High Temperature Corrosion of Cr(III) Coatings in N₂/0.1%H₂S Gas

한국표면공학회지 J. Korean Inst. Surf. Eng.

Vol. 52, No. 3, 2019.

https://doi.org/10.5695/JKISE.2019.52.3.111

<연구논문>

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

High Temperature Corrosion of Cr(III) Coatings in N

/0.1%H

S Gas

Dong Bok Lee and Shi Yuke*

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

(Received 26 March, 2019 ; revised 26 April, 2019 ; accepted 9 May, 2019)

ABSTRACT

Chromium was coated on a steel substrate by the Cr(III) electroplating method, and corroded at 500-900

oC for 5 h in N₂/0.1%H₂S-mixed gas to study the high-temperature corrosion behavior of the Cr(III) coating in the highly corrosive H₂S-environment. The coating consisted of (C, O)-supersaturated, nodular chromium grains with microcracks. Corrosion was dominated by oxidation owing to thermodynamic stability of oxides compared to sulfides and nitrides. Corrosion initially led to formation of the thin Cr₂O₃ layer, below which (S, O)-dissolved, thin, porous region developed. As corrosion progressed, a Fe₂Cr₂O₄ layer formed below the Cr₂O₃ layer. The coating displayed relatively good corrosion resistance due to formation of the Cr₂O₃ scale and progressive sealing of microcracks.

Keywords : Chromium, carbon, electroplating, H₂S corrosion, oxidation

1. Introduction

Chromium electroplating on metal substrates is widely used in industries owing to its decorative color, high hardness, and good corrosion resistance. It is usually performed in the hexavalent chromium (Cr⁺⁶) bath having CrO₃. Shortcomings of Cr(VI) coating are however low cathode efficiency that results in bad throwing power, serious health and environmental problems. To overcome these shortcomings, trivalent chromium (Cr⁺³) plating is developed as an alternative [1]. Coatings of Cr(III) were previously investigated, including their electroplating condition [2], hardness [3], wear resistance [4], corrosion resistance [5,6], electrical contact resistance [7], and high-temperature oxidation behavior [8]. In this study, Cr(III) coating was electroplated onto a steel substrate, and corroded at high temperatures in N₂/H₂S-mixed gas in order to study the corrosion behavior of Cr(III) coating in the serious H₂S-containing atmosphere. Since steels

corrodes fast in H₂S gas, forming thick FeS scales incorporated with hydrogen, it is imperative to develop optimum corrosion-resistant coatings for steels. H₂S comes off as a by-product during processing in oil refinery, chemical and petrochemical units, and gasification of fossil fuels [9]. It dissociates into sulfur and hydrogen. Sulfur reacts with metals to form nonprotective sulfides, while hydrogen ingresses into alloys interstitially, forms hydrogen clusters, increases corrosion rates, and causes hydrogen embrittlement [9,10]. This study aims to investigate the high-temperature corrosion behavior of Cr(III) coating in H₂S-environment at high temperatures in order to find the feasibility of Cr(III) coating in protecting steels. The microstructural and compositional variation of the coating during corrosion was studied.

2. Experimental

Chromium was electroplated on a low carbon steel substrate (AISI 1024 with a nominal composition of Fe-0.2C-1.5Mn-0.05S-0.04P in wt%) with a size of 2×0.5×0.3 cm³ to a thickness of 30-100μm. The bath composition and electrolysis condition are listed in

*

School of Advanced Materials Science and Engineering, Sungkyunkwan University

Tel: +82-3411-290-7355 ; Fax: +82-290-7379 E-mail: [email protected]

Table 1. Bath solution consisted of chromium sulphate (Cr₂(SO₄)₃·nH₂O) as a source of Cr⁺³, complexing agent (HCOOK) that supersaturates carbon in the Cr coating with an amorphous structure [5,11], conductive improvement agent (KCl, NH₄Cl), buffer agent (H₃BO₃), anti-oxidant agent (NH₄Br), and an additive (polyethylene glycol). The surface area of the anode was twice than that of the cathode.

The prepared samples were charged into the quartz reaction tube, and corroded at 500-900^oC for 5 h in N₂/0.1%H₂S-mixed gas in a horizontal tube furnace.

Employed N₂ and H₂S gases were 99.999% and 99.99% pure, respectively. The samples were characterized by a field-emission scanning electron microscope (SEM) equipped with an energy dispersive spectroscope (EDS), a field-emission electron probe microanalyzer (EPMA), an X-ray photoelectron spectrometer (XPS), and a high power X-ray diffractometer (XRD) with Cu-Kα radiation at 40 kV and 150 mA.

3. Results and Discussion

Figure 1 shows SEM/EDS/XRD/XPS analytical results of Cr(III) coating. This consisted of nodular grains with tens of micrometer in size (Fig. 1(a)).

