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What options exist for corrosion control of nuts, bolts and washers?

There are several different methods whereby a zinc coating can be applied to fasteners to provide a protective coating. These include zinc or cadmium electro deposition (electroplating), mechanical plating, sherradizing, and hot dip galvanizing. All these processes provide a degree of protection. For practical reasons, the most commonly used systems are hot dip galvanizing and zinc or cadmium electroplating which is frequently and misleadingly described as electro galvanizing.

The two most important factors to consider when selecting the most effective protective system are resistance to mechanical damage of the protective coating during assembly and tensioning and the long-term protective properties of the coating. Let us consider these two important attributes and the degree to which they apply to both zinc electroplating and hot dip galvanizing. Incidentally, cadmium electroplating is frowned upon by environmentalists and in fact it is banned in many countries due to the toxic properties of this metal, particularly when the coating is cut or welded.

The resistance to mechanical damage and adhesion properties of both electroplating and hot dip galvanizing are far superior to those provided by organic coatings although the mechanism whereby adhesion is achieved is somewhat different in each case.

Hot dip galvanizing has the added benefit of a series of hard iron/zinc alloys within the coating structure. This significantly reduces the possibility of coating damage during tensioning.

As far as corrosion resistant durability is concerned, the thickness of a zinc coating determines its corrosion resistant life. Unfortunately, the thickness of a zinc coating that can be applied by the electro-deposition process is distinctly limited to the extent that the zinc electroplated coating applied to bolts and nuts is unlikely to exceed a thickness of about 10µm. In contrast, the coating applied by hot dip galvanizing normally averages between 50 and 60 µm in thickness which equates to at least five times the corrosion resistant life of a relatively thick zinc electroplated coating. In other words, hot dip galvanizing provides heavy-duty corrosion protection for fastener assemblies.

Heavy-duty coatings as in an additional 25% thickness that may be specified for structural steel cannot be achieved in the case of hot dip galvanized fasteners. The reason for this is that after the galvanizing process, all bolts and nuts must be efficiently centrifuged to remove excess zinc and if this were not done successfully, the oversized nut would not be able to freely screw onto the bolt.

The influence of silicon and or phosphorus content on hot dip galvanized coatings of steels is an interesting phenomenon.

The coating is therefore limited to about 60µm, which when viewed from a corrosion perspective, can be less protective when used to couple hot dip galvanized structural steel together. Structural steel normally results in a coating thickness of greater than 85µm and in the case of aggressive environments, when a heavy duty coating is specified, at least 105µm. To ensure that the corrosion resistant life of the hot dip galvanized fastener roughly equates to the life of the coating on the structure, additional protection in the form of a suitable paint may be used.

Most paints will adhere to well degreased galvanized surfaces but alkyd enamel paints should not be applied directly onto zinc.

A hot dip galvanized coating can be applied to bolts from M8 and upwards. Internal nut threads are tapped oversize to predetermined limits in order to accommodate the coating thickness provided by hot dip galvanizing. This does not have a negative effect on the mechanical properties of tensioned fasteners. Internal nut threads are not coated as this would necessitate oversize tapping to an extent that the strength of a fastener assembly could be compromised if tensioned to provide the specified clamping force as laid down in relevant specifications. The absence of a zinc coating on internal nut threads does not impact negatively on the corrosion resistant properties of hot dip galvanized fasteners due to the sacrificial properties of the surrounding zinc coating.

High strength bolts and nuts up to class 10.9 are now hot dip galvanized on a regular basis.