ES2711205A1 - Procedure for coating the interior of barrel tubes based on nickel deposition (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Procedure to coat the interior of barrel tubes based on nickel deposition. The present invention relates to a method for coating the interior of barrel tubes comprising a step of depositing a nickel layer from a solution containing nickel salts, preferably nickel sulfate, and which may be for a autocatalytic process or by an electrodeposition process. Optionally, a subsequent heat treatment can be applied additionally. This procedure is an alternative with less environmental impact to the chroming process currently used. In addition, the invention also relates to the nickel-plated coatings obtained by said process which improve the properties of the tube. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Procedure to coat the inside of canon tubes based on nickel deposition

The present invention relates to a process for nickelizing the interior of the canon tubes and the coating obtained by this process.

BACKGROUND OF THE INVENTION

Current and developing weapons systems have tubes (canon tubes) of firearms that use electrodeposited chrome as a protective layer on the inner surface of the tube. This coating protects the inner surface of the tube against the harsh environment of the hot propellant gases, and the mechanical effects of the projectile thus increasing the life of the canon tube. The inner lining of the canon tubes with Cr (VI) meets the high demands required in its use such as:

- A high hardness and resistance to abrasion.

- Good resistance to heat, chemical agents and corrosion.

- Low coefficient of friction.

- Decorative effect and good appearance.

- Machinable by grinding, ability to repair if the chrome layer is damaged.

However, chromium is a heavy metal that is deposited on the surface of the tube by an electrolysis process called hard chrome, using an aqueous solution based on a chromic acid concentration (CrO3) of 228 250 gr / l. The chromic acid, EC number 1333-82-0, CAS number 215-607-8 used in the deposition process is a dangerous substance, since it contains hexavalent chromium.

Article 59 (10) of the REACH Regulation, legislation of the European Union for the safe use of chemical substances and products, has in its list of chemical substances of great concern for Human Health and the Environment (Candidate List) the chromic acid, all its salts and oligomers (hexavalent chromium), which are prohlbe their use, according to Annex XIV of said regulation (The revision of the January 17, 2013, Commission Regulation (EU) n ° 348/2013), as of September 21, 2017, and is there, unless there is an exemption for Defense for not having a possible technical alternative, where it may affect to the process of hard chromium inside the canon tubes, because it could not be done anymore.

The Cr (VI), coming from the (CrO3) (chromium trioxide), which intervenes in the hard chrome process is considered in the 57th of said regulation as a CMR substance. CMRs are substances classified as carcinogenic (C), mutagenic (M) or toxic for reproduction (R), categories 1 and 2 according to Directive 67/584 / EEC. Specifically, chromium trioxide is considered Carcinogen (category 1A) and Mutageno (category 1B). It has the transitory dispositions of the REACH regulation, the deadline for requesting authorization of use on March 21, 2016 and the date of expiration of use. September 21, 2017.

In chromium (VI), the limit value of professional exposure does not exist although it is believed that below 0.05 mg / m3 the risks to human health diminish but do not disappear.

From the environmental point of view the Cr (VI), the worn baths of the process of hard chrome plating are treated until reaching concentrations of Cr (VI) 0.05 mg / m3, depending on the limits established by the law of waste in each community . The waste can contaminate the groundwater, in fact it is considered the second most abundant metal in them, and causes genetic damages in the organisms while the Nickel (Ni2 +) does not have these harmful effects on the organisms.

Therefore, taking into account the high toxicity of Cr (VI), it would be necessary an alternative to the currently used coating that solves the current problems of contamination of this metal and that in turn maintains the properties required for this particular use.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a technical alternative to the chromium coating, which consists of an internal coating of the nickel-based canon tubes that has been deposited and optionally subjected to a subsequent heat treatment (from now, covering the invention). This coating must therefore meet or exceed the high requirements required for hard chrome plating for this specific application of the canon tubes.

Therefore, a first aspect of the present invention relates to a method for coating the interior of canon tubes comprising a step of depositing a layer of nickel inside the canon.

In a preferred embodiment, the deposition step of the nickel layer is a nickel autocatalytic process comprising the use of a bath of nickel salts as a source of nickel 2+ metal and a nickel reducing agent. Provided that said interior coating does not contain chromium.

