Case History: P Preferential Weld Corrosion
Transcript of Case History: P Preferential Weld Corrosion
58 MATERIALS PERFORMANCE March 2008
Case History: Preferential
Weld Corrosion ALI BABAKR, SABIC Technology Center,
Jubail Industrial City, Saudi Arabia
Preferential weld and heat-aff ected zone corrosion
is directly proportional to the process fl ow,
temperature, pH, location of welds within the
piping, and how active the weld metal is
electrochemically when coupled to the base metal.
Accelerated decay and possible reasons for this
attack are illustrated in this case study.
Preferential weld corrosion (PWC)
in stainless steel (SS) construction
is not new to the industry, but re-
mains a signifi cant concern. The
term describes selective attack of the weld
itself rather than of the base plate, and
sometimes the heat-affected zone (HAZ).
Preferential attack often occurs because
the weld material is more active than the
base metal. This occurs because differ-
ences in the two microstructures make the
weld more susceptible to corrosion than
the base metal.1 The influence of the
coupling is enhanced by the unfavorable
area ratio, a small anodic weld/large
cathodic base metal. Selection and ap-
proval of compatible and appropriate
welds should be done during material
selection at the start of the design phase.
In this case, the designers attempted
to prevent PWC and HAZ corrosion by
enhancing the induction and the integrity
of the protective corrosion fi lm. This was
done by adding elements such as nickel,
chromium, molybdenum, copper, alumi-
num, vanadium, etc.2
Unfortunately, these additions may
cause the HAZ to be preferentially anodic
to the base metal through galvanic ac-
tion.2-3 In addition, precipitation within
grains, inter-dendretic regions, and at
grain boundaries has a direct effect on the
weld’s corrosion potential.4 Thus, grain
boundaries and inter-dendretic regions
tend to attain a more active corrosion
potential than the neighboring grains. For
instance, micro-segregation of Cr and Mo
in welds of SS will lead to selective pitting
corrosion of the weld.5 Welds are also
susceptible to microbiologically influ-
enced corrosion (MIC).6-8
Selection of the proper weld material
is the best way to combat PWC.9 Another
method of combating PWC in fl owing
streams is inhibition. Proper knowledge
of inhibitors and their selection is crucial,
however, as inhibitors may actually lead
to accelerating corrosion rather than in-
hibiting it.10-11
March 2008 MATERIALS PERFORMANCE 59
M A T E R I A L S S E L E C T I O N & D E S I G N
Six-in. elbow and straight pipe section as received.
FIGURE 1
FIGURE 2
Weld condition inside both elbow and pipe; photos correspond to Figure 1.
In some cases, the HAZ may be selec-
tively corroded, leaving behind the weld
bead. The HAZ is metallurgically more
complex than the base metal and the weld
materials. Hence, the HAZ becomes
electrochemically more active than the
base or weld materials. The relatively
smaller size of the HAZ causes it to cor-
rode rapidly, leading to penetration and
subsequent leakage.
Case HistoryA pipeline conveying ethylene dichlo-
ride (EDC) from a reactor to storage as
dry EDC leaked in two places. Both leaks
were reported to be at weld areas. The
leaks were clamped until replacement
with the same pipe materials was made.
Many failures of the same nature had
occurred in the past. In addition, no his-
tory of any previous failures was kept. No
failure analysis was ever carried out. The
piping material is 6-in. (152-mm) carbon
steel (CS). The pressure and temperature
are 135 kPA and 50 to 55°C, respectively.
One failure occurred at the weld be-
tween a fl ange and an elbow, and the
other at the weld between a fl ange and
straight pipe (Figure 1). The pipes were
cut in half to reveal the internal condition.
There was excessive corrosion damage
60 MATERIALS PERFORMANCE March 2008
M A T E R I A L S S E L E C T I O N & D E S I G N Case History: Preferential Weld Corrosion
apparent on the inner surface of the pipe,
and the weld appeared as if selectively
attacked circumferentially (Figure 2). No
excessive corrosion deposit was found; no
Cross section sample of the pipe, weld, and fl ange showing preferential attack at the weld area.
Photograph of cross section of pipe sample showing signifi cant attack at the fusion boundary of the elbow, weld, and fl ange, respectively. (Original magnifi cation 500X).
FIGURE 3
FIGURE 4
surface cracking was apparent. Cross sec-
tion samples were taken at each failure.
Figure 3 shows a cross section sample
containing pipe weld and fl ange areas.
Metallography and optical micro-
scopic examination revealed the condi-
tion of the pipe samples. Figure 4 is an
optical microscope photograph showing
signifi cant attack at the fusion boundary
of the elbow-fl ange weld. The microstruc-
tures show no abnormality or cracking.
Figure 5 shows a magnifi ed portion of the
most-attacked fusion boundary micro-
structure. In Figure 6, a scanning electron
microscopy (SEM) photomicrograph
shows elemental analysis of three areas
adjacent to a hole where EDC had
leaked. All areas showed high content of
chlorides. In addition, measurement
along the line of fusion also showed high
content of chlorides.
Discussion
Dry EDC, in the absence of moisture,
is harmless to practically all metals used
in the chemical and petrochemical indus-
try. If moisture is present, however, hy-
drolysis can occur, releasing hydrochloric
acid (HCl). When this situation occurs,
material selection must be restricted to
those materials that resist HCl in the
suspected existing concentrations and at
the existing temperatures. It is advisable
not to select the type 300 series SS if there
is the slightest chance that the EDC could
be wet, or might become wet. The hydro-
lysis and resulting HCl formation could
lead to chloride stress cracking.
