Compositions And Methods for Pickling and Passivation of Stainless Steel Welds

Abstract

Compositions and methods for simultaneous pickling and passivation of stainless steel are presented that have exceptional shelf life, are free of hydrofluoric acid, exhibit no syneresis, and are non-corrosive to carbon steel. Moreover, contemplated compositions can be safely sprayed or otherwise applied onto various surfaces and will adhere to treated surfaces without significant runoff.

Description

This application claims priority to our U.S. Provisional Patent application with the Ser. No. 63/321,053, which was filed Mar. 17, 2022, and which is incorporated by reference herein.

FIELD OF THE INVENTION

The field of the invention is composition and methods for pickling and passivation of stainless steel welds, especially as it relates to non-corrosive and hydrogen fluoride (HF) free compositions.

BACKGROUND OF THE INVENTION

The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

All publications and patent applications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

Heat tint is commonly observed in furnace treated stainless steel and in the heat affected zone of weldments, and is the result of the thickening of a naturally occurring transparent oxide layer on the surface of stainless steel. The colors formed are similar to the “temper colors” seen on other steel surfaces following heat treatment and range from pale straw hues to dark blue. As heat tints are formed on the surface of stainless steel, chromium is drawn to the surface due to its preferential susceptibility to oxidation relative to iron in the steel. This leaves a layer at and just below the surface with a lower chromium level than in the bulk of the steel, and consequently a surface with greatly diminished corrosion resistance. Other oxides present in the heat tint include ferrite that can initiate accelerated corrosion of the weld area.

Heat tint on stainless steel fabrications can be removed using acidic brush-on pastes or gels, spray pickling, or immersion tank pickling. Pickling is a metal surface treatment that removes impurities, such as rust or scale from metals. Most commonly, hydrofluoric acid is the base component that is used for the pickling of stainless steel. Unfortunately, products containing this harmful and toxic acid require special handling procedures, and its fumes are dangerous to the user and environment. For at least this reason, hydrofluoric acid containing pickling compositions are unsuitable for spray application as such application creates a highly toxic aerosol to a user. Passivation is a metal surface treatment in which a light coat of a protective material is applied on the metal surface to create a barrier against corrosion. This is typically achieved by nitric acid, which is somewhat easier to handle and by itself is a less dangerous acid.

For example, U.S. Pat. No. 6,844,304 to Lunner describes a composition comprising nitric acid, urea, and a filler to remove an oxide layer of stainless steel after heat treatment. Nevertheless, and especially in combination with hydrofluoric acid, such pickling and passivation compositions are still problematic in use and disposal. In other attempts to remove heat tint, as described in U.S. Pat. No. 2,765,271 to Kreml, discoloration is removed by electrolytic treatment of the weld area using phosphoric acid. Similarly, and as disclosed in WO 2013/036999 to Lewer, the heat tint is electrochemically removed using low voltage high amperage current with an electrolytic fluid comprising a potassium phosphate salt of an acid at neutral pH. While such compositions are generally benign to the environment and operator, use is complicated by the need for specific equipment and electricity.

Advantageously, pickling and passivation can be performed using a single composition, and U.S. Pat. No. 10,280,515 describes a method in which magnesium salts of hydrofluoric and nitric acid are used in an aqueous base. Here, magnesium nitrate and magnesium fluoride are present in a ratio of about 2:1 and are used to help prevent dissociation of magnesium fluoride that would otherwise lead to generation of hydrofluoric acid. Unfortunately, the production of magnesium nitrate is a highly exothermic process and tends to complicate manufacture. Moreover, at least some of such formulations have a shelf life that is less than desirable.

Thus, even though various compositions and methods of pickling and passivating stainless steel welds are known in the art, all or almost all of them suffer from several drawbacks, particularly where the compositions contain hydrofluoric acid. Therefore, there remains a need for improved hydrofluoric acid-free and non-corrosive pickling and passivating compositions that are environmentally friendly and safe to apply to a variety of surfaces.

SUMMARY OF THE INVENTION

The inventive subject matter is directed to various compositions and methods for simultaneous pickling and passivation of stainless steel that have exceptional shelf life, that are free of hydrofluoric acid, exhibit no syneresis, and that are non-corrosive to carbon steel.

