replica standard procedure

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R ep lica t ion M icrosc op y Techn iques

for N E

A.R. Ma rder Energy Research Cen ter Lehigh Universi ty

SURFACE REPLICATION is a well-

developed electron microscopy sample

preparation technique that can be used to

conduct in situ measurements of the micro-

structure of components. The

in situ

deter-

mination of microstructural deterioration

and damage of materials subjected to vari-

ous environments is an objective of any

nondestructive evaluation (NDE) of struc-

tural components. The need to assess the

condition of power plant and petrochemical

metallic components on a large scale recent-

ly led to the application of surface replica-

tion to the problem of determining remain-

ing life. The usual method of metallographic

investigation, which may involve cutting

large pieces from the component so that

laboratory preparation and examination can

be performed, usually renders the compo-

nent unfit for service or necessitates a cost-

ly repair. As a result, metallographic inves-

tigations are avoided, and important

microstructural information is not available

for evaluating the c omponent for satisfacto-

ry performance. Therefore, an

in situ

or

field microscopy examination is needed to

aid in the proper determination of compo-

nent life.

The replica technique for the examination

of surfaces has been extensively used for

studying the structure of polished-and-etched

specimens and for electron fractographic ex-

amination (see the article Transmission

Electron Microscopy in Volume 12 of the

9th Edition of

Metals andbook

for a discus-

sion of replication techniques in fractogra-

phy). Surface replication was the predomi-

nant technique in electron microscopy prior

to being supplemented by thin-foil transmis-

sion and scanning electron microscopy. Re-

cently, the replication microscopy technique

has become an important NDE method for

microstructural analysis, and an American

Society for Testing and Materials specifica-

tion has been written for its implementation

(Ref 1).

Specimen reparation

Mec hanical Pol ishing Methods. Compo-

nents in service usually have a well-devel-

oped corrosion or oxidation product or a

decarburized layer on the surface that must

be removed before replication. Coarse-

grinding equipment can be used as long as

the proper precautions are taken to prevent

the introduction of artifacts into the struc-

ture due to overheating or plastic deforma-

tion. Sandblasting, wire wheels, flap

wheels, and abrasive disks have all been

used. After the initial preparation steps are

completed, standard mechanical polishing

techniques can be used. Field equipment is

commercia lly available to help the metallog-

rapher reproduce the preparation steps nor-

mally followed in the laboratory. Depending

on the material, various silicon carbide

abrasive disks of different grit size, together

with polishing cloth disks with diamond

paste or alumina of varying grit size, can be

used to prepare for the etching step. Final-

ly, any appropriate etchant for the material

being examined can be applied to develop

the microstructure. For the proper identifi-

cation of such microstructural features as

creep cavities, a maximum double or triple

etch-polish-etch procedure should be used

(Ref 2). The etchants used for the various

materials investigated by the replication

technique are described in Volume 9 of the

9th Edition of Metals andbook and in Ref

3.

Electrolytic Prep aration Tech nique.

Al-

though electrolytic polishing and etching

techniques have often been employed as the

final mechanical polish step in sample prep-

aration, inherent problems still exist in this

process. The electropolishing technique

uses an electrolytic reaction to remove ma-

terial to produce a scratch-free surface.

This is done by making the specimen the

anode in an electrolytic cell. The cathode is

connected to the anode through the electro-

lyte in the cell. Specimens can be either

polished or etched, depending on the ap-

plied voltage and current density, as seen in

the fundamental electropolishing curve in

Fig. 1. However, the pitting region must be

avoided so that artifacts are not introduced

into the microstructure. It is virtually im-

possible to prevent pitting without precise

control of the polishing variables, and pits

~ E t c h i n g l P o l i s h i n g

/ i ~ ~,~ >1 ~

i t t i n g

V o l t a g e

- ,u © .

C u r r e n t d e n s i t y v o l t a g e c u r v e f o r

e l e c t

i s h i n g

can often be mistakenly identified as c

voids.

Several portable electropolishing u

are commercially available. The most

portant variables (time, bath temperat

electrolyte composition, and the cur

density-voltage relationship) have been

vestigated for a selected group of elec

lytes (Ref 4). A direct comparison of e

tropolishing units and the precaut

necessary for handling certain electrol

are given in Ref 5.

It should be noted that there are area

both fossil and nuclear plants in w

neither acid etches nor electropolis

methods and materials are allowed beca

of the potential for intergranular stre

corrosion cracking. Stainless steel pipin

nuclear plants can be replicated to de

mine defects by manual polishing with

etchants. Generator retaining rings h

been replicated by manual polishing to

solve NDE indications, because they

extremely sensitive to stress-corro

cracking and no acids or caustics are

lowed to be used (Ref 6).

