WO2008100648A1

WO2008100648A1 - High speed tin plating process

Info

Publication number: WO2008100648A1
Application number: PCT/US2008/050168
Authority: WO
Inventors: Nicholas M. Martyak; Gary E. Stringer; Gary S. Smith
Original assignee: Arkema Inc.
Priority date: 2007-02-13
Filing date: 2008-01-04
Publication date: 2008-08-21

Abstract

Methods for the electrolytic preparation of tin coated metals are disclosed. The methods comprise the use of an alkanol sulfonic acid electrolyte or a salt thereof, preferably isethionic acid (ISA) as the electrolyte in electroplating operations. An aqueous solution of an alkanol sulfonic acid and/or an alkanol sulfonic acid salt can be used as rinse or flux immediately preceding reflow. The methods of the invention produce plated material that is free of blue haze.

Description

HIGH SPEED TIN PLATING PROCESS

Field of the Invention

This invention relates to the preparation of tin coated metals. In particular, this invention relates to a method for the electrolytic preparation of tin coated metals.

Background of the Invention

Tin is resistant to corrosion and is used as a protective coating on less resistant metals, such as steel. One method of applying a tin coating is to dip a steel plate into molten tin. However, this method is wasteful because it typically produces a thicker layer of tin than is necessary. Consequently, electrolytic methods, which produce a thinner and more uniform layer of tin, have been developed. Electroplating of tin onto steel strip is disclosed, for example, in Kitayama, U.S. Pat. No. 4,181,580, the disclosure of which is incorporated herein by reference.

In the high speed tinning of strips of steel, the strips of steel are first cleaned in a series of alkaline cleaners to remove oils and greases. Then the steel passes through several water rinses and then into a dilute acid ("pickling") solution before passing into the electrolyte plating bath, which produces a layer of tin on the steel surface. The layer of tin, as deposited, typically has a smooth matte surface.

Two tin plating solutions are commonly used in strip steel tin plating baths. The FERROSTAN® system contains phenolsulfonic acid (HOC₆H₄SO₃H, PSA) and stannous sulfate, while the RONASTAN® system contains methanesulfonic acid (CH₃ SO₃H, MSA) and stannous methanesulfonate. The use of MSA in electrolyte baths is disclosed, for example, in Thompson, U.S. Pat. No. 5,312,539, and in Copping, U.S. Pat. No. 6,251,255, the disclosures of which are incorporated herein by reference. The use of PSA acid electrolyte baths is disclosed, for example, in Ooniwa, U.S. Pat. No. 4,936,965, and in Dulcetti, U.S. Pat. No. 6,921,472, the disclosures of which are incorporated herein by reference.

After plating, the plated strip is typically rinsed twice with water or diluted portions of the plating electrolyte. After rinsing, the plated strip then enters a fluxing solution (e.g., an "acid flux" solution), followed by air drying. The term "flux" refers to a substance that aids the reflow operation. The plated strip is then heated in a reflow oven to slightly above the melting point of tin (about 232°C), typically in a reflow oven heated to about 240⁰C. The tin layer is melted, forming a surface layer of tin and a subsurface diffusion layer containing tin and tin-iron alloy on the steel substrate. After heating ("reflow"), the plated strip is rapidly cooled or quenched by immersion in water, producing a tin surface layer that has a bright finish.

The purpose of the rinse steps that follow plating is to remove as much of the components of the plating electrolyte solution from the tin surface as possible. Some of the plating electrolyte will be retained on the tin surface as "dragout" as it is removed from the plating bath. The dragout composition can include water, the plating acid (i.e., PSA or MSA), stannous salts, and dissolved electroplating additives. Because dragout of the components of the plating bath represents an economic loss, and because some water is lost from the plating bath due to evaporation or entrainment with gases evolved during the electroplating operation, the rinse solutions typically have a counter-current flow so that the rinse water and the plating bath components dragged into the rinse solutions with the plated strip are returned to the plating solution.

