Size
Facilities exist to hot dip galvanize articles of virtually any size and shape. (See list of member's kettle sizes - available on the website or from the Association). When an article is too big for single immersion in the largest bath available it may be possible to hot dip galvanize it by double-end dipping (figure 8), depending on the handling facilities and layout of the hot dip galvanizing plant (check with the galvanizer).

Large cylindrical objects can often be galvanized by progressive dipping (figure 9).

Modular Design
Large structures are also hot dip galvanized by designing in modules for later assembly by bolting or welding. Modular design techniques often produce economics in manufacture and assembly through simplified handling and transport.

Steel Grade
It is possible to hot dip galvanize all structural steels and the ultimate coating thickness achieved is determined by steel analysis, immersion time and to a lesser degree, zinc temperature. It is for this reason that hot dip galvanizing specifications provide for minimum coating thickness and no maximum limit is set. Reactive levels of silicon in steel and excessively high phosphorus combined with relatively low silicon levels can result in thicker coatings. Thicker coatings provide extended corrosion protection but can occasionally be prone to brittleness. The resultant coating could be aesthetically less pleasing sometimes displaying dull grey to black surface patches.

Fabrication

Bending
Steels that are susceptible to embrittlement and fatigue failure should be bent over a smooth mandrel with a minimum radius 2 to 3 times material thickness. Where possible hot work at red heat. Cold bending is unlikely to affect steels less than 3mm thick. Before bending, edges should be radiused over the full arc of the bend.

Bending and Forming after Hot Dip Galvanizing
Components which have been hot dip galvanized should not be bent or formed by applying heat above the melting temperature of zinc (419ºC) as this can cause embrittlement due to intergranular liquid zinc penetration between steel crystal boundaries.

Burrs
Unlike a paint coating, burrs will be overcoated by hot dip galvanizing but the removal of a burr after galvanizing may result in the presence of a small uncoated surface and for this reason, burrs must be removed prior to galvanizing.

Edges
Because a hot dip galvanized coating is formed by metallurgical reaction between molten zinc and steel, the coating thickness on edges and corners is equal to, or thicker that that on flat surfaces. Thus the rounding of sharp edges, as required for paint coatings, is not necessary. If subsequent painting is required, sharp edges should be rounded during fabrication to a radius of 3mm or 50% of steel thickness.

Edge Distances
In accordance with SABS 0162 minimum edge distances from the centre of any bolt to the edge of a plate or the flange of a rolled section should be used.

Punching
The limitations specified in SABS 1200H (to be superseded by SABS 2001-CSI) on the full-size punching of holes in structural members must be observed.

Material of any thickness may be punched at least 3mm undersize and then reamed, or be drilled. Good shop practice in relation to ratios of punched hole diameter to plate thickness, and punch/die diametral clearance to plate thickness should be observed.

For static loading, holes may be punched full size in material up to mm thick where Fy is material yield stress up to 360Mpa.

Shearing and Flame Cutting
Edges of steel sections greater than 16mm thick subject to tensile loads should be machined or machine flame cut. Edges of sections up to 16mm thick may be cut by shearing.

Sheared edges to be bent during fabrication should have stress raising features such as burrs and flame gouges removed to a depth of at least 1.5mm. Temperatures associated with flame cutting alter the surface properties of steel and if such surfaces are not thoroughly ground, a thinner galvanized coating will be formed (usually below the minimum specified).

Welding and Weld Slag
Welds should be continuous and free from excessive pin-holing and porosity. Weld slag, normally associated with stick welding, is not readily removed by acid cleaning and such slag must be removed by abrasive blast cleaning, chipping, grinding, flame cleaning or a pneumatic needle gun, prior to hot dip galvanizing. Shielded arc welding is preferred since this method does not result in the presence of tightly adhering slag (figure 10).

In case of double-sided fillet welds, the weld must be continued around the ends to prevent the unnecessary penetration of acid into any conceivable crevice (figure 11).

Weld Spatter
Weld spatter does not reduce the protective properties of a hot dip galvanized coating to the same extent as a paint coating, but it is recommended practice to remove spatter prior to hot dip galvanizing.

