TECHNICAL FIELD

The present invention relates to an apparatus and a method for hot dip galvanising ferrous articles. Throughout this specification the expression "ferrous" is to be understood to embrace iron and alloys of iron such as steel. BACKGROUND ART

Hot dip galvanising has long been practised to prevent corrosion of ferrous articles ranging in size from small articles such as nuts and bolts to very large articles such as bridge beams.

Hot dip galvanising protects ferrous articles from corrosion by providing a tough metallic zinc envelope which completely covers the surface of the article and seals it from the corrosive action of its environment. Hot dip galvanised coatings have very good abrasion resistance and where there is damage or a minor discontinuity in the zinc coating, protection of the ferrous article is maintained by the cathodic action of the neighbouring galvanised coating.

When a ferrous article is hot dipped galvanised, it is placed in a galvanising bath of a molten galvanising solution. The galvanising solution is typically zinc but the expression "galvanising solution" is to be understood to include alloys of zinc such as GALFAN ^® (approximately 95% zinc, 5% aluminium and small amounts of rare earth metals) . (GALFAN is a registered trade mark of International Lead Zinc Research Organisation Inc.) The ferrous article typically remains in the galvanising bath until it reaches the temperature of the molten galvanising solution and hence its residence time in the galvanising bath is largely dependent upon its mass and the thickness of its thickest portion. The galvanising solution reacts with the ferrous article to form layers of zinc- iron alloy which contain progressively greater proportions of zinc. The outermost layer is typically

metallic zinc which is strongly resistant to the corrosive action of normal environments.

Ferrous articles are typically cleaned and fluxed prior to hot dip galvanising. On immersion in the molten galvanising solution, the protective flux layer is removed leaving a clean surface which is wetted by the galvanising solution resulting in reaction between zinc in the galvanising solution and iron in the ferrous article. The thickness and alloy structure of a galvanised coating are most significantly influenced by the surface condition of the ferrous article and the metallurgical composition of the ferrous article. With the exception of some silicon steels, an excessive period of immersion in a galvanising bath does not tend to result in increased coating thickness.

It is theoretically possible to hot dip galvanise a ferrous article of any size. The upper size limit is effectively dictated by the size of the galvanising bath. In the case of small ferrous articles, the economics of hot dip galvanising are limited by the fact that it is not feasible to individually dip articles such as nuts and bolts. In these cases, it is a common practice to load a multiplicity of small articles into a basket and dip them at the same time. However, this approach has a number of limitations. For economic reasons, it is desirable that a ferrous article spend the minimum amount of time in a galvanising bath required to produce a satisfactory galvanised coating. When a multiplicity of small articles are immersed within a basket, those articles closer to the extremities of the basket are galvanised more quickly than those in the centre of the mass of articles and hence the total immersion time can be prohibitively long. Failure to immerse the basket of articles for a sufficient period of time can result in variations in coating thickness and quality from article to article. Even where the period of immersion is ideal, there is a tendency for the articles to stick together

during the galvanising process due to their close proximity with consequential variations in coating thickness.

The conventional approach of hot dip galvanising a multiplicity of small ferrous articles in a basket is typically labour intensive because it necessitates an operator loading the articles into the basket, lowering the loaded basked into the galvanising bath, and removing the basket of galvanised articles from the galvanising bath.

The foregoing problems have precluded hot dip galvanising from being used to provide corrosion protection for some small articles such as self tapping fasteners which are commonly treated by electrogalvanising techniques. Electrogalvanising of fasteners produces relatively light uniform coatings which have excellent appearance but which are generally unsuitable for outdoor exposure without additional corrosion protection. Where possible, ferrous articles are preferably treated by hot dip galvanising rather than electrogalvanising because of relatively less expensive manufacturing costs and enhanced corrosion protection of the resultant coating. DISCLOSURE OF THE INVENTION In a first aspect, the present invention provides an apparatus for hot dip galvanising a ferrous article in a galvanising bath of a galvanising solution, the apparatus comprising: means for immersing the ferrous article in the galvanising bath for a period of time to galvanise the ferrous article, and means for withdrawing the galvanised ferrous article from the galvanising bath under the influence of a magnetic field. In a second aspect, the present invention provides a method for hot dip galvanising a ferrous article, the method comprising the steps of:

immersing the ferrous article in a galvanising bath of a galvanising solution, retaining the ferrous article in the galvanising bath for a period of time to galvanise the ferrous article, and withdrawing the galvanised ferrous article from the galvanising bath under the influence of a magnetic field.

