GB2227255A

GB2227255A - Galvanizing with compositions including tin

Info

Publication number: GB2227255A
Application number: GB8924712A
Authority: GB
Inventors: Gabriella Pallos
Original assignee: John Lysaght Australia Pty Ltd
Current assignee: John Lysaght Australia Pty Ltd
Priority date: 1988-11-08
Filing date: 1989-11-02
Publication date: 1990-07-25
Anticipated expiration: 2009-11-02
Also published as: GB2227255B; GB2226332B; GB8924712D0; GB8924691D0; GB2226332A

Abstract

To prevent or lessen the darkening of galvanized steel of the kind in which the galvanizing coating contains about 1.3% by weight of lead and a minor amount of aluminium, an addition of from 0.11% to 0.3% by weight of tin or a mixture of tin and antimony is included in the coating composition. If both metals are present a typical preferred concentration would be 0.03 - 0.04% antimony and 0.15 - 0.2% tin.

Description

GALVANIZING WITH COMPOSITIONS INCLUDING TIN This invention relates to mini

-galvani z ing processes as described, for example, in Australian Patent 425t028 of 14th September 1971. The mini - galvani z ing process is so called because it provides a cost effective method of continuously galvanizing steel strip in relatively small quantities, as may be needed for specialised products requiring a high quality ductile coating but commanding a relatively small market.

In the mini -galvani z ing process steel strip to be coated is pretreated, usually by being de-greased, descaled, acid etched, rinsed and finally wet fluxed, and then directed downwards into a molten lead bath and then upwards through a molten zinc bath floating on the lead bath.

For descriptive convenience herein the lead and zinc baths are referred to as such, although, as explained in the aforesaid Australian Patent, mini-galvanizing processes, including those of this invention, require minor proportions of aluminium to be maintained in each bath.

The lead bath serves to heat the strip to a suitable temperature before it enters the zinc bath. The fluxing may be conveniently effected by passing the strip into the lead bath through a pool of flux also floating on the lead bath, in which event the lead bath also serves to separate the flux pool from the zinc bath. In any event, the zinc and lead baths are in intimate contact, and thus each becomes contaminated with metal from the other, to an extent limited by the respective saturation levels. In practice, the zinc bath acquires about 1.3% of -dissolved lead, and the lead bath a similar concentration of zinc.

Thus galvanizing melt in the mini-galvanizing process contains about 1.3% lead, because the zinc layer is "W 1 2 saturated with lead, and about 0.2% aluminium, by deliberate addition.

During -storage of coated product of a minigalvanizing process, especially in hot humid atmospheres, the passivated surface oxidizes and the appearance changes from shiny and bright to dull and often dark. This can occur after about ten weeks of storage. This is objectionable because it detracts from the appearance of the product, although it does not reduce its effective life.

Published work on galvanized steel with coatings having a low lead content, for example 0.2%, has shown that the darkening of the surface is due to a number of factors, one of which is lead. Scanning electron microscopy of darkened surfaces of galvanized steels produced by the minigalvanizing process has shown that a corrosion process occurs around lead particles on the surface which leads to the formation of a thick zinc oxide. As this has a very rough surface, it is non-reflective and appears dark grey to black.

Extensive investigations into the darkening of the mini-galvanized surface leading to this invention have shown that it is worse (a) at low coating mass (120-160 g/M2), (b) the higher the surface chromium, (c) the higher the cooling rate of the strip during solidification, (d) on the shiny spangles, and it does not occur on well defined frosty ones.

It was shown that both (a) and (c) lead to a high number of small lead particles on the surface and this, coupled with relatively high surface chromium (over 8 mg/m2), leads to darkening.

1 i 3 Scanning electron microscopy showed that shiny spangles have on average a large number of small lead particlesq while well-defined frosty spangles have on average a smaller number of large lead particles. This would explain the greater tendency of the shiny spangles to darken.

The present invention flowed from the discovery that the addition of small quantities of tin or mixtures of tin and antimony to the galvanizing melt retards and/or minimizes the darkening of the surface.

