Corrosion on Weldment

8/11/2019 Corrosion on Weldment

1/5

o rro s io n o f W e l d m e n t s

A . W a h i d D . L . O l so n a n d D . K . Ma t l o ck C o l o r a d o S ch o o l o f M i n e s

C .E . C r o ss Ma r t i n Ma r i e t t a A s t r o n a u t i cs G r o u p

WELDMENTS exhibit special microstruc-

tural features that need to be recognized and

understood in order to predict acceptable corro-

sion service life of welded structures (Ref 1).

This article describes some of the genera l char-

acteristics associated with the corrosion of weld-

ments. The role of macrocompositional and mi-

croeompositional variat ions , a feature common

to weldments, is emphasized in this article to

bring out differences that need to be realized in

comparing corrosion of weldments to that of

wrought materials. A more extensive presenta-

tion, with data for specific alloys, is given in

Volume 13 of the

SM Handbook

(Ref 2).

Weldments inherently possess compositional

and microstructural heterogeneities, which can

be classified by dimensional scale. On the larg-

est scale, a weldment consists of a transition

from wrought base metal through a heat-affected

zone and into solidified weld metal and includes

five microstructurally distinct regions normally

identified (Ref 3) as the fusion zone, the un-

mixed region, the partially melted region, the

heat-affected zone, and the base metal. This mi-

crostructural transi tion is illustrated in Fig. 1.

The unmixed region is part of the fusion zone,

and the partially melted region is part of the

heat-affected zone, as described below. Not all

five zones are present in any given weldment.

For example, autogenous (that is, no filler

metal) welds do not have an unmixed zone.

T h e f u s io n z o n e

is the result of melting

which fuses the base metal and filler metal to

produce a zone with a composition that is most

Weld nugge t

We ld i n te r face

/

Unmixed reg ion

P a r t i a l l y mel ted ~ /

reg ion

Una f fec ted base meta l

S c h e m a t ic s h o w i n g t h e r e g i o ns o f a h e t e r o g e -

F i g 1 n e o u s w e l d . S o u r c e : R e f 3

often different from that of the base metal. Th is

compositional difference produces a galvanic

couple, which can influence the corrosion proc-

ess in the vicinity of the weld. This dissimilar-

metal couple can produce macroscopic galvan ic

corrosion.

The fusion zone itself offers a microscopic

galvanic effect due to microstructural segrega-

tion resulting from solidification (Ref 4). The

fusion zone also has a th in region adjacent to the

fusion line, known as the unmixed (chilled) re-

gion, where the base metal is melted and then

quickly solidified to produce a composition sim-

ilar to the base metal (Ref 5). For example,

when type 304 stainless steel is welded using a

filler metal with high chromium-nickel content,

steep concen tration gradients of chromium and

nickel are found in the fusion zone, whereas the

unmixed zone has a composition similar to the

base metal (Fig. 2).

Heat-Affected Zone. Every position in the

heat-affected zone relative to the fusion line ex-

periences a unique thermal experience during

welding, in terms of both maximum temperature

and cooling rate. Thus, each position has its own

microstructural features and corrosion suscepti-

bility.

The partially melted region is usually one or

two grains into the heat-affected zone relative to

the fusion line. It is characterized by grain

boundary liquation, which may result in liqua-

tion cracking. These cracks, which are found in

the grain boundaries one or two grains below the

3 0

D 28

2 4

0 2 2

2O

1 6

. . . . . . . . + o o i = : 0 1 , : z H A z I. . . . . . . .

CHROMIUM

2

18

16 ~

14 z

12 o

NICKEL 10 m

I

0 500 1000 1500

DISTANCE MICRONS)

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

F i g . 2 n i c k e l a c ro s s t h e w e l d f u s i o n b o u n d a r y r e -

g ion o f t ype 304 s ta inless s tee l . Source : Re f 5

fusion line, have bee n identified as potentia

tiation sites for hydrogen-promoted unde

cracking in high-strength steel.

M i c r o s t r u c t u r a l G r a d i e n t s

On a fine s

microstructural gradients exist with in the

affected zone due to different time-temper

cycles experienced by each element of mat

Gradients on a similar scale exist within s

fied multi-pass weld metal due to bead-to

variations in thermal experience. Composi

gradients on the scale of a few microns, ref

to as microsegregation, exist within indiv

weld beads due to segregation of major and

elements during solidification (Ref 4).

F o r m s o f W e l d o r r o s i on

Weldments can experience all the cla

forms of corrosion, but they are particularly

ceptible to those affected by variations in m

structure and composition. Specifically,

vanic corrosion, pitting, stress corro

intergranular corrosion, and hydrogen cra

must be considered when designing w

structures.

