Corrosion Behavior of 316L and Alloy 182 Dissimilar Weld Joint with Post-Weld Heat Treatment

Austenitic stainless steel and nickel-base alloys welds are widely used in nuclear reactor components, plants of energy generation, chemical, and petrochemical industries, due to their high corrosion resistance. The post weld heat treatments (PWHT) are generally applied to welding in order to relieve the welding residual stress. The aim of this work was to evaluate the influence of different PWHT on corrosion behavior of a dissimilar weld joint of two AISI 316L austenitic stainless steel plates with a nickel-base alloy as filler material in saline environments. The material was submitted to heat treatments for three hours at 600, 700 and 800 °C. The weld joint was examined by optical microscopy to determine the effects of PWHT in the microstructure. The corrosion behavior of the samples before and after heat treatment was evaluated using cyclic potentiodynamic polarization (CPP) in sodium chloride solutions (19% v/v) and pH 4.0 at room temperature. Metallographic analyses showed that delta ferrite dissolute as PWHT temperature increased. CPP curves demonstrated an increase of pitting corrosion resistance as the PWHT temperature rises, although the pit size has been increased. The heat treated weld joint at 600 °C showed a similar corrosion resistance compared to as-welded material.


INTRODUCTION
Dissimilar metal welds (DMWs) or bimetallic welds refer to welds between two different metals [1]. Austenitic stainless steel and nickel-base alloys are generally used in DMWs, and such alloys are employed in nuclear reactor components, plants of energy generation, chemical, and petrochemical industries due to their high corrosion resistance [2][3][4][5][6][7]. The welds in nuclear power plants are employed in high temperature operating components [7]. For plant safety it is essential to keep the structural integrity of these DMWs during service life [1].
Austenitic stainless steel and nickel-base alloys may require a post weld heat treatment (PWHT) in order to remove the welding residual stress and the PWHT is generally applied to the weldment [8][9][10]. Residual stresses can be significantly reduced by PWHT in austenitic materials at a temperature range from 480 to 925 °C. At the higher end of this range, nearly 85% of the residual stresses may be relieved. However, PWHT at this range may result in sensitizing susceptible material, hence, these heat treatments should be only used for low carbon grades "L" and the 321 and 347 stabilized steels [1,5,8,11].
The mechanical properties and corrosion resistance of welded joints can be affected by the welding process and PWHT resulting in microstructural variations such as carbide precipitation and sigma phase [12][13][14][15][16][17][18][19]. The dissolution of ferrite occurs as heat treatment temperature increases above 500 °C in stainless steel. During its dissolution, part of delta ferrite is transformed into austenite plus carbides and second phases. The main intermetallic, in austenitic stainless steel AISI 316 or 316L, is sigma constituted with higher levels of carbon, chromium and molybdenum, and lower levels of nickel and iron [2,9,10,[20][21][22].
The corrosion behavior of the metals and alloys depends on the transformation of the microstructure and the chemical composition caused by the weld thermal history that occurs during the welding process. Hence, the corrosion resistance of weld may be inferior to the resistance of base metal, due to precipitation, microsegregation, the formation of unmixed zones, contamination of the solidifying weld pool, secondary phase and recrystallization and grain growth in the weld heat-affected zone (HAZ) [23,24]. The corrosion resistance can be preserved in the welded condition by balancing alloy compositions to avoid undesirable precipitation reactions, by selecting the proper welding parameters, by shielding molten hot metal surfaces from reactive gases in the weld environment, by extracting chromium-enriched oxides, and chromiumdepleted base metal from thermally discolored surfaces [25][26][27].
In this work, the influence of different PWHT on corrosion behavior of a dissimilar welds involving AISI 316L stainless steel and nickel-base alloy 182 was investigated using the electrochemical technique: cyclic potentiodynamic polarization (CPP) in sodium chloride solutions (19 %v/v) and pH 4.0 at room temperature.

Materials and welding procedure
The base metal (BM) used for welding in the present investigation was an austenitic stainless steel AISI 316L plates with 300 mm (length), 150 mm (width), and 12.7 mm (thickness). The filler metal was nickel-base alloy 182 with diameters of 2.5 and 3.2 mm. The nominal chemical composition of the base and the filler metal are shown in Table 1. The plates of stainless steel AISI 316L were prepared for the weld employing a single V groove edge preparation with an included angle of 30°. The top plates were welded with a narrow gap of 2.5 mm, with pre-heating at 150 °C, and interpass temperature between 100 to 150 °C by shielded metal arc welding (SMAW). The weld joint (WJ) was performed with 90 -100 A for the root weld (filler metal of 2.5 mm), and 110 -120 A for the other beads (filler metal of 3.2 mm). In order to compensate the deformations due to the welding process a lift was designed in the central region of the weld, supported by a copper plate 300 mm x 50 mm x 10 mm as shown in Figure 1. After welding, a PWHT was performed at 600, 700 and 800 °C for 3 hours to relieve residual stresses of the welded joints. All samples were taken from similar places of the welds. The sampling location was the root welding.

Microstructure analysis
The cross-section of the welded specimens were mechanically polished with 1 µm and electrolytically etched in a 10% oxalic acid solution at 2.2 V DC for 60 s in order to examine the microstructure and to identify grains, dendrite boundaries, and precipitates. The microstructures analyses were carried out by an optical microscope.

