THE ENGINEERS CI/SfB & ARCHITECTS' Yu1 GUIDE:
HOT DIP GALVANIZING Galvanizers Association provides RIBA CPD assessed material: Literature The Engineers & Architects' Guide: Hot Dip Galvanizing The Engineers & Architects' Guide to Nuts and Bolts Seminar "Corrosion protection of steel by Hot Dip Galvanizing" For further information contact Nicky Smith on 0121 355 8838 or email [email protected]
Front Cover: Bellmouth Passage Footbridge, Techniker & Patel Taylor, Peter Cook / View THE ENGINEERS & ARCHITECTS' GUIDE:
HOT DIP GALVANIZING
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If you prefer, an online comment form is available at www.hdg.org.uk/feedback THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING INTRODUCTION
STEEL IS A STRONG, VERSATILE AND INEXPENSIVE Hot dip galvanizing has many benefits Our technical team is on hand to give an MATERIAL WITH USES IN MANY DIFFERENT as a method of corrosion protection. immediate answer to all your queries about Importantly, it will: galvanizing. GA also monitors regulations, INDUSTRIES. IT HAS, HOWEVER, ONE MAJOR takes a leading role in standards development DISADVANTAGE: IT IS PRONE TO CORROSION, s¬PROVIDE¬STEEL¬WITH¬A¬COATING¬WHICH and oversees research projects to support EVEN IN INTERIOR ENVIRONMENTS CORROSION has a long, predictable and and develop the industry. PREVENTION IS THEREFORE ESSENTIAL IF STEEL maintenance free life. GA can also visit your company to give a STRUCTURES ARE TO BE ECONOMICAL. s¬BE¬HIGHLY¬COMPETITIVE¬ON¬A¬FIRST¬ presentation which is approved by RIBA for cost basis Continuing Professional Development (CPD). TODAY, INCREASED AWARENESS OF THE LONG-TERM GA publishes a wide range of technical SAVINGS POSSIBLE FROM REDUCED MAINTENANCE s¬BE¬THE¬MOST¬ECONOMIC¬WAY¬ literature, surveys the market and its to protect steel over long periods trends and improves communications COSTS HAVE HIGHLIGHTED THE NEED FOR DURABLE about galvanizing. PROTECTIVE SYSTEMS FOR STEEL. THIS GUIDE s¬BE¬A¬SUSTAINABLE¬SOLUTION HAS BEEN PRODUCED TO ASSIST DESIGNERS, SPECIFIERS, FABRICATORS AND USERS OF This Guide is published by Galvanizers Association, which represents the hot dip STEELWORK TO DEVELOP THE MOST EFFECTIVE, galvanizing industry of the UK and Ireland. PRACTICAL AND ECONOMIC SOLUTIONS. It contains the essential information needed by architects, consulting engineers and others in selecting a corrosion protection system.
The Guide details the protection given by hot dip galvanizing, where and how to use it and how to specify it. There is also information on the process itself and the economic advantages of galvanizing. Galvanizers Association (GA) has provided authoritative information and advice on hot dip galvanizing to users and potential users since it was first established in 1949. E&A INTRODUCTION 04 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING CONTENT
SUBJECT CONTENT PAGE _ _ _
ONE - Galvanizing and the environment - Zinc - natural & essential for health 06 GALVANIZING & SUSTAINABLE - Recycling & the environment CONSTRUCTION - GalvAction 21 - taking the galvanizing industry forward
TWO - Preparation - The coating, Coating thickness 10 AN INTRODUCTION TO - The galvanizing process - Post-treatments for galvanizing, HOT DIP GALVANIZING Size of articles
THREE - Cohesion - Toughness 14 PHYSICAL PERFORMANCE
FOUR - Atmospheric corrosion resistance - Performance in other environments 16 CORROSION PERFORMANCE - The Zinc Millennium Map
FIVE - Barrier protection - Protection by sacrificial action 22 HOW GALVANIZING PROTECTS STEEL
SIX - Economics - Lifetime costs 24 COSTS AND ECONOMICS - Initial cost
SEVEN - Standards - Lead times 28 SPECIFYING HOT DIP GALVANIZING - Fasteners - Appearance
EIGHT - Filling, venting and drainage - Strength 30 DESIGNING ARTICLES - Base metal and combinations - Welding FOR GALVANIZING - Size and shape - Labelling and marking - Fasteners - Masking, - Overlapping surfaces - Connections - Castings - Handling of articles - Moveable parts, - Surface contamination - Distortion NINE - Quality assurance - Coating finish 40 QUALITY AND INSPECTION - Coating weight or thickness - Renovating damaged coatings measurement TEN - Hot dip galvanized threaded fasteners - Rust prevention at welds 44 JOINING GALVANIZED STEEL - Welding galvanized steel ELEVEN - Preparation of galvanized steel - Guidelines for painting and 46 PAINTING AND POWDER COATING powder coating OF GALVANIZED STEEL
TWELVE - British Standards - Swedish Standards 48 RELEVANT STANDARDS - ASTM Standards - ISO Standards - DIN Standards - Australian Standards THIRTEEN - Advisory services 50 ADVISORY SERVICES
05 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ONE GALVANIZING AND SUSTAINABLE CONSTRUCTION
SECTION ONE _ TITLE GALVANIZING AND SUSTAINABLE CONSTRUCTION _ FUTURE TECHNOLOGIES WILL BE LOW ENERGY LOW WASTE. GALVANIZING IS A RECYCLABLE SOLUTION THAT REDUCES THE ENERGY DEMAND OF STRUCTURES.
The pressures on global industry to accept environmental responsibility GALVANIZING AND In the galvanizing process, iron or steel for its actions is wholly justified, given society’s quest for sustainability. THE ENVIRONMENT articles are dipped into a bath containing _ molten zinc just above its melting point. The philosophy of sustainability is quite simple and is concerned with Any zinc that does not form a coating on the ensuring a better quality of life for everyone - now and, importantly, Galvanizing, the coating of iron or steel metal remains in the bath for further re-use. for generations to come. The concept is common sense based on with zinc, is probably the most Galvanizing residues are recovered and zinc substantial criteria and principles. It sets out to meet four objectives: environmentally friendly process available recycled for further use. to prevent corrosion. It is estimated that s¬3OCIAL¬PROGRESS ¬WHICH¬RECOGNISES¬THE¬NEEDS¬OF¬EVERYONE corrosion costs around 4% of GDP in As well as zinc recovered from these the USA. Effective corrosion protection residues, recycled zinc from other sources, s¬-AINTENANCE¬OF¬HIGH¬AND¬STABLE¬LEVELS¬OF¬ECONOMIC¬GROWTH¬ is a vital means of reducing the energy such as zinc scrap, is often used in and employment demands of buildings and structures. galvanizing. Galvanized steel can be recycled easily with other steel scrap in the steel s¬%FFICIENT¬PROTECTION¬OF¬THE¬ENVIRONMENT Every 90 seconds, across the world, one production process. tonne of steel turns to rust; of every two s¬0RUDENT¬USE¬OF¬NATURAL¬RESOURCES tonnes of steel made, one is to replace rust. Improvements in gas burner technology have also greatly improved energy efficiency in In respect of efficient protection of the environment and prudent use Use of hot dip galvanizing to prevent rust heating the galvanizing bath. Exhaust heat is of natural resources, the hot dip galvanizing process stands up to means that for every one tonne of steel not wasted and is used to heat pre-treatment scrutiny and can be considered as a major contributor towards protected we conserve enough energy to chemicals or dry work prior to immersion. sustainable construction. satisfy an average family’s energy needs for several weeks. The galvanizing industry is committed to understanding and improving the life-cycle Galvanizing is efficient in its use of zinc to environmental performance of its process protect steel for very extensive periods - and products. Galvanizers Association has saving energy and resources with minimal recently helped establish a Pan-European impact on the environment. Galvanizing Life Cycle Inventory database for general will protect steel structures for decades galvanizing. This LCI data will allow and minimises maintenance. Environmental Product Declarations and other life cycle assessments to be made Zinc, the natural element responsible for this on structures involving galvanized steel. corrosion resistance, is indispensable for humans, animals and plants.
06 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ONE GALVANIZING AND SUSTAINABLE CONSTRUCTION
Process emissions
Process emissions from the galvanizing process are very low. Aqueous discharge - all waste liquids - which consist mainly of spent acids used to prepare the steel, are removed by licensed waste management companies in accordance with mandatory procedures, thus protecting surface and ground water. Spent acid is also increasingly used to neutralise other wastes and in the manufacture of water treatment chemicals. The industry has greatly improved its utilisation of process chemicals in recent years - reducing the volumes of acid used per tonne of steel processed.
Emissions to the atmosphere are inherently very low and are strictly governed by the Environmental Protection Act. Galvanizing baths must capture their particulate emissions to air. This is successfully accomplished by the use of bath enclosures together with filters. A survey by the Environment Technology Best Practice Programme concluded that “Galvanizers use less than 25 litres of water per tonne of product, compared with 2000 litres in the general metal finishing industry”.
07 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ONE GALVANIZING AND SUSTAINABLE CONSTRUCTION
Total Scrap
Old Scap automobiles, appliances, die-cast parts, galzanized steel
Secondary Zinc Process Scap offcuts, Zinc Ore from production trimmings, galvanizing mine production oxide, dust, slabs residues, ashes
Primary Zinc smelter
Production of materials and Fig. 1. products The recycling process for zinc in galvanized steel
RECYCLING Other examples of uses and markets ZINC - NATURAL AND The natural concentrations of zinc in different _ for recycled zinc: ESSENTIAL FOR HEALTH environments are referred to as background levels and can vary considerably between Zinc is the principal raw material AND THE ENVIRONMENT locations. The animal and plant species in galvanizing. s¬:INC¬OXIDES¬ ¬PHARMACEUTICALS ¬FOOD ¬ _ fertilisers and for curing rubber. within a particular area have evolved to take Zinc is essential to life. It is a natural element up zinc from their environment and use it Zinc is an inherently recyclable non-ferrous found in all plants and animals and plays a for specific functions in their metabolism. metal. It can be recycled indefinitely without s¬:INC¬DUST¬ ¬PAINTS ¬CHEMICALS ¬LUBRICANTS batteries and in gold recovery. crucial part in the health of our skin, teeth, Consequently, all organisms are conditioned any loss of physical or chemical properties. bones, hair, nails, muscles, nerves and brain to the bio-available zinc concentrations in This is a major advantage for the hot dip function. Zinc and its chemistry is found in their environment which are not constant but galvanizing process ensuring its environmental s¬!LLOYED¬WITH¬OTHER¬METALS¬ ¬CAST¬INTO¬ precision parts for appliances, over 200 enzymes and hormones in man. subject to seasonal variations. Organisms sustainability and its cost effectiveness. hardware, electronics and toys. have mechanisms to regulate their internal Zinc deficiency is a recognised health problem. zinc levels. If uptake levels drop too low, About 30% (2 million tonnes) of the world’s Hot dip galvanizing is very efficient in its The Recommended Daily Allowance (RDA) of deficiency can occur and adverse effects zinc consumption is from recycled sources. use of zinc, as any of the molten metal not zinc is 15mg for a male adult, a figure that is may be observed. A figure which is rising with increased forming a coating on the steel will run back easily met by a balanced diet containing meat environmental awareness and improvements into the galvanizing bath. and vegetables. However, certain people in recycling technology. require more zinc than others, pregnant and Three residual products are formed during lactating women for example, may need as Estimates suggest that 80% of zinc available the process; a zinc/iron mix called dross, much as 19mg a day. The elderly may be for recycling is in fact recycled (with today's zinc ash and flux skimmings. All of these zinc deficient because of reduced food tecnology). This means that much of the zinc contain valuable zinc and are recovered and consumption, especially of proteins, and so in use today has probably been used before. recycled by specialist firms and the recycled they may need to take a zinc supplement. zinc is often returned to the galvanizer. Zinc The presence of a zinc coating on steel does oxide is recovered from galvanizers’ ashes Zinc is the 17th most common element in not restrict its recyclability. Galvanized steel and used in pharmaceutical/beauty products. THE¬EARTHS¬CRUST¬-OST¬ROCKS¬CONTAIN¬ZINC¬IN¬ is recycled with other steel scrap in the steel varying amounts and zinc exists naturally in production process; it volatilises early in the air, water and soil. Due to natural weathering process and is collected for reprocessing. and erosion of rocks, soils and sediments together with volcanic eruptions and forest fires, a small but significant fraction of natural zinc is continuously being mobilised and transported in the environment.
