WO2013140099A1 - Process for depositing an organo-mineral sol-gel anticorrosion coating on stainless steel - Google Patents

Description

METHOD OF DEPOSITING ANTI-CORROSIVE SOLO-ORGANIC GEL COATING ON A STAINLESS STEEL

The present invention relates to a method of depositing an organ-mineral sol-gel anticorrosion coating on a stainless steel, as well as to an organo-mineral colloidal composition for carrying out said method.

 Stainless steels are widely used for street furniture or in the food industry, for example. They must resist atmospheric corrosion and be easy to clean. In addition, their surface appearance must retain an aesthetic appearance.

 Also, the stainless steels then used for these applications are generally austenitic stainless steels type AISI 304 or AISI 316 according to US nomenclature. Thanks to their nickel content, these stainless steels are easy to shape and resist corrosion.

 In order to further improve this corrosion resistance, it has been imagined to coat the surface of these stainless steels with an organo-mineral hybrid sol-gel coating including a corrosion inhibitor. Thus, not only is there a barrier to preserve the surface of the steel agents likely to corrode, but also, in the case where these agents nevertheless cross this barrier, the corrosion inhibitor acts to preserve the metal .

 Reference may be made to the document entitled "Cerium hybrid silica coatings on stainless steel AISI 304 substrate", J Sol-Gel Sci Techn (2006) 39: 131-138, which describes the implementation of such a coating.

 However, the sol-gel organic-mineral coating seems to be able to degrade over time, and even if the corrosion inhibitor plays its role, this degradation has a disadvantage in particular for the implementation of street furniture for which the appearance aesthetic is important.

 In addition, stainless steels including nickel are relatively expensive.

Also, a problem that arises is that aims to solve the present invention is to provide a method of depositing an anticorrosion coating which not only more resistant, but also that allows to coat stainless steels less noble and therefore less expensive.

 For this purpose, the present invention proposes, according to a first object, a method of depositing an organ-inorganic sol-gel anticorrosion coating on a stainless steel, said method comprising in the following order the following steps: a) providing a stainless steel element having a surface; b) treating said surface in a treatment step; c) applying to the treated surface a layer of an organo-mineral colloidal composition to form said organ-inorganic sol-gel anticorrosive coating; according to the invention, on the one hand, said treatment step b) comprises the following sub-steps: said surface of said stainless steel element is mechanically and chemically etched according to mechanical etching and chemical etching steps; and forming a passive layer on said etched surface so as to be able to apply said layer of said organo-mineral colloidal composition to the passivated surface; and, on the other hand, according to step c), a layer of an organo-mineral colloidal composition comprising a reactive mixture having organic phases and inorganic phases, and further poly (oxyethylene) is applied.

 Thus, a feature of the invention lies in the implementation of the surface of the stainless steel, and more specifically the implementation of two successive steps of mechanical etching and chemical etching before the passivation step, then application of the organo-mineral colloid composition comprising a reactive mixture having organic phases and inorganic phases, as well as poly (oxyethylene).

Mechanical etching essentially removes the elements of stainless steel from its processing and forming phases. Chemical etching allows it to dissolve the metal oxides present on the surface. And passivation, allows to reform a homogeneous oxide layer able to preserve the steel of the corrosion. It is then on this homogeneous oxide layer that the organo-mineral colloidal composition is applied. As will be explained hereinafter, thanks to the surface treatment of the stainless steel, the organ-inorganic sol-gel anticorrosive coating adheres to it perfectly. According to a particularly advantageous embodiment of the invention, a ferritic stainless steel element is provided in said step a). Thus, a less expensive stainless steel is used than an austenitic stainless steel, and thanks to the organo-mineral sol-gel anticorrosive coating, it is totally preserved from corrosion. In addition, the coating retains its integrity over time and gives the surface of ferritic stainless steel its aesthetic appearance.

 Advantageously, the method which is the subject of the invention further comprises a sub-step of degreasing between said steps of mechanical etching and chemical etching, so as to eliminate any organic contaminants.

