Zinc Nickel plating is a high corrosion resistant finish specified by the automotive industry and increasingly the aerospace market. By co-depositing zinc and nickel in the ratio of 85% zinc and 15% nickel the alloy coating with passivation can achieve in excess of 1000 hours in a salt spray test. Another very interesting feature of this alloy is that (unlike zinc) it does not generate a galvanic cell when in contact with aluminium. Such a galvanic effect speeds up corrosion and product failure. It is for this reason that its use is growing in the aerospace industry as a replacement for cadmium plating. Once deposited the alloy is normally passivated with a trivalent chromate either clear or black in colour.
These metals are used for different applications. Nickel plating is used either for decorative purposes (as an undercoat to chrome plating) or as a conductor of electrical current on terminals or connectors. It is a hard metal with good wear resistance and so is ideal for moving contacts, battery terminals and alike but not ideal for corrosion protection on steel parts. The reason for this is that unlike zinc metal it is not sacrificial which means that if the nickel coating is damaged then red rust will quickly appear at the damaged area. Conversely zinc will corrode sacrificially forming zinc oxide which will cover the damaged area protecting the steel substrate.
No they are different metals, zinc is a softer metal used in corrosion resistant coatings and nickel is a harder metal used mainly in decorative applications and as an alloy in the manufacture of stainless steel.
Zinc chromate refers to a zinc plated finish which has subsequently been passivated with a chromate solution. The purpose of this is to protect the zinc plating from oxidising which will happen due to the moisture in the atmosphere. The chromate layer is produced by immersing the freshly zinc plated part in a chromate solution, this then generates a zinc chromate film of about 1 µm. Many various chromates are available offering different levels of corrosion resistance and a range of colours from yellow, clear, black and green. Today chromates are normally specified to be trivalent for environmental reasons.