Preferential weld corrosion (PWC)

Transcription

1 Case History: Preferential Weld Corrosion ALI BABAKR, SABIC Technology Center, Jubail Industrial City, Saudi Arabia Preferential weld and heat-affected zone corrosion is directly proportional to the process flow, 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 remains a significant 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 differences 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 approval 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 film. This was done by adding elements such as nickel, chromium, molybdenum, copper, aluminum, vanadium, etc. 2 Unfortunately, these additions may cause the HAZ to be preferentially anodic to the base metal through galvanic action. 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 influenced 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 inhibiting it MATERIALS PERFORMANCE March 2008

2 FIGURE 1 Six-in. elbow and straight pipe section as received. FIGURE 2 Weld condition inside both elbow and pipe; photos correspond to Figure 1. In some cases, the HAZ may be selectively 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 corrode rapidly, leading to penetration and subsequent leakage. Case History A pipeline conveying ethylene dichloride (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 history 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 between a fl ange and an elbow, and the other at the weld between a flange and straight pipe (Figure 1). The pipes were cut in half to reveal the internal condition. There was excessive corrosion damage March 2008 MATERIALS PERFORMANCE 59

3 Case History: Preferential Weld Corrosion FIGURE 3 Cross section sample of the pipe, weld, and flange showing preferential attack at the weld area. FIGURE 4 Photograph of cross section of pipe sample showing significant attack at the fusion boundary of the elbow, weld, and flange, respectively. (Original magnification 500X). 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 surface cracking was apparent. Cross section samples were taken at each failure. Figure 3 shows a cross section sample containing pipe weld and flange areas. Metallography and optical microscopic examination revealed the condition of the pipe samples. Figure 4 is an optical microscope photograph showing significant attack at the fusion boundary of the elbow-flange weld. The microstructures show no abnormality or cracking. Figure 5 shows a magnified portion of the most-attacked fusion boundary microstructure. 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 industry. If moisture is present, however, hydrolysis 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 hydrolysis and resulting HCl formation could lead to chloride stress cracking. The history of HCl content obtained from the plant personnel showed concentrations 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 degraded by HCl. Since HCl is part of the process and cannot be eliminated, material replacement is inevitable. Generally, when 60 MATERIALS PERFORMANCE March 2008

4 FIGURE 5 chlorinated hydrocarbons hydrolyze, they release HCl in concentrations often <0.5% Still, SS will be prone to failure in this process. But, HCl concentration may increase with time. Among the many alloys that can be selected for this service, price and availability are controlling factors. Thus, it seems for this service and wide use, Monel will be suitable If CS is selected as the preferred material, then HCl and moisture content must be lowered to the allowable minimum content. According to plant personnel, process HCl can be in the thousands of ppm. Many other EDC manufacturing company 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 because of the high HCl content and moisture in the system. References 1 W. Nimmo, A.J. Griffiths, L. Orkney, A. Mensah, A. Turnbull, Evaluation of Techniques for Measuring Corrosion Activity of Carbon Steel Welds, British Corrosion J. 37, 3 (2002): p H.M. Herro, MIC Myths Does Pitting Cause MIC?, CORROSION/98, paper no. 278 (Houston, TX: NACE International, 1998). 3 Y. Chung, R. Pytlewski, D.M. McGarry, Microbiologically Influenced 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, observations 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. Optical micrograph showing an etched left region of the weld above, tube sample. (Original magnification 500X). FIGURE 6 Spectrum Location C O Na Al Si Cl Mn Fe A A A SEM photomicrograph showing three locations along the edge of the hole where EDC had leaked. All three locations show high content of chlorides. March 2008 MATERIALS PERFORMANCE 61

5 Case History: Preferential Weld Corrosion 7 G. Kobrin, ed., A Practical Manual on Microbiologically Influenced Corrosion (Houston, TX: NACE, 1993). 8 S.W. Borenstein, Microbiologically Influenced Corrosion of Austenitic Stainless Steel Weldments, MP 30, 1 (1991): p 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 Corrosion 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. Griffiths, L. Orkney, A. Mensah, A. Turnbull, Evaluation of Techniques for Measuring Corrosion Activity of Carbon Steel Welds, British Corrosion J. 37, 3 (2002): p OxyChem Ethylene Dichloride Handbook, 15 Corrosion in the Chemical Processing Industry, in Corrosion, vol. 13 (Materials Park, OH: ASM, 1995), p. 1, Corrosion and Its Effects, Emerson Process Management, Technical Data Sheet Online Only , 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. New for your library! NEW BOOK! Environment-Induced Cracking of Materials Two-Volume Set Edited by Sergei Shipilov, Russell Jones, Jean-Marc Olive, and Raúl Rebak NEW BOOK! Environment-induced cracking is a critical corrosion concern in many industries throughout the world. Although the problem has been known for many years, the debate on the effects and possible remedies available under different environmental conditions is ongoing and topical. Now offered by NACE International, Environment-Induced Cracking of Materials, Volume 1, discusses the chemistry, mechanics, and mechanisms involved with this pervasive form of corrosion. Volume 2 presents the latest information on prediction, industrial developments, and evaluation by Elsevier, 6 1/2 x 9 1/2, hardbound, two-volume set, 1,000 pages Item # 38474, $265 Visit our online store at or contact NACE FirstService NACE (U.S. & Canada), (Outside U.S. & Canada) 62 MATERIALS PERFORMANCE March 2008