Film forming treatment agent for composite chemical conversion film for magnesium alloy, and film forming process

Abstract

A film forming treatment agent for a composite chemical conversion film for magnesium alloy, and a film forming process method, and a composite chemical conversion film are provided. Components of the film forming treatment agent for a composite chemical conversion film for magnesium alloy comprise a water solution and a suspension of reduced graphene oxide flakes to the water solution. The water solution comprises strontium ions at 0.1 mol/L to 2.5 mol/L and phosphate ions at 0.06 mol/L to 1.5 mol/L, and pH of the water solution is 1.5 to 4.5. Concentration of the reduced graphene oxide varies between 0.1 mg/L and 5 mg/L. The film forming process method for a composite chemical conversion film for magnesium alloy comprises the following steps of: 1) pretreatment on surface of magnesium alloy matrix; 2) immersion of magnesium alloy matrix in the film forming treatment agent; and 3) removal of magnesium alloy pieces for drying in air. The composite chemical conversion film for magnesium alloy is formed by immersing magnesium alloy matrix in the film forming treatment agent. The composite chemical conversion film for magnesium alloy has excellent corrosion-resistance performance in 3.5 wt % NaCl solution.

Claims

1. A film forming treatment agent for a composite chemical conversion film for magnesium alloy, comprising: an aqueous solution having a pH value of 1.5-4.5, wherein the aqueous solution comprises strontium ions at a concentration of 0.1 mol/L to 2.5 mol/L and phosphate ions at a concentration of 0.06 mol/L to 1.5 mol/L, wherein the phosphate ions are derived from at least one selected from the group consisting of ammonium dihydrogen phosphate, sodium phosphate, sodium hydrogen phosphate, potassium phosphate, and potassium hydrogen phosphate; and a reduced graphene oxide insoluble to the aqueous solution, wherein the reduced graphene oxide has a concentration of 0.1 mg/L to 5 mg/L; and wherein the film forming treatment agent forms a composite chemical conversion film on the surface of a magnesium alloy substrate, and the film forming treatment agent does not contain chromate and fluoride.

2. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 1, wherein a ratio of the strontium ions to the phosphate ions ranges from 1:0.2 to 1:0.9.

3. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 1, wherein the strontium ions are derived from at least one selected from the group consisting of strontium nitrate, strontium chloride, strontium acetate, strontium borate, and strontium iodate.

4. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 3, wherein the strontium ions are derived from strontium nitrate.

5. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 1, wherein the phosphate ions are derived from ammonium dihydrogen phosphate.

6. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 1, wherein the aqueous solution further comprises an acidic buffering agent.

7. The film forming treatment agent for a composite chemical conversion film for magnesium alloy according to claim 6, wherein the acidic buffering agent is selected from at least one selected from the group consisting of nitric acid, sulfuric acid and organic acid.

8. A film forming process for forming composite chemical conversion film of magnesium alloy using the film forming treatment agent according to claim 1, comprises the steps of: (1) performing pretreatment on the surface of a magnesium alloy matrix; (2) immersing the magnesium alloy matrix in the film forming treatment agent; and (3) taking out the magnesium alloy matrix and drying in air.

9. The film forming process according to claim 8, wherein the pretreatment of the step (1) comprises: (1a) polishing; and (1b) ultrasonic-cleaning the magnesium alloy matrix with alcohol and acetone, respectively, at room temperature.

10. The film forming process according to claim 9, wherein the pretreatment of the step (1) further comprises: (1c) activating the magnesium alloy matrix in a concentrated phosphoric acid solution; (1d) cleaning the magnesium alloy matrix in citric acid; (1e) allowing the magnesium alloy matrix to react in a dilute sodium hydroxide solution for 5-15 min under a hydrothermal condition of 80-150° C.; (1f) cleaning with citric acid at room temperature; and (1g) ultrasonic-cleaning the magnesium alloy matrix with alcohol and acetone, respectively, at room temperature.