Intense hydrogen evolution during electroplating generated microcracks inter- and intra-granularly.

Nodular grains grew bigger, and microcracks became wider and deeper with the increment of plating time and coating thickness [1,2]. Generation of microcracks is the major drawback of the Cr(III) coating. The EDS-analyzed composition of the spot 1, 2, and 3, which were marked in Fig. 1(b), was 69.7Cr-19.3O-11C, 68.5Cr-19.2O-12.3C, and 68.5Cr- 21.6O-10C in at%, respectively (Fig. 1(c)). Although the quantification of light element such as carbon

was notoriously difficult, Cr, O, and C seemed to be uniformly distributed in the adherent, microcracked coating. The amorphous Cr(III) coating displayed a diffuse pattern around 44^o (Fig. 1(d)). It was reported that supersaturated carbon in the coating precipitated as chromium carbides such as Cr₂₃C₆ [2,11], Cr₇C₃ [12], and Cr₃C₂ [13] when annealed at 400-700^oC, being accompanied with increment of the coating hardness. XPS spectra of Cr, C and O of the coating are shown in Fig. 1(e). The Cr_2p spectrum indicates that the presence of metallic Cr and Cr₂O₃. The C_1s spectrum indicated C-C bond (amorphous carbon) [12,14] and COOH bond [15]. The oxygen dissolution in the Cr(III) coating made the intensity of the O_1s spectrum high.

Figure 2 shows SEM/EDS/XRD/XPS results of Cr(III) coating after corrosion at 500^oC for 5 h.

Fig. 1. Cr(III) coating. (a) SEM top view, (b) SEM cross-sectional image, (c) EDS spectra of spot 1, 2 and 3, (d) XRD pattern, (e) XPS spectra of Cr_2P, C_1S, and O_1S.

Table 1. Bath composition and electrolysis conditions Bath composition

Electrolysis condition Chemicals Content

Cr₂(SO₄)₃·nH₂O 140 g/l Temperature 30^oC

HCOOK 1 M pH 2

KCl, NH₄Cl 1 M, respectively

Current

density 20 A/dm² H₃BO₃ 0.65 M Anode graphite NH₄Br 10 g/l Cathode low carbon

steel polyethylene

glycol 2 g/l Agitation air bubbling

Microcracks propagated across the coating surface consisting of nodular grains (Fig. 2(a)). The scale was hardly recognizable owing to good corrosion resistance of the coating (Fig. 2(b)). In Fig. 2(c), no oxides were detected, and the diffuse amorphous pattern changed to the crystalline Cr pattern. In order to identify the thin scale that formed on the surface, XPS analysis was performed, as shown in Fig. 2(d).

The XPS-analyzed composition of the thin scale was 54.7O-17.8Cr-16.7C-9.2Fe-1.3S-0.3N in at%. Here, the carbon concentration is particularly suspicious, because carbon signal can come out as a background noise. Sulfur and nitrogen that originated from N₂/ 0.1%H₂S-mixed gas were apparently dissolved in the chromia scale, together with Fe, to a small amount.

Iron diffused out from the substrate across the coating.

The Cr_2P3/2 peak position was at 576.5 eV,

corresponding to Cr₂O₃. The C_1S peak binding energy (Eb = 284.7 eV) corresponded to carbon. The O_1s spectrum had a peak (Eb = 530.3 eV), corresponding to metal oxides. The Fe_2P3/2 peak value corresponded to Fe₂O₃ (Eb = 710.9 eV). The N_1S peak binding energy (Eb = 399.9 eV) shifted to a higher value than nitrides (Eb= 396.4-398.3 eV), probably to coexistence of oxygen and sulfur at the surface. The S_1S peak binding energy (Eb= 168.7 eV) corresponded to sulfate (SO₄²⁻).

The corrosion at 600^oC for 5 h formed a superficial scale on the Cr(III) coating (Fig. 3(a)). Here, microcracks were partially sealed. Figures 3(b-c) indicates that iron diffused outwardly along microcracks that were an easy diffusion path. This is responsible for the partial sealing of microcracks with oxides of iron and chromium. The oxygen source for this oxidation reaction was impurity oxygen in the employed N₂/0.1%H₂S gas. The Fig. 2. Cr(III) coating after corrosion at 500^oC for 5 h in

N₂/0.1%H₂S gas. (a) SEM top view, (b) SEM cross- sectional image, (c) XRD pattern, (d) XPS spectra of Cr_2P, C_1S, O_1S, Fe_2P, N_1S, and S_2P.

Fig. 3. Cr(III) coating after corrosion at 600^oC for 5 h in N₂/0.1%H₂S gas. (a) SEM top view, (b) SEM cross- sectional image, (c) EDS line profiles along A-B shown in (b).

carbon peak at the front of the carbon line profile shown in Fig. 3(c) came from carbon in the epoxy mount.

