Steel Protection
By Hot Dip Galvanizing & Duplex Systems
Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA
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 fulfill 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 19 & 20.
10.1 HOT DIP GALVANIZING SPECIFICATIONS
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 - Specification for hot dip galvanized coatings applied in automatic plants. (Table 23).
Note: The above specifications supercede 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 supercedes SABS 0214SABS 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 supercede SABS 934.SABS 675:1993
Zinc coated fencing wire.SABS 935:6993
Hot dip (galvanized) zinc coatings on steel wire.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 1: The specification does not stipulate a maximum upper coating thickness limitation, however, excessively thick coatings on threaded articles are undesirable. In order to ensure effective tensioning, the coating thickness on fastener, should not exceed a maximum of 9Opm, 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.
10.2 LEAD TIMES
As a general guide, most articles can be hot dip galvanized and returned to the fabricator within 7 days after receipt.
In the case of large contacts, 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 galvanizing and inspection is not provided in the overall programme, costly delays may occur at the erection stage.
| MINIMUM COATING THICKNESS ON ARTICLES THAT ARE NOT CENTRIFUGED | |||||
| ISO 1461 | SABS 763 | ||||
| Category and thickness (t) mm |
Local coating thickness m* |
Mean coating thickness m* |
Article type |
Thickness, m General Application |
|
| Profiles | t ≥ 6 | 70 | 85 | A1, B1, D1 E1, F1 |
85 |
| 3 ≤ t < 6 | 55 | 70 | A2, B2, F2 | 65 | |
| 1.5 ≤ t < 3 | 45 | 55 | |||
| t < 1.5 | 35 |
45 |
A3 B3 F3 |
45 55 45 |
|
| Castings | t ≥ 6 | 70 | 80 | D1 | 85 |
| t < 6 | 60 | 70 | |||
| # Thickness legend - 3 ≤ t < 6 = thickness less than 6mm but greater and equal to 3mm. | |||||
Table 19.
| MINIMUM COATING THICKNESS ON ARTICLES THAT ARE CENTRIFUGED | |||||
| ISO 1461 | SABS 763 | ||||
| Category and thickness (t) mm |
Local coating thickness m* |
Mean coating thickness m* |
Article type |
Thickness, m General Application |
|
| Profiles | ≥ 6 | 45 | 50 | C1 | 55 |
| 6 ≤ < 20 | 35 | 45 | C2 | 45 | |
| < 6 | 20 | 25 | |||
| Castings | t ≥ 3 | 45 | 55 | C1 Washers | 55 |
| D2 | 45 | ||||
| t < 3 | 35 | 45 | C2 Washers | 45 | |
| # Thickness legend - 6 ≤ < 20 = diameter less than 20mm but greater and equal to 6mm. | |||||
Table 20.
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 overage of the local coating thickness values from each reference area.
# Where only one reference area is required according to 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 cooling (25% greater than the standard in table 39 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.