Flux compositions for steel galvanization

Abstract

This invention relates to a flux composition for treating a metal surface, comprising (a) more than 40 and less than 70 wt. % zinc chloride, (b) 10 to 30 wt. % ammonium chloride, (c) more than 6 and less than 30 wt. % of a set of at least two alkali metal chlorides including sodium chloride and potassium chloride, (d) from 0 to 2 wt. % lead chloride, and (e) from 0 to 15 wt. % tin chloride, provided that the KCl / NaCl weight ratio of said set of at least two alkali metal chlorides ranges from 2.0 to 8.0.This invention also relates to a fluxing bath comprising this flux composition dissolved in water for use in galvanizing processes, by batch or continuously, of metal articles such as iron or steel long products and flat products including wires, plates, coils, rods, reinforcing bars, tubes, strips and sheets.

Description

[0001]This application claims the benefit of British Patent Application No. 1219211.8 filed Oct. 25, 2012, the disclosure of which is incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]The present invention relates to the field of galvanization, more specifically hot dip galvanization or hot-dip zinc coating. In particular the present invention relates to the galvanization of ferrous materials such as, but not limited to, iron, cast iron, steel and cast steel. More particularly the present invention relates to a range of flux compositions for treating the surface of a ferrous material such as iron and steel before it is dipped into a zinc-based molten bath. The present invention also relates to (1) galvanization processes, in particular hot dip galvanization, making use of the flux compositions in at least one process step, and (2) galvanized products, including galvanized ferrous products (e.g. steel flat and long products), made by a process wherein the product ...

AI Technical Summary

Benefits of technology

This patent is about a new flux composition that helps create smooth and void-free coatings on metal articles, like iron or steel, using hot dip galvanization with pure zinc or zinc alloys. The solution is to use a flux composition with a specific ratio of potassium and sodium chlorides. This new flux composition and process help create better quality coatings on metal, regardless of their shape or size.

Problems solved by technology

Oxides and other foreign materials (“soil”) on the surface of the steel article interfere with the chemistry of the galvanizing process and prevent formation of a uniform, continuous, void-free coating.

However, zinc-aluminum galvanizing is particularly sensitive to surface cleanliness so that various difficulties, such as insufficient steel surface wetting, are often encountered when zinc-aluminum alloys are used in galvanizing.

Wet fluxing has several disadvantages such as, consuming much more zinc than dry fluxing, producing much more fumes, etc.

However this requirement makes it extremely difficult to use this technology for shaped parts, or for steel wire since wires break too often and the annealing furnace method does not allow discontinuity.

Although this does not require processing under reducing atmospheres, it is disadvantageous because an additional metal-coating step required.

Because of the procedural and scale differences between batch and continuous operations, techniques particularly useful in one type of operation are not necessarily useful in the other.

For example, the use of a reducing furnace is restricted to continuous operation on a commercial or industrial scale.

However, before a zinc-based alloy coating with high amounts of aluminum (and optionally magnesium) can be introduced into the general galvanizing industry, the following difficulties have to be overcome:zinc alloys with high aluminum contents can hardly be produced using the standard zinc-ammonium chloride flux.

Fluxes with metallic Cu or Bi deposits have been proposed earlier, but the possibility of copper or bismuth leaching into the zinc bath is not attractive.

Thus, better fluxes are needed.high-aluminum content alloys tend to form outbursts of zinc-iron intermetallic alloy which are detrimental at a later stage in the galvanization.

This phenomenon leads to very thick, uncontrolled and rough coatings.

Hence bare spots due to a poor wetting of steel are easily formed, and hence a need to lower the surface tension of the melt.a poor control of coating thickness was reported. in Zn—Al alloys with high-aluminum content, possibly depending upon parameters such as temperature, flux composition, dipping time, steel quality, etc.

% Al and the balance being zinc, provides a poor plating ability, pin hole defect, some dross, and poorly flat.

However the prior art has still not resolved most of the technical problems outlined hereinbefore.