Corrosion of Fe-Cr Steels at 600-800℃ in NaCl Salts

한국표면공학회지 J. Korean Inst. Surf. Eng.

Vol. 51, No. 6, 2018.

https://doi.org/10.5695/JKISE.2018.51.6.354

<연구논문>

ISSN 1225-8024(Print) ISSN 2288-8403(Online)

Corrosion of Fe-Cr Steels at 600-800 ^o C in NaCl Salts

Dong Bok Lee

, Min Jung Kim

, Poonam Yadav

and Xiao Xiao

a

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea

(Received 2 November, 2018 ; revised 20 November, 2018 ; accepted 20 November, 2018)

Abstract

NaCl-induced hot corrosion behavior of ASTM T22 (Fe-2.25Cr-1Mo), T91 (Fe-9Cr-1Mo), T92 (Fe-9Cr- 1.8W-0.5Mo), 347HFG (Fe-18Cr-11Ni), and 310H (Fe-25Cr-19Ni) steels was studied after spraying NaCl on the surface. During corrosion at 600-800

C for 50-100 h, thick, non-adherent, fragile, somewhat porous oxide scales formed. All the alloys corroded fast with large weight gains owing to fast scaling and destruction of protective oxide scales. Corrosion rates increased progressively as the corrosion temperature and time increased. Corrosion resistance increased in the order of T22, T91, T92, 347HFG, and 310H, suggesting that the alloying elements of Cr, Ni, and W beneficially improved the corrosion resistance of steels. Basically, Fe oxidized to Fe

O

, and Cr oxidized to Cr

O

, some of which further reacted with FeO to form FeCr

O

or with NiO to form NiCr

O

.

Keywords : Fe-Cr steel, Stainless steel, Hot corrosion, NaCl salt

1. Introduction

Gas turbines, marine engines, and boilers are exposed to high-temperature corrosive environments during their service. Corrosion limits the lifetime of structural components through metal wastage, increases local temperatures, and decreases creep resistance. One of the most serious corrosion is NaCl–induced hot corrosion [1-7]. NaCl melts at 801

C, and accelerates corrosion of metals seriously, and thereby depletes protective elements at the alloy surface. The inward transport of salts and the outward diffusion of dissolved metallic ions occur fast through the porous or cracked scales that formed in salts. In this study, commercial chromium steels were corroded at 600-800

C after spraying a thin layer of NaCl on the surface, because NaCl-induced hot corrosion has been a major problem for high- temperature structural components in marine atmospheres. The aim of this study is to examine the

corrosion behavior of commercial steels in NaCl salts. NaCl-induced hot corrosion needs to be understood further because of the difficulty in studying the complex interactions between salts and alloys.

Corrosion rates, corrosion products, and morphologies of scales that formed on steels were examined as a function of corrosion temperature and time.

2. Experimental

Table 1 lists five different commercial alloys that were corroded in this study. ASTM T22 (viz. Fe- 2.25Cr-1Mo) is the low alloy ferritic steel. ASTM T91 (viz. Fe-9Cr-1Mo) and T92 (viz. Fe-9Cr-1.8W- 0.5Mo) are martensitic steels. ASTM 347HFG (viz.

Fe-18Cr-11Ni) and 310H (viz. Fe-25Cr-19Ni) are heat-resistant austenitic steels. Above alloys were cut into 2 × 8 × 10 mm

-size coupons, ground to 1000- grit SiC finish, cleaned in acetone and alcohol. After spraying aqueous NaCl solution on the surface to 2 mg/cm

NaCl, alloys were corroded at 600, 700, and 800

C for 50, 100, and 200 h in air using a muffle furnace. After corrosion, samples that were contained inside alumina crucibles were quickly taken out of the furnace, cooled to room temperature

* Corresponding Author: Xiao Xiao

School of Advanced Materials Science and Engineering, Sungkyunkwan University

Tel: +82-31-290-7355 ; Fax: +82-31-290-7371

E-mail: xiaoxiao2303@outlook.com

Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 51 (2018) 354-359 355

in air, mounted in epoxy, and polished without using water because not only NaCl but also both sodium ferrate (Na

FeO

) and sodium chromate (Na

CrO

) that could form during corrosion [6,7] were deliquescent. Corrosion rates were determined by measuring weight changes using a microbalance before and after corrosion testing. The formed scales were inspected by a field-emmision scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), and a high power X- ray diffractometer (XRD) with Cu-Kα radiation at 40 kV and 150 mA in a θ/2θ configuration.

