[2011] Effect of Hydrogen in Inconel Alloy 600 on Corrosion in High Temperature Oxygenated Water

8/3/2019 [2011] Effect of Hydrogen in Inconel Alloy 600 on Corrosion in High Temperature Oxygenated Water

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Short Communication

Effect of hydrogen in Inconel Alloy 600 on corrosion in high temperature

oxygenated water

J. Hou a,b, Q.J. Peng a,*, K. Sakaguchi a, Y. Takeda a, J. Kuniya a, T. Shoji a

a Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-01, Aramaki Aoba, Aoba-ku, Sendai 980-8579, Japanb State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China

a r t i c l e i n f o

Article history:

Received 8 October 2009

Accepted 25 November 2009

Available online 3 December 2009

Keywords:

A. Inconel Alloy 600

B. XPS

B. TEM

C. Hydrogen enhanced corrosion

C. Corrosion in high temperature water

C. Oxide film

a b s t r a c t

Corrosion test on hydrogen charged and uncharged coupons of Inconel Alloy 600 in high temperature

oxygenated water showed more weight loss of charged coupon. Observation of the oxide film by trans-

mission electron microscopy (TEM) showed a defective, thicker oxide layer on charged coupon. Analyses

of the oxide film by TEM-energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy

indicated enrichment of Ni but depletion of Cr in the oxide film on charged coupon. The changes in cor-

rosion behavior and microstructure of the oxide film were most likely due to the hydrogen enhanced

preferential dissolution of Cr cations in the water.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

The corrosion of structural alloys in light water reactors has

been one of the main concerns to the materials degradation man-

agement in nuclear power plants. While hydrogen dissolved in

water and from metal oxidation in water can dissolve into the me-

tal, little attentions have been paid to the role of hydrogen in metal

in the corrosion in high temperature water. Most efforts relating to

hydrogen effects have been focused on a mechanism of hydrogen

facilitated cracking [13]. Since an amount of hydrogen is expected

to enter and accumulate in the metal during a long-term operation

of light water reactors, it is necessary to understand the role of

hydrogen in metal in the corrosion behavior in high temperature

water.

Dissolved hydrogen in metals has shown to increase the anodic

dissolution of an austenitic stainless steel and chromium in chlo-ride and sulfate containing solution at room temperature [4,5],

and the oxidation rate of iron, chromium and chromium ferritic

steels in oxygen or steam at high temperatures [69]. It has been

suggested that hydrogen could decrease the film stability [813].

While these works have been done, to date the effect of hydrogen

in metal on the corrosion behavior in high temperature water had

remained unknown. Further, direct observation and analysis of the

oxide film are required to clarify the effect of hydrogen on the film

microstructure.

In the present work, the effect of hydrogen in Inconel Alloy 600

on corrosion in high temperature oxygenated water was studied by

employing an exposure test, X-ray photoelectron spectroscopy

(XPS), scanning electron microscopy (SEM) and transmission elec-

tron microscopy (TEM) analyses in an effort to develop the under-

standing of the role of hydrogen in metal in the corrosion behavior

in high temperature water. The Alloy 600 was selected for the

study because it has been used extensively in light water reactors

as structural materials.

2. Experimental method

The material used for the experiment is mill-annealed Alloy 600

with a chemical composition (wt.%): 0.07 C, 0.37 Mn, 9.46 Fe, 0.34

Si, 0.20 Cu, 15.41 Cr and 74.15 Ni. The corrosion test employed cou-

pons of the alloy with the size of 30 mm 20 mm 0.5 mm,

which were ground using emery papers up to 4000 grit. Hydrogen

was charged into the coupon cathodically with a current density of

1 mA/cm2 in sulfuric acid solution at 50 C and pH 3.03.5 [2]. A

charging time of 1680 h was used to obtain a high and uniform

hydrogen concentration in the coupon. The time needed to achieve

a near-uniform hydrogen concentration is determined when Dt/

L2 > 1.5 [14], where D: diffusivity (cm2/s), t: time and L: half thick-

ness of the sample. Suppose the hydrogen diffusivity in Alloy 600

at 50 C is 1.7 1010 cm2/s [15], the time needed is about 1560 h.