Eletrochemical measurements
The specimens for electrochemical measurements were weld to a copper wire, and embedded in epoxy resin.
The working surface was then abraded with SiC papers of grit size from 80 to 600 mesh and cleaned with distilled water and alcohol. The surface area of the samples was measured by Image J software. The average size of sample area was about 2.0 cm².
Electrochemical measurements were performed using an Autolab PGSTAT 20 potentiostat and an electrochemical cell of three electrodes: base metal and the welded samples as working electrode, platinum mesh as counter electrode and silver chloride electrode (Ag/AgCl, 3M KCl) as reference electrode. The electrochemical tests were carried out at 25 ºC and pH 4.0 containing 190 g/L NaCl and 0.4 g/L sodium acetate (5 mM CH 3 COONa). The open circuit potential (OCP) was recorded for 1 h or until stabilization, and the CPP curves were collected by scanning to the anodic direction at a scan rate of 0.167 mV s -1 , -0.02 V Ag/AgCl from the stabilized OCP. When the current reached 1 mA/cm 2 the scan was reversed. All the electrochemical measurements were repeated at least three times to ensure the reproducibility. The pitting potential (E pit ) was defined as the potential value at which the current density sharply rose. After the electrochemical tests, surface morphology was examined using an optical microscope and scanning electronic microscope (SEM).

RESULTS AND DISCUSSION
The microstructure of the base metal (AISI 316L) consists of austenite grains and stringers of delta ferrite, and the PWHT caused dissolution of delta ferrite as shown in Figure 2. The weld metal microstructure consists of dendrite grains, with inclusions and precipitates. The amount of precipitates in the weld metal decreased with an increase in temperature of the PWHT. The delta ferrite was partially dissolved in the HAZ with an increasing temperature of the PWHT. The delta ferrite content of the base and weld metals decreased with an increase in the annealing temperature [14,15]. It was measured for the base metal by image analyzing software for microstructure analyzing Quantikov, and shown in Table 2 [28].  The polarization curves for base metal and weld joints, before and after PWHT (600 -800 °C) in a 19% wt. NaCl solution (pH 4.0), are shown in Figure 3. According to literature, chloride-containing acidic environments lead to more severe pitting corrosion [29]. Table 3 contains the results of polarization studies, which comprises the corrosion potential (E corr ), passive current (I pass ), pitting potential (E pit ) and protection potential (E prot ).  WJ No PWHT -211,0 ± 1,0 3,0 ± 1,3 -43,0 ± 15,1 29,6 ± 23,5 WJ PWHT 600 °C -217,5 ± 21,9 9,9 ± 6,2 -49,5 ± 0,7 8,9 ± 0,4 WJ PWHT 700 °C -197,0 ± 9,0 1,8 ± 1,2 -18,3 ± 9,6 -42,5 ± 25,9 WJ PWHT 800 °C -217,0 ± 30,0 0,6 ± 0,4 43,0 ± 11,7 -112,9 ± 32,0 All weld joints showed a passive behavior, although the specimen of PWHT at 800 °C clearly demonstrated a wider and more defined passive region. PWHT has no influence on E corr value; this parameter was measured around -200 mV Ag/AgCl for all the specimens. The weld joint heat treated at 600 °C has similar corrosion behavior compared to the as-welded joint. As the temperature of PWHT increased, E pit also increased, and E prot decreased. This increase of E pit can be explained by a higher dissolution of delta ferrite at higher temperatures, this phase has deleterious effects on corrosion resistance [4,[30][31][32][33][34][35]. It is known that delta ferrite provides a favorable sites for pitting initiation [34]. Pitting potential and current density shift towards higher values, the passive region becomes wider for the base metal when compared with welded joints [36]. Therefore, the welded joint showed lower corrosion resistance than the base metal [18,19,37] as summarized in Figure 4. Metallographic examination of the weld joints after the cyclic potentiodynamic polarization revealed pitting corrosion as shown in Figure 5. The pit morphologies are roughly circular or elliptical and their diameter depends on PWHT temperature. Higher PWHT temperature has led to larger pits, showing a large hysteresis loop (E pit -E prot ) due to the difficulty to attain the passive layer. According to Chen et. al. [39], after the pit initiation, the pit may continue growing between the E pit and E prot [38,39]. The majority pits were found near the fusion line in the weld metal, since the local properties of the fusion zone are often inferior to those of the base metal [19], which can be attributed to segregation during solidification [23,34]. It was observed that after PWHT the phosphorus and aluminum segregation decreased, and it is known that phosphorus segregation can enhance corrosion [40]. The overlapped regions are usually the weakest sites to resist the corrosion attack in the weld metal, due to their microstructure morphology differences which promote a galvanic attack [23].

CONCLUSIONS
Polarization curves showed that heat treatments at higher temperatures increase the weld joint tendency to passivate. The PWHT at 800 °C had a wider and more defined passive region and the highest pitting corrosion resistance. The PWHT at this temperature provided a better corrosion resistance to the weld joint. However, also presented the lowest protection potential, thus, once a pit is developed, it will grow to larger sizes until its repassivation. The pitting corrosion resistance increased with temperature due to higher dissolution of delta ferrite at higher temperatures. The corrosion resistance of the weld joints was lower than the base metal before and after PWHT.

ACKNOWLEDGMENTS
The authors are grateful to the Brazilian government agencies (CNPq, CNEN, CAPES, and FAPEMIG) for the financial support for this research and the Centro de Desenvolvimento da Tecnologia Nuclear -CDTN for the technical support.