11. The film forming process according to claim 8, wherein in the step (2), film forming temperature is from room temperature to 100° C., and immersion time is 5-15 min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows microstructure of the surface of magnesium alloy matrix of Example C2 before pretreatment.

(2) FIG. 2 shows microstructure of the surface of magnesium alloy matrix of Example C2 after pretreatment.

(3) FIG. 3 shows microstructure of the surface of magnesium alloy matrix of Example C4 before pretreatment.

(4) FIG. 4 shows microstructure of the surface of magnesium alloy matrix of Example C4 after pretreatment.

(5) FIG. 5 shows microstructure of the surface of magnesium alloy matrix of Example C5 before pretreatment.

(6) FIG. 6 shows microstructure of the surface of the magnesium alloy matrix of Example C5 after pretreatment.

(7) FIG. 7 is X-ray diffraction pattern of the composite chemical conversion film on the surface of magnesium alloys of Examples C1-C5.

(8) FIGS. 8-12 are scanning electron micrographs of the surfaces of magnesium alloys of Examples C1-C5, respectively.

(9) FIGS. 13-17 are microstructure photographs of magnesium alloy surfaces of Examples C1-C5 after immersed in sodium chloride solution for 5 days, respectively.

(10) FIG. 18 is a microstructure photograph of magnesium alloy surface of Comparative Example D1 after immersed in sodium chloride solution for 5 days.

(11) FIG. 19 is a graph comparing the weight loss rates of the magnesium alloys of Examples C1-C5 and of the magnesium alloys of Comparative Examples D1-D3 after immersed in sodium chloride solution for 5 days.

DETAILED DESCRIPTION

(12) The film forming treatment agent for a composite chemical conversion film for magnesium alloy and the film forming process according to present invention will be further explained with reference to the accompanying drawings and specific Examples, while the technical solutions of present invention are not limited by the explanations.

Examples C1-C5

(13) The composite chemical conversion films for magnesium alloy of Examples C1-C5 are prepared by the following steps:

(14) (1) performing pretreatment on the surface of the magnesium alloy matrix, the pretreatment including:

(15) (1a) grinding the surface of magnesium alloy with 1200 #silicon carbide sandpaper and polishing;

(16) (1b) ultrasonic-cleaning the magnesium alloy matrix with alcohol (95 wt. %) and acetone at room temperature, respectively, the cleaning time is 5-15 min.

(17) In Examples C3, C4 and C5, the following steps are added after step (1b);

(18) (1c) activating the magnesium alloy matrix in concentrated phosphoric acid solution (85 wt. %) for 20-50 s;

(19) (1d) cleaning the magnesium alloy matrix in citric acid for 5-15 s;

(20) (1e) reacting the magnesium alloy matrix in dilute sodium hydroxide solution for 5-15 min under hydrothermal conditions of 80-150° C.;

(21) (1f) cleaning with citric acid for 5-15 s at room temperature;

(22) (1g) ultrasonic-cleaning the magnesium alloy matrix with alcohol and acetone at room temperature, respectively, the cleaning time is 5-15 min.

(23) (2) immersing the magnesium alloy matrix in a film forming treatment agent, components of the film forming treatment agent comprise an aqueous solution and a reduced graphene oxide insoluble to the aqueous solution. The aqueous solution comprises strontium ions at 0.1 mol/L to 2.5 mol/L and phosphate ions at 0.06 mol/L to 1.5 mol/L, the pH value of the aqueous solution is 1.5-4.5. The concentration of the reduced graphene oxide is 0.1 mg/L to 5 mg/L. The molar ratio of strontium ions to phosphate ions is controlled to be 1:(0.2-0.9) and the chemical composition in aqueous solutions and the pH value of aqueous solutions are shown in Table 1. The film forming temperature is from room temperature to 100° C., and the immersion time is 5-15 min.

(24) (3) taking out the magnesium alloy piece and drying with a blow dryer in the air, and a composite chemical conversion film is formed on the magnesium alloy matrix.

