8.1 Cadmium and properties of cadmium coating
(1) Color: off-white
(2) Hardness: 300~500 MPa
(3) Atomic weight: 112.40
(4) Specific gravity: 8.65
(5) Melting point: 321C
(6) Atomic valence: 2
(7) Standard potential: -0.4029V
(8) Atomic number: 48
(9) Electronic configuration: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6 4d^10 5s^2
(10) Resistance: 6.83mW-cm
(11)Strength: 70MPa
(12) Boiling point: 767C
(13)Crystal structure: hcp
(14) Chemical properties: similar to zinc, but insoluble in lye, does not change color in air at room temperature, and will oxidize in humid air to form a protective film to prevent continued oxidation and corrosion . Cadmium oxides are insoluble in water , and cadmium is less soluble in acid than zinc , but has a stronger effect on nitric acid . When cadmium is melted, it produces toxic gas , which can
Soluble salts are also toxic. Cadmium is a cathodic protection coating on steel substrates, but it is a sacrificial anodic protection coating in high temperature or marine climates .
8.2 Cadmium plating
In marine environment and high temperature (above 70C) hot water, cadmium coating has strong corrosion resistance and is relatively stable . Good weldability and lubricity , strong contact resistance , good resistance to alkaline solution , low hydrogen embrittlement , good gloss and strong adhesion . Therefore, many parts in aviation , marine and electronic industries are plated with cadmium . The cadmium coating is easy to polish and can be used as the bottom layer of paint after phosphating . However, cadmium is poisonous to the human body , and food utensils cannot be plated with cadmium .
8.3 Types of cadmium plating baths
Can be divided into the following four types:
(1) Alkaline cyanide plating bath. (3) Acid boron and fluorine salt plating bath.
(2) Alkaline sulfate bath. (4) Neutral sulfate bath.
The type of plating bath should be selected according to the shape of the plated piece, the requirements of the plating layer, and the uniformity of the plating bath. Generally, the plated parts with simple shapes use acid plating bath, while the plated parts with complex shapes use alkaline cyanide plating bath. Alkaline plating bath coating has fine crystallization and good uniformity, but it is toxic, waste liquid treatment is troublesome, the electroplating bath is unstable, the current efficiency of cathode and anode is different, and carbon dioxide in the air will cause the bath to form sodium carbonate and hydrogen cyanide. Acidic baths are less susceptible to hydrogen embrittlement.
8.4 Alkaline cyanide cadmium plating bath formula
Cadmium oxide 3 oz/gal
Sodium carbonate 4~8 oz/gal
Sodium cyanide 13.5 oz/gal
Caustic soda (sodium hydroxide) 1.9 oz/gal
Current density 5~90 ASF
The bath temperature is 60~100 ℃. f
8.4.1 Preparation of alkaline cyanide cadmium plating bath
Its steps are as follows:
(1) First check whether the quality of the drug is in compliance.
(2) Strictly abide by the safety rules for the use of highly toxic substances and good ventilation equipment.
(3) Dissolve the required amount of sodium cyanide in a small amount of hot water (50-60°C).
The temperature is too high to avoid decomposition.
(4) Dissolve the required caustic soda in water in another container.
(5) Inject the caustic soda solution into the sodium cyanide solution.
(6) Oxidation: Mix the required cadmium with water to form a paste, slowly add it into the above-mentioned mixture, stir fully, and dissolve completely . The temperature can be raised but not higher than 85 °C.
(7) Dissolve other additives in an appropriate amount of water and add them sequentially.
(8) Add 1.5-2 pounds of zinc powder per 100gal, fully stir for 30 minutes, let the solution stand for 4 hours , and filter into the plating tank .
(9) Sampling analysis and correction of each component.
(10) Empty electrolysis with 0.25-0.5V current for 24-48 hours.
(11) Add gloss agent.
(12) Electroplate at 10ASF for about two days to normalize the plating bath.
8.4.2 Additives for alkaline cyanide cadmium plating baths
Additives affect the structure and appearance of the coating in the electroplating bath. Inorganic additives such as nickel, cobalt, molybdenum and selenium and other compounds mainly improve the appearance and physical properties of the coating. Organic additives such as acetaldehyde (aldehyde), glucose, milk, sugar , Glue, molasses, sulfonic acid (sulfonic acid), Turkish red oil, sulfonated castor oil, etc. make the coating fine-grained, improve the uniformity, and conceal metal impurities. When the additive exceeds a certain amount, the coating will become brittle and produce bubbles.