08 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ONE GALVANIZING AND SUSTAINABLE CONSTRUCTION
PUMP ROOMS TENBURY WELLS, GALVACTION 21 WORCESTERSHIRE TAKING THE GALVANIZING _ INDUSTRY FORWARD _ The restoration of this unusual building showed that the galvanized steel cladding Recently Galvanizers Association launched a and roof sheets, which have protected the major initiative to take the galvanizing industry building for nearly ninety years, can, at the into the 21st Century with particular focus on end of their life be re-galvanized to provide the sustainability of the industry. Continuous the same protection all over again. environmental improvement is a key feature.
Just one example of the initiative in action is a project that has attracted funding from the Department of Trade & Industry (as part of its Recycling Programme) and relates to the recycling of waste hydrochloric acid from the galvanizing process. The project takes spent acid, recycles it and turns it into ferric chloride, a valuable raw material used to remove phosphates from wastewater.
The demand for ferric chloride is driven by the EU’s urban wastewater directive, which imposes increasingly stringent controls on consent levels for the discharge of phosphates.
For an information pack on galvanizing and sustainablity including (GalvAction 21) please contact Galvanizers Association or visit our website at www.galvanizing.org.uk
09 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWO AN INTRODUCTION TO HOT DIP GALVANIZING
SECTION TWO _
TITLE AN INTRODUCTION TO HOT DIP GALVANIZING _ HOT DIP GALVANIZING, HAS GROWN ALMOST CONTINUOUSLY SINCE IT WAS FIRST USED TO PROTECT CORRUGATED IRON SHEETS 150 YEARS AGO. ITS ABILITY TO GROW IN THE FACE OF SOPHISTICATED COMPETITION IS THE RESULT OF THE SIMPLICITY OF THE PROCESS AND THE UNIQUE ADVANTAGES OF THE COATING.
PREPARATION Welding slag, paint and heavy grease will not _ be removed by the above cleaning steps and should be removed before the work is sent to The galvanizing reaction will only occur on the galvanizer. a chemically clean surface. Therefore most of the preparation work is done with this After a further rinsing operation, the objective in mind. In common with most components will then commonly undergo a coating processes the secret to achieving fluxing procedure. This is normally applied a good quality coating lies in the preparation by dipping in a flux solution - usually about of the surface of the iron or steel. It is 30% zinc ammonium chloride at around essential that this is free of grease, dirt and 65-80°C. Alternatively, some galvanizing scale before galvanizing. These types of plants may operate using a flux blanket contamination are removed by a variety of on top of the galvanizing bath. The fluxing processes. Common practice is to degrease operation removes any of the last traces using an alkaline or acidic degreasing solution of oxide from the surface of the component into which the component is dipped. and allows the molten zinc to wet the steel. The article is then rinsed in cold water and then dipped in hydrochloric acid at ambient temperature to remove rust and mill scale.
degrease rinsechemical rinse flux dry dry immerse water cleaning in molten zinc
10 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWO AN INTRODUCTION TO HOT DIP GALVANIZING
11 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWO AN INTRODUCTION TO HOT DIP GALVANIZING
pure 6% 10% base zinc Fe Fe steel
Fig. 2. Fig. 3. Fig. 4. -ICROSTRUCTURE¬OF¬A¬TYPICAL¬HOT¬DIP¬ -ICROSTRUCTURE¬OF¬A¬THICK¬COATING¬OBTAINED¬BY¬ -ICROSTRUCTURE¬OF¬THE¬THICK¬COATING¬OBTAINED¬ Schematic section through a typical hot galvanized coating. grit blasting steel prior to galvanizing. using a silicon-rich steel. dip galvanized coating.
THE GALVANIZING PROCESS THE COATING Centrifuged galvanized coatings Galvanizing reactive steels _ _ This involves a process covered in BS EN ISO A thicker zinc coating will be obtained if the When the clean iron or steel component When the reaction between iron and zinc 1461 and is used for galvanizing threaded article to be galvanized is manufactured from is dipped into the molten zinc (which is has virtually ceased the article is taken out of components and other small parts. The parts, a reactive steel. The constituents in steel that commonly at around 450°C) a series of zinc- the galvanizing bath complete with its outer after preparation, are dipped in the molten have the greatest influence on the iron/zinc iron alloy layers are formed by a metallurgical coating of free zinc, the process is complete. zinc in a perforated basket. After the coating reaction are silicon, which is frequently added reaction between the iron and zinc. A microsection of the galvanized coating has formed this is centrifuged or spun at high to steel as a deoxidant during its production, The rate of reaction between the steel and will look like the picture above (Fig 2). speed, to throw off the surplus zinc to ensure and phosphorous. Silicon changes the the zinc is normally parabolic with time and In reality there is no demarcation between composition of the zinc-iron alloy layers so so the initial rate of reaction is very rapid and A¬CLEAN¬PROFILE¬-INIMUM¬AVERAGE¬COATING¬ steel and zinc but a gradual transition through weights for centrifuged work are identified in that they continue to grow with time and the considerable agitation can be seen in the zinc the series of alloy layers which provide the BS EN ISO 1461 and in BS 7371 Part 6. rate of growth does not slow down as the bath. The main thickness of coating is formed metallurgical bond. layer becomes thicker (Figs 4 & 5). To a lesser during this period. Subsequently the reaction Thicker coatings may be produced by one of degree phosphorus exerts a similar influence slows down and the coating thickness is not COATING THICKNESS the following: on the formation of the coating. increased significantly even if the article is in _ the bath for a longer period of time. Thicker coatings by surface roughening When an article fabricated from a reactive A typical time of immersion is about four The coating thicknesses’ are normally steel is removed from the zinc bath, a zinc or five minutes but it can be longer for determined by the steel thickness and are This is the most common method of layer adheres to the alloy layer as with any heavy articles that have high thermal inertia set out in BS EN ISO 1461 (see Section 7). achieving thicker coatings. other steel article. However, the reaction or where the zinc is required to penetrate There are three exceptions to this rule, rate in these steels can be so high that this internal spaces. Upon withdrawal from the the first produces a slightly thinner coating, Grit blasting, to Sa 2½ (ISO 7079), the steel pure zinc layer is transformed completely galvanizing bath a layer of molten zinc will be the other two increase it. surface prior to immersion, using chilled to zinc-iron alloy before the article has had taken out on top of the alloy layer. Often this angular iron grit of size G24, roughens and time to cool. The result is a coating of equal cools to exhibit the bright shiny appearance increases the surface area of steel in contact or increased thickness which can be much associated with galvanized products. with the molten zinc. This generally increases darker in appearance as is described in the weight per unit area of a hot dip Section 9. The change in appearance Post galvanizing treatment can include galvanized coating by up to 50% (Fig. 3). does not alter the corrosion resistance quenching into water or air cooling. Any steel article can be treated in this way, of the coating. providing it is thick enough to withstand Conditions in the galvanizing plant such as blasting. It may not be possible to grit temperature, humidity and air quality do not blast the inside surface of hollow sections affect the quality of the galvanized coating. and fabrications, but these are almost By contrast, these are critically important for always the areas least prone to corrosion. good quality painting. Thicker coatings than those required by BS EN ISO 1461 should only be specified following consultation with the galvanizer or Galvanizers Association (see also Section 7).
12 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWO AN INTRODUCTION TO HOT DIP GALVANIZING
POST-TREATMENTS SIZE OF ARTICLES _ FOR GALVANIZING _ Galvanizing is a versatile process and articles No post-treatment of galvanized articles is ranging in size from nuts and bolts to long necessary. Paint or a powder coating may structural sections can be treated. be applied for enhanced aesthetics or for This range, together with the ability to bolt additional protection where the environment or weld fabrications after galvanizing, allows is extremely aggressive. Painting and powder almost any size of structure to be galvanized. coating is discussed in Section 11. Complex shapes, open vessels and most hollow articles can be galvanized, inside (μm) zinc coating Chemical conversion coatings and other and out, in one operation. Certain hollow barrier systems may be applied to minimise fabrications may be galvanized on the the occurrence of wet storage stain outside surface only, however, this requires (Section 9). special designs and galvanizing techniques. The capacity of individual galvanizing plants is detailed in the Directory of General Galvanizers, available free of charge from Galvanizers Association, or viewed at www.galvanizing.org.uk. silicon content of steel (%)
0 0.1 0.2 0.3 0.4 0.5
Fig. 5. Approximate thickness of the zinc coating in relation to the silicon (Si) content of the steel. (This is a schematic representation, please contact Galvanizers Association for detailed information)
13 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION THREE 0(93)#!,¬0%2&/2-!.#%
SECTION THREE _
TITLE PHYSICAL PERFORMANCE _ THE UNIQUE NATURE OF THE GALVANIZING PROCESS PROVIDES A TOUGH AND ABRASION RESISTANT COATING WHICH MEANS LESS SITE DAMAGE AND SPEEDY ERECTION OF STRUCTURES.
COHESION TOUGHNESS _ _ Unlike most protective coatings, which rely Resistance to mechanical damage of solely on the preparation of the steel to protective coatings during handling, storage, obtain adequate adhesion, hot dip galvanizing transport and erection is very important if produces a coating which is bonded the cost of ‘touching up’ on site is to be metallurgically to the steel. In other words, avoided. The outer layer of pure zinc is the iron and the zinc react together to form relatively soft and absorbs much of the shock a series of alloys which make the coating of an initial impact during handling. The alloy an integral part of the steel surface with layers beneath are much harder, sometimes excellent cohesion. even harder than the base steel itself. This combination provides a tough and abrasion resistant coating. (Fig 6)
hardness, HV values >
50 100 150 200 250
Zn
Zn + Fe
Fe
Fig. 6. -ICRO¬SECTION¬OF¬HOT¬DIP¬GALVANIZED¬COATING¬ showing variations in hardness through the coating. The zinc-iron alloys are harder than the base steel.
14 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION THREE 0(93)#!,¬0%2&/2-!.#%
15 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#%
SECTION FOUR _
TITLE CORROSION PERFORMANCE _
LARGE FALLS IN ATMOSPHERIC POLLUTANTS MEAN GALVANIZING NOW LASTS MUCH LONGER
15 90 ATMOSPHERIC Given the wide range of environmental CORROSION RESISTANCE conditions encountered and protective 80 _ systems available, data on performance 13 can become complex. A number of guides The resistance of galvanizing to atmospheric have been produced giving the design life of 70 corrosion depends on a protective film which
/year protective systems in different environments. 2 zinc corrosion rate g/m2/yr forms on the surface of the zinc. When the 11 60 BS 5493 ‘Code of Practice for Protection of steel is withdrawn from the galvanizing bath, Iron and Steel Against Corrosion’ provided the zinc has a clean bright, shiny surface. some information but its guidance on the
50 of air 09 3 With time this changes to a dull grey patina lifetime of galvanized coatings is now as the surface reacts with oxygen, water 40 out-of-date.