Ferrous articles are ferromagnetic and hence are capable of being positioned within a static magnetic field and being moved by a moving magnetic field. Molten galvanising solutions are transmissive to magnetic fields and hence a ferrous article can be moved within a galvanising bath under the influence of a magnetic field.

According to the present invention, a ferrous article is withdrawn from a galvanising bath under the influence of a magnetic field. For example, the walls of a galvanising bath may be formed of material transmissive to magnetic fields (eg. stainless steel or mild steel containing approximately 15% manganese) with the poles of a bipolar magnet located on either side of the galvanising bath forming a magnetic field between the poles and through the galvanising bath. Coordinated movement of the poles results in movement of the magnetic field relative to the galvanising bath and enables a ferrous article to be "picked up" from within the galvanising bath and withdrawn from the galvanising bath. Preferably, in accordance with the present invention, a magnetic field is used to immerse a ferrous article in a galvanising bath, retain the ferrous article in the galvanising bath for a period of time to galvanise the ferrous article, and withdraw the galvanised ferrous article from the galvanising bath. The present invention thus enables automation of the hot dip galvanising process . The poles of the magnet cannot travel through the galvanising bath and hence are isolated from the galvanising bath. The poles may be located outside the bath adjacent the walls of the bath. Preferably however,

for at least a portion of their travel, the poles are located in the bath below the level of the galvanising solution adjacent a channel passing through the galvanising solution and are isolated from the galvanising solution by a shielding structure.

The magnet is preferably an electromagnet and the magnetic field generated between the poles of the magnet can follow a variety of paths in accordance with the present invention. The path may be essential vertical, commencing above the surface of the galvanising solution where ferrous article (s) are positioned within the magnetic field and then moved downwardly into the galvanising bath for galvanising prior to being raised out of the galvanising bath for removal. In such an arrangement, a series of adjacent magnetic fields may be generated for use in one galvanising bath. Alternatively, movement of the poles may move the magnetic field through a flattened elliptical path analogous to a conveyor having an upper run moving horizontally in one direction above the surface of the galvanising solution and a lower run moving horizontally in the opposite direction below the surface of the galvanising solution. Preferably however each pole moves through a circular path with the planes of the paths being parallel and the paths having a common axis.

The apparatus preferably comprises a channel arranged to be partially located in the galvanising bath and a magnet assembly mounted above the channel . The channel may be an open ended arcuate channel with the open ends of the channel projecting above the surface of the galvanising solution. Alternatively, the channel may be an annular channel having openings above the level of the galvanising solution for introducing ferrous articles to be galvanised and for removing galvanised ferrous articles.

The magnet assembly may comprise a bipolar electromagnet having two extended side poles, each culminating in a pole shoe between which a magnetic field

is generated. The pole shoes may be arranged to move through a circular path having a radius the same as the arc of the channel. A shielding structure may be attached to the channel to isolate the pole shoes from the galvanising solution. The electromagnet is preferably controlled by a controller which enables the strength of the magnetic field to be varied and is mounted on a shaft which is preferably driven by a variable speed motor to enable the speed of rotation of the pole shoes (and hence the speed of rotation of the magnetic field) to be varied. The channel is formed from a magnetically transmissive material which is preferably stainless steel or ceramic coated stainless steel and is perforated on its underside to allow ingress of galvanising solution to the level of the galvanising bath. The channel may be of any suitable shape in cross- section including circular. Preferably however the channel is square or rectangular in cross-section for the maintenance of a constant air gap between the pole shoes and the walls of the channel.

