Keywords:Pipelines failureWeldingPitting corrosionGalvanic corrosion

cher test indicated no sensitization in the welded area. The Huey test conrmed that the

(15 cming in

austenite may suffer preferential pitting in alloy depleted regions [2]. This attack is independent of any weld metal precip-itation and is a consequence of micro-segregation or coring in weld metal dendrites. Furthermore, in autogenous (no addedller) GTA weld, some Cr-rich delta ferrite also forms between the Ni-rich cored austenite [3]. Such a microstructure isavoided by use of a suitably alloyed ller metal [2]. Therefore uniform corrosion and preferential pitting is more likely inautogenous (no ller) GTA welds [2,3], as in the present case. A proper annealing treatment of the welded pipe can alleviate

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* Corresponding author.E-mail address: alfantaz@interchange.ubc.ca (A. Alfantazi).

Engineering Failure Analysis 17 (2010) 810817

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Engineering Failure Analysisdoi:10.1016/j.engfailanal.2009.10.015tubing due to the chloride in the residual hydro-test water. The present case is an example of pitting Type 316L stainlesssteel due to chloride ions which can be from either hydro-test water or the marine atmosphere. Type 316L stainless steelis a grade resistant to pitting in contrast to Type 304.

Type 304 stainless steel is well dened and should respond reproducibly well-established specication. Unfortunately,this is not always so, and such well-known and extensively used materials sometimes exhibits unexpected behavior-mainlybecause important details such as surface cold work, or erroneous heat treatments of the materials are not identied [1].Type 316L stainless steel on the other hand is supposed to be resistant to pitting due to the addition of about 2% Mo tothe base composition of Type 304. Yet, spectacular pits occurred on the seam welded pipes after only a few months of instal-lation and exposure to the marine atmosphere. Under moderately oxidizing conditions, such as a bleach plant, weld metal1. Introduction

Pitting was observed on a 6-in.months of installation and before gocorrosion rate of samples from welded area were higher that of samples from base metalin a boiling nitric acid test.The results indicated the presence of a high level of inclusions in the welded area. Pitting

initiation in HAZ and FZ may be attributed to existence of large inclusions in the weldedarea. The general corrosion of the FZ can be attributed to the galvanic effect betweeninter-dendrite delta ferrite and the cored austenitic in the welded area which could be pre-vented by proper annealing after welding. It is plausible to claim that the general corrosionof these areas could trigger the pitting initiation as well.

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) are piping made of stainless steel ASTM A312 Gr. TP 316L, after only a fewto service. There have been numerous cases of pitting of Type 304 stainless steelPitting of 316L stainless steel in are piping of a petrochemical plant

E. Mohammadi Zahrani a, A. Saatchi b, A. Alfantazi a,*aDepartment of Materials Engineering, University of British Columbia, 6350 Stores Road, Vancouver, BC, Canada V6T1Z4bDepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

Article history:Received 20 July 2009Accepted 10 October 2009Available online 6 November 2009

a b s t r a c t

Pitting was observed on a 6-in. (15 cm) are piping made of stainless steel ASTM A312 Gr.Tp 316L, prior going into service in a petrochemical plant. The pits were in the heat-affected zone (HAZ) and fusion zone (FZ) boundary of the pipe seam welds. The FZ was alsouniformly corroded. The SEM photomicrographs showed large inclusions in the weldedarea, while EDS analysis indicated that the inclusions were rich in Al, Si, and S. The Strei-

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this problem. In the present case numerous inclusions were also observed in the HAZ/FZ, which accentuate pitting sensitivityof the material. Thus the poor quality of the material is responsible for the poor performance.

2. Experimental aspects

The chemical composition of the pipe metal was determined by spark atomic emission spectroscopy. The microstructureof the FZ and HAZ was studied by using SEM and optical microscopy. Composition of the inclusions which were observednear the pits in the welded area was determined by EDS analysis. The corrosion behavior of the samples from welded areaand base metal and susceptibility to intergranular attack after sensitization heat treatment at 675 C for 1 h, were studied byStreicher and Huey test (ASTM A262-55) [4].

