Technical Sheet #18
The vast majority of steels can be galvanized according to EN-ISO 1461. But substances such as silicon and phosphorus in the steel affect the zinc coating properties, coating thickness, roughness and appearance of the galvanized steel. In addition, there are other elements that play a role.
FORMATION AND COMPOSITION OF THE GALVANIZING LAYER
Unlike the case of paint and powder coating, among others, in hot-dip galvanizing the protective zinc layer is formed by a chemical compound of iron and zinc. During immersion in the zinc bath (at a temperature of 450°C), three hard, dark-gray iron-zinc (Fe-Zn) alloy layers are formed on the surface of the steel by a so-called diffusion process.
When the steel is removed from the zinc bath, the draining and simultaneously solidifying zinc (Figure 1) adheres to these alloy layers. This zinc solidified by cooling forms a silvery, shiny layer. The minimum thicknesses of the zinc coating are listed in the international standard EN-ISO 1461.
REACTIVITY OF STEEL TO MOLTEN ZINC
In principle, all low-alloy steels can be hot-dip galvanized. However, the chemical steel composition can affect the following properties of the hot-dip galvanizing layer: coating thickness, appearance (gloss, uniformity, roughness), resistance to local mechanical damage (impact resistance) and the layer structure of the galvanizing layer.
In the case of certain silicon contents in the steel, the reaction between the liquid zinc and the steel is very intense. In that case, we speak of “reactive steel. The higher the reactivity, the faster the accretion of Fe-Zn alloy layers. In reactive steels, the proportion of alloy layers in the total galvanizing layer is higher than that in non- or less reactive steels. In some cases, the galvanizing layer even consists exclusively of Fe-Zn alloys. With reactive steel, the thickness of the galvanizing layer is relatively large and there is a risk of a less attractive appearance due to light and dark gray spots. Many studies have shown that the silicon content and (to a lesser extent) the phosphorus content strongly influence the reactivity of the steel
INFLUENCE OF SILICON (SI)
In steelmaking, Si or Al (aluminum) is used to remove oxygen from the steel. This is called the “quenching” of steel. In practice, Si is mostly used for quenching so that some Si is always present in the steel. Steels with Si levels that increase the rate of zinc accretion are reactive. This phenomenon was studied by Sandelin, after whom this effect was named. As shown in Figure 2, we get greater coating thicknesses at Si contents between 0.03% and 0.14% and above 0.25%. The mentioned values are not absolute, different limit values occur in the literature (see Table 1). Increasingly, aluminum calibrated steels are coming on the market. Typically, these steels possess very low Si content.
| CATEGORIE | TYPERENDE NIVEAUS VAN REACTIEVE ELEMENTEN % (GEW. PERCENTAGE | AANVULLENDE INFORMATIE | TYPERENDE EIGENSCHAPPEN VAN DE DEKLAAG |
|---|---|---|---|
| a | ≤ 0,03% Si en 0,02% P | Zie opmerking 1 en opmerking 4 | 4 De deklaag heeft een glanzend uiterlijk met een fijnere textuur. De buitenste zink-laag maakt deel uit van de structuur van deklaag. |
| b | ≥ 0,14% Si tot ≤ 0,25% Si | Andere elementen kunnen ook invloed hebben op de reactiviteit van staal Met name fosforgehaltes boven de 0,035% geven verhoogde reactiviteit. | Een deklaag kan een glanzend of mat uiterlijk hebben. Afhankelijk van de staalsamenstelling kan de buitenste zinkdeklaag deel uitmaken van de deklaagstructuur of kan een ijzer-zinklegering doorlopen tot het oppervlak van de deklaag,. |
| c | > 0,03% Si tot < 0,14% Si | Er kunnen zich buitensporig dikke deklagen vormen. | De deklaag heeft een donkerder uiterlijk met een grovere textuur. IJzer-/zinklegeringen domineren de deklaagstructuur en lopen vaak door tot aan het oppervlak van de deklaag, met verminderde weerstand tegen hanteringsschade. |
| d | d > 0,25% Si | De dikte van de deklaag neemt toe naarmate het siliciumgehalte hoger is. | |
