Technical Info Sheet #10
Research on the effect of the zinc coating as a long-term protection for steel was started more than 100 years ago in practical situations. There are still practice sites distributed all over the world where test plates are exposed to all kinds of climatic conditions.
For most atmospheric exposures there is an almost linear relationship between coating thickness and service life of the zinc coating. Thus, a fairly accurate estimate of when the first major maintenance is needed can be made. That maintenance may consist of painting the steel on site or re-galvanizing the steel after it has been disassembled and brought to a galvanization plant.
CORROSIVITY CATEGORIES
In addition to European standard EN ISO 1461, which specifies the specifications to be met by coatings applied by hot-dip galvanizing, there is also standard EN ISO 14713 part 1 on principles of design and corrosion resistance. This contains a reference to EN ISO 9223 and EN ISO 9224 on corrosion resistance.
Both EN ISO 9223 and EN ISO 9224 divide atmospheric corrosion conditions into so-called corrosivity categories (C1 to C5 and CX).
A category is an arbitrary classification of an atmosphere from very mild to an environment of severe contamination. Associated with each of these categories is a minimum and a maximum decrease due to corrosion of iron, zinc, aluminum or copper.
In EN ISO 9223 a maximum and minimum decrease in zinc coating thickness during the first year is indicated for each category, in EN ISO 9224 the average annual zinc corrosion during the first 10 years and the first 30 years is indicated.
WHAT IS ZINC PATINA?
| Corrosiecategorie | Afname zinklaagdikte in het eerste jaar uitgedrukt in g/m² (Afname in μm) |
|||
|---|---|---|---|---|
| STAAL | ZINK | KOPER | ALUMINIUM | |
| C1 | ≤ 10 (≤ 1,3) | ≤ 0,7 (≤ 0,1) | ≤ 0,9 (≤ 0,1) | te verwaarlozen |
| C2 | 10 – 200 (1,3 – 25) | 0,7 – 5 (0,1 – 0,7) | 0,9 – 5 (0,1 – 0,6) | ≤ 0,6 (–) |
| C3 | 200 – 400 (25 – 50 | 5 – 15 (0,7 – 2.1) | 5 – 12 (0,6 – 1,3) | 0,6 – 2 (–) |
| C4 | 400 – 650 (50 – 80) | 15 – 30 (2,1 – 4,2) | 12 – 25 (1,3 – 2,8) 2 | 2 – 5 (–) |
| C5 | 650 – 1500 (80 – 200) | 30 – 60 (4,2 – 8,4) | 25 – 50 (2,8 – 5,6) | 5 – 10 (–) |
ZINC COATING LIFE PREDICTOR
In addition to the above classification, you can also consider the conditions of the actual atmosphere in which the material is placed. The corrosion of hot-dip galvanized steel, under atmospheric conditions, depends on the following climatic data; temperature, precipitation amount and relative humidity. Also important for calculation is the proximity to the coast (chlorides) and the presence of sulfur dioxide. In addition to the above aspects, it is important whether the material is placed in the open air or under a canopy, where it can get wet from rain or where it is shielded from rain but can be exposed to sea breezes or sulfur dioxide.
In a situation where there are elevated corrosive conditions (e.g. CX), the open air will provide a longer service life. This is because the materials are regularly cleaned by rainfall. For placement in a low corrosion category (e.g., C3), this need not be the case. Under a canopy, there will be no reduction in zinc coating thickness due to corrosion due to chlorides or sulfur dioxide, and thus the zinc coating will provide longer protection for the steel, which means that the first maintenance is also further away.
Because emissions of SO2 into our atmosphere have been reduced by about 90% since 1990 (fewer coal-fired power plants, switch from oil to gas, flue gas cleaning at refineries, industry and energy sector), this air pollution plays little to no role in zinc corrosion today. It should be noted that industrial activity involving the release of sulfur dioxide may cause accelerated zinc corrosion of a plant in which hot-dip galvanized steel has been used.
Based on the knowledge of these parameters and taking into account the current best available knowledge on corrosion of zinc, ILZRO (International Lead & Zinc Research Association) and Teck Cominco Metals Ltd have developed a software package: the Zinc Coating Life Predictor. This tool is managed by our US sister organization AGA. It allows the annual zinc coating thickness decrease (μm/year) to be calculated.
All necessary climatic data and the determining environmental contaminants are available from:
You can access the Zinc Coating Life Predictor here. This web page also contains PDF files with the necessary climatic and environmental contamination data. In addition to being able to estimate by calculation, the protection life of the zinc coating can also be assessed by doing a situation survey at the location in question. Often there is other hot-dip galvanized steel at or near the site. Based on its condition (appearance and layer thickness) and the period for which the galvanized steel is already in use there, this gives a realistic picture of when the first maintenance can be expected.
More information can be found in the article “Determining the durability of a hot-dip galvanized building product.
| Corrosie— categorie | Afname laagdikte ( μm/jaar) | Materiaaldikte en gemiddelde minimum zinklaagdikte volgens EN ISO 1461 |
|||
|---|---|---|---|---|---|
| < 1,5 mm | ≥ 1,5 mm tot ≤ 3 mm | > 3 mm tot ≤ 6 mm | > 6 mm | ||
| 45 μm | 55 μm | 70 μm | 85 μm | ||
| C1 | < 0.05 | ∞ | ∞ | ∞ | ∞ |
| C2 | 0,05 – 0,4 | > 100 | > 100 | > 100 | > 100 |
| C3 | 0,4 – 1,1 | 40 – > 100 | 50 – > 100 | 63 – > 100 | 77 – > 100 |
| C4 | 1,1 – 2,2 | 20 – 40 | 25 – 50 | 32 – 63 | 38 – 77 |
| C5 | 2,2 – 4,4 | 10 – 20 | 12,5 – 25 | 16 – 32 | 19 – 38 |
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
EN ISO 9223
Corrosion of metals and alloys – Atmospheric corrosion rate – Classification, determination and estimation
EN ISO 9224
Corrosion of metals and alloys – Atmospheric corrosivity – Guideline values for corrosivity categories