(25) In the above step (2), the strontium ions in the aqueous solution of the film forming treatment agent may be selected from at least one of strontium nitrate, strontium chloride, strontium acetate, strontium borate, and strontium iodate, wherein strontium nitrate is preferred. The acid ions may be selected from at least one of ammonium dihydrogen phosphate, sodium phosphate, sodium hydrogen phosphate, potassium phosphate, and potassium hydrogen phosphate, wherein ammonium dihydrogen phosphate is preferred. In addition, an acidic buffering agent may be added to the aqueous solution of the film forming treatment agent so that the pH value of the aqueous solution is 1.5-4.5. The acidic buffering agent may be at least one of nitric acid, sulfuric acid and organic acid, wherein nitric acid is preferred.

(26) It should be noted that the relevant process parameters in the above steps (1) to (3) are shown in Table 2.

(27) Table 1 shows the concentration of each chemical component and the pH value of the film forming treatment agent for immersing the magnesium alloy matrixes of Examples C1-C5.

(28) TABLE-US-00001 TABLE 1 ratio of reduced strontium ions graphene acidic strontium phosphate to phosphate oxide buffering pH Number magnesium alloy matrix ion (mol/L) ions(mol/L) ions (mg/L) agent value C1 Magnesium alloy AZ31 strontium ammonium 1:0.5 0.5 nitric acid 3.0 (Mg—3Al—1Zn—0.2Mn) phosphate dihydrogen phosphate 0.1  0.06 C2 Magnesium alloy strontium potassium 1:0.5 2 hydrochloric 2.5 Mg—1Al—1Zn—0.5Ca chloride phosphate acid 0.5  0.25 C3 Magnesium alloy strontium ammonium 1:0.9 3 sulfuric acid 1.8 Mg—1Al—1Zn—0.5Ca iodate dihydrogen phosphate, potassium hydrogen phosphate 1   0.9 C4 Magnesium alloy strontium sodium 1:0.2 1 nitric acid 2.5 Mg—1Ca—0.5Mn acetate phosphate 0.5 0.1 C5 Magnesium alloy AZ91D strontium sodium 1:0.4 5 carbonic 4.5 (Mg—9.1Al—0.7Zn—0.2Mn) borate hydrogen acid, phosphate lactic acid 2.5 1.0

(29) It should be noted that the number in front of the corresponding element of each magnesium alloy matrix in Table 1 indicates the mass percentage of the element, and Mg is the balance amount. For example, Mg-3Al-1Zn-0.2Mn indicates that the content of Al is 3 wt. %, the content of Zn is 1 wt. %, the content of Mn is 0.2 wt. %, and balance of Mg.

(30) Table 2 shows specific parameters of the film forming process of the composite conversion film for magnesium alloys of Examples C1-C5.

(31) TABLE-US-00002 TABLE 2 Step (1e) hydrothermal Step (2) temperature reaction time Film forming Number (° C.) (min) temperature immersion time C1 — — 100  5 C2 150 15 80 5 C3 — — 40 10 C4  80 10 60 15 C5 100  5 Room 5 temperature Note: “—” means hydrothermal treatment without step (1e).

(32) FIGS. 1 and 2 show the microstructure of the surface of the magnesium alloy matrix of Example C2 before and after the pretreatment, respectively. FIGS. 3 and 4 show the microstructure of the surface of the magnesium alloy matrix of Example C4 before and after the pretreatment, respectively. FIGS. 5 and 6 show the microstructure of the surface of the magnesium alloy matrix of Example C5 before and after the pretreatment, respectively.

(33) As shown in FIGS. 1, 3 and 5, the bright regions indicate that the surfaces of Example C2, Example C4 and Example C5 contain the intermetallic compounds of elements Ca, Mn and Al. After step (1), as can be seen from the microstructures shown in FIGS. 2, 4 and 6, the intermetallic compounds on the surface of the magnesium alloy are effectively removed, and the surfaces of these magnesium alloy matrice contain only magnesium element.

(34) FIG. 7 shows X-ray diffraction pattern of the composite chemical conversion film on the surface of magnesium alloys of Examples C1-C5.

