineers, Houston,

Texas (1 981 )

.

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions,

Pergamon Press, NY, p. 310 (1966).

6 .
10

Figure Captions

1. a. A summary of the equations proposed for subcoating events

leading to surface roughening, oxide film thickening and

the promotion of Cl~ ion accumulation.

b. Schematic showing Cl" ion entry into the hollow cone of anodic

corrosion products in the presence of a remote cathode.

c. Schematic suggesting one mode of anodic-cathodic "breakthrough".

2. Experimental curve showing change in the ellipsometric parameter A

and pH as subcoating process proceeds. Steady decline of pH after

1200 min. indicates a "breakthrough" between anodic and cathodic

sites in the vicinity of the subcoating pH probe.

3. Schematic showing the similarities in ellipsometric behavior with

time for uncoated iron at pH -v 15 and coated iron undergoing corrosion

in 0.05M NaCl.

f.

i;
oo

UJ

troQOZ< oao

XIoooE

C\J +

CNJ

u_t

(O

>»

XJ

>*co co uo

(U

O

liJ (TOO01

»-<o uo

(U

XJo

x: ^ o

XJ

— § 2 o — oc oEI Xo

C\J tX+

CJ o

-|ro h

<D

CM o o E

O)

CM

+• oCM

O

X CM + X

0)

CM+ I CM O

a>

li- X

toCM

Xo

to

<u t\ CM O

OJ U-X

CM

Naf

Cl'

HiO
A

( deg

)

Fe coated with collodion

14

13

12

1 1

10

9

8

7

pH

log t (min)
I t

(Min )

[Part III - To be published in the proceedings of a symposium commemorating the 75th brithday

'Of H. H. Uhlig by the Electrochemical Society at its National Meeting in Denver Colorado

in October 1981.]

DISSOLUTION OF PASSIVE FILMS ON IRON

IN NEARLY NEUTRAL SOLUTIONS

J. Kruger

Center for Materials Science

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

A description is given of a study of the dissolution of passive films formed by potentiostatic

anodic oxidation of iron in sodium borate-boric acid

solutions. Using an el 1 ipsometric-potentiostatic

technique, four different potential regions were

identified where different dissolution behavior could

be observed. Two different dissolution modes were

found at potentials in the passive region; one dissolution mode at potentials at or slightly below the

Flade potential, and one mode at potentials where

cathodic reduction becomes possible.

A great deal of attention has been paid to the formation of

passive films on iron [see (1) for reviews of the subject], but

considerably less has been written on the details of the dissolution of these films, a process of equal importance to an understanding of corrosion processes. The studies in the literature

on the dissolution of the passive film mainly deal with dissolution in acid solutions (2) and involve either gal vanostatic (3) or steady state open circuit (4) conditions that produce film

thinning. One major contribution to this area was that made by

, Prof. Uhlig, in whose honor this and the other papers of this

: symposium are dedicated; this was his study with Peter King of

the Flade potential (5) in 1959. The work described here differs

from past efforts in that dissolution is carried out under

potentiostatic control (fixed driving force) and in nearly neutral solutions. The rationale for such an approach was to derive

insights into the role played by reverse fields on dissolution

and to, thereby, seek to distinguish between field and chemical

dissolution effects that result in passive film thinning. In

more acidic solutions, it is likely that chemical dissolution

would be a more expected mode of film thinning than in neutral or

alkaline environments. There are two other reasons for studying

dissolution in nearly neutral solutions: (a) the dissolution

process is slower than in acidic media and, hence, easier to

study; and, (b) a nearly neutral solution is more closely related
to a number of natural environments (e.g., natural waters) in

which an understanding of protective film dissolution is important to corrosion prevention technology.

Experimental Approach

The metals (polycrystalline and single crystal iron) and

solutions (sodium borate-boric acid solutions) are described

elsewhere (6). The ell ipsometric-potentiostatic technique used

is also given in previous papers (6,7). To study dissolution

processes, these techniques were used in the following manner: A

passive film was grown on an iron surface by polarizing that

surface by means of a potentiostat in borate buffer solutions at

either pH 8.1 or pH 8.8 at a potential well into the passive

region 0.8v (SCE). After the film growth has proceeded to the

O point where the rate of growth was one A/hr or less (a few hours),

the potential was set by a potentiostat at a selected potential

below 0.8v and the change (if any) of the thickness of the film

on the iron surface observed by means of an ellipsometer. In

this way, the rate of dissolution or other changes on the metal

surface could be observed as a function of potential.

Dissolution Regions

Four distinct potential regions were identified where different dissolution modes were observed. These regions were as follows: Region I —from the film growth potential 0.8v to

-0.2v; Region II —from -0.2v to -0.5v; Region III —from -0.5v

to -0.7v; and. Region IV--from -0.7v to -0.8v. In Fig. 1, these

four regions are shown in a plot of steady state film thickness (the nearly constant values reached after a few hours at a potential lower than the film formation potential) versus the

potential that the iron was brought to by means of a potentiostat

after film formation at 0.8v (SCE). The characteristics of the

four regions shown in Fig. 1 will now be described and discussed.

1 . Region I

In going from the film formation potential 0.8v to any lower

potential in Region I, no changes in film thickness were observed. In fact, since after one hour the growth of a film at

0.8v was still going on, lowering the potential to a Region I value stopped this ongoing growth but did not cause dissolution.

If one assumes that anodic film growth on iron is a field assisted process, involving both forward and backward fields [see
Young (8)], Region I behavior would indicate that the forward

field was sufficiently high In this potential region to resist

dissolution. Growth was stopped, however, because the forward

field was not high enough at a potential lower than the formation,

potential (0.8v).

Figure 2 illustrates the role of the field (overpotential,

n, divided by the film thickness, x) in continuing, stopping, or

in going into Region II, reversing film growth. During the first

ten minutes, the film is growing at a potential of 0.8v. When

the potential is lowered to 0.4v (a potential in Region I), the

growth stops, and the thickness remains constant, because the

overpotential, and hence the field, is too low to promote further

growth. When the potential is again raised to the original

formation potential 0.8v, growth starts up again, because n/x is

again apparently large enough to cause further growth. When the

potential is dropped to a value in Region II, -0.2v, not only is

growth stopped, but the film starts to thin. Going to 0.8v again

causes growth to resume. The experiment illustrated in Fig. 2

strongly suggests that a field across the passive film can play a role in causing growth, stopping growth and, as the results to be

described next for Region II indicate, promoting dissolution.