The invention consists in a method of galvanizing steel by a minigalvanizing process of the kind utilising a galvanizing composition comprising zinc and minor amounts of lead and aluminium, characterised by the addition to the composition of from 0.11% to 0.30% by weight of tin or of a mixture of tin and antimony.

If both metals are present a typical preferred concentration would be from 0.03% to 0.04% antimony and from 0.15% to 0.2% tin.

Tin is present in low concentrations, typically up to 0.05% by weight, in some blast furnace zinc; however, the maximum concentration of tin can reach 0.1%, which is slightly less than the lower limit of the concentration according to the invention.

Tin has been added to some zinc compositions hitherto in processes other than the mini-galvanizing process with the aim of producing spangles. This has not been done in the case of mini-galvanizing processes hitherto as it is unnecessary in view of the aluminium content which also induces spangles.

Tin addition also increases the pereEntage o'L frosty Q spangles from about 301% max to 70% max. Since darkening of 4 the mini-galvanized surface occurs mostly on shiny spangles the percentage of the surface which may darken is decreased by the tin addition.

The limiting values of the range of the tin or combined tin and antimony content according to the invention were indicated by the aforesaid investigations and a confirmatory trial under commercial production conditions utilising tin and antimony at the indicated optimum levels was conducted. In that trial operation of the present invention tin and antimony were added to the galvanizing melt of a commercial, continuous mini galvanizing plant; for most of the trial period the substrate strip was hard iron (cold reduced steel) but a few soft iron (fully annealed steel) coils were also included.

Tin and antimony were added to the melt for 5 days and the concentrations ranged from 0.030% to 0.045% antimony and from 0.140% to 0.190% tin. On the sixth day the tin was held at 0.140% and the antimony allowed to drop to about 0.010%. Before the trials, samples were collected for 24 hours without additions to the melt and these were used as controls in all the darkening tests.

Eleven samples 300 mm x full width were taken every 2 to 3 hours during the trial. Eight of each batch were stacked in a hot, humid climate, while three were subjected to accelerated testing and surface analysis.

In the accelerated testing the size of the samples used was 50 x 50 mm. Five to seven samples were taken from different positions along the width of the coil (sheet). These were stacked top to bottom and tested for 10 days at 750C and 57% relative humidity.

A The discolouration was evaluated by two methods:

Measured by an integrating sphere colour spectrophotometer, namely a Micromatch 3000. The degree of darkening is expressed by a delta L value. The lower this value, the darker the surface.

2. Visually evaluated using a scale of 1 to 5 where 1 is for a black or dark grey sample and 5 for a sample with no darkening. The visual evaluation was compared to the delta L values and corrections were made where necessary.

Measurement of the discolouration by the spectrophotometer has two disadvantages: 1. A small general dulling of the surface which does not detract from the general appearance can result in a very low delta L value. 2. Strong discolouration of a very small area of the surface might not result in a very low delta L value if the rest of the surface remains bright.

The results of the trials are indicated in the accompanying drawings.

Figure 1 is a histogram showing the numbers of hard iron samples having various degrees of darkening as assessed by visual rating.

Figure 2 is similar to figure 1 showing the results for the same samples but with degrees of darkening as measured by the spectrophotometer.

It is considered that in practice the visual rating results shown in figure 1 are more relevant than the results indicated in figure 2.

1 6 When compiling the results the top and bottom surfaces of the samples were counted separately. Figure 1 shows that 46% of the control samples (without additions) darkened considerably (rating 1.5 to 3), 41% had some darkening, and only 13% had no discernible darkening, whereas of the samples containing tin and antimony 22% had little darkening and 78% had no discernible darkening. Figure 2, though not as persuasive as Figure 1, nevertheless shows that the tin and antimony had a significant effect in improving the resistance to darkening as seen by the spectrophotometer.

The soft iron samples showed more darkening in the accelerated tests than the hard iron samples, but the addition of tin or tin and antimony appears to improve the resistance to darkening in this case also. As only two control soft iron coils were run with no additions and four coils with additions of tin and antimony, no histograms could be constructed. However, three months field tests showed darkening of the controls over from 20% to 85% of their areas while the product with tin or tin and antimony additions showed no discolouration.

