Technical conditions for hot dip galvanizing of steel structures.
Hot dip galvanization is the process of coating parts with a layer of zinc. The part is dipped into mixture of molten zinc and necessary additives. During this process doped material layers are formed between the main material and the external pure zinc layer. In certain conditions the entire surface coating could be made of doped material layers.
II. Area of application
The below mentioned conditions apply to hot dip galvanization of products made of steel and other iron-based materials.
Materials are galvanized according to EVS-EN ISO 1461 and EVS-EN ISO 14713-2 standards.
III. Material and its suitability
1. Rimmed steel is ideal for hot dip galvanizing. The best material for galvanizing is a material with strength grade S235 and free of rolling defects, pores, soot and flux additives. In general, steel can be hot dip galvanized until strength grade S355 in which case the properties of steel meet the requirements of EN 10025. When hot dip galvanization of a higher grade steel is required, it should first be consulted with the company providing hot dip galvanization service. It is not recommended to hot dip galvanize Hardox and Weldox steel.
Rimmed or aluminium killed steel is highly suitable for hot dip galvanizing.
Hot dip galvanizing company cannot change the fact that silicon killed steel may form a thicker zinc layer which has lower adhesive properties. Other doping elements may also cause similar results. If sulphur content of steel is above 0.08%, it can be correctly hot dip galvanized only if it is free-cutting steel. It is impossible to galvanize steel structures where free-cutting steel has been combined with another type of steel.
a) Base material. The chemical composition of base material has a significant impact on the formation and thickness of the zinc layer. Doping elements such as silicon and phosphor have the greatest impact. If silicon content is within a range of 0.03%…0.12% or above 0.25%, an extremely aggressive reaction between iron and zinc and a thicker layer can be expected. Phosphor content should not exceed 0.020%. The client is obligated to inform the galvanizing company in writing, if there are any deviations in the content of additives. Even too high carbon or sulphur content may cause formation of a thicker and uneven layer (grooves).
b) External surface. High level of external surface roughness of the base material may cause uneven zinc layer formation and make the layer thicker.
c) Material thickness. Different wall thickness and size and shape of structures affects thickness of the zinc layer during galvanization depending on the dipping time and different thermal capacity.
2. Cast parts must not have spaces and they must be properly cleaned of sand and graphite residues (treated with a shot blaster). Gray cast iron can not be galvanized correctly since its high carbon content impairs the galvanization progress. Cast plates may break due to thermal stress. It is not allowed to galvanize cast and steel parts together, they must be dipped in bath separately because their material requires different processing. The procedure in case of galvanization orders regarding cast parts is that the first order is considered as a trial galvanization of a certain product type during which the hot dip galvanizing company disclaims all responsibility.
3. The client is solely responsible for the galvanization suitability of steel of the delivered parts.
IV. Properties of parts
In order to ensure correct galvanization of parts the client must pay attention to the following.
1. Dimensions of the parts to be galvanized depend on the bath size used by the galvanizing company.
2. Objects must be constructed so they can be galvanized in a single process, if possible. When dipping the objects in bath zinc must be able to flow unhindered to their lowest point and air must be able to escape through the highest point. Upon removal of objects from the bath, zinc must be able to flow off unhindered and quickly. It must be able to remove galvanization residues.
3. In order to avoid deformation of parts the material must be free of internal stress that may be created during cold working and welding. The galvanizing company is not responsible for all types of deformation.
4. Straightening works are not provided.
5. Before processing the parts to be galvanized their structure, shape and size must be coordinated with the hot dip galvanizing company. This is also applicable when using materials that could cause problems because of their properties. When drawings are provided, the hot dip galvanizing company will check structures for their galvanization suitability. The annex of these technical terms of delivery includes design instructions that meet the hot dip galvanization requirements.
6. The material must be delivered so it does not require any sorting or dismantling. If these works are still required before or after galvanization, the client will be invoiced for the actual expenses incurred. The parts delivered for galvanization must be sorted according to their length or similarity so they could be lifted with a crane or a forklift.
V. Hollow profiles
1. Boilers, containers and other hollow profiles are usually galvanized both inside and outside. Therefore they must have one or multiple adequate size inlets and outlets that are located diagonally from each other. The size of these openings must be a quarter or a third of the cross-section of the hollow profile. The requirement which states that parts must allow air purging applies also to very narrow spaces, e.g. small gaps, which are located between two flat parts that are stacked on top of each other, joined together on all sides and larger than the size stated in DIN A5 (space is not galvanized). Ignoring this requirement will form acid residues. Acid residues create rusty dribbles on the zinc layer after galvanization which, however, does not affect the corrosion protection.
2. Objects with closed spaces must not be galvanized due to danger of explosion. Such spaces must have openings through which overpressure caused by heating can be released from the object.