In one aspect of the inventive subject matter, the inventor contemplates a pickling and passivating composition that includes a magnesium fluoride salt, and nitric acid and/or a nitrate salt in an aqueous medium, and that further includes a water-swellable crosslinked copolymer in an amount sufficient to thicken the composition to allow for the composition to cling to a vertical surface. Most preferably, the crosslinked copolymer is chemically stable in the composition such that, upon storage for at least one year, the composition exhibits substantially no syneresis and exhibits a loss in viscosity of no more than 5%.

For example, suitable water-swellable crosslinked copolymers are inverse emulsion polymerization copolymers, which may be prepared from an acrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-based polyfunctional crosslinking agent (e.g., Solagum SH210). In further examples, the composition may further comprise sulfamic acid and/or calcined alumina. As will be readily appreciated, the magnesium fluoride salt, the nitric acid, and the nitrate salt are formed in situ by reaction of magnesium nitrate hexahydrate and hydrofluoric acid.

In further contemplated embodiments, the copolymer is present in an amount of at least 2.5 wt %, and/or the loss in viscosity is no more than 3%. Where desired, the composition can be packaged into a multi-use tube or multi-use container, or a container from which the composition can be drawn to be sprayed onto a surface. Therefore, contemplated compositions may be provided in association with an instruction to spray the composition to a surface.

Viewed from a different perspective, the inventor also contemplates a method of preparing a pickling and passivating composition that includes a step of dissolving magnesium nitrate hexahydrate in an aqueous medium to form a magnesium nitrate solution, and a further step of adding hydrofluoric acid to the magnesium nitrate solution to form a magnesium fluoride precipitate and nitric acid. In another step, a water-swellable crosslinked copolymer is added in an amount that is sufficient to thicken the composition to allow for the composition to cling to a vertical surface. Most preferably, the crosslinked copolymer is chemically stable in the composition such that, upon storage for at least one year, the composition exhibits substantially no syneresis and exhibits a loss in viscosity of no more than 5%.

Among other options, the water-swellable crosslinked copolymer is an inverse emulsion polymerization copolymer. For example, suitable copolymers may be prepared from an acrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-based polyfunctional crosslinking agent. Thus, exemplary copolymers include Solagum SH210. In addition, it is contemplated that sulfamic acid and/or calcined alumina may be added to the magnesium nitrate solution, and/or that the nitric acid reacts with the sulfamic acid to form NO₂ and sulfuric acid.

In some embodiments, the copolymer is added in an amount of at least 2.5 wt % of the composition, the magnesium nitrate hexahydrate is dissolved in an amount of about 43 wt % of the composition, and/or the hydrofluoric acid is added in an amount of about 9 wt % of the composition. In further embodiments, the water-swellable crosslinked copolymer is added in an amount of about 3 wt % of the composition.

Therefore, the inventor also contemplates a method of pickling and passivating a stainless steel surface in which the compositions presented herein are applied to the stainless steel surface for a time sufficient to pickle and passivate the stainless steel surface. Most typically, the stainless steel surface comprises a weld, and/or the composition is applied by spraying or from a multi-use container.

Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

DETAILED DESCRIPTION

The inventor has discovered various compositions and methods for simultaneous pickling and passivation of stainless steel that have exceptional shelf life, that are free of hydrofluoric acid, exhibit no syneresis, and that are non-corrosive to carbon steel. In further beneficial aspects, the compositions can be sprayed without adverse effects on the operator and will cling to the treated surface without significant runoff

In prior formulations, magnesium oxide was reacted with nitric acid to form magnesium nitrate in a highly exothermic process. The so produced magnesium nitrate was then reacted with hydrofluoric acid to generate magnesium fluoride and nitric acid, and the magnesium fluoride was then used as a pickling agent while the nitric acid was used as a passivating agent. As should be readily apparent, such production method was difficult to control and environmentally problematic. Moreover, while such formulations could be brushed onto a surface, cling to vertical surfaces was at least in some cases problematic.

