A new method to determine the synergistic effects of area ratio and microstructure on the galvanic corrosion of LAS A508/309 L/308 L SS dissimilar metals weld

doi:10.1016/j.jmst.2020.10.044

Abstract

Abstract:

The synergistic effects of area ratio and microstructure on the galvanic corrosion of A508/309 L/308 L dissimilar metals weld (DMW) are studied by a multi-analytical approach. It was demonstrated that decreasing the anode/cathode surface area ratio obviously enhances the corrosion rate of A508, both locally and globally. Deeper analyses of the AFM results enabled quantitative comparison of the corrosion behaviour of the different surface constituents. It was revealed that in the galvanic interaction of the DMW, the grain refined region corrodes most, followed by the partial grain refined region and base metal matrix of the A508, respectively. The electrochemical localization index (LI) estimation method and AFM analysis both confirmed the presence of a mixed (localized and uniform) corrosion phenomenon occurring on the surface of the A508 anode metal in the galvanic interaction of the dissimilar metals. Finally, the degree of synergism equation was utilized to describe the synergistic effects of anode/cathode area ratio and the microstructure of the samples on the galvanic corrosion of LAS A508/309L/308L SS DMW.

Key words: Low alloy steel, Stainless steel, SEM, AFM, Galvanic corrosion, Inclusion

Bright O. Okonkwo, Hongliang Ming, Jianqiu Wang, En-Hou Han, Ehsan Rahimi, Ali Davoodi, Saman Hosseinpour. A new method to determine the synergistic effects of area ratio and microstructure on the galvanic corrosion of LAS A508/309 L/308 L SS dissimilar metals weld[J]. J. Mater. Sci. Technol., 2021, 78: 38-50.

Figures/Tables 17

Fig. 1. Schematic of the LAS A508/309 L/308 L SS dissimilar metal weld.

Table 1 Chemical composition of the base metal and the weld metals (wt.%) used in this study.

Alloy	Cr	Ni	Mn	Si	C	Mo	Cu	S	Nb	Fe
LAS A508	0.14	0.83	1.39	0.23	0.194	0.490	0.039	0.0006	<0.001	Bal.
309 L	23.96	13.32	1.69	0.41	0.017	0.032	0.043	<0.001	0.001	Bal.
308 L	20.33	10.32	1.75	0.36	0.022	0.024	0.028	0.002	0.001	Bal.

Fig. 2. Schematic of the electrochemical setup.

Fig. 3. (a) OM micrograph of LAS A508/309L/308L SS dissimilar metal weld. ((b)-(d)) Higher magnification images of selected regions marked with red rectangular box in (a), revealing the microstructure transition in LAS A508 base metal: GR region (coarse martensite + little deposit of granular bainite (b)) → PGR region (bainite + little deposit of fine martensite (c)) → base metal matrix (bainite (d)).

Fig. 4. The potentiodynamic polarization plots of LAS A508 and 309 L/308 L SS dissimilar metals.

Fig. 5. (a) Galvanic current density vs. time curves for anode/cathode area ratios 2:1, 1:1, 1:2 and 1:4. (b) Galvanic potential vs. time curves for anode/cathode area ratios 2:1, 1:1, 1:2 and 1:4 and time dependent curves of the open circuit potentials for LAS A508 and 309L/308L SS uncoupled.

Table 2 Data extracted from ZRA measurements in Fig. 5.

Area ratio (anode/ cathode (cm²))	ZRA measured galvanic current density (μA/cm²)	ZRA measured galvanic potential (V)
2:1	8.06 ± 1	-0.598
1:1	11.32 ± 1	-0.607
1:2	13.17 ± 1	-0.608
1:4	19.59 ± 2	-0.550

Table 3 Localization index estimated data obtained from ZRA generated galvanic current density.

Area ratio (anode/ cathode (cm²))	σi(μA/cm²)	irms(μA/cm²)	LI
2:1	0.845	8.87	0.09
1:1	0.298	9.80	0.03
1:2	1.035	13.21	0.07
1:4	0.573	11.33	0.05

Fig. 6. AFM topography images of (a) base metal, (b) partial grain refined, and (c) grain refined regions along with the root mean square value of the surface roughness, which is calculated from AFM images. Images have been captured after 6 h immersion to PWR primary water simulated solution with sulphate impurity at 25 °C. The corresponding roughness RMS values are provided for each region.

Fig. 7. (a) Histogram and (b) PSD analysis of AFM images in Fig. 6. Histogram and PSD profiles have been provided based on multimodal Gaussian distribution and FFT analysis, respectively.

