6.1. Properties of chromium
(1) Color: silvery white, slightly blue.
(2) Atomic weight: 51.996.
(3) Specific gravity: 7.2.
(4) Melting point: 1900. C
(5) Hardness: 800~10OOHB.
(6) Standard potential: 0.71 volts.
(7) It is very stable in humid atmosphere and can keep the color for a long time.
(8) It is very stable in nitric acid, sulfide, carbonic acid and organic acids.
(9) Soluble in sodium hydroxide and hot concentrated sulfuric acid.
(10) Chromium anode is easily dissolved in caustic soda solution.
(11) The chrome coating has good heat resistance, it does not change color below 480"C, it starts to oxidize above 50O°C, and the hardness decreases at 700"C.
(12) The advantages of chromium coating are high hardness, good wear resistance and strong light reflection.
(13) The disadvantage of chrome plating is that it is too hard, brittle and easy to fall off.
(14) The potential of chromium is negative than that of iron. Chromium plating on steel is an anodic protective coating, and chromium itself is easy to form a very thin , so it is corrosion-resistant .
(15) The chromium plating layer is porous, so it is not very satisfactory to the corrosion of steel, so generally copper is plated first, then nickel is plated, and finally a layer of chromium is plated to achieve the purpose of anti-corrosion and decoration
(16) Chromium plating is widely used to improve the corrosion resistance, wear resistance, size repair, reflective light, and decoration of parts.
6.2 Types of chrome plating
1. Traditional chromic acid chrome plating.
2. Catalyst chromic acid chromium plating.
3. Auto-catalyzed chromic acid chrome plating.
5. Hee color chrome plating.
6. Chrome plated drum‥
7. Porous chrome plating‥
8. Micro-cracked chrome plating‥
9. Crack-free chrome plating
6.3 Characteristics of chrome plating
(1) Operating conditions such as bath temperature, current density, and pole distance must be strictly controlled
(2) The uniformity is poor, and the plated parts with complex shapes need to be properly treated.
(3) The current efficiency is very low, and a large current density is required.
(4) The anode is an insoluble anode, and the chromium must be supplemented with chromic acid.
(5) Do not interrupt during the electroplating process
(6) Plating pieces with different shapes should not be treated in the same tank, and different hangers must be used‥
(7) The preheating of the plated parts is consistent with the bath temperature, and the adhesion will be good.
(8) The plated parts must be effectively activated, and they must be charged into the tank, so that the adhesion is good‥
(9) Impulse current (greater than normal 50-100%) is required to start electroplating more complex shapes for about 2-3 minutes and then slowly drop to the normal current density range .
6.4 Influencing factors of chrome plating
(l) Relationship between CrO3 concentration and electrical conductivity
(2) The relationship between temperature and conductivity
(3) The relationship between CrO3 concentration and current efficiency
(4) Effect of sulfuric acid concentration When the concentration is low , the current efficiency is high at low current density , otherwise the current efficiency is low ‥
(5) Influence of trivalent chromium
1. When there is little trivalent chromium, the deposition rate decreases.