 Furthermore, in said step c), in a particularly advantageous embodiment, another layer of said organo-mineral colloidal composition is applied to said passivated surface so as to form said anticorrosive coating. In this way, two layers of the organo-mineral colloidal composition are successively applied to the surface. According to an alternative embodiment, another layer of said organo-mineral colloidal composition is applied to said surface so as to bring the number of successive layers to three. In addition, an anti-wear layer of the type applied to organic glasses is advantageously formed on the last layer of organo-mineral colloidal composition. The nature of these anti-wear layers will be described in greater detail in the remainder of the description.

 In addition, in said step b), said surface of said stainless steel element is brought into contact with sulfuric acid to etch said surface. This gives a perfect dissolution of the oxides present on the surface of the steel. The sulfuric acid is preferentially diluted in water, and advantageously it is mixed with other acids.

In addition, with respect to step b), said etched surface of said stainless steel element is contacted, preferably with nitric acid to form said passive layer. In this way, not only is the formation of the oxide layer accelerated, but moreover, it promotes the formation of a homogeneous layer. It will be observed that the steps of stripping Mechanical and chemical preliminaries of the surface of the steel allow a perfect penetration of the acid in all the anfractuosities of the surface, and thus, the formation of a homogeneous layer of oxide.

 Moreover, in said step c), thermal energy is supplied to the layer 5 of said organo-mineral colloidal composition applied to said passivated surface to form said organo-mineral sol-gel coating. Also, for example, the surface of the stainless steel coated with the layer of the colloidal composition is subjected to a temperature above 70.degree. C. and below 100.degree. For example, it is carried out for a period of between 15 minutes and 60 minutes. In addition, when two layers of the organo-mineral colloid composition are successively applied to the surface, the surface of the stainless steel is subjected to an increase in temperature after the application of each of the layers. In this way, the coating is thus formed of two perfectly cohesive and much better polymerized half-layers, in comparison with a coating obtained by applying a single layer of equivalent thickness.

 In addition, in step c), a layer of an organo-mineral colloidal composition preferably comprising a corrosion inhibitor is applied, so as to further preserve the stainless steel from corrosion.

Preferably, said poly (oxyethylene) of said organo-mineral colloidal composition has a molecular weight greater than 30000 mol.l ^-1 . For example, said organo-mineral colloidal composition comprises between 4% and 10% by weight of said poly (oxyethylene). ).

 In addition, said reaction mixture advantageously comprises 3-5 (trimethoxysilyl) propyl methacrylate and Tetraethyl orthosilicate.

 In addition, said organo-mineral colloidal composition comprises between 20% and 30% by weight of said reactive mixture.

 The advantages of these latter characteristics concerning said organo-mineral colloidal composition will be set forth below.

According to another object, the present invention provides an organo-mineral colloidal composition for forming an organ-inorganic sol-gel anticorrosive coating, said composition comprising a reactive mixture having organic phases and inorganic phases. According to the invention, the The composition further comprises poly (oxyethylene). Thus, thanks to poly (oxyethylene), the thickness of the layer of the organo-mineral colloid composition can be perfectly controlled. Indeed, by choosing for example a poly (oxyethylene) having a molecular weight greater than 30000 g. mol ^"1 , one can apply a layer of constant thickness regardless of the mode of application.

 According to a particularly advantageous embodiment of the invention, said reaction mixture comprises 3- (trimethoxysilyl) propyl methacrylate and Tetraethyl orthosilicate. Preferably, the colloidal composition comprises between 20% and 30% by weight of said reactive mixture. Furthermore, it preferably comprises between 4% and 10% by weight of said poly (oxyethylene).

 Advantageously, said organo-mineral colloidal composition comprises a corrosion inhibitor. To do this, cerium nitrate is used, for example Cerium (III) nitrate hexahydrate. Thanks to cerium, strong bonds appear on the surface between the metal and the oxides. Also, the surface of the metal has greater wettability and hence greater adhesion of the colloidal composition when applied.

 In addition, the composition is advantageously dissolved in an alcohol solvent, for example ethanol.

