Corrosion of New Zirconium Claddings in 500 °C/10.3 MPa Steam: Effects of Alloying and Metallography

doi:10.1007/s40195-018-0857-7

Abstract

Abstract:

With the aim of improving corrosion resistance of rod cladding for in-service and accident conditions, six new zirconium alloys (named N1-N6) have been designed. The contents of Sn and Nb were optimized for better behavior at high-temperature pressurized water, and Fe, Cr, V, Cu or Mo elements were added to the alloys to adjust the corrosion behavior. The current work focused on the rapid corrosion behavior in 500 °C/10.3 MPa steam for up to 1960 h, aiming to test the corrosion resistance at high temperature. The structure of matrix and properties of second-phase particles (SPPs) were characterized to find the main differences among these alloys. All the six alloys exhibited better corrosion resistance than N36, and N1 was shown to have the best performance. A careful analysis of the corrosion kinetics curves revealed that Cr was beneficial for severe condition. Elements Fe, Cr, V, Cu or Mo aggregated into SPPs with different concentrations and structures. This was demonstrated to be the main reason for different corrosion resistance. Due to good processing control, all alloys had a uniform structure and a uniform distribution of SPPs. As for N4, N6 and N36, the existing of large-size SPPs (450 nm) might be a contributing factor of the relatively poor corrosion resistance.

Key words: New zirconium cladding, Corrosion behavior, Alloying elements, Metallography, Second-phase particles

Jing-Jing Liao, Zhong-Bo Yang, Shao-Yu Qiu, Qian Peng, Zheng-Cao Li, Ming-Sheng Zhou, Hong Liu. Corrosion of New Zirconium Claddings in 500 °C/10.3 MPa Steam: Effects of Alloying and Metallography[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(8): 981-994.

Figures/Tables 13

Table 1 Characteristic chemical composition of the tubes used in this paper

Name	Sn (wt%)	Nb	Fe	Cr	V	Cu	Mo
N1	0.2-0.5	0.1-0.3	0.5	0.2	-	-	-
N2	0.2-0.5	0.1-0.3	0.5	-	0.2	-	-
N3	0.2-0.5	1.1-1.4	0.1	-	0.1	-	-
N4	0.2-0.5	0.4-0.7	0.3	-	-	-	0.1
N5	0.5-0.8	0.1-0.3	0.3	0.1	-	-	0.1
N6	0.5-0.8	0.1-0.3	0.3	-	-	0.1	0.1
N36	1.0	1.0	0.3	-	-	-	-

Fig. 1 Schematic sequence of production processes of the 200 kg-level zirconium alloys

Fig. 2 Weight gain profiles of the claddings corroded in 500 °C/10.3 MPa steam, a total weight gain profiles for the 7 alloys, b weight gain profiles within 300 h in the double logarithmic coordinates

Table 2 Fitting results for both pre-transition and quasi-linear period

Alloys	Pre-transition		Linear profile		Transition time (h)	Maximum ΔW (mg/dm²)
Alloys	k ₁	n	k ₂	R ²	Transition time (h)	Maximum ΔW (mg/dm²)
N1	7.69	0.402	0.27	0.9988	~?108	589.38?±?25.41
N2	8.35	0.408	0.46	0.9996	~?94	904.66?±?59.73
N3	8.35	0.438	0.45	0.9961	~?88	942.65?±?68.71
N4	9.70	0.374	0.54	0.9954	~?57	1016.28?±?65.63
N5	9.63	0.370	0.35	0.9984	~?100	731.24?±?34.27
N6	11.73	0.337	0.53	0.9986	~?43	1106.84?±?67.52
N36	10.09	0.393	0.83	0.9993	~?47	1625.55?±?53.15

Fig. 3 Grain orientation images of EBSD analysis in the axial direction

Table 3 Recrystallization fractions of all the claddings after the final annealing at 580 °C

Alloy	R (%)	D (%)	Alloy	R (%)	D (%)
N1	96.1	0.6	N2	90.6	2.6
N3	91.6	1.7	N4	98.6	0.7
N5	98.6	0.6	N6	96.7	1.3
N36	91.0	2.2

Fig. 4 Frequency distribution of grain size and average grain size

Fig. 5 Size distributions and morphologies of SPPs for N1-N6 and N36 claddings. Except for N4, N6 and N36, in which unordinary large SPPs up to 461 nm exist, the others show similar sizes and distributions

Table 4 Average SPP size, maximum SPP size, the area fraction and counted SPP numbers for N1-N6 and N36 claddings

Alloy	Average (nm)	Maximum (nm)	Area fraction	Numbers studied
N1	71	278.4	0.020	957
N2	92.6	287.3	0.019	1006
N3	89.7	266.2	0.022	1161
N4	91.8	461.5	0.022	979
N5	111.2	294.9	0.017	963
N6	68.7	448.2	0.010	963
N36	104.1	439.2	0.024	1049

Fig. 6 TEM data for the all the claddings. The bright-field images, SPP, SAED pattern and EDS results are shown. For different kinds of composition, SAED patterns were analyzed for crystal structure

Table 5 Structure calibration of all the alloys

Alloy	Stoichiometry	Structure	Crystal parameter
N1	Zr(Fe,Nb,Cr)_x	FCC	a?=?0.7271 nm
N2	Zr(Fe,Nb,V)_x	FCC	a?=?0.7441 nm
N3	β-Nb	BCC	a?=?0.3377 nm
N3	Zr(Fe,Nb,V)_x	HCP	a?=?0.5447 nm, c?=?0.8878 nm
N4	Zr(Fe,Nb,Mo)_x	FCC	a?=?1.2381 nm
N5	Zr(Fe,Nb,Cr,Mo)_x	FCC	a?=?0.7284 nm
N6	Zr(Fe,Nb,Cu,Mo)_x	FCC	a?=?1.2416 nm
N6	Zr(Fe,Nb,Cu,Mo)_x	HCP	a?=?0.5423 nm, c?=?0.8773 nm
N36	Zr(Fe,Nb)_x	HCP	a?=?0.5434 nm, c?=?0.8823 nm

Fig. 7 Fracture morphologies of alloy N1-N6 and N36. The difference between N1 and N36 corroded for different time is explicit. The differences lie on the oxide thickness and the distance between lateral crack bands

Fig. 8 Morphologies of interface undulation of alloy N1-N6 and N36. The differences lie on the oxide grain size and the dark spots

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