CA1065204A - Zinc-aluminum eutectic alloy coating process and article - Google Patents
Zinc-aluminum eutectic alloy coating process and articleInfo
- Publication number
- CA1065204A CA1065204A CA233,849A CA233849A CA1065204A CA 1065204 A CA1065204 A CA 1065204A CA 233849 A CA233849 A CA 233849A CA 1065204 A CA1065204 A CA 1065204A
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- zinc
- hot
- bath
- coating
- aluminum
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Abstract
ABSTRACT OF DISCLOSURE
A process of hot-dip coating a ferrous metal strip with the zinc - 5% aluminum eutectic alloy in which the clean strip is continuously passed through a zinc-aluminum eutectic alloy bath maintained at a temperature between about 775°F
and 825°F and impinging on the fluid eutectic alloy coating as it is withdrawn from the bath jets of steam heated to at least 900°F to control the coating weight and retard the solidification of the zinc-aluminum eutectic alloy coating.
The resulting zinc-aluminum eutectic alloy coated strip is formable at room temperature, and the coating is free of surface ripples and large spangles.
A process of hot-dip coating a ferrous metal strip with the zinc - 5% aluminum eutectic alloy in which the clean strip is continuously passed through a zinc-aluminum eutectic alloy bath maintained at a temperature between about 775°F
and 825°F and impinging on the fluid eutectic alloy coating as it is withdrawn from the bath jets of steam heated to at least 900°F to control the coating weight and retard the solidification of the zinc-aluminum eutectic alloy coating.
The resulting zinc-aluminum eutectic alloy coated strip is formable at room temperature, and the coating is free of surface ripples and large spangles.
Description
The present invention relates generally to an improved zinc-aluminum alloy coated ferrous metal article and more particularly to an improved process of continuously hot-dip coating a ferrous metal strip in a bath comprised essentially of the zinc-aluminum eutectic alloy and to the coated ferrous metal strip produced by such process.
Zinc based coatingshave long been applied by hot-dip coating a ferrous metal surface in order to improve the corrosion resistance of the ferrous metalO Many alloying metals have been used in the zinc hot-dip coating bath to improve the protective coating. Among the alloying metals which have been used in a zinc coating bath are lead, tin, silicon, cadmium, antimony, magnesium and aluminum.
The prior art patent processes relating to zinc-aluminum hot-dip coatings (see U. S. Patent Nos. 1,741,388;
1,764,132; 3,320,040; 3,343,930 and 3,505,043) have failed to make use of the unique improvements in a hot-dip coating process which are made possible by employing the zinc-aluminum eutectic alloy as the coating bathO ThUs, UO S. Patent NosO
1,741,388 and 1,764,132 disclose a process for applying zinc-aluminum alloy coatings in which the aluminum content ranges from about two percent up to about 20% by wt. aluminum with specifie references to using zinc-aluminum alloy containing .
6% and 8% by wt. aluminum. U~ S. Patent No. 3,320,040 teaches adding from about 0.1 to a maximum of 3.5% by wt. aluminum to a zinc hot-dip coating bath with a preferred range of from 0.1% to 0.5% by wt. aluminum along with a small amount of magnesiumO U. S. Patent NoO 3,343,930 describes hot-dip coating using a coating bath containing from 25% to 70% by weight aluminum with the balance zinc. And, U. S. Patent No.
Zinc based coatingshave long been applied by hot-dip coating a ferrous metal surface in order to improve the corrosion resistance of the ferrous metalO Many alloying metals have been used in the zinc hot-dip coating bath to improve the protective coating. Among the alloying metals which have been used in a zinc coating bath are lead, tin, silicon, cadmium, antimony, magnesium and aluminum.
The prior art patent processes relating to zinc-aluminum hot-dip coatings (see U. S. Patent Nos. 1,741,388;
1,764,132; 3,320,040; 3,343,930 and 3,505,043) have failed to make use of the unique improvements in a hot-dip coating process which are made possible by employing the zinc-aluminum eutectic alloy as the coating bathO ThUs, UO S. Patent NosO
1,741,388 and 1,764,132 disclose a process for applying zinc-aluminum alloy coatings in which the aluminum content ranges from about two percent up to about 20% by wt. aluminum with specifie references to using zinc-aluminum alloy containing .
6% and 8% by wt. aluminum. U~ S. Patent No. 3,320,040 teaches adding from about 0.1 to a maximum of 3.5% by wt. aluminum to a zinc hot-dip coating bath with a preferred range of from 0.1% to 0.5% by wt. aluminum along with a small amount of magnesiumO U. S. Patent NoO 3,343,930 describes hot-dip coating using a coating bath containing from 25% to 70% by weight aluminum with the balance zinc. And, U. S. Patent No.
-2- ~
.
.
lOt;SZ04
.