3. If a hollow profile is to be galvanized only on the outside, a pipe must be installed for each closed space for air purging. Galvanization of these structures is only possible up to a limited size due to lifting capacity and therefore it must be previously coordinated with the hot dip galvanizing company.
4. When welding joints it is important to ensure that they do not reach inside, form edging and are always located at the lowest or highest point of the object in galvanization position, so molten zinc could flow completely out of the space.
5. Welded pipe structures must be equipped in all non-penetrating butt joints with air outlets necessary for the zinc inflow and outflow. These openings must be located as close as possible to the welded connections.
6. To ensure high quality galvanizing of pipe structures and profile pipe ladders, the area of the drilled air outlets must not be smaller than a quarter up to a third of the cross-section area, otherwise the drilled openings may become clogged during galvanizing (see the table on the last page of terms of delivery).
Air outlets with a smaller area cause problems and additional expenses during hot dip galvanizing. Hot dip galvanizing company has the right to present an invoice for these additional expenses. This will also reduce the quality of galvanization.
Light and extra large objects may deform to a small or large extent as a result of relatively intensive heating (up to 450°C). Round containers and steel sheet structures with bent arch-shaped walls or reinforcement ribs are more suitable for full bath galvanization than steel sheet structures with plane walls that could deform very easily. Based on our experience we can say that riveted or welded profile steel reinforcement elements or frames, e.g. sheet metal doors, deform quite often and very seriously and it is almost impossible to straighten them properly afterwards. Therefore sheet metal and reinforcement elements must be transported separately, i.e. unpacked, so individual parts could be straightened after galvanizing, if required. They must be fixed with rivets or bolts only after galvanizing.
VII. Riveted structures
Riveted parts are not very suitable for galvanizing since during pre-treatment process acid enters between supporting surfaces and its residues may cause corrosion. It is also important to ensure that only steel rivets are used because rivets made of non-ferrous metals or their alloys may come loose during heating. In this case perfect quality can not be assured.
VIII. Brazed parts
These parts can be dipped into zinc bath only briefly since brazing loses part of its strength in molten zinc and it can not be put under mechanical load any more. Therefore galvanizing possibilities for such products are very limited. The client is liable for all expenses regarding losses or damages.
IX. Solder parts
Solder seams melt immediately in zinc bath which is why they must not be used with hot dip galvanized objects.
X. Moving parts
Hinges, latches, locks and other parts with movable joints must be removed. In terms of structure they must have an adequate clearance (many millimetres) so they could be moved or made to be moved after galvanization as well. Dismantling is performed if expenses are covered and the client wishes it. Hinges and other movable parts must be delivered as individual components and they must have a clearance of one millimetre or more (depending on the bolt diameter).
XI. Threads, screws, openings and bolts
According to ISO 10684 standard threaded bolts must have an undersized thread and threaded nuts must have an oversized thread. Threaded parts with a rigid connection to the structure must be threaded again by the client, unless agreed otherwise. According to empirical values the bolt holes must be as follows:
a) from M8 to M20 (included): +1.5…+2.0 mm larger than rated diameter;
b) from M24: +2.0…+3.0 mm larger than rated diameter.
In order to ensure suitability nut thread must be 0.3…1 mm smaller than rated diameter depending on its size.
XII. Bolted joint structures
The client must deliver these structures disassembled, otherwise the hot dip galvanizing company will disassemble them and present an invoice for disassembly.
XIII. Overlaps and rollers
If overlaps, enclosures, rollers etc. are not tightly sealed or they have a clearance of 3…4 mm, etching liquid and flux residues will enter these parts which may cause corrosion or stains.
XIV. Welding seams
Welding seams must be completely cleaned of slag and they must not have pores or flux residues. Seams must be continuous and not stitched, otherwise some areas may not be galvanized. Welding slag must be removed completely. In case of spot-welding the spots must be located as close as possible to each other. Electrical arc welding and CO2 welding residues must be completely removed by the client. All welding seams must be cleaned of welding slag. It is not allowed to use welding aerosols that contain silicone. Galvanizing company will not be checking this.
XV. Cleanliness of the external surface
Completely or partially painted, lacquered, oiled, greased, pre-galvanized, heavily corroded or products that have been treated with corrosion protection agents must be cleaned before galvanization. Therefore the client is recommended to pay attention when buying raw material that the material is not covered with any of the above mentioned surface coatings. To avoid zinc layer defects the client should ensure that the external surface of steel is free of oil, any surface coatings, tar, grease and different surface protection products. The client must remove contaminated areas of the external surface of materials and structure parts by using specific processing at own expense.
Pre-cleaning of objects that are covered with regular corrosion is not required from the client.
If objects are covered with flammable surface coatings or they contain such residues, the galvanizing company must be informed of this in writing prior to the delivery of goods.