The samples were polished with silicon carbide sand papers (grit range of 804000). In the Streicher test, they wereetched in the oxalic acid 10% (Merck, GR) for 1.5 min after polishing. The current density was 1 amp/cm2. Prepared samplewas used as anode and stainless steel sheet was used as cathode during etching process. The etched surfaces of each samplewere studied by optical microscopy.

In the Huey test, the corrosion rate (mils per year) of the polished samples in boiling nitric acid (Merck, GR) was measuredby determining the loss of weight for each sample after 2, 4, and 6 day [4]. The corrosion rate was reported as mils per year(mpy), calculated in the following rate of corrosion equation [4,5]:

E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817 811Fig. 1. Two of many samples of the perforated are pipe.

Mils per year mpy : 534WDAT

1

T is the time of exposure (h), A is the total surface area (in2),W is the weight loss (mg), and D is the density of the sample (g/cm3).

3. Results and discussion

Fig. 1 shows two of the many samples of perforated pipe. The pits were located in the fusion zone (FZ) and heat-affectedzone (HAZ) boundary of the pipe seam welds along a straight line and the FZ also was uniformly etched. It can be seen thatthe seam weld has been located at the 6 oclock position, which is the ideal situation for the accumulation of moisture con-densate from the marine atmosphere, and also ideal for pitting corrosion. The problem would not have occurred in such ashort time if the seam weld was at any other position except 6 oclock, due to the gravity effect in pitting.

Fig. 2. SEM photomicrograph of FZ, HAZ and base metal.

812 E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817Fig. 3. SEM photomicrograph of smaller pits in their early stage of growth adjacent to a well developed pit in the welded zone.

SEM photomicrograph of FZ, HAZ and Base Metal can be seen in Fig. 2. It seems that HAZ has a coarse grain structure be-cause of high heat input in the welding process. Fig. 3 shows the SEM photomicrograph of smaller pits in their early stage ofgrowth adjacent to a well developed pit in the welded area. Pits had diameters up to 2 mm and had penetrated into thewhole 4 mm thickness of the pipe. Fig. 4 shows SEM photomicrograph of area near the pits that uniformly corroded.Fig. 5 shows developed pits in the FZ (after polishing the sample and without etch) and Fig. 6 shows the corroded areaand pits in the HAZ.

The chemical composition of the pipe is presented in Table 1. According to the WRC-1992 diagram, chromium and nickelequivalents are as follows [3,6]:

Creq CrMo 0:7Nb 17:4 2:49 0:70:11 19:967Nieq Ni 35C 0:25Cu 11:5 350:0234 0:250:205 12:37025

Thus the amount of delta ferrite in the FZ is about 8% according to WRC-1992 diagram (for predicting weld ferrite contentand solidication mode) [6]. The presence of 48% delta ferrite is optimal to reduce solidication cracking in the welds [6],but it seems that this amount of Cr-rich ferrite could increase the uniform corrosion and pitting susceptibility of weld areas.Rapid cooling from the molten state in the FZ also causes the coring in the austenite. Coring in the Ni-rich austenite and the

Fig. 4. SEM photomicrograph of uniformly corroded area near the pits.

E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817 813Fig. 5. Developed pits in the FZ.

Cr-rich inter-dendrite delta ferrite can be observed in Fig. 7. The general corrosion of the FZ was due to galvanic effect be-tween inter-dendrite Cr-rich delta ferrite and the cored Ni-rich austenitic in the weld area. The general corrosion of theseareas could have triggered the pitting initiation as well.

Figs. 811 show SEM photomicrographs of inclusions found in the HAZ and FZ. EDS analysis conrmed that the inclusionswere rich in S, Al, and Si. It seams that the existence of large inclusions in the welded area can be the reason for occurrence ofpitting. In these gures, the overall corrosion of the HAZ/FZ area is also observed.