2 . Region II

As indicated by the results shown in Fig. 2, Region II is an interval of potentials where slow dissolution of a film formed at

0.8v takes place. Figure 3 shows the progress of such dissolution for two buffered borate solutions, pH 8.8 and pH 8.1.

Continuing the suggestion made in the discussion of Region I that

the field across the passive film plays an important role in

growth and dissolution, it is now useful to examine in more

detail the interplay of the forward and backward fields involved

in the formation of films by anodic oxidation. Young (8) describes the anodic oxidation process as involving the movement of

ions in an oxide lattice under the influence of an applied field.

He describes two ion currents produced by a field; a forward

current, T, and a reverse current, T. Making a number of simplifying assumptions, e.g., neglecting concentration gradients.

Young calculates a ratio, R, of the forward to the backward ion

currents as follows:

= 1 - exp - [(W - qaE)/kt' ~ exp - L(W + qaE)/kt" [

1 ]
where W = the activation energy for a mobile ion to jump from one lattice position to an adjacent position; q

= the charge on a moving ion; a = the half barrier width or half jump distance;

and, T = the temperature.

Equation [1] can now be used to calculate R in order to

examine whether the backward field would be expected to promote

dissolution in Region II, but not in Region I.

Making the following assumptions:

(a) The equilibrium reaction for iron in a solution of pH 8.8 is

3Fe + 4 H 2 O = Fe 304 + SH'*’ + 8e” (this assumption is needed to

calculate the overpotential used to calculate the field, E);

(b) W has the typical value of 1.0 ev (8);

(c) The moving ion is Fe^^ and q-is, therefore, two;

O (d) The value for a is 2.6 A, as reported by Moshtev (9) for the

passive films formed on iron in nearly neutral solutions;

(e) The temperature is 300 °K;

R was found to have the values of:

5

R (for Region I potential 0.4v) = 4 x 10 ;

R (for Region II potential -0.35v) = 181.

It can thus be suggested that the significant difference between

Region I, where no dissolution is observed, and Region II, where

slow dissolution is found, is that in Region II, the backward ion

current or field which promotes dissolution is around, three

orders of magnitude higher than it is for Region I. These re- sults suggest that the ratio of T/t (R) is more important than

chemical dissolution for passive film thinning on iron in a nearly neutral solution.

To test this hypothesis. Region II dissolution was carried

out in solutions with two different pH values (8.8 and 8.1).

Since the overpotential is higher for the higher pH, the ratio of

the forward to the backward current should be higher for the

films formed in the pH 8.8 solution and the rate of dissolution,

therefore, lower than that found for the pH 8.1 solution. When R

is calculated for the two solutions, the following results are

found:
R (for pH 8.8) = 181;

R (for pH 8.1) = 107.

As Fig. 3 shows, the expected higher dissolution rate for the

pH 8.1 solution was observed experimentally.

Another factor besides pH that controls the rate of dissolution in Region II is the nature of the passive film. This nature

is influenced by, among many other variables of formation, the

time of growth. For example, the induction time for pit initiation depends strongly on the time that a passive film on iron is

allowed to grow before it is exposed to breakdown conditions (10).

As Fig. 4 shows, the Region II dissolution rate is also dependent

on film growth times. The complete film (a film that was held at

0.8v long enough to reach steady state growth, a few hours) shows

a lower dissolution rate than an incomplete film (a film that was

held at 0.8v for a few minutes). These results indicate that

another factor, the nature of the dissolving film, may play a role in the dissolution process in addition to that played by the

forward and backward fields.

Finally, it should be pointed out that the potential where

the transition between Region I and Region II occurs is near the

potential called by Nagayama and Cohen (3). was the potena d

tial they found for iron in buffered borate solutions above which

virtually-no ferrous ions were found in solution and below which

detectable concentrations were found. If R, the value of the

ratio of forward to backward ion currents, is important in determining whether passive film dissolution takes place, as suggested

above, then it is responsible for the existence of E^, and Ej is

o d

the boundary potential between Region I and Region II.

% 3. Region III

This region straddles the active-passive transition, the

Flade region. The optical results were not as straightforward as that observed for Regions I and II. Instead of the observation

that the el 1 ipsometric parameter A increased as film thickness

decreased, as found in Region II, A, the relative phase retardation, increased at the beginning of the dissolution process and

then began to decrease (Fig. 5).

Earlier studies (7) have shown that the decrease was the

result of surface roughening after the initial dissolution of the

passive layer. Figure 5 suggests that the roughening, as sig-
naled by going from + to also occurred when the current

went from + to This change in sign may indicate the transition from the passive to the active state. The earlier studies (7) also suggested, on the basis of rather speculative

optical calculations, that the metal dissolution that resulted in'

surface roughening occurred at discrete sites, either to form

pits or to attack the grain boundaries on the polycrystalline

iron surface that was used.

To determine whether these suggestions had any merit.

Region III dissolution was carried out on single crystal surfaces

where, of course, there were no grain boundaries. Figure 6 shows

the results obtained for two different orientations, (Oil) and

(311). If grain boundaries are the only sites of attack, the

reversal in the direction of change for L should not be observed.

As Fig. 6 shows, this is what was observed for the iron surface

with the (311) orientation. The (Oil) surface, however, did show

the same reversal in direction of change for A that was observed

for the polycrystall ine iron surface. These results may not

throw doubt on the interpretation of the optical results (7)

described, because it has been found that the (Oil) orientation

for iron is more susceptible to pitting attack than other orientations (11) and that the optical observations, therefore, do

indicate the surface roughening that results from pitting.

4. Region IV

This region contains potentials where the passive layer

should be rapidly reduced. The el 1 ipsometric measurements indicated this was so but also indicated that there are two rates of

reduction (see Fig. 7). The first stage was more rapid than the

final stage. These results confirm the gal vanostatic reduction

results of Nagayama and Cohen (3), who ascribed the two stages to

the initial more rapid reduction of yFe203 , followed by a slower

reduction of Fe 304 .

Summary

1. Four potential regions of dissolution of the passive layer

on iron have been identified.

2. The ratio of the forward to the backward currents produced

by the field across the passive layer appears to be important in separating Region I from Region II. This observation suggests that Region II dissolution occurs because of

field effects rather than being the result of chemical

dissolution. . -

j
3. Region III dissolution behavior involved both passive film

dissolution and metal dissolution.

4. Region IV dissolution occurs in two stages, with the first ^ stage being the more rapid of the two.

Acknowledgement

I am grateful to the Office of Naval Research, who supported

part of this work under Contract NAONR 18-69 NRO 36-082.
REFERENCES

"Passivity of Metals". R. P. Frankenthal and J. Kruger,

Eds., Electrochem. Soc., Princeton (1978).