μg/m and carbon dioxide in the atmosphere. SO μg/m3 of air 2 2 A complex but tough, stable, protective layer 07 30 SO Since BS 5493 was published in 1977, is formed which is tightly adherent to the zinc. atmospheric sulphur dioxide levels (SO2) Contaminants in the atmosphere affect the
corrosion rate of zinc g/m 20 have reduced considerably. A direct 05 nature of this protective film. relationship exists between the corrosion
10 rate of zinc and airborne SO2 levels (Fig 7). The most important contaminant for zinc is Therefore, the life of galvanized coatings 03 03 sulphur dioxide (SO2) and it is the presence has increased significantly since the of SO2 which largely controls the atmospheric 1970’s and it is important to use corrosion of zinc. up-to-date information in assessments of coating life. BS 5493: 1977 has been
1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 The corrosion rate for zinc is generally linear replaced in part by the new guidance for a given environment. A major advantage document BS EN ISO 14713: 1999 (Table 1) of this is that it allows predictions of ultimate life to be made on the basis of interim Fig. 7. assessments of coating thickness. Reduction in sulphur dioxide levels in Stockholm since 1978 and accompanying decrease in corrosion rate of zinc.1
1(Source: Knotkova D and Porter F 1994 Longer life of galvanized steel due to reduced sulphur dioxide pollution in Europe, Ed. proc. Intergalva 94, p GD 8/1-8/20 pub. EGGA, London)
16 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#%
The environments in most corrosion guides Comparison of data obtained from The Zinc Table 1. are necessarily general. Specific corrosivity -ILLENNIUM¬-AP¬PAGE¬ ¬WITH¬RESULTS¬ Indicative zinc corrosion rates for different environments Source: BS EN ISO 14713 values in the UK have been mapped by the from previous maps (1982 and 1991) show (Corrosivity categories from ISO 9223) Agricultural Development Advisory Service a clear, and very significant, drop in the (ADAS). The information was based on data corrosion rate for zinc for most atmospheric obtained from exposure of zinc reference exposures across the UK and the Republic average annual zinc samples at National Grid Reference points of Ireland. corrosivity category corrosion rate (μm/year) in a large number of 10km square reference areas of the UK. In particular, urban locations which, in the C1 interior: dry <0.1 past may have been considered aggressive The results indicated varying rates of C2 interior: occasional condensation for zinc, will yield significantly enhanced corrosion for zinc in exterior locations. exterior: rural 0.1 to 0.7 lives for galvanized coatings. In more remote Galvanizers Association sponsored the revision of the map, to provide specifiers locations these downward trends in SO2 C3 interior: high humidity, some air with the very latest information on zinc levels are not so pronounced and the pollution exterior: urban inland corrosion. For the first time the survey was (already low) corrosion rates for zinc would or mild coastal 0.7 to 2 extended to the Republic of Ireland. The not be anticipated to be further reduced to same experts that were involved in the any significant extent. C4 interior: swimming pools, chemical development of previous corrosivity maps plants exterior: industrial inland were contracted to co-ordinate the project. A brief inspection of Table 1 and Figure 8 or urban coastal 2 to 4 confirms that for most exterior applications of galvanizing the corrosion categories across C5 exterior: industrial with large areas of the UK and the Republic of high humidity or high Ireland are C2 / C3 ie 0.1μm/yr - 2μm/yr. salinity coastal 4 to 8
4HE¬:INC¬-ILLENNIUM¬-AP¬RESULTS¬SHOW¬THAT¬A¬ standard 85μm galvanized coating may now achieve a coating life of 50 years in most environments. Similarly, a thicker 140μm galvanized coating, often produced on structural steel, may achieve a coating life of over 100 years.
17 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#% Zinc corrosion rates are represented by represented rates are Zinc corrosion five categories indicated by the colour codes shown below. Fig. 8 THE ZINC MILLENNIUM MAP Atmospheric Corrosion Annual Average of Zinc, UK and Republic Ireland, 1998-2000 guide and is most This is an approximate exterior-exposed to stationary, relevant will need to take account You structures. of any site specific factors which may affect rate. the corrosion Detailed data for individual sites and advice (e.g. the possible on its interpretation on the of a local micro-climate effects rate actually experienced by corrosion is available the galvanized structure) Galvanizers Association. from Discover the background rate for your town corrosion @ www.galvanizing.org.uk How to use the map s¬,OCATE¬YOUR¬PROJECT¬ON¬THE¬MAP s¬-ATCH¬THE¬COLOUR¬OF¬THE¬SQUARE¬TO¬THE¬KEY s¬2EAD¬OFF¬THE¬AVERAGE¬BACKGROUND¬CORROSION¬ rate in μm per annum s¬)DENTIFY¬THE¬MINIMUM¬AVERAGE¬GALVANIZED¬¬ coating thickness for steelwork in μm (see section 7) s¬$IVIDE¬THE¬COATING¬THICKNESS¬BY¬THE¬ rate to obtain the expected corrosion 18 minimum life of the galvanized coating THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#% IUM¬-AP¬ Cartographic reproduction by Lovell Johns Ltd, Copyright: Galvanizers Association Cartographic reproduction Survey 1:1,500,000 map, with the permission of 2001 Based upon the Ordnance of The Stationery Office. the controller of survey sites in grateful to Orange Plc for provision ADAS Consulting and GA are for survey sites that allowed corrosion the UK and Irish Electricity Supply Board RATES¬AT¬KEY¬)RISH¬LOCATIONS¬TO¬BE¬INCLUDED¬IN¬THE¬:INC¬-ILLENN planned to extend survey coverage in the Republic of Ireland. Further studies are may be used the rates indicated for comparable areas not yet covered, For areas rates. as an indication of likely corrosion
Acknowledgements s¬ !GRICULTURAL¬$EVELOPMENT¬!DVISORY¬3ERVICE¬!$!3¬#ONSULTING management for project s¬ -R¬4OM¬3HAW¬FOR¬CONSULTANCY¬SERVICES¬THROUGHOUT¬THE¬PROJECT¬ data and for application of the Thiessen method to project s¬ "RITANNIA¬:INC¬,IMITED¬FOR¬PROVISION¬OF¬LABORATORY¬FACILITIES s¬ -R¬*OHN¬#URTIN¬CONSULTANT ¬AND¬-R¬-ICHAEL¬4AYLOR¬ assistance (Britannia Zinc Limited) for valuable project s¬ !LL¬THOSE¬WHO¬PROVIDED¬SAMPLE¬SITES¬FOR¬THE¬PROJECT 19 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#%
PERFORMANCE IN Where this situation could arise, sacrificial In contact with wood High temperature OTHER ENVIRONMENTS protection can be assured by installing a _ magnesium anode as ‘back-up’ Very acidic woods such as oak, sweet Galvanized coatings will withstand continuous to the zinc coating. chestnut, western red cedar and douglas exposure to temperatures of approximately Interior environments fir can be used in conjunction with 200°C and occasional excursions up to 275°C Immersed: Sea water galvanized steel, provided they are isolated without any effect on the coating. Above A common misconception is that corrosion from direct contact. these temperatures there is a tendency for may not be a problem for interior steelwork Sea water is more aggressive than freshwater. the outer zinc layer to separate, but the which is protected from the elements. For UK waters the corrosion rate normally In contact with other metals alloy-layer, which comprises the majority If there is frequent condensation on its lies in the range 10 - 15μm/yr for continuous of the coating, remains intact. Adequate surface, corrosion of inadequately protected immersion. Tidal immersion, regular sea spray There is only slight additional corrosion of protection may often, therefore, be provided steel will be significant. Under these or immersion in warm tropical sea water can zinc as a result of contact with the majority up to the melting point of the alloy layer conditions, hot dip galvanizing may give more lead to an increased corrosion rate. of metals in most atmospheric conditions. (around 530°C). than 40 years life. Hot dip galvanizing has Bimetallic corrosion can occur in immersed also been used extensively to protect interior Underground conditions or in locations where rainwater In contact with building materials steelwork in harsh environments such as is unable to drain easily or dry out at the swimming pools and breweries. Corrosion The life of a buried galvanized coating can contact surfaces. Guidance is given in BSI’s Damp mortar, cement and plaster have may also arise on exposed ungalvanized vary depending on, for example, the type PD6484: ‘Commentary on corrosion at a slight etching action upon galvanized steelwork intended for interior environments if of soil - its acidity and whether it has been bimetallic contacts and its alleviation’. coatings whilst drying or setting. there is a delay in assembly or construction. disturbed, (contact Galvanizers Association This effect ceases once the action for further advice). A pH range of 5.5 to 12.5, In contact with chemicals has finished. Immersed: Cold water ie. weakly acid to alkaline, is favourable. Soils containing ashes and clinkers are Contact with chemicals requires special -OST¬WATERS¬CONTAIN¬SCALE FORMING¬SALTS¬ especially harmful. In many cases, the consideration. A wide range of chemicals are which can form a protective layer on the application of a bituminous solution (to compatible with galvanized steel. Prolonged inside surfaces of galvanized water BS3416 Type 1) over the zinc coating is or frequent contact with acids and strong distribution systems, hence coatings can beneficial - particularly where galvanized steel alkalis is not advisable. have their lives enhanced usually to greater is buried in the ground or at the point where it than 40 years. If these salts are not present, emerges from concrete. Galvanized steel can as is often the case in soft waters, longer life be safely embedded into concrete. For even can be obtained by applying two coats of greater protection in soils, thicker galvanized bituminous solution (to BS3416 Type II coatings can be specified (see Section 7). for drinking water).
Immersed: Hot water
The scale-forming properties of water are also important in hot water giving normal life expectancies of over 10 years. Above 60°C zinc may become cathodic to steel in some waters and no longer provide sacrificial protection if the coating is damaged.
20 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FOUR #/22/3)/.¬0%2&/2-!.#%
21 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FIVE HOW GALVANIZING PROTECTS STEEL
SECTION FIVE _
TITLE HOW GALVANIZING PROTECTS STEEL _
GALVANIZING IS UNIQUE - TOUGH, LONG LASTING, SELF HEALING AND COVERS INTERNAL AND EXTERNAL SURFACES.
BARRIER PROTECTION _ Galvanizing provides a barrier between all internal and external steel surfaces and their environment. Galvanizing is a term often wrongly used to describe zinc coatings in general. Fig. 9. illustrates how the different types of zinc coatings vary in terms of coating thickness. The life expectancy of a zinc coating thickness: μm measured from steel surface coating is largely determined by its thickness. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Thicker coatings give longer life. Hot dip 85 galvanizing provides fabricated iron or steel steel surface coating weight: g/m 2 610 products with maximum protection through a continuous, tough, metallurgically bonded steel of > 6mm section coating of much greater thickness. hot dip galvanized to EN ISO 1461
thick hot dip galvanized coating
centrifuge galvanizing to BS 7371 Pt.6
zinc spraying to EN ISO 2063
continuous galvanized sheet to EN 10346 key: dispersed zinc pigment zinc plating to Fe/Zn5 of EN 12329 zinc alloy layers sherardizing to grade 1 of BS 4921 pure zinc paints and coatings incorporating zinc dust
Fig. 9. Zinc coatings compared in terms of coating thickness
22 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION FIVE HOW GALVANIZING PROTECTS STEEL
PROTECTION BY SACRIFICIAL ACTION _ Fig. 10. Schematic to illustrate the consequence of damage to Zinc corrodes in preference to steel and different types of coating offering corrosion protection. sacrifices itself to protect the steel, hence hot dip galvanizing will provide this sacrifical action Fig 10. The corrosion products from the zinc are deposited on the steel resealing Anodic it from the atmosphere and therefore stopping (electronegative) end - more active metals corrosion. With paint coatings, additional protection would have to be applied -AGNESIUM immediately after the damage occurred or the Zinc steel would rust with eventual break down of Aluminium Zinc protects steel Cadmium the whole coating as rust crept underneath Steel the paint film. Lead Tin Nickel Brass Copper
Cathodic hot dip paint coating coating of more (electropositive) end - less active metals galvanized coating electro-positive The steel rusts metals than steel A galvanic cell is where the paint film Zinc’s position in the Galvanic Series formed. The zinc has been damaged. Nickel, chromium, around the point of Rust creeps under and copper - give damage corrodes. the paint film, which rise to more rapid Corrosion products is lifted up from corrosion at the precipitate on the the steel surface. point of damage steel surface and Corrosion continues than if the steel protect it. The steel until the damage had been uncoated. is also protected has been repaired. The corrosion often because it is takes the form of cathodic in relation pitting, which can to the zinc coating. even go through the steel.