Venting, Filling and Drainage
External stiffeners, welded gussets and webs on columns and beams and gussets in channel sections should have their corners cropped. The gaps created should be as large as possible without compromising structural strength. If welding is required around the edge created, a radiused corner is desirable to facilitate continuity of the weld around the cut end to the other side. Circular holes are less effective: if used, they should be as close to corners and edges as practicable. Where more convenient, the cropped corners or holes may be in the main beam. Consultation with the galvanizer, regarding the appropriate vent and drainage hole sizes is recommended (figures 5 & 26, table 1).

Welded Pipe Sections
Closed sections must never be incorporated in a fabrication. Sections should be interconnected using open mitred joints as illustrated in figure 27, or interconnecting holes should be drilled before fabrication as in figure 28. Alternatively external holes may be positioned as in figure 29, a method which is often preferred by the galvanizer, since quick visual inspection shows that the work is safe to galvanize.

Pipe ends can be left open, or provided with removable plugs. (see unwanted vent holes).

*Vent/Fill drainage holes can be drilled before fabrication.

Small Tubular Fabrications
Small tubular fabrications must be vented, preferably with holes not less than 10mm diameter (see table 1).

Unwanted Vent Holes
These may be closed by hammering in lead or aluminium plugs after hot dip galvanizing and filing off flush with surrounding surfaces.

Tubular Fabrications / Hollow Structural's
Drain/vent hole sizes should be preferably 25% of internal diameter or diagonal dimension for sections yielding a maximum cross sectional area of 180cm. This percentage can be dependent on the shape of the fabrication, therefore consultation with the galvanizer at the design stage is recommended.

Tubular Fabrication after Hot Dip Galvanizing
The requirement for bending tubes after hot dip galvanizing, i.e. for the fabrication of gates etc. must be carried out according to the method set out in the Bend Test (Galvanized Tube).

Note: Should the above requirement of bending be implemented for the fabrication of gates, etc. after hot dip galvanizing, the maximum coating thickness should be no greater than 40% more than the minimum required in table 5.

Adhesion of the Coating
Acceptable adhesion is related to the practical conditions pertaining during transportation, erection and service. Hot dip galvanized coatings should be sufficiently adherent to withstand normal handling without peeling or flaking regardless of the nature and thickness of the coating. Bending or forming, other than straightening by the galvanizer after hot dip galvanizing, is not considered to be normal handling.

When reactive grades of steel or very thick sections are hot dip galvanized, coatings which are thicker than usual may occur. The galvanizer has limited control over the formation of thicker coatings, since this is a function of the chemical composition of the
steel. Extended immersion time also plays a role. Heavy hot dip galvanized coatings greater than 250µm thick, may have brittle tendencies. Interpretation of the standard adhesion tests must take this into consideration. The requirements for careful transportation, handling and erection should be evaluated against the additional corrosion protection afforded by these thicker coatings.

Testing for Adhesion
Testing for adhesion is not necessarily a true measure of the adhesive strength of the metallurgical bond between the hot dip galvanized coating and the base steel, but it does serve as an indicator of the adhesion properties of the coating.

Paring Test
This simple but effective test is conducted by cutting or prying the hot dip galvanized coating with a sharp knife. Considerable pressure is exerted in a manner tending to remove a portion of the coating. Adherence is considered satisfactory when it is possible to remove only small particles of the coating. It should not be possible to peel any portion of the coating in the form of a layer so as to expose the underlying iron or steel in advance of the knife. Although not mentioned in ISO 1461, this test has shown practical significance as a test for adhesion.

Cold Flattening Test (Hot Dip Galvanized Tube)
For compliance with EN 10240, the most popular test is cold flattening in accordance with EN 10233. Test pieces not less than 40mm in length are flattened between parallel flat platens as shown in table 2. No cracking or flaking of the coating shall occur on the surface away from the cut surface.

Bend Test (Hot Dip Galvanized Tube)
The bend test shall be carried out using a tube bending machine, and the test piece shall be bent through 90º round a former having a radius at the bottom of the groove equal to eight times the outside diameter of the tube.

Tanks and Closed Vessels
When both internal and external surfaces are to be hot dip galvanized at least one filling and draining hole must be provided, with a vent hole diagonally opposite to allow the exit of air during immersion. For each 0,5 cubic metres of volume, provide one fill/drain hole of minimum size Æ60mm and vent hole of minimum size Æ40mm (figure 12).