2. When the trivalent chromium is high, the coating becomes dark.
3. As the trivalent chromium increases, the conductivity decreases and a larger voltage is required
4. The more trivalent chromium, the smaller the gloss range.
(6) Effect of current density and temperature
1. As the temperature of the plating bath increases, the current efficiency decreases.
2. The higher the current density, the higher the current efficiency.
3. High current density, low temperature will make the coating dark, high hardness, high brittleness, coarse crystallization‥
4. 高温而低电流密度，镀层硬度小，呈乳白色，延性好，无网状裂纹，结晶细致，适合装饰性的镀件。
5. 中等温度及中等电流密度，镀层硬度高，有密集的网状裂纹，光亮硬质铬镀层。
(7)杂质的影响
1. 铁杂质，电解液不稳定，光泽镀层范围缩小，导电性变差,电压须增高，去除铁杂质比三价铬还困难，要尽量防止铁污染，不要超过10g/l
2. 铜、锌杂质，含量低时，对镀层影向不大，铜更好不要大于3g/l
3. 硝酸，是镀铬很有害的杂质，镀浴须严禁带入硝酸污染‥
(8) 阳极及电流分布之影响
1. 阳极较大，电流分布较不均匀使镀层厚度不均勺‥
2. 阳极面积大，三价铬形成较多。
3.复杂镀件，阳极宜用象形电极或辅助电极，使电流分布均匀。
4.阳极的铅易氧化，形成黑色的氧化铅及黄色的过氧化铅。过氧化铅导电性不良，应立刻除去。
5.电流因尖端及边缘效应，造成镀层厚度不均，可采用绝缘物遮盖尖端或边缘。
6．5 镀铬之挂架(Rack)
镀铬其镀浴均一性极差，电流效率很低，须使用较高电流密度，所以挂架之设计要求对镀铬质量影响很大。其设计要点如下
(l) 安定不溶解。
(2) 导电顺利不发熟，需足够截面积。
(3) 与镀件接触良好。
(4) 结构以焊接方式，导电钩要弯成直角
(5) 非电镀部份要用绝缘物覆盖，以减少电流消耗。
(6) 结构要简单、易制造、轻便。
(7) 镀件放置位置要使气体自由逸出容易。
(8) 应用辅助电极、双极电极。
(9) 依镀件的形状、尺寸、数量及镀层用途等因素决定挂架之设计
6．6 镀铬之缺陷及其原因
(l) 镀层粗糙有颗粒
1. 电流太大。
2. 阴极保护不当或末装。
3. 阴阳极太近。
4. 表面前处理不好，
5. 镀浴有浮悬杂质，
6.硫酸太少。
(2)镀层脱落
1. 前处理不良。
2. 中途断电。
3. 中途加冷水。
4. 预热不够。
(3) 局部无镀层
1. 电流太小。
2. 镀件互相遮盖。
3. 装挂不当，气体停滞。
(4) 镀层不均匀
1 挂具接触不良。
2 气体不易逸出。
3 阳极型状不当
(5) 沈积速度慢
1. 电流太小。
2. 三价铬太小。
3. 二极间距太大。
4. 镀件过大。
5. 槽内镀件过多。
(6) 镀层暗色
1. 温度太低。
2. 硫酸此例太少。
3. 三价铬太多。
(7) 镀层针孔
1. 前处理不佳。
2. 气体停滞镀件表面上。
3. 镀件被磁化。
4. 浮悬杂质。
5. 表面活性剂。
6. 镀浴有磁性粒子。
6．7 三价铬电镀
三价铭的电镀历史此六价铬久，其特点是只要六价铬所需的电流一半，电流效率较高，废液毒性小处理容易，但所用化学品昂贵，所以一直没有被广泛采用，三价铬电镀一直积极地被研究，目前已有些专利及伤业配力公开发行，其中特点在良好的被覆性及低铬浓度，使废液处理的问题减小。详细资料由厂商提供，三价铬电镀的主要困难之一是六价铬会在镀浴阳极形成，有专利配方可以防止或还元六价铬。
6．8 镀铬之管理与维护
(1) 每 天
1. 将镀槽添满镀浴。 ．
2. 用低压空气有效搅拌镀液。
3. 检查温度控制器，绸整温度于正常范围内‥
4. 检查挂架并修护之。
5. 用最大宁流15垩J30分钟做净化罔解处理‥
6. 哈氏槽试验。
(2) 每 周
1. 蒸发浓缩回收铬镀液倒回镀槽内。
2. 检验催化剂含量并补充需要。
(3) 每 月
1. 检查镀浴金屏杂质(Zn 、Fe 、Cu 、Nj ) ‥
2. 清洁及整理阳极。
3. 检查三价铬含量。
(4) 每 年
1. 检查安培计及安培小时计。
2. 检查及校正温度控制器。
3. 清洁及修理所有槽外设施。
4. 整修通风罩、输送管路。
5. 镀液出清，去除积泥，清洁、检查及修理镀槽、加热管及导电棒头。阳极整理及更换。
6．9 镀铬的氢脆性
镀铬的电流效率非常低，所以产生大量的氢气，会引起氢脆,尤其是硬化钢、高强度钢更需注意，氢脆性增加的因素有
(1) 钢铁之硬化处理。 (4) 酸浸。
(2) 研磨 。 (5) 硬化厚度与镀层厚度‥
(3) 表面缺陷。
去除氢脆方法有 :
(l) 镀前先做应力消除(stress relieving) : 镀铬表面需要没有应力存在，一般镀件经机械加工、研磨，或硬化热处理都有残留应力(residual stre6s) ， 可加热 150至230"C消除残留应力。
(2) 镀后烘焙(baking) : 可依照规范QQ-C-320B及Mil-S-13165 处理。
6．10 镀铬之回收及废物虚理
铬酸废液可用回收法或消毁( destroy )法处理，其选择因素有 :
(1) 投资金额。 (6) 厂液童。 '
(2) 人工及维护成本。 (7) 铬酸之价格。
(3) 化学品价格。 (8) 水费‥
(4) 空间。 (9)环境保护之规范‥
(5) 公共设备成本。
更好是减少废液生成，其方法有 :
(1) 尽量滴干。 (4) 回收洗净水。
(2) 应用滴干囤收槽。 (5) 控制水的带入(drag-in)，
(3) 挂架避免使镀件装水(cupping action) 。
6．11 多孔铬电镀
During World War II, porous chrome plating was used in the manufacture and repair of aircraft and engine piston rings (piston, rings) and cylinder bores. Porous chromium is a general hard chrome plating with a thickness of more than 0.004” containing many holes ( porous) imbibes lubricants, so prevents
To prevent parts from scratching, engine overheating and pinching (seizing) and galling (galling), etc. There are three methods of electroplating porous chromium:
(I) Hard chrome plating on rough or etched substrate surfaces.
(2) Chemical or electrochemical etching after hard chrome plating.
(3) Control the composition, operating conditions and temperature of the plating bath