23 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SIX #/34¬!.$¬%#/./-)#3
SECTION SIX _
TITLE COST AND ECONOMICS _
GALVANIZING PROVIDES UNRIVALLED WHOLE LIFE COST BENEFITS FOR STEEL STRUCTURES AND COMPONENTS - AND CAN ALSO BE COMPETITIVE ON AN INITIAL COST BASIS
ECONOMICS INITIAL COST _ _ 140 The true cost of protecting steelwork from Hot dip galvanizing is often perceived to corrosion has to take into consideration two be more expensive than it is. There are 4 coat paint important elements: two reasons for this: Firstly, that such a 120 high performance coating is automatically s¬4HE¬INITIAL¬COST¬OF¬PROTECTION assumed to be expensive. Secondly, the initial cost of galvanizing relative to paint 3 coat paint 100 s¬4HE¬LIFETIME¬COST ¬WHICH¬INCLUDES¬THE¬COST¬ has changed significantly over recent years. of maintenance. This is the cost of ensuring Painting costs have steadily increased whilst that steelwork is protected from corrosion galvanizing costs have remained stable. throughout its service life. 80 Galvanizers Association recently
commissioned independent consultants, relative cost The Steel Protection Consultancy Ltd (SPC), 60 to investigate the cost competitiveness hot dip galvanize of galvanizing. SPC in conjunction with consulting engineers, WS Atkins, designed 40 a typical, 240 tonne, steel framed building and sent it out to tender. Two corrosion protection systems were specified; (i) hot dip 20 galvanizing and (ii) a good quality, grit blast and three coat paint system of 250μm dry film thickness. Quotations from eight 2 3 5 6 8 10 14 19 25 fabricators in different parts of the UK were steel thickness (mm) obtained and averaged. The paint system was found to be 35% more expensive Fig. 11. than hot dip galvanizing. Comparison between initial costs
Fig. 11 illustrates that for many applications the cost of hot dip galvanizing is lower than that of applying alternative coatings. The reason for this is simple: alternatives such as painting are very labour intensive compared with galvanizing, which is a highly mechanised, closely controlled, factory process.
24 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SIX #/34¬!.$¬%#/./-)#3
25 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SIX #/34¬!.$¬%#/./-)#3
WHOLE-LIFE COST _ Design Build Operate Dispose Total 2,500 The whole-life cost of a building can be defined as: Run/Maintain “The cost of acquiring, operating and £ - 40% 2,000 maintaining a building over its whole life through to disposal” 100% £ £ Repair £ Cost 1,500 Whole-life costing can be characterised as 3% 17% £ - 30% ?% of a system that quantifies financial values for Ownership buildings from inception and throughout the Cost Periodic building’s life. It is an approach that balances £ 1,000 capital with revenue costs to achieve an Replacement/Refurbish optimum solution over a building’s whole life. £ - 10% 1 Year 2 Year 25 Years 1 Year Total This technique, whilst not in itself new, has 500 over recent years become accepted best practice in construction procurement. Whole-life costing can be used at any stage Table 2. of the procurement process and can be used Capturing Whole Life Costs 0 at the levels of facility, function, system and component. This includes everything from initial design to end-of-life.
It is estimated that up to 80% of a building’s Sep 06 Sep 08 Sep 10 Sep 12 Sep 14 Sep 16 Sep 18 Sep 20 Sep 22 Sep 24 Sep 26 Sep 28 Sep 30 Sep 32 Sep 34 Sep 36 Sep 38 Sep 40 whole-life cost can be attributed to running, maintenance and refurbishment costs. Consequently, there are spikes in Fig. 12a. expenditure at 10 years and every five Smoothing the Expenditure years after that (see Fig. 12a). Life cycle expenditure tends to inherently produce “spiky” profiles with large peaks at 10, 15, 20, 25 years. The initial choice of materials and the way that they are protected obviously plays an important role within the maintenance and refurbishment costs of a building over its lifetime. They therefore have a very large influence on the whole-life cost profile of the project.
Above information supplied by Turner and Townsend, Construction and Management Consultants 26 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SIX #/34¬!.$¬%#/./-)#3
LIFETIME COSTS EXAMPLE Paint: system 3 _ _ 200 The overall cost of protecting a steel Take the case of a steel structure that has a A superior paint system consisting of blast cleaning followed by three coats 197 fabrication throughout its life depends on the projected life of 25 years and for which the cost and durability of the initial coating in discount cost of capital is 5%. of a higher quality paint. This system the particular environment and, on the costs has a life expectancy of 11 years and 169 150 and frequency of any subsequent treatments Galvanize: system 1 will need to be repainted twice in 25 years. where the required life exceeds that of the The initial cost is higher than the other initial coating. Hot dip galvanize to BS EN ISO 1461 with a paint system at 135 units. The cost of minimum average coating of 85μm on steel repainting is half this value at 67.5 units. In the majority of applications, hot dip 100 of 6mm or more thick. As galvanizing to this (NPV = 197.5) 100 galvanizing will provide a long, maintenance- standard has an average life expectancy of free life without any requirement for more than 50 years in the UK (see Section 4), Conclusion maintenance painting. relative cost % it is very conservative to project a life of 25 years without further maintenance. Let the It can be seen that over a 25 year project 50 There are ways of calculating the benefits life the cost of a ‘cheaper’ paint system or disadvantages of different methods of cost of galvanizing be a base figure of 100 units. There are no further maintenance costs. is almost 70% more than the cost of corrosion protection. The most common galvanizing. Likewise the cost of a more method is to calculate the Net Present Value (NPV = 100) ‘expensive’ paint system is almost double (NPV) of each method and compare the that of galvanizing. In initial, or first, cost 0 results. This calculation takes into account Paint: system 2 terms hot dip galvanizing is comparative the cost of borrowing money, the initial cost system 1 system 2 system 3 of protection, subsequent maintenance costs A paint system consisting of cleaning with a good quality paint system. (galvanize) (paint) (paint) and the lifetime of the project. It is frequently followed by an undercoat and two top However, when looking at lifetime costs, used by companies to measure the likely coats of paint. This system has a life hot dip galvanizing works out to be outcome of a capital investment project. expectancy of 8 years and so will need considerably cheaper than most to be repainted three times in 25 years. other systems. intial first second third NPV = I ¬¬¬¬¬-¬¬¬¬ ¬¬¬¬-¬¬¬ The initial cost is slightly cheaper than cost repaint repaint repaint + P + P + etc (l+r) 1 (l+r) 2 hot dip galvanizing at 90 units. The cost of repainting for the first two Where I = Initial cost of protective system Fig. 12b. occasions is 45 units but goes up to ¬ -¬ ¬#OST¬OF¬MAINTENANCE¬IN¬YEAR¬0 90 units for the third repaint when the Net Present Value compared ¬ -¬ ¬#OST¬OF¬MAINTENANCE¬IN¬YEAR¬0¬ original paint must be removed. r = Discount rate (NPV = 169)
27 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SEVEN SPECIFYING HOT DIP GALVANIZING
SECTION SEVEN _
TITLE SPECIFYING HOT DIP GALVANIZING _
STANDARDS UK National Building Specification (NBS) _ 300
) There are many references to specifying The basic specification for hot dip galvanized t / coatings on iron and steel articles is defined 2 hot dip galvanizing throughout the National m ( Building Specification (NBS) system, the by a single standard, BS EN ISO 1461 200 ‘Hot dip galvanized coatings on iron and steel main location being G10 - Structural steel articles - specifications and test methods’. framing. General guidance on corrosion protection is also given. The galvanizer acts as a specialist subcontractor to a steel fabricator and, In the unlikely event of being unable to 100 as such, his contractual relationship is with identify the correct clause for a particular
ace area per tonne galvanizing application, please contact the fabricator, not with the ultimate user. f 80 It is important, therefore, that the user’s Galvanizers Association for specific advice. sur requirements for galvanizing are made clear l 60
to the fabricator and that all communications stee concerning galvanizing are channelled through the fabricator. To ensure the best quality and technical back-up service, it 40 should be stated that the work be ‘processed by a member of Galvanizers Association’. 30
When hot dip galvanizing is specified, the surface of the steel is completely covered 20 with a uniform coating whose thickness is determined principally by the thickness of 15 the steel being galvanized (Tables 3 and 4).
This is an important advantage of the 10 galvanizing process; a standard coating thickness is applied almost automatically. The actual thickness of galvanized coating achieved varies with steel section size, 5 surface profile and surface composition. Actual coating weights are often much more than the minimum specified in the standard. 1 2 4 6 8 10 20 40 60mm As coating life expectancy figures quoted are based on the minimum coating thickness they are therefore usually very conservative. .187 .25 .375 .5 .75 1.0 1.5 2.0 ins
18 16 14 12 10 8 Steel Wire Gage steel thickness
Fig. 13. Relationship between steel thickness and surface area/tonne
28 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION SEVEN SPECIFYING HOT DIP GALVANIZING
Table 3. The requirements of BS EN ISO 1461 include FASTENERS EN ISO 1461: coating minimum masses/ cleaning and preparation of steelwork as _ thickness on articles that are not centrifuged. well as galvanizing. Guidance on the use Specifications for fasteners should clearly and performance of hot dip galvanizing is state ‘that the fastener coating should articles & its local coating mean coating contained in BS EN ISO 14713 ‘Protection conform to BS 7371: Part 6: 1998’. thickness (minimum) (minimum) against corrosion of iron and steel in -ERELY¬TO¬SPECIFY¬@GALVANIZED¬IS¬OPEN¬TO¬ g/m22 μm g/m μm structures - zinc and aluminium coatings - misinterpretation and zinc electroplated, guidelines’ and further guidance is available mechanically plated or sherardized steel > 6mm 505 70 610 85 from Galvanizers Association. components, which provide less protection, MAY¬BE¬SUPPLIED¬-ORE¬INFORMATION¬ON¬ THICKER COATINGS steel > 3mm to ≤ 6mm 395 55 505 70 _ fasteners is given in Section 10. steel ≥ 1.5mm to ≤ 3mm 325 45 395 55 Thicker coatings than those set out in LEAD TIMES BS EN ISO 1461 can give additional _ steel < 1.5mm 250 35 325 45 protection for use in particularly aggressive Provided reasonable notice is given, most environments and can be specified in articles can be galvanized and returned castings ≥ 6mm 505 70 575 80 conjunction with BS EN ISO 1461. to the fabricator within a week. A typical It should, however, be emphasised that turn-around, depending on size of the order, castings < 6mm 430 60 505 70 for most applications, thicker coatings is three days. Galvanized bolts and nuts are are rarely necessary. now widely stocked, but it is advisable that orders for galvanized fasteners should be The means of achieving thicker coatings are placed as early as possible. Table 4. described in Section 2. Grit blasting prior to EN ISO 1461: coating minimum masses/ galvanizing is usually the most appropriate APPEARANCE thickness on articles that are centrifuged. method and a requirement for a nominal _ coating thickness of 1000 g/m2 (140μm) articles & its local coating mean coating has been successfully specified for steel Galvanizing is primarily used to protect thickness (minimum) (minimum) of 6mm section thickness. For structural steelwork against corrosion. Acceptable g/m2 μm g/m2 μm steelwork, it is advisable to ascertain whether appearance under BS EN ISO 1461 is articles with threads: thicker coatings could be achieved through discussed in Section 9. In cases where their greater section thickness and without appearance and smoothness are particularly > 6mm ø 285 40 360 50 grit blasting. important, the galvanizer should be consulted at an early stage. ≤ 6mm ø 145 20 180 25 Achieving thicker coatings through specification of a reactive steel is normally DUPLEX COATINGS _ other articles (incl. castings): only appropriate for specific applications. The use of paint or powder coatings over ≥ 3mm 325 45 395 55 Specification of thicker coatings must only a galvanized coating may be used for be made following consultation with the aesthetics or for additional protection < 3mm 250 35 325 45 galvanizer concerning viability and the means (see Section 11). by which they will be achieved.