In a more preferred embodiment the nickel salt is selected from nickel sulfate, nickel acetate, nickel carbonate and nickel chloride. In a still more preferred embodiment, the nickel salt is nickel sulfate. The salt concentration used is preferably 5.4 to 6.6 g / l and more preferably 6 g / l.

In a more preferred embodiment, the nickel reducing agent is sodium hypophosphite among others. The concentration of the reducing agent in the bath is preferably 24 to 42 g / l and more preferably greater than 30 g / l.

In another more preferred embodiment, the nickel autocatalytic process is carried out at a pH of between 4 to 6. In a still more preferred embodiment, the nickel autocatalytic process is performed at a pH of between 4.6 and 5.0.

In another more preferred embodiment, the nickel autocatalytic process is carried out at a temperature of between 80 to 100 ° C. In a still more preferred embodiment, the nickel autocatalytic process is carried out at a temperature between 82 and 91 ° C.

In a still more preferred embodiment the autocatallotic process comprises the immersion of the canon tube in a solution comprising nickel sulfate and sodium hypophosphite, at a pH of between 4.6 and 5.0 and is carried out at the temperature between 82-91 ° C and preferably at a deposition rate between 14-22 pm / h.

In a more preferred embodiment the solution used in the autocatalltico process of the Furthermore, it comprises accelerating substances of the reaction, complexing agents or inhibitors of the reaction.

In another preferred embodiment, the deposition step of the nickel layer is an electrodeposition process comprising the use of a nickel sulfate bath and a tungsten / tungsten salt.

In a more preferred embodiment, the electrodeposition process is performed at a pH of between 8 and 9.8. In a still more preferred embodiment, the electrodeposition process is carried out at a pH of between 9.3 and 9.5.

In another more preferred embodiment, the electrodeposition process is carried out at a temperature between 50 and 70 ° C. In a still more preferred embodiment, the electrodeposition process is carried out at a temperature between 60-65 ° C.

In another still more preferred embodiment, the deposition step of the nickel layer is electrodeposition by immersion of the canon in a solution comprising sodium tungstate, nickel sulfate, sodium citrate, ammonium chloride and sodium bromide at pH 8- 9.8 and at a temperature of 50-70 ° C.

The canon tube on which the deposition process is applicable, both the autocatalltico and electrodeposited, is generally made of steel, preferably alloy steel, although it may be of other metals usually used in this field.

In a preferred embodiment, prior to the deposition step of the nickel layer, pretreatment of the tubes is carried out. Preferably the pretreatment comprises the sub-steps:

i. degrease and wash the inner surface of the tube, and

ii. mechanically stripping by means of aluminum oxide at a pressure of between 4-6kg / mm2 or pickling with an HCl solution.

The applicable degreasing methods can be any of those known in the state of the art, such as immersion in aqueous solutions of NaOH, silicofluorides, mixtures of glycols, amines, silicates and surfactants, etc. In addition to the degreasing methods, degreasing-deoxidizing and washing with water can to be replaced by degreasing in vapor phase with an organic solvent that can be perchlorethylene.

In another preferred embodiment, after the deposition step of the nickel layer, a thermal treatment step is carried out at a temperature between 365-410 ° C for at least 60 min in an atmosphere of N2.

This thermal treatment results in an increase in hardness and wear resistance inside the canon tubes. In a preferred embodiment, the duration of the heat treatment should be a minimum of one hour after the parts have reached the temperature specified for the heat treatment, that is, once the time of heating has passed. The thermal treatment also serves to improve the adhesion of the nickel layer to the substrate.

In another preferred embodiment of the method of the invention, the tubes are washed and dried once or several times between the deposition stage and the thermal treatment stage.

The nickel coating layer has a thickness that is given by the design requirements of the canon tube, preferably between 8.50 to 150 microns and more preferably between 40-60 microns, and still more preferably 50 microns. The required layer thickness is obtained by varying the chemical nickel process time.

The nickel coating will have an average phosphorus content of 5-9% or a percentage of tungsten of 40-45%, depending on the deposition process used. This coating confers to the interior of the tube hardness, resistance to abrasion and wear, good resistance to heat, chemical agents and corrosion, low coefficient of friction, decorative effect and good appearance.

After comparing the mechanical properties of the products obtained by both processes, the following was observed:

- Hardness: in both is very similar, you can say 1000 HV 0.1.

- Resistance to corrosion: the resistance is good in both cases.

- Coefficient of friction: it is low in both cases.

