Steel fabrications that are to be hot dip galvanized must be DESIGNED for GALVANIZING.
The temperature of a galvanizing bath is about 450°C and immersion time is usually around 5 minutes, and may be longer with larger sections.
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| External stiffeners should have the corners cropped to eliminate 'dead pockets'. |
This immersion has little effect on the mechanical properties of steel used for structural applications. However, stresses can arise from welding or from cold working operations such as severe forming or bending, shearing and punching.
Some steels are susceptible to strainage hardening, and galvanizing can cause accelerated aging and embrittlement. Such steels are usually incorrectly specified if they cause any such problems and galvanizing simply reveals the possibility of failure at an early stage.
When fabricating work is to be galvanized, welding techniques must be adopted to prevent locking up stresses into the work. Where stresses cannot be avoided, it may be possible to galvanize components separately and bolt them together. These problems should be discussed with the galvanizer if any doubt exists.
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| Internal diaphragms on small box sections must have large 'crops' in the corners. |
Drainage of molten zinc from fabrications
One of the advantages of the hot dip galvanizing process is its ability to penetrate all parts and surfaces of the work, both internal and external. With any hollow sections or work with fabricated compartments, it is essential that its design allows molten zinc to flow freely into and out of the work.
Sealed compartments in fabrications are an explosion hazard when dipped in molten zinc, especially if they contain any liquid that can vaporise.
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| Internal diaphragms on large box sections to have cropped corners and a 'manhole'. |
Holes in work for venting and draining should be as large as possible, with 10mm holes should be considered minimum for small fabrications and much larger holes are needed on bigger work.
Holes for venting and draining should be positioned in the work so that regardless of the attitude of the work as it is dipped in the bath, the zinc can flow freely in and out without being trapped behind gussets or stiffeners.
Where hollow sections are joined together, holes must be provided connecting the sections, or each section must be individually vented to ensure complete zinc protection and to prevent any explosion hazards.
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| If holes can be provided in the locations indicated, a cleaner blemish free coating will be obtained. |
Holes should be positioned to take advantage of the natural drainage characteristics of the work. For this reason it is not usually necessary to plug drainage holes as the same degree of zinc protection is provided to the fabrication inside and out. The drain holes will prevent any accumulation of water in the work under normal circumstances when it is in service.
Identification
Permanent identifications should be provided on the work to be galvanized. Chalk and crayon are usually removed in the pickling process and are not a satisfactory means of marking and identifying work. Stamped, welded or metal tagged identification is best used for permanence. These methods remain visible after galvanizing and assist both customer and galvanizer.
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| Angle bracings should, if possible be stopped short of the main boom flange. |
Cost savings in galvanizing
When work is specified to be galvanized, it is often possible for the fabricator to obtain a lower cost galvanizing job by ensuring the work can be galvanized with a minimum of handling. This may mean providing lifting lugs on the work to facilitate slinging and ensuring the work is as clean as possible.
Oils, greases and paints must be removed prior to galvanizing. The galvanizer can remove these coatings by using hot caustic stripping solutions, but this may add to the cost of hot dip galvanizing operation. Some coatings, like black japan and special marking paints, are extremely difficult to remove - even in caustic baths.
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| Structural hollow sections. Provision must be made for venting and draining. On vertical members, drilled holes should be provided diagonally opposite each other at top and bottom. Alternatively, a 'v' notch can be cut in the end of the member. |
This difficulty is further aggravated if the coatings are over weld affected areas, as the high temperatures convert black japanned coating into a refractory film that is extremely difficult to remove.
Work kept free of unnecessary markings and coatings will be easier and quicker to galvanize with attendant cost savings. Some fabrications may be able to be designed to be assembled after galvanizing, and this may save the galvanizer having to double dip the work.
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| External stiffeners for beams and channels should have the corners cropped. |
Designing for galvanizing will ensure job satisfaction is achieved by both the customer and the galvanizer, at the lowest possible cost.
The galvanizer involved in the project will be pleased to offer practical advice on any specific job and further technical and design information on designing for galvanizing can be obtained from Sulphide Corporation (PO Box 42 Boolaroo NSW 2284) or through the Galvanizers Association of Australia, 95 Collins Street, Melbourne Vic 3000.
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| If contacting surfaces cannot be avoided, as with these channels, then the edges of the contacting areas should be continuously welded and a hole drilled through both members to eliminate the danger of an explosion in the galvanizing bath. If this is not possible then welding should be intermittent; in service there may be some weeping rust stains from acid residues trapped between the plates but this will not be detrimental to the protection given by the coating. It will not generally be necessary to make any provision if the enclosed area is less than 70cm. |