Mg在各种介质下之腐蚀情形
CrO3 HF Alkali 有机酸 中性介质 钝化状态 MgF2保护膜 Mg(OH)2非常耐蚀 不耐蚀 不耐蚀
S2O62-、CL-、SO42SiO32-、CrO42-、 Cr2O72-、PO43-
腐蚀速率大 促进保护膜形成
Anodizing bath type
I. Acid electrolytes: currently being phased out
Anodmag (Magnesium technology Licensing Ltd.)
Anodizing (Ⅰ)
Type Magoxide-coat (GmbH Ltd.) Description & Properties 1. The coating consists of three layers, a thin (100µm) barrier layer at the metal surface followed by a low porosity oxide ceramic layer. The final layer acts as a good base for paint adhesion 2. The coating has been shown to consists of mainly MgAl2O4.The process is capable of producing uniform coating even on edges and cavities 1. At a lower voltage a 5-µm thick, light tan sub coating is produced. At a higher voltage a dark brown, thicker(30µm)film is produced, upon sealing the HAE treatment provides excellent corrosion resistance
• • • II. Dow9 Dow17 Cr22 V≤300V I:10~50A/ft2 T:50~90℃
Alkaline electrolytes • • • • HAE Tagnite Magoxid-coat Anodmag V:100~600V I:10~40A/ft2 pH:10~13 T:20~60 ℃
Anodic film properties
Acid anodizing Anodic coatings Coating thickness Mg oxide, chromate, fluoride, and phosphate <~30 µm 20~50 µm Rough and 1. Non-porous with randomly distributed micro scale holes 2. Light grays to white Soft Alkaline anodizing
镁合金皮膜之阳极极化曲线
1. 基材铝含量愈高，抗腐
蚀性愈强！
2. 以电压3V成长之皮膜耐 蚀性较佳
3. 电压80V成长之皮膜耐蚀
性较10V佳
Corrosion interfaces between solution and (A)AZ21,(B)AZ501 and (C)AZ91 in solution at their corrosion potentials.
70
Film thickness
µm
皮 膜 50 硬 度
VHN 30
热处理70℃
2Hrs
15 10g/L 25g/L 50g/L 10g/L 25g/L 50g/L
Electrolyte concentration g/L
电解液浓度g/L
（三）电解液pH值与膜厚关系图
25
（四）电解液温度与膜厚关系图
25
皮 膜 硬 度
µm
20
皮 膜 硬 度
µm
20
15 4
15
电解液pH值
6
20
40
60
℃
电解液温度
镁合金在碱性溶液的阳极处理
基材：AZ31、AZ91（经铝粉0.05 µm机械抛光） 电解液：1N NaOH for 10 mins、25℃ SEM photographs of each specimen 电压：3V、10V、80V anodized in 1 N NaOH solution 皮膜腐蚀测试：0.1%NaCL（wt.%)、25 ℃ 中性及碱性介质中主要腐蚀行为： 总腐蚀反应： Mg+2H2O Mg(OH)2+H2 阳极反应： Mg Mg2++2e阴极反应： 2H2O+2eH2+2OHSurface oxide: Mg2++2OHMg(OH)2 高电压处理皮膜厚度较薄且平整！
•General alkaline cleaners can be used for degreasing
magnesium alloys •Typical acid picking solution •100~200 g/L CrO3 •100~200 g/L Na2Cr2O7 •100~200 g/L NHO3 •10~30 g/L NH4HF •150~250 g/L 50%HF •Sealing solution •25~55 g/L Na2Si4O9 •20~30 g/L Na2Cr2O7 •60~100 g/L NH4HF2
Alkaline clean Water rinse Acid picking Water rinse
Anodizing (AC or DC)
Cold DI water rinse
Sealing
Warm DI water rinse
Air dry
Water rinse
Compositions of electrolyte
Anodizing (二)
Type Dow 17 (Dow Chemicals) Description & Properties 1. Produce 2-phase, two layer coating 1st layer:5µm,light green coating, at a lower V 2nd layer:30µm,thick dark green coating, at a higher V 2. The film produces on AZ91D has a thin barrier layer at the interface with an overlying, cylindrical pore structure layer. The crystallization of MgF2 and NaMgF3 Proceed in the over layering porous layer. 1. The coatings produced are semi transparent to pearl color. The formation of a porous film with a pore size of about 6µm and porosity of 13%sealing and painting were shown to reduce the pore size and porosity to 3µm and 4%. 2. The three-step treatment was also shown to reduce galvanic corrosion rates and no change in the fatigue properties.
Pickling Bath Choice
I. 去除前加工成型所造成表面污染物
II. 去除表面2mil左右之喷砂后表面
III. 可移除表面氧化膜，及旧有化成皮膜
IV. 提供一清洁表面，供另做一道不会与金属
表面产生作用之化成皮膜处理，尤显重要
Black anodizing of Magnesium Alloy
•Thicker coating •Good corrosion-resistance barriers •Improve wear resistance, surface hardness, and
paint adhesion
Layout of a typical Mg anodizing line
镁合金阳极处理机制探讨
工业材料研究所
Surface treatment before painting
I. Chromating II. Phosphating III. Anodizing Enhance paint durability and adhension Slight corrosion protection
Black Anodizing bath
Potassium dichromate Ammonium sulfate pH Temperature Cathode Current Voltage
25g/L 25g/L 4.5 24±1℃ Anodizing tank (SUS)
1.Electrolyte reactions: K2Cr2O7 2K++Cr2O72(NH4)2SO4 2NH4++SO42H2O H++OH2.Anodic reactions: Mg0 Mg2++2e2Mg2++2Cr2O42-+6OH2MgCrO4+2Cr(OH)3+2e-
HAE
Anodizing (Ⅱ)
Type Tagnite Surface Treatment (Technology Application Group) Description & Properties 1. A chromate-free, anodic electrode position surface treatment 2. The pretreatment can consist of a) Immersion in an ammonium fluoride bath b) Electrolytic treatment in a aqueous bath containing hydroxide and fluoride compound 3. This produces a ceramic like coating of SiO2 at the surface.This process has been shown to holes of a gearbox. 4. The abrasion resistance, wear resistance paint adhesion and corrosion resistance were all improved compared to the Dow and HAE processes even prior to organic finishing.