.
lOt;SZ04
3,505,043 teaches hot-dip coating a metal surface with a zinC-aluminum-magnesium alloy containing about 3 to 17% by wt.
aluminum in combination with from 1% to 5% by weight magnesiumO
In an article by Cameron and Ormay entitled "The Effects of Agitation, Cooling and Aluminum On The Alloying In Hot-Dipping in Zinc" published in "Edited Proceedings 6th International Conference on Hot-Dip Galvanizing", Interlaken, June 1971, pgs. 296-297, the authors discuss the metallurgical struc~ures and drosses formed when a ferrous metal panel is immersed for 24 hours in a wide range of zinc-aluminum alloy bath compositions, including a zinc-5% aluminum alloy coating bath, and describe the effects of agitation and rate of cooling. The authors do not disclose a continuous hot-dip coating process, nor do they evaluate the properties or use-fulness of any of the zinc-aluminum alloy coatings after immersing the panel for 24 hours in the coating bath. Also, Brauer and Peirce in a paper entitled "The Effect of Impurities On The Oxidation and Swelling of Zinc Aluminum Alloys" (TransO
Am. Inst. Met. Eng. (1923) Vol. 68,796) describe studies of zinc die casting alloys, including 5% aluminum-0 5% to 5%
copper-zinc die casting alloys, which contain impurities in the zinc~ such as lead, tin or cadmium.
Despite the considerable efforts which have been made to improve the hot-dip continuous coating of a ferrous metal strip, there remains considerable room for improvement in both the hot-dip zinc-base coatings and in the method by which hot-dip zinc-base coatings are continuously applied to a ferrous metal strip in order to obtain uniformly in a more economical manner a smooth, attractively appearing, formable zinc-aluminum alloy hot-dip coated strip. For example, in a ~0~;5Z~)~
conventional hot-dip galvanizing ba~h a substantial amount of a heavy, compact iron-zinc dross is formed in the coating bath which sinks to the bottom of the bath and requires periodic removal in order to allow continued use of the bath.
Careful control over the length of time the ferrous metal strip is allowed to re~ain immersed in the hot-dip coating bath is also required, particularly when the bath is maintained at the usual temperature of 850F or above. An objectionably thick intermetallic iron-zinc layer is formed which seriously interferes with the desired formability of the coated product, if the strip is allowed to remain for more than a few seconds in a conventional hot-dip coating bath. Also, careful control of the temperature of the strip entering the hot-dip coating bath is required so that the temperature of the strip is not so high as to form a thick intermetallic layer which cannot be deformed at room temperature without interfering with the corrosion resistance of the coating.
The paintability of a hot-dip zinc-base coating is impaired by the presence of large spangle on the surface of a zinc coated ferrous metal strip, and the weldability of a zinc-coated product is unsatisfactory when the zinc coating is not smooth. Any pronounced spangles in a hot-dip coating are clearly evident through the conventional paint or enamel coatings of the type which are normally applied to building ~ -panels or automobile body panels, and it is considered highly desirable ~o minimize spangle formation on hot-dip coatings to be painted or enameled. Temper rolling of the usual zinc-hot-dip coated products having pronounced spangles is required to smooth out and remove the objectionable surface pattern.
The size of the spangles which form in a conventional zinc-base , ' 1065Z~4 hot-dip coatings tends to be large because the hot-dip zinc coating bath temperature is relatively high and the period during which the hot-dip coating remains in the liquidus phase or in the transition phase between liquidus and solidus tends to be prolonged. And, the addition of certain metallic elements, such as lead, antimony, and tin which are used to reduce surface ripples and improve the smoothness of the hot-dip coatings, further increase the grain size or spangles.
Thus, in the prior art zinc-base hot-dip coating processes the use of such additives as lead to control coating smoothness is limited and cannot be used in an amount sufficient to insure a smooth zinc coating without causing the formation of objectionably large spangles in a conventional zinc-base hot-dip coated surface.
It is therefore an object of the present invention to provide an improved zinc-aluminum alloy hot-dip coated ferrous metal base in a more efficient and economical manner.
Another object of the present invention is to provide an improved process of applying a zinc-aluminum alloy hot-dip coating on a ferrous metal strip.
In one broad aspect the invention comprehends a corrosion resistant continuous hot-dip coated ferrous metal strip having applied directly on a surface thereof a hot-dip coating comprising a zinc-aluminum eutectic alloy which contains 5.0 - 0.5 wt. % aluminum with the balance being essentially zinc. The coating provides a readily paintable surface which is free of large spangles interferring with good paintability. The coating may contain at least one metal additive selected from the group consisting of lead, tin and antimony, preferably about 0.1 wt. %.
`,E~' ' 106SZ(~4 Another aspect of the invention pertains to a process for applying a zinc-aluminum alloy hot-dip coating to a ferrous metal strip in which the strip is continuously immersed in a molten zinc-aluminum alloy hot-dip coating ;
bath and withdrawn continuously from the bath. The improvement comprises forming a molten metal hot-dip coat ng bath of a zinc-aluminum eutectic alloy which contains 5.0 - 0.5 wt. % aluminum with the balance being essentially zinc, and continuously passing the ferrous metal strip through the bath while the surface of the strip is free of oxides and metallic and non-metallic surface films. The bath is characterized by being free of accumulations of -heavy ~ross at the bottom thereof after prolonged passage Of the strip through the bath. The bath may contain a - maximum of about 0.1 wt. % of at least one metal additive selected from the group consisting of lead, tin and antimony.
Other aspects of the present invention will be apparent from the detailed description and claims to follow.
An improved method has now been discovered for continuously providing a hot-dip zinc-aluminum alloy coated ferrous metal base, such as an endless strip of galvanizing steel, in an efficient and economical manner by using as the hot-dip coating bath the zinc-aluminum eutectic alloy wherein the aluminum content of the bath is substantially 5% by wt.
with the balance being substantially zinc which can contain .