8.4.3 Defects and causes of cyanide cadmium plating
(1) The coating is discolored and stained. The reasons are:
1. The underlying metal has holes.
2. The pre-treatment is not clean.
3. Contact with dirty hands.
(2) The coating is scorched black, rough and dark. The reasons are as follows:
1. The current density is too high.
2. Too little free cyanide.
3. There are suspended impurities.
(3) Poor coating adhesion and surface blistering, the reasons are:
1. Insufficient cyanide.
2. 碱度过高。
3. 前处理不良。
4. 酸性时镀件吸收氢气。
(4) 阴极大量氰气产生，阳极呈结晶状态，被覆性不良，其原因有
1. 游离氰化物过高。
2. 金属盐不足。
(5) 阳极溶解不均匀，有沉渣，其原因有
1. 游离氰化物不足。
2. 氢氧化物不足。
8.5 非氰化物镀镉浴(Non-Cyanide Plating Baths)
近些年由于公害管制及减少电镀时氢气的形成，已开发出许多非氰化物镀镉浴，可分为中性硫酸盐镀浴、酸性硼氟酸镀浴及酸性硫酸盐镀浴。非氰化物镀浴其电流效率高，氢脆性小，所以高强度之钢铁要用非氰化物镀浴。其中硼氟酸镀浴在高电流密度下仍保持高电流效率。
8.5.1 中性硫酸盐镀浴配方
氯化铵(ammonium chloride) 1.5~3 oz/gal
硫酸铵(ammonium sulfate) 10~15 oz/gal
镉(含量) 0.5~1.5 oz/gal
电流密度 2~15 ASF
浴温 60~100 。F
8.5.2 酸性硼氟酸盐镉镀浴配方
硼氟酸铵(ammonium fluoborate) 8 oz/gal
硼酸(boric acid) 3.6 oz/gal
镉 12.6 oz/gal
硼氟酸镉(cadmium fluoborate) 32.2 oz/gal
电流密度 30~60 ASF
浴温 70~100 。F
8.5.3 酸性硫酸盐镀镉浴配方
氧化镉(cadmium oxide) 1~1.5 oz/gal
硫酸(salfuric acid) 4.5~5 oz/gal
电流密度 10~60 ASF
浴温 60~90 。F
8.6 镀镉镀层之剥离及补镀
(1) 钢铁、铜及铜合金镀件： 列溶液溶解剥离镉镀层
1. 稀盐酸。
2. 100g/l之硝酸铵。
3. 废铬酸溶液。
(2) 铝及其合金镀件： 用5~10%硝酸溶解剥离镉镀层。
(3) 外观不良镀件： 有些镀件可进行直接补镀，不需将镀层剥离。专业行1~2分钟电解洗净，将表面污物及薄膜去除，而后进行活化、清洗，并停留于镀镉浴中10~30秒，再进行补镀，时间依镀件情况而定。
8.7 镀镉工程流程
镀前检验→溶剂脱脂→化学或电解洗净→热水洗→冷水洗
↓
电解洗净←冷水洗←上架←中和←冷水洗←酸浸除锈
↓
热水洗→冷水洗→酸洗活化→冷水洗→镀镉→冷水洗→吹干
↓
冷水洗←钝化←干燥←热水洗←冷水洗←出光←除氢
↓
温水洗→干燥→检验
流程说明:
步骤1： 检查镀件表面状况、尺寸。
步骤2： 零件内外清洗干净。
步骤3： 使零件表面全部为水润湿为止，电解洗净先阴极后阳极，阳极时间要比较长，薄片及弹簧片用阳极处理。电解液成份30~50g/lNaOH,20~30g/l Na2CO3，30~50g/l Na3PO4及10~15g/lNa2SiO3。
步骤6： 其溶液成份为50~100g/l的硫酸及100~150g/l之盐酸。
步骤8： 其溶液为50~70g/l之碳酸钠。
步骤11：同步骤3。
步骤14：用30~50g/l硫酸溶液去除氧化物膜。
步骤19：烘培2~4小时，温度200~250℃。
步骤20：铬酸溶液浸泡0.1~0.5分钟。
步骤24：钝化溶液成份为80~150g/l重铬酸钠及8~10g/l硫酸。
8.8 有关镀镉之标准与规范
(1) QQ-A-671 Federal Specification(镉阳极标准规范)
(2) JIS H8611 Electroplated Coatings of Cadmium on Iron and Steel
(3) ASTM A165 Specification for Electrodeposited Coatings of Cadmium on Steel
8.9 镀镉专利文献资料
美国专利编号：
1770839 1692240 1564414 3442776 1383174
1383175 1383176 1536858 1536859 2222398
2377228 2862860 2990344 2938840 2107806
2085747 2085748 2085749 2085750 2085776
2090049 2350165 1818179 1537020 1537047
1818229 1893368 2097630 1681509 2803593
2833705 2848393 2892761 2912370 3802150
3411996 2383798 2149344 2164924 1868052
2058702 2035633 1816837 2070871 2021592
2194498 2186579 2154468 2416294 2035380
2088429 2172171 2393640 2393663 2393664
2393943 2293716 2434525 1816152 1833422
1838423 1833450 2703311
