Electrogalvanizing is a process in which a layer of zinc is bonded to steel to protect against corrosion, enhance adhesion, or give an aesthetic appeal.
The process involves electroplating, running a current of electricity through a saline-/zinc-based electrolytic solution with a zinc anode and steel cathode.
Compared to hot-dip galvanization, electrogalvanization offers these significant advantages: Zinc plating was developed and continues to evolve to meet the most challenging corrosion protection, temperature, and wear resistance requirements.
Electroplating of zinc was invented in 1800, but the first bright deposits were not obtained until the early 1930s with the alkaline cyanide electrolyte.
the reinforcement of the corrosion specifications of major European car makers and the End of Life Vehicles Directive (banishing the use of hexavalent chromium conversion coating) required greater use of alkaline Zn/Ni containing between 12 and 15% Ni (Zn/Ni 86/14).
These oxide films range in thickness from 10 nm for the thinnest blue/clear passivates to 4 μm for the thickest black chromates.
Additionally, electroplated zinc articles may receive a topcoat to further enhance corrosion protection and friction performance.
The ratio of NaCN : Zn can vary between 2 and 3 depending upon the bath temperature and desired deposit brightness level.
The following chart illustrates the typical cyanide electrolyte options used to plate at room temperature: Contain zinc and sodium hydroxide.
Boric acid may be used as a pH buffer and to reduce the burning effect at high current densities.
The corrosion protection is primarily due to the anodic potential dissolution of zinc versus iron.
During corrosion, the attack of zinc is preferred and the dezincification leads to a consistent increase of the potential towards steel.
that the first nucleation on steel starts with pure nickel, and that this layer is plated 2 nm thick prior to the Zn-Ni.
Additionally, all the zinc alloys receive the new CrVI free conversion coating films which are frequently followed by a top-coat to enhance corrosion protection, wear resistance and to control the coefficient of friction.