As discussed in O'Driscoll, U.S. Pat. No. 6,409,850, and in Allen, U.S. Pat. No. 2,719,820, the disclosures of which are incorporated herein by reference, the purpose of the fluxing agent is to remove oxide from the tin surface and to reduce the surface tension of the melting tin during reflow, thus preventing uneven flow of the tin during reflow. Such uneven flow can result in a non-uniform surface (e.g., "woodgrain") after quenching. Examples of fluxing agents include hydrogen chloride, stannous chloride, zinc chloride, ammonium chloride, palm oil, gluconic acid, glutamic acid, citric acid, tartaric acid, citrazinic acid, chelidamic acid, chelidonic acid, cyclohexene-l,2-dicarboximide, various naptholdisulfonic acids, and various hydroxybenzenesulfonic acids, including PSA. Although PSA serves as a good fluxing agent, MSA is not suitable as a fluxing agent due to formation of blue stains, as discussed below.

When a FERROSTAN® plating solution, which contains PSA and an organic grain-refining additive (ethoxylated naphthalenesulfonic acid), is used, the concentration of PSA in the acid flux solution, due to dragin from the plating bath and the prior rinse, typically is about 0.1-1.0% of PSA. An acid flux solution that contains 0.1 to 1.0% of PSA produces a bright, adherent surface layer after reflow. However, because of the presence of free phenol in a plating solution that contains PSA and because PSA has a low inherent electrical conductivity, electrolytes other than PSA have been sought.

A plating solution that contains MSA is more worker friendly because it does not contain phenol and also more conductive than a plating solution that contains PSA. In addition, MSA is a non-oxidizing acid and minimizes the oxidation of stannous ion (Sn⁺²) to stannic ion (Sn⁺⁴). Stannic ion forms stannic sludge, an insoluble oxide sludge which precipitates from solution, resulting in a loss of tin from the electroplating system. When MSA is used in the plating solution, the acid flux solution contains MSA and organic grain refining additives (polyoxy ether compounds) due to dragin from the plating bath. When MSA is present in the acid flux solution, after reflow the surface layer sometimes has an undesirable blue haze, which may be deleterious to the appearance of the tin surface and may also affect the corrosion resistance of the surface layer.

Thus, a need exists for tin plating processes that do not have the disadvantages of the process that uses PSA or MSA and does not lead to the formation of an undesirable blue haze after reflow.

Summary of the Invention

In one aspect, the invention is a method for electroplating, the method comprising the steps of:

a) electroplating tin onto a steel strip in an acidic electroplating bath comprising an alkanol sulfonic acid electrolyte, stannous ion and an anion, optionally grain refining additives, optionally one or more antioxidants, optionally surfactants, and forming a plated strip comprising a plated tin surface comprising a surface layer of tin;

b) performing one or more rinses; c) heating the plated strip to at least the melting point of tin but to less than the melting point of the steel strip; and d) either (i) quenching the plated strip in water or (ii) quenching the plated steel strip in a solution of about 0.01 vol% to 10 vol% of an organic compound in water.

In another aspect, the invention relates to the components of the plating baths, rinses and/or solution employed in the tin electroplating operations. The components of the aqueous plating baths, rinses and/or solutions of the invention comprise an alkanol sulfonic acid electrolyte and optionally one or more weaker acid functionalities, salts or anhydrides thereof, and mixtures thereof, optionally grain refining agents, optionally one or more antioxidants and optionally surfactants including anionic, non-ionic or cationic and mixtures thereof. The plating baths of the present invention are either a non-oxidzing media or include oxidation stabilizers to minimize the oxidation of the stannous ion. For example, the invention relates to aqueous plating solutions that comprise an alkanol sulfonic acid electrolyte, for example, to aqueous plating solutions that comprise stannous ion, and about 0.01 wt% to 10 wt% of (1) an alkanol sulfonic acid electrolyte, such as isethionic acid (2- hydroxyethane sulfonic acid), (2) an anhydride thereof, or (3) a salt thereof, grain refining agents, antioxidants and surfactants.

In another aspect, the invention relates to the tin-plated steel thus produced by the uses of the methods described above. Detailed Description of the Invention

Unless the context indicates otherwise, in the specification and claims the terms an alkanol sulfonic acid, anhydride, salt, organic compounds, antioxidants, surfactants and similar terms also include mixtures of such materials. Unless otherwise specified, all percentages are percentages by weight and all temperatures are in degrees Centigrade (degrees Celsius).