29 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
SECTION EIGHT _
TITLE DESIGNING ARTICLES FOR GALVANIZING _ EARLY CONSULTATION BETWEEN GALVANIZER, FABRICATOR AND DESIGNER IS THE KEY TO OBTAINING THE BEST RESULTS FROM THE GALVANIZING PROCESS. DESIGN FEATURES, WHICH AID THE ACCESS AND DRAINAGE OF MOLTEN ZINC, WILL IMPROVE THE QUALITY AND APPERANCE OF THE COATING.
FILLING, VENTING General principles size of hollow minimum diameter Where venting of a hollow section is required, AND DRAINAGE section (mm) of hole (mm) for a longer section (e.g. > 3 m) there may _ 1. Holes for venting and draining should be a need for additional or larger vent holes be as large as possible. The minimum hole < 25 10 to help achieve the best possible surface Good design requires: diameters are given in Table 5. finish and advice should be sought from ≥ 25-50 12 the galvanizer. s¬MEANS¬FOR¬THE¬ACCESS¬AND¬DRAINAGE¬ 2. Holes for venting and draining should be of molten zinc diagonally opposite to one another at the high > 50 - 100 16 Holes that have been drilled for venting can point and low point of the fabrication as it is be plugged, but this is mainly necessary for s¬MEANS¬FOR¬ESCAPE¬OF¬GASES¬FROM¬INTERNAL¬ suspended for galvanizing. Very long hollow > 100 - 150 20 aesthetic reasons, because a galvanized compartments (venting) sections may require additional vent holes coating covers all surfaces. If required, to aid drainage and to help produce a better > 150 consult galvanizer tapered aluminium or plastic plugs are It is important to bear in mind that the surface finish. available and will prevent undesirable steelwork is immersed into a bath of molten ingress of water. zinc at a temperature of 450°C. Thus any 3. With hollow sections sealed at the ends, features which aid the access and drainage holes should be provided, again diagonally Detailed design advice is available from of molten zinc will improve the quality of the opposite one another, as near as possible Table 5. Galvanizers Association or directly from the coating and reduce costs. to the ends. In some cases it may be more Suitable sizes of vent holes in galvanizer. Some points to bear in mind for economical to provide V or U shaped notches tubular structures venting and draining are illustrated in the With certain fabrications, holes which are in the ends, or to grind corners off rectangular diagrams on pages 34 - 38. present for other purposes may fulfil the hollow sections - these procedures provide requirements for venting and draining; ideal locations for venting and draining. in other cases it may be necessary to provide extra holes for this purpose. 4. Where holes are provided in end plates or capping pieces, they should be placed For complete protection, molten zinc diagonally opposite one another, off centre must be able to flow freely to all surfaces of and as near as possible to the wall of the a fabrication. With hollow sections or where member to which the end plate is connected. there are internal compartments, the galvanizing of the internal surfaces 5. Internal and external stiffeners, baffles, eliminates any danger of hidden corrosion diaphragms, gussets etc., should have during service. the corners cropped to aid the flow of molten zinc.
30 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
BASE METAL AND COMBINATIONS SIZE AND SHAPE CASTINGS _ _ _ 1. Controlling welding procedures Plain carbon steel, some low-alloy steels and In recent years, the size and capacity Castings must be grit blasted before during fabrication. iron and steel castings can all be galvanized. of galvanizing plants has increased galvanizing, as embedded sand from the significantly. Reference should be made casting process cannot be removed by 2. Arranging weld seams symmetrically. Soldered or brazed components should not to the Directory of General Galvanizers or conventional chemical cleaning. When The size of weld seams should be be galvanized. www.galvanizing.org.uk for an indication designing castings to be galvanized, features kept to a minimum. of the bath sizes available in the UK and such as sharp corners and deep recesses A fabrication consisting of a variety of Republic of Ireland. When the length or depth should be avoided as these may develop 3. Avoiding large changes in structural materials with different surface conditions of the item exceeds the size of the bath, excessive distortion and thermal stresses cross-section, which may increase should be avoided, as this could affect the special techniques may be employed to during hot dip galvanizing. Large fillet distortion and thermal stress in the uniformity and appearance of the coating. facilitate dipping - in this case advice must radii and uniform section thickness are galvanizing bath. Where differing materials are used, grit be sought from the galvanizer. also desirable. blasting the entire assembly can minimise any differences which may arise due to FASTENERS MOVEABLE PARTS Where there is an inherent tendency to differing effects of pre-treatment. _ _ distort, e.g. in asymmetrically shaped fabrications, the effect can be minimised Preferably, the fabrication should be of To accommodate the thickness of zinc when Adequate clearance on mating surfaces, similar steel type throughout. or possibly eliminated by restricting the galvanizing threaded components, extra such as hinges, should be allowed if they fabrication to such a size and design that it clearance must be provided on the female are to move freely after galvanizing. An extra Steel fabrications that have been subject to can be rapidly immersed in a single dip. THREADS¬-ORE¬INFORMATION¬ON¬THE¬USE¬OF¬ clearance of at least 1mm is usually sufficient. The galvanizer should be consulted for advice heavy cold work (e.g. bent through a tight galvanized fasteners is given in Section 10. radius) may be susceptible to strain age at an early stage if this is being considered. embrittlement and should be stress relieved DISTORTION The size and position of filling and drainage prior to galvanizing. OVERLAPPING SURFACES _ holes in fabricated vessels can have a major _ effect on distortion, as can the size and If steel fabrications distort during galvanizing, position of lifting holes or lugs, particularly When the steel to be galvanized is a high- Overlapping surfaces should be avoided this is usually due to ‘built-in’ stresses on hollow fabrications. strength steel with a crack initiator and a as far as possible. Care must be taken not being released, as the steel is heated to the high level of residual stress, consideration to specify sealed articles for galvanizing. If galvanizing temperature. Stresses may be overlaps are completely sealed by welding STRENGTH should be given to reducing potential for steel inherent in the steel, but they can also be _ cracking to occur. Such a combination is very there is a risk of explosion during dipping due introduced by welding, cold forming, rare but when it does exist, advice should to increased pressure of any entrapped air. If and hole punching. The tensile properties of structural steels be sought from Galvanizers Association or overlaps are not completely sealed there is are not affected by galvanizing, reference made to Publication No 40/05, a danger of cleaning fluid entering the cavity Efforts can be made at the design stage and as shown in Table 6 (page 32). ‘Galvanizing structural steelwork - and then weeping out and causing localised elsewhere to minimise residual stresses, An approach to the management of staining (see page 36). for example: liquid metal assisted cracking’, published by the British Constructional Steelwork Association (BCSA) 31 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
Cold Rolled 10% Cold Rolled 40% With Punched Holes Welded Steel
As Galvanized Not Galvanized Not Galvanized Not Galvanized Not Not Received Galvanized Galvanized Galvanized Galvanized Galvanized
EN 10025-2 Tensile strength (Pa) 453 461 563 560 741 706 S275 0.5% proof stress (Pa) 294 281 550 502 732 659
Elongation (%) 45 46 18 22 8 15 *10 10 28 38
EN 10025-2 Tensile strength (Pa) 531 522 644 635 811 784 S355 0.5% proof stress (Pa) 367 362 634 587 807 746
Elongation (%) 41 43 16 20 8 15 +15 16 25 33
EN 10028-3 Tensile strength (Pa) 585 597 714 734 905 860 P460 0.5% proof stress (Pa) 451 446 692 683 896 842
Elongation (%) 36 34 21 21 10 13 ^6 5 29 30
key * As Received + 10 % Cold Rolled ^ 40% Cold Rolled
Table 6. Tensile properties of structural steel tests on plates 12.7mm thick (source: ILZRO, 2006)
32 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
33 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
POINTS TO CONSIDER WHEN DESIGNING FABRICATIONS FOR HOT DIP GALVANIZING.
WELDING MASKING SURFACE CONTAMINATION _ _ _ Welding slag is not removed in the cleaning If certain areas of steelwork need to remain Clean steel surfaces are an essential process and may result in black bare spots uncoated this can be achieved by masking, requirement for good hot dip galvanizing. after hot dip galvanizing. To avoid this using high temperature tape, grease or paint problem, efforts should be made to use gas and other anti-galvanizing treatments. Again, Contamination in the form of grease, tar, SHIELDED¬WELDING¬PROCESSES¬EG¬-)'¬AND¬IF¬ the galvanizer should be consulted about any paint and weld slag cannot be removed coated electrodes are used, welds should be areas required to be masked. by chemical cleaning and may result in thoroughly de-slagged. In order to minimise black bare spots after hot dip galvanizing. the incidence of raised weld seams after CONNECTIONS Specifiers should ensure that the fabricator hot dip galvanizing, the silicon content of _ takes responsibility for articles being the welding rod material should be less than delivered free from contamination. Galvanized articles can be joined by bolting 0.04%. Anti-spatter weld sprays should be (including friction grip connections), welding, water soluble and should be oil and silicone Steel sections, which have been cut or drilled riveting and adhesive bonding. Bolted joints free. Further information on welding is given using suds type oil can give similar problems are best made after galvanizing. Refer to in Section 10. to burnt on anti-spatter sprays. The cutting Section 10. fluid, which has been burnt or baked onto the LABELLING AND MARKING steel, should be removed prior to sending _ HANDLING OF ARTICLES the steelwork for galvanizing. _ Water soluble paint or detachable metal labels can be used for temporary Depending upon size and shape, articles for Contamination is sometimes difficult to identification marks on fabrications. Enamel galvanizing may require suspension holes or detect on the steel surface and will only based paints or oil based markers must lifting lugs. They may alternatively be handled show up after the galvanizing treatment. not be used. For permanent identification by chains or, for smaller articles, on racks or The article may then have to be regalvanized, -ORE¬EXTENSIVE¬GUIDANCE¬ON¬DESIGN¬FOR¬HOT¬ to be legible after galvanizing, large heavily in baskets. at additional cost. dip galvanizing can be found in punched or embossed marks are necessary. BS EN ISO 14713 and other In the case of tanks (in particular, open tanks), Galvanizers Association publications. cross stays may be necessary to ensure the shape of the vessel is maintained during handling.
34 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
Extenal stiffeners, welded gussets Cropping the corners of these brackets will Angle bracings should, if possible, be On structural hollow sections, provision and webs on columns and beams, aid access and drainage of molten zinc and stopped short of the main boom flange. must be made for venting and draining. and gussets in channel sections a cleaner coating will be obtained. This will allow the free flow of molten zinc With vertical members, drilled holes or V should have their corners cropped. across the surface of the flange, enhancing notches should be provided, diagonally drainage from the structure. This will assist opposite each other, at top and bottom. the development of a smoother galvanized coating, reduce the potential for retention of ash on the surface of the flange and help to avoid air traps within the structure, which could lead to uncoated areas.
35 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
Good venting of these sections will aid Every sealed section of a fabrication must Overlapping or contacting surfaces are access and drainage of molten zinc and be vented for reasons of safety and to allow potentially dangerous as pretreatment a cleaner coating will be obtained. access and drainage of molten zinc. solutions trapped between surfaces Holes diagonally opposite each other should are converted to superheated steam be as close as possible to the sealed end. in the galvanizing bath and can lead to an explosion.
