CA1051759A - Low fuming prefluxes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Low fuming prefluxes are provided which can be used in operations in which a preheating step is involved which contain from 8 to 30% by weight of ammonium chloride and zinc chloride at a weight ratio of 0.1 to 0.4 in water.

Description

75g BACKGR UND OF THE INV~NTION
Flux compositions, particularly preflux composi-tions, are used in galvanizing as well as other indus-tries to aid in the effective cleaning and wetting of the surfaces to be treated. Particularly in continuous strip ~alvanizing, the preflux must have a high degree of chemical activity which is provided by ammonium chloride which is generally present in commercial fluxes at concentrations of 40% by weight or moreO In the continuous process the rapid evaporation of the ammonium chloride from the strip is required to release flux from the strip as it enters the zinc pot and to provide turbulence in the vortex to keep the sheet from picking up skimmings from the pot.
Unfortunately, when such quantities of ammonium chloride are used, a high degree of fuming occurs from the flux composition at the preheat stage, at the galvanizing kettle, and at the stack. Generally, the quantity of fuming which occurs is sufficient to violate antipollution standards in many areas.
~O Compromise formulations for ~luxing compositions have~been proposed in which the ammonium chloride level is reduced. Howevert particularly in a continuous strip - galvanizing operation, the reduction o the ammonium chloride has in the past resulted in a reduction in the chemical activity of the preflux so that the oxides on the surface of the strip to be galvanized are not adequately removed or the surEace is not adequately wetted or flux consumption is increased as are the skimmings on the urface o the xinc pot and adequate turbulence ls not maintain~d in the vortex around the strip where it enters the molten zinc.

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SUMMARY OF THE INVENTION
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It has now been ~ound that a low fuming preflux composition substantially devoid of prior art disadvantages even at lowex concentrations of ammorlium chloride can be prepared when the preflux solution contains from 8 to 30%
by weight, preferably 10 to 25% by weight of ammonium chloride and zinc chloride at a weig~lt ratio of Oo l to 0.4, preferably 0.2 to 0.4, in water. ~n the practice of this invention, the chemical activity of the preflux solution is maintained at a su*ficiently high level to allow it to remove the last traces of rust from the strip in a prefluxing and prehea~ng operation and to maintain an adequate degree of turbulence in the vortex around the strip where it enters the molten metal ollowing pre-fluxing and preheating. The preflux solution also shows significantly better thermal stability in the preheating step. me flux consumption and the degree o~ skimmings which result when the preflux solution of this invention is used are reduced by as much as 30 to 35%. In addition, it is possible to achieve a signiicant r~duction (up to 5~/0) i~ the cons~mption o~ the aluminum which is present in - the zinc bath to keep a brittle alloy of iron and zinc from forming. mese advantages are obtained by the practice of this invention in addition to the fact that the rate o~
fuming at the kettle and duriny the preheating step is sharply xeduced when the w~ig~t ratio of ammonium chloride to zinc chloride in the preflux is in the range o~ O.1 to 0.4, preferably 0.2 to 0.4~
DETAILED DESCRIPTION OF THE INVENI'ION
: In galvanizing operations, particularly continuous :~ ' ' ' ` ' . ' ' .
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strip galvanizing, prefluxes are often employed. In 'the case of continuous strip galvanizing, the strip to be galvaniæed is first run through an aqueous solution of preflux and then subjected to a prehLeating treatment as an auxiliary means of supplying heat to help maintain the galvanizing temperature at 454C.
The concentration of the ammonium chloride and zinc chloride in aqueous solution i9 from ~/0 to 30% by weight and preferably 10% to 25% by weight. The higher weight ratios of ammonium chloride to zinc chloride should be used at the lower concentrations for best results.
The preflux solution is generally applied so that about 2.5 to 4 grams of solids is available per kilogram of steel galvanized, For a typical product, this amounts to about 0.001 g./cm. o~ steel surface. The `w~t strip which is preheated in a vertical heater on the upleg after the strip leaves the prefluxing pot is heated to a temperature of about 121C. me flux must be dry before it reaches the top o~ the upleg and begins the downward leg to the galvanizing pot, In the downleg the temperature reaches 204 to 232C. The hot fluxed strip enters the molten æinc at speeds of up to 137~2 meters per minute~
In a continuous operation, no top flux is employed since the "boiling action" of the vaporizing preflux as the strip enters the zinc pot ~fectively blows away all oxides on the molten zinc surface. -The accumulated oxides are periodically skimmed ~rom the pot by hand. A l'sinker"
roll which changes the direction of the strip within 30 th~ molten metal and also keeps the strip submerged in ~ 4 ~

