ZINC coating, in its various forms, gives sacrificial protection to iron or steel in most environments; That is, the zinc will be attacked in preference to the underlying metal, even when this base metal has been exposed to a corrosive environment. Zinc finishes are used extensively in the fastener industry, the most commonly available coatings being: ­

Galvanizing : A dip and spin process where components are dipped into a bath of molten zinc and then spun to remove the excess zinc. The subsequent coating is silvery grey in appearance and is relatively thick (35-75 microns), which makes it unsuitable for threaded fasteners below M8. It is used extensively on fasteners supplied to the construction industry.

The galvanizing build up on the thread of one component requires an extra clearance
of four times the coating thickness to enable assembly of the mating part. Normal
practice is to tap the nuts 0.4mm (0.015") oversize after galvanizing. When assembled,
the uncoated nut thread is protected by direct contact with the coating on the
mating bolt.

Sheradizing : A diffusion process where components are heated in contact with
zinc dust in an inert atmosphere. The subsequent coating is quite thick (15-35 microns)
and is matt grey in appearance. This coating has now been largely superseded b y the
organic type coatings, which will be explained later.

Electro zinc plating (barrel zinc) is a wet process where components are loaded into perforated barrels and immersed in the plating bath. The barrels are then rotated to give a tumbling action and an electric current is passed through the plating solution to enable electroplating to take place. Electroplated zinc is a bright silver metallic finish which gives reasonable protection at fairly low thickness' (typically 3-8 microns),so is ideal for small components with fine threads.

Note: Hydrogen embrittlement is an effect often associated with galvanizing and electroplating high

Tensile steel components. All with a tensile strength of 100g/mm 2 and above. Its occurrence is usually minimized by baking the parts immediately after plating, although this cannot be guaranteed to be completely effective.It is not easily detected, but when present, hydrogen embrittlement can cause parts to fail in use, usually within a short time of assembly. Mechanical zinc plating is usually specified to avoid the risk of hydrogen embrittlement in high tensile fasteners

The process, which essentially uses mechanical energy to apply the coating, involves tumbling the components in a barrel containing zinc powder, very small glass beads and other reagents. This effectively . cold welds' the coating to the surface of the components, resulting in a dull or matt silvery grey finish.

Passivation or Conversion Coating

Corrosion resistance of electro/mechanically plated zinc components can be further enhanced by (Chromate) Passivation, a process which converts the surface layer to a complex compound and further delays corrosion. Passivation can be obtained in various colours, the most popular ones being: Clear, Yellow and Black
Accelerated Corrosion (Salt Spray) Tests.

These are used to give some idea of the relative performance of different coatings, or as a test for Quality Control purposes. The usual failure point under accelerated testing of zinc on steel is the appearance of white corrosion products (wcp), followed by rusting of the base metal. The onset of white (zinc) corrosion products is dependant on the effectiveness of the passivate coating, whilst the appearance of rust is dependant on both the passivate and the zinc coating thickness

Table I: Typical Salt Sp ray Resistance for various zinc based coatings

The electroplated finishes, which can include Nickel, Chromium, Copper, Brass and Chemical / black are typical cosmetic type finishes whose uses are now mainly confined to the Domestic Appliance Industry. Unlike zinc, which offers barrier and sacrificial coatings on steel; these coatings offer barrier type protection only. The effectiveness of their corrosion resistance is therefore dependant on maintaining the coating envelope intact. If the underlying steel is exposed by damage to the coating, rusting may occur swiftly.

ORGANIC COATINGS

These are usually applied by a dip/spin process and consist of a coloured pigment bonded with a heat curing resin polymer, which is formulated to be tough and hard wearing Organic coatings offer good barrier type corrosion resistance with no risk of hydrogen embrittlement and can be supplied in various colours, although black and silver are the most popular. It is not particularly suited to very small fasteners, as some thread/recess fill may be experienced with thread diameters below M5.

These proprietary finishes are specified under various trade names, e.g. Xylan, Polyseal, Deltaseal/Deltatone, Dacromet etc. and are used extensively in the Automotive and associated industries. (Dacromet is probably the most popular organic finish, and consists of zinc-flake aluminium in a binding matrix of zinc chromate, which gives a pleasing silver grey appearance and excellent surface protection, offering both barrier and sacrificial protection to steel components).

PHOSPHATING

Zinc or Manganese Phosphate coatings are conversion coatings formed by reaction of the base metal with the appropriate phosphate salt. Crystals are formed on the metal surface, the grain size of which are critical to corrosion and torque tension performance. Although phosphate alone does not give corrosion protection, its absorption properties allow it to be used with special oils, which give high levels of protection. It is often used as a primer or base coat for some organic finishes and because of its excellent torque/tension characteristics is frequently specified for fasteners requiring precise tightening controls, e.g. engine cylinder head bolts.