Galvanic C ouples.Although some alloy

be autogenously welded, fill er metals are

commonly used. The use of filler metals

compositions different from the base ma

may produce an electrochemical potent ial d

ence that makes some regions of the weld

more active. For example, Fig. 3 depicts

metal deposits that have different corrosio

havior from the base metal in three alum

alloys (Ref 6).

For the majority of aluminum alloys, the

metal and the heat-affected zone become

noble relative to the base metal, as demonst

in Fig. 3(a) and (b) for a saltwater environ

(Ref 6). Certain aluminum alloys, how

form narrow anodic regions in the heat-affe

zone and are prone to localized attack. A

7005 and 7039 are particularly susceptib

this problem (Fig. 3c).

There are a number of other common

deposit/base metal combinations that are k

to form galvanic couples. It is common pra

to use austenitic stainless steel welding con

ables for field repair of heavy machinery, p

ularly those fabricated from high-strength

ASM Handbook, Volume 6: Welding, Brazing,and Soldering

D.L. Olson, T.A. Siewert, S. Liu, and G.R. Edwards,editors,p 1065-1069

Copyright 1993ASM InternationAll rights reser

www.asminternationa


2/5

1 0 6 6 / S p e c i a l W e l d i n g a n d J o in i n g T o p i c s

Distance from we ld centerl ine, in.

o

1 2

3 4

o 900 ' I Edg~ o f we l~ l bead ' =

,

I Hardness

i= f - -

~-E

7 5 0

~I~

Co~oslon po ten t ia l

70C I I

0 25 50 50 1O0

Distance from we ld centerl lne, mm

65 =

5 5 -r

5

~

5

Distance from we ld centertine, in.

O 1 2 3 4

800

=-~mm 75 0 llLIIEdge l o f w e l d = b e a d l 1 5 m

e= n Hardnes s :=

~ 60 0 If t I I I I 5

0 25 50 50 1O0

Distance from weld center l ine

mm

Distance from we ld centerl lne, in.

1 2 3 4

uo 10500 ,

[ I E d o e ' w ' d ' a d ' 1

1DO0 ~ Hardness

/ , . . . . . . . . . . .

o

= 950

iT 90 0 Cor rosion po ten t ia l

o 8 5 0 I J I

0 25 50 50 100

Distance from weld center line mm

a) b) c)

E f f ec t o f we l d i n g h e a t o n m i c r o s t r u c t u r e , h a r dn e s s , a n d c o r r o s i o n p o t e n t i a l o f t h r e e a l u m i n u m a l l o y we l d e d a s s e m b li e s . ( a) A l l o y 5 4 56 -H3 2 1 b a s e m e t a l w i t h

Fig 3

5556 i l l e r . ( b ) A l lo y 2219-T87 base me ta l w i th a l loy 2319 i l l e r . ( c ) A l loy 7039-T651 base me ta l w i th a l loy 5183 i l l e r . S ou rce : Re f 6

i n t r a g r a n u l a r ~ g ra in b o u n da r y

~ E 2 , , ~ ~ d e p l e t i o n

~ 3 E I ~

El: potentia l for ppt. boundary

E 2 : p o t en t ia l f o r d e p le t e d z o n e p r e c i p i t a t e

E3: potentia l for matr ix

Dep le ted reg ions ad jacen t to p rec ip i ta tes .l a

4

= '~ These reg ions cause an e lec t rochemica l po -

ten t ia l (E) d i f fe rence tha t can p rom ote loc a l i zed co r ro -

s ion a t the micros t ruc tu ra l leve l .

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

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

w i t h t h e s t e e l . I n t h e p r e s e n c e o f c o r r o s i v e e n v i -

r o n m e n t s , h y d r o g e n i s g e n e r a t e d a t t h e a u s t e n -

i t i c w e l d m e t a l c a t h o d e , w h i c h i s c a p a b l e o f

m a i n t a i n i n g a h ig h h y d r o g e n c o n t e n t w i t h o u t

c r a c k i n g . H o w e v e r , t h e c a t h o d i c b e h a v i o r o f t h e

a u s t e n i t i c w e l d d e p o s i t m a y i n c r e a s e t h e s u s c e p -

t i b i l i t y f or s t r e s s - c o r r o s io n c r a c k i n g i n t h e h e a t -

a f f e c t e d z o n e o f t h e h i g h - s tr e n g t h s t e el . A 4 0 %

t h e r m a l e x p a n s i o n m i s m a t c h b e t w e e n t h e a u s t e -

n i t i c s t a i n l e s s s te e l a n d f e r r it i c b a s e m e t a l p r o -

d u c e s a s i g n i f i c a n t r e s i d u a l s t r e ss f i e l d i n t h e

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

u t e s t o c r a c k i n g s u s c e p t i b i l i t y . A s i m i l a r , b u t

m o r e l o c a l i z e d , b e h a v i o r m a y e x p l a i n t h e c o r r e -

l a t i o n b e t w e e n s t r e s s - co r r o s i o n c r a c k i n g s u s c e p -

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

h i g h - s t r e n g t h s t e e l w e l d d e p o s i t s.

