Effect of vacancy condensation on the corrosion susceptibility of subgrain boundaries in aluminum

LETTERS

morphologique,

et

dans

la

rapprochement

de resultats

positions

identiques,

non

mesure

qu’autorise

tires d’alliages les

TO

le

de com-

phenomenes

qui

se

THE

It may first be noted that the sub-boundary pattern

in aluminum

be strongly

oxygene

ties.(435)

differents

de

The

boundaries

est, au contraire, triis pauvre en oxygene.

impurities.(2)

La relation Bventuelle entre les deux cas particuliers du profil d’equilibre de la surface et delasous-structure

sub-boundary

l’objet

en handes

et en stries meriterait

d’experiences

systematiques,

de la chemisorption defauts

dans

Notons, dans des

le reseau

lente

qui

expliquent

et progressive

Ce moyen

determination en

pour le premier n’exclue

pas les

le second.

enfin, l’interet du tampon Blectrolytique recherches de ce genre impliquant la

dissolution ficielles.

de faire

car l’hypothese

spectrographique

composants

prolong&e

est d’ailleurs

mineurs

et a haute

au

des couches

super-

deja utilise pour la de l’appauvrissement

tours

de

temperature

l’oxydation

d’alliages

com-

plexes.(6)

corrosion

There

of

acid

are several

susceptibility to impurity

Xaint-Made

(Xeine) Dau

depends

reasons

why

in hydrochloric

* Received

is

For

is low after slow cooling

after

cooling

(present practice involves

stream of air from a temperature 645°C).

high

ratec2)

cooling a 1; mm sheet in a in the range 550”-

For another, this cooling rate seems too high

to permit

formation

atmosphere purpose

of the relatively

presumed

For

this, it is supposed

that

of estimating

atomic

concentration

concentrated

to be necessary.(l) of roughly

the

0.05 extending

an for

the three atom layers on each side of the boundary

is

required.

in

The total

atomic

solute

concentration

September

present

participate

and

have

will be much too dilute, even at equilibrium,

16, 1959.

say, 350°C.

The boundary

and

selective

postulated diffusion

atmosphere coefficient,

estimate@

which

to form.

The volume chemical

for the solute given in Nowick’s has the highest

diffusion

along

aluminum

subgrain

cooled

rate from elevated temperatures

by exposure to hydrochloric

composition.

(ly2) Explanation

susceptibility

in terms

seems unsatisfactory.

at an can be

acid of appropof the unusual

of atmospheres The possibility

of that

the susceptibility is a result of the condensation of lattice vacancies on the sub-boundaries during cooling has been investigated

by comparing

an etch pattern

rate in

aluminum, is lo-l1 cmz/sec at 350°C and lo-l2 at 300°C (these figures agree with recent data for 0.0% corrosion

which cool through

would

boundary

have,

Cottrell

on

present

during

a generous

atmospheres

cooling

observations

substantially

since

Doherty

solute

atmosphere

corrosion

observed

in five

99.996%,

and has been

atom

and Davis’

show that the excess vacancies

selves migrate to the boundaries. to the impurity

submore

The excess vacancies enhance

boundary

in

in specimens estimate,

cannot

to boundaries,

Zn(7)).

can be produced

the range 350”-300°C

dilute than supposed necessary. diffusion

corrosion

in high-purity

intermediate

on the corrosion boundaries

of diffusion

coefficient and time must be greater than 1O-g for the

Since the sub-boundary

Effect of vacancy condensation susceptibility of subgrain in aluminum*

except

must drain more

than lo3 atom layers so that the product

which

boundaries

interaction

of 0.3 eV, the atmosphere

in less than 15 see, it can be produced

impurities

acid

at a fairly

specimens

riate

on high

one thing, the susceptibility

below,

Bibliographie

corrosion

grain

also

Cottrell atmospheres.

energies with the boundary

1. R. H. HEIDENREICR, E. A. NESBITT et R. D. BURBANK, J. A@. Phys. 30, 995 (1959). %. P. A. JACQUET, C.R. Amd. Sci., Paris 243, 2068 (1956); Note Technique ONERA No. 40 (1957). 3. A. R. WEILL, J. Phys. Radium, Phyls. Appliq. 19, 181A (1958). 4. J. MOREAU et J. BENARD, J. Inst. Met. 83, 87 (1954-55). 5. J. MOREAU et J. BENARD, J. Ghim. Phys. 53, 787 (1956). 6. Melle. H. MALAMAND. Communication priv6e.

produced

of impuri-

high-angle

and greatest

solutes

JACQUET

Rue Jeanne- D’Arc

Rapid

aqua

these studies is 1O-4 or less so that, even if most of the P. A.