Re plication Techniques

Replication techniques can be classi

as either surface replication or extrac

replication. Surface replicas provide an

age of the surface topography of a sp

ASM Handbook, Volume 17: Nondestructive Evaluation and Quality ControlASM Handbook Committee, p 52-56

Copyright © 1989 ASM InternationaAll rights reserv

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T a b l e 1 C o m p a r i s o n o f r e p l i c a

t e c h n i q u e s

T y pe dv a n t a g es D isa dv a nt a g es

Surface replicas

Acetate ... . . . . Excellent resolution Coating equired

Acry lic . . . . . . . Di rec t v iewin g Adhes ion

Rubber . . . . . . . Easy rem ova l Reso lu t ion

Extraction replicas

Direct stripped

plastic .. . . . . Easy preparation Particle etention

Positive

carbon .. . . . . Excellent particle Coating equired

retention with

two-stage etching

Direct carbon.. Excellent resolution Not applicable to

n s tu studies

m e n , w h i l e e x t r a c t i o n r e p l i c a s l i f t p a r t i c l e s

f r o m t h e sp e c i m e n . T h e a d v a n t a g e s a n d

d i s a d v a n t a g e s o f s o m e t y p i c a l r e p l i c a t i o n

t e c h n i q u e s a r e g i v e n in T a b l e 1 .

5 u d a t e R e p l i c as

R e p l i c a t i o n o f a s u r -

f a c e c a n i n v o l v e e i t h e r d i r e c t o r i n d i r e c t

m e t h o d s . I n t h e d i r e c t , o r s i n g l e - s t a g e ,

m e t h o d , a r e p l i c a is m a d e o f t h e s p e c i m e n

s u r f a c e a n d s u b s e q u e n t l y e x a m i n e d i n t h e

m i c r o s c o p e , w h i l e i n t h e i n d i r e c t m e t h o d ,

t h e f i n a l r e p l i c a i s t a k e n f r o m a n e a r l i e r

p r i m a r y r e p l i c a o f t h e s p e c i m e n s u r f a c e .

O n l y t h e d i r e c t m e t h o d w i l l b e c o n s i d e r e d i n

t h i s a r t i c l e b e c a u s e i t l e n d s i t s e l f m o r e f a -

v o r a b l y t o o n - si t e p r e p a r a t i o n . T h e m o s t

e x t e n s i v e l y u s e d d i r e c t m e t h o d s i n v o l v e

p l a s t i c , c a r b o n , o r o x i d e r e p l i c a m a t e r i a l .

A l l d i r e c t m e t h o d s e x c e p t p l a s t i c m e t h o d s

a r e d e s t r u c t i v e a n d t h e r e f o r e r e q u i r e f u r t h e r

p r e p a r a t i o n o f th e s p e c i m e n b e f o r e m a k i n g

a d d i t i o n a l r e p l i c a s .

P l a s t i c r e p l i c a s le n d t h e m s e l v e s t o i n - p l a n t

n o n d e s t r u c t i v e e x a m i n a t i o n b e c a u s e o f t h e i r

r e l a t i v e s i m p l i c i t y a n d s h o r t p r e p a r a t i o n

t i m e . P l a s t i c r e p l i c a s c a n b e e x a m i n e d w i t h

t h e l i g h t o p t i c a l m i c r o s c o p e , t h e s c a n n i n g

R e p l i c a t i o n M i c r o s c o p y T e c h n i q u e s f o r N D E /

e l e c t r o n m i c r o s c o p e , a n d t h e t r a n s m i s s i o n

e l e c t r o n m i c r o s c o p e , d e p e n d i n g o n t h e r e s -

o l u t i o n r e q u i r e d . A s i l l u st r a t e d i n F i g . 2 , t h e

p l a s t i c r e p l i c a te c h n i q u e i n v o l v e s s o f t e n i n g a

p l a s t i c f i l m i n a s o l v e n t , a p p l y i n g i t t o t h e

s u r f a c e , a n d t h e n a l l o w i n g it t o h a r d e n a s t h e

s o l v e n t e v a p o r a t e s . A f t e r c a r e f u l r e m o v a l

f r o m t h e s u r f a c e , t h e p l a s t i c f i l m c o n t a i n s a

n e g a t i v e i m a g e , o r r e p l i c a , o f t h e m i c r o s t r u c -

t u r e t h a t c a n b e d i r e c t l y e x a m i n e d i n t h e l i gh t

m i c r o s c o p e o r , a f t e r s o m e p r e p a r a t i o n , i n

t h e e l e c t r o n m i c r o s c o p e . D o u b l e - f a c e d t a p e

i s u s e d t o b o n d t h e r e p l i c a t o t h e g l a s s s l i d e

i n o r d e r t o o b t a i n l a r g e , f i a t , u n d i s t o r t e d

r e p l i c a s u r f a c e s .