K. J. Vetter and F. Corn, Electrochim . Acta , 18 , 321 (1973)

0*962^^^^^^ -* Electrochem . Soc. , 109 , 781

K. J. Vetter, J. Electrochem. Soc . , 110 , 597 (1963).

0*959)^*^^^'^ Electrochem . Soc . , 106 , 1

(1967)^^*" Calvert, J. Electrochem . Soc ., 114 , 43

J. Kruger, Corrosion , 88 (1966).

L. Young, "Anodic Oxide Films", Academic Press, NY, 1961,

p. 15.

R. V. Moshtev, Electrochim . Acta , 16, 2039 (1971).

J. R. Ambrose and J. Kruger, Proc. Sixth Inti. Cong, on Metallic Corr., Amsterdam, NACE, Houston, 1972, p. 698.

J. Kruger, Electrochem . Soc ., 106 , 736 (1959).
I

Fig. 1 . The dissolution

regions observed when a film formed by anodic oxidation at 0.8v (SCE) in*"

buffered borate solution

pH 8.8 is brought to a lower potential. The

thickness values A shown

are the steady state values

observed after a few hours

when dA/dt < lA/hr.

Fig 2 . The effect of

changes of potential on film growth and dissolution in Region I, pH = 8.8.

Fig. 3 . Region II dissolution. The change in

film thickness (A) with

time (min) for pH 8.8 and

pH 8.1 solutions. Dissolution potential, -0.35v.

MINUTES
Fig. 4 . The Region II dissolution of an incomplete (less

than a few minutes growth at

0.8v) and a complete film

(growth at 0.8v for over one

hour). Dissolution potential,

-0.35V.

Fig. 5 . Region III dissolution. The change in A,

the relative phase retardation, with log time (min) in

a pH 8.1 solution using a polycrystal line iron surface.

Dissolution potential, -0.55v.
Fig. 6 . Region III dissolution. The change in A

with log time (min) in a pH 8.1 solution on (311) and

(Oil) iron single crystal

surfaces. Dissolution

potential, -0.65v.

Fig. 7 . Region IV dissolution.

The change in film thickness

(A) VS log time (sec) for a pH 8.1 solution. Dissolution

potential, -0.8v.
t 1

,. ..4 L, ;; 4

''j

-,if'

:S'?^ . :*»

iK'il

iiJ

. -* " .'. • f-M MA

!,

*7,r

•» -l£-« J « 2‘ I < 4

* 'f r^J

Jt

M"

w'- ' 0 , m

yJ3%’

'.'u . " i -It; j(J^'^j(^|^^-^’^."ii'- f.^*¥ .ii^

JvrT«* W V

lii

01^'

PftgcT^ . fto.fe fdis' « -B Hq

ii'.TTF/

I mLiJ*.

'' "'l?M

JU'

jta •

WiJ

fe'"' '

" v.''k

a

'*

-.0

i 4> - . '.'i'
[Part IV - To be submitted for publication in Surface Science.]

FIELD ION MICROSCOPY OF ALPHA URANIUM

J. J. Carroll and A. J. Melmed*

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

Field ion micrographs of alpha uranium show atomic details of

many orthorhombic crystal planes. Images of the (010) and (001)

plane edges are examined and discussed in particular. Micrographs

indicate that a surface hydride phase is formed readily under

hydrogen imaging conditions. Some micrographs indicate possible

hydride particle precipitation at a major crystal defect boundary.

No evidence was found, howcver, of hydrogen/stress induced surface

cracks. Procedures used to prepare alpha uranium for field ion microscopy are described.

*Surface Science Division
yi ‘ Is sr^afiju^: <i’f '10'^

,

' * .'A ims ftO*i^'"

ACSO<r*

(T^O)

.

.

' p,

'’

. .

' IS* V ® . B1

. "^': '^ y . ® .''' ‘^ y- i:.y

'

.:oes

'.— f

^ ' ' '.•'

;f}0 ^ ;t-- ^0> : Til . .. r,ia‘ ,' "" ' ""'"
Field Ion Microscopy of Alpha Uranium

J. J. Carroll and A. J. Melmed

1 . INTRODUCTION

Orthorhombic alpha uranium provides a rich source of interesting

surface structural characteristics that should be imageable on an atomic

scale in the field ion microscopy (FIM). In addition to the orthorhombic

lattice dimensions. Fig. 1^^^ indicates schematically the spatial arrangement of atoms in the (001), (100), and (010) planes. The overall crystal

structure is similar to hep but less symmetric and, therefore, more complicated. The orthorhombic (001) plane is comparable to the hep (0001)

plane. The planar (001) atoms are arranged as a centered rectangle

and consecutive layers shift back-and-forth in the <010> directions yielding an A,B,A,B,..., stacking sequence. The orthorhombic (100) plans is

comparable to hep (1120) plane. Consecutive (100) planes are shifted

similarly to the (001) planes, resulting in an A,B,A,B,..., stacking

sequence. Of the three major faces, the orthorhombic (010) plane is

most complicated. It is comparable to the hep (1010) plane with the

atoms lying in a plane having an A,B,C,D.A.B,C,D, . . . , stacking sequence.

The (010) plane and all orthorhombic planes of the type (h, k>0,il) are

described^^^ alternatively as having a corrugated arrangement of atoms

lying in a thin selvedge. This description leads to an A,B,A,B,...,

stacking sequence. Although alternating stacking sequences were observed

in FIM studies of hep metals, no micrographs of crystal planes showing

(4-6)

any stacking sequences were observed in FIM studies of alpha uranium.

Moreover, an indexed helium-hydrogen field ion micrograph of alpha

uranium^^^ shows a central (010) plane lacking the necessary two-fold
symmetry about tv;o orthogonal axes perpendicular to the <01 0> direction.

Micrograph quality was inadequate in the earlier studies^^'^^ for such

important features to be observed, thus indicating a need for improved

image quality. Studies of uranium using STEM have proceeded to the

stage of imaging isolated atoms, and lattice spacings in a crystalline

form .

Uranium is known to interact strongly with hydrogen at room temperature and, under certain circumstances, to embrittle. A surface

study of the change in work function due to hydrogen chemisorption

on uranium thin films prepared in ultra-high vacuum indicates substantial

surface hydride formation without significant hydrogen absorption. A

metal lographic electron microscope study^^^ of uranium specim§Hs charged

with hydrogen indicates the primary cause of embrittlement is precipitation

of a hydride phase, preferentially but not exclusively at grain boundaries,

leading to specimen fracture under stress.