 According to yet another object, the present invention provides a ferritic stainless steel member having an organo-mineral sol-gel anticorrosive coating obtained according to the deposition method as described above.

 Other features and advantages of the invention will emerge on reading the description given below of a particular embodiment of the invention, given for information but not limiting.

 Firstly, the formulation of a colloidal composition in accordance with the invention and then the method of treating the surface of a stainless steel will be described. Then, we will describe the modes of application.

According to the invention, there is provided an organo-mineral hybrid colloidal composition, that is organic / inorganic based on silicon alkoxide and 3- (Trimethoxysilyl) propyl methacrylate. The composition is here formulated in absolute ethanol and comprises, according to the invention, poly (oxyethylene). In addition, it contains according to the example below, a corrosion inhibitor. Other possible colloidal compositions based on alkoxysilanes have been formulated with, with the polymerizable organic group, 3-glycidoxypropyltrimethoxysilane (GPTMS).

 Example of formulation by weight of the colloidal composition

 • 3- (Trimethoxysilyl) propyl methacrylate (MAP) 2% -5%

• Tetraethylorthosilicate (TEOS) 20% -25% · Cerium (III) nitrate hexahydrate 0-1%

• Poly (oxyethylene) aqueous solution 35,000 g. mol ^"1 15% -25%

• Absolute ethanol 45% -65% According to the example above, the reaction mixture containing 3-

(Trimethoxysilyl) propyl methacrylate and Tetraethylorthosilicate, represents 25% by weight of the colloidal composition, while Poly (oxyethylene) in solution and ethanol represent respectively 15% and 60%. In addition, 0.5% of Cerium (III) nitrate hexahydrate is substituted for 0.5% of ethanol.

 According to one particular embodiment of the invention, 3- (trimethoxysilyl) propyl methacrylate represents 3.5%, Tetraethylorthosilicate 23%, Poly (oxyethylene) 6.5% and absolute ethanol 51.5%. .

The aqueous solution of poly (oxyethylene) is prepared independently by introducing 30% by weight of Poly (oxyethylene) 35,000 g. mol- ¹ in demineralized water Poly (oxyethylene) is here in solid form and is mixed with the water until completely dissolved.

 The colloidal solution, or sol, is prepared separately at room temperature.

The Cerium (III) nitrate hexahydrate is first dissolved in the total volume of absolute ethanol kept hermetically to prevent evaporation. The necessary amounts of Tetraethylorthosilicate and 3- (trimethoxysilyl) propyl methacrylate are then added with stirring.

The organo-mineral colloidal composition is then made by adding a sufficient amount of the aqueous poly (oxyethylene) solution to the colloidal solution and then stirring the mixture. The organo-mineral colloidal composition thus produced is intended to be applied to the surface of a stainless steel, preferably of the ferritic type. This type of steel is indeed less noble than austenitic steels, for example, and steels of this type corrode more easily. Also, thanks to the aforementioned composition, the corrosion resistance of these steels is greatly improved.

 Thus, a ferritic steel element is chosen and prior to the application of the aforementioned organo-mineral colloidal composition, its surface must be prepared. This preparation consists essentially of two stripping steps, one mechanical, the other chemical, advantageously separated by a degreasing step and followed by a repassivation step. The various steps of treating the surface of the stainless steel will be detailed below.

 The first stripping step is mechanical and aims to eliminate the metal elements resulting from the transformation and shaping of the steel and the various oxides. It is sandblasted, for example white corundum, which is an aluminum oxide with very low iron and soda content.

 Preferably, in an intermediate step, the surface thus mechanically etched is degreased. Advantageously, this degreasing is carried out using an alkaline solution while hot, for example at 60 ° C.

 For example, a first alkaline solution containing 45 g / l of sodium hydroxide, 20 g / l of trisodium phosphate, 20 g / l of sodium silicate and 10 g / l of dodecyl is prepared at a temperature in the region of 50 ° C. sodium sulfate. In another example, a second alkaline solution containing 13.5 g / l of sodium hydroxide, 5 g / l of trisodium phosphate, 17 g / l of sodium silicate and 13 g / l of gluconate is also prepared at 50 ° C. of sodium and 5 g / l of sodium dodecyl sulfate.