A conventional tin plating facility uses the following steps in the following order:

plating -> first water rinse -> second water rinse -> acid flux (with same acid used in plating or an added fluxing agent) -> air dry -> reflow -> quench in water -> dry The terms "flux" and "fluxing agent" generally refer to materials that aid in the fusing and/or flowing of the tin layer during annealing. Tin plating processes in which MSA is present in the acid flux can, after reflow, produce a surface layer that has a blue haze. The presence of this blue haze may affect the corrosion resistance of the surface layer. We have found that blue haze on the surface layer after reflow can be eliminated by the methods described below.

Use of an alkanol sulfonic acid electrolyte

Blue haze after reflow can be eliminated by the use of an alkanol sulfonic acid electrolyte or a salt thereof, preferably isethionic acid (ISA). The alkanol sulfonic acid and/or an alkanol sulfonic acid salt electrolyte bath typically comprises about 0.01 wt% to about 30 wt% of acid and/or acid salt. Preferably, at least enough of the acid is present so that the electrolyte bath is acidic having a pH < 3.

The alkanol sulfonic acid may be mixed with other sulfonic acids, for example, methane sulfonic acid, methanedisulfonic acid, para toluenesulfonic acid and phenol sulfonic acid, and/or inorganic acids, such as sulfuric acid. Any of these mixtures of alkanol sulfonic acid, with or without added acid, may also be used as acid/current carrier in the tin plating solution. If a mixture of acids is used, at least half of the acid should be an alkanol sulfonic acid.

The measured conductivity of solution of sulfuric acid is less than a solution of an alkanol sulfonic acid, such as ISA, at the same normality and temperature. For example, the conductivity of a 0.4 N sulfuric acid solution at 4O⁰C is 107.3 mS/cm while the conductivity of a 0.4 N ISA solution at the same normality and temperature is 160 mS/cm.

Mixtures of ISA and alkyl sulfonic acids may also be used, provided the normality of the alkyl polysulfonic acid is at least about equal to that of the ISA. For example, when a 0.4 N acid that 3/1 MSA:ISA was used in the plating bath, a visible blue stain was observed. However, when a 0.4 N acid that 1/1 or 1/3 MSA:ISA was used in the plating bath, no visible blue stain was observed.

Mixtures of ISA and alkyl polysulfonic acids may also be used, provided the normality of the alkyl polysulfonic acid is at least about equal to that of the ISA. For example, when a 0.4 N acid that 3/1 methanedisulfonic acid (MDSA):ISA was used in the plating bath, no blue stain was observed. Similarly, when a 0.4 N acid that 1/1 or 1/3 MDSA:ISA was used in the plating bath, no blue stain was observed.

Further, mixtures of alkanol sulfonic acids and sulfuric acid may be used, provided the ratio of the normality of the alkanol sulfonic acid is at least about twice that of the sulfuric acid. For example, when a 0.4 N total acid solution in which the ratio of sulfuric acid to ISA was 3/1 used in the plating bath, no visible blue stain was observed and the tin deposit was not difficult to reflow.

Use of a Water/Organic Compound Mixture

Though not being bound by any theory of explanation, it is believed that the blue haze that forms when MSA is used as the electrolyte, may be, at least in part, organic in nature. When the TP-SR Additive, the additive used with MSA electrolyte in the RONASTAN® system, was omitted from the plating bath, no blue haze was formed on conventional washing and reflow. However, the tin deposit was coarse- grained and difficult to reflow. When the TP-SR Additive was replaced with ENSA additive (ethoxylate of α-naphthol sulfonic acid), the additive used in the FERROSTAN® process, during plating using MSA electrolyte, no blue haze was formed, but following reflow the plated tin surface was not as bright as that formed using TP-SR Additive.

Formation of blue haze is eliminated by use of an acidic electroplating bath comprising an alkanol sulfonic acid electrolyte.

