THE RELATION OF OXYGEN VALUE, HEAT TINT AND PITTING CORROSION IN SUPER DUPLEX PIPE

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1 THE RELATION OF OXYGEN VALUE, HEAT TINT AND PITTING CORROSION IN SUPER DUPLEX PIPE Adisak Aumpiem Department of Production Engineering, Faculty of Engineering, King Mongkut s University of Technology Thonburi, Bangmod, Toong-kru, Bangkok 10140, Thailand Asa Prateepasen Department of Production Engineering, Faculty of Engineering, King Mongkut s University of Technology Thonburi, Bangmod, Toong-kru, Bangkok 10140, Thailand Abstract In weldment, the presence of oxygen around the molten weld can lead to several kinds of welding defects. Discoloration is unsightly and in some instances might produce metallurgical imbalance, especially with some stainless steels. This paper presents the relation among oxygen value, heat tint (discoloration) and pitting corrosion rate of Super duplex stainless steel (SDSS) in pipe UNS32750 alloy. The SDSS pipes were welded with Gas Tungsten Arc Welding (GTAW) process. The argon purging gas was used to protect the root side of stainless steel pipe weldments. The percentage of the oxigen content was varied in eight levels (in PPM) by using the differt amount of the argon purging. After that the test samples were subjected to corrosion tests as per ASTM G48 Method A test for the determination of pitting corrosion rate. The heat tint (dis-colorlation) was then observed to compare with the oxigen content. The results show that the oxygen contend above 445 PPM around the weld causes the weldment do not meet the acceptance standard due to its dis-colorlation. Purge free (> 999 ppm) in level eight shows not only the rejection of dis-colorlation but also an imperfection in weld. Slightly amount of pitting corrosin was found in all of the test samples. Keywords Oxygen contain, Heat tint, Pitting corrosion, Weld, Super duplex stainless steel Introduction Super duplex stainless steels (SDSS) are defined as the steels for which the pitting resistance equivalent number (PREN) is greater than 40. The pitting corrosion resistance of super duplex stainless UNS S32750 is strongly influenced by the annealing temperature. From the previous researches, the best pitting corrosion resistance for the alloy was found annealed at approximately 1080 C, with the highest critical pitting temperature value 96 C and when the annealing temperature is lower than 1080 C, raising the annealing temperature enhances the critical pitting temperature of the specimen; while the annealing temperature is higher than 1080 C, the critical pitting temperature falls with increase of the annealing temperature [1]. The effect of backing gas composition (100 % Ar, 100 % N2 and 90 % N2+10 % H2) on the corrosion performance of super duplex UNS S32750 has been investigated. GTAW was used for the weld roots and FCAW for filling passes. The influence of backing gas composition had only a small effect on the weld metal hardness, but the highest values were found in the root and when welding with 90 % N2+10 % H2 as backing gas. The impact toughness of the 100 % Ar and 100 % N2 samples was higher when welding without a gap 50