. ' ' ' , ' ' - . .
-106S'~0'~
one or more conventional incidental metals, such as lead, normally found in commercial zinc spelter and preferably including lead in amounts not exceeding about 0.1% by wt.
The zinc-aluminum eutectic alloy when applied directly to the surface of a ferrous metal strip in accordance with the present invPntion provides an adherent, formable, weldable and paintable hot-dip coated strip which has minimum spangle formation and which is highly resistant to a wide range of corrosive conditions, including resistance to both marine and heavy industrial atmosphere corrosion.
When using the zinc-aluminum eutectic alloy as the hot-dip coating bath in a process for the continuous coating of an endless steel strip, however, unexpected difficulties and advantages are present due to the bath composition itself.
Thus, in the practice of the present invention an endless steel strip of the type which is hot-dip coated in a con-ventional continuous galvanizing hot-dip coating line is first cleaned to remove surface contamination and oxides, preferably by means of a Sendzimir-type oxidation-reduction process. While the oxide free strip is at a temperature of between about 700F and 1000F, but preferably at about the temperature of the hot-dip coating bath~ the strip is immersed in the zinc-aluminum eutectic alloy coating bath maintained at a temperature between about 775F to about 825F for a period ranging between about 5 seconds and 30 seconds. As the strip is withdrawn from the coating bath, the thickness of the hot-dip coating is controlled by gas impingement means.
And3 it has been found necessary to use a gas, such as steam, which is heated to at least about 900F, and preferably to about 950F, in order to produce an acceptable product when ; -7-~ ........ . ~ --- - . : - -, :
:
~ 106S2~)4 coating with the zinc-aluminum eutectic alloy. Gas blowing apparatus which can be used in accordance with the present invention is described in such prior art patents as U. SO
Patent Nos. 3,406,656; 3,459,587; 3~449,418 and 3,667,425.
The use of the zinc-aluminum eutectic alloy as the hot-dip coating bath makes it possible to achieve important improvements in a continuous zinc hot-dip coating process.
For example, be~ause the zinc-aluminum eutectic alloy has a melting point of 720F, as compared with a melting point of -790F for a conventional galvanizing bath containing about .2% by wt. aluminum, the hot-dip coating bath in the present process can be kept at a temperature between about 775F and about 825F and preferably at a temperature of about 800F~
This bath temperature is substantially lower than the 850F
bath temperature normally used in a conventional continuous hot-dip galvanizing process. The relatively low hot-dip coating bath temperature in the present process requires substantially less energy than in a conventional galvanizing process. And~
since the rate of reaction between zinc and iron doubles with about every 18F increase in the coating bath temperature, there is less iron contamination of the bath and less inter-metallic compound formed on the strip even when the strip is immersed in the eutectic alloy bath for longer periods of time. ThUS, with the present process greater latitude is possible in the hot-dip coating operating conditions, such as longer immersion times and higher strip temperatures, without adversely affecting the hot-dip coating. But, contrary to what might be expected, using a hot-dip coating bath temperature substantially below the temperatures normally used in con-ventional galvanizing does not have an adverse effect on the . - :. ~ . . -520~
adherence of the hot-dip eutectic alloy coating to the ferrous metal strip and the eutectic alloy coating is metallurgically bonded to the surface of the steel by a thin ductile iron-aluminum intermetallic layer with good strip formability at ambient or room temperature.
Another unexpected improvement achieved by the present zinc-aluminum eutectic alloy continuous hot-dip coating process is the complete elimination of the heavy iron-zinc dross which normally collects at the bottom of the hot-dip coating bath and which is difficult to remove. And, any light aluminum-zinc-iron dross which is formed on the surface of the eutectic coating bath can be readily removed by periodically skimming the bath.
When the high fluidity~ low viscosity fused zinc-aluminum eutectic alloy is used as the hot-dip coating bath in accordance with the present process, the fused hot-dip coating levels out quickly and is transformed from the liquidus phase to the solidus phase very soon after the coated strip is removed from the coating bath. ThUs, the period during which excess intermetallic compound can form and crystals or spangles can grow is very short, and the formation of a smoother hot-dip coating with minimum spangles is facilitated. How-ever, the rate of phase transformation from liquidus to solidus is so rapid when using the zinc-aluminum eutectic allOy that it has been found necessary to slow down the rate of transformation in order to avoid surface ripples and irregularities in the hot-dip coating. The most convenient means for controlling the rate of solidification is by using a heated fluid or gas in gas-wiping means for controlling the thickness of the hot-dip coating. Jets of steam having a ' :
_g_ . ~ .. .. .- -~065Z()4 temperature of at least about 900F have been found satis-factory for retarding the rate of solidification of the coating sufficiently to allow the coating to level out and avoid the formation of ripples or like irregularities in the surface of the coating which impairs the weldability and appearance When the steam temperature is allowed to fall appreciably below 900F solidification of the liquid coating is too rapid and the objectionable waves or ripples are formed in the hot-dip coating. Other heated fluids such as nitrogen, -carbon dioxide or air can be used in place of steam. The presence of even a small fraction of one percent by wt. of -magnesium in the hot-dip coating bath greatly increases the difficulty of forming a smooth hot-dip coating, and magnesium additions are therefore avoided.