75,

on the distribution

rapid

in hydrochloric

not ascribed

etch

by Lacombe’s

acid reagent has been shown to

dependent

ceux qui se localisent sur cette surface lorsque le milieu

interne

produced

regia plus hydrofluoric

deroulent sous la surface exposee a un milieu riche en ne seraient pas essentiellement

49

EDITOR

them-

The third objection

hypothesis

is that sub-

at about the same rate has been lots

of aluminum observed

from in both

99.97

to

ends of

crystals grown from the melt, where solute content would differ because of its redistribution during solidification. The vacancy

condensation

hypothesis

was tested on

with the pattern, described recently by Doherty and Davis,c3) of surface pits formed under the oxide film

a 14 X 10 X 100 mm crystal of approximately (122) [2i2] orientation of 99.99 + o/0 aluminum (0.004% Fe,

by the condensation

0.002%

of vacancies.

Cu, 0.0007%

Xi, 0.0001%

Na, <0.0002%

for

ACTA

50

METALLURGICA,

VOL.

8,

1960

than they can absorb,7

and the depleted

zone from

which the boundary has drained vacancies and abThe etched surface (Fig. 1b) also shows sorbed them. distributed

pits (flat-bottomed

$,u) and a depleted zone;

and not deeper than

however the boundary

itself It

is now visible as an etohed groove (about 3p deep). is to be noted that the etching was conducted continuous

microscopic

examination

and

under the

pit

density is known to have been changing slowly, if at all, with time when the etching avoid excessiveimpingement

was termina~d

to

of the pits on one another.

In both Pigs. la and lb, the pit density is about 1 x 10’ cm-2 and the total width of the depleted zone about 20,~~. This comparison suggests that dislocations and dislocation

boundaries

on which vacancies

densed have enhanced the conditions

employed

dislocation

have con-

susceptibility

and provide

under

sites at which

In the case of the boundary,

attack is initiated. the

corrosion

density

faster than it spreads

is high,

attack

where

penetrates

out from the narrow zone in

which it starts. Fig. 2 shows the similarity of condensation

and etch

pit patterns for a variety of boundaries (the orientation differences determined from Schulz X-ray

photographs

are 5’4 for the lineage boundaries on the left and 2’-5’ for the long polygonization boundaries on the right).

There is as expected

FIG. 1. Region including a. lineage boundary in unbent portion of crystal. x 500.

one boundary

(a) As air-cooled from 645°C showing vacancy condensation pits. Fine lines are believed to be cracks in the oxide film. Dark field illumination.

to another.

bound-

grooves on the

side surface as well as on the face. Where the boundaries

are near one another,

they

would be expected to compete for vacancies and have lower susceptibility.

It was electropolished,

in the

The polygonization

aries appear as more or less continuous

(b) Structure of the same field after subsequently electropolishing and etching anodicallg. Vertical illumination.

Mg, Mn, Ca).

some variation

character of the depleted zone and the depth of the etched groove (not visible at this nlagni~cation) from

bent over part

areas in Fig. 2b;

This was not,ed in some of the

studies on other specimens indicate

of its length to a 5 cm radius, and studies made of the

that it depends somewhat

flat portion and t,he first part of the bent portion after

tation di~erenee of the boundary, but boundaries much

annealing

in air at 645°C for 48 hr and cooling

stream of air.

pits observed on the previously after

cooling

structure

in a

Figs. 1 and 2 show the condensation from

obtained

electropolished

the

anneal,

after

again

then etching anodically 8 hr at 0.05 mA/cm2.

together

surface with

electropolishing

the and

in 12% hydrochloric acid for Anodic etching was desirable in

the present, case since it gives essentially

on the character

closer than the 20,~ width of the depleted

or orienzone ob-

served here always have distinctly lower susceptibility. A behavior

comparable

to that of the matrix would

be expected in boundaries of su~ciently low orientation difference---a simple 1’ tilt, boundary has a dislocation

concentration

the value

for the distributed

only a few times more than dislocations

deduced

the same

from the pit density in Fig. 1. A number of segments

behavior and is capable of much closer control than the straight chemical methods involving hydrochloric

of what are believed to be such low orientation difference boundaries are visible in Fig. lb.

acid.