2 [2]. The weld root region was susceptible to pitting attack as compared to weld cap region. This is due to the precipitation of hazardous secondary austenite in weld root due to multiple heating cycles. Lower welding heat input was found to give better corrosion properties than higher heat input. At high heat input, there was a formation of secondary austenite and intermetallic phases due to repetitive heating cycles. Welding can encounter a number of different problems that can trigger premature failure in service or costly incidents during production if adequate controls are not exercised by the manufacturer. In this paper the effect of the amount of the oxygen around the weld was investigated in term of the discoloration and pitting corrosion resistance 1. Theories Super duplex stainless steels (SDSS) [25% Cr] are composed of unique ferrite/austenite microstructure which makes them superior material than conventional AISI 316 austenitic stainless steels [3,4]. Super duplex stainless steels are upgraded versions of DSS. They exhibit a higher pitting resistance [Pitting Resistance Equivalent Number (PREN) > 40]. The combination of mechanical properties and the higher corrosion resistance make DSS and SDSS attractive materials in aggressive corrosive seawater environments. GTAW uses a non-consumable tungsten electrode (EWTh2) to produce arc and filler wire to join the material and it is shielded by an inert gas like helium or argon to protect the molten weld pool from the atmospheric contaminants [5]. GTAW is a major fabrication process for DSS and SDSS materials being used in offshore and marine industries. Despite these advantages, mechanical and corrosion properties could be deteriorated if the weld parameters are not controlled during a welding operation. The purging gases most commonly used purging gas in Europe is commercial quality argon in the USA helium is in more general use, being less expensive. The presence of oxygen to a around the molten weld can lead to wide-ranging defects. Weld purging is the act of removing, from the region of the joint, oxygen and any other gases or vapours that might be harmful to a welding joint as it is being welded and immediately after welding. Weld purging is generally necessary for the first weld run when joining to separate parts. This sealing weld will be called a root run when it takes more than one run or (pass) to fully seal the root area from weldment above. Argon gas, which is heavier than air, it should be entered from the bottom of the enclosed space, where it will move slowly across the base area. An Oxygen analizer is the instrument used to monitor the oxygen levels while using inert gas to purge stainless steel. A measure unit is ppm (parts per million). NORSOK standard recommends the oxygen content should be lower than 100 ppm to ensure the compliance with the heat tint [6]. Discoloration is unsightly and in some instances might produce metallurgical imbalance, especially with some stainless steels. Gross oxidation inevitably results in reduction in mechanical properties and can cause catastrophic loss of corrosion resistance. The corrosion testing presented above suggests even with color in the heat affected zone (HAZ) when performed properly will further improve the corrosion resistance in that area concern. Furthermore, the information above indicate that any resultant weld discoloration up to and including an O 2 exposure of 50 ppm will have no effect on the corrosion resistance recognized in the biopharmaceutical industry for electro polished 316L stainless steel material [7]. The oxygen exposure causes the increasing of heat input 51

3 and then heat tint. The relation between heat tint and the temperature was shown in table 1 TABLE 1 Showed the Heat tint as various temperature Approx. Color Formed Temperature C Pale yellow 290 straw yellow 340 Dark yellow 370 Brown 390 Purple brown 420 Dark purple 450 Blue 540 Dark blue Experimental setups 3.1. Materials The details of the materials used in this study were 50.8 mm (2 inch) pipes of SDSS, UNS S32750, Pitting Resistance Equivalent Number (PREN) = 42.6 designated with 5.54 mm thickness and 50 mm length. Table 2 presents the chemical composition of materials used. TABLE 2 Chemical composition of SDSS inside pipe (back-purging gas). Three layers or numbers of weld passes were performed. The hydrogen addition to the torch shielding gas is considered to have no effect on the Heat affected Zone (HAZ) heat tint oxide on the inside surface. TABLE 3 Welding procedure specification (WPS) To provide vary amounts of the oxygen content in the purging gas a compressed tank of medical grade air was added to 99.99% minimum pure Argon (< 1ppm oxygen, moisture, and hydrocarbons) on Table 4 and the oxygen was measured with an oxygen analyzer. The amount of the oxygen content in the backing gas in ppm was illustrated in Table 5 TABLE 4 Welding parameters to the study effect of argon shielding & purging gas and inter-pass temperature Welding process The welding parameters are given in Table 3. Filler wires used for our welding trials are manufactured by ESAB, and 2.4 mm diameter wires were used for all welding trials. All welding parameter as shown in table 3 are in accordance with qualified welding procedure specification (WPS). Gas Tungsten Arc Welding (GTAW) with Direct Current Electrode Negative (DCEN) used polarity to weld the pipes. Welding was carried out by varying a % of oxygen 3. 3 Pitting corrosion test Pitting behavior of the welded samples was studied using ASTM G48, Method A [8], After welding, specimens were cut into 50mm 25mm size across the weld for testing, and the initial weight was measured by digital weighing machine. The specimens were immersed in 6% of ferric chloride solution for a period of 24 h at a constant test temperature of (35±1) C. After 24 hrs. period of immersion, specimens were rinsed with water, dipped into acetone in an ultrasonic cleaner, and air-dried. The specimen has pickled 52