It has also been found that the uniformity of the eutectic zinc-aluminum alloy coating on a steel strip can be improved by allowing the content of lead, a normal incidental impurity in zinc spelter of the eutectic alloy, to rise to a level of about 0.1% by wt. Although lead used in an amount of about 001% by wt. normally has the effect of increasing spangle size to the point where the appearance is undesirable and paintability is impaired, because of the more rapid transformation of the zinc-aluminum eutectic alloy from the liquidus phase to the solidus phase, larger amounts of lead can be tolerated in the zinc-aluminum eutectic alloy coating bath without causing objectionable spangle formation than is possible in conventional zinc coating baths It is, thus, possible with the process of the present invention to provide a smoother brighter appearing coating having only relatively small unobjectionable spangles, and temper rolling ~ . , , ~: ' .
. --\
~(~65Z04 of the hot-dip coated product of the present invention can be eliminated~
A series of hot-dip coated ferrous metal strips embodying the present invention were formed on an experimental coating line which closely simulates a production Sendzimir-type continuous hot-dip zinc coating line, wherein strips of 20 gauge full hard steel about 6 inches wide having a chemical composition of about 0.1% carbon, 0.29% to 0.35% manganese, 0.01% to 0.011% phosphorus, 0.019% to 0.020% sulfur and 0O04%
copper with the balance being essentially iron were hot-dip coated with the zinc-aluminum eutectic alloy containing a maximum of 0.1% by wt. lead. The hot-dip coating bath was prepared by maintaining a quantity of preformed zinc-aluminum eutectic alloy in an induction heated ceramic lined pot at a temperature of about 825F to form a coating bath which contained 5% ~ 0.5% by wt. aluminum with the balance comprising essentially zinc with a maximum lead content of about 0.1%
by wt. The steel strip to be coated was passed continuously through a controlled oxidizing atmosphere in which the surface contaminants were burned off and the surface of the strip reduced in a hydrogen atmosphere to remove surface oxides, generally in accordance with a conventional Sendzimir process.
In the alternative the strip can be chemically cleaned, as with - an alkaline cleaning bath~ where desired. The clean ferrous metal strip free of oxides and other metallic and non-metallic surface films or coatings at an inlet temperature of 850F
was then continuously passed through the zinc-aluminum eutectic alloy coating bath maintained at 825F at a rate of about 30 to 60 ft. per minute with a dwell time in the bath between about 4 seconds and about 8 seconds. Steam at a temperature ~ `:
of 950F was impinged upon the hot-dip coating as the strip was removed from the coating bath to control the thickness of the hot-dip coating to about 0.5 ounce per square foot for general coil coating. If culvert stock coil were desired, a - coating weight of about 1.0 ounce per square foot would be - requiredO The strip was air quenched and the hot-dip coating formed directly on the surface of the steel strip had a smooth ~-bright appearance with minimum spangles being evident.
During the continuous hot-dip coating process periodic -bath additions of zinc-aluminum eutectic alloy were made as required to maintain the bath at the proper level. Top skimming formation was not excessive and appaared to be about the same as encountered in normal continuous hot-dip galvanizing line operations. No bottom dross was formed~ The iron content of the coating bath did not increase significantly, and during a period extending over about five months, the iron analysis of the bath held constant at about 0.01% ironO
- Molten metal corrosion of the sinker roll arms extending into the hot-dip coating bath appeared negligible.
Additional hot-dip coating runs were made in the above described manner at a bath temperature of 800F with 22-gauge rimmed steel strips, some of which were differentially coated.
Accelerated corrosion tests on the above products have shown an approximately 2 to 1 advantage in corrosion resistance over regular hot-dip galvanized steel. The results of the 5% salt fog test (ASTM B117 Test) showed that with the present eutectic alloy coating one could expect to obtain twice the exposure time from equal coating thicknesses or equal exposure time with one-half the coating thickness when . ' , ! , .
- , . , : : : .
lO~SZ04 compared with conventional hot-dip galvanized coatings. The results of the S02 cabinet exposure test [Kesternick (German) DIN50018] showed that a zinc-aluminum eutectic alloy hot-dip coating having approximately 40% of the coating thickness of a conventional hot-dip galvanized coating exhibited an improve-ment in corrosion resistance of approximately 1.5 to 2.0 times.
Similar improvements were found in outdoor exposure tests (ASTM G-l Test) where the test panels were exposed to a marine type atmosphere and a heavy industrial atmosphereO The corrosiOn rate (mils/year) was significantly lower for the zinc-aluminum eutectic alloy hot-dip coated product and the rate decreased as the exposure time increased in contrast with ' the hi~her and normal linear rate of corrosion for the con-ventional 0.25% aluminum, lead-free hot-dip galvanized control panels.
Spot weld tests carried out on the zinc-aluminum eutectic alloy hot~dip coated steel of the present invention showed - satisfactory performance for both the 20 and 22 gauge steelstrips when coated on both surfaces with a 0O3 - 0.4 mils eutectic coating or when differentially coated with 0.5 mils on one side and 0.1 - 0.2 mils coating thicknesses on the ' opposite side and where the electrodes were in contact with the lighter side of the coating.
' ~ The formability of the zinc-aluminum eutectic alloy hot-dip coating which is one of the essential properties of a useful hot-dip coated strip was equal or superior to that of conventional hot-dip galvanized coa,tings and successfully passed The olsen suttOn Draw Test, the Reverse Impact Test, the 180Bend Test and The Lock Seam Test.