Painted structures and especially old ironwork have a high risk of damage since they very often have welding seams and slag that do not meet the requirements and it is impossible to check them before processing. In such cases the possibility of breaking must always be considered with. If any parts of such objects are damaged in various processes, it will not be compensated. In addition slight deformations may occur and warping is possible during hot dip galvanizing. Expenses claims for the resulting recovery, straightening and other similar work are not accepted.
Paint can not be completely removed from narrow gaps, joints etc. between flat metal parts that are tightly against each other as well as from ironwork bundles, low quality welding seams (pores, flux residues) and garden mesh. It is unavoidable that after hot dip galvanizing etching liquid and flux residues may appear on the object which is recognized by rusty dribbles. Therefore it is recommended to clean and cover the defective areas thoroughly.
XVI. Product marking
The products must be marked with water-soluble dispersion paint. It is not allowed to mark objects with oil paints, crayons or aerosols that contain silicone because they must be removed before galvanizing or these areas will not be galvanized.
XVII. Processing residues
Make sure that only water-soluble emulsions are used when drilling, punching and pressing parts that are going to be galvanized. Also do not use welding aerosols that contain silicone since they burn partially into the object during welding and etching liquid does not remove these defects. Zinc accumulates into drilling and cutting burrs that have not been removed and into cut marks during hot dip galvanizing. The hot dip galvanizing company does not remove them. The client must clean the inside of containers completely from drill chips.
XVIII. Food contact objects
Galvanization of food contact objects has to comply with special requirements and this must be agreed upon separately with the hot dip galvanizing company.
XIX. Galvanizing method
The hot dip galvanizing company will determine the most appropriate pre-treatment and galvanizing method according to the conditions.
XX. Quality of zinc
In general, zinc with mass percentage of at least 98.5% is used for hot dip galvanizing according to the EVS-EN ISO 1461 standard.
XXI. Quality properties of the zinc layer
1. Appearance. The zinc layer must be complete without defects that cause corrosion. Grey zinc layer is caused mostly by the chemical composition of steel material (silicone content) and it does not reduce quality. Same applies to the forming of zinc oxide. The forming of zinc oxide may cause in case of certain after-treatment, e.g. surface coating, adhesion problems for the client. Besides the chemical composition of steel material the reason for the greyish colour also depends on material type and thickness and different material thickness of the structure parts.
2. The surface area per unit weight and coating thickness of the zinc layer is regulated by EVS-EN ISO 1461 standard.
3. Adhesion strength of zinc coating. In case of correct load conditions the zinc coating must not peel off or come loose in any other way. This does not apply to excessively thick zinc coating caused by base material. Incorrect load conditions include among others later deformation, cutting, uneven straightening and transport. Excessively thick zinc coating may also form on large and difficult to galvanize structures due to longer dipping time which also affects the adhesion strength of the zinc coating.
4. Inspecting the zinc coating. The thickness of zinc coating is measured according the requirements of ISO 1461 and a report is provided upon request.
5. Testing. Galvanized parts can be tested at the galvanizing company upon special request by the client. The related expenses are covered by the client.
XXII. Removal of defects
It is allowed to repair individual ungalvanized or poorly galvanized defects. The repaired defect must comply with the corrosion protections characteristics of undamaged zinc coating. The repair method will be determined by the hot dip galvanizing company according to the requirements of EVS-EN ISO 1461.
XXIII. Additional fees
1. Based on the requirements on material properties and product suitability described in these technical terms the hot dip galvanizing company has the right to present an invoice for additional fees.
Prices shall be determined by the hot dip galvanizing company.
Avoiding large parts
Large parts may cause transport and galvanization related problems; plane parts can be galvanized with higher quality and more efficiently. In case of hollow profiles there must be openings for zinc inflow and air purging.
Avoiding closed internal and external corners – making openings in overlays
Air purging and zinc outflow openings must also be insured in case of open pipes in frame structures.
Partial welding of profiles
The openings for zinc inflow and air purging must be located as crosswise as possible to the hooking points.
XXIV Storing recently hot dip galvanized parts
Recently hot dip galvanized coatings are in danger of forming white rust. Therefore recently galvanized parts must be stored in a dry and well ventilated room. Parts must not come into contact to avoid water condensation on the contact surface. The galvanizing company is not responsible for the formation of white rust.
XXV Transporting recently hot dip galvanized parts
Recently hot dip galvanized parts must be transported in a moisture-resistant environment. We recommend using closed vehicles for transport. This is particularly important during winter period when besides moisture the zinc coating may also be damaged by road salt.
The galvanizing company is not responsible for possible damages due to unsuitable transport method.
XXVI Requirements for steel brands intended for laser/plasma cutting
In case of steel brands intended for laser/plasma cutting, to achieve a zinc coating according to the requirements of ISO 1461 it is necessary to roughen the steel surface before hot dip galvanizing (shot blaster).
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