The condensation

pit pattern in Fig. la near a

wandering lineage boundary (about 5’ tilt) shows the distributed pits formed by vacancies piped to the surface along dislocat,ions which have gathered

more

The most likely explanation rate at dislocations

for the higher corrosion

in these studies is believed

to be

7 This view of the mechanism of pit formation differs from t,he one given in reference 3.

LETTERS

FIG. 2. Transition polygonized zone.

TO

region from the unbent to the Oblique illumination. x 30.

(a) As air cooled from 645°C. (b) After electropolishing

and etching anodically.

broadly as follows, along lines suggested by the work of Pryor (*f. The aluminum exposed to the acid is soon protected by a very thin film of low ionic and electronic conductivity, so that the corrosion rate is quite low except where dislocations or impurities in the metal make it easier to form lattice defects in the film and there is thus some slow attack. Edge dislocation

THE

EDITOR

51

lines on which vacancies have condensed have high jog densities; when the slowly corroding surface arrives at a jog, a defect is injected into the film, e.g. when the jog is at the layer just below the surface of the metal it will be destroyed by the movement of one atom from one surface site to an adjacent one. Continued rapid attack will occur if the enhanced local corrosion due to increased local film conductivity progresses to the next jog before the effect of the additio~~al conductivity is lost. Vacancies or vacancy pairs associated with screw dislocations would have the same effect. The vacancy concentration at sufficiently elevated temperatures is more than enough to -produce high jog densities even if some are piped to the surface; a vacancy concentration of 10-6, for example, is equivalent to several vacancies per site along the dislocation lines at a dislocation density of lo8 cm-s. The sub-boundary corrosion s~ceptibility in 99.9%~~ aluminum eater-quenched from 600°C is low(l,s) and has been observed to remain so after aging 4 years at room temperature. This is not inconsistent with the fact that relaxation times of days or less are associated with effects on the plastic properties of aluminum quenched and aged at room temperature(sJe)-the plastic properties will be responsive to the formation of only a small number of jogs. The vacancies in quenched specimens would be expected to be largely tied up at in~pu~ty trap@ or as sheets which have collapsed to form small dislocation loops.(ls) It is noteworthy that the sub-boundary susceptibility in specimens water-quenched from 600°C and reheated for 4 hr periods was restored by reheating to 3OO’C but not below this temperature(2)-this is in accord with the view that most of the vacancies in the asquenched state were strongly bound at locations other than pre-existing dislocations and dislocation boundaries. hub-boundary etching after aging at room temperature has been observed in aluminumzinc alloys air cooled from elevated temperatures.(raW Since the change in etching behavior occurs in as low as 15 min for the 12% zinc alloy and since the roomtemperature interdiffusion coefficient estimated from recent data(‘) is only lo-l9 cm2/sec for this alloy, the change in etching behavior cannot represent the formation of zinc atmospheres unless atomic diffusion is enhanced by excess vacancies. A large excess of vacancies can apparently be trapped in the lattice at room temperature in these alloys.(15’ In this case it appears possible that vacancies freed from their traps may significantly enhance atomic diffusion before they are annihilated at dislocations, so that it is not clear whether the change in sub-boundary etching behavior

METALLURGICA,

ACTA

ii2

is associated dislocation

with

zinc

atmospheres

or vacancy-

intera.ctions.

VOL.

8,

phenomenons

1960

a fun~mental

investigation

versible hydrogen embrittlement

This work was supported

by the Office of Naval

Research.

0. P. ARORA M. METzGER

to obtain

sufficient accurate

knowledge

in order to derive a suitable

on irre-

has been undertaken of the ellect

mechanism.