' -652~4 The zinc-aluminum eutectic alloy~hot-dip coated surface, after the application of a conventional zinc phosphate pretreatment, exhibited satisfactory paintability on application of conventional enamel coatings of the type used on automobile body panels. No flaking was observed on the cross-hatch adhesion test (Adaptation of ASTM D2197 Test).
Test results using a coil coating paint system show that the zinc-aluminum eutectic alloy hot-dip coating is at least equal to conventional hot-dip galvanized steel.
.
: .
aluminum in combination with from 1% to 5% by weight magnesiumO
In an article by Cameron and Ormay entitled "The Effects of Agitation, Cooling and Aluminum On The Alloying In Hot-Dipping in Zinc" published in "Edited Proceedings 6th International Conference on Hot-Dip Galvanizing", Interlaken, June 1971, pgs. 296-297, the authors discuss the metallurgical struc~ures and drosses formed when a ferrous metal panel is immersed for 24 hours in a wide range of zinc-aluminum alloy bath compositions, including a zinc-5% aluminum alloy coating bath, and describe the effects of agitation and rate of cooling. The authors do not disclose a continuous hot-dip coating process, nor do they evaluate the properties or use-fulness of any of the zinc-aluminum alloy coatings after immersing the panel for 24 hours in the coating bath. Also, Brauer and Peirce in a paper entitled "The Effect of Impurities On The Oxidation and Swelling of Zinc Aluminum Alloys" (TransO
Am. Inst. Met. Eng. (1923) Vol. 68,796) describe studies of zinc die casting alloys, including 5% aluminum-0 5% to 5%
copper-zinc die casting alloys, which contain impurities in the zinc~ such as lead, tin or cadmium.
Despite the considerable efforts which have been made to improve the hot-dip continuous coating of a ferrous metal strip, there remains considerable room for improvement in both the hot-dip zinc-base coatings and in the method by which hot-dip zinc-base coatings are continuously applied to a ferrous metal strip in order to obtain uniformly in a more economical manner a smooth, attractively appearing, formable zinc-aluminum alloy hot-dip coated strip. For example, in a ~0~;5Z~)~
conventional hot-dip galvanizing ba~h a substantial amount of a heavy, compact iron-zinc dross is formed in the coating bath which sinks to the bottom of the bath and requires periodic removal in order to allow continued use of the bath.
Careful control over the length of time the ferrous metal strip is allowed to re~ain immersed in the hot-dip coating bath is also required, particularly when the bath is maintained at the usual temperature of 850F or above. An objectionably thick intermetallic iron-zinc layer is formed which seriously interferes with the desired formability of the coated product, if the strip is allowed to remain for more than a few seconds in a conventional hot-dip coating bath. Also, careful control of the temperature of the strip entering the hot-dip coating bath is required so that the temperature of the strip is not so high as to form a thick intermetallic layer which cannot be deformed at room temperature without interfering with the corrosion resistance of the coating.
The paintability of a hot-dip zinc-base coating is impaired by the presence of large spangle on the surface of a zinc coated ferrous metal strip, and the weldability of a zinc-coated product is unsatisfactory when the zinc coating is not smooth. Any pronounced spangles in a hot-dip coating are clearly evident through the conventional paint or enamel coatings of the type which are normally applied to building ~ -panels or automobile body panels, and it is considered highly desirable ~o minimize spangle formation on hot-dip coatings to be painted or enameled. Temper rolling of the usual zinc-hot-dip coated products having pronounced spangles is required to smooth out and remove the objectionable surface pattern.
The size of the spangles which form in a conventional zinc-base , ' 1065Z~4 hot-dip coatings tends to be large because the hot-dip zinc coating bath temperature is relatively high and the period during which the hot-dip coating remains in the liquidus phase or in the transition phase between liquidus and solidus tends to be prolonged. And, the addition of certain metallic elements, such as lead, antimony, and tin which are used to reduce surface ripples and improve the smoothness of the hot-dip coatings, further increase the grain size or spangles.
Thus, in the prior art zinc-base hot-dip coating processes the use of such additives as lead to control coating smoothness is limited and cannot be used in an amount sufficient to insure a smooth zinc coating without causing the formation of objectionably large spangles in a conventional zinc-base hot-dip coated surface.
It is therefore an object of the present invention to provide an improved zinc-aluminum alloy hot-dip coated ferrous metal base in a more efficient and economical manner.
Another object of the present invention is to provide an improved process of applying a zinc-aluminum alloy hot-dip coating on a ferrous metal strip.
In one broad aspect the invention comprehends a corrosion resistant continuous hot-dip coated ferrous metal strip having applied directly on a surface thereof a hot-dip coating comprising a zinc-aluminum eutectic alloy which contains 5.0 - 0.5 wt. % aluminum with the balance being essentially zinc. The coating provides a readily paintable surface which is free of large spangles interferring with good paintability. The coating may contain at least one metal additive selected from the group consisting of lead, tin and antimony, preferably about 0.1 wt. %.
