Galvanic Corrosion in Gas Tungsten Arc Welding (GTAW) 304L Austenitic Stainless Steel Weldment M. H. Moayed1*, A. Davoodi2, A. Ale-Yassin1 1
Metallurgical and Material Engineering Department Faculty of Engineering, Ferdowsi University of
Mashad, Mashad, Iran. 2
Materials Engineering Department, Faculty of Engineering, Tarbiat Moallem University of Sabzevar,
Sabzevar 397, Iran. *
Corresponding author address: Metallurgical and Material Engineering Department, Ferdowsi
University, Mashad, 91775-11, Iran, Tel: +98-5118763305, E-mail:
[email protected]
Abstract Galvanic corrosion occurrence between the base alloy, HAZ and weld zone in a GTAW of 304L StSt in 3.5%wt NaCl solution was studied. The results indicated that the weld zone had the lowest corrosion and pitting potential and the highest passivity current density. Therefore in galvanic cell, weld and base zones act as anode and cathode, respectively. Coupled current density and potential measured by ZRA showed that galvanic current density in weld-base couple was almost 40 times more than HAZ-base. To elucidate the ZRA results, a descriptive approach including constituents of weldment is proposed. Microscopic observations also confirmed formation of several pits in the weld zone after 60 days exposure time. Keywords: GTAW, 304L, galvanic corrosion, potentiodyanimc polarization, ZRA.
1. Introduction Austenitic StSt 304L is a widespread usable alloy for highly aggressive environments in petrochemical, marine and food industries. Gas tungsten arc welding (GTAW) is commonly used as components assembling technique [1] and reaching to the desired corrosion resistance is crucial [2-6].
304L microstructure contains a variety of
complex austenite-ferrite, sigma phase and precipitated carbides [1,3-4]. The microstructural changes during welding causes electrochemical dissimilarity of individual phases [6-9], causing pitting and galvanic corrosion. However, number of reports in which elucidate galvanic current extension are scarcely available. In this work, galvanic corrosion occurred between galvanic couples; weld-base and HAZbase was studied by microscopic studies and measuring pitting parameters by
potentiodyanimc polarization. Coupled potential and galvanic current density was also measured by zero resistance ammetery technique in 3.5%wt NaCl solution.
2. Materials and experimental methods 304L StSt (400×100×10 mm) were welded by GTAW operation according to ASME using ER309L filler [10]. Individual samples of base, weld and HAZ regions were obtained (Fig. 1) with cross-section area was 8×4 mm2.
Figure 1. Schematic illustration of locations of prepared specimens from weld zone and HAZ
ACM Instrument was used for electrochemical tests in a 3.5%wt NaCl at room temperature. Coupled potential and current density were measured by Zero Resistance Ammeter for one hour. Finally, the whole polished sample was immersed in 3.5%wt NaCl for 60 days.
3. Results and discussion 3.1. Microstructure evaluation – While 304L contains equiaxial grains, step etched grain boundaries and the twin lines [11-14], in HAZ, the grain boundaries suffered more dissolution (Fig. 2(a)) due to the formation of continuous precipitated carbide.
(a)
(b)
25 μm
Figure 2. OM images of HAZ coarse grain microstructure next to the weld zone (a) continuous network of precipitated of carbides at austenite grain boundaries and (b) delta ferrite bands in austenite grains.
Near to the base, a decrease in amount of continuous precipitated carbide is observed while the remnant of delta ferrites band is observed as shown in Fig. 2(b) [11-14]. The HAZ length in the form of a crescent was 10 mm. SEM of etched weld zone (Fig. 3) revealed dissolved skeleton of delta ferrite network formed in weld zone [11-14]. Figure 3. SEM micrograph of 304L SS weld zone, etched in 10%wt oxalic acid, showing dissolved streak of skeleton delta ferrite network formed during solidification of weld zone.
3.2. Potentiodynamic measurements - The potentiodynamic polarization curves of individual base alloy, HAZ and weld zone sample for 304L steel are shown in Fig. 4.
Figure 4. Potentiodynamic polarization curve of base, weld and HAZ in 304L in 3.5%wt NaCl solution at 25º. Scan rate 1 mV/s.
The weld has the lowest OCP value. The pitting potential of HAZ (in current density of 100 µA/cm2) is about 200 mV lower than that of the base alloy zone, while its passivity current density is 5 times more, indicating of lower pitting corrosion resistance in HAZ compared to base zone [8-9]. Weld zone reveals a poor passivation behavior with a high passive current density of almost 3 orders of magnitude greater than base alloy. It also shows the lowest pitting potential with an average value of 460 mV.
3.3. ZRA measurements - The galvanic potential current densities were measured by ZRA method for the galvanic couples of base /weld and base/HAZ, presented in Fig.