Organic compounds that are miscible with water or that have sufficient solubility in water to form at least an about 1% (volume:volume) solution in water may be used. The water/organic compound mixture should be a single phase. Preferred organic compounds include those that are substantially non-foaming under the high speed electroplating conditions. Preferably, the organic compound is an alkylene oxide condensation compound of an aliphatic hydrocarbon having between one and seven, and preferably less than six, carbon atoms and at least one hydroxy group, or solution soluble derivatives thereof. In addition, preferred organic compounds impart to the solution a cloud point of above about 110° F. When bright deposits are desired, the electrolyte may also include a brightening agent One preferred organic compound is an alkylene oxide condensate of an alcohol, such as butyl alcohol. Also, to achieve the desired cloud point, the alkylene oxide compound may be ethylene oxide wherein between about four and 40 moles of ethylene oxide, and preferably between six and twenty-eight, are used to form the condensation compound. Some of the moles of ethylene oxide may be replaced with propylene oxide

Another suitable organic compound is an alkylene oxide condensation compound of an aromatic organic compound having 20 carbon atoms or less; or solution soluble derivatives thereof. This aromatic compound may preferably contain one or two rings, preferably containing between 10 and 12 carbon atoms when two rings are utilized. Also, the aromatic organic compound may include an alkyl moeity of six carbon atoms or less, and one or more hydroxyl groups. Preferably, the aromatic organic compound is phenol, bisphenol A, styrenated phenol, or an alkylated derivative thereof. Also, benzene, naphthalene and toluene, each having at least one hydroxy group, or alkylated derivatives thereof, may likewise be used

Therefore, the most preferred organic compounds include an organic compound having 20 carbon atoms or less condensed with a sufficient amount of an alkylene oxide compound or solution soluble derivatives thereof to impart a cloud point of above 110°F to the solution

The method is carried out in high speed electroplating equipment of the type described above. Such equipment includes an electroplating cell, an overflow reservoir adjacent the cell, a pump for returning solution from the reservoir to the cell through one or more sparge pipes, and means for directing the steel strip to be plated from an entry point at one end of the cell to an exit at a second end of the cell. The electrolytes of the invention are introduced into the equipment in a manner such that the cell is substantially filled with the electrolyte. Also, the electrolyte continuously overflows into the reservoir and is continuously returned into the cell so that vigorous agitation and circulation of the electrolyte within the cell is achieved. Thus, the steel strip is continuously electroplated as it passes through the cell.

Antioxidants useful in this invention include those organic and inorganic materials that are effective in maintaining a low stannic ion concentration. Organic reducing agents useful in this invention include hydroquinone, substituted hydroquinones, 1,4 benzoquinone, recorsinol, catechol, gallic acid and its esters, pyrogallate, hydroxylamine and its derivatives or mixtures thereof. Inorganic reducing agents include vanadium in an oxidation greater than zero.

The temperature of the plating solution can vary from about room temperature to as high as 7O⁰C but the preferred temperatures are between 35⁰C to about 5O⁰C.

Industrial Applicability

The methods of the invention can be used for the preparation of tin coated metals, especially tin coated steel, known as "tinplate." The tin layer on each surface is typically about 0.38 micron to about 1.6 micron thick. The tin coated steel strip is typically about 0.15 mm to about 0.60 mm thick. Cans made of tin plated steel ("tin cans") are widely used in packaging, such as in the packaging of food and beverages, as well as in the packaging of other materials, such as paint and motor oil.

The advantageous properties of this invention can be observed by reference to the following examples, which illustrate but do not limit the invention.

EXAMPLES

Glossary

MSA Methanesulfonic acid (CH₃SO₃H)

ENSA Additive Ethoxylate of onaphthol sulfonic acid; electroplating additive (Rohm & Haas, Philadelphia, PA)

PSA Phenolsulfonic acid (HOC₆H₄SO₃H)

Sn(CH₃SO₃) 2 Tin(II) methanesulfonate

TP-SR Additive RONASTAN® TP-SR tin plating additive (Rohm &

Haas, Philadelphia, PA)

ISA isethionic acid (2-hydroxyethane sulfonic acid), Example 1: A solution of sodium isethionate was converted to the acid form by passing it through an ion-exchange resin in the acid form. The effluent from the ion-exchange resin was tested for acid normality using sodium hydroxide as a titrant and titrating the isethionic acid(ISA) to a pink end-point with a phenolphthalein indicator. The solution contained 1.77 N ISA

Example 2: To an aqueous solution containing 0.4 N isethionic acid was added stannous methanesulfonate so the final concentration was 12 g/liter as Sn(II). To this solution was added 1 gram/liter hydroquinone. Tin was deposited on a clean steel cathode at 4 amper/dm² for 25 seconds at 4O⁰C. After plating, the deposit was rinsed in distilled water and heated to about 24O⁰C to melt the tin. Immediately after melting the tin, the sample was quenched in water. The deposit was semi-bright.