T h e r e a r e s o m e s i g n i f i c a n t a d v a n t a g e s o f

t h e r e p l i c a t e c h n i q u e o v e r t h e u s e o f p o r ta -

b l e m i c r o s c o p e s i n t h e fi e l d R e f 5 ) :

* A p e r m a n e n t r e c o r d o f t h e s p e c i m e n i s

o b t a i n e d

• B e t t e r r e s o l u t i o n a n d h i g h e r m a g n i f i c a -

t i o n c a n b e u s e d

• C o n t a m i n a t i o n o f th e p o l i s h e d s u r f a c e is

m i n i m i z e d

• T i m e s p e n t i n a n u n p l e a s a n t o r h a z a r d o u s

e n v i r o n m e n t i s m i n i m i z e d

• S c a n n i n g e l e c t ro n m i c r o s c o p y c a n b e u t i-

l i z e d

S e v e r a l m a t e r i a l s , i n c l u d i n g a c e t a t e ,

a c r y l i c r e s i n , a n d r u b b e r , c a n b e u s e d i n t h e

s u r f a c e r e p l i c a t e c h n i q u e R e f 5 ). T h e

c h o i c e o f m a t e r i a l d e p e n d s o n t h e g e o m e t r y

o f t h e c o m p o n e n t a n d t h e m i c r o s t r u c t u r a l

f e a t u r e s t o b e e x a m i n e d .

I n t h e a c e t a t e m e t h o d , a n a c e t a t e t a p e i s

w e t t e d w i t h a c e t o n e a n d a p p l i e d t o t h e

s u r f a c e ; o t h e r l e s s v o l a t i l e s o l v e n t s , s u c h a s

m e t h y l a c e t a t e , c a n b e u s e d w h e n l a r g e

a r e a s a r e r e p l i ca t e d . F o r i m p r o v e d , r e s o l u -

t i o n , t h e b a c k s i d e o f t h e r e p l i c a c a n b e

p a i n t e d w i t h a n y f a s t - d r y in g b l a c k p a i n t o r

i n k p r i o r to r e m o v a l , o r f o r t h e s a m e e f f e c t ,

e v a p o r a t e d c o a t i n g s o f c a r b o n , a l u m i n u m ,

S o f te n e d c e t a t ea p e

/ ~ T a p ea p p lie

s u r fa c e n d d

Po l i sh e d - a n d - /

e tc h e d a r t ~ [ ~

T a p e e m o v e d

w i th n e g a t iv e

r e p l i ca f

su r fa ce

F i g 2

Schematicof the plastic replica techni

o r g o l d c a n b e a p p l i ed a t a s h a d o w a n g l

4 5 ° t o t h e f r o n t s i d e o f t h e r e p l i c a a

r e m o v a l .

I n th e a c r y l i c c a s t i n g re s i n m e t h o d , d

a r e r e q u i r e d b e c a u s e a p o w d e r i s m i

w i t h a l i q u i d o n t h e s u r f a c e t o b e r e p l i c a

A f t e r h a r d e n i n g , t h e r e p l i c a c a n b e e x

i n e d d i r e c t l y in a n o p t i c a l m i c r o s c o p e w

o u t f u r t h e r p r o c e s s i n g . I f a d h e s i o n i

p r o b l e m , a c o m p o s i t e r e p l i c a c a n b e m

o f a n i n i ti a l l a y e r o f P a r l o d i a n l a c q u e r

f o r e t h e a c r y l i c l a y e r i s a p p l i e d .

I n t h e d e n t a l i m p r e s s i o n r u b b e r m e t h

u n c u r e d l iq u i d r u b b e r m a t e r i al f o r e x

p l e , G E R T V 6 0 s i l i c o n r u b b e r c o m p o u n d

p o u r e d o n t o t h e s u r f a c e t o b e re p l i c a t e d

i s c o n t a i n e d b y a d a m . A f t e r r e m o v a l ,

r e p l i c a c a n b e e x a m i n e d d i r e c t l y o r c a n

c o a t e d f o r b e t t e r r e s o l u t i o n .

