High Temperature Corrosion of A106 Gr B Steel Weldments at 600-800oC in N2/H2S-mixed Gases

Chunyu Xu and Dong Bok Lee* School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, South

Korea *Corresponding author: Email dlee@skku.ac.kr

Abstract: The A106 Gr B steel with a composition of Fe-1Mn-0.25Si (wt.%) was welded, and its corrosion behavior was studied at 600, 700 and 800oC at 1 atm of N2/H2S-mixed gases. Corrosion occurred mainly via sulfidation owing to the H2S gas. The base metal had better sulfida-tion resistance than the weld metal. How-ever, the morphology and the composi-tion of the scales formed on base metal and weld metal were similar. The scales were generally thick and fragile. Since FeS was present throughout the whole scale, the steel was non-protective. The alloying elements of Mn and Si tended to incorporate in the lower part of the scale. 1 Introduction

The A106 Gr B low carbon steels are used as main stream pipes and headers in the electric power plants and heavy chemical plants [1,2]. Pipes are joined by welding, and exposed to high-temperature corrosive atmospheres dur-ing service. Their service lifetimes are limited by creep, fatigue and corrosion. Although creep and fatigue properties of weldments are extensively studied, their high-temperature corrosion was not ade-quately investigated yet. The fundamental knowledge on high-temperatures corro-sion of welded joints is of great impor-tance. Traditionally, materials designed for use at high temperatures have been developed primarily for their mechanical properties, but there is now a growing re-alization that corrosion also limits lifetime, either directly through metal wastage or indirectly through raising local tempera-tures due to low thermal conductivity of the scale.

On the other hand, H2S gases are ma-jor concerns in current and future power generation systems, high-temperature gas turbines, and petrochemical units [3].

They dissociate into sulfur and hydrogen, and reacts with metal according to the reaction; H2S+M → MS+H2. Hydrogen atoms from H2S ingress into the steels interstitially, form hydrogen clusters, and cause hydrogen embrittlement, accelerat-ing the corrosion rates significantly. In this study, the A106 Gr steel was corroded between 600 and 800oC in N2/H2S-mixed gases in order to understand its corro-sion. 2 Experimental Procedure

An eleven-pass welded A106 Gr B steel pipe was tested in this study. Its chemical composition is listed in Table 1. It is widely used in industrial facilities such as nuclear power plants and heavy chemical plants. The cross-section of the multi-pass welded pipe is shown in Fig. 1. The weldments were corroded at 600, 700 and 800 oC in N2/H2S-mixed gases, and characterized by SEM/EDS, XRD, and EPMA

Table 1. Chemical composition of A106 Gr B steel (wt.%).

C Mn Si P S Ni Cr Mo Cu

0.2

1.02

0.25

0.017

0.005

0.02

0.06

0.01

0.01

Fig. 1. Multi-pass welded pipe.

3 Results and Discussion

Weight gains measured after corrosion are displayed in Fig. 2. Large weight gains were recorded, indicating vastly fast sulfidation kinetics. The scale failure be-came more serious as sulfidation pro-gressed. The sulfidation appeared to fol-low the linear rate law, implying negligible protection owing to the formation of FeS. The scales formed were usually very thick and highly susceptible to spallation during corrosion and subsequent cooling stage.

Figure 3 shows the SEM cross sec-tional image and EDS line profiles of the base metal. The outer thick FeS scale de-tached off and the inner fragile FeS scale split into a few layers. The scale was ~400 µm thick, implying poor sulfidation resistance. The outer FeS layer had ei-ther an equiaxed or a columnar structure, and the inner layer consisted of fine grains. Mn and Si were rather uniformly distributed. Cracks developed in the scale owing to stresses developed in the thick oxide scale. Similar scale morphology was also obtained in the weld metal, indi-cating that the principal sulfidation mode was the same in H2S/N2 atmospheres. The main difference was the acceleration of corrosion rates in weld metal.

Fig. 2. Sulfidation kinetics of A106 Gr B base metal and weld metal at 600-800oC for 20 h in H2S/N2 gases .

Fig. 3. A106Gr B base part after sulfidation at 700oC for 20h in H2S/N2 gases,(a) SEM cross sectional image, (b) line profiles.

4 Conclusions The A106Gr B steel had poor sulfidation

resistance. Both the base metal and weld metal displayed similar scale structures. FeS was the main corrosion product. The formed scales were usually very thick and quite fragile. The base metal sulfidized slower than the weld metal.

Acknowledgements

This work was supported by the Human Resource Development Project of the Korea Institute of En-ergy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114010203020).

References [1] M.J. Kim, D.H. Bae, D.B. Lee, “Analysis on

the scales formed on the heat affected zone of low carbon steel weld in NaCl and H2S water solutions,” J. Kor. Inst. Surf. Eng. vol. 4 (2010) pp. 205-210.

[2] G.Y. Lee, D.H. Bae, “Assessment of the sulfide corrosion fatigue strength for a multi-pass welded A106 Gr B steel pipe below the low SSCC limit,” J. Mech. Sci. Technol. vol. 23 (2009) pp. 3191-3198.

[3] D.J. Young, High Temperature Oxidation and Corrosion of Metals, Elservier: 2008, pp. 361-396.