Following hydrogen charging, the coupon was ground again

using the emery papers of 1500 and 4000 grits, in order to remove

any surface damage by the cathodic charging. Then the charged

0010-938X/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2009.11.037

* Corresponding author. Tel.: +81 22 7957520; fax: +81 22 7957543.

E-mail address: [email protected] (Q.J. Peng).

Corrosion Science 52 (2010) 10981101

Contents lists available at ScienceDirect

Corrosion Science

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http://dx.doi.org/10.1016/j.corsci.2009.11.037mailto:[email protected]://www.sciencedirect.com/science/journal/0010938Xhttp://www.elsevier.com/locate/corscihttp://www.elsevier.com/locate/corscihttp://www.sciencedirect.com/science/journal/0010938Xmailto:[email protected]://dx.doi.org/10.1016/j.corsci.2009.11.037


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coupon and an uncharged coupon were weighted, and hung into a

refreshed autoclave equipped with a recirculation loop. The auto-

clave loop was then filled with high-purity water, pressurized to

8.5 MPa and heated to 288 C. Dissolved oxygen (DO) in the inlet

water was kept at 8 ppm by purging the mixture of N2 and O2 into

the water tank of the loop. Water purity was controlled by using

ultra-high-purity water at the inlet and by purifying the outlet

water using ion-exchangers. During the exposure, the water con-

ductivity at inlet was 0.06 ls/cm and < 0.2 ls/cm at outlet.

The coupons were exposed in the water for a period of 100 h.

Post-test analyses conducted include weigh gain measurements

and microstructural and compositional analyses of the oxide using

XPS, SEM, TEM and TEM-energy dispersive X-ray spectroscopy

(EDX). The XPS analysis was done by sequential sputtering of the

oxide scale with a rate of 4.2 nm/min using a 2 kV argon ion beam

rastering an area of 2 mm diameter. Samples for the TEM analysis

were prepared by focus ion beam technique after coated the oxide

by carbon and platinum.

3. Results and discussion

Both charged and uncharged coupons showed weight loss fol-

lowing the exposure, Fig. 1. However, the charged coupon has a

weight loss of 1.7 times of the uncharged coupon, indicating the

enhanced dissolution of Alloy 600 in high temperature oxygenated

water by hydrogen. Depth profile analysis of the oxide by XPS

showed a thicker film on the charged coupon, Fig. 2(a) and (b).

The thickness of the oxide film estimated by the half height of oxy-

gen is about 70 nm on charged coupon and 40 nm on uncharged

coupon. Further, the figures also show that the oxide film on

charged coupon has a higher Ni concentration but lower Cr concen-

tration throughout the thickness of the film than the oxide film on

uncharged coupon, indicating that hydrogen increased Ni concen-

tration but decreased Cr concentration in the film. The maximum

concentrations of Ni and Cr in the film on the charged coupon is

32 at.% and 17.5 at.%, respectively, which changed to 23.5 at.%

and 23 at.% in the film on the uncharged coupon. The concentration

of Fe, however, did not show difference in the two oxide films. The

difference in the corrosion behavior and concentrations of Ni and

Cr in the oxide film are most likely indications that hydrogen influ-

ences the cation transport behavior in the oxide, which will be dis-

cussed later.

Surface morphologies of the oxide films were observed by SEM,

Fig. 3. An amount of white, irregular networks of oxide was ob-

served on the uniform oxide layer on the charged coupon

(Fig. 3a), but few on uncharged coupon (Fig. 3b). Cross-sections

of the oxide films were analyzed by TEM. On the charged coupon,

a layer of loose, defective oxide was observed with a thickness of

about 30100 nm, Fig. 4(a). Scattered, needle- and pyramid-like

oxides were observed on this layer, which should correspond to

the white oxides on the outmost surface shown in Fig. 3(a). It is

worthwhile to note that the oxide layer has a high density of

defectiveness, suggesting it is unprotective. The diffraction pattern

reveals that the defective layer is a spinel-type oxide, shown in

Fig. 4(a) as an inset. The oxide scale on the uncharged coupon

has a typical double-layer structure that consists of an inner

thin, compact layer and an outer thick, continuous layer of

needle- and pyramid-like oxides with a total thickness of about

2050 nm, Fig. 4(b). The numbered points 1 and 2 in both

Fig. 4(a) and (b) designate the locations for the TEMEDX analyses

of the oxides in inner and outer layers. The concentrations of Ni, Fe,

Cr, O and the ratio of the concentration of Ni to Cr obtained from

the analyses are listed in Table 1. Concentrations of Ni and Cr aswell as their ratio clearly indicate the enrichment of Ni but deple-

tion of Cr in the oxide on the charged coupon. Further, it is noted

that the enrichment of Ni is more in the outer layer than that in

the inner layer. The depletion of Cr, however, is more in the inner

layer than that in the outer layer.