Uranium presents itself through field evaporation in the FIM, usually

in a relatively inert imaging gas, as a multifaceted single or polycrystalline surface while undergoing tensile stress. Interacting

such a surface with hydrogen gas under imaging conditions can provide,

in principle, information regarding the hydrogen-uranium surface interaction, such as the occurrence of hydrogen/stress induced crystal defects.

The technique was used successfully in other hydrogen/metal /stress systems.

The following includes a description of the preparation of alpha uranium

specimens for field ion microscopy, a method of obtaining improved micrographs of alpha uranium in an unbaked microscope, and an FIM view of the

hydrogen/alpha uranium/stress interaction.

2
2. EXPERIMENTAL

An ingot of depleted uranium was swaged at room temperature, with

intermediate vacuum annealing treatments at 600 C, to about 0.5 mm

diameter wires. Wires were annealed finally in vacuum at 1000 C for

several hours and cooled slowly to obtain the alpha phase. (An

( 12 ) attempt to electrotransport ' purify a wire segment was not successful

_ 5 after annealing in 1.3 x 10 Pa vacuum with a (dc) current at 1000 C

for twelve days.) Spectrographic analysis of the wire revealed that

the uranium was 99.4% pure with 0.5 wt.% iron as the major impurity

(13

)

constituent.' '

The following electropolishing steps were used to make sharp alpha

uranium tips suitable for the FIM: (a) the wire diameter was significantly reduced and tapered by electropolishing between 40-20V(ac)

using a solution of four g citric acid, one ml HNO^Cconc.) in 200 ml H

2

O,

vs. a carbon rod electrode; and, (b) the final polishing step, performed

while observing the process at 150 magnification, used the abovementioned electrolyte contained in a loop of 0.13 mm diameter platinum

wire at 20-25 V(ac). Tip shaping and sharpening were controlled with

the aid of a micromanipulator that adjusted the position of the platinum

loop about the tip-end, and an on/off push-button switch in the, circuit. Specimens were rinsed with methanol.

The unbaked field ion microscope was equipped with a 90 mm

diameter microchannel plate and reagent grade imaging gases. A typical

- cold specimen base pressure of 3.3x10" Pa was obtained in the imaging

chamber. The tip temperature was controlled by adjusting the flow of

3
cold gaseous helium from a liquid helium reservoir over a specimenmount feed-thru pin. Specimens were cooled to <30 K.

3. RESULTS

3.1 . Neon, Helium, and Hydrogen Field Ion Micrographs of Alpha Uranium

Our best micrographs were obtained using neon imaging gas. An

example is shown in Fig. 2. Neon field ion micrographs have sufficient

overall contrast and yield adequate imageability for most crystal planes.

Specimens were stable at neon imaging fields provided that hydrogen

and residual gas partial pressures remained low enough. However, neon

image quality deteriorated with prolonged imaging or by reimaging, at

a later time, a previously field evaporated specimen. In addition to

extra bright spots bestrewing the image, as in Fig. 3 (a), patterns

developed signs of preferential field attack through the (010) plane

and nearly to the (120) and (120) planes, as in Fig. 3 (b). These

image features were removed consistently, as in Fig. 3(c), by exposing

specimens to large enough electric fields in hydrogen to obtain zone

line and major crystal plane development as shown in Fig. 4, and

then evacuating hydrogen before readmitting neon imaging gas, while

maintaining the previously applied voltage. Subsequent neon images

remained optimum for 10-20 s without regard to the amount of field

evaporation. In time, extra bright spot development occurred and,

with continued field evaporation, preferential etching occurred. This

suggests that the extra bright spots originated from a gas-surface

interaction rather than from the bulk alpha uranium.

4
Alpha uranium images obtained with helium were relatively dim and

marginally stable with most crystal planes visible but with less image

quality than with neon.

Hydrogen field ion micrographs of alpha uranium (Fig. 4) showed

satisfactory overall contrast but most crystal areas were disordered,

indicating a significant chemical component of the imaging process

between hydrogen and alpha uranium. None of the image enhancement

effects were found that typify hydrogen promoted helium or neon images

observed with other material s

.

3.2. Crystal Plane Indexing of Alpha Uranium Field Ion Micrographs

It is known that alpha uranium wire formed by sv;aging at room

temperature develops grains oriented predominantly with their <010>

poles aligned parallel to the wire axis,^^^^ and it was expected that

this situation would prevail following the presently used swaging and

annealing treatments. Among twenty specimens successfully imaged,

thirteen prominently showed the same crystal pole having two twofold axes of symmetry. Thus, the surface plane perpendicular to this

pole was identified as the (010) plane. Other crystal planes were

indexed with the aid of stereographic projections obtained from several

( 15 ) sources.^ ' The remaining specimens were divided almost equally

between tips centered near the <001> and <110> poles. These results

( 14 ) are consistent with results from other studies' ' that showed almost

no^<100> pole development parallel to the cold swaged wire axis.

5
3.2.1. Image Properties of the (010) Plane Edges

Neon field ion micrographs of the top-most (010) plane and succeeding (010) plane edges of progressively larger diameter, examples of

which are shown in Fig. 5, have important image features that relate

to surface structural properties of the field evaporated (010) plane

of alpha uranium. The micrographs show a sequential alternation in

the relative roughness of succeeding planes and spacings between planes

leading to an apparent A,B,A,B,..., planar stacking sequence. Designating the more rough looking planes as A and the more smooth looking planes

as B, the micrographs of Fig. 5 indicate that A planes more closely

adjoin the next larger diameter B planes than the previous, next

smaller diameter B planes, judging by the relative ring separations.

These image features suggest that the step-height or interplanar spacing is larger between a B plane and the next larger A plane than between

an A plane and the next larger B plane, assuming that the local radius

of curvature is approximately constant and that conventional field ion

imaging theory^^^^ is applicable.

The foregoing image features may be understood by considering a

schematic profile of a field evaporated (010) plane viewed from the

<100> direction, as shown in Fig. 6. The corrugated layers are noted

by zig-zag lines connecting atom positions in the (010) plane. Atom

positions in the plane of the paper are open circled and those close

circled are out of the plane of the paper by a spacing. Assuming

normal field evaporation, the process exposes alternately the fully

corrugated (010) planes and partial (010) planes, where the once

6
protruding apex atoms (every other one) were field evaporated. As

a result, succeeding step-heights alternate between 0.12 and 0.17

succeeding field evaporated (010) planes alternate between those having

more closely packed and more loosely packed atoms. The larger interplanar spacing, shown schematically in Fig. 6, occurs between a B plane

and the next largest A plane. This is consistent with the micrographs

of Fig. 5 which show a more obvious gap between the more smooth looking

planar rings and the next larger, more rough looking planar rings.

