镀锌、镀镉、镀铬、镀镍都有啥区别

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镀锌
特点：锌在干燥空气中相对稳定，不易变色。在水和潮湿的大气中，与氧气或二氧化碳反应形成氧化物或碱性碳酸锌膜，可防止锌进一步电镀和氧化，起到保护作用。
锌在酸、碱和硫化物中容易腐蚀。在铬酸或铬酸盐溶液中钝化后，所形成的钝化膜不易与潮湿空气反应，因而大大提高了防腐能力。弹簧件、薄壁件（壁厚<0.5m）和机械强度高的钢件必须进行除氢处理，铜及铜合金件不能。
镀锌成本低，加工方便，效果好。锌的标准电位为负，所以锌镀层是许多金属的阳极镀层。
用途：镀锌广泛应用于大气环境和其他良好的环境中。但不适用于摩擦部件
镀镉
特点：与海洋大气或海水、热水接触70℃以上的零件，镉镀层相对稳定，耐腐蚀性强，润滑性好。它在稀盐酸中溶解很慢，但很容易溶于硝酸，不溶于碱，其氧化物也不溶于水。镉镀层比锌镀层软，镀层氢脆小，结合力强，在一定电解条件下，得到的镉镀层比锌镀层更美观。但是，镉熔化产生的气体是有毒的，可溶的镉盐也是有毒的。
在一般情况下，镉是钢的阴极涂层，在海洋和高温大气中是阳极涂层。
用途：主要用于保护零件免受海水或类似盐溶液及饱和海水蒸汽引起的大气腐蚀。航空、航海和电子工业中的许多零件、弹簧和螺纹零件都镀镉。可用于抛光、磷化、喷漆，但不能用作餐具。
镀铬
特点：铬在潮湿的大气、碱、硝酸、硫化物、碳酸盐溶液和有机酸中非常稳定，易溶于盐酸和热浓硫酸。在直流电作用下，以铬层为阳极，易溶于苛性钠溶液中。
铬层附着力强，硬度高，800～1000V，耐磨性好，反光性强，耐热性高。480℃以下不变色，500℃以上氧化，700℃时硬度明显下降。铬的缺点是硬、脆、易脱落，特别是在交变冲击载荷下。它是多孔的。
金属铬在空气中容易钝化形成钝化膜，从而改变铬的电位。因此，铬对铁是一种阴极涂层。
用途：钢铁零件表面直接镀铬作为防腐层是不理想的。一般情况下，可通过多层电镀（即镀铜→镀镍→镀铬）来达到防锈和装饰的目的。目前，它被广泛应用于提高零件的耐磨性、修复尺寸、反光性和装饰性等方面。

Galvanized

Features: zinc is relatively stable in dry air and is not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form oxide or alkaline zinc carbonate film, which can prevent zinc from further plating and oxidation and play a protective role.
Zinc is easy to be corroded in acid, alkali and sulfide. After passivation in chromic acid or chromate solution, the passivation film formed is not easy to react with humid air, so the anticorrosion ability is greatly enhanced. For spring parts, thin-walled parts (wall thickness < 0.5m) and steel parts with high mechanical strength, hydrogen removal must be carried out, while copper and copper alloy parts can not.
Zinc coating has low cost, convenient processing and good effect. The standard potential of zinc is negative, so zinc coating is anode coating for many metals.
Application: galvanizing is widely used in atmospheric conditions and other good environments. But it is not suitable for friction parts
Cadmium plating
Features: for parts in contact with marine atmosphere or sea water and hot water above 70 ℃, cadmium coating is relatively stable, with strong corrosion resistance and good lubricity. It dissolves very slowly in dilute hydrochloric acid, but it is very easy to dissolve in nitric acid, insoluble in alkali, and its oxide is also insoluble in water. The cadmium coating is softer than the zinc coating, the hydrogen brittleness of the coating is smaller, the adhesion is stronger, and under certain electrolysis conditions, the obtained cadmium coating is more beautiful than the zinc coating. However, the gases produced by cadmium melting are toxic, and the soluble cadmium salt is also toxic.
Under general conditions, cadmium is a cathode coating for steel, and an anode coating in marine and high temperature atmosphere.
Application: it is mainly used to protect parts from atmospheric corrosion caused by seawater or similar salt solution and saturated seawater vapor. Many parts, springs and threaded parts in aviation, navigation and electronic industries are cadmium plated. It can be used for polishing, phosphating and painting, but not as tableware.
Chrome plating
Features: chromium is very stable in humid atmosphere, alkali, nitric acid, sulfide, carbonate solution and organic acid, and easy to dissolve in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if chromium layer is used as anode, it is easy to dissolve in caustic solution.
The chromium layer has strong adhesion, high hardness, 800 ~ 1000V, good wear resistance, strong light reflection, and high heat resistance. It does not change color below 480 ℃, oxidizes above 500 ℃, and decreases hardness significantly at 700 ℃. The disadvantage of chromium is hard, brittle and easy to fall off, especially when subjected to alternating impact load. It is porous.
Chrmium metal is easy to passivate in the air to form a passivation film, thus changing the potential of chromium. Therefore, chromium to iron is a cathodic coating.
Application: it is not ideal to directly plating chromium on the surface of iron and steel parts as anti-corrosion coating. Generally, the purpose of anti rust and decoration can be achieved by multi-layer plating (i.e. copper plating → nickel → chromium). At present, it is widely used to improve the wear resistance, repair size, light reflection and decorative lights of parts.