5. The galvanic current density is gradually decreases to 5 μΑ/cm2 after one hour. Gradual increasing in base/weld couple galvanic potential from -475 to -435 mV is attributed to the anodic polarization of the anode, weld zone. A considerably lower galvanic current density was measured for base/HAZ couple with an average value of ca 1 μΑ/cm2. Interestingly, a negligible change in couple galvanic potential measurement was observed for base/HAZ couple indicating of stable polarization condition of base alloy and HAZ. Base of corrosion potential differences, the most probable galvanic corrosion is base/weld [7-8].
Figure 6. Galvanic couple potential and current density measured by ZRA of base alloy-weld zone and base alloy-HAZ pairs for 304L StSt in 3.5%wt NaCl at 25ºC.
To better understanding of overall current density, a schematic of galvanic couple is presented in Fig. 6 [15]. Galvanic cell of base/weld and base/HAZ are called cell A and B, respectively. The weld zone contains at least two major phases including δ ferrite and austenite phases, γ. The overall anodic and cathodic current densities in weld are
r r r I weld ( A) = I pass / weld + I pit / weld r
&
s s s I weld ( A = I γ / weld + I δ / weld
s
Since I weld ( A) > I weld ( A) , a net anodic current is generated by weld zone as follow:
r r s I net / weld ( A) = I weld ( A) − I weld ( A) On base alloy, the anodic reactions is austenite passive current, cathodic current in base alloy is
r I pass / base( A) . The
s s r I net / base( A) = I base( A) − I base( A) The overall current measured by ZRA method is exactly the net current associated with the anode that flows toward cathode, expressed as:
r s I ( measured ) weld / base = I net / weld = I net / base
Similar approach can be addressed for the formation of a galvanic cell between base alloy and HAZ zone as below
r s I measured ( B ) = I HAZ ( B ) − I HAZ ( B )
&
s r I measured ( B ) = I γ / base( B ) − I pass / base( B )
Higher current density measured for cell A is associated to the higher portion of δ ferrite in weld in compared to HAZ.
r I pass / weld
s I γ / weld
γ
r I pit / weld
s I δ / weld
r I pass
I ( measured ) weld / base
/ base ( A )
/ base ( A )
γ
δ
Weld
s Iγ
Base
r s = I net / weld ( A) = I net / base
Figure 6. Schematic representation of the electrochemical reactions occurring on a weld-base (cell A) galvanic couples composed of two phases (γ+δ) . The arrow length is an indication of current magnitude.
In next experiment, the whole weldment was immersed in 3,5% NaCl for 60 days. While no pit was observed in base region, a few and several pits were formed in HAZ and weld, respectively, confirming formation of galvanic corrosion leading to pitting corrosion of weld and HAZ which is in agreement with previous results and in-situ observations by scanning reference electrode technique (SRET) [7-9,13,16-18].
4. Conclusion Galvanic corrosion occurrence among individual parts of a 304L type welded StSt in 3.5% NaCl was studied. Potentiodyanimc polarization measurement revealed the highest passive current density and the lowest pitting potential in weld zone. One hour measurement of base/weld galvanic couple through ZRA configuration revealed much higher average current density7.28 in compared to base/HAZ couple 0.195 µA/cm2). Therefore, weld and HAZ zone act as the anode and base metal as the cathode of the galvanic couples. Contribution of individual current density is associated to the microstructure constituent. Immersion of weldment after 60 days in 3.5% NaCl also showed formation of several pits in weld zone.
5. Acknowledgement Financial support from Ferdowsi University of Mashad is appreciated.
6. References 1. ASM Handbook Vol. 6: Welding, Brazing, and Soldering. ASM international, USA;1993. 2. Sanchez-Cabrera et al, Mater Sci Eng A 2007;452-453:235. 3. Hunter A, Ferry M., Scripta Materialia 2002;45:253. 4. Ma JC, Yang YS, Tong WH, Fang Y, Yu Y, Hu ZQ., Mater Sci Eng A 2007;444:64. 5. Elmer JW, Wong J, Ressler T. Second international symposium on the joining of advanced materials ASM Materials Solutions 99, Cincinnati, Ohio, November 1-4;1999. 6. Dadfar M et al, Mater Letters 2007;61:2343. 7. Garcia C et al, Corr Sci 2008;50:1184. 8. Lu BT et al, Electrochimica Acta, 2005;50:1391. 9. Kwok CT et al, J Mater Process Technol 2006;176:168. 10. ASME, Boiler & Pressure Vessel Code- Section II- Materials, 2nd ed; 2000. 11. Vander Voort GF et al, Metallography and Microstructures, ASM Handbook Vol 9, 2004, p. 670. 12. de Lima-Neto P et al, Corr Sci 2008;50:1149. 13. Matula M et al, Mater. Charact 2001;46:203. 14. Park SHC, Sato YS, Kokawa H, Okamoto K, Hirano S, Inagaki M., Scripta Materialia 2004;51:101. 15. Idrac J. et al, Electrochimica Acta 2007;52:7626. 16. Cui Y, Lundin CD, Mater Design. 2007;28:324. 17. Cui Y, Lundin CD., Mater Letters 2005;59:1542. 18. Akid R, Mills DJ, Corr Sci 2001;43:1203.