Preferably, one or more ferrous articles are introduced into the apparatus at one open end of an arcuate channel or through an opening of an annular channel where they fall within the magnetic field between the rotating pole shoes and are transported through the galvanising solution within the channel until they reach the other open end of an arcuate channel or another opening of an annular channel where they are unloaded. Ferrous articles may be loaded into the apparatus by a vibratory feeder or the like and may be unloaded by striking a plate which restricts continued movement in the magnetic field. Galvanised ferrous articles are preferably removed by action of another magnetic field which is the subject of the present applicant's Australian provisional patent application no. PN9730.

Where the channel is an annular channel, the apparatus is preferably arranged such that a charge of ferrous articles to be galvanised is held in an upper

portion of the channel above the galvanising solution by resting on or against a plate or the like until they are picked up by the magnetic field and moved into the galvanising solution under action of the magnetic field. The charge of ferrous articles is preferably delivered from a vibratory feeder mounted on load cells and the level and temperature of the galvanising bath are preferably monitored and maintained substantially constant. Consistency of product quality can therefore be achieved because the variables of charge mass, bath depth, bath temperature and galvanising duration can be controlled. Introduction of a charge from the vibratory feeder may be manually controlled. Alternatively, the vibratory feeder may be arranged to introduce a charge of predetermined mass by movement of the magnetic field and the charging process may thereby be automated. Once a charge sitting in the upper portion of the channel has been picked up by the magnetic field, movement of the magnetic field beyond a further point on the channel can be detected and trigger introduction of a further charge from the vibratory feeder which then sits in the upper portion of the channel until the magnetic field returns to pick up the further charge with the process then continuing. The above described preferred embodiments of the present invention refer to a bipolar electromagnet having one set of pole shoes between which a magnetic field is generated. The apparatus preferably includes a magnetic assembly having two or more magnetic fields generated between respective sets of pole shoes at the ends of respective extended side poles in a manner analogous to spokes on a wheel with the electromagnet forming the hub of the wheel. It will be appreciated that such an arrangement facilitates increased throughput of ferrous articles.

It is sometimes desirable to galvanise ferrous articles in the absence of atmospheric oxygen, particularly where the galvanising solution is GALFAN ^Φ .

The apparatus of the present invention may be arranged to be encased or covered by a shroud to be operated in an inert gas atmosphere such as nitrogen or in a reducing atmosphere such as nitrogen/hydrogen. The channel can be engineered to accommodate ferrous articles of various sizes and the maximum size of an article capable of being galvanised according to the present invention will only be limited by the strength of the magnetic field which can be generated. A plurality of articles can be galvanised with each pass of a magnetic field and self tapping fasteners have been successfully galvanised according to the present invention.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic side elevation of an embodiment of the present invention,

Figure 2 is a schematic end elevation of the embodiment of Figure 1,

Figure 3 is a schematic side elevation of another embodiment of the present invention, and

Figure 4 is a schematic plan of the embodiment of Figure 3. Referring firstly to Figures 1 and 2, a galvanising bath 10 of a molten zinc galvanising solution is contained within a kettle 12. A galvanising apparatus 14 comprising an arcuate product channel 16 and magnet assembly 18 is suspended above and partially in the galvanising bath 10. The product channel 16 includes a number of perforations 19 permitting ingress of the galvanising solution. The product channel 16 is formed from 316L stainless steel which is magnetically transmissive and resistant to attack by the galvanising solution. The galvanising solution occupies the product channel 16 to the level 20 of the galvanising bath 10 leaving the entry 22 and exit 24 of the product channel 16 above the galvanising solution level 20. The product