The second stripping step is chemical and it aims essentially to dissolve by means of acids, oxides present on the surface that would not have been removed during mechanical etching or that would be formed after. Several chemical stripping solutions that can be used are described below.

 A first solution of pickling with sulfuric acid consists of an aqueous solution containing 4.5% by weight of 34% hydrochloric acid and 9.5% of 96% sulfuric acid.

 A second fluoronitric acid etching solution consists of an aqueous solution containing 20% by weight of 60% nitric acid and 1.5% of sodium fluoride. According to an alternative embodiment, hydrofluoric acid is substituted for sodium fluoride,

 A third hydrochloric acid pickling solution consists of an aqueous solution containing 34% by weight of 35% hydrochloric acid and 3% sodium fluoride.

 A fourth phosphoric acid etching solution consists of an aqueous solution containing 35% by weight of 85% phosphoric acid and 1% sodium fluoride.

 A fifth permanganate pickling solution consists of an aqueous solution containing 10% sodium hydroxide and 4% potassium permanganate.

 The pickling solutions described above are used on purpose depending on the stainless steel used. Preferably, this chemical etching is carried out hot, for example between 45 and 55 ° C for the sulfuric solution and for a period of 15 minutes for this same solution.

 After having subjected the surface of the stainless steel to the two stripping steps and the intermediate degreasing step, and before the application of the organo-mineral colloidal composition, the steel surface is passivated, to form a homogeneous oxide layer. This oxide layer will be on the one hand a barrier to protect the metal from corrosion, and on the other hand to provide a better grip for the coating.

The surface of the stainless steel is thus brought into contact with an aqueous solution of nitric acid containing, for example, 20% by weight of 60% nitric acid. This step is carried out at room temperature, and by example, for a period of 15 minutes. In this way, a passive layer is obtained which is perfectly regular and homogeneous.

 The steel then has a surface state whose roughness, measured through the parameter Ra, is of the order of 0.5 μητι, while the ratio of the developed surface, or specific surface area, and the gross area, to polished steel surfaces is greater than two. In this way, it will obtain a better attachment of the colloidal composition with the best compromise between the mechanical attachment and the chemical bonding.

 The surface of the stainless steel thus prepared is then coated with the organo-mineral colloidal composition as described above, according to a defined protocol.

 Different modes of application are provided depending on the nature of the stainless steel elements to be coated.

 When the elements are pieces of finished and preformed dimensions, for example crankcases, credenzas or metal furniture, they are immersed in the colloidal composition and then removed and drained before being heated to a temperature of about 100 ° C for a period of time. period of time between 20 and 60 minutes, for example. In this so-called soaking mode, Poly (oxyethylene), thanks to its effects on the viscosity and the thixotropy of the composition, allows the formation of a layer of homogeneous composition and constant thickness.

 The viscosity is, according to a particularly advantageous embodiment, between 10 and 100 mPa.s, for example 45 mPa.s.

 In the case of continuous elements, for example coil sheets, they can be continuously coated at the parade.

 The colloidal composition can also be projected under pressure onto the surface of the steel. This method is also suitable for parts of finished dimensions, in manufacture or maintenance. The application with a cloth or brush is also suitable for the maintenance or repair of the coatings.

It will be observed that Poly (oxyethylene), confers the same beneficial properties to the colloidal composition for the other modes that the mode of implementation dipping. The surface of the treated stainless steel is advantageously covered with said colloidal composition in two steps followed respectively by a heat treatment, so as to form a coating with a total thickness of a few microns, preferably between 2μιη and 5μηη, plus precisely between 3pm and 4pm and for example 3.5pm.

 Thus, a first layer of the colloidal composition as defined above is applied to the surface of the pickled and passivated stainless steel, and then the surface thus coated is subjected to a temperature of between 85.degree. and 100 ° C, for example 90 ° C, and for a time between 20 minutes and 40 minutes, for example 30 minutes. In this way, a densified, perfectly solid, continuous and homogeneous first sub-layer is obtained.