According to the model shown in Fig. 6, the more rough looking A olanar

rings of Fig. 5 appear so because atoms are imaged in a corrugated

selvedge layer whereas, the more smooth looking B planar rings

of Fig. 5 appear so because only atoms of a single crystal layer are

imaged. Similarly, the A,B,A,B,..., stacking seouence observed with

FIM is consistent with the A,B,C,D,A,B,C,D, . . . , stacking sequence in

the crystal lattice. The FIM A plane images atoms of the A and B

crystal layers, the succeeding FIM B plane images only atoms of the

B layer, the succeeding FIfl A plane images atoms of the C and

D layers, and the next FIM B plane images only atoms of the D layer.

The C and D crystal layers are indistinguishable from the A and

B crystal layers, respectively, since they do not differ in geometry,

but differ only in relative lateral positions.

3.2.2. Image Properties of the (001) Plane Edges

Neon field ion micrographs of the (001) plane region are shown in

Fig. 7. They indicate that consecutive (001) planes image differently

along <010> directions. The edge of the top-most (001) plane is

imaged in one <010> direction and environs but fails to image in the

7
opposite <010> direction and environs. The edge of the next larger

(001) plane images in the directions where the previous (001) plane

failed to image. The sequence repeats itself with the result that

(001) planes 1,3,5,.,., are imaged in one <010> direction and environs

and (001) planes 2,4,6,..., are imaged in the opposite <010> direction

and environs. Consecutive (001) plane edges are imaged in alternate

<010> directions and environs leading to an A,B,A,B,..., stacking

sequence. Consecutive (001) planes, however, are imaged in the

<100> directions.

The image features of the (001) plane may be understood with the

aid of Fig. 8 which schematically indicates a profile configuration of

atoms in field evaporated (001) plane layers viewed from the <100>

/ direction. It is assumed that normal field evaporation prevails and

produces single step-height plane layers. The imaging process in

field ion microscopy is influenced by the geometrical structure of

( 18 ) surface atoms' ' and the availability of surface dangling bonds,

favorably or unfavorably disposed for efficient electron tunnelling

( 19 ) from the imaging gas.

' Figure 8 illustrates that the geometries of

step-edge atoms and dangling bonds are significantly different in

opposite <010> directions on the same (001) plane, and that similar

geometries are repeated iii again opposite <01 0> directions on consecutive (001) planes. The numbered (001) planes are lettered with an A

and B to emphasize the two different step-edge geometries. If an

image of a (001) plane-edge is formed primarily by the availability of

surface dangling bonds then, plane-edges IB and 3B might be imaged in

8
one <010> direction and 2B and 4B in the opposite direction. If,

however, local radius of curvature at the step-edges primarily

influences imageabil ity, then plane-edges lA and 3A might be imaged

in one <010> direction and 2A and 4A in the opposite direction. Close

inspection of micrographs reveals some evidence that the dim parts of

the top-most (001) plane are preferential ly field evaporated exposing

an unusually large terrace of the second (001) plane layer. See Figs.

7(a) and 9. We conclude, therefore, that sten-edge geometry of tyne A

is exposed to a larger local field due to a smaller radius of curvature

and that (001) plane imaging is controlled by imaging gas orbital overlan

with dangling bonds at step-edge type B.

As the viewing direction is changed from the <100> to the <01 0>

direction, the profile schematic of the (001) plane dramatically changes

due to the loss of the corrugated (010) plane layers in the plane perpendicular to the paper. There are equivalent step-edge geometries on

consecutive (001) planes in both <100> directions leac'inq to the imageability of consecutive (001) planes in both <100> directions.

3.3 Field Electron Emission from Alpha Uranium

Fig. 10 shows a field electron emission pattern obtained from an

alpha uranium specimen previously imaged and field evaporated with

neon. The microscope was not baked, so the imaged surface probably is

not free completely from adsorbed residual gas. Nevertheless, these

experiments clearly indicate that clean alpha uranium surfaces for

field electron emission investigations can be prepared by field

9
evaporation at low temperature, at least in neon, and probably in vacuum

as well. Moreover, the pattern shown in Fig. 10 is a substantial improvement over a previously published micrograph. '''

> Further work must be

done to ascertain the stability of the field electron pattern after

elevated temperature treatment in ultra-high vacuum.

3.4. Effects Due to Hydrogen

Experiments performed using hydrogen imaging gas indicate that a surface

hydride phase is formed readily. For example, prolonged imaging of an as-sharpened alpha uranium tip specimen in hydrogen yields micrographs

(see Fig. 11(a)) that show significant grain structure. Subsequent

micrographs indicate that the grain structured material is removed by neon field evaporation to reveal a single crystalline alpha-uranium

substrate. Conversely, micrographs indicate nucleation and growth of

surface hydride (see Figs. ll(b,c)) due to exposure of a previously

neon field evaporated surface to hydrogen at lower fields, to minimize

evaporation. Figs. ll(b,c) show the formation and lateral growth of

bright centers of emission that we interpret as hydride nuclei. Assessments of hydride formation are derived from qualitative differences

among micrographs within the limitations of conventional field ion

microscopy since an atom-probe was not available for these

studi es

.

Neon field ion micrographs of alpha uranium failed to show any

significant lattice defects resulting from prior hydrogen surface

treatments, as in Fig. 4, or those that formed surface hydride nuclei,

as in Figs. ll(b,c). Alpha uranium specimens usually shov/ single

10
crystal emission patterns. However, one specimen that showed a major

twin or grain boundary following exposure to hydrogen imaging conditions,

also showed clusters of brighter. emission centers on the boundary. See

Fig. 12. These might correspond also to hydride particles, an inter-

(21 9) pretation consistent with an earlier finding' ’ ' that the hydride

phase precipitates predominantly at grain boundaries.

4. SUMMARY

Significantly improved field ion micrographs from alpha uranium

were obtained and major crystal planes were identified. The preparation

procedures used to make field ion emitters from alpha uranium were

described as well as imaging techniques used to obtain optimum micrographs in ordinary vacuum. The FIM resolved interesting morphological

details of the (010) and (001) crystal plane regions. Exposure to,

hydrogen most probably produced a surface hydride phase and may have

produced hydride precipitates at a defect boundary found in one

specimen

.

