111

한국표면공학회지J. 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 N2/0.1%H2S 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 500-900oC for 5 h in N2/0.1%H2S-mixed gas to study the high-temperature corrosion behavior of the Cr(III) coatingin the highly corrosive H2S-environment. The coating consisted of (C, O)-supersaturated, nodular chromiumgrains with microcracks. Corrosion was dominated by oxidation owing to thermodynamic stability of oxidescompared 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 belowthe 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 30-100 μm. The bath

composition and electrolysis condition are listed in

*Corresponding Author: Shi Yuke

School of Advanced Materials Science and Engineering,Sungkyunkwan University Tel: +82-3411-290-7355 ; Fax: +82-290-7379E-mail: 5124dlee@hanmail.net

112 Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 52 (2019) 111-116

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 500-900oC 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 44o (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 400-700oC,

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 Cr2p spectrum indicates

that the presence of metallic Cr and Cr2O3. The C1s

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 O1s spectrum high.

Figure 2 shows SEM/EDS/XRD/XPS results of

Cr(III) coating after corrosion at 500oC for 5 h.

Fig. 1. Cr(III) coating. (a) SEM top view, (b) SEMcross-sectional image, (c) EDS spectra of spot 1, 2and 3, (d) XRD pattern, (e) XPS spectra of Cr2P, C1S,and O1S.

Table 1. Bath composition and electrolysis conditions

Bath compositionElectrolysis condition

Chemicals Content

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

HCOOK 1 M pH 2

KCl, NH4Cl1 M,

respectivelyCurrent density

20 A/dm2

H3BO3 0.65 M Anode graphite

NH4Br 10 g/l Cathodelow carbon

steel

polyethylene glycol

2 g/l Agitation air bubbling

Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 52 (2019) 111-116 113

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 Cr2P3/2 peak position was at 576.5 eV,

corresponding to Cr2O3. The C1S peak binding energy

(Eb = 284.7 eV) corresponded to carbon. The O1s

spectrum had a peak (Eb = 530.3 eV), corresponding to

metal oxides. The Fe2P3/2 peak value corresponded to

Fe2O3 (Eb = 710.9 eV). The N1S 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 S1S peak binding

energy (Eb = 168.7 eV) corresponded to sulfate (SO42−).

The corrosion at 600oC 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 500oC for 5 h inN2/0.1%H2S gas. (a) SEM top view, (b) SEM cross-sectional image, (c) XRD pattern, (d) XPS spectra ofCr2P, C1S, O1S, Fe2P, N1S, and S2P.

Fig. 3. Cr(III) coating after corrosion at 600oC for 5 h inN2/0.1%H2S gas. (a) SEM top view, (b) SEM cross-sectional image, (c) EDS line profiles along A-Bshown in (b).

114 Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 52 (2019) 111-116

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

700oC 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 ~15 μm-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 900oC

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 700oC for 5 h inN2/0.1%H2S gas. (a) SEM top view, (b) SEM cross-sectional image, (c) XRD pattern.

Fig. 5. Cr(III) coating after corrosion at 800oC for 5 h inN2/0.1%H2S gas. (a) XRD pattern, (b) EPMA cross-sectional image, (c) EPMA maps of (b).

Dong Bok Lee et al./J. Korean Inst. Surf. Eng. 52 (2019) 111-116 115

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 500oC, the coating crystallized to Cr. At

600oC, a superficial oxide scale formed. Microcracks

began to be filled with oxides of Cr and Fe. At 700oC,

a thin Cr2O3 scale formed owing to thermodynamic

stability of oxides compared to the corresponding

sulfides or nitrides. When corroded at 800oC, a Cr2O3

scale formed through the outward diffusion of Cr,

which formed voids underneath the Cr2O3 scale.

When corroded at 900oC, 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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