3. Results and discussion

Figure 1 shows corrosion kinetics of steels at 600- 800

C for 50-200 h. Since chlorine in NaCl cracks and detaches protective Al

O

or Cr

O

oxide scales [8], weight gains displayed in Fig. 1 were inaccurate.

Nonetheless, all the samples gained weights progressively as the corrosion temperature and time increased. They tended to corrode almost linearly, as the corrosion temperature increased [9]. The corrosion time was limited to 100 h at 800

C owing to excessive corrosion. Generally, corrosion resistance increased in the order of T22 (Fe-2.25Cr-1Mo), T91 (Fe-9Cr-1Mo), T92 (Fe-9Cr-1.8W-0.5Mo), 347HFG (Fe-18Cr-11Ni), and 310H (Fe-25Cr-19Ni), suggesting that alloying elements such as Cr, Ni, and W beneficially improved the corrosion resistance of steel.

Figure 2 shows typical XRD patterns of corroded steels. In Figs. 2a-b, NaCl was vaguely detected because NaCl (m.p.= 801

C) tended to vaporize, and reacted with oxides, resulting in destruction of the protective Cr

O

scale. NaCl increased the tendancy of oxide scales to crack and spall, leading to fast corrosion [10]. Fe

O

, which was the oxide of the base element, Fe, was the most strongly detected for T22 (Fe-2.25Cr-1Mo), T91 (Fe-9Cr-1Mo), T92 (Fe- 9Cr-1.8W-0.5Mo), and 347HFG (Fe-18Cr-11Ni). In Fig. 2d, the primary alloying element, Cr, oxidized to the stable FeCr

O

spinel. In Fig. 2e, Cr

O

and

NiCr

O

were additionally detected, besides Fe

O

and FeCr

O

. Basically, Fe oxidized to Fe

O

, and Cr oxidized to Cr

O

, some of which further reacted with FeO to form FeCr

O

. This oxide was detected when the Cr content in the alloy was 18 and 25%.

Nickel in 310H oxidized to the stable NiCr

O

spinel through the reaction of NiO with Cr

O

. Other corrosion products of minor alloying elements such as W, Mo, Mn, Si, V, and Nb were not detected in Fig. 2 due to their small amounts or dissolution in oxides.

During the NaCl-induced corrosion of stainless steel, Fig. 1. Weight gain versus corrosion time curves of steels after corrosion in NaCl. (a) 600

C, (b) 700

C, (c) 800

C.

Table 1. Nominal compositions of chromium steels (wt.%)

C Cr Ni W Mo Mn Si V Nb

T22 0.12 2.25 1 0.45 0.3

T91 0.10 9 1 0.45 0.4 0.20 0.08

T92 0.07 9 1.8 0.5 0.45 0.06 0.20 0.05

347HFG 0.08 18 10 1.6 0.6 0.8

310H 0.05 25 21 1 0.3

NaCl initially reacted with the Cr

O

and Fe

O

to form Na

CrO

and Na

FeO

, which depleted Cr and Fe in the oxide, and thus destroyed protective properties.

Resultantly, rapidly growing scales that consisted of an outer Fe

O

layer with chromate particles on top and an inner spinel oxide layer formed [7]. In this study, Na

CrO

(m.p.= 762

C) and Na

FeO

were not detected probably due to their volatility.

Figure 3 shows SEM results of T22 steel after corrosion at 700

C for 100 h. The scale was ~ 205 μm-thick, and had some pores (Fig. 3a). Cr and Cr

O

reacted with NaCl, evolving gas, according to the following reactions [6].