5. ACKNOWLEDGEMENTS

We recognize the assistance of F. Biancaniello and J. Smit who

formed wires from a depleted uranium ingot, and the Office of Naval

Research for partial financial support with Contract NAONR-13-69.

11
REFERENCES

(1 ) A. N. Holden, Physical fletallurgy of Uranium (Addison-Wesl ey

,

Reading, Massachusetts, 1958) ^. 21 . The present Figure 1 is

taken largely from Fig. 3-1.

(2) ibid., p. 87.

(3) For example, A. J. Melmed and R. Klein, "Field-ion microscopy of

ruthenium at 77 K," J. Less-Common Metals 1_0 (1966) 225-228;

A. J. Melmed, "The structure of field evaporated hexagonal closepacked metal surfaces: rhenium and ruthenium," Surface Sci . ^

(1966) 359-379; A. J. Melmed and J. J. Carroll, "Field ion

microscopy of osmium," J. Less-Common Metals ^ (1973) 199-204.'

(4) A. L. Suvorov, G. M. Kukavadze, T. L. Razinkova, B. V. Sharov,

V. A. Fedorchenko, A. F. Bobkov, and B. Ya . Kuznetsov, "Field

ion microscopy of uranium. Preliminary results," Soviet Atomic

Energy ^ (1974) 14-18.

(5) A. L. Suvorov and G. M. Kukavadze, "Field ion microscopy of

uranium," Problems of Atomic Science and Engineering, Ser.:

Physics of Radiation Damages and Radiation Material Science, 1(2),

Khar'kov (1975) 72-73.

(6) A. L. Suvorov, G. M. Kukavadze, D. M. Skorov, A. F. Bobkov, B. Ya.

Kuznetsov, B. A. Kalin, V. B. Volkov, and S. V. Zaitsev, "Field

ion microscopy of uranium in a continuous field evaporation system,

Soviet Atomic Energy ^ (1975) 72-77.

(7) M. Isaacson, M. Ohtsuki , and M. 'Jtlaut, "Electron microscopy of

individual atoms," in: Introduction to Analytical Electron

Microscopy , Eds. J. J. Hren, J. I. Goldstein, and D. C. Joy

(PI enum , New York, 1979) Chapter 13.

(8) J. C. Riviere, "The Surface potential of hydrogen on uranium,"

Nuovo Cimento Suppl . 5 0967) 466-471 .

(9) C. J. Beevers and G. T. Newman, "Hydrogen embrittlement in uranium,

J. Nucl . Materials ^ (1967) 10-18.

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

Appl ications (Elsevier, New York, I9by) pp. lbb-169.

(11) J. J. Carroll, J. Smit and A. J. Melmed, "Field ion imaging of

titanium and effects due to hydrogen," Surface Sci. ^ (1976)

640-648, J. J. Carroll and A. J. Melmed, "Field ion microscopy of

zirconium," Surface Sci. ^ (1976) 601-604; J. _J. Carroll,

12
A. J. Melmed and J. Kruger, "Hydrogen interaction with stressed

titanium-palladium alloys and stressed vanadium explored with

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

Stress Corrosion Susceptibility (NBSIR 80-21 01 ) August, 1980.

(12) D. T. Peterson, F. A. Schmidt, and J. D. Verhoeven, "Electrotransport of carbon, nitrogen and oxygen in thorium," Trans, .‘letall.

Soc. AIME

^

(1956) 1311-1315; D. T. Peterson and F. A. Schmidt,

"Separation of high purity thorium and thorium single crystals,"

J. Less-Common Metals, ^ (1971) 223-228.

(13) The other impurities identified were 0.003 wt. % calcium, 0.002

wt. % silicon, 0,002 wt. % copper, and 0.005 wt. % magnesium. The

analysis, performed by Herbert M. Ochs, Denver, Colorado 80223,

excluded nonmetal lie impurities.

(14) Ref (1), pp. 97-99.

(15) The stereograms of Fig. 5, Ref. (4) were not available to the authors

in the beginning of this study. Many stereograms were furnished

kindly by Ron Anderson, IBM Corp., Hopewell Junction, N. Y. 12533.

Other useful stereograms are found in Ref. (1), pp. 79-83.

(16) Ref. (10), Chapters II, III, and VI.

(17) C. W. Tucker, Jr., "The crystal structures of metallic uranium,"

Trans. ASM ^ (1950) 762-770.

(18) A. J. W. Moore, "The structure of atomically smooth spherical

surfaces," J. Phys . Chem. Solids ^ (1962) 907-912.

(19) Z. Knor and E. W. Muller, "A refined model of the metal surface

and its interaction with gases in the field ion microscope,"

Surface Sci . 1_0 (1968) 21-31.

(20) Ref. (10), pp. 130-134.

(21) H. R. Gardner and J. W. Riches, "The effect of uranium hydride

distribution and recrystallization on the tensile properties of

uranium," Trans. ASM ^ (1960) 728-747.

13
FIGURE CAPTIONS

Figure 1- Orthorhombic crystal lattice showing <01 0>, <100> and

<001> directions and the lattice dimensions of alphg /,

v

uranium. This is not the unit cell as given by Holden.' '

Figure 2 A neon field ion micrograph obtained from alpha uranium,

23kV.

Figure 3 Neon field ion micrographs obtained from alpha uranium,

(a) 26kV; (b) 24kVj (c) 23kV.

Figure 4 A hydrogen field ion micrograph obtained from alpha

uranium, 17kV.

Figure 5 Neon field ion micrographs obtained from the (010) plane

region of alpha uranium, 20kV.

Figure 6 A schematic profile of a field evaporated (010) plane in

the orthorhombic lattice viewed from the <100> direction.

Figure 7 Neon field ion micrographs obtained from the (001) plane

of alpha uranium, 20kV.

Figure 8 A schematic profile of a field evaporated (001) plane in

the orthorhombic lattice viewed from the <100> direction.

Figure 9 A neon field ion micrograph obtained from the (001) plane

of alpha uranium, 24kV.

Figure 10 A field electron emission micrograph, 2kV, obtained from

alpha uranium previously field evaporated. Pattern

indexed from the specimen's field ion micrograph.

Figure 11 (a) A hydrogen field ion micrograph from alpha uranium,

8.6kV, obtained after prolonged imaging of an as-sharpened

specimen.

(b) A hydrogen field ion micrograph, 15kV, obtained from

alpha uranium previously neon field evaporated and exposed

to hydrogen imaging conditions for 15 minutes.

