To inhibit corrosion, steel may be coated with metallic or organic finishes. Metallic finishes are typically zinc (Z), zinc–iron alloy (ZF), zinc–aluminium alloy (ZA), aluminium–zinc alloy (AZ), aluminium–silicon–alloy (AS) and aluminium (A) all of which may be applied by hot-dipping of steel into the molten metal (Table 5.9). These metallic coatings, except for pure alu- minium, are covered by the standard BS EN 10346:
2009. The standard BS EN ISO 14713: 1999 indicates an
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 8 3 Table 5.9 Typical examples for zinc- and aluminium-based coatings
on steel
Coating Designation Coating mass (g/m2) (total mass)
Nominal thickness (m) (on each face)
Zinc Z275 275 20
Zinc–iron ZF120 120 8
Zinc–aluminium ZA255 255 20
Aluminium–zinc AZ185 185 25
Aluminium–silicon AS100 100 17
Note:
The standard BS EN 10346: 2009 gives a range of coating thicknesses for each of the coatings.
anticipated annual corrosion loss of between 0.7 and 2m of zinc per year from galvanised steel in most UK locations. Organic coatings (BS EN 10169: 2003) may be divided into liquid paints, powder coatings and films. Certain products are suitable only for interior applications.
ZINC-COATED STEEL
The zinc coating of steel has for many years been a stan- dard method for its protection against corrosion. The zinc coating may be applied by hot-dipping or spray- ing with the molten metal, sherardising in heated zinc powder (BS EN 13811: 2003) or electrolytically (ZE) to BS EN 10152: 2009. In hot-dip galvanising the steel is cleaned by pickling in acid followed by immersion in molten zinc at 450◦C. The zinc coating (Z) protects the steel by acting as a physical barrier between the steel and its environment, and also by sacrificially protecting the steel where it is exposed by cutting or surface dam- age. If annealed, the pure zinc coating becomes alloyed from the substrate steel producing an alloy of 8–12%
iron (ZF) which gives a better surface for painting or welding.
The durability of the coated steel is dependent on the thickness of the coating and the environment. Coating designations to BS EN 10346: 2009 (Table 5.9) indi- cate the total mass and nominal thickness of applied metallic coatings.
Coastal situations and industrial environments with high concentrations of salt and sulphur dioxide, respectively, may cause rapid deterioration. The alkalis in wet cement, mortar and plaster etch zinc coatings, but once dry, corrosion is slow; however, calcium chlo- ride used as an accelerator in plaster is aggressive and
should only be used sparingly. Fixings for zinc-coated sheet should be carefully chosen to avoid the formation of bimetallic couples, which can cause accelerated cor- rosion. In particular, no copper or brass should make contact with either zinc or iron–zinc alloy coated steel.
Other metals, such as lead, aluminium and stainless steel, have less serious effects in clean atmospheres, but generally all fixings should be sealed and insulated by rubber-faced washers. Where zinc-coated steel is to be fixed to unseasoned timber or timber impregnated with copper-based preservatives, the wood should be coated with bitumen paint. Where damaged in cutting, fixing or welding, the zinc coating should be repaired with the application of zinc-rich paint.
Zinc-coated steel may be painted for decoration or improved corrosion resistance. However, thenormal spanglezinc finish will show through paint and the minimised spangleor iron–zinc alloy uniform matt grey finish is more appropriate for subsequent painting.
ALUMINIUM–ZINC ALLOY COATED STEEL
The two distinct zinc/aluminium alloys used for coat- ing steel contain 5% (ZA) and 55% (AZ) aluminium.
The ZA coating to BS ISO 14788: 2005 is usually a substrate for an organic finish. For profile sheet metal roofing to BS EN 508-1: 2008, the minimum coating mass totals are 255 and 185 g/m2, respectively, com- pared to 350 g/m2 for pure zinc. Steel coated with the AZ alloy of aluminium (55%), zinc (43.4%) and silicon (1.6%) is more durable than that coated with an equiv- alent thickness of pure zinc, and may be used without further protection in non-aggressive environments. It is also used as the substrate for certain organic coatings.
The finish is a metallic lustre due to crystal formation.
ALUMINIUM-COATED STEEL
Hot dip aluminium on steel for profiled roofing sheet or tile to BS EN 508-1: 2008 may be applied to three grades with total mass levels of 195, 230 or 305 g/m2. However, the 195 g/m2product is only suitable for sub- sequent organic coating. Usually, an alloy containing 8–11% silicon is used, which produces a durable alloy finish of aluminium, iron and silicon.
TERNE-COATED AND LEAD-CLAD STEEL
Lead and terne, an alloy of lead (80–90%) and tin (20–10%), are used as finishes to steel and stainless steel for cladding and roofing units. Terne can be
applied to sheet stainless steel as a 20m layer by immersion in the molten alloy. Terne-coated stainless steel does not suffer from bimetallic corrosion and can normally be used in contact with lead, copper, aluminium or zinc. Thermal movement is similar to stainless steel, allowing for units up to 9 m in length to be used for roofing and cladding. The compos- ite material, lead-clad steel, is produced by cold-roll bonding 0.75 mm lead to 1.0 mm terne-plated steel or to 0.8 mm terne-plated stainless steel. Lead-clad steel is suitable for cladding and roofing systems and has the appearance and corrosion resistance of milled lead.