`,E~' ' 106SZ(~4 Another aspect of the invention pertains to a process for applying a zinc-aluminum alloy hot-dip coating to a ferrous metal strip in which the strip is continuously immersed in a molten zinc-aluminum alloy hot-dip coating ;
bath and withdrawn continuously from the bath. The improvement comprises forming a molten metal hot-dip coat ng bath of a zinc-aluminum eutectic alloy which contains 5.0 - 0.5 wt. % aluminum with the balance being essentially zinc, and continuously passing the ferrous metal strip through the bath while the surface of the strip is free of oxides and metallic and non-metallic surface films. The bath is characterized by being free of accumulations of -heavy ~ross at the bottom thereof after prolonged passage Of the strip through the bath. The bath may contain a - maximum of about 0.1 wt. % of at least one metal additive selected from the group consisting of lead, tin and antimony.
Other aspects of the present invention will be apparent from the detailed description and claims to follow.
An improved method has now been discovered for continuously providing a hot-dip zinc-aluminum alloy coated ferrous metal base, such as an endless strip of galvanizing steel, in an efficient and economical manner by using as the hot-dip coating bath the zinc-aluminum eutectic alloy wherein the aluminum content of the bath is substantially 5% by wt.
with the balance being substantially zinc which can contain .
. ' ' ' , ' ' - . .
-106S'~0'~
one or more conventional incidental metals, such as lead, normally found in commercial zinc spelter and preferably including lead in amounts not exceeding about 0.1% by wt.
The zinc-aluminum eutectic alloy when applied directly to the surface of a ferrous metal strip in accordance with the present invPntion provides an adherent, formable, weldable and paintable hot-dip coated strip which has minimum spangle formation and which is highly resistant to a wide range of corrosive conditions, including resistance to both marine and heavy industrial atmosphere corrosion.
When using the zinc-aluminum eutectic alloy as the hot-dip coating bath in a process for the continuous coating of an endless steel strip, however, unexpected difficulties and advantages are present due to the bath composition itself.
Thus, in the practice of the present invention an endless steel strip of the type which is hot-dip coated in a con-ventional continuous galvanizing hot-dip coating line is first cleaned to remove surface contamination and oxides, preferably by means of a Sendzimir-type oxidation-reduction process. While the oxide free strip is at a temperature of between about 700F and 1000F, but preferably at about the temperature of the hot-dip coating bath~ the strip is immersed in the zinc-aluminum eutectic alloy coating bath maintained at a temperature between about 775F to about 825F for a period ranging between about 5 seconds and 30 seconds. As the strip is withdrawn from the coating bath, the thickness of the hot-dip coating is controlled by gas impingement means.
And3 it has been found necessary to use a gas, such as steam, which is heated to at least about 900F, and preferably to about 950F, in order to produce an acceptable product when ; -7-~ ........ . ~ --- - . : - -, :
:
~ 106S2~)4 coating with the zinc-aluminum eutectic alloy. Gas blowing apparatus which can be used in accordance with the present invention is described in such prior art patents as U. SO
Patent Nos. 3,406,656; 3,459,587; 3~449,418 and 3,667,425.
The use of the zinc-aluminum eutectic alloy as the hot-dip coating bath makes it possible to achieve important improvements in a continuous zinc hot-dip coating process.
For example, be~ause the zinc-aluminum eutectic alloy has a melting point of 720F, as compared with a melting point of -790F for a conventional galvanizing bath containing about .2% by wt. aluminum, the hot-dip coating bath in the present process can be kept at a temperature between about 775F and about 825F and preferably at a temperature of about 800F~
This bath temperature is substantially lower than the 850F
bath temperature normally used in a conventional continuous hot-dip galvanizing process. The relatively low hot-dip coating bath temperature in the present process requires substantially less energy than in a conventional galvanizing process. And~
since the rate of reaction between zinc and iron doubles with about every 18F increase in the coating bath temperature, there is less iron contamination of the bath and less inter-metallic compound formed on the strip even when the strip is immersed in the eutectic alloy bath for longer periods of time. ThUS, with the present process greater latitude is possible in the hot-dip coating operating conditions, such as longer immersion times and higher strip temperatures, without adversely affecting the hot-dip coating. But, contrary to what might be expected, using a hot-dip coating bath temperature substantially below the temperatures normally used in con-ventional galvanizing does not have an adverse effect on the . - :. ~ . . -520~
adherence of the hot-dip eutectic alloy coating to the ferrous metal strip and the eutectic alloy coating is metallurgically bonded to the surface of the steel by a thin ductile iron-aluminum intermetallic layer with good strip formability at ambient or room temperature.
Another unexpected improvement achieved by the present zinc-aluminum eutectic alloy continuous hot-dip coating process is the complete elimination of the heavy iron-zinc dross which normally collects at the bottom of the hot-dip coating bath and which is difficult to remove. And, any light aluminum-zinc-iron dross which is formed on the surface of the eutectic coating bath can be readily removed by periodically skimming the bath.