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

t i o n i n v o l v e s t h e u s e o f h i g h - n i c k e l a l l o y s f o r

w e l d r e p a i r o f c a s t i r o n . F e - 5 5 N i w e l d i n g e l e c -

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

h o l d i n s o l i d s o l u t i o n m a n y o f t h e a l l o y i n g e l e -

m e n t s c o m m o n t o c a s t i r o n . F u r t h e rm o r e , w e l d

d e p o s i t s m a d e w i t h F e - 5 5 N i w e l d i n g c o n s u m -

a b l e s h a v e a n a c c e p t a b l e t h e r m a l e x p a n s i o n

m a t c h t o t h e c a s t i r o n . B e c a u s e c a s t i r o n i s a n -

o d i c t o t h e h i g h - n i c k e l w e l d d e p o s i t , c o r r o s i v e

a t t a c k o c c u r s i n t h e c a s t i r o n a d j a c e n t t o t h e w e l d

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

w i t h h i g h - n i c k e l d e p o s i t s b e c o a t e d ( p a i n t ed ) t o

r e d u c e th e su sc e p t ib i l i t y to s e l e c t iv e c o r r o s io n

a t t ac k .

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

g a l v a n i c a t ta c k . F o r e x a m p l e , t h e E 6 0 1 3 w e l d -

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

A 2 8 5 b a s e m e t a l i n a s e a w a t e r e n v i r o n m e n t ( R e f

7 ) . I t i s i m p o r t a n t t o s e l e c t a s u i t a b l e f il l e r m e t a l

w h e n a n a p p l i c a t i o n i n v o l v e s a h a r s h e n v i r o n -

m e n t .

W e l d D e c a y o f S t a i n l e s s Stee l . D u r i n g

w e l d i n g o f s t a i n l e s s s t e e l s , l o c a l s e n s i t i z e d

z o n e s ( i . e . , r e g i o n s s u s c e p t i b l e t o c o r r o s i o n ) o f-

t e n d e v e l o p . S e n s i t i z a t i o n i s d u e t o t h e f o r m a -

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

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

r e g i o n a d j a c e n t t o t h e g r a i n b o u n d a r y ( R e f

8 - 1 4 ) . T h i s c h r o m i u m d e p l e t i o n p r o d u c e s v e r y

l o c a l i z e d g a l v a n i c c e l ls ( F i g . 4 ) . I f t h is d e p l e t i o n

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

1 2 w t % t h a t i s r e q u i r e d t o m a i n t a i n a p ro t e c t i v e

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

t o c o r r o s i o n , r e s u l t i n g i n i n t e r g r a n u l a r a t t a c k .

T h i s t y p e o f c o r r o s i o n m o s t o f t e n o c c u r s i n t h e

h e a t - a f f e c t ed z o n e . I n t e r g r a n u l a r c o r r o s i o n

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

t h e w e l d d e p o s i t ( F i g . 5 ) . T h i s c o r r o s i o n b e h a v -

io r i s c a l l e d w e ld d e c ay (R e f 1 3 ) .

T h e f o r m a t i o n o f s u f f ic i e n t c h r o m i u m c a r b i d e

t o c a u s e s e n s i t i z a t i o n c a n b e d e s c r i b e d b y t h e

C - s h a p e d c u r v e s o n t h e c o n t i n u o u s c o o l i n g d i a -

g r a m i l l u s t r a t e d i n F i g . 6 . T h e f i g u r e s h o w s s u s -

c e p t i b i l i t y t o s e n s i t i z a t i o n a s a f u n c t i o n o f t e m -

p e r a t u r e , t i m e , a n d c a r b o n c o n t e n t ( R e f 1 5 ). I f

t h e c o o l i n g r a t e i s s u f f i c i e n t l y g r e a t ( c u r v e A i n

F i g . 6 ) , t h e c o o l i n g c u r v e w i l l n o t i n t e r s e c t t h e

g i v e n C - s h a p e d c u r v e f o r c h r o m i u m c a r b i d e a n d

t h e s t a i n l e s s s t ee l w i l l n o t b e s e n s i t iz e d . B y d e -

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

( c u r v e B ) e v e n t u a l l y i n te r s e c t s t h e C - s h a p e n u -

c l e a t i o n c u r v e , i n d i c a t i n g t h a t s e n s i t i z a t i o n m a y

o c c u r . A t v e r y l o w c o o l i n g r a t e s , t h e fo r m a t i o n

o f c h r o m i u m c a r b i d e o c c u r s a t h i g h e r t e m p e r a -

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

r e s u l t i n g i n a m o r e e x t e n s i v e c h r o m i u m - d e -

p l e t e d r e g i o n .