4 before being weighed and tested. Pickling performed for 5 min at 60 0 C in a solution of 20 % HNO3 + 5 % HF. Subsequently, the test specimens were examined for visible pits and weighed to obtain the weight loss due to corrosion attacks. According to ASTM G48, Method A test, the stable pitting is said to be initiated when weight loss is more than 1g/(m 2.day). SDSS weldments, a weight loss of more than 1 g/(m 2.day) was observed at 35 C for all conditions. 4. Results and discussion The picture of the welded joints is displayed in Fig. 3. The pitting corrosion tested was focused in a specimens which contends the oxygen more than 100 ppm. (Test no.# 3-# 8 ) However, the test no. #1-# 2 it is complied with NORSOK standard a oxygen contain less than 100 ppm thus it is no need to perform the pitting corrosion test. The results are given in Table 5 Relation of oxygen contain with pitting corrosion Test Oxygen no Weight contain Weight loss (g/m 2 ) loss (g) (ppm) # 1 25 Not require Not require # 2 29 Not require Not require # # # # # # 8 > Fig.4 Theimperfection of welded # 8 (Before test) 4.2 Heat tint (Dis-coloration) Fig.3 Longitudinal view form welded pipe 4.1 The amount of oxygen in welded root and imperfections The amount of O 2 in the pipeline reacting with molten water (weld pool) makes molten weld imperfections. When the amount of argon is not enough to replace the existing pipe. The amount of exceeded greatly oxygen causes the poor quality of the weld. Fig.4 shows the welding defects in the weld of the test number 8 which is purge free (> 999 ppm). TABLE 5 The oxygen contain and pitting corrosion weight loss Fig.5 The amounts of oxygen is effective on weld root 4.3 Pitting corrosion resistance Table 5 shows the weight loss (corrosion rate) of the specimen number 1 to 8. Table 6 presents the example 53

5 of the details of the weight loss of the test number 8. It was found that the weight loss is very low. It is only to g. It may cause of the super duplex used in this paper are high corrosion resistance. Its resistance equivalent number (PREN) is More over the heat input used in the experiments is controlled under the qualified PQR. TABLE 6. The weight loss of imperfaction welded specimens Pitting corrosion Result of test specimens no.# 8 No. Description Average 1 Measure width (mm) Measure length (mm) Measure thickness (mm) Weight before immersion (g) Weight after immersion (g) Conclusions This paper presents the relation among oxygen value, heat tint (discoloration) and pitting corrosion rate of Super duplex stainless steel (SDSS) in pipe UNS It was found that the color of the weld are unacceptable in accordance with the NORSOK M601 standard, when the oxigen oroud the weld pool is 445 PPM or obove. Argon purge free can couse the imperfection in weld. For the reason that the materails used in this paper contain high Pitting Resistance Equivalent Number (PREN = 42.6), the weight loss cannot be found for all oxygen value around the weld pool. 6. Acknowledgment Authors would like to express gratitude the support from National Research University (NRU), King Mongkut s University of Technology Thonburi. References [1] Hua Tan, Yiming Jiang, Bo Deng, Tao Sun, Juliang Xu, Jin Li, Effect of annealing temperature on the pitting corrosion resistance of super duplex stainless steel UNS S32750, Materials characterization 60, 2009, [2] E. M. Westin & M. M. Johansson Effect on microstructure and properties of super duplex stainless steel welds when using backing gas containing nitrogen and hydrogen, International Institute of Welding 2014, [3] M.A. M, K.A. Shrikrishana, P. Sathiya, Finite element modelling and characterization of friction welding on UNS S31803 duplex stainless steel joints, Eng. Sci. Technol. 2015, [4] M.A. M, K.A. Shrikrishna, P. Sathiya, S. Goel, The impact of heat input on the strength, toughness, microhardness, microstructure and corrosion aspects of friction welded duplex stainless steel joints, J. Manuf. Process. 18 (2015) [5] Prabhu Paulraj, Rajnish Garg, Effect of welding parameters on pitting behavior of GTAWof DSS and super DSS weldments, Engineering Science and Technology, an International Journal 19 (2016) [6] NORSOK Standard M-601, Welding and inspection of piping, 2016 [7] Ken Kimbrel, Determining acceptable levels of weld discoloration on mechanically polished and electro polished stainless steel surface, Pharmaceutcal Engineering, Vol.31 no.6, 2011 [8] ASTM Standard G48-03, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution, ASTM International, West Conshohocken, PA, 2003 [9] Ken Kimbrel, Determining acceptable levels of weld discoloration on mechanically polished and electro polished stainless steel surface, Pharmaceutcal Engineering, Vol.31 no.6,