In contrast, in one exemplary production process according to the inventive subject matter, magnesium nitrate hexahydrate is dissolved in water to a concentration of about 43.03% (w/w). Finely ground calcined alumina is added as a colorant in an amount of about 4.31% (w/w), and sulfamic acid is added in an amount of about 2.94% (w/w). Hydrofluoric acid is added to the so prepared mixture in an amount of about 9.7% (w/w), which will react with magnesium nitrate, thereby forming a magnesium fluoride precipitate and nitric acid. Some of the nitric acid in turn reacts with sulfamic acid to produce NO₂ and sulfuric acid. To this mixture is then added about 2-3% (w/v) of a synthetic acrylate polymer that is inert to and long-term stable in acidic conditions in the composition. Preferably, the synthetic polymer is made by inverse emulsion polymerization (e.g., using an acrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof and an amine-based polyfunctional crosslinking agent) and is used as an emulsifying thickener to produce a stable composition with gel-like consistency.

Notably, the inventor has discovered that such polymers, when added in proper quantities and appropriate ratio to MgNO₃/HF (e.g., water-swellable crosslinked copolymers, emulsifying thickeners), had the unexpected effect to stabilize contemplated formulations over significant periods of time even in the absence of a hydrophobic phase. Among other benefits, such polymers produced thickened formulations without syneresis and so enabled extended storage and usability without any degradation and loss in viscosity. While conventional viscosity agents will contract and expel water under the acidic conditions and cannot be recombined by subsequent stirring/mixing (in a manner similar to coagulated blood that cannot be ‘un-coagulated’ by stirring/mixing), the compositions presented herein can be formulated to any desired thickness and will not separate into a contracted polymeric phase and an expelled water phase.

Suitable polymers therefore include water-swellable crosslinked copolymers that are chemically stable at the pH and in the compositions presented herein. Viewed from a different perspective, suitable polymers will have a drop in viscosity of no more than 15%, or no more than 10% or no more than 8%, or no more than 6%, or no more than 4%, or no more than 3%, or no more than 2%, or no more than 1% after storage of at least 6 months, or at least 12 months, or at least 18 months, or at least 24 months, or at least 36 months with a storage temperature of about 20-25° C. Likewise, suitable polymers will exhibit no (less than 1% of water expelled and separable from the composition) or substantially no (less than 5% of water expelled and separable from the composition) syneresis after storage of at least 6 months, or at least 12 months, or at least 18 months, or at least 24 months, or at least 36 months with a storage temperature of about 20-25° C. Exemplary polymers suitable for use are described in U.S. Pat. Nos. 8,936,797, 6,683,144, FR 2,943,677, and EP 1 116 733 all of which are incorporated by reference herein (e.g., selected polymers are commercially available from SEPPIC S.A.). In addition to SEPPIC-type water swellable polymers, poly(ethylene oxide) based polymers such as Polyox N 3000 (commercially available from DuPont) can also be used, individually or in combination with SEPPIC-type polymers. In contrast, conventional viscosity modifiers such as various gums, cellulosics, pectins, and polyvinyl alcohols have shown little to no stability under the acid conditions of the formulations presented and have led to significant reduction in viscosity and syneresis. As such, these conventional viscosity modifiers do not allow for long-term storage.

It should further be appreciated that where a composition is prepared from hydrofluoric acid and magnesium nitrate, all or substantially all (e.g., at least 95 mol %, more typically at least 98 mol %) of the hydrofluoric acid has reacted with magnesium nitrate to so form a mixture of magnesium nitrate and magnesium fluoride salts where the magnesium fluoride is very little or insoluble in water. Without wishing to be bound by any theory or hypothesis, the inventor contemplates that such mixture can be obtained by combination of magnesium nitrate in molar excess with hydrofluoric acid. Viewed from a different perspective, molar excess of magnesium nitrate will ensure that formation of hydrogen fluoride will not take place or is greatly reduced, therefore rendering the composition to be free or substantially free (e.g., at least 5 mol %, more typically at least 2 mol %, most typically less than 1 mol %) of hydrofluoric acid. In most typical embodiments, a composition that is substantially free of hydrofluoric acid comprises an amount of less than 5 ppm (parts per million) hydrofluoric acid, and more preferably less than 2 ppm hydrofluoric acid.

