Metallic Coatings

We can arrange for your steels to be hot-dip galvanised, thermal sprayed, electroplated and sherardizing. Electroplated and sherardizing coatings are normally used for small items, fasteners and fittings.

Hot-Dip Galvanizing

The steel is immersed in a bath of molten zinc.  This results in a durable and tough abrasion resistant coated steel.

Hot-dip galvanizing is a process that involves immersing the steel component to be coated in a bath of molten zinc (at about 450°C) after pickling and fluxing, and then withdrawing it. The immersed surfaces are uniformly coated with zinc alloy and zinc layers that form a metallurgical bond with the substrate. The resulting coating is durable, tough, abrasion resistant, and provides cathodic (sacrificial) protection to any small damaged areas where the steel substrate is exposed.

As the zinc solidifies, it usually assumes a crystalline metallic lustre, often referred to as spangling. The thickness of the galvanized coating is influenced by various factors including the size and thickness of the workpiece, the steel surface preparation, and the chemical composition of the steel. The typical minimum average coating thickness for structural steelwork is 85 µm. Thick steel parts and steels which have been abrasive blast cleaned tend to produce relatively thick coatings up to 140 µm.

Since hot-dip galvanizing is a dipping process, there is obviously some limitation on the size of components that can be galvanized. However, ‘double-dipping’ can often be used when the length or width of the workpiece exceeds the size of the bath. The longest tank in the UK is currently 21 metres in length, the maximum double-dip dimension is 28 metres, and the maximum lift weight is 16 tonnes.

Some aspects of the design of structural steel components need to take the galvanizing process into account, particularly with regards the ease of filling, venting and draining and the likelihood of distortion. To enable a satisfactory coating, suitable holes must be provided in hollow to allow access for the molten zinc, the venting of hot gases, and the subsequent draining of zinc. Further guidance on the design of articles to be hot-dip galvanized can be found in BS EN ISO 14713-1[1].

The suitability of steels for hot-dip galvanizing should also be considered. Structural steel that is to be hot-dip galvanized should be clearly specified, by invoking the appropriate options in the material standards, e.g. Option 5 in BS EN 10025-1

Thermally Sprayed Metal Coatings

The metal is sprayed with aluminium, zinc and zinc-aluminium to give long-term robust corrosion protection to steel constructed in external environments. The metal is fed through a spray gun with a heat source of either electric arc or an oxygas flame.

Thermally sprayed coatings of zinc, aluminium, and zinc-aluminium alloys provide long-term corrosion protection to steel structures exposed to aggressive environments. They are an important component of coating systems that are currently specified by Network Rail and they are commonly used on steel bridge decks prior to surfacing with mastic asphalt systems.

For bridge components, thermally sprayed aluminium is usually preferred and it acts as a barrier coating. However, for rail bridges likely to be subjected to collision damage, zinc is often preferred due to its sacrificial nature.

The metal, in powder or wire form, is fed through a special spray gun containing a heat source, which can be either an oxygas flame or an electric arc. Molten globules of the metal are blown by a compressed air jet onto the previously grit blast cleaned steel surface. No alloying occurs and the coating consists of overlapping platelets of metal and is porous. The adhesion of sprayed metal coatings to steel surfaces is considered to be essentially mechanical in nature. It is therefore necessary to apply the coating to a clean roughened surface and blast cleaning with coarse grit abrasive is normally specified.

he pores are subsequently sealed by applying a thin organic coating, which penetrates into the surface. Sealers may be either un-pigmented, with colouring agents or aluminium flake. Typically specified coating thicknesses vary between 100-200 µm (microns) for aluminium and 100-150 µm for zinc.

Thermally sprayed metal coatings can be applied in the shop or at site. No drying time is required, they do not sag or run, and can be applied to the required thickness in a single operation. There is no limitation on the size of the workpiece that can be coated, as there is with hot-dip galvanizing, and since the steel surface remains cool, there are no distortion problems. Guidance on the design of articles to be thermally sprayed can be found in BS EN ISO 14713-1^[1]. Thermal spraying is considerably more expensive than hot-dip galvanizing.

For some applications (e.g. bridges), thermal spray coatings are over-coated with paint coatings (after the application of a sealer coating) to form a ‘duplex’ coating system. The combination of metal and paint in a duplex protective treatment has greater durability in comparison with that of the individual components.

The protection of structural steelwork against atmospheric corrosion by thermal sprayed aluminium or zinc coatings is covered in BS EN ISO 2063-1^[4] and BS EN ISO 2063-2^[5].

For further information on thermally sprayed metal coatings, refer to Steel Bridge Group Guidance Note GN 8.04.

Electroplated

A steel plate can be coated from a metal ion combination in a electrochemical reaction. No heat is involved and this type of coating is used in for electronics components.

Some Electroplating Metal Finishes Include;

Bright nickel Chrome Metal Finish
Metal plating for the bathrooms, architectural finishes, automotive, marine, lighting, shop fitting, and furniture.

Stainless Steel Electropolishing
Attractive, bright finish with improved corrosion resistance

Black Chrome Finish
Excellent finish for architectural hardware, machine parts which is highly durable.

Sherardizing

A steel part can be placed in a closed rotating drum that contains zinc dust. This leaves the part with a long-term anti-corrosion layer of protection. This is normally applied to small components like washers, bolts, nuts, castings, forgings and springs or unusually shaped components.