channel 16 is formed with two wings 26 which project laterally and subsequently upwardly from the product channel 16 above the level of the galvanising bath 20 to isolate the magnet assembly 18 from the galvanising bath 10. The surfaces of the wings 26 remote from the galvanising bath 10 are covered with heat insulating material (not shown) to reduce the temperature under which the magnet assembly 18 must operate. The magnet assembly 18 comprises a bipolar electromagnet 28 mounted on a shaft 30 which is driven by a variable speed electric motor 32 which rotates the electromagnet 28 in the direction of the arrow 33 in Figure 1 (ie. clockwise) . The electromagnet 28 comprises a rotating air cooled magnetic coil fed by a set of slip rings which are supplied current from a 240V DC power supply. For enhanced cooling, the magnetic coil may be cooled by a flow of oil or other coolant rather than air. The electromagnet 28 is formed with two sets of extended poles 34,35 and 36,37 which culminate in two pairs of pole shoes 38,40 and 42,44 respectively. When current is applied to the electromagnet 28 by a variable magnet controller 46 two strong magnetic fields are induced; the first between pole shoes 38 and 40 and the second between pole shoes 42 and 44. The electromagnet 27 is designed to provide an electromagnetic field of sufficient strength to move the desired ferrous article (s) through the galvanising bath 10. Electromagnets of the kind described above can generate magnetic fields equivalent to 0-150,000 ampere turns. In use, current is applied to the magnet controller 46 and the electromagnet 28 is continuously rotated about shaft 30 to sequentially expose the product channel 16 to the magnetic field between pole shoes 38 and 40 and then pole shoes 42 and 44. The product channel 16 and the galvanising solution are transmissive of the magnetic field and hence a ferrous article can be carried through the galvanising solution under influence of the magnetic field. One or more ferrous articles are introduced into

the product channel 16 at entry 22 where they are retained in the magnetic field between pole shoes 38 and 40 above the level of the galvanising bath 20. Rotation of the electromagnet 28 results in the ferrous articles being maintained in the magnetic field between pole shoes 38 and 40 with consequential immersion in the galvanising bath 10 at the point marked A and transport through the galvanising bath 10 until they are withdrawn from the galvanising bath 10 at the point marked B with continued rotation of the electromagnet 28 moving the now galvanised ferrous articles to the exit 24 where they meet plate 42 and are prevented from continuing to move under influence of the magnetic field. The galvanised ferrous articles are then preferably removed from the apparatus 14 under the influence of another magnetic field which draws them substantially horizontally away from the plate 42.

Referring now to Figures 3 and 4, like reference numerals to those used in relation to Figures 1 and 2 have been used to avoid repetition of description.

The arcuate channel 16 of Figures 1 and 2 has been replaced by an annular channel 50 which is rectangular in cross-section (see Figure 4) , the magnet assembly 18 is configured for counter-clockwise rotation as viewed in Figure 3, and for simplicity of illustration, only one set of extended poles are illustrated in Figure 3. Figure 4 is a schematic cross-sectional plan view of Figure 3 in which both sets of extended poles 34, 35 and 36, 37 are illustrated in horizontal positions. Ferrous articles to be galvanised are introduced into the channel 50 from a vibratory feeder (not shown) mounted on load cells (not shown) through a hopper 52 and entry 22. A charge of ferrous articles so introduced strike the inner wall of channel 50 and rest in channel 50 against plate 54 until picked up by the magnetic field between pole shoes 38 and 40 as it moves in a counter¬ clockwise direction past plate 54. As illustrated in Figure 3, the charge of ferrous articles has been picked

up and moved to a position at about 10.00 o'clock. Thereafter, the charge of ferrous articles continue counter-clockwise movement until immersed in the galvanising bath at the point marked A, galvanised in the galvanising bath 10 between the points marked A and B, withdrawn from the galvanising bath 10 at the point marked B, and arriving at the exit 24 where they meet plate 42 and are prevented from continuing to move under influence of the magnetic field. The magnetic field then continues in counter-clockwise rotation where it picks up a further charge of ferrous articles at plate 54 and the galvanising process is repeated.

The centre wall of the channel 50 is interrupted adjacent immersion and withdrawal points A and B respectively at apertures 56 and 58. Air is continuously blown from air line 60 through air nozzles 62 and 64 to clear the surface 20 of the galvanising bath 10 at immersion and withdrawal points A and B to improve the quality of the galvanised product. The apertures 56 and 58 allow ash and other impurities on the surface 20 of the galvanising bath 10 to be blown out of the channel 50 by air passing through air nozzles 62 and 64 respectively.