Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Patent
1998-12-09
2000-11-28
Beck, Shrive
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
427433, 427436, 428681, 106 105, 106 117, B32B 1518, B05D 118, C23C 206
Patent
active
061533149
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a bath for hot-dip galvanizing consisting of alloyed zinc, that is particularly useful for batch-wise galvanizing steel articles, the silicon content of which is variable or the composition of which is unknown.
BACKGROUND OF THE RELATED ART
When galvanizing steel in a conventional non-alloyed zinc bath, serious problems arise, when the steel contains more than 0.02 wt % of silicon: the resulting zinc coating is both too thick and too brittle and in addition it has a greyish aspect. This is due to the fact that the iron--zinc alloy layer, which forms on the surface of the steel when the latter is in contact with a conventional zinc bath, grows linearly with time during the entire duration of the immersion, when the steel contains more than 0.02 wt % of silicon. This is not the case with steels containing less silicon, as the growth rate is here proportional to the square root of the immersion time. The influence of the silicon content of the steel on the coating thickness is illustrated in FIG. 1, wherein the thickness peak on steels with 0.03-0.15 wt % Si is called the Sandelin peak.
Efforts have already been made in the past to cope with this problem. The Technigalva.RTM. process uses a zinc bath alloyed with 0.05-0.06 wt % of nickel. As shown in FIG. 1, the Sandelin peak disappears in the Technigalva.RTM. bath, but the coating thickness still increases with the silicon content of the steel. The Polygalva.RTM. process uses a zinc bath with 0.035-0.045 wt % of aluminium and 0.003-0.005 wt % of magnesium. As shown in FIG. 1, the Polygalva.RTM. bath gives rather good results; however it presents the drawback that its aluminium content has to be controlled very strictly, because the reaction between the steel and the bath blocks almost completely once the aluminium content of the bath exceeds 0.05 wt %.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a bath for hot-dip galvanizing consisting of alloyed zinc, which makes the coating thickness much less dependent on the silicon content of the steel than is the case with the Technigalva.RTM. bath and much less dependent on small variations in the bath composition than is the case with the Polygalva.RTM. bath.
This aim is achieved according to the invention by a bath that contains 1-5 wt % of tin and 0.01-0.1 wt % of nickel and that may contain lead at a concentration up to saturation and at least one of aluminium, calcium and magnesium at a concentration up to 0.06 wt %, the rest being zinc and unavoidable impurities.
When the bath does not contain nickel, its preferred tin content is 3.5-14 wt %, the most preferred tin content being 5-10 wt %. When it contains nickel, its preferred tin and nickel contents are respectively 2.5-5 wt % and 0.03-0.06 wt %.
The nickel content of the bath with 1-5 wt % of tin has to be at least 0.01 wt %; otherwise, the coating thickness may vary substantially with the silicon content of the steel. However, the nickel content mustn't exceed 0.1 wt %; otherwise there is a risk of formation of floating dross.
An addition of lead at a concentration that may attain saturation, for example 0.1-1.2 wt %, may be useful in order to decrease the surface tension of the bath.
An addition of at least one of aluminium, calcium and magnesium, preferably at a concentration of 0-0.03 wt % and more preferably of 0.005-0.015 wt %, may also be useful in order to protect the zinc from oxidation; otherwise a yellowish pellicle is formed on the surface of the bath, which fouls the galvanized articles.
However the aluminium content preferably should not exceed 0.03 wt %; otherwise there is a risk of obtaining uncovered spots. The magnesium and/or calcium contents mustn't exceed 0.03 wt %; otherwise MgO or CaO floating on the surface of the bath may spoil the coating; moreover the bath becomes less fluid, and may result in a degraded finishing of the coating.
It should be noted here that LU-A-81 061 describes a process consisting of a galvanisation bath which contains at
REFERENCES:
patent: 3962501 (1976-06-01), Ohbu et al.
patent: 4439397 (1984-03-01), Dreulle
patent: 4451541 (1984-05-01), Beal
patent: 5049453 (1991-09-01), Suemitsu et al.
patent: 5455122 (1995-10-01), Carey et al.
C.H. Mathewson, Zinc, The Science and Technology of the Metal, Its Alloys and Compounds (1959).
Kirk-Othmer, Encyclopedia of Chemcial Terminology, vol. 24, p. 833 (1984).
Zinc de deuxieme fusion (NF A 55-111), ICS: 77.120.60 (Jun. 1995).
Gilles Michael
Sokolowski Richard
Barr Michael
Beck Shrive
Jain Mishrilal
N. V. Union Miniere S.A.
Nirmel Chittaranjan N.
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