BS 4568.1:1970 - for conduitSection 4 requires Conduits to be classified for type and protectionConduits and fittings are classified as1) Plain conduits and fittings.2) Screwed conduits and fittings.

These are further subdivided according to the type of protection applied:Class 1. Light protection both inside and outside.Example: priming paint.Class 2. Medium protection both inside and outside.Example: stoved enamel; air-drying paint.Class 3. Medium heavy protection. Inside as Class 2, outside as Class 4.Example: stoved enamel inside. Sherardized outside. (Pls note - Not hot dip zinc coating)Class 4. Heavy protection both inside and outside.Example: hot-dip zinc coating, sherardizing.Conduits may be solid drawn or seam weldedConduits should also be marked with the name or identification mark of the maker, the BS Number and the class of finish

Galvanizing methodsThere are a variety of metal coatings that can be used to protect small steel components. Taking the example of using zinc, there are at least three different ways of using it in such instances, all of which offer totally different coating characteristics and performance.These three systems are:1. Centrifuge or spin hot dip galvanizing2. Metallic Coating with zinc dust sherardizing3. Zinc electroplating

Centrifuge or spin galvanizingThe terms spin galvanizing or centrifuge galvanizing are used to describe the process for hot dip galvanizing threaded components and other small parts. The process is very similar to galvanizing larger steel sections. Small components however are placed in perforated containers, allowing items down to 8mm in diameter to be processed. After immersion in the galvanizing bath the container is centrifuged at high speed so that the spinning action throws off any surplus zinc and ensures a clean profile.When the articles are immersed in the galvanizing bath, a series of zinc-iron alloy layers are formed with a metallurgical bond to the steel surface. These alloys are harder than mild steel and are normally covered by an outer layer of comparatively soft zinc which remains after the spinning operation. This structure is unique and gives the galvanized coating very good resistance to rough treatment - direct blows are cushioned by the outer layer of zinc and the hard alloy layers resist abrasion. As a general rule, nuts, bolts and washers down to M8 can readily be galvanized and threaded components up to 2m long can be processed. Special arrangements can often be made to centrifuge galvanized components outside this range.Metallic Coating with Zinc Dust SherardizingArticles to be sherardized are first chemically cleaned or blasted and need to be thoroughly dried. Any remaining water can cause problems when mixed with zinc powder. The clean, dry work is then placed in the sherardizing barrel with the appropriate amount of zinc dust. In some cases fine sand has to be added to aid even distribution of the dust and the temperature raised, prior to tumbling, until the desired coating is achieved. The dust should be very fine as coarser powders take longer to form the coating. The need to ensure even temperature within the barrels imposes a size limitation on the process, which is generally used for small and fairly intricate components.Zinc ElectroplatingA zinc salt solution is used to electrolytically deposit a layer of zinc on a cleaned steel surface. Acid or alkaline electrolytes can be used. After cleaning the work is transferred, as quickly as possible, to the plating solution. Good adhesion of the plated layer to the substrate depends on a very close physical conformity of the plating with the base metal. To achieve this, the base metal must be scrupulously clean immediately before plating. The table below summarises the coating characteristics and general coating thicknesses that are available via each process. It is crucial from a corrosion protection point of view that the correct system is specified for the relevant environment to achieve the desired performance. One difficulty that arises is that all of these processes use zinc and their different properties, coating thicknesses and performance may be confused into an overarching description of galvanizing. In fact zinc electroplating is often known - misleadingly - as electrogalvanizing. It is not therefore enough to merely specify galvanizing if long life is required. If hot dip galvanizing is to be used, the specification should clearly state that the fastener coating should conform to BS 7371 : Part 6 + A1 and to be galvanized by a member of Galvanizers Association.

The tables of standard systems for steelwork presented in this article relate to environment categories which are based upon those given BS EN ISO 12944-2[1] and BS EN ISO 9223[2], which are described in the table below. Atmospheric corrosivity categories and examples of typical environments (BS EN ISO 12944-2[1])

Corrosivity category and risk Low-carbon steel Thickness loss (m)a Examples of typical environments in a temperate climate (informative only)

Exterior Interior

C1very low 1.3-Heated buildings with clean atmospheres, e.g. offices, shops, schools, hotels

C2low> 1.3 to 25Atmospheres with low level of pollutionMostly rural areasUnheated buildings where condensation may occur, e.g. depots, sports halls

C3medium> 25 to 50Urban and industrial atmospheres,moderate sulphur dioxide pollutionCoastal area with low salinityProduction rooms with high humidity and some air pollution e.g. food-processing plants, laundries, breweries, dairies

C4high> 50 to 80Industrial areas and coastal areas with moderate salinityChemical plants, swimming pools, coastal, ship and boatyards

C5-Ivery high(industrial)> 80 to 200Industrial areas with high humidity and aggressive atmosphereBuildings or areas with almost permanent condensation and high pollution

C5-Mvery high(marine)> 80 to 200Coastal and offshore areas with high salinityBuildings or areas with almost permanent condensation and high pollution

Notes: 1m (1 micron) = 0.001mm a The thickness loss values are after the first year of exposure. Losses may reduce over subsequent years. The loss values used for the corrosivity categories are identical to those given in BS EN ISO 9223[2]. In coastal areas in hot, humid zones, the mass or thickness losses can exceed the limits of category C5-M. Special precautions must therefore be taken when selecting protective paint systems for structures in such areas. In many instances, steelwork will be in a warm dry interior where it will not corrode, and the structural stability of the building will not be threatened during its design life (generally taken as 50 years). In such conditions (classified as C1) no corrosion coating is required. Examples include steelwork inside dry buildings with neutral atmospheres such as multi-storey office buildings, shops, schools, hotels, residential buildings, airport terminals, and hospitals, etc. However, when steelwork is exposed to moisture, corrosion will occur at a rate depending on the severity of the environment. In such cases, a coating system appropriate to the environment category should be provided. Note that some buildings may contain areas where different environment conditions apply e.g. hospitals would normally be C1, but may contain kitchens and laundry areas that would normally be C3. Some types of buildings, such as car parks may fall into any of the above categories or combinations of them depending upon their location, design and construction. Higher risk categories for interior environments (e.g. C4 and C5) are not covered in this article and users are advised to seek specialist advice if their project involves such situations. The protective systems in the tables have unique identities. The film thicknesses given in the tables are nominal dry film values (m = micron = 0.001mm). The tables also identify the nearest equivalent system in BS EN ISO 12944-5[3] , and the required surface preparation to BS EN ISO 8501-1[4]

BS 4678 Part 1 :1971 - for cable trunking

Class 1: Electroplated zinc having a minimum thickness of zinc coating of 0.0012 mm, inside outside.

Class 2: As Class I but with additional coating of stoved or air drying paint, applied at least to the external surface.

Class 3: Hot-dip zinc coated steel to BS 2989 and with a minimum coating designation of G275