| Opmerking 1: Staalsoorten met een samenstelling volgens de formule Si ≤ 0,03% en Si + 2,5P ≤ 0,09% hebben naar verwachting deze eigenschappen. Voor koudgewalst staal gelden deze eigenschappen naar verwachting ook, mits de samenstelling van het staal voldoet aan de formule Si + 2,5P ≤ 0,04 %. Opmerking 2: De aanwezigheid van legeringselementen (bijv. nikkel of aluminium) in het gesmolten zink kan grote invloed hebben op de eigenschappen van de deklaag, zoals aangegeven in deze tabel. De aanwijzingen in deze Tabel 1 zijn niet van toepassing bij hoge temperatuur verzinken (d.w.z. dompelen in gesmolten zink van 530 °C tot 560 °C). Opmerking 3: De staalsamenstellingen aangegeven in deze tabel verschillen onder invloed van andere factoren (bijv. warmwalsen) en de grenswaarden van elk bereik zullen daardoor verschillen. Opmerking 4: Staalsoorten met samenstellingen < 0,01% silicium en met een aluminiumgehalte > 0,035% kunnen een lagere reactiviteit vertonen, die kan resulteren in een deklaagdikte die minder is dan verwacht. Bij deze staalsoorten kan de deklaag een verminderde deklaagcohesie vertonen. Opmerking 5: Het ontwerp van het werkstuk dat zal worden verzinkt kan ook invloed hebben op de deklaagkenmerken. |
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INFLUENCE OF PHOSPHORUS
When the Si content of the steel is up to 0.03%, the P content also comes into play: then Si + 2.5P must be less than or equal to 0.09% to limit the reactivity of hot-rolled steel. For steels with higher Si contents, normal P contents do not affect the reactivity of steel. Regardless of Si content, the following rule applies: P < 0.035%.
INFLUENCE OF OTHER ELEMENTS IN THE SAMPLE
Apart from the Si and P contents, the C (carbon) content must be less than 0.25% and the Mn (manganese) content less than 1.35%. Aluminum (Al), nickel (Ni) and sulfur (S) can also affect the build-up of alloy layers. For example, an Al content greater than 0.045% combined with Si contents lower than 0.02% will increase reactivity. In contrast, a Si content lower than 0.01% and an Al content greater than 0.035% will potentially produce a layer thickness that is (too) low. In case of manganese content above 1.35%, it is necessary to blast the materials before galvanizing them.
ORDERING STEEL
So the steel composition is very important for the galvanizing result.
EN-ISO 14713-2:2019 states what the expected result will be of steel with a certain Si or P content.
Table 1 is taken from this standard. If you buy steel, you can rely on this with the understanding that due to rolling processes the contents listed in the table may differ. The standard for technical delivery conditions for unalloyed structural steel (EN 10025-2:2019) also refers to the classification according to EN-ISO 14713. For a uniform appearance of a project, it is important to place the order for the steel from the same steel shop. Additionally, you can require in your order that the steel parts ordered must come from the same batch or, if this is not possible; be at least equivalent with respect to chemical composition. By the way, the steel strength indication does not give a good indication of the chemical steel composition.
FINALLY
Despite the fact that the usual galvanizing procedure has been applied, it may occasionally occur that from a batch of steel the prescribed zinc coating thickness, as described in EN-ISO 1461, is not achieved. For example, very low alloy steel or steel with very low surface roughness may be the cause of this. In particular, this applies to steel from the category of weldable fine-grained structural steels obtained by thermomechanical rolling as defined in EN 10025-4. In such cases, client and galvanizing plant should consult with each other. Indeed, an insufficient zinc coating thickness obtained after galvanizing these steels is not a reason for rejection.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO14713-2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 2: Hot dip galvanizing
EN 10025-2
Hot-rolled structural steel products – Part 2: Technical coating conditions for unalloyed structural steels
EN 10025-4
Hot rolled structural steel products – Part 4: Technical delivery conditions for thermomechanical rolled weldable fine grain structural steels
