Case History: P Preferential Weld · PDF file60 MATERIALS PERFORMANCE March 2008 MATERIALS SELECTION & DESIGN Case History: Preferential Weld Corrosion apparent on the inner surface

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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