(35) Examples C1-C5 were sampled, and the composition of the composite chemical conversion film on the surface of the magnesium alloys of Examples C1-C5 was determined by X-ray diffraction. As shown in FIG. 7, in addition to the magnesium element, the main components in Examples C1-C5 are strontium-containing salts and hydroxy strontium phosphate, and the minor components thereof are magnesium phosphate, magnesium hydroxide, magnesium hydrogen phosphate and the like.

(36) Examples C1-C5 and Comparative Examples D1-D3 were sampled, wherein Comparative Examples D1-D3 are uncoated Mg—Al—Zn—Ca-based magnesium alloys, uncoated AZ910 magnesium alloys and uncoated aluminum alloys 6061, respectively. Samples in Examples C1-C5 and Comparative Examples D1-D3 were immersed in a sodium chloride solution having a concentration of 0.1 mol/L for 5 days at room temperature. After immersing for 5 days, samples in Examples and Comparative Examples were taken out and photographed by an optical microscope. Meanwhile, the weight losses due to corrosion were measured, and the weight loss rates are shown in Table 3.

(37) TABLE-US-00003 TABLE 3 Number C1 C2 C3 C4 C5 D1 D2 D2 weight loss rate 0.13 ± 0.02 ± 0.065 ± 0.085 ± 0.12 ± 0.61 ± 0.29 ± 0.078 ± (mg/cm.sup.3 .Math. h) 0.04 0.003 0.015 0.014 0.002 0.01 0.02 0.014

(38) FIGS. 8-12 show scanning electron micrographs of the surfaces of magnesium alloys of Examples C1-C5, respectively. As can be seen from FIGS. 8-12, the surfaces of Examples C1-C5 are densely and completely covered by regular columnar strontium phosphate crystal particles.

(39) FIGS. 13-17 show microstructure photographs of magnesium alloy surfaces of Examples C1-C5 after immersed in sodium chloride solution for 5 days, respectively. FIG. 18 shows the microstructure photograph of magnesium alloy surface of Comparative Example D1 after immersed in sodium chloride solution for 5 days. FIG. 19 shows comparison results of the weight loss rate of the magnesium alloys of Examples C1-C5 and of the magnesium alloys of Comparative Examples D1-D3 after immersed in sodium chloride solution for 5 days.

(40) According to Table 3 and FIG. 19, although the magnesium alloys of Examples C1-C5 were immersed in a corrosive solution for 5 days, the weight loss rate thereof was much lower than that of Comparative Example D1 (uncoated Mg—Al—Zn—Ca-based magnesium alloys) and Comparative Example D2 (uncoated AZ91 D magnesium alloys). Therefore, compared with the uncoated magnesium alloys, the corrosion resistance of the magnesium alloy in the Examples is significantly improved due to the coated composite chemical conversion film, which improves the corrosion resistance of the magnesium alloy. In particular, the weight loss rate of the magnesium alloys of Examples C2-C3 is even lower than that of Comparative Example D3 (the existing aluminum alloy 6061), which further demonstrates that the magnesium alloy of the present invention has excellent corrosion resistance and is not easily corroded by corrosive liquid.

(41) As shown in FIGS. 13-17, no severe corrosion occurred on the surfaces of the magnesium alloys of Examples C1-C5 after immersed in the sodium chloride solution for 5 days. Referring specifically to FIG. 14, the surface of the magnesium alloy of Example C2 has substantially no corrosion and no significant change. On the other hand, referring specifically to FIG. 18, severe corrosion occurred on the surface of Comparative Example D1 (bare magnesium alloy Mg—Al—Zn—Ca), and precipitations of corrosion products covered on the surface of the magnesium alloy. It can also be seen from the comparison of the microstructures shown in FIGS. 13-17 and FIG. 18 that coated magnesium alloys has better corrosion resistance.

(42) It should be noted that the above is only specific Examples of present invention. It is obvious that present invention is not limited to the above Examples, and there are many similar changes. All variations that a person skilled in the art derives or associates directly from the disclosure of present invention shall fall within the protection scope of present invention.