Internal baffles should be cropped as illustrated (figures 6 and 12). Manholes or pipes should finish flush inside to prevent trapping excess zinc (figure 13).

Lifting lugs should be provided opposite the biggest and most accessible filling / draining hole and adjacent to the vent hole on the opposite end (see figure 1). The lugs must be designed to accommodate the excess mass of molten zinc within the cylinder / pipe on withdrawal.

Large vessels require an appropriate size manhole in the baffle (figure 6).

When vessels and heat exchangers etc. are not to be hot dip galvanized inside, "snorkel" tubes or extended vent pipes must be fitted after discussion with the galvanizer, to allow air to exit above the level of molten zinc in the galvanizing bath (figure 14).

Masking, Welding, Handling, Minimizing Distortion, Clearance for moving parts and Identification

Masking
Masking materials have been developed, which if applied prior to hot dip galvanizing, will prevent the formation of the hot dip galvanized coating on surfaces where it is not desired.

Combinations of Ferrous Surfaces
Fabrications containing a combination of castings and steels, or rusted and mill scaled surfaces must be abrasive blast cleaned before hot dip galvanizing.

Provision of Handling
Work not suitable for handling with chains, baskets, hooks or jigs must be provided with suspension holes or lifting lugs (figure 1). If in doubt check with the galvanizer.

Tubular Fabrications / Hollow Structural's
Drain/vent hole sizes should be preferably 25% of internal diameter or diagonal dimension for sections yielding a maximum cross sectional area of 180cm. This percentage can be dependent on the shape of the fabrication, therefore consultation with the galvanizer at the design stage is recommended.

Materials Suitable for Hot Dip Galvanizing
All ferrous materials are suitable, including sound stress-free castings. Brazed assemblies may be hot dip galvanized but check first with the galvanizer. Assemblies soft soldered or aluminium riveted cannot be hot dip galvanized.

Distortion
Distortion can be minimised by:

Clearance for Moving Parts
Drop handles, hinges, shackles, shafts and spindles require a minimum radial clearance, to allow for the thickness of the hot dip galvanized coating (figure 23).

Identification Markings
For permanent identification use heavily embossed, punched or welded lettering (figure 24). For temporary identification use heavily embossed metal tags wired to the work, water soluble paint or the correct marking pen (figure 24).

Do not use enamel / oil paints, adhesive labels or any other coating that cannot be readily removed by degreasing or pickling (figure 25). If present, these coatings require to be removed by paint stripper or abrasive blasting prior to pickling and hot dip galvanizing.

Hot Dip Galvanized Fasteners
Hot dip galvanized fasteners are recommended for use with hot dip galvanized or painted structures, but if ISO 1461 is not specified, there is every likelihood that thinner zinc or cadmium electroplated coatings will be supplied.

Oversize Tapping Allowance for Hot Dip Galvanized Nuts
The zinc coating on external threads shall be free from lumps and shall not have been subjected to a cutting, rolling or finishing operation that could damage the zinc coating. The zinc coating of an external standard metric thread that has not been undercut shall be such as to enable the threaded part to fit an oversized tapped nut in accordance with the allowances given in the table below.

On bolts greater than M24, undercutting of bolt threads is frequently preferred to oversizing of nut threads. The allowance should be increased to 0,4mm.

Specifying Hot Dip Galvanizing
Packaging and transporting of Hot Dip Galvanizing goods.
Even though the hot dip galvanized coating is capable of withstanding fairly rough treatment it should be handled with care during storage and transportation. In the case of long goods, simple packaging and binding into bundles not only protects against mechanical damage but it often facilitates transportation itself. However, packaging and binding should be done in such a way as to avoid the risk of wet storage stain. Spacers should be used to prevent such attacks.

Specifying Hot Dip Galvanizing
The galvanizer acts as a sub-contractor to a steel fabricator and as such, his contractual relationship is normally with the fabricator, not with the ultimate user or specifier. It is important, therefore, that the users' or specifiers' requirements for hot dip galvanizing are made clear to the fabricator and that all instructions are channeled via the fabricator to the galvanizer.