- Resistance to wear and abrasion: it is very similar in both cases.

- Both are machinable by grinding and in both it is possible to remove the coating and re-apply it, very important characteristic from the point of view of manufacturing. The condition of being machinable means that if it exceeds the contribution of chrome or nickel layer can be rectified to put it in tolerances. - Both have resistance to high temperatures of up to 250 ° C.

The advantages of the coating obtained by the method of the invention are summarized in that it has the same mechanical properties as those of hard chromium plating for the specific application of the internal coating of the canon tubes. It exceeds slightly to the hard chromium in the resistance to the corrosion.

In addition, the environmental and occupational health and safety problems of hard chrome plating are eliminated due to the REACH legislation, since the source of Ni (Ni2 +) is nickel sulphate (NiSO4, CAS number 7786-81-4, CE number 028-009 00-5) present in Ni baths, is not included in the list of REACH candidate substances as SVHC substances (Substance of Very High Concern), therefore, not dangerous. Neither the rest of salts considered as possible sources of metallic nickel (nickel acetate, nickel chloride, nickel carbonate) are included as SVHC.

With the method of the invention the risks to human health are eliminated if a suitable aspiration is installed. Regarding occupational safety and health, the Spanish regulations on professional exposure limits for chemical agents of 2015, establishes a value of 0.1 mg / m3 for nickel sulphate (Ni2 +) (for a working day of 8 hours a day, 5 per week) below which the risks to human health disappear, with an adequate aspiration values below 0.1 mg / m3 can be achieved.

The cost of the process of the invention does not exceed the cost of hard chrome plating process with regard to the formation of the baths, in the additions, that is, the maintenance of the bath and renovation is similar to the hard chrome plating process, economically the process is profitable and its cost is not higher than that of hard chromium plating.

Throughout the description and the claims the word "comprises" and its variants do not intend to exclude other technical characteristics, additives, components or Steps. For those skilled in the art, other objects, advantages and characteristics of the invention will emerge partly from the description and partly from the practice of the invention. The following examples and figures are provided by way of illustration, and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1: Shows the nickel coating on the base material inside a canon tube 30 mm in diameter. A nickel layer thickness is 50 microns or 0. 05 mm.

EXAMPLES

1. Procedure for coating the inside of the canon tubes by means of autocatalytic nickel plating.

In a first stage, the alloy steel canon tubes were placed in the appropriate tools to carry out the process. To this end, the canon tubes were mounted on frames designed for them, according to their dimensions.

The tubes were degreased by several stages, as follows:

- Degrease by immersion in aqueous solution composed of Na (OH)> 20% (2mg / m3), Na2SiO3> 40%, Na2CO3> 20%, at a temperature between 75 ° -85 ° C for 5-10 minutes;

- degreasing by immersion in aqueous solutions of Na> 40% silicofluoride at a concentration of 30-45 g / l, at a temperature of 75 ° -85 ° C, for 5-10 minutes; - Degrease by immersion in aqueous solution of glycols, amines, silicates and surfactants at a concentration of 10-12% by volume, at a temperature between 75 ° -85 ° C and for 5-10 minutes

- degrease and deoxidize the tubes by immersing them in an aqueous solution containing Na (OH)> 50% and aliphatic petroleum solvents <1% in a concentration of 10-12 g / l, at a temperature of between 75 ° -85 ° C and for 5-10 minutes. (It may be that there is hydrogenation with the pieces in this bath, but it is removed heated above 190-210 ° C)

After the different stages of degreasing, the pieces were washed by immersion in a water bath for a period of 2-5 minutes at room temperature. Subsequently, the pieces were washed by immersion in a demineralized water bath for 3-5 minutes at room temperature.

In the next step, the inside of the tubes was protected and subsequently nickel-plated using PTFE or silicone plugs and immersed in a lacquer bath. The properties of the lacquer were as follows

Viscosity in poises at 25 ° C 14 +/- 2 Brookfield at 21 ° C / 60 rpm Flash point 4.4 ° C to 7.2 ° C closed cup Content in solids by weight: 34.5%

Density: 1.00 / - 0.03

Red color

Autoignition temperature: 480 ° C

The lacquer must have the viscosity recommended by the manufacturer for its application, if for some reason it is too viscous, it can be diluted with the SF66072 or SF66095 thinner.