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75~t the kettle is located about 1.38 meters under the surface of the molten zinc so that the flux must come off of the preflux sheet in less than 0~6 second or else the flux will be picked up by the roll and cause imperfections in the coating. The total residence time of the steel in the molten 7.inc depends on the speed at which the strip moves and ranges from about 1-1/2 to 2-1/2 seconds.
The evolution of fumes has presented a serious problem both at the prehat stage and at the galvanizing kettle, the fumes ~rom which are generally vented through a stack. A scale of 1-10 is used to e~aluate the amount of smoke evolved in any specific galvanizing operation. For ~x~mple, in one evaluation the smoke level reached using an aqueous preflux solution containing 24% by weight o~ an ammonium chloride/zinc chloride composition having a weight ratio of 0.79, under normal gaivanizing conditions is rat d at 10. The amount o smoke liberated during the preheat stage does not decrease makerially when an aqueous solution containing 17% by weight of a composition of ammonium chloride and zinc chloride at a weight ratio of 0.5 is used. Howeverf at a concentration of 20% by weight of an ammonium chLoride/zinc chloride composltion having a weight ratio of 0O3~ the smoke level decreases dramatically to about 6. The smoke level decreases even further ~o about 4 when the concentration of the same weight ratio of ammonium chloride and zinc chloride in the preflux solution is decreased to between 8 and 11%~ Very li~tle smoke is observed when a preflux solution containing a 10% by weight of ammonium chloride and zinc chloride at a weight ratio of about 0.2 is used; the rating on a scale , ,' ' ~:.

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~IS~7~9 of l-l0 is about 30 me amount o smoke liberat~d at the kettle can also ~e evaluated using 1:he same or other standard.
One of the primary functions of the preflux solution in a continuous strip galvanizing operation is to provide a continuous violent boiling action around the metal strip as it enters the molten zinc. The boiling action keeps the flux skimmings blown away from the vortex in the molten zinc created by the rapid movement of the strip. Without this boiling action, the skimmings would stick to the strip and cause defects in the zinc coating.
m e boiling action h~retofore achieved by the decomposition of ammonium chloride in the flux is maintained at a high degree in the practice of this invention even at low ammonium chloride to æinc chloride ratios. It is believed ,~
- that this results because the flux film obtained at the lower ratio of ammonium chloride to zinc chloride (that is, at higher zinc chloride contents) are more di~ficult to ; dry than the higher ratio flux films because zinc chloride ZO has a greater tendency to retain water than does ammanium chloride. As a conseque~ce, the flux film on the strip at the ratios of this invention is somewhat l'wet" at the time that strip is dipped into the molten zinc. It is believed that the steam Erom the water in the flux provides a ~iolent boiling action which was hexetofore provided by the evolution o:E ammonia and hydrogen chloride from the decomposition of a~monium chloride in the ~lux. This is a unique applicatlon of the operation of steam as a preflux~
The aluminum content o~ the molten zinc in the ~ . :::

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kettle is normally kept at a concentration o~ about 0.1 to 0.2%, preferably 0.1~. The aluminum ls present in the molten æinc in order to control the formation of a zinc-- iron alloy which, because of its brittle natureO is deleterious to the galvanized coating. It has been found ¦
that the consumption of aluminum in the molten zinc is markedly reduced in the practice of this invention. It is ~
believed that the reason for the reduction in the con- -; ~ sumption of aluminum is due to the fact that less aluminum is lost as aluminum chloride vapors because of the smaller amount o~ ammonium chloride present in the preflux which in turn provides less hydrogen chloride to react with the aluminum to form aluminum chloride.
It has also been noted that the practice of this invention results in a reduced consumption of preflux solution per ton o material to be galvanized, Generally, only about 2/3 of the ~lux solu~ion ordinarily consumed in a prefluxing operation is consumed in the practice of this invention.
m e prefluxes of this invention are advantageously employed whether the bath metal is zinc or an alloy and can be used in any metal coating operations that require the ware to be preheated prior to the final treating step. In addition, the prefluxes may contain any desired stabilizing material, wetting a~ent or other additlve generally employed in fluxing and prefluxing compositions.
Further, while the invention has been described with particular reference to continuous strip galvanizing, it is to be understood that the prefluxes of this invention may be used in any type of galvanizing operation and that - 7 ~
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~;05~759 articles of any desired shape can be coated with the bath metal as discussed, for example, in ot Dip Galvaniz nq Practice, William H, Bowers, Jr., The Penton Publishing Company, Cleveland, Ohio, 1938, pages 32-70. me prefluxes of this invention are most advantageous for the coating ~~!
of ferrous and tin articles including cold rolled and "~ull hard" steel.
The invention is further illustrated but is not intended to ke limited by the following examples in which .
all parts and percentages are by weight unless otherwise specified.
EV~LUATION .
The tests used to evaluate. the 1uxes of the examples were run on the Wheeling-type continuous strip galvanizing lines with two variations, ~ and B, of surfac :
preparations of the steel before the steel was galvanized. ~ .-A. As a sheet of mild steel (0.25% carbon or less) is ~
unwound continuously from a roll (30~5 - 137.~ m,/min.~, .
it is degrPased with caustic and pickled in a sul- .
20 ~ furic acid solution to remove rust, with water rinsing ~a~ter each treatment. The strip then goes through an aqueous solu~ion of preflux solution and is :~
,, :. preheated~at 204 - 232C. before being submerged ;.
~or a short time (2-4 seconds) in a kettle of molten :
zinc at 454 - 460C., and cooled~ :~
- B. me steel strip is pickled as in "A" and then pre-plated electrol.ytically with an éxtremel~ thin layer :~
of zinc before being prefluxed:and galvanized.
: The following properties were e~aluated: .
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Fuming from the galvanizing ~luxes can be :, :