Most typically, the pH of the composition will be equal of less than 6.0, or equal of less than 5.5, or equal of less than 5.0, or equal of less than 4.5, or equal of less than 4.0, or equal of less than 3.5, or equal of less than 3.0, or equal of less than 2.5, or equal of less than 2.0, or even more acidic. Contemplated compositions can further include sulfamic acid or oxalic acid in an amount between 0.5-50% by weight of the composition, and more typically between 1-15% by weight, and most typically 2-5% by weight of the composition. The sulfamic acid or oxalic acid further ensures that the magnesium fluoride salt does not dissociate. When there is a large amount of dilution, there is a risk that the magnesium fluoride salt can dissociate, and the fluoride ion is used to produce hydrofluoric acid. However, using sulfamic acid or oxalic acid acts as a barrier and creates protons to prevent magnesium fluoride salt from dissociating even when there is a large amount of dilution. In addition, the sulfamic acid or oxalic acid acts as a chelating agent to remove iron oxide from stainless steel, such that the iron oxide dissolves so that it can be rinsed off the stainless steel. Therefore, the composition can further be used to remove rust from steel. Most notably, contemplated compositions were shown to be non-corrosive, even when used on carbon steel.

Contemplated compositions and formulations may further include additional ingredients to provide one or more desired functionalities, and particularly preferred additional ingredients include surfactants, chelators, fillers, pigments, and odor masking agents. Notably, as the preferred polymers also act as emulsifying thickener, surfactants may be omitted. However, it should be noted that surfactants not excluded and may be present in an amount of between 0.5-15% by weight of the composition, and more typically between 0.5-5% by weight of the composition. Suitable surfactants for the composition include at least one of a linear alkyl benzene sulphonic acid and alcohol ethoxylates (C_9-11).

Chelators can be included in the composition between 1.0-15% by weight of the composition, and more typically between 1.0-5% by weight of the composition. Suitable chelators include one or more mono-, bi, and polydentate chelators. For example, contemplated chelators include gluconic acid, citric acid, tartaric acid, gluconate salts, and ethylenediaminetetraacetic acid (EDTA). While not limiting to the inventive subject matter, chelating agents are generally preferred in contemplated formulations as chelating agents assist with the removal of pickled oxides of chromium and iron.

It should be appreciated that regardless of the type of formulation, the composition suitable for treating heat tint by pickling and passivation of stainless steel can be applied in cold temperatures. It is contemplated that the composition can include an antifreeze component that does not create auxiliary reactions. Typically, the antifreeze component is propylene glycol. The addition of the antifreeze component allows the composition to be applied in temperatures below 0° C. and even below −20° C. or −50° C. Advantageously, the composition can be applied in gel, paste, or spray formulation throughout the year without incident. The antifreeze component can be included in an amount between 2-30% by weight of the composition, and more typically between 10-20% by weight of the composition. Other suitable antifreeze components include methanol or a glycol (e.g., ethylene glycol).

In further aspects of the inventive subject matter, a method of treating a heat tint by pickling while simultaneously passivating a stainless steel is contemplated. The method includes a step of contacting the stainless steel with a formulation that comprises a magnesium fluoride salt, a nitrate salt, an acid, and an acrylate copolymer such that the nitrate salt and the acid are present in an amount sufficient to prevent formation of hydrofluoric acid caused by dissociation of the magnesium fluoride salt, and such that the acrylate copolymer provides sufficient thickening to maintain the formulation on the treated surface for an extended time. Contemplated acids that prevent formation of hydrofluoric acid include oxalic acid and sulfamic acid.

When the formulation is applied to a heat tint, the stainless steel is pickled to remove the heat tint while the steel is simultaneously passivated to protect it from corrosion. It should be appreciated that the magnesium fluoride salt is present in an amount effective to reduce or remove weld marks by pickling while the nitrate salt is present in an amount effective to promote passivation of the stainless steel. Therefore, the stainless steel is the heat tint is removed and the stainless steel is protected by passivation using the formulation.

After applying the formulation and allowing time for pickling and passivation of the stainless steel, the formulation is removed from the stainless steel. Contemplated contact times for the formulation onto the stainless steel are between 1-4 hours. The formulation can be removed by using pressurized water. However, since the formulation will be extremely diluted by the addition of pressurized water, there can be a problem with the dissociation of the magnesium fluoride salt. Such dissociation can result in the formation of hydrofluoric acid using the fluoride ion of the dissociated magnesium fluoride salt, which is a toxic residue harmful to the environment. Advantageously, using a nitrate salt and an acid (e.g., oxalic acid or sulfamic acid) as described herein prevented or substantially reduces the formation of hydrofluoric acid by reducing the likelihood that the magnesium fluoride salt will dissociate.