Pl a s t i c

' - - -~ u ~ i , i - - F i rs te t c h /

~ 'JJ '. ~ . /nc ;us ion

°

i r s t e t c h

• \ \ ® / / e

• ( ~ ~ ) ~ , / / / ~ ) M e ta l

(a) (b)

C a r b o n

? ~ / J n L

~ ~ J

C a r b o n

(c) (d)

' 'ul¢|~ 3 Positivecarbon extraction replication steps, (a) Placem entof plastic after the first etch. (b) After the second etch . (c) Afte r the deposition of c arbon . (d) The p

replica offer the plastic is dissolved



5 4 / M e t h o d s o f N o n d e s t r u c t i v e E v a l u a t i o n

a)

c)

= . == = -. . 4 P r o p a g a t i o n o f d i f f e r e n t c r o c k t y p e s . a )

c o r r o s i o n

E x t r a c t i o n R e p l i c a s . Several different ex-

traction replica techniques can be used to

characterize small particles that are embed-

ded in a matrix, such as small second-phase

particles in a steel (see the article Analyt-

ical Transmission Electron Micros copy in

Volume 10 of the 9th Edition of Metals

Handbook . More detailed descriptions of

the various extraction replica techniques

can be found in Ref 7 and 8.

After careful preparation of the surface

using normal polishing methods, the first

step in producing an extraction replica is to

etch the alloy heavily to leave the particles

of interest in relief. In the positive carbon

extraction replica, as shown in Fig. 3, a

piece of solvent-softened polymeric film

(cellulose acetate tape) is pressed onto the

surface exposed by this first etch (Ref 5).

Once the solvent has evaporated, one of

two steps can be taken. The tape can be

carefully pulled from the specimen to pro-

duce a negative of the surface, or the spec-

imen can undergo a second etch to free the

particles exposed by the first etch (Fig. 3).

In the second etch, the specimen can be

etched through the plastic; most plastics are

quite permeable to etching solutions, and

the specimen etches almost as rapidly as

without the plastic film (Ref 9). Carbon is

then evaporated in a vacuum onto the plas-

tic replica. The carbon and plastic contain-

ing the particles now make up the positive

replica. The cellulose acetate is then dis-

solved, and the positive carbon replica is

allowed to dry. It should be noted that for

the negative carbon extraction replica tech-

b)

d )

Cre e p . b ) F a t ig u e • c )

S t r e s s c o r r o s io n . d ) I n t e r g r a n u l a r

nique, vacuum deposition of carbon onto

the surface of the specimen is required, and

therefore this replica method is not applica-

ble to NDE.

i c r o s t r u c t u r a l A n a l y s i s

C r a c k d e t e r m i n a t i o n

is important to help

establish the root cause of a potential failure

in a component. After a preliminary evalu-

ation of the crack to assess crack shape and

length by using magnetic flux or dye pene-

trant, the replica method is then used on

unetched specimens to assist in the crack

evaluation. Figure 4 schematically shows

the propagation of different types of cracks

in a steel structure (Ref 10). Each cr ack has

its own characteristics, and it is often pos-

sible to make a correct determination of

crack type. It is important to determine

whether the crack is the original defect or

has been caused by service conditions or

damage. Once the crack type is identified,

the proper corrective action, such as elim-

inating a corrosive environment or reducing

stress levels, can be attempted. Figure 5

shows the replication of surface cracks in a

boiler tube.

Creep Damage

Creep defects cause the

majority of failures in power plant c ompo-

nents operating under stress and thermal

load, and the replica method is especially

suitable for the detection of these defects.

Therefore, the replica method has become

an especially important tool in the deter-

mination of remaining life in such compo-

nents as boiler tubes, steam piping, and

@

j -

a)

~,

• 2 0 _ _

b )

S u r fa c e c r a c k i n a b o i l e r t u b e .

C o m p a

F i g 5 o f t h e a ) a c t u a l m i c r o s t ru c t u r e a n d b

r e p l i c a o f t h e c r a c k

turbine components. The replica met

reveals defects due to creep at a m

earlier stage than other NDE techniqu

Creep defects begin as small holes or c

ities at grain boundaries or second pha

With time and stress, these holes or c

ties can link up and form cracks

eventually lead to failure of the compon

(Fig. 6). Creep cracks are usually v

localized, and they form in welds, ben

or other highly stressed regions. Determ

ing the remaining life of components n

mally depends on assessments of reg

inspections, as indicated in Table 2. Fig

7 shows a comparison of creep voids

surface replica and the cor responding b

microstructure.

