Metallurgical Characteristics of Galvanized Coatings

Iron and Steel Substrates. The chemical composition of irons and steels, and even the form in which certain elements such as carbon and silicon are present, determines the suitability of ferrous metals for hot dip galvanizing and may markedly influence the appearance and properties of the coating. Steels that contain less than 0.25% C, less than 0.05% P, less than 1.35% Mn, and less than 0.05% Si, individually or in combination, are generally suitable for galvanizing using conventional techniques. The results of a recent study suggest that a silicon level of 0.15% to 0.25% should also produce acceptable results. Another study suggests that optimum amounts of silicon and phosphorus can be defined by the formula %Si + 2.5(%P) = 0.09.

To avoid brittleness of the iron-zinc alloy layer in cast iron materials, substrate iron must be low in phosphorus and silicon; a preferred composition may contain about 0.01% P and about 0.15% Si.

Type of Zinc. Any grade of zinc in ASTM B 6-87(92) can be used in galvanizing. ASTM A 123-89 now stipulates that the zinc being used for galvanizing must contain 98% pure zinc. The remaining 2% may be such other elements as desired to produce an optimum coating. ASTM A 123 does not require prime western zinc, but some other galvanizing specifications do. Most galvanizers use either prime western (98.0% Zn) or high grade (99.90% Zn). Prime western has a lead alloy content of 0.5 to 1.4%, which is beneficial for hot dip galvanizing, and higher allowed levels of iron and other elements as impurities. When high grade zinc is used, lead is often added separately to the kettle. During galvanizing, there is a buildup of iron in the bath caused by dissolution of iron from the surface of steel work and the tank walls so that the equilibrium iron content of the bath is nearly equal regardless of the grade of zinc used. High purity special high grade (99.99% Zn) zinc coatings have the same metallurgical properties as those obtained with prime western zinc. High purity zinc has little metallurgical advantage for use on fabricated items.

Bath Alloying Elements. Cadmium and iron are usually present in zinc baths as contaminants, but are not intentionally added to the bath as alloying elements. An aluminum concentration up to 0.01% will improve drainage and increase the brightness of the galvanized coating. Small amounts of lead may be added to promote proper spangle and better drainage and to aid with drossing the bath. Other alloying elements have been tried with success. Nickel, vanadium, antimony, titanium, and rare earth metals are known to produce positive results under some circumstances.

An aluminum concentration less than 0.01% is generally maintained in the zinc bath when a preflux and/or a bath flux are used. The high chloride content of the fluxes reacts with the aluminum in the bath, producing a surface film of dross, oxide, and chloride on the bath surface.

Coating Thickness. In addition to base metal chemistry and surface profile, the thickness of coatings applied by hot dipping is primarily a function of:

• The duration of immersion, which controls the thickness of alloy layer (Fig. 4)

• The speed of withdrawal from the bath, which controls the amount of unalloyed zinc adhering (Fig. 5)

• The temperature of the bath, which affects both the alloy and the free zinc layers (Fig. 6)

Coating weight can be further affected by the amount of zinc removed by wiping, shaking, or centrifuging after the dipping process.

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Fig. 6 Coating thickness versus galvanizing temperature for a typical silicon-killed steel at two different immersion times. Source: Ref 4

The protection against corrosion provided by zinc coatings is essentially determined by the thickness of the coating (Fig. 2). Many comprehensive studies have shown that all other factors, such as method of applying the zinc coating, purity of the zinc, and the extent to which it is alloyed with the iron, are minor in determining life, as compared with the thickness of the coating.

Zinc coatings applied by hot dipping after fabrication are measured in mils of zinc on the surface. However, the weight of galvanized coatings on sheet is stated in ounces per square foot (grams per square meter) of sheet. Since the sheet is coated on both sides, the coating weight per square foot (meter) of surface on each side is approximately one half the average weight of coating per square foot (grams per square meter) of sheet. ASTM A 123 and A 153 give coating weight requirements as a function of thickness and type of material to be coated.

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