Cr + 2NaCl + 2O

(g) = Na

CrO

+ Cl

(g) (1) Cr

O

+ 4NaCl + 5/2 O

(g) = 2Na

CrO

+ 2Cl

(g) (2) Although corrosion resistance increased with increasing the chromium content, the above reactions

depleted Cr and destroyed the Cr

O

scale [9,11].

Cl

(g) evaporated, and partly penetrated through the partially destroyed oxide scale to form volatile CrCl

at the metal/oxide interface [3]. Hence, the interface between the metal and the porous scale was uneven, and porous (Fig. 3a). The scale consisted primarily of Fe

O

, where Cr was incorporated (Fig. 3b). Except Fe-Cr-O, no other elements including Na-Cl were detected due to their evaporation and/or small amounts. The scale morphology shown in Fig. 3 was similarly observed in the case of T91 steels that was corroded under the same condition given in Fig. 3.

Figure 4 shows SEM cross-sectional images of corroded T91 and T92 steels. Generally, the oxide scales that formed at 600

C tended to split into pieces, and spalled off (Figs. 4a-b), whereas those that formed at 800

C tended to agglomerate because m.p. of NaCl is 801

C (Figs. 4c-d). All the oxide scales that formed at 600-800

C were loosely adherent, and somewhat porous. The thickness of oxide scales was about 115 μm (Fig. 4a), 35 μm (Fig. 4b), 490 μm (Fig. 4c), and 350 μm (Fig. 4d).

Such inconsistent thickness of oxide scales was partly attributed to the breakage and detachment of oxide scales formed. The loss of Cr in Fe- and Ni- base alloys owing formation of Na

CrO

was primarily responsible for formation and breakdown Fig. 2. X-ray diffraction patterns after corrosion at

700

C for 100 h. (a) T22 (Fe-2.25Cr-1Mo), (b) T91 (Fe-9Cr-1Mo), (c) T92 (Fe-9Cr-1.8W-0.5Mo), (d) 347HFG (Fe-18Cr-11Ni), (e) 310H (Fe-25Cr-19Ni).

Fig. 3. T22 (Fe-2.25Cr-1Mo) steel after corrosion at

700

C for 100 h. (a) SEM cross-sectional image, (b)

EDS line profiles along A-B.

Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 51 (2018) 354-359 357

of thick, (Fe, Cr)-rich scales [7,9].

Figure 5 shows SEM results of T92 steel after corrosion at 700

C for 100 h. The oxide scale was ~ 95 μm-thick, somewhat porous, and friable (Fig. 5a).

It split into pieces, and partially spalled off. As did in Fig. 3b, the major alloying element, Cr was incorporated in the oxide scale that consisted primarily of Fe

O

(Fig. 5b). Such was plausible because Fe

O

and Cr

O

have the corundum

structure. Additionally, the minor alloying element, W, was incorporated in the oxide scale (Fig. 5b).

Corrosion products of other minor alloying elements and Na-Cl were not detected due to their absence and/or small amounts in the oxide scale. The scales that formed on stainless steels consisted of an outer Fe

O

layer with chromate particles on top and an inner layer consisting of spinel type oxide, (Fe,Cr,Ni)

O

[7]. This oxide and Na

Fe

O

[7] were not detected in Fig. 2, because the X-ray beam was not able to penetrate deeply.

Figure 6 shows SEM results of 347HFG after corrosion at 700

C for 100 h. The oxide scale was ~ 130 μm-thick, detached, somewhat porous, and partially broken (Fig. 6a). As did in Figs. 3b and 5b, the major alloying element, Cr, was incorporated in the oxide scale that consisted primarily of Fe

O

and some FeCr

O

(Fig. 6b). The other major alloying element, Ni, did not readily oxidize because Ni is thermodynamically more noble than Fe and Cr. Thus, Ni was accumulated below the oxide scale. In Fig. 1, Ni in austenitic stainless steel was found to improve the corrosion resistance [6], because the following Ni/NaCl reaction was thermodynamically unfavorable [12].

