Metal treatment – Compositions – Fluxing
Patent
1987-11-04
1989-02-07
Rosenberg, Peter D.
Metal treatment
Compositions
Fluxing
148 26, B23K 3534
Patent
active
048029327
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to fluoride-free flux compositions preferably intended for hot galvanization in zinc baths having a high aluminium content (.gtoreq.0.15%), with or without other elements.
"Conventional" hot galvanization by dipping the objects to be galvanized in a molten zinc bath requires extensive surface preparation, which is carried out in a plurality of stages, namely: degreasing, stripping, rinsing, fluxing and drying.
The fluxing operation can be carried out dry. In that case, it consists of depositing a film of flux on the pieces to be galvanized: this deposit is applied by dipping the pieces in a flux solution, followed by suitable drying (this being called the prefluxing method).
The basic products employed in fluxing are generally zinc chloride, ammonium chloride and mixed chlorides such as ZnCl.sub.2.2NH.sub.4 Cl and ZnCl.sub.2.3NH.sub.4 Cl.
It is well known that a higher content of aluminium in zinc baths favours hot galvanization because it slows down the formation of the various layers of Fe-Zn alloy, giving rise to a coating having well-defined mechanical properties, especially a high ductility.
Furthermore, coatings having a high aluminium content, such as Galvalume (55% Al-43.5% Zn-1.5% Si), Galfan (5% Al-95% Zn-0.05% Mischmetall), Supergalva (Al: 0.1 to 30%-Na: 0 to 1.0%-Mg: 0 to 5%-Zn: balance) and others, are known to have better corrosion resistance while affording the same cathodic protection as 100% zinc coatings.
The fluxes employed in the "conventional" hot galvanization, either as a covering flux in so-called wet galvanization or as an aqueous flux in so-called dry galvanization, in no way prove satisfactory when they are employed on, or with, zinc baths containing 0.15% or more of aluminium.
The special fluxes which have been developed for use with zinc baths containing a higher percentage of aluminium usually contain not insignificant percentages of fluorides. Examples of the use of fluorides in galvanization fluxes are given in U.S. Pat. Nos. 1,914,269 and 4,496,612. These fluorides are very troublesome because of their acute toxicities. They present major problems both in respect of workplace hygiene and in respect of depollution. Furthermore, the use of aqueous flux containing fluorides sometimes requires the addition of acid, because of the low solubility of the majority of fluorides in water. An excessively acid flux ultimately leads to a flux polluted with iron, which has an adverse effect on the galvanization.
The flux compositions according to the present patent application do not contain fluorides but nevertheless make it possible to achieve correct galvanization, especially by the dry method (aqueous flux), when using zinc baths containing 0.15% or more of aluminium.
The present invention is based on the unexpected fact that flux compositions containing zinc chloride and ammonium chloride in well-defined ratios, in the presence of a wetting agent and, preferably, a foaming agent and/or a soluble rare earth salt, make it possible to achieve the same result, or a better result, in hot galvanization using zinc baths with a high aluminium content as do flux formulations containing fluorides, without however having the disadvantages of the latter.
The compositions according to the invention contain 80 to 90% of zinc chloride, 10 to 20% of ammonium chloride and--based on the total of these two products--0.01 to 5% of a preferably nonionic wetting agent, 0 to 5% of a foaming agent and/or 0 to 5% of a soluble rare earth salt.
The foaming agent can be of the polyalcohol type such as glycerol, sorbitol, mannitol, pentaerythritol and others, or a polyglycol or molecules such as hexamethylenetetramine or tetradecylamine etc. (which can at the same time serve as attack-limiting compounds) or a combination of these products.
Amongst the soluble rare earth salts, the salts of La or of Ce are preferred when working with the Galfan alloy.
In general, flux solutions having the compositions mentioned are prepared by dissolving the mixtures in water in an amount of 100 g/l
REFERENCES:
patent: 3350244 (1967-10-01), Shanklin
patent: 3814637 (1974-06-01), Scott
patent: 4042731 (1977-08-01), Chay
patent: 4062703 (1977-12-01), Vastag
patent: 4647308 (1987-03-01), Beal
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