(c) A hydrogen field ion micrograph, 16kV, obtained from

alpha uranium previously neon field evaporated and exposed

to hydrogen imaging conditions for 30 minutes.

Figure 12 Neon field ion micrographs obtained from alpha uranium, 27kV

with a crystal defect boundary. Two and three clusters of

material at the boundary are indicated by circles in (a)

and (b), respectively, and are assumed to be uranium

hydride particles. Particle dimensions range from 6 to

1 9 nm

.

14
<00l>




O.I2nm O.ITnm O.I2nm

STEP STEP STEP

'T' ' C?

i'






Part V - From Preprints of Symposium on Advances in Coatinq Metals for Corrosion Protection,

Division of Polymer Chemistry, American Chemical Society, Vol . 43, p. 575 (1980).

A Neiiz El lipsometric-Electrochetiiical Approach to

the Study of Corrosion Under Organic Coatings

J. J. Ritter and J. Kruger

National Measurement Lciboratory

Center for Materials Science

National Bureau of Standards

Washington, DC 20234

ABSTRACT

Qualitative ellipsometry was used in conjunction with electrochemical pH and potential measurements as a technique for the study of

corrosion processes occurring under organic coatings. The objective of

the v7ork was to gain a deeper understanding of the mechanisms goveriiing

the corrosion protective actions of paints on metal surfaces. Computer

modeling and experiments with collodion coatings on iron substrates

indicated tliat changes in the ellipsometric parameter A could, for the

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

oxide film and roughening of the substrate. Experiments with the Fecollodicu system in dilute chloride solutions exhibited three sequential

stages of activity, two of v^hich 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.
1 (ItXfIt )
A New F.llipsomctrlc-F.lectrochemical Approach to

the Study of Corrosion Under Organic Coatings

J. J. Ritter and J. Kruger

National Measurement Laboratory

Center for Materials Science

National Bureau of Standards

Washington, DC 20234

Recently, a description of a feasibility study^ was presented to the

4th International Conference on Ellipsometry that detailed the application

of qualitative ellipsometry to the study of metal substrates covered by

transparent organic coatings. It was sho\m that such an approach is feasible and that it can provide insight into the corrosion processes that take

place under paints. This paper is a somewhat updated version of that

presentation and is concerned with the application of ellipsometry to a study of painted surfaces. This new approach is being pursued because

there is still a great lack of understanding of the processes that control

the corrosion protection actions of paints. A recent book. Corrosion

Control by Coatings^ , contains many references to unsolved problems in this

field. Of special value for addressing this need would be a technique or techniques that could carry out 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 w’ould 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.

The approach described in this paper seeks to provide a way to carry

out such studies by simulating the painted metal circumstance through the

use of 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 corro- sion 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 subcoating environment.

EXPERIMENTAL

The detailed procedures for specimen preparation, ellipsometer data

interpretation and computer modeling have been described previously^ . The

work to be described in this paper adds an experimental step to those

described previously; namely, at the end of each experiment with a coated

specimen, the collodion coatings were stripped with methanol and the ex- posed specimen surfaces cathodically reduced in deaerated borate buffer

solutions (pH 8.5) at -758 mV S.H.E. Ellipsometric measurements on the

reduced surfaces wore recorded and an estimate of total oxide film thick- ness was obtained. The effects of a high pH environment on the coating

material were evaluated by ellipsometric studies of collodion-coated gold

specimens in saturated NaOH solutions (pH 15) . The electrochemical

potential and ellipsometric behaviors of uncoated iron specimens were

similarly evaluated in this high pH environment.

Obviously, such a system is too complicated to permit rigorous quantitative ellipsometric determination of film or environmental optical

constants or film thicknesses. Instead, the technique employed in the work

described here Is the qualitative one described elsewhere^. This approach

mea?nires the time dependence of changes which occur as Indicated by significant alterations in the c- 1 11 psometr ic parameters, A and \j>

,

and correlates
Lhepc changes wit.h concomitant electrocliemlcal measurements. Thus, for

example, tlie rate of change of metal oxide film thickness (assumed proportional to A) can be monitored. To help interpret the changes of A and ij; observed, and to judge whether the magnitude and direction are realistic

and reasoiiabl.e , model calculations using the Drude equations and reasonable

optical parameters were carried out^

.

RESULTS AND DISCUSSION

Collodion Coating on Iron

Figure 1 indicates that on the basis of changes in A, the Fe-collodion

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

A rises gradually, \p decreases as does pb increased during this

period due to the reduction of O2 on cathodic regions of the specimen. The

next stage covering from 1000 to 6000 min. shows a considerable decline in

A ('V. 25°) concomitant with a continued decrease in i];, (p . The pH tends to

higher and higher values. It is proposed that the initiation of the second

stage corresponds to the presence of active corrosion somewhere on the

metal surface. In the third stage, A undergoes a series of reversals, while

and pH increase. By 5400 min. an anodic area was well developed at the

upper corner of the specimen, remote from the pH probe and ellipsometer

observation regions.

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

where A increases and ip 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 fi].m combined v>?ith an increase of the liquid film refractive

index and a radical decrease in the refractive index 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.

Undoubtedly, O 2 , H2 O and some Cl penetrate the coating and arrive at the

metal surface during this period.

In the second and third stages the behavior of A and \p is due to a combination of at least three events; (1) a roughening of the metal surface, (2) and growth of metal oxide film*, and (3) possible chemical

changes in the coating material due to the high pH environment.

The high pH sub-coating environment could promote the dissolution of

Fe with the formation of either HFe02 or Fe04 ^ ions depending on the speci- men potential.** Since both ions are colored, an optically absorbing

liquid film may be present while metal dissolution results in surface

roughness. Experiments V7ith uncoated iron in S3.mulated sub-coating en- vironments (high pH) showed that both surface roughening and oxide film

growth occurred. Moreover, experiments with collodion coated gold suggested that the fluctuations in A noted in the third stage may be due in part

to a deterioration of the coati73g material after prolonged exposure to a high pH environment.

*Leidheiser and co-workers have previously reported on the growth of oxide

films on cobalt coated with polybutadiene and immersed in 3 percent NaCl.

The sub-coating oxide films were detected by Mossbauer spectroscopy’.