T h e m i n i m u m t i m e r e q u i re d f o r s e n s i t i za t i o n

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

l e s s s t e el a l l o y i s d e p i c t e d i n F i g . 7 . B e c a u s e t h e

W E L D D E C A Y

In te rg ran u la r co r ros ion (we ld decay) o f

F i g . 5 l e s s s t e e l we l d m e n t s . F Z , f u s i o n z o n e

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

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

c a r b o n c o n t e n t m u s t n o t e x c e e d 0 . 0 7 w t

a v o i d s e n s i t i z a t i o n . N o t i c e t h a t t h e c a r b i d

c l e a t i o n c u r v e s o f F i g . 6 m o v e d o w n a n

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

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

g i v e n c o o l i n g r a t e .

T h e c o n t r o l o f s t a in l e s s s t e e l s e n s i t i z

m a y b e a c h i e v e d b y u s i n g :

A p o s t w e l d h i g h - t e m p e r a t u r e a n n e a l

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

b o u n d a r i e s , a n d h i n d e r c h r o m i u m c a r b i d

m a t i o n o n c o o l i n g

A l o w - c a r b o n g r a d e o f s t a i n l e s s s t e e l (

3 0 4 L o r 3 1 6 L ) t o a v o i d c a r b i d e f o rm a t i o

A s t a b i l i z e d g ra d e o f s t a i nl e s s s t e e l c o n t a

t i t a n i u m ( a l l o y 3 2 1 ) o r n i o b i u m ( a l l o y 3

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

c h r o m i u m i n s o l u t i o n . ( T h e r e i s t he p o s

i t y o f k n i f e - l i n e a t t a c k i n s t a b i l i z e d g r a d

s t a in l e s s s t e e l . )

A h i g h - c h r o m i u m a l l o y ( e . g . , a l l o y 3 1 0 )

Ro le o f D e l ta F er r i te in S tain less

W e l d D e p o s i t s . A u s t e n i t i c w e l d d e p o s it

f r e q u e n t l y u s e d t o j o i n v a r i o u s f e r r o u s a l l o

h a s b e e n w e l l e s t a b l i s h e d t h a t i t i s n e c e s s a

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

f e r r it e , a l s o k n o w n a s a ~ f e r r it e , i f h o t c r a c k

t o b e m i n i m i z e d ( R e f 1 6 , 1 7 ) . T h e a m o u n


3/5

C o r r o s i o n o f W e l d m e n t s / 1

9 0 0

8O0

P

7

o

E 6 0 0

5 0 0

4 0 0

10

~ o . o s o , t

\ o 0

1 min 10 min 1 h 10 h

Tim e to sen s i t iza t ion

- 1 6 o o

1 4 0 0

1 2 0 0 ~ .

E

1 9 C - l O O O ~

8 0 0

100 h 1000 h

T i m e - t e m p e r a t u r e - s e n s i t i z a t i o n c u r v e s f o r t y p e 3 0 4 st ai n le s s s t e e l i n a m i x t u r e o f C u S O 4 a n d H S O 4

F i g . 6 c o n t a i n i n g c o p p e r . S o u r c e : R e f 1 5 . C u r v e s A a n d B i n d ic a t e h i g h a n d m e d i u m c o o l i n g r a te s , r e s p e c -

t i ve ly .

104

~ 103

d

E

1 02

p

. - 1 0

i-

\

0.1

0 . 0 1 0 . 0 3 0 . 0 5 0 . 0 7 0 . 0 9 0 . 1 1

C a r b o n c o n t e n t , w t

M i n i m u m s e n s i t i z a t i o n t i m e f r o m

a t ime- tem-

=::nr,~. 7 pe ra tu re -se ns i t i za t i on d iag ram as a func t i on

o f ca rbo n co n ten t fo r a t yp i ca l 300-se r ies s tain less s tee l

a l l oy . Source : Re f 15

form of ferrite in the weld metal can be con-

trolled by selecting a filler metal with the appro-

priate chr omium and ni ckel equivalent. A high

chromium/nickel ratio favors primary ferrite for-

mation, whereas a low ratio promotes primary

austenite (Fig. 8). An optimum condition can be

attained for ferrite contents between 3 and 8

vol in the weld deposit. Ferrite contents above

3 vol usual ly guarantee primary ferrite forma-

tion and thus reduce hot cracking susceptibility.

However, ferrite above 10 vol can degrade

mechanical properties at low- or high-tempera-

ture service. At low temperatures, excess ferrite

can promote crack paths when the temperature is

below the ductile-brittle transition temperature.

At high temperatures, continuous brittle sigma

phase may form at the interface between the

austenite and the ferrite. The ferrite content can

be confirmed using magnetic measuring equip-

ment (Ref 16, 17).