Figs. 12 and 13 show the microstructures of the samples after Streicher test. The presence of steps between grainswas observed in the microstructure after Streicher test which is acceptable etch structure after oxalic acid etch for this kindof material [4]. Observation of this step-like structure conrmed that the samples were certainly free of any probable

Table 1Chemical composition of the pipe.

V Ti Nb Cu Mo Cr Ni Mn P S Si C

0.13 0.0679 0.11 0.205 2.49 17.4 11.5 1.32

susceptibility to the intergranular attack. Obviously, chromium carbide wasnt precipitated in the grain boundary even aftersensitization heat treatment.

Fig. 8. SEM photomicrograph of inclusions found in the FZ.

Fig. 9. SEM photomicrograph of inclusions found in FZ and contain high levels of Si, S, and Al and some pits very closed to this area.

E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817 815Fig. 10. SEM photomicrograph of inclusions found in the HAZ.

Fig. 11. SEM photomicrograph of inclusions found in the HAZ contain high levels of Al.

Fig. 12. Microstructure of the heat treated sample of base metal after oxalic acid etch.

Fig. 13. Microstructure of the heat treated sample of welded area after oxalic acid etch.

816 E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817

10

15

20

25

ion

Rat

e (m

py)

Welded Area Base Metal

E. Mohammadi Zahrani et al. / Engineering Failure Analysis 17 (2010) 810817 817to claim that these observed impurities and inclusions are the reason for pitting initiation in the welded area.

4. Conclusion

(1) Although the presence of 48% delta ferrite is optimal to reduce solidication cracking in the welds, it could increasethe corrosion and pitting susceptibility. The general corrosion of the FZ was due to galvanic effect between Cr-richinter-dendrite delta ferrite and the Ni-rich cored austenite in the weld area which could be prevented by properannealing after welding. The general corrosion of these areas could have triggered the pitting initiation as well.

(2) The seam welded stainless steels are particularly susceptible to preferential pitting. The proper levels of impuritiesand inclusions and subsequent heat treatment are essential for their corrosion resistance. In the present case, the highcorrosion rate of the sample fromwelded area in boiling nitric acid indicated the presence of high level of inclusions inthe welded area which was the main reason of pitting initiation in the welded area.

(3) Step-like structure in the samples after oxalic acid etches showed that the samples were free of any susceptibility tointergranular attack and chromium carbide was not precipitated in the grain boundary after sensitization heattreatment.

Acknowledgment

The authors are grateful to the Iranian National Gas Company for nancial support of this project.The corrosion rate of the samples from base metal and welded area after 2, 4, and 6 days immersion in the boiling nitricacid was shown in Fig. 14. It was conrmed that the corrosion rate of the sample from welded area was higher than thecorrosion rate of the sample from base metal in the boiling nitric acid after 2, 4, and 6 days immersion. This is an indicationfor the presence of high levels of impurities and inclusions after welding process in the welded area. Therefore, it is plausible

0

5

2 4 6Time ( day )

Cor

ros

Fig. 14. Corrosion rate of samples in the boiling nitric acid test.References

[1] Staehle RW. Lifetime prediction of materials in environments. In: Hoboken NJ, Revie RW, editors. Ughlighs corrosion handbook. Springer; 2000. p. 53.[2] Krysiak KF. Corrosion of weldments. In: Metals handbook. Corrosion, vol. 13. ASM International; 1987. p. 348.[3] Kou S. Welding metallurgy. 2nd ed. John Wiley & Sons; 2003.[4] Annual book of ASTM standards, Standard practice for detecting susceptibility to intergranular attack in austenitic stainless steel, A262, 93a, vol. 01.03.;

1997. p. 626.[5] Fontana MG. Corrosion engineering, translated by Ahmad Saatchi to Farsi, Jahad daneshgahi of Esfahan University of Technology. 3rd ed.; 1982.[6] Brooks JA, Thompson AW. Austenitic stainless steel welds. Int Mater Rev 1991;36:1543.

Pitting of 316L stainless steel in flare piping of a petrochemical plantIntroductionExperimental aspectsResults and discussionConclusionAcknowledgmentReferences