Although

this study is still in progress, the experimental

results

~~~~~~~~~rg~&~~~ng~n~~~~ng

describing

the kinetics

of hydrogen

appear

University of lllinois

sufficiently

significant

to be presented

~~p~~t~~nt of Mining ad

since no quantitative

Urbanu, Illinois

a

(1954).

2. M. METZGER

and J. INTRIITER,NACA

Technicnl Note 3281

(1955).

3. P. E. DOHERTY and R. 8. DAVIS, Actn Met. ‘4, 118 (1969). and P. LAC~MBE, Conference 0% Defects in 4. G. WYOK ~~~~~tffZl~~~e Solids, p. 187. Physical Society, London (1955). 5. G. WYON. J.-M. n$ARCNIN and P. L~COXBE, Rev.MBtall. 53, 945 ( 1956).

J. A&. 6. A. S. SOWCK, I. J.E. HIL~IARD,B.L.

Phys.

7, 86 (1959).

(1955). 12. P. B. HIRSCH, J. SILCOX, R. E. SMAL~MAN and K. H. WESTMACOTT, Phil. Mug. 9, 897 (1958). nnd A. B~RGHEZ~N,C. R. Aearl.Sci.,Pn& 13. P. LACOMBE

226, 2152(1948). 14. A. BEIEGWGZAN, Rev. iklktnll. 48, 99 (1952). 15. E. C. ELLWOOD, J. Inst.-Wet. 80, 217 (1052). April 8, 1959;

low-carbon,

zdrogen

killed

steel

(205°C) in a nitrogen

atmosphere

embrittlement.

at about to remove

any

cause

Hydrogen

of

attack

is defined here as the per cent loss in reduction of area, and was measured

on miniature

mens (0.090 in. diameter, at room

temperature

round tensile speci-

0.5 in. gauge length) tested

in an Instron

testing

machine

at a cross head speed of 0.5 in./min. Fig.

1 presents

per

cent

hydrogen

attack

vs.

exposure time curves at 800”, 900°, lOOO”, and 1100°F (427*, 482”, 538”, and 593°C) respectively; each point represents a mean value obtained

revised version June 23, 1959.

a

to ex-

and for the desired

were baked

and thus eliminate

reversible hydrogen

to

Subsequent

posure at the desired temperature length of &me, the specimens 400°F

subjected

pressure of 700 lb/in2.

residual hydrogen,

22, 1182 (1951).

AVERBACH~~~M.COHEN, ActaMet.

8. M. J. PRYOR, 2. Electrochem. 61, 783 (1958). &fag. 46, 735 9. H. MAD~XN and A. H. COTTRELL, FhiE. (1955). M. LEVY sand M. &fETzGER, Phil. &fag. 46, 1021 (1955). 2: W. hf. LOMER and A. H. COTTRELL, Phil. Mug. 46, 711

* Received

pre-

The data which are to be presented were obtained

iVuture, Land. 1’74, 547

and 3. INTRATER,

immediately,

of this kind have

viously been reported.

References 1. M. METZGER

data

attack

from three or four

The curves all have the same shape and specimens. each can be considered to be made up of four segments,

namely:

(1) an incubation

which no permanent embrittlement by

the

criterion

selected;

time,

t,, before

occurs, as detected

(2) an “instantaneous”

steep rate of attack over a short, time interval, of 24 hr, On the kinetics hydrogen

of irreversible

Irons and steels subjected environment periods

at elevated

to a pressurized hydrogen

temperatures

of time are susceptible

brittlement accompanied

for sufficient

to a permanent

This

phenomenon

‘Lhydrogen attack”.

embrittlement

is a rever.Gle

has

It is differentiated

type

been

has generally reaction

decarburization) methane

of embrittlement;

rate;

and finally (4) a low steady-state

Whether

(3) a decreasing

transient

rate of attack.

or not this same kinetic pattern

occurs for

all materials and all types of exposure conditions still to be determined.

termed

by the fact t-hat the latter

of phenomenon

be restored by a low temperature chemical

stage

from the usual while hydrogen

attack is irreversible in that original ductility attack

em-

manifested as intergranular fractures by reduced ductility and strength pro-

perties.(l-4) hydrogen

in which the material ra.pidly proceeds to an advanced

embrittlement*

been thought

of hydrogen

anneal.

cannot

Hydrogen

to be due to the

and carbides

at the grain boundaries,

(internal

resulting in

whose pressure builds up causing localized

grain boundary fissuring and thus giving rise to deterioration of the material. Since this model fails satisfactorily to explain many observations of this

tE

FIG. 1. Hydrogen

(hr)

attack vs. exposure time at 700 lb/in2 H, pressure.

has