Figure 8 can be used to predict the type of

ferrite (primary or eutectic) and the ferrite con-

tent when a difference exists between the stain-

less steels being joined, such as when welding

type 304 to type 310 stainless steel (Ref 18).

This diagram shows the compositional range for

the desirable primary solidification mode. The

dotted lines on the diagram indicate the various

transitions in the primary solidification phase.

Because not all ferrite is primary ferrite (i.e.,

some is a phase compone nt of a ferrite-austenite

eutectic), this diagram can be used to ensure that

ferrite is the first solid (primary) phase to form.

This condition occurs when the weld deposit has

a composition in the range labeled FA in Fig.

8. Because primary ferrite is the preferable mi-

crostructure, use of this diagram should re-

duce problems of hot cracking during welding.

Also, the corrosion behavior of stainless steel

weld deposits and castings is measurably dif-

ferent depending on whether the stainless steel

has a microstructure generated with primary

ferrite or primary austenite (Ref 19-25). Thus,

knowledge of the weld metal ferrite content

and form is necessary in order to be able to

properly characterize and predict corrosion be-

havior.

Pitt ing is a form of localized attack caused by

a breakdown in the thin passive oxide fi lm that

protects material from the corrosion process.

Pits are commonly the result of a concentration

cell established by a variation in solution com-

position that is in contact with the alloy material.

Such compositional variations result when the

solut ion at a surface irregularity is different from

that of the bulk solution composition. Once a pit

has formed, it acts as an anode supported by

relatively large cathodic regions. Pitting has a

delay time prior to nucleation and growth, and

nucleation is very site-selective and microstruc-

ture-dependent. Pits are often initiated at spe-

cific microstructural features in the weld deposit

(Ref 26). Pitting occurs when the material/

solution combination achieves a potential that

exceeds a critical value, known as the pitting

potential. The tendency for a given alloy/

solution combination to pit can often be charac-

terized by critical potentials for pitting and

repassivation determined by a cyclic potent

namic polarization technique.

Pits develop more readily in metallurg

heterogeneous materials. For example, w

austenitic stainless steel is heated to temp

tures where sens itizat ion takes place (Re

27), the resulting chromium-depleted regi

subject to pitting. Pits may also initiate a

austenite-ferrite interfaces in stainless steel

metal.

Although weld metal has a higher proba

of being locally attacked because of micros

gation in the dendritic structure, filler meta

now available that have better pitting resist

than their respective base metals; inform

about these filler metals can be obtained

consumable suppliers. However, even whe

proper filler metal is used, pitting may still o

in the unmixed zone.

Duplex stainless steels, with ferrite con

in the range of 40 to 50 vol , are often us

decrease the tendency of stress-corrosion c

ing in chromium-nickel high-alloy steels.

welding practice for duplex stainless steels

be given special attention (Ref 20-22, 24

27) to avoid reduction in corrosion resista

The combination of a low carbon content a

carefully specified nitrogen addition have

reported to improve resistance to pit ting c

sion, stress-corrosion c racking, and interg

lar corrosion in the as-welded condition.

low carbon content helps avoid sensitiza

while the addition of nitrogen slows the pre

tation kinetic s associated with the segregati

chromium and molybdenum during the we

process (Ref 1). On rapid cooling from

temperature, nitrogen also has been report

form deleterious precipitates (for exam

Cr2N) in the ferrite, thus reducing the corr

resistance (Ref 28). Nitrogen also increase

formation of austeni te in the heat-affected

and weld metal during cooling. A mi nimum

ting corrosion rate is achieved at a ferrite co

of about 50 vol .

Stress Corros ion Cracking. Weldment

be susceptible to stress-corrosion cracking

specific environmental conditions. This c

ing requires the proper combination of corr

media, susceptible microstructure, and te

stress. Welds are often loaded in tension (d

residual stress) to a level approaching the

strength of the base metal. A weld, with its

ious heterogeneous microstructural fea

thus becomes an excellent candidate for s

corrosion cracking.

Stress-corrosion cracks have an anodic

tip and often leave apparent corrosion pro

along the fracture. Cracking is often chara

ized by crack branching and usually has a

time prior to crack initiation, with i nitiatio

curring at corrosion pits. Increasing the f

content in stainless steel weld metal red

stress-corrosion cracking susceptibility.

proximately 50 vol ferrite gives opt

stress-corrosion cracking resistance.

