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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
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8/11/2019 Corrosion on Weldment
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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
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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
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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 .
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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 ,
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