Example 3: To an aqueous solution containing 0.4 N isethionic acid was added stannous methanesulfonate so the final concentration was 12 g/liter as Sn(II). To this solution was added 1 gram/liter of hydroquinone and incremental amounts of the TP-SR grain refining additive (available from Rohm-Haas; Freeport, NY) were added from 10 ml/liter to 80 ml/1. Tin was deposited on a clean steel cathode at 4 amper/dm² for 25 seconds at 4O⁰C. After plating, the deposits were rinsed in distilled water and heated to about 24O⁰C to melt the tin. Immediately after melting the tin, the samples were quenched in water. The deposits plated in solutions with less than or equal to about 25 ml/1 of the TP-SR additive were bright whereas those plated in solutions containing greater than 50 ml/1 of the TP-SR additive had the blue stain.

Example 4: Tin solutions containing mixed free acids were prepared. To solutions containing a total of 0.4 N free acid based on mixtures of isethionic acid and sulfuric acid was added stannous mathanesulfonate so the final tin(II) concentration was 12 g/1. To these solution was added 1 gram/liter hydroquinone and 50 ml/1 of the TP-SR additive. The solutions were heated to 42°C and tin deposited on clean steel. After plating, the steel was rinsed in solutions containing a portion of the tin plating electrolyte followed by drying. The tin deposit was then reflowed and quenched in water. The following results were obtained:

1. The solution containing 0.4 N H₂SO₄ poor reflow; dull tin deposit. 2. The solution containing 0.3 N H₂SO₄IO.1 N ISA good reflow; semi- bright tin deposit.

3. The solution containing 0.2 N H₂SO₄:0.2 N ISA good reflow; bright tin deposit.

4. The solution containing 0.1 N H₂SO₄:0.3 N ISA good reflow; bright tin deposit

5. The solution containing 0.4 N ISA good reflow; bright tin deposit

Example 5: Tin solutions containing mixed free acids were prepared. To solutions containing a total of 0.4 N free acid based on mixtures of isethionic acid and methanesulfonic acid (MSA) was added stannous mathanesulfonate so the final tin(II) concentration was 12 g/1. To these solutions was added 1 gram/liter hydroquinone and 50 ml/1 of the TP-SR additive. The solutions were heated to 42⁰C and tin deposited on clean steel. After plating, the steel was rinsed in solutions containing a portion of the tin plating electrolyte followed by drying. The tin deposit was then reflowed and quenched in water. The following results were obtained:

1. The solution containing 0.4 N MSA good reflow; blue haze in tin deposit.

2. The solution containing 0.3 N MSA:0.1 N ISA good reflow; very slight blue haze in tin deposit.

3. The solution containing 0.2 N MSA:0.2 N ISA good reflow; bright tin deposit.

4. The solution containing 0.1 N MSA:0.3 N ISA good reflow; bright tin deposit

5. The solution containing 0.4 N ISA good reflow; bright tin deposit

As long as the normality concentration of isethionic acid is greater to or equal than the sulfonic acid, there is no blue stain. Example 6: Tin solutions containing mixed free acids were prepared. To solutions containing a total of 0.4 N free acid based on mixtures of isethionic acid and ethanesulfonic acid (ESA) was added stannous mathanesulfonate so the final tin(II) concentration was 12 g/1. To these solutions was added 1 gram/liter hydroquinone and 50 ml/1 of the TP-SR additive. The solutions were heated to 42⁰C and tin deposited on steel. After plating, the steel was rinsed in solutions containing a portion of the tin plating electrolyte followed by drying. The tin deposit was then reflowed and quenched in water. The following results were obtained:

1. The solution containing 0.4 N ESA good reflow; blue haze in tin deposit.

2. The solution containing 0.3 N ESA:0.1 N ISA good reflow; slight blue haze in tin deposit.

3. The solution containing 0.2 N ESA:0.2 N ISA good reflow; bright tin deposit.

4. The solution containing 0.1 N ESA:0.3 N ISA good reflow; bright tin deposit

5. The solution containing 0.4 N ISA good reflow; bright tin deposit

As long as the normality concentration of isethionic acid is greater to or equal than the sulfonic acid, there is no blue stain.