Welding parameters influence the amoun

distribution of residual stress, because the e

of the stressed region and the amount of d


4/5

1 0 6 8 / S p e c i a l W e l d i n g a n d J o in i n g T o p i c s

18

/ /

17 / /

12 ; , /

13 / / /

5

/ .o.~z ~ /

1 7 18 1 9 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 3 1

Chromium equ iva len t (Creq = Cr + M o + 0 .7Nb)

Weld ing Research Counc i l (WRC-1988) d iag ram used to p red ic t we ld meta l fe r r i te con ten t . Source : Re f

F i g . 8 1 8

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

w e l d d e p o s i t ; t h i s d e p o s i t i s d i r e c t l y r e l a t e d t o

t h e h e a t in p u t . T h e t h e r m a l e x p e r i e n c e o f w e l d -

i n g i s o f t en v e r y l o ca l i zed , r e s u l t i n g i n s t r a i n s

t h a t ca n ca u s e d i s t o r t i o n a n d r es i d u a l s t r e s s .

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

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

a s s i st e d c r a c k i n g . T h e u s e o f s m a l l w e l d d e p o s -

i t s r ed u ces t h e s t r e s s a n d t h u s r ed u ces t h e s u s -

c e p t i b i l i t y o f e n v i r o n m e n t a l l y e n h a n c e d c r a c k -

i n g .

I t i s k n o w n t h a t p o s t w e l d h e a t t r e a t m e n t c a n

r e d u c e s t r e s s - c o r r o s i o n c r a c k i n g b y r e d i s t r i b u t -

i n g t h e l o c a l i z e d l o a d a n d b y r e d u c i n g t h e m a g -

n i t u d e o f t h e r es i d u a l t en s i l e s t r e s s a v a i l a b l e t o

i n d u c e c o r r o s i o n c r a c k i n g ( R e f 2 9 ) . I n a r e c e n t

s t u d y o n a ca s t a u s t en i t i c s t a i n l es s s t ee l , p o s t s o -

l i d i f i c a t io n h e a t t r e a tm e n t s w e r e a l s o s h o w n t o

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

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

m i c r o s t r u c t u r e t o s t r e s s- c o r r o s i o n c r a c k i n g ( R e f

30) .

H y d r o g e n d a m a g e r e s u l t s f r o m t h e c o m -

b i n e d a c t i o n s o f h y d r o g e n a n d s t r e s s ( R e f 3 1 ) ;

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

p o o l i n t h e l i q u i d s t a t e h a s a m u c h h i g h e r s o l u -

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

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

p e r s a t u r a t e d i n t h e w e l d p o o l a s h y d r o g e n a t -

t e m p t s t o t r a n sp o r t f r o m t h e w e l d d e p o s i t. W h i l e

s o m e o f t h e h y d r o g e n l e a v e s t h e w e l d m e n t , s i g -

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

l i n e a n d i n t o t h e h e a t - a f f e c t e d z o n e .

T h e f u s i o n l i n e r e g i o n a l s o h a s t h e h i g h e s t

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

f r a c t u r e - s e n s i t iv e m i c r o s t r u c t u r e s ( e . g . , m a r t e n -

s i te ) . T h e c o m b i n a t i o n o f h i g h h y d r o g e n c o n t e n t

a n d b r i t t l e m i c r o s t r u c t u r e s c a n p r o d u c e c r a c k -

i n g . T h e r e s u l t i s u n d e r b e a d c r a c k i n g , c h a r a c t e r -

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

t r a v e l a d ja cen t t o t h e f u s i o n l i n e i n t h e h ea t -

a f f e c t e d z o n e . T h e t y p i c a l h y d r o g e n c r a c k t r a v -

e l s o n e o r t w o g r a i n s f r o m t h e f u s i o n l i n e i n th e

h e a t - a f f e c t e d z o n e .

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

f o r m o f c h e v r o n c r a c k s ( R e f 3 2 - 3 4 ) . C h e v r o n

c r a c k f o r m a t i o n a n d p r o p a g a t i o n a r e i n f l u e n c e d

b y t h e c o m b i n a t i o n o f s t r e s s a n d o r i e n t a t i o n o f

t h e c o l u m n a r g r a i n b o u n d a r i e s . C h e v r o n c r a c k -

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

a p p e a r s t o b e i n a c t i v e a t l o w l e v e l s . T h i s t y p e o f

c r a c k i n g a p p e a r s t o b e s e n s i t i v e t o r e h e a t in g d u r -

i n g m u l t i - p a s s d e p o s i t i o n o f w e l d i n g b e a d s .