It should be mentioned that while the element concentrations

shown in Fig. 2 and Table 1 follow similar dependence on dissolved

hydrogen, there are also discrepancies in the film thickness and

element concentration between Fig. 2 and Table 1. A main cause

for the discrepancy may lie in the large area of the XPS analysis

0 5 10 15 20 25 30 35

0

10

20

30

40

50

60

70

80

at%

Sputtering time (min)

Ni

Cr as oxideCr as metalFeO

oxide

0 5 10 15 20 25 30 35

0

10

20

30

40

50

60

70

80

at%

Sputtering time (min)

Ni

Cr as oxideCr as metalFeO

oxide

ba

Fig. 2. Depth profile of the oxide film on hydrogen charged coupon (a) and uncharged coupon (b) of Alloy 600 analyzed by XPS.

-0.04

-0.03

-0.02

-0.01

0.00

Unch

argedcoupon

Weightgain(g/cm

2)

Chargedcoup

on

Fig. 1. Weight gain of the hydrogen charged and uncharged coupons of Alloy 600

after the exposure in 288 C, oxygenated water.

J. Hou et al. / Corrosion Science 52 (2010) 10981101 1099


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(2 mm in diameter) that incorporated scatters in the concentration

generated by the unevenness of the surface, the interface between

the inner and outer layers of the oxide film, and the interface be-

tween the oxide film and the substrate. In addition, certain errors

may have been generated by the EDX analysis of the light element

(oxygen).

The results of the experimental works described above showed

that both the corrosion behavior and the microstructure of the

oxide film were affected by hydrogen dissolved in the alloy. In

addition to the higher weight loss, hydrogen also resulted in a

thicker, defective inner layer and a discontinuous outer layer of

the oxide, in conjunction with the enrichment of Ni and depletion

of Cr. One possible cause for the enhanced corrosion by hydrogen is

the interactions of hydrogen with defects in the alloy and with thelattice of the alloy [4]. Defects in the alloy trapped hydrogen can

become to the active sites. On the other hand, hydrogen may de-

crease the interatomic cohesion, increase the activity and conse-

quently the corrosion propensity. The simulation of quantum

chemical molecular dynamics on the interaction of hydrogen with

metal also suggested that hydrogen diffusing into the metal could

be negatively charged, which weakened metal atomic bond

strength and accelerated the oxidation [16,17].

While the interactions of hydrogen with the defects and lattice

of the alloy can generally interpret the enhanced corrosion by

hydrogen, the change in the microstructure and composition of

the oxide film by hydrogen can not be clarified by this mechanism

since details of the interactions remain unknown. The most possi-

ble cause for the effect of hydrogen is the hydrogen enhanced pref-

erential dissolution of Cr cations in the water. Hydrogen in metals

dissolves in the oxide film as interstitial protons [5,9]. The protonsat the metaloxide interface or in the oxide film are commonly

bonded to oxygen ions, forming substitutional hydroxide point de-

fects, which are finally compensated by the metal vacancies in the

film [5,9,1820]. Therefore, the increase of dissolved interstitial

protons in the film resulted from the diffusion of hydrogen in the

metal to the film can increase the concentration of metal vacancies

in the film. This is the cause for the defective nature of the oxide

shown in Fig. 4(a).

Although it is difficult to demonstrate directly the enhanced

transport of cations in the oxide by the presence of hydrogen-re-

lated defects, it is possible the increased concentration of metal

vacancies enhance the diffusivity of metal cations [9,1820]. In

fact, the enhanced transport of Cr cations in chromia due to the

presence of dissolved hydrogen has been reported [9]. Dependingon the solubility of the metal cations in the water, the enhanced

Fig. 3. SEM observation of the surface morphology of the oxide on hydrogen charged coupon (a) and uncharged coupon (b) of Alloy 600.