If contacting surfaces can not be avoided, as with these channels, then the edges of the contacting areas should be continuously welded. One hole should be drilled through both members for each 100 cm2 of overlap, the minimum hole diameter being 10 mm, or the thickness of the section, whichever is greater, in order to eliminate the danger of an explosion in the galvanizing bath.
36 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
If this is not possible, then welding should Internal diaphragms in large box sections External stiffeners for beams or channels Alternative designs for venting sections be intermittent: in service there may be some should have cropped corners and a should have the corners cropped. fixed to base plates. weeping of trapped pretreatment solutions ‘manhole’. Internal diaphragms on small from between the plates leading to brown box sections should have cropped corners. staining, but this will not be detrimental to Where any hollow section is vented internally, the protection given by the coating. It will not it is essential for safety reasons, that the generally be necessary to make any provision galvanizer is able to view such venting. if the enclosed area is less than about 100 cm2 (e.g.10 cm x 10 cm).
Where designs cannot avoid larger areas of overlapping surfaces (e.g. flange to flange and plate along flange) specialist advice must be sought from the galvanizer or Galvanizers Association.
37 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
Examples of how weldments should be designed in order to avoid acid traps (narrow crevices).
Welded joints should be continuous if they are To minimise the risk of distortion, flat panels Large open top tanks should be stayed to Vents should be diametrically opposite and not enclosing an otherwise unvented surface. should be braced, e.g. dished or ribbed. minimise distortion. Where angles are used at least 50mm in diameter. Internal baffles Bolted joints are best made after galvanizing. Openings should be provided in the corners. to rim the tank, appertures must be provided should be cropped top and bottom. Lifting in the corners. Angles or flats used as stays lugs are required as indicated. It should be should be as close as possible to the tank possible to view the baffles through either wall thickness. the vent holes or an inspection hole - the placement of the inspection hole should be discussed with the galvanizer.
For further guidance please refer to BS EN ISO 14713 and the "Design for Galvanizing" wallchart available from Galvanizers Association.
38 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION EIGHT DESIGNING ARTICLES FOR GALVANIZING
39 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION NINE QUALITY AND INSPECTION
SECTION NINE _
TITLE QUALITY AND INSPECTION _
QUALITY ASSURANCE COATING WEIGHT OR THICKNESS appearance acceptability of protection _ (not necessarily of appearance) MEASUREMENT Galvanizers Association always insists upon _ dull grey coating acceptable the highest quality standards from its member The nature of the process ensures that, in (all alloy, no free zinc) companies, who are required to process work most cases, if the coating has formed it will to BS EN ISO 1461. The requirements of automatically be of sufficient weight to meet cracking in pooled zinc acceptable these standards ensure that the zinc coating the requirements of BS EN ISO 1461. is continuous and of the required thickness. There are a number of inspection techniques rust stains acceptable which can be used when necessary. easily removed by stiff brushing) Quality Assurance for industry has been
enhanced by the introduction of the BS A definitive check of the weight or thickness general roughness acceptable unless otherwise agreed EN ISO 9000 series of standards - ‘Quality of a zinc coating can only be made by Systems’. Already the majority of hot dip destructive methods, i.e. a ‘strip and weigh’ lumpiness and runs acceptable unless otherwise agreed galvanizing plants are registered and others test or by preparing and measuring thickness (uneven drainage) will be doing so in the near future. on a microsection. However, for most purposes, non-destructive instruments are pimples acceptable unless dross adequate. The magnetic instruments available contamination is heavy are of two types - one measures the magnetic attraction between a permanently magnetized bulky white deposit acceptable (provided coating weight needle and the base steel or iron, and the (wet storage stain) remains in compliance with other works on a magnetic induction BS EN ISO 1461) principle. Non-destructive tests can be carried out at any stage in the life of a flux staining not acceptable galvanized article to establish the thickness OF¬THE¬REMAINING¬ZINC¬COATING¬-AGNETIC¬ bare spots generally not acceptable, damaged areas testing of coating thickness is covered in can be repaired following guidance given BS EN ISO 2178. in the standard COATING FINISH _ Table 7. Table 6 summarises variations in finish which Summary of variations in may be observed. The variations are often galvanized finish (illustrated on pages 41- 43). caused by surface features of the steel itself, and the acceptability of a coating should usually be judged primarily on its long-term performance and corrosion resistance.
40 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION NINE QUALITY AND INSPECTION
DULL GREY OR Staining and discolouration by rust To avoid rust staining, all parts of a structure General roughness DARK GREY COATING should receive comparable effective corrosion _ Sound galvanized steel with many years protection where possible. Nuts and bolts BS EN ISO 1461 demands that a galvanized of corrosion-free life still remaining, can and other fasteners should also be hot dip coating shall be ‘smooth’, but points out Silicon is sometimes added to steel as a sometimes be rust stained or discoloured. galvanized (see Section 10). Welds should be that smoothness is a relative term and that deoxidant during production and this speeds This may give an incorrect impression that the continuous wherever possible to minimise the coatings on fabricated articles should not up the reaction between the steel and the coating has failed and may occasionally be retention of cleaning residues and should also be judged by the same standards as those molten zinc. When the galvanized article is visually unacceptable. It may be the result of be slag-free. Structures should be designed applied to mechanically wiped products, removed from the bath, but still remains hot, one or more of the following factors: to avoid run-off water from other metals such as galvanized sheet, tube and wire. the reaction may continue and convert all or onto galvanized steel. Where welding after part of the surface zinc layers to zinc-iron s¬ $IRECT¬CONTACT¬OF¬GALVANIZED¬ galvanizing is necessary, welded areas should An uneven coating is usually due to excessive alloys. These are dark grey compared with the articles with unprotected or be thoroughly cleaned and the zinc coating or uneven growth of the alloy layers, because light grey of pure zinc, although after a period inadequately protected steel restored (see page 43). of the composition or surface condition of of exposure the difference in grey colour (e.g., galvanized steel sections the steel. An uneven coating is often thicker becomes less pronounced. fastened with unprotected, Discolouration and staining from most than a conventional coating and therefore, electroplated or painted external sources have no effect on the life of has a longer life, but on rare occasions it may Zinc-iron alloy coatings on reactive steel steel bolts). the coating. However, affected areas may be be unsatisfactory or it may interfere with the are thicker and hence longer lasting, than cleaned to improve the appearance of the intended use of the article. those on rimmed or aluminium-killed steels. s¬ $EPOSITS¬OF¬IRON¬AND¬STEEL¬DUST¬AND¬¬ structure. Generally, wire brushing or the use Corrosion resistance, thickness for thickness, swarf from other operations or of a scouring powder will remove the stain is at least as good as unalloyed zinc and may sources on the galvanized surface. and leave a sound galvanized coating. be better in acidic industrial atmospheres. Zinc-iron alloys are more abrasion resistant s¬ 7ATER¬DRAINING¬FROM¬UNPROTECTED¬ than zinc, but thick coatings have a greater or poorly protected steelwork, tendency to flake if handled roughly and e.g. from damaged areas on appropriate care must be taken. painted steelwork. The dark grey coating surface may develop s¬ &ROM¬CLEANING¬RESIDUES¬IN¬WELDS¬ staining after a relatively short period of During cleaning, acid may exposure, even in mild, damp conditions. This penetrate into the weld area via pin is only a surface effect and does not indicate holes or other gaps in the welding. serious deterioration of the coating: the galvanized coating remains and continues to s¬ 2USTING¬OF¬AREAS¬WELDED¬AFTER¬ protect the steel. galvanizing and subsequently left unprotected or inadequately protected.
s¬ 7ATER¬RUNNING¬OFF¬OTHER¬MATERIALS ¬ notably metals such as copper and certain hardwoods (e.g. oak). This effect may occur whenever water can dissolve materials from one surface and deposit them on the galvanized steel.
41 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION NINE QUALITY AND INSPECTION
Lumpiness and runs Pimples Wet storage stain Flux and dirt staining
Lumps and runs caused by uneven drainage Pimples are caused by inclusions of dross Wet storage stain is the white corrosion Where flux is used during the dipping of zinc from an article when it is removed (a pasty zinc/iron alloy residual that forms products and dark stains, which may be seen process, flux residues may adhere to the from the bath may occur due to shape or in the galvanizing bath) in the coating. on the surfaces of newly galvanized articles surface after immersion and pick up moisture thinness of the component and are not These may arise from iron salts carried over when they have been closely stacked and to form white corrosion products. Although harmful to the life of the coating. Sharp points on the work from the cleaning tank and stored, or transported under damp or wet this is a surface effect, flux stains may be of excess solidified zinc are not acceptable as unable to escape from the surface of the conditions. Where wet storage stain has detrimental to the life of the coating and they may present a hazard during handling. coating. Contamination may also arise from formed, the coating beneath may be stained should be removed. Fabrications with areas where spikes have agitation of the dross layer at the bottom of dark grey or black. been knocked off, exposing bare steel, the bath. Dross has a similar corrosion rate to Dirt may be picked up on the surface of require remedial repair of the coating, that of zinc and its presence in the coating as To prevent wet storage stain, zinc coated the coating from the site, truck beds or which is explained overleaf. finely dispersed particles, is not objectionable. articles must be transported and stored under from contact with other articles. These are But, major dross inclusions tend to embrittle dry and well ventilated conditions. If stored readily washed off to reveal a sound coating the coating and the galvanizer will avoid this. outdoors, the surfaces should not be in close underneath and are not, therefore, harmful. contact: free circulation of air is necessary to prevent condensation and retention of moisture. Nesting or close packing must be avoided as capillary action can attract water into closely contacting surfaces. Articles should not be stored in direct contact with the ground.
Barrier coatings, such as clear lacquers, may be used to preserve a bright finish where appearance is paramount. A range of proprietary products are available, some formulated for specific applications. Heavy deposits of wet storage stain should be removed. This can usually be achieved with a stiff bristle brush or light abrasives. Chemical methods should be used as a last resort and require thorough rinsing with fresh water after use.
42 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION NINE QUALITY AND INSPECTION
Bare spots RENOVATING DAMAGED s¬ 4HOROUGHLY¬WIRE¬BRUSH¬THE¬AFFECTED¬AREA Zinc rich paint is much the simplest to apply, COATINGS and apply sufficient coats of zinc rich especially on site. Thermal zinc spraying Due to the sacrificial action of zinc, small _ paint (by brush or aerosol spray) to give is usually only economic when applied in localised flaws up to 5mm maximum width a coating thickness in accordance with the workshop. Small areas of the coating may be damaged are usually self-healing and have little effect the supply standard e.g. EN ISO 1461 by operations such as cutting or welding on the life of coating. Articles with uncoated BS EN ISO 1461 requires that the coating after galvanizing and although a galvanized areas due to faulty processing will be s¬ 4HOROUGHLY¬WIRE¬BRUSH¬THE¬AFFECTED¬AREA ¬ thickness on renovated areas shall normally coating has excellent resistance to rough detected by the galvanizer on inspection and heat it with a blow torch to 300°C and be 30μm more than the local coating treatment, small areas of damage may retreated but, occasionally bare spots can apply a special zinc alloy rod or powder thickness requirement for the galvanized occasionally occur in transport and erection. also be caused by rolling defects in the steel to obtain required repair coating coating. An exception to this may be Due to the sacrificial action of zinc, small such as laps and folds, laminations and thickness. This may, however, be difficult when the coating is to be overcoated and localised flaws do not reduce protection. non-metallic impurities rolled into the surface. or too time-consuming to carry out on the renovated area then requires an Nevertheless, it is often aesthetically erected steelwork, or on awkward, or equivalent thickness. desirable to renew the coating even over inaccessible areas. such small areas. Care should be taken in applying heat by Purchasers and applicators of subsequent a blowtorch as excessive heat may coatings should ensure that the applied Adequate corrosion resistance will be damage the surrounding galvanized system is compatible with the method achieved at any damaged area if a weight coating, especially on the heavier coatings and materials used for renovation. of zinc is deposited equivalent to the weight resulting from reactive steels. of the undamaged coating. The following techniques are acceptable according to s¬ 'RIT¬BLAST¬THE¬AFFECTED¬AREA¬AND¬THERMAL¬ BS EN ISO 1461: zinc spray. A 100μm thermally sprayed zinc coating confers corrosion protection equivalent to an 85μm galvanized coating.