It should be appreciated that the formulation can also be applied to rust on metals. Thus, the formulation can be used in areas of the steel that are not affected by a heat tint. Indeed, the formulation described herein comprising oxalic acid or sulfamic acid will act as a chelating agent dissolving the iron oxide so that it could easily be removed.

In the context of application of the formulations, it should be appreciated that the formulations presented herein cannot only be prepared as a liquid, but also as a brushable or sprayable formulation that has sufficient cling. For example, exemplary compositions with desirable cling will remain on a vertical surface such that after application of a quantity of the composition at least 75%, or at least 80%, or at least 85%, or at least 90%, or at least 95%, or at least 98% of the applied quantity will remain on a vertical surface where the surface is steel and where the applied composition has an average thickness of at least 0.5 mm, or at least 0.7 mm, or at least 0.9 mm, or at least 1.2 mm, or at least 1.5 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, and even thicker.

Moreover, due to their exceptional stability, it is contemplated that the formulations may be stored in single- or multi-use tubes or other containers at relatively small volume (e.g., less than 1,000 mL) from which the formulation can be directly dispensed to the surface to be treated (e.g., by use of a tube with applicator tip or a caulking gun). This will advantageously allow portability of the formulations to the point of use in a tool box or other container and avoid the need for large containers and/or on-site preparation. Indeed, contemplated formulations have shown remarkable storage stability and maintained substantially identical working parameters for at least 3 years. However, storage in larger containers is also contemplated, and such containers may be portable (e.g., enclosing a volume of up to 5,000 mL, or up to 10 L, or up to 20 L), or in a larger form factor for on-site or whole sale storage from which portions can be dispensed over an extended period (e.g., at least 1 year, or at least 2 years, etc.)

In still further contemplated embodiments, the compositions presented herein can also be formulated to avoid freezing, and even be effective at temperatures as low as minus 40 degrees Celsius. This can be achieved by replacing 50% or less (w/w) of the amount of water in any of the given variations of the formulation with an antifreeze agent. Among other suitable choices, preferred antifreeze agents include propylene glycol due to its non-toxic and biodegradable nature. Alternatively, further antifreeze agents can be used, such as ethylene glycol, and the water to antifreeze ratio will generally be determined by the azeotropic curve of the mixture of the two components as a function of the freezing point value.

EXAMPLES

The following experimental data is disclosed to provide exemplary formulas that achieve pickling and passivation of stainless steel by using non-water soluble salts under conditions that lack of free hydrofluoric acid.

In particular, the inventor prepared a variety of formulations as shown in Table 1 below and then tested pickling and passivation performance along with measurements in drop in viscosity and ability to cling to stainless steel at a 90° angle to normal. Performance indicated in numeric values with 5 being best and 0 being worst.

TABLE 1
Ingredient (wt %/wt %)	A	B	C	D
Magnesium Nitrate × 6H2O	43.03	38.00	43.03	43.03
HF (49%)	9.74	8.60	9.74	9.74
Sulfamic Acid	2.94	2.94	2.94	2.94
Alumina Calcinated	4.31	4.31	4.31	4.31
SolaGum SH210	2.94	2.94	1.5	N/A
Polyox WSR N3000	N/A	N/A	N/A	6.00
Water	Balance	Balance	Balance	Balance
Pickling/Passivation	5	3	4	4
Performance
Drop in viscosity (12 months)	1-3%	1-3%	30-40%	40-60%
90 Degrees Cling to SS panel	5	5	2	1
Syneresis	none	visible	visible	visible

As can be seen from the table, most formulations had a desirable pickling/passivation performance. Notably, cling to a vertical surface was superior with water-swellable crosslinked copolymers/emulsifying thickeners, and desirable stability of viscosity was achieved when the water-swellable crosslinked copolymers, emulsifying thickeners was present in an amount of more than 1.5 wt %. The physicochemical properties were even further enhanced where MgNO₃/HF and the water-swellable crosslinked copolymers/emulsifying thickeners had a ratio of at least 16.5, and more typically at least 17, or at least 17.5, or at least 18.0.