P r e c i p i t a t e

A n a l y s i s . The detection

various deleterious precipitates in com

nents subjected to high temperature

stress can lead to improved life assessm



R e p l i c a t i o n M i c r o s c o p y T e c h n i q u e s f o r N D E /

a) b) c ) d)

F i g , 6 Sc he ma t i c o f c r e e p c r oc k format ion. S ma ll cavit ies a ) l ink up over time b) and form intergranulor crocks c) and eventually mac rocracks d ).

T a b l e 2 C r e e p d a m a g e c l a s s i f i c a t i o n

la s s Na tu re Action

1 No creep defects

2 A few cavities

3 Coalesce nt cavities

4 Microscopic creep cracks

5 Macroscopic creep cracks

Source: Ref 11

None

Reinspection after 20 000 h of service



Management must be informed immedi

a)

a) b)

F i g . 8 C o m p a ris o nof cT-phase ormation as seen in a) a repl ica and b) the actua l microstructure

b )

F i g

7 Compar ison of c r e e p voids in a) a repl ica

and b) the ac tual micros t ruc ture

analysis of these components. The extrac-

tion replication technique is an excellent

nondestructive method of detecting these

precipitates.

Sigma phase is a deleterious FeCr com-

pound that can form in some stainless

steels, and its presence can severely limit

remaining life. Extraction replicas have

been used to determine the amount of r

phase in the microstructu re Ref 12), and

the amount of ~ phase has been directly

related to the creep rate Ref 13). Figure 8

shows an example of cr phase in an extrac-

tion replica.

The composition of carbides, and their

stability with time and temperature of expo-

sure, can indicate the remaining life of a

component. Extraction replicas have been

used to evaluate carbides, and it has been

suggested that changes in morphology and

chemistry can be used to assist the esti

tion of effective exposure temperature

use in determining the remaining life

components Ref 14). Figure 9 shows

example of precipitates extracted fro

200 000-h exposed sample, toget her

the ac companying chemical analysis.

A C K N O W L E D G M E N T

The author would like to acknowledge

contributions of his colleagues A.O. B

scoter, S.D. Holt, and T.S. Hahn in

preparation of this article.



5 6 / M e t h o d s o f N o n d e s t r u c t i v e E v a l u a t i o n

a )

F ig . 9 Ex t rac t ion rep l ica o f the mic ros truc ture a ) and the

b )

precipitate microchemical analysis b ) from an e xtraction repl ica

R E F E R E N C E S

1 . S t a n d a r d P r a c t i c e f o r P r o d u c t i o n a n d

E v a l u a t i o n o f F i e l d M e t a l l o g r a p h i c

Re pl ic as , E 512-87 , A n n u a l B o o k o f

A S T M S t a n d a r d s A m e r i c a n S o c i e t y

f o r T e s t i n g a n d M a t e r i a l s

2 . A . M . B i s s e l , B . J . C a n e , a n d J . F . D e -

L o n g , R e m a n e n t L if e A s s e s s m e n t o f

S e a m W e l d e d P i p e w o r k , P a p e r p r e -

s e n t e d a t t h e A S M E P r e s s u r e V e s s e l

a n d P i p i n g C o n f e r e n c e , A m e r i c a n S o c i -

e t y o f M e c h a n i c a l E n g i n e e r s , J u n e 1 9 88

3 . G . F . V a n d e r V o o r t , M e t a l l o g r a p h y :

P r i n c i p l e s a n d P r a c t i c e M c G r a w - H i l l ,

1984

4 . T . S . H a h n a n d A . R . M a r d e r , E f f e c t o f

E l e c t r o p o l i s h i n g V a r i a b l e s o n t h e C u r -

r e n t D e n s i t y - - V o l t a g e R e l a t i o n s h i p ,

M e t a l l o g r a p h y Vol 21 , 1988, p 365

5 . M . C l a r k a n d A . C e r v o n i , I n S i t u

M e t a l l o g r a p h ic E x a m i n a t i o n o f F e r -

r o u s a n d N o n - F e r r o u s C o m p o n e n t s ,

C a n a d i a n E l e c t r i c a l A s s o c i a t i o n , N o v

1985

6 . J . F . D e L o n g , p r i v a t e c o m m u n i c a t i o n

7 . D . K a y , E d . , T e c h n i q u e s f o r E l e c t r o n

M i c r o s c o p y Blackw el l Sc ien t i f ic Publ i -

ca t ions , 1965

8 . J . W . E d i n g t o n , P r a c t i c a l E l e c t r o n M i -

c r o s c o p y i n M a t e r i a l s S c i e n c e V a n

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