43 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TEN JOINING GALVANIZED STEEL
SECTION TEN _
TITLE JOINING GALVANIZED STEEL _ THERE IS NO REASON WHY STRUCTURES SHOULD HAVE THEIR LIFE EXPECTANCIES REDUCED BY THE SERVICE LIFE OF THE THREADED FASTENERS USED IN ASSEMBLY, SINCE THEY TOO CAN BE HOT DIP GALVANIZED.
HOT DIP GALVANIZED Surface finish and appearance Specification of hot dip Friction grip connections THREADED FASTENERS galvanized fasteners _ Galvanized fasteners usually have a bright Initially, the coefficient of friction with light grey appearance, but with certain grades Selected zinc-based protective coatings on galvanized contact surfaces is low - an Sizes of high yield and high tensile bolts, the threaded fasteners have been listed in average of about 0.19. As slip commences coating may be matt grey, because the higher Table 8. One difficulty arises because however, friction rapidly builds up and As a general rule, nuts, bolts and washers silicon content of the steel makes them more electroplating is often known - misleadingly ‘lock-up’ occurs, due to cold welding down to 8mm diameter can be galvanized reactive towards the molten zinc. Fasteners - as electrogalvanizing. It is not, therefore, between the coated surfaces. If a small and a wide range of threaded components hot dip galvanized at high temperature, enough merely to specify ‘galvanizing’ if a amount of slip can be tolerated it is therefore can now be processed using special (around 550°C) tend to take on a uniform, long life is required. The fastener specification unnecessary to treat the surfaces, but if equipment. For ISO metric fasteners, matt grey appearance due to the structure should clearly state ‘that the fastener coating all slip must be avoided, the coefficient of the galvanizing of one thread either internal of the coating formed when the component should conform to BS 7371: Part 6: 1998’ friction can be raised by roughening the or external requires an extra clearance of four is cooled. and the addition of a clause ‘to be galvanized surface of the galvanized coating. times the coating thickness. In practice, it is by a member of the Galvanizers Association’ Wire brushing will raise it to 0.35 and a normal for standard bolts from stock to be Storage will ensure the high quality provided by its figure of 0.5 can be achieved by a light grit fully galvanized, but for nuts to be galvanized member companies and the technical blasting, or by roughening with a pneumatic as blanks and then tapped up to 0.4mm Galvanized fasteners should be stored under back-up service of GA. chisel hammer, or needle gun. In the United oversize with the threads then lightly oiled. dry well ventilated conditions to minimise States, galvanizing is one of the few coatings When assembled, the nut thread is protected the occurrence of wet storage stain Relative costs permitted on the contact surfaces in the by contact with the coating on the bolt. Even (see Section 9). specification for friction grip joints. This after many years of service, galvanized nuts The initial cost of hot dip galvanizing threaded is approved by the Research Council for can readily be unfastened even though the Table 8. fasteners is generally a little higher than Riveted and Bolted Structural Joints of the threads have never been galvanized. Further zinc plating to BS3382: Part 2. However, Engineering Foundation. details of the dimensions, processing and type coating thickness in terms of cost per year of rust-free life, performance of hot dip galvanized nuts and hot dip galvanizing is by far the most The ‘lock-up’ effect described above can bolts is given in ‘The Engineers & Architects' zinc electroplated 5 - 12 μm economical coating. cause galling in the threads of galvanized Guide to Hot Dip Galvanized Nuts and Bolts’, (typical) fasteners used in friction grip connections published by Galvanizers Association. High strength bolts and lubrication may be required in order that sherardized class I 30 μm the required clamping force is developed. Coating uniformity. General grade high strength bolts (ISO grade Beeswax has been found to be a most sherardized class II 15 μm ¬TO¬"3¬¬0ART¬ ¬EQUIVALENT¬TO¬!34-¬ effective lubricant and molybdenum Whilst there is some tendency for hot dip A325) can be galvanized without difficulty. disulphide, or tallow, have also been galvanizing to be thicker in thread roots, a hot dip galvanized 43 μm (min) ISO Grade 10.9 bolts (BS4395: Part 2 or specified for this purpose. near uniform coating can be obtained with BS 7371: !34-¬! ¬ARE¬GALVANIZED¬IN¬THE¬5+ ¬*APAN ¬ modern equipment. Any thickening that does part 6: 1998 Italy, France and Germany but may require occur can be accommodated in the normal blast-cleaning as an alternative treatment truncation on the thread of the nut. prior to hot dip galvanizing. Grade 12.9 bolts and higher strength fasteners should not be galvanized.
44 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TEN JOINING GALVANIZED STEEL
WELDING GALVANIZED STEEL Detailed recommendations for this and other _ fusion welding processes are available from Tests at The Welding Institute sponsored Galvanizers Association. by the International Lead Zinc Research Organisation (ILZRO) have established that While zinc is a necessary trace element in satisfactory high quality welds can be made the human diet and it does not accumulate on hot dip galvanized steel and that the in the human body, the inhalation of freshly tensile, bend and fatigue properties of such formed zinc oxide fumes can cause a welds can be virtually identical to those transient ‘metal fume fever’ with symptoms of similar welds made on uncoated steel. similar to influenza. To maintain fume levels Welding speeds are slower and there is more within acceptable levels, extraction should be provided when welding galvanized steel spatter, particularly when CO2 welding. in confined areas, as indeed it should when All the fusion welding processes can readily welding uncoated steel. Wherever welding be used on galvanized steel, but minor takes place, due note must be taken of the variations in technique may be required, relevant COSHH regulations. depending on the welding process used, the RUST PREVENTION AT WELDS type of joint and the welding position. For _ example, with manual metal arc welding: All welds made in galvanized articles should s¬ !¬SLIGHT¬@WHIPPING¬ACTION¬MOVING¬ be protected against rust as soon as welding the electrode forward and is finished, as the top surface is then free backward along the joint. from rust and easy to treat. Treatment can be carried out as described in Section 9. s¬ LINE¬OF¬THE¬JOINT¬ENCOURAGES¬ volatization of the zinc in front of the weld pool. s¬ 3LIGHTLY¬WIDER¬GAPS¬ARE¬RECOMMENDED¬ in butt joints to give complete penetration. s¬ !¬SHORT¬ARC¬LENGTH¬GIVES¬BETTER¬CONTROL¬ of the weld pool and helps to prevent either intermittent excess penetration, or undercut. s¬ "OTH¬BASIC¬AND¬RUTILE¬COATED¬ELECTRODES¬ can be used but, simple procedural tests should be made before undertaking production welding.
45 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ELEVEN PAINTING AND POWDER COATING OF GALVANIZED STEEL
SECTION ELEVEN _
TITLE PAINTING AND POWDER COATING OF GALVANIZED STEEL _ HOT DIP GALVANIZING BY ITSELF IS A LONG LASTING AND COST EFFECTIVE MEANS OF CORROSION PROTECTION. WHEN ORGANIC COATINGS SUCH AS PAINT OR POWDER COATINGS ARE APPLIED OVER GALVANIZED STEEL, THE RESULTING COMBINATION IS KNOWN AS A DUPLEX COATING. THESE COATINGS ARE USED TO:
Add colour for aesthetic, The reasons for the need for effective surface GUIDELINES: PRE-TREATMENT T-Wash (or its proprietary equivalent) camouflage, or safety purposes preparation in most cases are quite straight FOR PAINTING forward. When the steel is withdrawn from the _ Despite the fact that this preparation process Increase the economic life of a structure galvanizing bath it has a clean, bright, shiny has been available for some considerable Guidelines for the pre-treatment of galvanized surface. With time this changes to a dull time, T-Wash is still generally considered to surfaces have been drawn up as a result Provide additional protection in grey patina as the surface zinc reacts with be the best pre-treatment method for painting of (i) a study carried out by an independent aggressive environments oxygen, water and carbon dioxide in the galvanized steel. T-Wash is a modified zinc research centre and a leading UK paint atmosphere to form a complex, but tough, phosphate solution, which contains a small manufacturer into the performance of Paint coatings may be applied soon after stable, protective layer, which is tightly amount of copper salts. When applied, a commercially available pre-treatment and galvanizing, or later in the lifetime of the adherent to the zinc. The patina takes time to dark grey or black discolouration of the paint systems and the parameters affecting structure when the galvanized coating develop and the exact time depends upon the zinc surface will result. T-Wash must not be their performance on hot dip galvanized has weathered, or later still when further climate around the galvanizing. Typically, the allowed to pool on horizontal surfaces, or this coatings, and (ii) many years experience protection is essential to maintain protection. time can vary from six months to two years will prevent maximum paint adhesion. with duplex coatings. or more. During this transition of the zinc Any excess should be removed by water. When paint is to be used to further extend outer layer into its final state, simple oxides T-wash is most suitable for application to Although pre-treatment of galvanized the life of a galvanized structure, it is often and carbonates form, which do not adhere new galvanizing and should not be used components is best carried out immediately most economic to defer painting until a long strongly to the surface. As most duplex on weathered galvanizing after galvanizing, before the surface has maintenance-free life has been achieved from coatings are applied whilst the galvanizing is (see etch primers below) become contaminated in any way, this is the galvanized coating. in this condition, the surface layer must be not always practical. Pre-treatment can be modified by chemical or mechanical means. The constituents of T-Wash are phosphoric carried out later, but it is vital that the surface Coatings may be applied directly to the acid (9.0%), ethyl cellusolve (16.5%), PREPARATION OF is adequately cleaned to remove all traces of initial pure zinc surface, or to the weathered methylated spirit (16.5%), water (57.0%) and contaminants such as oil, grease and dirt. GALVANIZED STEEL surface, but the results are not always copper carbonate (1.0%). Variations to this _ The cleaning operation must leave no consistent and “taking a chance” composition may exist and so it is wise to residues on the cleaned surface and any wet As with all protective treatments of steelwork, is not recommended. consult the supplier if a successful result is storage staining should be removed using a it is of great importance that preparation of to be achieved. stiff brush. Washing down the coating with the galvanized surface is carried out in a Where the aesthetic requirements for a water will help to remove soluble salts. thorough and considered manner. duplex system are particularly high, a degree Sufficient time must be allowed for the In particular, failure to degrease the of surface finishing or fettling may be required T-Wash to react and dry thoroughly before There are four recognised methods of surface galvanized steel surface properly is a on galvanized coatings, as small surface the first coat of paint is applied. pre-treatment that produce a sound substrate common source of failure of duplex coatings. projections may become more obvious after (Suppliers’ information will give recommended for paint coating: the application of an organic coating. time intervals). While research has shown As with many other substrates, organic This is particularly the case for powder that T-Washed surfaces can be left for up coatings can not usually be applied directly coating systems. Care must be taken when to 30 days before painting and good paint onto galvanized steel. However, simple smoothing a galvanized coating, because the adhesion can still result, it is advisable to direct application paint systems, designed zinc coating may be damaged by heavy or minimise the time between pretreatment specifically to adhere to non-ferrous metals excessive grinding. and paint application. Any white salt formed such as zinc, are becoming more popular and by the exposure of the T-Washed surface are available in a full range of colours. to moisture must be removed before painting using a stiff brush. If the T-Washed surface has become contaminated, it must be cleaned in accordance with the suppliers’ recommendations.
