Chloride-Induced Stress Corrosion Cracking of 316 Stainless Steel Oil Tube

Hi everybody, today, I would like to show my case study to you. In this case, the most fashion of failure mode in SS is present.

Abstract

The mineral oil tube failed after being in service for nearly 10 years. Pipe leakage was detected during operation. One of the failed pipes was selected for failure analysis by visual examination, chemical analysis, and microstructure analysis techniques. Examination revealed that the oil tube failed by chloride-induced stress corrosion cracking. Chloride contaminants were exposed to come from the service environment. Failure prevention can be achieved by relieving the tube residual stress after forming, removing contaminants from the tube exterior periodically and applying chloride-free protective coating.

Introduction
Corrosion failure of pipelines in petrochemical plants in Thailand was common, especially, in the zone exposed to the coastal environment. The majority of the pipelines used in petrochemical and chemical plants are mostly made of either carbon steel or austenitic stainless steel. Carbon steels mostly degrade in form of general corrosion, while the stainless steel parts mostly fail due to stress corrosion cracking [1]. This failure mode was generally found in stainless steel pipelines that are exposed to coastal environment. Stress corrosion cracking is the formation of cracks in susceptible metals in the presence of certain corrosive media and tensile stresses. In all cases of failure by stress corrosion cracking, the following three factors must be present: tensile stress, corrosive media, and the sensitive material [2]. This paper described the mechanism of chloride-induced stress corrosion cracking of oil tube as a result of improper material selection and lack of maintenance. Although austenitic stainless steels provide excellent resistance to general corrosion, they are susceptible to localized corrosion, such as pitting corrosion, intergranular corrosion, and stress corrosion cracking, especially in chloride containing environments [3]. In this paper, the failure mode of the stainless steel tube was found to be stress corrosion cracking and preventive procedures of such failure were provided. The failed component for analysis was the mineral oil tube which had been in service for nearly 10 years. It was made of AISI 316 stainless steel and used for feeding mineral oil into electronic part production process. The tube was fabricated and installed in the hood. The service temperature at the exterior was around 90 C and the pressure interior was 1 bar. Contaminants and stains on the surface of the tube were removed every shutdown period. During operation, the failure was detected by oil leaking from cracks.

Investigation Methods
Visual and stereo microscopic examinations were carried out on the failed oil tube. Then, the tube was sectioned and prepared for microstructural evaluation by mounting, polishing, and etching. The etchant was the mixture of 3 parts of HNO3, 2 parts of HCl, and 1 part of glycerol. The microstructure was observed under a reflected light microscope. A bulk composition of the failed pipe was determined by a spark emission spectrometer. The corrosion products on the tube surface were analyzed by energy dispersive spectrometry.

Results

Visual Examination

The cracks were oriented about 45° angle to the tube axis as shown in Fig. 1b. Dark brown corrosion products were found on the external tube surface. Cracks were found in single lines. After removing corrosion products, pits were found all over the tube surface. In contrast, the internal tube surface was smooth and free from corrosion product.

Fig. 1: As-received oil tube for failure analysis a) dark brown corrosion products b) appearance of internal surface and crack orientation

Chemical Composition Analysis

The results of chemical analysis of the tube show that its chemical composition complies with AISI 316 specification. Chemical compositions of corrosion products and the normal area are shown in Fig. 2a and b, respectively. The results indicated the presence of chlorine, calcium, potassium, phosphorus, sodium, and sulfur in the corrosion products. Significant high peak of chlorine was found in the pits.

Fig. 2 : EDS spectra taken from (a) dark brown corrosion products and (b) normal area

Microstructural Analysis

Microstructure of cross section of the failed specimen as shown in Fig. 3 indicated that the tube was seamed stainless steel tube. Fig. 3a showed the cross section of the area where the crack was initiated. From Fig. 3a, micro-cracks were initiated from external tube surface, most likely at the pits which acted as stress concentrators. When the micro-cracks coalesced and became continuous throughout the cross section, they became one single visible macro-crack as shown in Fig 1b. Fig. 3a showed that micro-cracks propagated from macro-crack, this supported the argument that macro-crack resulted from coalescence of micro-cracks. Fig. 3b showed microstructure and crack morphology in the tube. The microstructure was mostly austenitic structure and twin grains typical of austenitic stainless steel. Twin grains indicated that the tube was subject to cold forming and retained residual stresses. Cracks were branched and blunt, indicative of stress corrosion cracking. In some area, it was found that corrosion attack tend to propagate along grain boundaries. Furthermore, blunt crack tip was found at the corrosion attacked path.