When the high fluidity~ low viscosity fused zinc-aluminum eutectic alloy is used as the hot-dip coating bath in accordance with the present process, the fused hot-dip coating levels out quickly and is transformed from the liquidus phase to the solidus phase very soon after the coated strip is removed from the coating bath. ThUs, the period during which excess intermetallic compound can form and crystals or spangles can grow is very short, and the formation of a smoother hot-dip coating with minimum spangles is facilitated. How-ever, the rate of phase transformation from liquidus to solidus is so rapid when using the zinc-aluminum eutectic allOy that it has been found necessary to slow down the rate of transformation in order to avoid surface ripples and irregularities in the hot-dip coating. The most convenient means for controlling the rate of solidification is by using a heated fluid or gas in gas-wiping means for controlling the thickness of the hot-dip coating. Jets of steam having a ' :
_g_ . ~ .. .. .- -~065Z()4 temperature of at least about 900F have been found satis-factory for retarding the rate of solidification of the coating sufficiently to allow the coating to level out and avoid the formation of ripples or like irregularities in the surface of the coating which impairs the weldability and appearance When the steam temperature is allowed to fall appreciably below 900F solidification of the liquid coating is too rapid and the objectionable waves or ripples are formed in the hot-dip coating. Other heated fluids such as nitrogen, -carbon dioxide or air can be used in place of steam. The presence of even a small fraction of one percent by wt. of -magnesium in the hot-dip coating bath greatly increases the difficulty of forming a smooth hot-dip coating, and magnesium additions are therefore avoided.
It has also been found that the uniformity of the eutectic zinc-aluminum alloy coating on a steel strip can be improved by allowing the content of lead, a normal incidental impurity in zinc spelter of the eutectic alloy, to rise to a level of about 0.1% by wt. Although lead used in an amount of about 001% by wt. normally has the effect of increasing spangle size to the point where the appearance is undesirable and paintability is impaired, because of the more rapid transformation of the zinc-aluminum eutectic alloy from the liquidus phase to the solidus phase, larger amounts of lead can be tolerated in the zinc-aluminum eutectic alloy coating bath without causing objectionable spangle formation than is possible in conventional zinc coating baths It is, thus, possible with the process of the present invention to provide a smoother brighter appearing coating having only relatively small unobjectionable spangles, and temper rolling ~ . , , ~: ' .
. --\
~(~65Z04 of the hot-dip coated product of the present invention can be eliminated~
A series of hot-dip coated ferrous metal strips embodying the present invention were formed on an experimental coating line which closely simulates a production Sendzimir-type continuous hot-dip zinc coating line, wherein strips of 20 gauge full hard steel about 6 inches wide having a chemical composition of about 0.1% carbon, 0.29% to 0.35% manganese, 0.01% to 0.011% phosphorus, 0.019% to 0.020% sulfur and 0O04%
copper with the balance being essentially iron were hot-dip coated with the zinc-aluminum eutectic alloy containing a maximum of 0.1% by wt. lead. The hot-dip coating bath was prepared by maintaining a quantity of preformed zinc-aluminum eutectic alloy in an induction heated ceramic lined pot at a temperature of about 825F to form a coating bath which contained 5% ~ 0.5% by wt. aluminum with the balance comprising essentially zinc with a maximum lead content of about 0.1%
by wt. The steel strip to be coated was passed continuously through a controlled oxidizing atmosphere in which the surface contaminants were burned off and the surface of the strip reduced in a hydrogen atmosphere to remove surface oxides, generally in accordance with a conventional Sendzimir process.
In the alternative the strip can be chemically cleaned, as with - an alkaline cleaning bath~ where desired. The clean ferrous metal strip free of oxides and other metallic and non-metallic surface films or coatings at an inlet temperature of 850F
was then continuously passed through the zinc-aluminum eutectic alloy coating bath maintained at 825F at a rate of about 30 to 60 ft. per minute with a dwell time in the bath between about 4 seconds and about 8 seconds. Steam at a temperature ~ `:
of 950F was impinged upon the hot-dip coating as the strip was removed from the coating bath to control the thickness of the hot-dip coating to about 0.5 ounce per square foot for general coil coating. If culvert stock coil were desired, a - coating weight of about 1.0 ounce per square foot would be - requiredO The strip was air quenched and the hot-dip coating formed directly on the surface of the steel strip had a smooth ~-bright appearance with minimum spangles being evident.
During the continuous hot-dip coating process periodic -bath additions of zinc-aluminum eutectic alloy were made as required to maintain the bath at the proper level. Top skimming formation was not excessive and appaared to be about the same as encountered in normal continuous hot-dip galvanizing line operations. No bottom dross was formed~ The iron content of the coating bath did not increase significantly, and during a period extending over about five months, the iron analysis of the bath held constant at about 0.01% ironO
- Molten metal corrosion of the sinker roll arms extending into the hot-dip coating bath appeared negligible.
Additional hot-dip coating runs were made in the above described manner at a bath temperature of 800F with 22-gauge rimmed steel strips, some of which were differentially coated.
Accelerated corrosion tests on the above products have shown an approximately 2 to 1 advantage in corrosion resistance over regular hot-dip galvanized steel. The results of the 5% salt fog test (ASTM B117 Test) showed that with the present eutectic alloy coating one could expect to obtain twice the exposure time from equal coating thicknesses or equal exposure time with one-half the coating thickness when . ' , ! , .
- , . , : : : .
lO~SZ04 compared with conventional hot-dip galvanized coatings. The results of the S02 cabinet exposure test [Kesternick (German) DIN50018] showed that a zinc-aluminum eutectic alloy hot-dip coating having approximately 40% of the coating thickness of a conventional hot-dip galvanized coating exhibited an improve-ment in corrosion resistance of approximately 1.5 to 2.0 times.