In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” As used herein, the terms “about” and “approximately”, when referring to a specified, measurable value (such as a parameter, an amount, a temporal duration, and the like), is meant to encompass the specified value and variations of and from the specified value, such as variations of +/−10% or less, alternatively +/−5% or less, alternatively +/−1% or less, alternatively +/−0.1% or less of and from the specified value, insofar as such variations are appropriate to perform in the disclosed embodiments. Thus, the value to which the modifier “about” or “approximately” refers is itself also specifically disclosed. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. As also used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously.

It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the scope of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification or claims refer to at least one of something selected from the group consisting of A, B, C . . . and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.

Claims

1. A pickling and passivating composition, comprising:

a magnesium fluoride salt, and nitric acid and/or a nitrate salt in an aqueous medium;

a water-swellable crosslinked copolymer in an amount sufficient to thicken the composition to allow for the composition to cling to a vertical surface; and

wherein the crosslinked copolymer is chemically stable in the composition such that, upon storage for at least one year, the composition exhibits substantially no syneresis and exhibits a loss in viscosity of no more than 5%.

2. The composition of claim 1, wherein the water-swellable crosslinked copolymer is an inverse emulsion polymerization copolymer.

3. The composition of claim 2, wherein the copolymer is prepared from an acrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-based polyfunctional crosslinking agent.

4. The composition of claim 3, wherein the copolymer is Solagum SH210.

5. The composition of claim 1, wherein the composition further comprises sulfamic acid and/or calcined alumina.

6. The composition of claim 1, wherein the magnesium fluoride salt, and nitric acid and/or a nitrate salt are formed in situ by reaction of magnesium nitrate hexahydrate and hydrofluoric acid.

7. The composition of claim 1, wherein the copolymer is present in an amount of at least 2.5 wt %.

8. The composition of claim 1, wherein the loss in viscosity is no more than 3%.

9. The composition of claim 1, wherein the composition is packaged into a multi-use tube or multi-use container.

10. The composition of claim 1 in association with an instruction to spray the composition to a surface.

11. A method of preparing a pickling and passivating composition, comprising:

dissolving magnesium nitrate hexahydrate in an aqueous medium to form a magnesium nitrate solution;

adding hydrofluoric acid to the magnesium nitrate solution to form a magnesium fluoride precipitate and nitric acid; and

adding a water-swellable crosslinked copolymer in an amount sufficient to thicken the composition to allow for the composition to cling to a vertical surface;

wherein the crosslinked copolymer is chemically stable in the composition such that, upon storage for at least one year, the composition exhibits substantially no syneresis and exhibits a loss in viscosity of no more than 5%.

12. The method of claim 11, wherein the water-swellable crosslinked copolymer is a copolymer is an inverse emulsion polymerization copolymer.

13. The method of claim 12, wherein the copolymer is prepared from an acrylamide, an acrylamidoalkyl-sulfonic acid, and/or a salt thereof, and an amine-based polyfunctional crosslinking agent.

14. The method of claim 13, wherein the copolymer is Solagum SH210.

15. The method of claim 11, further comprising a step of adding sulfamic acid and/or calcined alumina to the magnesium nitrate solution, wherein the nitric acid reacts with the sulfamic acid to form NO2 and sulfuric acid.

16. The method of claim 11, wherein the copolymer is added in an amount of at least 2.5 wt % of the composition.

17. The method of claim 11, wherein the magnesium nitrate hexahydrate is dissolved in an amount of about 43 wt % of the composition, wherein the hydrofluoric acid is added in an amount of about 9 wt % of the composition, and/or wherein the water-swellable crosslinked copolymer is added in an amount of about 3 wt % of the composition.

18. A method of pickling and passivating a stainless steel surface, comprising a step of applying the composition of claim 1 to the stainless steel surface for a time sufficient to pickle and passivate the stainless steel surface.

19. The method of claim 18, wherein the stainless steel surface comprises a weld.

20. The method of claim 18, wherein the composition is applied by spraying or from a multi-use container.