Corrosion performance of atmospheric plasma sprayed alumina coatings on AZ31B magnesium alloy under immersion environment

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Figures & data

Table 1 Microstructure observation for fixing the working range of parameters.

SI no.	Parameters	Parameter range	Microstructure	Name of the defect
1	Power	<18 kW		Coating delamination
2	Power	>18 kW		Poor deposition efficiency
3	Stand-off distance	<10 cm		Peeling-off of the coating
4	Stand-off distance	>13 cm		Poor coating adhesion

Fig. 1 Experimental details.

Table 2 Important APS process parameters and their levels.

Factors	Notations	Units	Levels
			−1.682	−1	0	+1	1.682
Power	P	kW	18	19.4	21.5	23.6	25
Stand-off distance	S	cm	10	10.6	11.5	12.4	13
Powder feed rate	F	gpm	15	20	25	30	35

Table 3 Design matrix and experimental results.

Spray condition	Coded values			Original value			Porosity (vol%)	Corrosion rate (mm/year)
	P	S	F	P (kW)	S (cm)	F (gpm)
1	−1	−1	−1	19.4	10.6	20	12	15.9172
2	1	−1	−1	23.6	10.6	20	7	7.92
3	−1	1	−1	19.4	12.4	20	14	14.3527
4	1	1	−1	23.6	12.4	20	6	9.9182
5	−1	−1	1	19.4	10.6	30	10	11.2399
6	1	−1	1	23.6	10.6	30	9	8.902
7	−1	1	1	19.4	12.4	30	18	15.8024
8	1	1	1	23.6	12.4	30	13	15.568
9	−1.682	0	0	18	11.5	25	14	19.02
10	1.682	0	0	25	11.5	25	5	12.9808
11	0	−1.682	0	21.5	10	25	9	9.8012
12	0	1.682	0	21.5	13	25	15	15.344
13	0	0	−1.682	21.5	11.5	15	8	6.122
14	0	0	1.682	21.5	11.5	35	12	6.8973
15	0	0	0	21.5	11.5	25	5	5.0241
16	0	0	0	21.5	11.5	25	6	4.9898
17	0	0	0	21.5	11.5	25	5	5.5344
18	0	0	0	21.5	11.5	25	6	4.892
19	0	0	0	21.5	11.5	25	5	5.9254
20	0	0	0	21.5	11.5	25	5	4.9994

Fig. 2 Measure the porosity level using computer image analyzing software.

Table 4 Results summary of porosity analysis.

Total area measured			0.0432 mm^2
Porosity	8.6%	Pores/mm^2	2796.8
Classes	Area %	Pores/mm^2	Type
0 > 10 μm	1.8	2098.69	A08
10 > 25 μm	3.9	642.52	B08
25 > 75 μm	3.5	49.87	–
75 > 125 μm	0	0	–
>125 μm	0	0	–

Fig. 3 (a) SEM image of alumina coating, (b and c) high magnification images of pore and crack and (d) cross-section of the alumina coated specimen after immersion in NaCl solution.

Fig. 4 Optical micrograph of the Al₂O₃ powder.

Fig. 5 SEM surface morphology of alumina coating.

Fig. 6 Optical microstructure and SEM image of the coating.

Fig. 7 EDAX analysis.

Table 5 ANOVA test results.

Source	Sum of squares	df	Mean square	F value	p-Value Prob > F
Model	404.847	9	44.983	249.2319	<0.0001	Significant
P	46.35474	1	46.35474	256.8322	<0.0001
S	32.24242	1	32.24242	178.6417	<0.0001
F	1.623079	1	1.623079	8.992797	0.0134
PS	4.013228	1	4.013228	22.23561	0.0008
PF	12.15097	1	12.15097	67.32342	<0.0001
SF	14.56596	1	14.56596	80.70388	<0.0001
P^2	217.629	1	217.629	1205.791	<0.0001
S^2	103.0545	1	103.0545	570.9817	<0.0001
F^2	4.056546	1	4.056546	22.47561	0.0008
Residual	1.804865	10	0.180487
Lack of Fit	0.961151	5	0.19223	1.139189	0.4449	Not significant
Pure error	0.843715	5	0.168743
Cor. total	406.6519	19
Std. dev.	0.424837	R-squared		0.995562
Mean	10.05754	Adj R-squared		0.991567
CV%	4.224065	Pred R-squared		0.978082
PRESS	8.912924	Adeq precision		46.90273

df: degrees of freedom; CV: coefficient of variation; F: Fisher ratio; p: probability.

Fig. 8 Normal probability plot for the response.

Fig. 9 Correlation plot for the response.

Fig. 10 Effect of power and stand-off distance on corrosion rate.

Fig. 11 Effect of powder feed rate and power on corrosion rate.

Fig. 12 Effect of APS process parameters on porosity level.

Fig. 13 Effect of APS process parameters on corrosion rate.

Fig. 14 (a) Cross section of as-sprayed alumina coating on AZ31B magnesium alloy after immersion in NaCl solution for 6 h (b and c) EDX and XRD pattern analysis.

Fig. 15 SEM micrographs (a) and X-ray diffraction analysis (b) of the corroded surface after 6 h exposure of coatings.

Fig. 16 (a) SEM micrograph showing corrosion product in the cross section of as-coated coating exposed for 6 h and (b) EDS analysis.

Fig. 17 Relationship graph for porosity and corrosion rate.