Similar improvements were found in outdoor exposure tests (ASTM G-l Test) where the test panels were exposed to a marine type atmosphere and a heavy industrial atmosphereO The corrosiOn rate (mils/year) was significantly lower for the zinc-aluminum eutectic alloy hot-dip coated product and the rate decreased as the exposure time increased in contrast with ' the hi~her and normal linear rate of corrosion for the con-ventional 0.25% aluminum, lead-free hot-dip galvanized control panels.
Spot weld tests carried out on the zinc-aluminum eutectic alloy hot~dip coated steel of the present invention showed - satisfactory performance for both the 20 and 22 gauge steelstrips when coated on both surfaces with a 0O3 - 0.4 mils eutectic coating or when differentially coated with 0.5 mils on one side and 0.1 - 0.2 mils coating thicknesses on the ' opposite side and where the electrodes were in contact with the lighter side of the coating.
' ~ The formability of the zinc-aluminum eutectic alloy hot-dip coating which is one of the essential properties of a useful hot-dip coated strip was equal or superior to that of conventional hot-dip galvanized coa,tings and successfully passed The olsen suttOn Draw Test, the Reverse Impact Test, the 180Bend Test and The Lock Seam Test.
' -652~4 The zinc-aluminum eutectic alloy~hot-dip coated surface, after the application of a conventional zinc phosphate pretreatment, exhibited satisfactory paintability on application of conventional enamel coatings of the type used on automobile body panels. No flaking was observed on the cross-hatch adhesion test (Adaptation of ASTM D2197 Test).
Test results using a coil coating paint system show that the zinc-aluminum eutectic alloy hot-dip coating is at least equal to conventional hot-dip galvanized steel.
.
: .
Claims (12)
1. A corrosion resistant hot-dip coated ferrous metal strip having applied directly on a surface thereof a hot-dip coating comprising a zinc-aluminum eutectic alloy which contains 5.0 ? 0.5 wt. % aluminum with the balance being essentially zinc, and said coating providing a readily paint-able surface which is free of large spangles interferring with good paintability.
2. A corrosion resistant hot-dip coated ferrous metal strip as in Claim 1, wherein said coating contains at least one metal additive selected from the group consisting of lead, tin and antimony.
3. A corrosion resistant continuous hot-dip coated ferrous metal strip as in Claim 2, wherein said coating contains a total of about 0.1 wt. % of the said metal additive.
4. A corrosion resistant hot-dip coating ferrous metal strip as in Claim 2 or Claim 3, wherein said metal additive is lead.
5. A corrosion resistant hot-dip coating ferrous metal strip as in Claim 2 or Claim 3, wherein said metal additive is antimony.
6. In a process for applying a zinc-aluminum alloy hot-dip coating to a ferrous metal strip in which the said strip is continuously immersed in a molten zinc-aluminum alloy hot-dip coating bath and withdrawn continuously from said bath, the improvement comprising; forming a molten metal hot-dip coating bath of a zinc-aluminum eutectic alloy which contains 5.0 ? 0-5 wt. % aluminum with the balance being essentially zinc, and continuously passing said ferrous metal strip through said bath while the surface of said strip is free of oxides and metallic and non-metallic surface films, and said bath is characterized by being free of accumulations of heavy dross at the bottom thereof after prolonged passage of said strip through said bath.
7. A process as in Claim 6, wherein said bath contains a maximum of about 0.1 wt. % of at least one metal additive selected from the group consisting of lead, tin and antimony.
8. A process as in Claim 6 or Claim 7, wherein said metal additive is lead.
9. A process as in Claim 6 or Claim 7, wherein said metal additive is antimony.
10. A process as in Claim 6 or Claim 7, wherein said coating bath is maintained at a temperature between about 775°F and 825°F.
11. A process as in Claim 6 or Claim 7, wherein said coating bath is maintained at a temperature of about 800°F.
12. A process as in Claim 6 or Claim 7, wherein said strip has a temperature of about 850°F when immersed in the said bath while said bath is maintained at a temperature of about 825°F.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US52475374A | 1974-11-18 | 1974-11-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1065204A true CA1065204A (en) | 1979-10-30 |
Family
ID=24090542
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA233,849A Expired CA1065204A (en) | 1974-11-18 | 1975-08-21 | Zinc-aluminum eutectic alloy coating process and article |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1065204A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0111039A1 (en) * | 1982-12-07 | 1984-06-20 | James W. Hogg | Process for the high speed continuous galvanizing and annealing of a metallic wire |
| GB2226332A (en) * | 1988-11-08 | 1990-06-27 | Lysaght John | Galvanizing with compositions including antimony |
-
1975
- 1975-08-21 CA CA233,849A patent/CA1065204A/en not_active Expired
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0111039A1 (en) * | 1982-12-07 | 1984-06-20 | James W. Hogg | Process for the high speed continuous galvanizing and annealing of a metallic wire |
| GB2226332A (en) * | 1988-11-08 | 1990-06-27 | Lysaght John | Galvanizing with compositions including antimony |
| GB2227255A (en) * | 1988-11-08 | 1990-07-25 | Lysaght John | Galvanizing with compositions including tin |
| GB2226332B (en) * | 1988-11-08 | 1992-11-04 | Lysaght John | Galvanizing with compositions including antimony |
| GB2227255B (en) * | 1988-11-08 | 1993-04-07 | Lysaght John | Galvanizing with compositions including tin |
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