Electroplating
Widely used by industry in general, and automotive manufacturers in particular, electrodeposited zinc gives sacrificial protection to the underlying iron or steel, that is the zinc corrodes in preference to the substrate. This has the additional benefit that steel exposed at cut or abraded areas will not easily rust. Bright zinc gives reasonable protection at fairly low thicknesses, typically 8µ, so is ideal for small components such as machine screws with fine threads. The corrosion resistance of zinc can be improved by a post plating dip called a passivate (or more correctly, a conversion coating). This applies a thin coating on the zinc surface that usually contains hexavalent chromium. The types of solutions used have been extended recently because of the ELV Directives requirement for hexavalent chromium free coatings.
The new trivalent passivates developed fall into two categories - a clear (light) coating or an iridescent (light green, or heavyweight) coating which requires heated solutions and long immersion times. These coatings can be improved by using a seal (often including a lubricant) which will extend the corrosion performance, such as the Zinklad® system from MacDermid plc. The previously used leach and seal coatings (such as JS500®) do not work very well with trivalent passivates, so new seals have been developed for this application.
The performance of zinc coatings (and others) is often assessed using neutral salt spray testing. The typical performances from the common zinc based finishes are as below. The performance from vat plated coatings is also given, as due to the absence of damage after passivation, these coatings perform better than bulk (barrel) plated coatings.