46 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION ELEVEN PAINTING AND POWDER COATING OF GALVANIZED STEEL
Etch primers Weathering Two-pack polyurethane and acrylic urethanes However, the pre-treatment of the galvanized are commonly used as top coats and surface will depend upon which of the Etch primers have also been used This process only becomes fully effective offer good durability and colour retention. many powder types such as polyester, successfully. Their major disadvantage is after a galvanized surface has been exposed Alternatives include acrylic epoxies and epoxy, or hybrid is being used. This usually the absence of any visible colour change to the atmosphere for a period of at least six polysiloxanes, the latter offering increased includes a form of chemical pretreatment in contrast to T-Wash. Therefore, there months. The surface is prepared using either abrasion resistance along with good gloss such as chromating or phosphating, gentle can never be complete confidence that all abrasive pads, or a stiff brush, to remove all and colour retention. heat treatment, followed by application of surfaces have reacted with the primer. loose adherent materials and making sure the powder. The successful application of Etch primers are most suited to application that the bright zinc surface is not restored. At present, high-build epoxies are widely used a powder coating to any metallic surface on older, weathered galvanizing although water-based products, including requires the multistep instructions provided This is followed by a hot detergent wash and polyurethanes, are now being specified. by the powder manufacturer to be respected Sweep blasting rinsing with fresh clean water. The surface They are less tolerant of poor pre-treatment, in every detail. For this reason an experienced must be fully dry before any paint is applied. but their use could to grow as the Solvent or approved applicator should be asked to A mechanical method of pretreatment is Weathering should not be used as a method Emissions Directive becomes more stringent. do the work. As with wet painting, a full range sweep blasting using fine copper slag, J blast of surface preparation in marine environments of colours is available. Powder coating of or carborundum powder with a blast pressure with high chloride levels. GUIDELINES: galvanized steel for architectural applications of no greater than 40psi (2.7 bar). This will POWDER COATING is covered at present by BS EN 13438. ensure that only the minimum amount of GUIDELINES: PAINTING _ oxide is removed and the zinc surface is left _ It is important that the galvanizer is advised Powder Coating is a fast growing method of in a slightly roughened condition. Care should that work is to be subsequently powder All paint systems should be specifically adding colour to metal surfaces. be taken when carrying out sweep blasting coated and post-galvanizing treatments formulated for use on galvanized steel Like galvanizing it is carried out under on very thick galvanized coatings to avoid agreed with the powder coater. and applied in accordance with the paint carefully controlled conditions in a factory. damage to the coating. The optimum nozzle- manufacturer’s recommendations. For this reason the maximum size of the to-work piece distance and angle of blasting A limited number of direct application steel fabrication to be powder coated will be needs to be identified for all surfaces on the products are now available for use on The choice of paint system will depend upon limited, but powder coatings can be applied galvanized steel work, if optimum results galvanized steel work. Given adequate both the application and service environment. successfully to hot dip galvanized surfaces. are to be achieved. Angular iron blasting grit With the decline in the use of chlorinated preparation work, the use of a direct must not be used under any circumstances. application paint negates the need for rubber and alkyd paints, high- build epoxy The thermal characteristics of galvanized Sweep blasting is often used in addition to any chemical or mechanical pre-treatment products and vinyl/vinyl co-polymer systems steel are almost identical to those of the chemical preparation stage. to be conducted. These products are are increasingly utilised with glass reinforced un-galvanized steel for powder coating known to have been used in a variety epoxy being an option for more severe purposes and there are many examples of applications. environments. In multi-coat systems the use of steel which has been galvanized and OF¬A¬MICACEOUS¬IRON¬OXIDE¬-)/ ¬PRIMER¬HAS¬ then powder coated. been shown to give improved adhesion.
47 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWELVE RELEVANT STANDARDS
SECTION TWELVE _
TITLE RELEVANT STANDARDS _ LISTED BELOW ARE NATIONAL AND INTERNATIONAL STANDARDS ON HOT DIP GALVANIZING AND OTHER ZINC COATINGS. THERE ARE OVER 100 BRITISH STANDARDS AND CODES OF PRACTICE WHERE HOT DIP GALVANIZING IS SPECIFIED AS THE REQUIRED OR OPTIONAL FINISH. _
BRITISH AND EUROPEAN Other zinc coating standards Other useful British Standards BS EN 23270: 1991 STANDARDS Specification for temperature and _ BS EN 12329: 2000 BS 7361: Part 1: 1991 humidities for conditioning and testing paints, varnishes and their BS EN ISO 1461: 1999 Corrosion protection of metals. Cathodic protection. raw materials. Hot dip galvanized coatings on fabricated Electrodeposited coating of zinc with
iron and steel articles – specifications supplementary treatment on iron or steel. BS EN 13636: 2004 BS 4395: Parts 1 & 2: 1969 and test methods. Cathodic properties of buried metallic tanks BS EN ISO 2063: 2005 and related piping. High strength friction grip bolts. BS EN ISO 14713: 1999 Sprayed aluminium and zinc coatings. BS EN 14399: Parts 1-6: 2005 Protection against corrosion of iron BS 7773: 1995 High strength structural bolting and steel in structures – Zinc and aluminium BS 3382: Part 2: 1961 Cleaning and preparation of metal surfaces. assemblies for preloading. coatings – Guidelines. Electroplated zinc on threaded components. PD 6484: 1979 BS EN ISO 1460: 1995 BS 7371: Part 6: 1998 BS 4921: 1988 Corrosion at bimetallic contacts. Determination of hot dip galvanizing Coatings on metal fasteners – specification Sheradized coatings on iron coating mass: gravimetric method. for hot dip galvanized coatings. and steel articles. BS 7079: 1994 Surface preparation of steel substrates BS EN ISO 1463: 2004 BS EN 10244: Part 2: 2001 before application of coatings. Steel Wire & Wire Products – Non ferrous -EASUREMENT¬OF¬COATING¬THICKNESS¬ microscopic method. metallic coatings on steel wire. BS EN ISO 2178: 1995 -EASUREMENT¬OF¬COATING¬THICKNESS¬ BS EN ISO 2064: 2000 BS EN 10326/10327: 2004 magnetic method. Definition and convention concerning Hot dip zinc coatings on steel sheet. BS EN ISO 9001: 2000 coating thickness measurement. BS 3083: 1988 Quality management systems. BS EN ISO 12944: 1998 Hot dip coated corrugated sheet Paints and varnises - corrosion for general purpose. BS EN ISO 11124: Parts 3 & 4 Chilled iron shot and grit for grit blasting. protection of steel structures by protective paint systems. BS EN 971-1: 1996 Paints and varnishes terms and definitions for coating materials. General terms.
48 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION TWELVE RELEVANT STANDARDS
ASTM STANDARDS DIN STANDARDS SWEDISH STANDARDS AUSTRALIAN STANDARDS _ _ _ _ A 123/A 123M – 02 DIN 267 SS 3192 AS/NZS 4680 Hot dip galvanized coatings Hot dip galvanized fasteners Hot dip galvanized threaded components Hot dip galvanized (zinc) coatings on fabricated products. on fabricated ferrous articles DIN EN ISO 1461 SS 055900 A90/A 90M – 01 Hot dip galvanized coatings on fabricated Blast cleaning steel AS/NZS 4534 Test method for weight of zinc coating iron and steel articles - Specifications Zinc and zinc/aluminum alloy coatings on iron and steel articles. and test methods SS 3583 on wire Principles and requirements for A143 – 03 DIN 50978 hot dip galvanizing AS/NZS 4791 Safeguarding against embrittlement. Testing of adhesion of hot dip Hot dip galvanized (zinc) coatings on ferrous galvanized coatings SS EN ISO 1461 open sections applied by an in-line process A153/A 153M – 05 Hot dip galvanized coatings on fabricated iron Hot dip galvanized coating on iron DIN 50933 and steel - Specifications and test methods AS/NZS 4792 and steel hardware. -EASUREMENT¬OF¬COATING¬THICKNESS¬USING¬ Hot dip galvanized (zinc) coatings on ferrous stylus instrument ISO STANDARDS hollow sections, applied by a continuous or A325 – 06 _ a specialised process High strength carbon steel bolts. DIN 51213 ISO 1459 Testing of zinc coatings on wire Protection by hot dip galvanizing: guiding A384-02 principles Safeguarding against distortion. DIN 59231 Galvanized corrugated sheet ISO 2063 A385 – 05 Practice for providing high quality DIN 50961 -ETAL¬SPRAYING¬BY¬ZINC¬AND¬ALUMINIUM galvanized coating. Electroplating zinc ISO 2081 Electroplated zinc coatings A653/A 653M – 06 DIN EN ISO 2063 Hot dip galvanized steel sheet. Sprayed aluminium and zinc coatings ISO 3575 Continuous hot dip galvanized sheet A767/A 767M – 05 Hot dip galvanized rebar.
A780 – 01 Repair of damaged hot dip galvanizing.
49 THE ENGINEERS & ARCHITECTS' GUIDE HOT DIP GALVANIZING SECTION THIRTEEN ADVISORY SERVICES
SECTION THIRTEEN _
TITLE ADVISORY SERVICE _ UNITED KINGDOM AND REPUBLIC OF IRELAND _GALVANIZERS ASSOCIATION
Galvanizers Association was set FOR ADDITIONAL INFORMATION up in 1949 and comprises the leading _ galvanizing companies throughout the British Isles and others throughout så Visit www.galvanizing.org.uk the world. In the British Isles more than 90% of work is processed by members så Subscribe to “Hot Dip Galvanizing” magazine and internationally there are over 60 (quarterly publication highlighting interesting overseas affiliate members in projects across Europe) 28 countries. Contact Nicky Smith 0121 355 8838 email [email protected] Galvanizers Association is a non-profit making organisation promoting hot dip galvanized steel and providing a service of technical advice and information to producers and users. This information and advice is generally available free of charge.
LISTED BELOW ARE SOME OF THE SUBJECT AREAS COVERED BY GA PUBLICATIONS. _
så !PPLICATIONSå så /RGANICåCOATINGSå on galvanizing så &ASTENERS så $ESIGNåIMPLICATIONSå for galvanizing så 7ELDING så 0ERFORMANCEåINFORMATION så 3USTAINABILITY så #ASEåHISTORIES
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PLEASE NOTE ACKNOWLEDGEMENTS WITH THANKS TO _ _
Care has been taken to ensure that the Page 6 Eden Project contents of this publication are accurate, Nicholas Grimshaw & Partners but Galvanizers Association does not accept Paul Armiger responsibility for errors or for information which is found to be misleading. Suggestions for, Page 7 Eden Project or the end use or application of, galvanized Nicholas Grimshaw & Partners products are for information only and Peter Cook / View Galvanizers Association accepts no liability in respect thereof. Before having products hot Page 9 The Pump House dip galvanized, the customer should satisfy Jaqueline Demaus, themselves of their suitability. Tenbury Wells
Page 10 Wedge Group Galvanizing
Page 11 & 13 IKEA Stubbs Architects Diem Photographs
Page 15 Free University, Berlin Foster & Partners Nigel Young
Page 17 Clydebank Pool Hall Doyle Partnership Guthrie Photography
Page 20 & 21 Yorkshire Sculpture Park Feilden Clegg Peter Cook / View
Page 25 Houses in Constance Schaudt Architecten Reiner Blünck, Germany
Page 30 & 31 Silversmith’s Workshop Plasma Studios
Page 32 & 33 Gormley Studio David Chipperfield Architects Arcaid, Richard Bryant
Page 35 & 36 Joseph Ash Ltd
Page 39 Raines Court Allford Hall Monaghan Morris Tim Soar
Page 45 St Augustine’s School CTM Architects
Page 46 & 47 Bellmouth Passage Footbridge Techniker & Patel Taylor Peter Cook / View Wren’s Court, 56 Victoria Road, Sutton Coldfield, West Midlands, B72 1SY _
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www.galvanizing.org.uk DESIGN: UN.TITLED TEL: 0116 2471111 WWW.UN.TITLED.CO.UK printed 10/07