Stress Corrosion Induced Failure of 316L Stainless Steel Pipe

S. Kaewkumsai, Nirut Bunchoo, and E.Viyanit

National Metal and Material Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), 114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120 THAILAND

Phone 66-2564-6500 ext. 4734-9, Fax.66-2564-6332, E-Mail: famd@mtec.or.th

Abstract
The present work aims to highlight how to conduct the failure analysis of 316L stainless steel pipe which has been in service for nearly 10 years. The key factor to cause the serious failure involved a complex reaction of stress corrosion cracking, which was initiated at HAZ of the welded pipe. The laboratory investigation including visual and metallographical examinations with using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) was performed. The results indicate that the failure of the welded pipe is due to chloride-induced stress corrosion cracking (CISCC). Chloride could be derived from the contamination caused by sea-shore environment. Moreover, the fabrication of the stainless steel pipe by an arc welding process introduces residual stress necessary for SCC. The cross-sectional examination reveals that the crack initiation takes place at HAZ on the outer surface of the pipe. Significant evidence of carbide precipitates, whose surrounding areas have decreased Cr content necessary for the formation of stable oxide film, was also observed in the originated zones. Form those results, it is recommended to control the microstructural aspects and residual stress levels after the arc welding process.

1. Introduction
Stress corrosion cracking (SCC) is a term used to describe service failure in engineering materials that occur by slow environmentally induced crack propagation[1].This phenomenon is associated with a combination of stress (applied or residual) above the threshold value[2].

The failed component was 316 stainless steel pipe used in chemical industry for feeding the chemical solutions composed of Ethylene Dichloride (C2H4Cl2) and catalyst (solid Fe2Cl3 powder) at temperature of 35C and pressure of 3 Bars. The outer surface of the welded pipe was exposed to ambient industrial environment.

Before failure, the welded pipe has been in service for nearly 10 years. The failed position was the T-shape joint as shown in Figure 1a.

2. Experimental
Visual examinations with the aid of a stereo microscope were thoroughly carried out on the failed pipe. Then, the pipe was cross-sectioned at the failed site to prepare the convenient samples for microstructural analysis under an optical microscope and a scanning electron microscope (SEM). The spark emission spectrometer was used for chemical composition analysis of the stainless steel pipe. Additionally, the corrosion products on the pipe surface were analyzed by energy dispersive spectroscopy (EDS).

3. Results
3.1 Visual Examination
Visual examination revealed that the outer pipe surface was covered with the paint coating. Some areas showed the signs of damaged coating, especially at the T-shape joint. Under the pilling-off of coatings, the oxidation of material as evidence by a dark-brown scale was found. On the other hand, corrosion attacks on the inner pipe surface were observed. On the pipe end, it shows mechanical damage, plastic deformation, with the presence of localized cracks.

Figure 1 a) failed stainless steel pipe; b) branched-cracks; and c) crack initiated at HAZ

 
3.2 Cross-section and Microstructure Analysis

Optical microscopic examination of cross-section specimen reveals the branched crack which is a typical aspect of the stress corrosion cracking (See Figure 1b). Cracks are initiated at HAZ of the outer pipe surface as shown in Figure 1C. The cracking mode is transgranular as observed in the area beneath the outer pipe surface.

3.3 Chemical Composition Analysis

The chemical composition of the pipe conforms to the AISI specification of 316L stainless steel. The chemical composition analysis of the corrosion products indicats the impurity element such as chlorine and sulfur.

4. Discussions

Visual examination on the crack area clearly shows that the failure of the welded stainless steel pipe is caused by the excessive stresses and corrosive atmosphere outside the pipe. The stresses are consisted of residual stress, which was introduced by welding fabrication, and stress introduced during service. The crack initially occurs at a localized region on the outer pipe surface. It could be produced from the corrosion attack at any susceptible points to form pitting corrosion which subsequently acts as the stress concentrators. The branched crack exhibits characteristics of the stress corrosion cracking (SCC). From EDS analysis, the results show that the corrosion products have untypical elements of chlorine and sulfur, which are the aggressive species for stress corrosion cracking of austenitic stainless steel[3].


During welding fabrication of stainless steel pipe, it is expected that increased residual stress in austenitic stainless steel should be attributed to its higher coefficient of thermal expansion.


The interaction between mechanical stress and the presence of chlorine and sulfur results in increasing susceptibility of austenitic stainless steel to cracking.


In general, the corrosion resistance of austenitic stainless steels to SCC depends on the following factors[3]: 1) Alloying composition. Increasing nickel and molybdenum content of austenitic stainless steel enhances the resistance to SCC, 2) Tensile stresses. Lowering residual or applied stresses increases time to failure. Stress relieving or shot peening is usually used to release tensile stresses or introduce compressive stresses, 3) Microstructures. Delta ferrite in austenitic stainless steels generally improves resistance to chloride SCC. Sensitization can promotes SCC; 4) Environmental factors. The formation of SCC is usually promoted by higher temperature, higher concentration of chloride, and lower pH levels.

5. Conclusions
- Stress corrosion cracking is a major reason to explain the failure of the welded austenitic stainless steel pipe
- Metallographic examination is a powerful tool, which can be used for identification of the stress corrosion cracking characteristic

References:
[1] C. Manfredi, J.L. Otegui, 2002, “Failure by SCC in buried pipelines”, Engineering Failure Analysis, Vol.9, pp.495-509.
[2] Kaishu Guan, Xinghua Zhang, Xuedong Gu, Longzhan Cai, Hong Xu, Zhiwen Wang, 2005, “Failure of 304 stainless bellows expansion joint”, Engineering Failure Analysis, vol.12, pp.387-399.
[3] R.C. Yin, A.H.Al-Shawaf, W.Al-Harbi, 2007, “Chloride-induced stress corrosion cracking of furnace burner tubes”, Engineering Failure Analysis, vol.14, pp.36-40.