C h e v r o n c r a c k i n g i s m o s t l i k e l y f o u n d w i t h f l u x -

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

M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n

( M I C ) i s a p h e n o m e n o n i n w h i c h m i c r o o r g a n -

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

r o l e m a y b e t o i n i t i a te o r a c c e l e r a t e t h e c o r r o s i o n

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

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

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

s u r f a c e . T h e r e s u l t in g n o n u n i f o r m c o v e r a g e m a y

l e a d t o a c o n c e n t r a t i o n c e l l a n d e v e n t u a l l y i n i -

t i a t e c o r r o si o n . I n a d d i t i o n , t h e m e t a b o l i c p r o c -

e s s o f th e m i c r o o r g a n i s m c a n p r o d u c e a l o c a l -

i z e d a c i d e n v i r o n m e n t t h at c h a n g e s t h e c o r r o s i o n

b e h a v i o r o f t h e e x p o s e d m e t a l b y , f o r e x a m p l e ,

a l t e r i n g a n o d i c a n d c a t h o d i c r e a c t i o n s , d e s t r o y -

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

i t s ( R ef 35 ) .

I n a u s t en i t i c s t a i n l es s s t ee l w e l d m en t s , t h e e f -

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

o r a d j a c e n t t o w e l d s ( R e f 3 6 , 3 7 ) . M I C a t t a c k s

e i t h e r y o r oL p h a s es , a n d ch l o r i d es a r e s o m e-

t i m e s f o u n d i n a p i t , e v e n w h e n t h e w a t e r h a s

e x t r e m e l y l o w c h l o r i d e c o n t e n t . P i t s a r e fo u n d i n

r e g i o n s o f t h e h e a t - a f f e c t e d z o n e a t t h e f u s i o n

l i n e , a n d i n t h e b a s e m e t a l n e a r t h e w e l d f o r

r e a s o n s n o t w e l l u n d e r st o o d . T h e r e i s s o m e e v i -

d e n c e t h a t M I C t a k e s p l a c e a l o n g w i t h s t

c o r r o s i o n c r a c k i n g i n w e l d m e n t s o f a u s te

s t a i n le s s s t e e l . W e l d i n g d e s i g n a n d p l a n t o

t i o n c a n m i n i m i z e M I C a t t a c k , m a i n l y b y

v e n t i n g a n a c c e p t a b l e e n v i r o n m e n t f o r m i c

g a n i s m s .

Heat -T int Oxide Format ion . T h e w e

p r o c e s s , e s p e c i a l l y w i t h p o o r g a s s h i e ld i n g

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

v a t i n g o x i d e . T h e v a r i a t i o n i n o x i d a t i o n w i

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

d ep l e t i o n a d ja cen t t o a s t a i n l es s s t ee l w e l d .

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

c o r r o s i o n ( R e f 1 ). A n i n d i c a t io n o f t h i s

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

( R e f 3 8 ) .

Welding Pract ice to

Min imize orros ion

S e v e r a l m e t h o d s a r e a v a i l a b l e t o m i n

c o r r o s i o n i n w e l d m e n t s ( R e f 3 9) . T h e m o s

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

M a t e r i a l a n d W e l d i n g C o n s u m a b l e S

t i o n . C a r e f u l s e l e c t i o n o f m a t e r i a l s a n d w e

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

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

a n d t h u s r e d u c e t h e g a l v a n i c e f f e c ts .

Surface Preparat ion . A p r o p er l y s e l

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

t en s i t e s f o r co r r o s i v e a t t a ck i n a g g r es s i v e

r o n m e n t s . H o w e v e r , t h e c l e a n i n g p r o c e s s

a l s o b e a s o u r c e o f t r o u b l e . F o r e x a m p l e

m e c h a n i c a l l y c l e a n e d s u r f a c e ( i . e . , c l e a n e

s a n d b l a s t i n g o r g r i n d i n g ) ca n l ea v e i m p u

o n t h e s u r f a c e . T h e t y p e o f w i r e b r u s h u s e

a l s o b e a n i m p o r t a n t c o n s i d e r a t i o n ( R e f

S t a i n l es s s t ee l b r u s h es a r e g en er a l l y p r e f

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

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

W e l d i n g d e s ig n

s h o u l d p r o m o t e d e p o s i t

h a v e r e l a t i v e l y fl a t b e a d s w i t h l o w p r o f i l e

h a v e m i n i m a l s l a g e n t r a p m e n t . A p o o r d

ca n g en er a t e c r ev i ces t h a t t r a p s t a g n a n t

t i o n s , l e a d i n g t o p i t t i n g a n d c r e v i c e c o r r o

I r r e g u l a r w e l d d e p o s i t s h a p e s c a n p r o m o t e

b u l en t f l o w i n a t u b u l a r p r o d u c t a n d r es u

e r o s i o n c o r r o s i o n .

W e l d i n g P r a c t i c e . C o m p l e t e p e n e t r a t i

p r e f e r r e d t o a v o i d u n d e r b e a d g a p s . S l a g s h

b e r e m o v e d a f t e r e a c h p a s s w i t h a p o w e r g r

o r p o w e r c h i p p i n g t o o l . I f t h e w e l d i n g m e

u s e s f l u x , t h e g e o m e t r y o f t h e j o i n t m u s t p

t h o r o u g h f l u x r e m o v a l , b e c a u s e m a n y f l u x

d u e s a r e h y d r o p h i l i c a n d c o r r o s i v e .