 Then, a second layer of the colloidal composition is applied to the first densified underlayer, and then subjected to a temperature of 90 ° C for example, for a period of between 40 minutes and 80 minutes. In this way, a second densified, continuous and homogeneous sub-layer is obtained.

 The two sub-layers thus form a coating with a thickness of between 3 μm and 4 μm.

 It is planned to be able to apply still additional layers with a thickness between 1, 5 pm and 2 pm each. In addition, an end-of-wear layer is advantageously applied to the coating. These transparent layers, applied according to the same process, are of the type of which the organic glasses are coated. They consist for example of titanium oxide, alumina, zirconia or silicon or a mixture thereof.

 In addition, the colloidal composition advantageously comprises a dye, for example an organic pigment. The latter is then incorporated at the end of preparation with the aqueous solution of poly (oxyethylene). For example, the organic pigment has the general formula C27H34N2O4S, and gives a turquoise blue color.

 The invention also relates to a stainless steel part covered with such a coating.

Claims

1. A method of depositing an organominer sol-gel anticorrosion coating on a stainless steel, said method comprising in the following steps:  a) providing a stainless steel element having a surface; b) treating said surface in a treatment step;  c) applying to the treated surface a layer of an organo-mineral colloidal composition to form said organ-inorganic sol-gel anticorrosive coating;  characterized in that said processing step b) comprises the following substeps:  said surface of said stainless steel element is mechanically and chemically etched according to mechanical etching and chemical etching steps;  a passive layer is formed on said etched surface so as to be able to apply said layer of said organo-mineral colloidal composition to the passivated surface; and,  in that in step c), a layer of an organo-mineral colloidal composition comprising a reactive mixture having organic phases and inorganic phases, and further poly (oxyethylene) is applied.  2. deposition process according to claim 1, characterized in that provides in said step a) a ferritic stainless steel element.  3. deposition process according to claim 1 or 2, characterized in that it further comprises a degreasing sub-step between said steps of mechanical etching and chemical etching.  4. Deposition process according to any one of claims 1 to 3, characterized in that in said step c) is further applied another layer of said organo-mineral colloidal composition on said passivated surface to be able to form said anticorrosive coating . 5. Deposition process according to any one of claims 1 to 4, characterized in that in said step b), said surface of said stainless steel element with sulfuric acid for etching said surface.  6. deposition process according to any one of claims 1 to 5, characterized in that in said step b), said etched surface of said stainless steel element is brought into contact with nitric acid to form said passive layer. .  7. deposition process according to any one of claims 1 to 6, characterized in that in said step c), thermal energy is supplied to the layer of said organo-mineral colloidal composition applied to said passivated surface to forming said organo-mineral sol-gel coating.  8. Deposition process according to any one of claims 1 to 7, characterized in that in step c), a layer of an organo-mineral colloidal composition comprising a corrosion inhibitor is applied. 9. Deposition process according to any one of claims 1 to 8, characterized in that said poly (oxyethylene) has a molecular weight greater than 30000 mol.l ^"1 .  10. The deposition method according to any one of claims 1 to 9, characterized in that said reactive mixture comprises 3- (trimethoxysilyl) propyl methacrylate and Tetraethyl orthosilicate.  11. A deposition method according to any one of claims 1 to 10, characterized in that said organo-mineral colloidal composition comprises between 20% and 30% by weight of said reactive mixture.  12. Deposition process according to any one of claims 1 to 11, characterized in that said organo-mineral colloidal composition comprises between 4% and 10% by weight of said poly (oxyethylene).  13. Organo-mineral colloidal composition for forming an organomineral sol-gel anticorrosion coating, said composition comprising a reactive mixture having organic phases and inorganic phases, characterized in that it further comprises poly (oxyethylene).  14. A ferritic stainless steel element having an organomineral gel-sol-gel anticorrosion coating obtained according to the deposition method according to any one of claims 1 to 12.