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1~5~59 observed at three locations: the prehating oven~ the kettle and the stack. Two methods were used to measure the level of fuming:
- Visual - Fume levels were measured by visual means both at the preheating oven and at the Xettle using a scale of 1-10, the higher the rating the greater the fume level. The fume level reached using a preflux solution containing 15% by weight of the commercial flux described below was assigned a rating of 10~
Instrument - EPA method ~o. 5, a procedure for sampling particulate emission from a stationary source, was - used to determine the fume level at the stack.
me amount-of particulate emission per unit time was then converted into a rating scale of 1-10, the higher the number the hi~her the level o~
~ume emission. me same standard used in the visual test was applied.
'~u~ t~c~ -~ While the steel strip is being galvanized continuouslyt its line speed fluctuates and as a result, .
the temperature to which the strip is prehated varies somew~at. Generally, the slower the speed the higher the preheat temp~rature. Too high a preheat temperature ,-causes the flux to "burn-out" or decompose ~eading to defects in the zinc coating~ j The "burn-out" resistance of the fluxes of the invention was measured by rating it on a scale of 1~5, the higher the nw~ber the lower the resistanceO Ihe prehsat burn-out resistance for the commercial preflux ; : . ' I' _ g _ '' ' . .

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described below was assigned the rating of 5 in this evaluation.
Aluminum Content __ , The aluminum content of the molten zinc was determined by dissolving a sample o~ molten zinc in hydrochlori~ acid and analyzinq it by the atomic absorption spectroscopic method.

The skimmings o~ flux produced during the con-tinuous strip galvanizing operation are removed periodicallyfrom the molten zinc surface with a skimmer and weighed~
The greater the amount of skimmings the higher the costs in }abor and zinc loss.
C MMERCI~L PREFLUX
A commercial flux presently used in the Wheeling-type continuous strip galvanizing process contains an ammonium chloride to zinc chloride ratio of 1:1.27~ The cvncentration of the ammonium chloride/zinc chloride composition in the preflux and the characteristics o such fluxes are given in Table 1 to serve as a basis of compari son with the fluxes of this invention.

The fluxes of the composltions given in Table 2 were prepared and used under various conditions as prefluxes in galvanizing steel sheets of various widths and thick-nesses by the Wheeling-type continuous strip galvani~ing process. me results are given in Table 3.
In comparing Table 3 with Table 1, it ls evident that the fluxes of this invention havs superior properties in that they have lower levels of fuming, higher "burn-out' : , .

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resistance in the preheat oven, and lower amounts o~ Al consumed and skimmings generated.

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While the invention has been described in con-siderable detail in the foregoing, it is to be understood that such detail is solely for the purpose of illustration and that variations can be made by those skilled in the art without departing from the spirit and scope of the invention except as set forth in the claims.

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Claims (7)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. A low fuming preflux composition comprising from 8 to 30% by weight of a weight ratio of ammonium chloride to zinc chloride of from 0.1:1 to 0.4:1 in water.

2. The low fuming preflux of Claim 1 wherein the ratio of ammonium chloride to zinc chloride is 0.2:1 to 0.4-1.

3. The low fuming preflux of Claim 1 wherein the composition is from 10% to 25% by weight in water.

4. The low fuming preflux of Claim 2 wherein the composition is from 10% to 25% by weight in water.

5. In a process for galvanizing an article by passing it through a preflux solution, heating the article containing a film of preflux and then passing the article through a bath of molten zinc, the improvement which com-prises passing the article through the preflux solution of Claim 1.

6. In a process for galvanizing an article by passing it through a preflux solution, heating the article containing a film of preflux and then passing the article through a bath of molten zinc, the improvement which com-prises passing the article through the preflux solution of Claim 4.

7. The process of Claim 5 in which a con-tinuous strip of metal is continuously prefluxed, heated and passed through the molten zinc bath.