Fig. 4. SEM cross-sectional images after corrosion.

(a) T91 (Fe-9Cr-1Mo) at 600

C for 100 h, (b) T92 (Fe- 9Cr-1.8W-0.5Mo) at 600

C for 100 h, (c) T91 (Fe-9Cr- 1Mo) at 800

C for 50 h, (d) T92 (Fe-9Cr-1.8W-0.5Mo) at 800

C for 50 h.

Fig. 5. T92 (Fe-9Cr-1.8W-0.5Mo) steel after corrosion

at 700

C for 100 h. (a) SEM cross-sectional image, (b)

EDS line profiles along A-B.

Ni+ 2NaCl + O

(g) = NiO + Na

O + Cl

(g) (3) Instead of eq. (3), Ni is known to be corroded according to eq. (4) [12].

Ni+ Cl

(g) = NiCl

(4)

Nickel chloride vaporized from the surface, and the following oxychlorination occurred.

NiCl

+ O

(g) = 2NiO + Cl

(g) (5) In this study, NiO reacted further to form NiCr

O

(Fig. 2e). The minor alloying element, Mn, readily oxidized owing to its strong oxygen affinity, resulting in its incorporation in the whole oxide scale (Fig.

6b). Like Cr, iron and other alloying elements would be chlorinated or oxidized to make the alloys nonprotective [13]. Chlorine is known to accelerate corrosion, establishing a self-sustained cyclic process of chloride- and oxide-formation, a corrosion phenomenon referred to as ‘active oxidation’ [14].

Figure 7 shows SEM results of 310H after corrosion at 700

C for 100 h. The oxide scale was ~ 70 μm-thick, somewhat porous, and partially broken (Fig. 7a). In Fig. 2e, the oxide scale was found to be consisted of Fe

O

, Cr

O

, FeCr

O

, and NiCr

O

. Basically, Fe, Cr, and Ni were distributed in the whole scale, in the outer scale, and in the inner scale, respectively (Fig. 7b). Such distribution might be associated with abundance of Fe in the alloy, and thermodynamic nobility of Ni and Cr. Comparatively active element, Mn, was segregated in the outer scale. In this study, the NiCr

O

spinel delayed the corrosion to a certain extent, as shown in Fig. 1.

Principal causes of accelerated corrosion was attributed to NaCl, which accelerated oxidation of all the alloys through oxychlorination and chlorination/

oxidation cyclic reactions, and catalytic actions of chloride or chlorine [12].

4. Conclusions

NaCl-induced corrosion behavior of ASTM T22 (Fe-2.25Cr-1Mo), T91 (Fe-9Cr-1Mo), T92 (Fe-9Cr- 1.8W-0.5Mo), 347HFG (Fe-18Cr-11Ni), and 310H Fig. 6. 347HFG (Fe-18Cr-11Ni) steel after corrosion at

700

C for 100 h. (a) SEM cross-sectional image, (b) EDS line profiles along A-B.

Fig. 7. 310H (Fe-25Cr-19Ni) steel after corrosion at

700

C for 100 h. (a) SEM cross-sectional image, (b)

EDS line profiles along A-B.

Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 51 (2018) 354-359 359

(Fe-25Cr-19Ni) steels was studied at 600-800

C in air. During corrosion, oxidation dominated. The oxide scales were generally thick, non-adherent, and somewhat porous, because NaCl increased corrosive attack significantly through the rapid scaling and subsequent destruction of oxide scales. The oxide scales consisted primarily of Fe

O

for T22, T91 and T92, (Fe

O

, FeCr

O

) for 347HFG, and (Fe

O

, Cr

O

, FeCr

O

, NiCr

O

) for 310H. Generally, Fe, Cr, and W existed in the whole oxide scale, while Ni tended to accumulate in the lower part of the oxide scale, depending on the amount and reactivity of each element. Although Cr, Ni, and W improved the corrosion resistance, current steels displayed poor corrosion resistance.

Acknowledgment

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03028792).

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