Due to frequent misrepresentation and incorrect specifying of other coatings such as "cold galv" and "electrogalv" which no doubt fulfil a requirement in the market, the specifier, who requires hot dip galvanizing for corrosion protection reasons, should specify that all components be hot dip galvanized to ISO 1461 or EN 10240 as applicable.

To ensure the best quality and technical support, a galvanizer who is a member of the Hot Dip Galvanizers Association Southern Africa should be preferred.

When hot dip galvanizing is specified, the surface of the base steel is completely covered with a relatively uniform coating of zinc and the minimum thickness specified is related to the thickness of the steel being hot dip galvanized, as shown in table 3 and 4.

Thickness legend - 3 £ t < 6 = thickness less than 6mm but greater and equal to 3mm Table 3

Minimum Coating Thickness on Articles that are Centrifuged
Thickness legend - 6 £ Ø < 20 = diameter less than 20mm but greater and equal to 6mm Table 4

NOTES*
Local coating thickness is defined as the mean of the measurements taken within a specified reference area. Mean coating thickness is the control sample number average of the local coating thickness values from each reference area.

Where only one reference area is required according to the size of the article, the mean coating thickness within that reference area shall be equal to the mean coating thickness given in the above tables.

Deviation from standard coating thickness. A requirement for a thicker coating (25% greater than the standard in table 3) can be requested for components not centrifuged, without affecting specification conformity.

Where steel composition does not induce moderate to high reactivity, thicker coatings are not always easily achieved.

Coating qualifies 'A' and 'B' refer to end application with quality 'A' being for gas and water installations and 'B' for other applications. The number following the quality letter refers to specific requirements in terms of coating thickness.

NOTE: In South Africa, EN 10240 to quality A1 replaces the previous SABS 763, B4 coating.

Table 5

Hot Dip Galvanizing Specifications
All ferrous materials are suitable, including sound stress-free castings. Brazed assemblies may be hot dip galvanized but check first with the galvanizer. Assemblies soft soldered or aluminium riveted cannot be hot dip galvanized.

Distortion
ISO 1461:1999 - Hot dip galvanized coatings on fabricated iron and steel articles - Specifications and test methods

EN 10240:1999 - Internal and/or external protective coatings for steel tubes - Specifications for hot dip galvanized coatings applied in automatic plants.

Note: The above specifications supersede SABS 763.

SABS 763:1997 - Hot dip (galvanized) zinc coatings (other than on continuously zinc coated sheet and wire).

ISO 14713:1999 - Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings - Guidelines.

Note: The above specification supersedes SABS 0214

SABS ISO 4998:1996 - Continuous hot dip zinc coated carbon steel sheet of structural quality.

SABS ISO 3575:1996 - Continuous hot dip zinc coated carbon steel sheet of commercial, lock forming and drawing grades.

Note: The above two specifications supersede SABS 934

SABS 675:1993 - Zinc coated fencing wire.

The coating thicknesses specified in SABS 675 are also recommended for other applications in corrosive environments.

General hot dip galvanizing specifications state the local (minimum) and the (mean) coating thicknesses. The thickness actually achieved varies with steel composition and this can range from the minimum up to at least 50% greater. As life expectancy predictions are normally based on the minimum coating thickness, they are usually conservative.

Note: The specification does not stipulate a maximum upper coating thickness limitation. Excessively thick coatings on threaded articles are undesirable. In order to ensure effective tensioning, the coating thickness on fasteners should not exceed a maximum of 90µm, this applies particularly to high strength bolts and nuts.

In South Africa, the South African Bureau of Standards (SABS) has adapted ISO 1461:1999, EN 10240:1999 and ISO 14713. The specifications are therefore published by the SABS as SABS ISO 1461: 1999, SABS EN 10240:1999 and SABS ISO 14713:1999.

Lead Times
As a general rule, most articles can be hot dip galvanized and returned to the fabricator within 7 days after receipt.

In the case of large contracts, the galvanizer should be involved at the programming stage with the fabricator and the end user. Hot dip galvanizing is normally the final process after fabrication and prior to delivery and erection. If sufficient time for hot dip galvanizing and inspection is not provided in the overall programme, costly delays may occur at the erection stage.