In a raft of the appropriate dimensions, the piece to be treated is immersed in the solution. When removing the submerged piece, it must be done slowly, since the drip of the lacquer can introduce air bubbles in the bathroom. The coated part is allowed to dry at room temperature until it reaches a surface condition of drying to the touch, then it is rotated 180 ° in vertical position and submerged again to give a second layer. After these steps it is dried for a time of 30-60 min. at room temperature.

The protection of the inside of the tubes was removed and they were previously degreased by immersion in aqueous solution of HCl at 40-50% by volume at room temperature for 2-5 minutes. After this pickling, the pieces were washed by immersion in a mains water bath for 3-5 minutes at room temperature and in demineralised water by immersion in demineralized water bath for 3-5 minutes at room temperature.

This phase of pickling in hydrochloric acid is replaced in order to give a better adhesion of the nickel layer to the base substrate (steel) by the internal blasting of the canon tubes with aluminum oxide at a pressure of between 4-6 kg / mm2. The phase is included in the process after the degreasing process and before protecting the tubes with lacquer.

After all the preparation stages described, all parts were chemically nickel-plated by immersion in autocatalltico solution composed of a nickel source that was nickel sulfate (NiSO4), a reducing agent that supplies the electrons necessary for the reduction of the nickel , and rest: complexing agents, accelerating substances, inhibitors (stabilizers) as specified in table 2, under the following conditions:

Table 1.

Table 2

The equipment used were PVDF or stainless steel rafts (type 316 / 1.4571 with an anodic potential of approximately 600-800 mV), with stainless steel tools. The agitation was performed with stainless steel serpentine.

After deposition of the nickel inside the canon tube to obtain the After coating, the tubes were washed in a net-water bath for 2-6 minutes at room temperature and with demineralized water bath for 2-6 minutes at room temperature.

The pieces were dried by immersion in a hot water bath for at least 5 minutes at a minimum temperature of 80 ° C. The time between the washing and drying baths will be the minimum possible to avoid problems of final appearance on the nickel-plated surfaces. Finally the pipes were blown with clean and dry compressed air until the complete elimination of solution residues in the process was guaranteed and the lacquer was removed.

To improve the adhesion of the nickel layer to the base metal, the inside of the tubes can be mechanically stripped with Al2O3, corundum at a pressure of 5 kg / mm2.

Before thermally treating the tubes, an inspection was made of the appearance and thickness of the nickel coating obtained:

a) Appearance: the nickel coating should be shiny, and when visually inspecting it should be free of bites, blisters, exfoliation, nodular growths, cracks and other defects damaging the final finish, unless otherwise specified. Standard specimens with the specified appearance should be used to compare.

b) Thickness: the specified layer thickness will be the minimum local thickness. The minimum local thickness of the layer will be that measured at any point on the treated surface that can be felt by a ball of 20 mm diameter, unless otherwise specified.

After deposition of the nickel coating, a subsequent thermal treatment of the surface is carried out, which consists in maintaining the tube at a temperature of 400 ° C for a time of 1 hour minimum.

2. Checking the properties of the coated canon tubes by the procedure described in example 1.

To study the properties of the coating obtained by the procedure of the invention described in example 1, special tests were performed with test pieces, which were treated together the canon tubes to measure / test adhesion, layer thickness, porosity, corrosion resistance, hardness and other properties when the pieces to be treated are of a size, shape or material that is not suitable for testing, or when it is not practical to subject the treated articles to destructive tests because they are few or very expensive as is the case with canon tubes. The test specimens must be of the same material, must be in the same metallurgical condition and must be treated in the same load together with the rest of the articles they represent.

The properties of autocatallic chemical nickel with a medium phosphorus content are:

Density: 7.75 gr / cm3

Melting point: 890 ° C

Electric Resistivity: 90 pQ.cm

Thermal conductivity: 4 W / m.K

Coefficient of thermal expansion (22-100 ° C or 72-212 ° F): 12 pm / ° C.m

Magnetic properties: it is not magnetic

Resistance to traction: 700

Ductility,% Elongation: 1.0

Modulus of elasticity: 200 GPa

Hardness after deposition: 500 HV100

Hardness after heat treatment (precipitate) at 400 ° C for 1 hour minimum: 1100 HV100

Coefficient of friction versus lubricated steel: 0.13

Resistance to wear after deposition: Taber 18 mg / 1000 cycles Resistance to wear after heat treatment (precipitate) for 1 hour minimum: Taber 9 mg / 1000 cycles.