When the Cr(III) coating was corroded further at 700^oC for 5 h, microcracks were partially sealed, and a thin scale covered the surface along the contours of the coating surface (Fig. 4(a)). A thin scale was shown in Fig. 4(b), indicating that the coating still had good corrosion resistance despite of microcracks in the coating. The scale consisted of slowly growing Cr₂O₃ (Fig. 4(c)). Here, neither sulfides nor nitrides, which could form owing to N₂/0.1%H₂S gas, were detected because of their thermodynamic nobility compared to the corresponding oxides.

The scale consisting primarily of Cr₂O₃ grew thicker as the corrosion progressed, leading to detection of strong Cr₂O₃ peaks (Fig. 5(a)). Here, there was a faint indication of FeCr₂O₄ peaks, implying that Cr₂O₃ reacted with FeO to a small extent. In Fig. 5(b), microcracks were sealed further.

The scale was ~15μm-thick (Fig. 5(c)). Sulfur dissolved in Cr₂O₃. Its amount was not large enough to form any sulfides as shown in Fig. 5(a), because

the solubility of sulfur in Cr₂O₃ was limited [16,17].

Chromia grew by the outward diffusion of Cr³⁺ ions [18], which led to formation of Kirkendall voids beneath the scale (Fig. 5(b)). Oxygen and sulfur could diffuse easily along voids and along microcracks (Fig. 5(c)). Voids and mechanical weakness arisen by the dissolution of sulfur and oxygen were responsible for formation of cavernous area below the scale (Fig. 5(b)). A close look at the carbon map shown in Fig. 5(c) indicates the presence of carbon in the uncorroded coating. The Fe map shown in Fig. 5(c) indicates the outward diffusion of Fe especially along microcracks and toward voids.

The crack healing effect was similarly reported when Cr(III) coating was oxynitrocarburized and steam- oxidized in order to form Fe₃O₄, Fe₂O₃, and/or Fe₄N [4]. Nitrogen in N₂/0.1%H₂S-mixed gas could not penetrate the scale due mainly to its negligible solubility in Cr₂O₃.

When the Cr(III) coating was corroded at 900^oC for 5 h, the scale consisted of Cr₂O₃ as the major phase and FeCr₂O₄ as the minor one (Fig. 6(a)).

Microcracks were almost completely sealed off (Fig.

6(b)). The EPMA maps shown in Fig. 6(c) indicates Fig. 4. Cr(III) coating after corrosion at 700^oC for 5 h in

N₂/0.1%H₂S gas. (a) SEM top view, (b) SEM cross- sectional image, (c) XRD pattern.

Fig. 5. Cr(III) coating after corrosion at 800^oC for 5 h in N₂/0.1%H₂S gas. (a) XRD pattern, (b) EPMA cross- sectional image, (c) EPMA maps of (b).

that FeCr₂O₄ formed beneath the Cr₂O₃ scale, apparently suggesting that the outward migration of Fe was blocked by the Cr₂O₃ scale. Sulfur diffused inwardly to dissolve in the bi-layered scale with a thickness of ~30 µm. Oxygen also diffused inwardly, but to a large amount particularly along microcracks.

The inner FeCr₂O₄-containing scale was mechanically weak, because it formed around pre-formed voids that were outlined in Fig. 5(c). Additional factors that deteriorate the soundness of the matrix were anisotropic volume expansion owing to simultaneous formation of Cr₂O₃ and FeCr₂O₄, stress that accumulated owing to thickening of the scale, and hydrogen escape from H₂S gas. The carbon map shown in Fig. 6(c) indicates that there still existed carbon in the uncorroded coating.

4. Conclusions

The amorphous Cr(III) coating consisted of oxygen-dissolved nodular grains with microcracks. Its corrosion behavior in N₂/0.1%H₂S gas for 5 h was studied. At 500^oC, the coating crystallized to Cr. At 600^oC, a superficial oxide scale formed. Microcracks

began to be filled with oxides of Cr and Fe. At 700^oC, a thin Cr₂O₃ scale formed owing to thermodynamic stability of oxides compared to the corresponding sulfides or nitrides. When corroded at 800^oC, a Cr₂O₃ scale formed through the outward diffusion of Cr, which formed voids underneath the Cr₂O₃ scale.

When corroded at 900^oC, an outer Cr₂O₃ layer and inner (Cr₂O₃, FeCr₂O₄)-mixed layer formed. The increment of corrosion temperature resulted in the thickening of the scale, the progressive sealing of microcracks, and the enhancement of outward migration of Cr and Fe as well as the inward transport of oxygen and sulfur.

Acknowledgement

This research 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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