The history of HCl content obtained
from the plant personnel showed concen-
trations were higher than expected. This
explains the many repeated failures that
had occurred. Acid corrosion at localized
weak points thinned the weld. Instead of
an even crown, a cross section of the
surface weld revealed a cresting wave
pattern. The circumferential weld on the
lower head appeared to have been de-
graded by HCl.
Since HCl is part of the process and
cannot be eliminated, material replace-
ment is inevitable. Generally, when
March 2008 MATERIALS PERFORMANCE 61
Spectrum
Location C O Na Al Si Cl Mn Fe
A1 5.27 45.78 0.51 0.17 7.18 0.30 40.80
A2 4.77 45.25 7.66 42.31
A3 37.15 0.75 0.52 7.42 54.16
FIGURE 6
Optical micrograph showing an etched left region of the weld above, tube sample. (Original magnifi cation 500X).
SEM photomicrograph showing three locations along the edge of the hole where EDC had leaked. All three locations show high content of chlorides.
FIGURE 5
chlorinated hydrocarbons hydrolyze,
they release HCl in concentrations often
<0.5%.12-13 Still, SS will be prone to fail-
ure in this process. But, HCl concentra-
tion may increase with time. Among the
many alloys that can be selected for this
service, price and availability are control-
ling factors. Thus, it seems for this service
and wide use, Monel† will be suitable.13-15
If CS is selected as the preferred material,
then HCl and moisture content must be
lowered to the allowable minimum con-
tent. According to plant personnel, pro-
cess HCl can be in the thousands of ppm.
Many other EDC manufacturing com-
pany brochures mention that their HCl
content is no more than 10 to 20 ppm.16
In conclusion, in view of the available
evidence, the failure was preferential
HAZ attack to the EDC line welds be-
cause of the high HCl content and mois-
ture in the system.
References
1 W. Nimmo, A.J. Griffi ths, L. Orkney, A. Mensah, A. Turnbull, “Evaluation of Techniques for Measuring Corrosion Activity of Carbon Steel Welds,” British Corrosion J. 37, 3 (2002): p. 182.
2 H.M. Herro, “MIC Myths—Does Pit-ting Cause MIC?,” CORROSION/98, paper no. 278 (Houston, TX: NACE International, 1998).
3 Y. Chung, R. Pytlewski, D.M. McGarry, “Microbiologically Infl uenced Corrosion of TP304l Stainless Underground Piping with Tape Wrapped ER/E316l Welds Steel.”
4 S. Turgoose, J.W. Palmer, G.E. Dicken, “Preferential Weld Corrosion of 1% Ni Welds: Effect of Solution Conductivity and Corrosion Inhibitors,” CORRO-SION/2007, paper no. 275 (Houston, TX: NACE, 2007).
5 Author’s personal experimentations, ob-servations and analysis, to be published.
6 B. Messer, S. Seitz, D. Roth, A. Gray, T. Phillips, “Selection of Dissimilar Metal Welds in Severe Environments for Today’s Petrochemical Plants,” CORROSION/2007, paper no. 568 (Houston, TX: NACE, 2007).
†Trade name.
M A T E R I A L S S E L E C T I O N & D E S I G N
62 MATERIALS PERFORMANCE March 2008
7 G. Kobrin, ed., A Practical Manual
on Microbiologically Infl uenced
Corrosion (Houston, TX: NACE,
1993).
8 S.W. Borenstein, “Microbiologically Infl uenced Corrosion of Austenitic Stainless Steel Weldments,” MP 30, 1 (1991): p. 52-54.
9 D. Thierry, Ed., “Aspects of Microbially Induced Corrosion,” papers from EUROCORR ‘96 and The EFC Working Party on Microbial Corrosion (London, U.K.: The Institute of Materials, 1997).
10 W.H. Kearns, Welding Handbook, Seventh Ed., Vol. 4: Metals and Their Weldability (Miami, FL: AWS, 1982).
11 J.W. Palmer, J.L. Dawson, T. Ulrich, A.N. Rothwell, “Inhibition of Weld Cor-rosion Under Flowing Conditions—The Development of a Test Procedure,” CORROSION/93, paper no. 119 (Houston, TX: NACE, 1993).
12 I.G. Winning, N. Bretherton, A. McMahon, “Evaluation of Weld Corrosion Behavior and the Application of Corrosion Inhibitors and Combined Scale/Corrosion Inhibitors,” CORROSION/2004, paper no. 538 (Houston, TX: NACE, 2004).
13 W. Nimmo, A. J. Griffi ths, L. Orkney, A. Mensah, A. Turnbull, “Evaluation of Techniques for Measuring Corrosion Activity of Carbon Steel Welds,” British Corrosion J. 37, 3 (2002): p. 182.
M A T E R I A L S S E L E C T I O N & D E S I G N Case History: Preferential Weld Corrosion
14 OxyChem Ethylene Dichloride Hand-book, www.oxy.com.
15 Corrosion in the Chemical Processing Industry, in Corrosion, vol. 13 (Materials Park, OH: ASM, 1995), p. 1,163.
16 Corrosion and Its Effects, Emerson Pro-cess Management, Technical Data Sheet Online Only 00816-0100-3045, Rev. CA (May 2003).
ALI BABAKR is a senior materials and corrosion engineer at SABIC, PO Box 11669, Jubail Industrial City, 31961, Saudi Arabia. He has worked at the company for the past eight years as a senior failure analyst and researcher dealing in corrosion, failure analysis, materials selection, turbine alloys, and metallurgy. He has an M.S. degree and Ph.D. from the University of Idaho.
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