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Influence of Conversion Coating on Magnesium and Aluminum Alloys

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Influence of Conversion Coating on Magnesium and Aluminum Alloys Materials Transactions, Vol. 49, No. 5 (2008) pp. 1065 to 1070 Special Issue on Platform Science and Technology for Advanced Magnesium Alloys, IV #2008 The Japan Institute of Metals Influence of Conversion Coating on Magnesium and Aluminum Alloys by Adhesion Method Manabu Takai* and Matsufumi Takaya Department of Mechanical Science and Engineering, Chiba Institute of Technology, Narashino 275-0016, Japan Magnesium alloys are lightweight materials with high specific strength and have electromagnetic shielding characteristics. On the other hand, chromate is known as carcinogen and it leads a limited use of the conversion coating on magnesium. In recent years, adhesion technology is essential and widely used in producing electronic equipment and sporting goods from magnesium alloys. Adhesives can easily bind different materials. Adhesives may even inhibit the contact corrosion of magnesium, and we may expect adhesion to be used to effectively join different metals. This paper describes and compared with the influence of various non-chromate conversion coatings on the bonding strength of magnesium and aluminum alloys. In this study, specimens of AZ31B and A2017, A6022 were used. The adhesive cements used were epoxy and acrylic resin. The results of a pull-off test and tensile lap-shear strength test ware evaluated. The Mg(OH)2 coating on AZ31B and Boehmite on A6022 exhibited a higher tensile shear strength (16 MPa) with epoxy resin. In the acrylic resin, Mg(OH)2 coating on AZ31B and Boehmite on A6022 had a tensile shear strength of 14 MPa. It is obvious that these conversion coatings can be alternatives to chromate conversion coatings as surface treatment for adhesion of magnesium alloys and aluminum alloys. [doi:10.2320/matertrans.MC2007104] (Received October 19, 2007; Accepted February 15, 2008; Published April 9, 2008) Keywords: magnesium alloy, aluminum alloy, conversion coating, adhesion method, bond strength 1. Introduction after drying. Table 3 presents the adhesive conditions. The adhesive cements used were epoxy and acrylic resin. The Magnesium alloys are lightweight materials with high bond strength was measured by an adhesion tester. The specific strength and have electromagnetic shield character- tensile lap-shear strength was measured by a tensile test istics. On the other hand, chromate is known as carcinogen machine. Figure 1 depicts a test piece for the pull-off test (JIS and it leads a limited use of the conversion coating on K 5600-5-7); Figure 2 depicts a test piece for the tensile shear magnesium.1) In recent years, adhesion technology is strength test (JIS K 6850). The surface of the conversion essential and widely used in producing electronic equipment coatings on magnesium and aluminum alloys was observed and sporting goods from magnesium alloys.2) Adhesion can by scanning electron microscopy (SEM). bond different materials easily. Adhesion materials may even inhibit contact corrosion of magnesium, and we may expect 3. Results and Discussions adhesion to be used in the effective joining of different metals. We previously reported magnesium alloys bonding 3.1 Observation of conversion coating surfaces 3) with Mg(OH)2 conversion coating. Figure 3 presents SEM images of various conversion This paper describes and compared with the influence of coatings on AZ31B magnesium alloy. The chromate con- various non-chromate conversion coatings on the bonding version coating was scale and the Mg(OH)2 coating was a strength of magnesium alloy and aluminum alloys. thin film. Figure 4 presents SEM images of various con- version coatings on A6022 aluminum alloy. The chromate- 2. Experimental Procedure phosphate conversion coating was scale, the manganese conversion coating was cracked and the boehmite conversion The specimens used in this study were AZ31B magnesium coating was dimple. alloy, A2017 and A6022 aluminum alloys. Table 1 lists the solution composition and conditions of conversion with 3.2 Evaluation of bond strength by pull-off test4) magnesium alloy. Table 2 lists solution composition and Figure 5 illustrates the relationship between bond strength conditions with aluminum alloys. For pre-treatment of with epoxy resin and various conversion coatings on AZ31B magnesium alloy, the specimen was polished with #100 magnesium alloy and A2017 aluminum alloy and, Figure 6 emery paper and degreased with 2-propanol. Following this, illustrates the relationship between bond strength with the specimen was treated by conditions of conversion coating acrylic resin and various conversion coatings on AZ31B to show in Table 1. Aluminum alloys was immersed in magnesium alloy and A2017 aluminum alloy. The Mg(OH)2 10 mass% NaOH for 60 s at 333 K, desmutting 10 vol% coating on AZ31B magnesium alloy and boehmite on HNO3 solution for 15 s at 293 K. After pre-treatment, the A2017 aluminum alloy exhibited a higher bond strength specimen was treated by conditions of conversion coating to (10 MPa) with a 373 K cure temperature by epoxy resin. In show in Table 2. These specimens