1 7

Claims

CLAIMS:

1. A method of galvanizing steel by a mini -galvani z ing process of the kind utilising a galvanizing composition comprising zinc and minor amounts of lead and aluminium, characterised by the addition to the composition of from 0.11% to 0.30% by weight of tin or a mixture of tin and antimony.

2.

A method of coating a steel strip comprising the steps of passing the strip downwardly through a pool of flux floating on a lead bath and then upwardly through a pool of galvanizing composition also floating on the lead bath characterised in that the composition includes from 0.11% to 0. 30% by weight of tin or a mixture of tin and antimony.

3. A method of reducing the darkening of galvanized steel strip comprising the step of including from 0.11% to 0.30% by weight of tin or a mixture of tin and antimony in the galvanizing composition applied to the strip.

4. A method according to any one of the preceding claims wherein there is from 0.140% to 0.0.190 by weight of tin and from 0.030% to 0.045% antimony in the galvanizing composition.

5. A galvanized steel strip whereof the galvanizing coating comprises up to substantially 1.3% lead, up to substantially 0.2% aluminium, from 0. 11% to 0.3% tin or a mixture of tin and antimony and the balance zinc, all the percentages being by weight.

6. A strip according to claim 5 wherein the coating includes from 0.140% to 0.190% of tin and from 0.030% to 0.045% antimony.

8

7. A galvanizing composition comprising from 1.2% to 1.4% lead, from 0.1% to 0.3% aluminium, from 0.11% to 0.3% tin or a mixture of tin and antimony, and the balance zinc, all the percentages being by weight.

8. A composition according to claim 7, wherein there is from 0.140% to 0. 190% of tin and from 0.30% to 0.045% of antimony.

a i Published 1990 at The Paten Office State House. 66 71 High Holborn. Lop don WC1 R 4TP Further copies may be obtained from The Patent Office Sales Branch. SL Mai7 Cray. Orpington. Kent BR5 3RD Printed by Multiplex techniques ltd. St Mary Cray. Kent. Cor. . 1,87

1 L- 1 - q_ Amendments to the claims have been filed as follows 1. A method of galvanizing steel by a mini -galvani z ing process of the kind utilizing a galvanizing composition comprising zinc naturated with lead and a minor amount of aluminium, characterised by the addition to the composition of from 0.11% to 0.30% by weight of tin or a mixture of tin and antimony.

2. A method of coating a steel strip comprising the steps of passing the strip downwardly through a pool of flux floating on a lead bath and then upwardly through:a pool of galvanizing composition also floating on the load bath, characterised in that the componition includes from 0.11% to 0. 30% by weight of tin or a mixture of tin and antimony.

3. A method of reducing the darkening of galvanized oteel otrip whereof the galvanizing composition comprises zinc saturated with lead, comprising the step of including from 0.11% to 0.30% by weight of tin or a mixture of tin and antimony in said galvanizing composition.

4. A method according to any one of the preceding claims wherein there is from 0.140% to 0.190% by of tin and from 0.030% to 0.04.5% antimony in the galvanizing composition.

5. A galvanized steel strip whereof the galvanizing coating comprises substantially 1-3-70 1 ead ' up t c. substantially 0.2% aluminium, from 0. 11% to 0.3% tin or a mixture of tin and an'-9-on,.r an--' '-h,,? hn--.97, tn! zine, all the percentages being by weight.

f. n -, the coatinR.

0 A strip accordLne, ti clairr. 5 wherei, includes from 0.140% to 0.190% of tin -ind fron 0.030% tC, 0.045% antimony.

1 1 1 1 i 4k - -to- 7. A galvanizing composition comprising from 1.2% to 1.4% load, from 0.1% to 0.3% alum:Lniumt from 0.11% to 0.3% tin or a mixture of tin and antimony, and the balance zinc, all the percentages being by weight.