3. Procedure for coating the inside of the canon tubes by electroplating with Ni-W.

Another embodiment for depositing nickel on the interior of the canon tube would be by an electroplating process of nickel-tungsten alloys (NiW) which is described below:

The step that changes from the previous procedure described in example 1 would be the electrodeposition phase of the coating. The electrolysis bath consisted of sodium tungstate, nickel sulfate, sodium citrate, ammonium chloride and sodium bromide, with a pH of 9.5. The working temperature was 65 ° C and the solution was stirred magnetically.

NiW coatings were obtained, at the laboratory level, with a tungsten percentage of 40-45%, from an ammonium-citrate electrolyte. These coatings were subsequently characterized in terms of morphology and structure, and their most relevant properties were analyzed, such as hardness, corrosion resistance and abrasion resistance.

In addition, it was found that the application of a subsequent thermal treatment can significantly improve properties such as hardness, as well as the autocatalltico process.

Claims (26)

1. Procedure for coating the interior of canon tubes comprising a step of depositing a layer of nickel inside the canon. Process according to claim 1, wherein the step of depositing the nickel layer is a self-catalytic nickel process comprising the use of at least one nickel salt and a nickel reducing agent. 3. Process according to claim 2, wherein the nickel salt is selected from nickel sulfate, nickel acetate, nickel carbonate or nickel chloride. 4. Process according to the preceding claim wherein the nickel salt is nickel sulfate. 5. Process according to any of claims 2 to 4 wherein the concentration of the nickel salt is between 5.4 to 6.6 g / l. 6. Process according to any of claims 2 to 5 wherein the concentration of the reducing agent is from 24 to 42 g / l. 7. Process according to the preceding claim wherein the concentration of the reducing agent is greater than 30 g / l. 8. Process according to any of claims 2 to 7, wherein the nickel reducing agent is sodium hypophosphite. 9. Process according to any of claims 2 to 8 wherein the nickel-plating step is carried out at a pH of between 4 to 6. 10. Process according to the preceding claim wherein the nickel plating step is carried out at a pH of between 4.6 and 5.0. 11. Method according to any of claims 2 to 10 wherein the nickel plating step is carried out at a temperature of between 80 to 100 ° C. 12. Method according to the preceding claim wherein the nickel-plating step is carried out at a temperature between 82 to 91 ° C. 13. Process according to any of claims 2 to 12, wherein the solution also comprises reaction-accelerating substances, complexing agents or reaction inhibitors. 14. Process according to claim 1 wherein the nickel plating step is an electrodeposition step comprising the use of a nickel sulfate bath and a tungsten salt. 15. Process according to the preceding claim wherein the canon tube is immersed in a solution comprising nickel sulfate, sodium tungstate, sodium citrate, ammonium chloride and sodium bromide. 16. Process according to any of claims 14 or 15 wherein the electrodeposition is carried out at a pH between 8 and 9.8. 17. Process according to the preceding claim wherein the electrodeposition is carried out at a pH of between 9.3 to 9.5. 18. Process according to any of claims 14 to 17 wherein the electrodeposition is carried out at a temperature of 50-70 ° C. 19. Process according to the preceding claim wherein the electrodeposition is carried out at a temperature between 60-65 ° C. 20. Method according to any of the preceding claims wherein the canon tube is made of steel. 21. Method according to any of the preceding claims, wherein after the nick deposition stage a thermal treatment step is carried out at a temperature between 365-410 ° C for at least 60 min in an atmosphere of N2. 22. Procedure according to the previous claim where between the stage of The deposition of nickel and the heat treatment step are washed and the tubes are dried. 23. Method according to any of the preceding claims, wherein pre-treatment of the tubes is carried out prior to the deposition step. 24. Procedure according to the previous revindication where the pre-treatment comprises the following sub-steps: i. degrease and wash the inner surface of the tube, and ii. Mechanically stripping by means of aluminum oxide at a pressure of 4-6kg / mm2 or pickling with an HCl solution. 25. Coating of canon tubes comprising a nickel layer obtained by the process according to any of claims 1 to 24. 26. Coating according to claim 25 which also comprises phosphorus in a proportion of 5 to 9% by weight. ^{27. Coating according to re} v ^{indication 26 which also comprises tungsten.}