were bonded immediately the acrylic resin, Mg(OH)2 coating on AZ31B magnesium alloy and boehmite on A2017 aluminum alloy had a bond *Graduate Student, Chiba Institute of Technology strength of 8 MPa with a cure temperature of 293 K. The 1066 M. Takai and M. Takaya Table 1 Solution composition and condition with magnesium alloy. Polishing & Concentration Temperature Time Specimen Conversion coating Composition Degrease (kg/m3) (K) (s) # CrO3 159.0 100 Chromate NaSO4 12.0 293 600 AZ31B emery paper (JIS K 6848-2) & Ca(NO3)2 13.9 2-propanol Mg(OH)2 coating NaOH 80.0 343 600 Table 2 Solution composition and condition with aluminum alloy. Polishing & Concentration Temperature Time Specimen Conversion coating Composition Degrease (kg/m3) (K) (s) CrO3 10.0 Chromate- H3PO4 4.0 323 180 100# phosphate NaF 3.0 A2017 emery paper & KMnO4 3.0 & A6022 Manganese MgSiF6 2.0 313 60 Acetone MnSiF6 2.0 Boehmite N(CH2CH2OH)3 0.003 363 1800 Table 3 Condition of adhesive. Grip t Item Condition 1 Cure temperature(K) 293 323, 373, 423, 473 Pressing time(hour) 24 1 Grip 100 0.2 5 Pressure(MPa) 0.06 12.5 0.25 50 1 0.25 Aging(hour) 48(293 K)Ã 25 ÃCure temperature 2 φ 20 Adhesive area :312.5mm (mm) 4 (Dolly) Fig. 2 Test piece of tensile lap-shear test. φ 9.5 9 14 2 on AZ31B magnesium alloy and A6022 aluminum alloy; t (Plate) 3 Figure 8 illustrates the relationship between tensile shear strength with acrylic resin and various conversion coatings on AZ31B magnesium alloy and A6022 aluminum alloy. The 45 Mg(OH)2 coating on AZ31B magnesium alloy and boehmite on A6022 aluminum alloy exhibited a higher tensile shear strength (16 MPa) with epoxy resin. In the acrylic resin, 45 (mm) Mg(OH)2 coating on AZ31B magnesium alloy and boehmite on A6022 aluminum alloy had a tensile shear strength of Adhesive area :314mm2 14 MPa. Fig. 1 Test piece of pull-off test. 3.4 Evaluation of failure patterns6) Figure 9 shows the model of the failure patterns. The bond strength of epoxy resin exhibited higher strength with failure patterns were evaluated by adhesive failure (AF), cure temperature at 373 K and, the bond strength of acrylic cohesive failure (CF), and delamination failure (DF). resin exhibited higher strength with cure temperature at Table 4 presents failure patterns with various conversion 293 K. coatings epoxy resin and acrylic resin (Pull-off test). The results of the pull-off evaluation with epoxy resin indicate 3.3 Evaluation of tensile shear strength by tensile lap- cohesive failure (CF) when bond strengths of Mg(OH)2 shear strength test5) coating on AZ31B magnesium alloy and boehmite on A2017 Figure 7 illustrates the relationship between tensile shear aluminum alloy exhibited higher strength with a cure strength with epoxy resin and various conversion coatings temperature of 373 K. However, a delamination failure Influence of Conversion Coating on Magnesium and Aluminum Alloys by Adhesion Method 1067 (a) 10µm (b) (c) 10µm 10µm Fig. 3 SEM images of various conversion coatings on AZ31B magnesium alloy. (a) as cleaned, (b) chromate, (c) Mg(OH)2 coating. (a) (b) 10µm 10µm (c) (d) 10µm 10µm Fig. 4 SEM images of various conversion coatings on A6022 aluminum alloy. (a) as cleaned, (b) chromate-phosphate, (c) manganese, (d) boehmite. (DF) occurred for chromate on AZ31B magnesium alloy. Failure patterns were different by various conversion For the acrylic resin, the failure pattern exhibited cohesive coating. In particular, the chromate coating on AZ31B failure (CF) when the bond strengths of as cleaned and magnesium alloy and chromate-phosphate coating on alumi- Mg(OH)2 coating on AZ31B magnesium alloy were all num alloys showed delamination failure. We consider that cure temperatures. small crack chromate coating showed delamination failure. Table 5 presents failure patterns with various conversion Therefore, chromate coating and chromate-phosphate coat- coatings epoxy resin and acrylic resin (tensile lap-shear ing combination exfoliated all chromate coating side. strength test). The results of the tensile lap-shear strength tests with epoxy resin and acrylic resin indicate delamination 3.5 Observation of galvanic corrosion by salt spray failure (DF) when tensile shear strengths of chromate on testing7) AZ31B magnesium alloy or chromate-phosphate and man- The specimens used in this study were AZ31B magnesium ganese on A6022 aluminum alloy increased with cure alloy and A6022 aluminum alloy; the adhesive cements temperature. were used epoxy and acrylic resin. 1068 M. Takai and M. Takaya (a) (b) A2017: As Cleaned ~Epoxy resin~ A2017: Chromate-phosphate ~Epoxy resin~ 14.0 14.0 293K 323K 373K 423K 473K 293K 323K 373K 423K 473K 12.0 12.0 10.0 10.0 8.0 8.0 6.0 6.0 4.0 4.0 Bond Strength (MPa) Bond Strength (MPa) 2.0 2.0 0 0 As cleaned Chromate Mg(OH)2 coating As cleaned Chromate Mg(OH)2 coating Conversion Coatings (AZ31B) Conversion Coatings (AZ31B) (c) (d) A2017: Manganese ~Epoxy resin~ A2017: Boehmite ~Epoxy resin~ 14.0 14.0 293K 323K 373K 423K 473K 293K 323K 373K 423K 473K 12.0 12.0 10.0 10.0 8.0 8.0 6.0 6.0 4.0 4.0 Bond Strength (MPa) Bond Strength (MPa) 2.0 2.0 0 0 As cleaned Chromate Mg(OH)2 coating As cleaned Chromate Mg(OH)2 coating Conversion Coatings (AZ31B) Conversion Coatings (AZ31B) Fig.
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