W e l d Surface Finishing. T h e w e l d d e

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

w e l d i n g . M a x i m u m c o r r o s i o n r e s i s t a n c e u s

d e m a n d s a s m o o t h u n i f o r m l y o x i d i z e d s u

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

t i e s ( R e f 3 7 ) . D e p o s i t s n o r m a l l y v a r y i n r o

n e s s a n d i n d e g r e e o f w e l d s p a t t e r , a c o n c e r n

c a n b e m i n i m i z e d b y g r in d i n g . F o r sm o o t h

d e p o s i t s , w i r e b r u s h i n g m a y b e s u f f i c i e n t

s t a i n l es s s t ee l , h o w ev er , b r u s h i n g d i s t u r b

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

s i v e a t t a ck .


5/5

C o r r o s io n o f W e l d m e n t s 1

S u r f a c e C o a t i n g . W h e n a v a r i a t i o n i n c o m -

pos i t i on ac ros s the w e l d m e t a l c an c aus e l oc a l -

i z e d a t t ac k , i t m ay be de s i rab l e t o us e pro t e c t i ve

c oa t i ngs . The c oa t i ng ne e ds t o c ove r bo t h t he

w e l dm e nt and t he pa re n t m e t a l and of t e n re -

qui re s s pe c i a l s ur f ac e pre pa r a t i on .

P o s t w e l d H e a t T r e a t m e n t A p o s t w e l d h e a t

t re a t m e nt c an be an e f f e c t i ve w ay t o re duc e c or -

ros i on s us c e pt i b i l i t y . Thi s i m prove d c or ros i on

re s i s t anc e i s ac c om pl i s he d t hrough a re duc t i on

i n re s i dua l s t re s s grad i e n t s t ha t i n f l ue nc e s t re s s -

c or ros i on c rac ki ng grow t h . Pos t w e l d he a t t re a t -

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

t h e w e l d m e n t a n d r e d u c e s u s c e p t ib i l i ty o h y d r o -

ge n c rac ki ng . The t re a t m e nt c an a l s o re duc e

c om p os i t i ona l g rad i e n t s ( i . e . , m i c ros e gre ga t i on )

a n d c o r r e s p o n d i n g m i c r o g a l v a n i c c e l l s .

P r e h e a t a n d I n l e r p a s s T e m p e r a t u r e T h e

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

a n d i n t e r p a ss t e m p e r a t u r e m a y p r e v e n t h y d r o g e n

c r a c k i n g i n c a r b o n a n d l o w - a l l o y s t ee l .

P a s s i v a ti o n T r e a t m e n t A

pas s i va t i on t re a t -

m e n t m a y i n c r e a s e t h e c o r r o s i o n r e s i s t a n c e o f

s t a i n l e s s s t e e l w e l de d c om pone n t s .

A v o i d a n c e o f F o r m i n g C r e v ic e s Sl ag t ha t i s

s t i l l adhe r i ng t o t he w e l d de pos i t and de fe c t s

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

re s u l t in c re v i c e s tha t c an prom ot e a l oc a l i z e d c on-

c e nt ra t i on c e l l , re s u l t i ng i n c re v i c e c or ros i on .

Prope r s e l ec ti on o f w e l d i ng c ons um abl e s , p ro pe r

w e l d i ng prac t ic e , and t horough s l ag re m ova l c an

a l l e v i a te h i s form of c or ros i on dam age .

R e m o v i n g S o u r c e s o f H y d r o g e n T h r o u g h

p r o p e r s e l ec t i o n o f w e l d i n g c o n s u m a b l e s ( t h a t

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

e l e c tr o d e s ) , p r o p e r d r y i n g o f f l u x , a n d w e l d i n g

c l e a n s u r f a ce s , t h e h y d r o g e n p i c k u p c a n b e d r a s -

t i c a l l y re duc e d .

REFERENCES

1 . O . I . S t e k l ov

e t a l . ,

M e t h o d o f E v a l u a t i n g

t h e I n f lu e n c e o f N o n - U n i f o r m i t y o f W e l d e d

J oi n t Prope r t i e s on Corros i on ,

S va r . Pro i z , ,

Vo l 19 (No. 9) , 1972, p 34-36

2 . K , F . K r y s i a k

e t a l . , M e ta l s Ha n d b o o k ,

V o l

1 3 , 9 t h e d . , A S M , 1 987, p 3 4 4 -3 6 7

3 . W . F . S a v a g e , N e w I n s ig h t i n to W e l d

C r a c k i n g a n d a N e w W a y o f L o o k i n g a t

W e l d s ,

W eld . Des . En g . ,

D e c 1 96 9

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