**Tlie potentials plotted in Figs. 1 and 2 were determined with the reference

electrode immersed in the bulk solution and probably do not represent the

true potential of the metal in its sub-coating environment. Potential

measurements on uncoated Fe in a pH 15 environment suggests that values

from -900 to -150 mV S.ll.E. are more realistic.
The contribution of these events to the total change in A (cf Fig. 1)

is estimated as follows. The amount of oxide film accumulation is typically 6 nm as determined from cathodic reduction of stripped specimens in

borate buffer media. This measurement accounts for 35 to 40 percent of the

total measured angular decrease in A. From experiments with collodioncoated gold in high p]I environments, it is estimated that an additional 10

to 15 percent of the change in A can arise from changes in the coating

itself. The remaining 45-55 percent of the change in A is attributed to

surface roughening.

For the purpose of comparison, oxide film accumulation and surface

roughening was assessed for uncoated iron specimens after prolonged immersion (10^^ min) in a pH 15 NaOH solution. Cathodic reduction indicated

that 30 percent of the total change in A measured in these experiments

was due to oxide film growth while the remaining 70 percent was ascribed to

surface roughening. Surface roughening was qualitatively demonstrated in

these specimens by cycling the specimen potential between -758 and +442 mV

S.ll.E. and measuring the changes in the A parameter. The observed increase

in A values as a result of potential cycling is an indication of surface

smoothing.

Collodion Coating on Iron with KpCr04 Islands

Figure 2 shows the first stage of the experiment lasts about 1200 min

and indicates that both A and decreases slightly. 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 con- comitant decrease in ijj is consistent with this interpretation, but the

magnitude of the change is not as readily explained and must wait additional experiments.

The second stage of the process is notably different from that of the

previous experiment in that both the rate of change in A and the overall

decrease in A, are considerably smaller. Thus, it appears that the Cr04^

greatly inhibits substrate roughening and film growth in this stage. No

third stage vjas observed.

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.

CONCLUDING COMMENTS

The main objective of this work was to gain a better understanding of

the corrosion processes occurring under organic coatings. The results

described in the prior section indicate that 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 techuicpie that:

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

the first minutes with the collodion coating system used in

this study. Moreover, one can correlate these optically measured

stages with time and pH measurements so that the presence of
active corrosion at the coated metal surface can be determined ^

(2) situ in a non-destructive manner.

The effects of ati inhibitor-ion such as Cr04 ^ can be detected and some preliminary insights gained into the inhibition phenomenon,

i.e. that both oxide film growth and surface roughening are

supressed

.

The implications of these findings to the processes which lead to

coating failure (e.g. delamination) are currently under investigation.

ACKNOWLEDGEMENTS

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

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

REFERENCES

(1) J. J. Ritter and J. Kruger, Proc. 4th Int. Conf. on Ellipsometry

,

Surf. Sci. (in press).

(2) Corrosion Control by Coatings , H. Leidheiser, Jr., Ed., Science

Press, Princeton (1979).

(3) J. Kruger and J. R. Ambrose, Surf. Sci., 394 (1976).

(4) Henry Leidheiser Jr., A. W. Simmons and Elsie Kellerman , J.

Electrochem. Soc. 120, 1516 (1973).
U)

Fig. 1- A,

relative

phase

retardation,

ip

relative

amplitude

reduction,

pH

and

potential

of

the

iron

surface

versus

standard

hydrogen

electrode

(S.H.E.)

versus

time

for

Fe

with

collodion

in

0.05

N

NaCl
•4

-13

12

tlpH

lO 1. A, relative phase retardation, relative am- plitude reduction, pH

and , potential of

Fe

the iron surface vs. standard hydrogen electrode (S.H.E.) vs. time

for Fe with collodion

in 0.05 N NaCl.

t { min)
potential regions were identified where different dissolution behavior could be observed.

Two different dissolution modes were found at potentials in the passive region; one dissolution mode at potentials at or slightly below the Flade potential, and one mode

at potentials where cathodic reduction becomes possible.

Part IV; Field ion micrographs of alpha uranium show atomic details of many orthorhombic

crystal planes. Images of the (010) and (001) plane edges are examined and discussed

in particular. Micrographs indicate that a surface hydride phase is formed readily

under hydrogen imaging conditions. Some micrographs indicate possible hydride particle

precipitation at a major crystal defect boundary. No evidence was found, however, of

hydrogen/stress induced surface cracks. Procedures used to prepare alpha uranium for

fipid ion microscopy are described.

Part V: Qualitative ellipsometry was used in conjunction with electrochemical pH

and potential measurements as a technique for the study of corrosion processes

occurring under organic coatings. The objective of the work was to gain a deeper

understanding of the mechanisms governing the corrosion protective actions of paints on ;

!T'' tal surfaces. Computer modeling and experiments with collodion coatings on iron

^ 'bstrates indicated that changes in the el 1 ipsometric parameter A could, for the most

part, be safely interpreted as thickness alterations in the substrate oxide film and

roughening of the substrate. Experiments with the Fe-collodion system in dilute chloride

-Tolutions 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 subcoating

; ocesses.
NBS«n4A iREv. 2 -PC) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See irstructions)

1, PUBLICATION OR

REPORT NO.

NBSIR 81-2396(ONR)

2. Performing Organ, Report No

561

3. Publication Date

November 1981

4.

TITLE and subtitle

Passive Films, Surface Structure and Stress Corrosion and Crevice Corrosion

Suscepti bi 1 i ty

5.

AUTHOR(S)

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

6.

PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. NR036-082/1-7-69

8. Type of Report & Period Covered

Annual Report

Aug. 1980 - Aug. 1981

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, Stole, 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 factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

Part I: Transparent organic coatings on iron are used to simulate painted metal

surfaces for simultaneous el 1 ipsometric and electrochemical measurements. These

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

the subcoating environment during prolonged immersion in dilute Cl~ media. The

relationship of these changes to phenomena such as metal passivation, coating

delamination, and inhibitor behavior are addressed.

Part II: The corrosion of metals protected by organic coatings is an incompletely

understood phenomenon. Recent investigations by an in situ el 1 i psometricelectrochemical technique combined with microanalytical studies have provided new

insights into the destructive processes. Events such as the roughening of the metal

surface, chloride ion accumulation and the thickening of the surface oxide have been

detected under the coating and are interpreted in terms of the chemistry which

develops between the microenvironment and the substrate.

Part III: A description is given of a study of the dissolution of passive films

formed by potentiostatic anodic oxidation of iron in sodium borate-boric acid

solutions. Using an el 1 ipsometric-potentiostatic technioue, four different (cont'd,

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

alpha uranium; chloride ion; chromate ion; electrochemistry; el 1 i psometry ; field

ion microscopy; inhi bi tors ; organic coatings; passive film dissolution

SUrfarp hyrlriHp

; pH measurements

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

71

15. Price

^^Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $8.00

USCOMM-DC 6043-P80

I1