Fig.3: Cracks propagated from the pits on external surface in transgranular mode.

Discussions

Observation of the failed tube revealed that the external surface of the tube was covered with high quantity of dispersed dark brown corrosion products. Cracks were aligned transversely inclined to the tube axis. In addition, the interior of tube near the crack was smooth and free from corrosion products. This was due to the exposure to oil that is normally not corrosive.

Comparing chemical composition of dark brown corrosion products with that of normal area revealed that the contaminants consisted of sulfur, calcium, potassium, sodium, and chlorine, especially for sulfur and chlorine, the aggressive species that induce stress corrosion cracking in austenitic stainless steel. The contaminants could come from the solution inside hood, washing water, and airborne NaCl in the atmosphere. Sulfur could probably be in form of CaSO4.

Because the passive film under the rust stain was destroyed and defects were produced on the surface of the tube. These defects acted as a preferred site for accumulation of corrosive media which reacted with the base metal. The tube surface lost its corrosion protection property. This combined with the presence of high density of twin grains, residual stress from cold forming give rise to failure of the part. Since the breakdown of passive films has been recognized as key role in pitting and stress corrosion cracking of austenitic stainless steel [3], it is reasonable to correlate the effects of microstructure and residual stress on corrosion resistance to the properties of passive film.

From the above reasons, it indicated that cracking was generated from the external tube surface. The effect of corrosion attack combined with residual stresses during shaping process give rise to corrosion failure of the tube in form of stress corrosion cracking. The chloride and sulfur containing environment were the contributing factors of this failure

Microstructure analysis indicated that the residual stress was generated during shaping process as evident by high density of twin grains during rolling and welding the stainless steel. In service condition, the external tube surface was subject to tensile stresses. Then, the residual tensile stress combined with the destroyed passive film and exposure to high temperature (~90 C) corrosive media made the oil tube prone to stress corrosion cracking. The failure mode was clearly explained by microstructure analysis; cracks were generated in the rust-stained area. Cracks were branched and propagated mostly in transgranular mode. The crack tips were sharp at the primary stage of failure. After long-term service, blunt crack tips were observed.

Normally, stainless steel has a protective film that resists corrosion damage from chemical species. For the failed part, the defects generated from rust caused discontinuities in protective film. Therefore, the failure of the tube was probably caused by the interaction between the base metal and corrosive media at the defected areas followed by crack initiation and propagation. R.A. Cottis [4] said that the temperature usually needs to be above 70 C before SCC can occur. In this case, the service temperature was operated at 90 C, which was ideal for generated chloride-induced stress corrosion cracking. The higher temperature, the higher concentration of chloride that promotes the occurrence of SCC.

Conclusion
The oil tube failed by stress corrosion cracking which was induced by chloride contamination in the ambient environment. Chloride-free coating and periodic cleaning was suggested for preventing the problem. Selection of tube in annealed condition was also recommended.

References

[1] M. Suresh Kumar, M. Sujata, M.A. Venkataswamy, S.K. Bhaumik, 2008, “Failure analysis of a stainless steel pipeline”, Engineering Failure Analysis, Vol.15, pp.497-504.
[2] J. Woodtli, R. Kieselbach, 2000, “Damage due to hydrogen embrittlement and stress corrosion cracking”, Engineering Failure Analysis, Vol.7, pp. 427-450.
[3] B.T. Lu, Z.K. Chen, J.L. Luo, B.M. Patchett, Z.H. Xu, 2005, “Pitting and stress corrosion cracking behavior in welded austenitic stainless steel”, Electrochemica Acta, Vol.5, pp.1391-1403.
[4] R.A. Cottis, “Stress Corrosion Cracking”, Guides to Good Practice in Corrosion Control [online], http://www.npl.co.uk.