Table 1

Concentrations of Ni, Cr, Fe, O and the ratio of the concentration of Ni to Cr in the

oxide films on hydrogen charged and uncharged coupons of Alloy 600 analyzed by

TEMEDX.

at.%

Ni Cr Fe O Ni/Cr

Charged 1 (inner layer) 19.1 10 6.2 64.6 1.9

2 (outer layer) 16.5 7.4 5.9 70.2 2.3

Uncharged 1 (inner layer) 18.0 14.3 4.7 63.0 1.3

2 (outer layer) 11.6 9. 9 6.1 72.8 1.2

Fig. 4. TEM observation of the cross section of the oxide film on hydrogen charged coupon (a) and uncharged coupon (b) of Alloy 600.

1100 J. Hou et al. / Corrosion Science 52 (2010) 10981101


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cation transport will result in the difference in the cation concen-

tration in the oxide. At high electrochemical corrosion potential

associated with oxidizing species, the solubility of Cr is higher than

that of Fe and Ni since Cr tends to go into solution as chromate

(CrO42), and hence, the Cr-deficient oxide is expected to be

formed by the preferential dissolution of Cr cation [21]:

Ni;Fe

Cr

2

O4

3

2

O2H2O

!Ni

2;Fe

2 2CrO

2

4 2H

1

It is therefore can be inferred that the enhanced cation trans-

port in the film will increase the concentration of Ni due to its rel-

atively lowsolubility in the oxygenated water but will promote the

dissolution of Cr cations in the outer layer of the oxide film. In fact

the results shown in Table 1 support the hypothesis on the hydro-

gen enhanced transport of cations. Suppose there are enhanced

transports of the cation in the film, it can be easily inferred that

the enrichment of Ni cation would be more in the outer layer but

the depletion of Cr cation will be more in the inner layer since

the transport direction of the cation is from the inner layer to the

outer layer. This is in consistence with the results of EDX analyses

of the inner and outer layers shown in Table 1. As for Fe, its concen-

tration showed little difference in the oxide film on the charged

and uncharged coupons. This likely implies that the transport ofFe cation is not dominant in the oxide due to the low Fe concentra-

tion in the alloy. However, the exact mechanism is not fully

understood.

The preferential dissolution of Cr cations in high temperature

oxygenated water also suggests it dominates the enhanced disso-

lution of Alloy 600. On the other hand, the enhanced transport of

Ni cation may promote the film growth due to their low solubility

and result in a thicker oxide film on the charged coupon.

As mentioned previously, there are difficulties in obtaining di-

rect proof of the hydrogen enhanced cation transport in the oxide

film. Further mechanistic study of the hydrogen enhanced corro-

sion of nickel-base alloys in high temperature water is required.

4. Conclusions

1. Exposure of hydrogen charged and uncharged coupons of Alloy

600 in288 C, oxygenated water showed that hydrogen in Alloy

600 enhanced the dissolution of the alloy.

2. The defective nature of the oxide film formed on the hydrogen

charged coupon of Alloy 600 was shown directly by TEM

observation.

3. Analyses of the oxide by XPS and TEM indicated a thicker oxide

film on hydrogen charged coupon of Alloy 600. The enrichment

of Ni and depletion of Cr in the film on the hydrogen charged

coupon were also revealed.

4. The enhanced corrosion of Alloy 600 by hydrogen is most likely

the result of enhanced preferential dissolution of Cr cations in

high temperature oxygenated water due to the hydrogen

enhanced cation transport in the oxide film.

Acknowledgements

This researchhas been supportedby a joint researchprogram on

Prediction and Evaluation of Environmentally Assisted Cracking in

LWR Structural Materials (PEACE-E program). The authors would

thank Prof. T. Miyazaki for his support to the TEM analysis. One

author (J. Hou) would also thank the support of the Special Funds

for the Major State Basic Research Projects G2006CB605000 in

China.

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[2011] Effect of Hydrogen in Inconel Alloy 600 on Corrosion in High Temperature Oxygenated Water