Fig. 8. AFM topography images of low alloy steel A508 (anode)/309 L welded (cathode) interface before (a), and after 6 h immersion in PWR primary water simulated solution with sulphate impurity at 25 °C with 96 ppm of sulphate at different anode/cathode surface area ratio of (b) 2:1, (c)1:1, (d) 1:2, and (e) 1:4.

Fig. 9. The multi-modal Gaussian histograms distribution analysis of the AFM results (Fig. 8) disentangling the contribution of each surface constituents in overall corrosion of the low alloy steel A508 /309 L welded interface before immersion (a) and after 6 h immersion in PWR primary water simulated solution with sulphate impurity at 25 °C containing 96 ppm of sulphate at different anode/cathode surface area ratio of (b) 2:1, (c) 1:1, (d) 1:2, and (e) 1:4.

Table 4 Gaussian distribution parameters extracted from the topography histograms in Fig. 8. Average corrosion depths and corrosion rates are calculated from mean value of topography along with Faraday law for major corroded part of the heterogeneous surface.

Anode/cathode ratio	Constituents	Z_phase value (nm)	Standard deviation of topography (nm)	Estimated surface area (10^-6 cm²)	Average corrosion depth Δz = z_base- z_phase (nm)	Corrosion rate (μA cm^-2)
2:1 Before corrosion test	A508	80	5.1
	309 L SS	55	12.3
2:1 After corrosion test	Inclusion	107.8	81.4	6.4	(347.6-107.8=) 239.8	25.2 ± 3
	Martensite	193.7	71.7	53.2	(347.6-193.7=) 153.9	16.15
	Bainite	264.5	55.7	48.6	(347.6-244.5=) 83.1	8.73
	309 L SS	347.6	73.3
1:1	Inclusion	168.2	62.3	4.32	(457.1-168.2=) 288.9	30.4 ± 2
	Martensite	250.5	79.5	62.6	(457.1-250.5=) 206.6	21.62
	Bainite	338.3	131.2	41.1	(457.1-338.3=) 118.8	12.38
	309 L SS	457.1	60.5
1:2	Inclusion	122.3	70.6	7.5	(571.4-122.3=) 449.1	48.6 ± 4
	Martensite	325.4	194.2	57.24	(571.4-325.4=) 245.6	25.71
	Bainite	376.2	74.3	43.2	(571.4-376.2=) 195.2	20.26
	309 L SS	571.4	95.1
1:4	Inclusion	190.5	80.4	8.64	(723.6-190.5=) 533.1	59.6 ± 4
	Martensite	314.5	102.3	50.7	(723.6-314.5=) 409.1	42.92
	Bainite	517.9	175.2	48.6	(723.6-517.9=) 205.7	22.2154
	309 L SS	723.6	98.3

Fig. 10. Calculated current density of most deteriorated surface constituents at welded interface (extracted from Fig. 9 and Table 4) for samples with different anode/cathode area ratios.

Table 5 Amount of synergism (S) and corrosion augmentation factor values calculated from the corrosion rates of individual surface constituents (shown in Table 4).

Phase constiutents	Area ratios (cm²)	T_a+m (μA cm^-2)	C_m (μA cm ^-2)	S (μA cm^-2)	Corrosion Augumentation Factor
	1:1 to 2:1	8.73	12.38	-3.65	0.71
Bainite	1:1 to 1:2	20.26	12.38	7.88	1.64
	1:1 to 1:4	22.22	12.38	9.84	1.79
	1:1 to 2:1	16.15	21.62	-5.47	0.75
Martensite	1:1 to 1:2	25.71	21.62	4.09	1.19
	1:1 to 1:4	42.92	21.62	21.30	1.99
	1:1 to 2:1	25.20	30.40	-5.20	0.83
MnS inclusion	1:1 to 1:2	48.60	30.40	18.20	1.59
	1:1 to 1:4	59.60	30.40	29.20	1.96

Fig. 11. The PSD profiles description of the AFM results for samples with different anode/cathode ratios (Fig. 8). Four individual contributions can be observed, appearing as the change in the slope of PSD profiles at distinct spatial frequency regions. These contributions correlate to different surface constituents in A508 /309 L welded interface (i.e. MnS inclusion, martensite phase, bainite phase, and 309 L SS base metal).

Fig. 12. The SEM micrograph of AFM test sample for area ratio 1:1 after immersion in a corrosive medium. The chemical composition of the marked pit with a yellow cross is provided at the right side.

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