Because of the support afforded by the steel substrate, lead-clad steel or stainless steel can be used for self- supported fascias, soffits, gutters and curved sections.
Joints can be lead burned and soldered as traditional lead. Cut ends should be protected by soldering in the case of lead-clad steel, although the stainless steel version requires no protective treatment. Unlike tra- ditional lead, the material is virtually theft-proof and does not suffer significantly from creep. Patination oil should be applied to the lead surface after installation to prevent staining effects. Details of the various steel and coating grades are listed in BS ISO 4999: 2005.
ORGANIC COATED STEEL
Since the 1960s, a range of heat-bonded organic coatings for steel has been developed, including PVC plastisol (Colourcoat), polyvinylidene fluoride (PVDF), polyesters and PVC film (Stelvetite). Within this product range the PVC plastisol currently has the largest market share for cladding and roofing within the UK.
PVC plastisol coating
PVC plastisol is applied to zinc or aluminium/zinc coated steel to a thickness of 0.2 mm. It has a tough leather grain finish and is available in a wide range of colours, although the pastel shades are recommended for roofing applications. The reverse side is usually coated with a grey corrosion-resistant primer and polyester finish, although PVC plastisol may be spec- ified for unusually aggressive internal environments.
Careful site storage and handling is required to prevent physical damage to the surface. For non-marine envi- ronments the most durable colours will give a period to first maintenance of greater than 20 years. Very deep colours, and the non-pastel shades in coastal locations, will have reduced periods to first repainting.
Polyvinylidene fluoride coating
Polyvinylidene fluoride (PVDF), an inert fluoro- carbon, when applied as a 0.027 mm coating to zinc-coated steel, has good colour stability at temper- atures up to 120◦C, making it suitable for world-wide use and for buildings which are likely to be extended at a later date. The finish is smooth and self-cleaning, although considerable care is required on site to pre- vent handling damage. A period to first maintenance of 15 years is typical within the UK for non-coastal locations. The wide colour range includes metallic sil- ver. Polyvinylidene fluoride finished zinc/tin (terne) coated steel or stainless steel is also available in a range of colour finishes which include copper, copper patina and stainless steel. The material can be used for cladding and roofing; the solar reflective paint finish reduces excessive solar gain.
Polyester coating
Polyester and silicone polyester coated galvanised steels are economic products, but offer only medium-term life in non-aggressive environments. Externally, the period to first maintenance will be typically 10 years in unpolluted inland locations, but they are suitable for internal use. Silicone polyester should not be used in marine or hot, humid environments. Polyester and silicone polyester coatings are smooth and typically 0.025 mm in thickness.
Polyurethane coating
Polyurethane is applied as a 0.05 mm coating to zinc or zinc/aluminium alloy coated steel. It is available in a range of solid and metallic colours with a 25 year period to first maintenance on zinc/aluminium-coated steel for roofs. A textured finish on zinc-coated steel with a 20 year period to first maintenance is available for exterior walls in non-marine environments.
Enamel coating
Organic enamel-coated steels offering good light reflectance are suitable for internal use as wall and roof linings. Coatings, usually 0.022 mm thick, are typically applied to hot-dip zinc/aluminium alloy-coated steel and are easily cleaned. The standard colour is brilliant white, but a range of light colours is also available.
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PVC film coating
PVC film (0.02 mm) in a range of colours, decorative patterns and textured finishes is calendered to zinc- coated steel strip. The product is suitable only for internal applications.
Paints
The wide range of paints used to protect steel from corrosion fall into several categories listed in BS EN ISO 12944-5: 2007. Paints including solvent-borne chlorinated rubber, acrylic polymers and vinyl chlo- ride copolymers dry by solvent evaporation which can be reversed by solvent action. The majority of other products dry irreversibly by a loss of solvent or coalescence from the water emulsion. Alkyds, urethane alkyds and epoxy esters dry by a combination of a loss of solvent and aerial oxidation. Water-borne acrylics, vinyls and urethanes coalesce as the water evapo- rates. Two-component paints such as epoxys and poly- urethanes set through chemical processes activated on mixing.
Paint is normally applied in three coats. The primer adheres to the cleaned surface and gives corrosion pro- tection. The second coat builds up thickness and the finishing coat provides protection from the environ- ment and the required aesthetic finish. The standard BS EN ISO 12944: 2007 gives guidelines on the suitability of a range of paint systems in relation to environmental conditions and durability.
Steel tiles and slates
Lightweight steel tile and slate units, manufactured from galvanised or aluminium–zinc alloy hot-dipped steel, coated with acrylic or polyester resin and a gran- ular finish, give the appearance of traditional slate or pantile roofs. The products have the advantage, particularly for refurbishment work, of lightness in comparison to the traditional materials. Units can typ- ically be used for roof pitches between 12◦ and 90◦. A span of 1200 mm allows for wider spacing of roof trussed rafters. Units in a range of traditional material colours are available with appropriate edge and ven- tilation accessories. The standard BS EN 14782: 2005 requires a minimum thickness of 0.4 mm (excluding any coating) for self-supporting sheet steel in construc- tion, but a typical 0.7 mm polyester coated galvanised sheet steel system will span 1 m.