Example 7: Tin solutions containing mixed free acids were prepared. To solutions containing a total of 0.4 N free acid based on mixtures of isethionic acid and methanedi sulfonic (MDSA) was added stannous mathanesulfonate so the final tin(II) concentration was 12 g/1. To these solution was added 1 gram/liter hydroquinone and 50 ml/1 of the TP-SR additive. The solutions were heated to 42⁰C and tin deposited on clean steel. After plating, the steel was rinsed in solutions containing a portion of the tin plating electrolyte followed by drying. The tin deposit was then reflowed and quenched in water. The following results were obtained:

1. The solution containing 0.4 N MDSA good reflow; no blue haze in tin deposit; bright tin deposit. 2. The solution containing 0.3 N MDSA:0.1 N ISA; good reflow; no blue haze in tin deposit; bright tin deposit.

3. The solution containing 0.2 N MDSA:0.2 N ISA; good reflow; bright tin deposit.

4. The solution containing 0.1 N MDSA:0.3 N ISA; good reflow; bright tin deposit

5. The solution containing 0.4 N ISA good reflow; bright tin deposit

When using a polysulfonic acid, the ratio of isethionic acid to the polysulfonic acid can vary to greater than 3:1 polysulfonic acid:isethionic acid.

Having described the invention, we now claim the following and their equivalents.

Claims

1. A method for plating tin, the method comprising the steps of: a) electroplating tin onto a steel strip in an acidic electroplating bath comprising an alkanol sulfonic acid electrolyte, stannous ion and an anion, and forming a plated strip comprising a plated tin surface comprising a surface layer of tin;

b) performing one or more rinses; c) heating the plated strip to at least the melting point of tin but to less than the melting point of the steel strip; and d) either quenching the plated strip in water or a solution of about 0.01 vol% to 10 vol% of an organic compound in water.

2. The method of claim 1 wherein said alkanol sulfonic acid electrolyte comprises an alkanol sulfonic acid, salts of an alkanol sulfonic, anhydrides of an alkanol sulfonic or mixtures thereof.

3. The method of claim 1 wherein said alkanol sulfonic is isethionic acid.

4. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises a sulfonic acid with the proviso that said sulfonic acid comprises less than 50% of the acid in said alkanol sulfonic acid electrolyte.

5. The method of claim 4 wherein said sulfonic acid is selected from the group consisting of methane sulfonic acid, methanedisulfonic acid, para toluenesulfonic acid, and phenol sulfonic acid.

6. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises an alkyl sulfonic acid with the proviso that the normality of said alkyl sulfonic acid is equal to or greater than the normality of the alkanol sulfonic acid in said alkanol sulfonic acid electrolyte.

7. The method of claim 6 wherein said sulfonic acid is a polysulfonic acid.

8. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises sulfuric acid with the proviso that the normality of the alkanol sulfonic acid in said alkanol sulfonic acid electrolyte is at least twice the normality of said sulfuric acid.

9. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises an antioxidant selected from the group consisting of hydroquinone, substituted hydroquinones, 1 ,4 benzoquinone, recorsinol, catechol, gallic acid and its esters, pyrogallate, hydroxylamine and its derivatives, vanadium in an oxidation greater than zero or mixtures thereof.

10. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises a grain refining additive.

11. The method of claim 1 wherein said alkanol sulfonic acid electrolyte further comprises a surfactant.

12. The method of claim 1 wherein the pH of said alkanol sulfonic acid electrolyte is less than about 3.

13. The method of claim 1 wherein the temperature of said alkanol sulfonic acid electrolyte is from ambient to about 70° C.

14. The method of claim 13 wherein the temperature of said alkanol sulfonic acid electrolyte is from about 35° C to about 50° C.