WO2015121790A2

WO2015121790A2 - Continuous trivalent chromium plating method

Info

Publication number: WO2015121790A2
Application number: PCT/IB2015/050974
Authority: WO
Inventors: Carlos Enrique MUÑOZ GARCIA; Sara Elisa MUÑOZ CASTAÑO; David MUÑOZ CASTAÑO; Marcela MUÑOZ CASTAÑO
Original assignee: Muñoz Garcia Carlos Enrique; Muñoz Castaño Sara Elisa; Muñoz Castaño David; Muñoz Castaño Marcela
Priority date: 2014-02-11
Filing date: 2015-02-09
Publication date: 2015-08-20
Also published as: US20170167040A1; CO7190036A1; EP3106544A2; EP3106544A4; RU2016135556A; WO2015121790A3; RU2016135556A3; MX2016010449A; BR112016018584A2; CN106164340A

Abstract

The invention pertains to the field of chemistry and metallurgy and specifically relates to a method for the electrolytic or electrophoretic production of electrochemical coatings (with a bath based on solutions of decorative trivalent chromium) on a metal or plastic substrate, with trivalent chromium ions, sulfate ions, graphite anodes, hexavalent chromium control and with purification of a dummy cathode in a continuous industrial operation.

Description

CONTINUOUS TRIVAL CHROME PROCESS TECHNOLOGICAL SECTOR

The present invention belongs to the field of chemistry and metallurgy and is specifically related to a process for the electrolytic or electrophoretic production of electrochemical coatings with a bath from solutions of decorative trivalent chromium on a metallic or plastic substrate with trivalent chromium ions, sulfate ions, graphite anodes, hexavanlent chromium control and with false cathode purification in a continuous industrial operation.

STATE OF THE TECHNIQUE

Chromium is a metal of great importance for a wide variety of industrial applications. For more than 70 years the decorative and functional chrome plating process has been carried out with electrolytes from chromic acid, a process with high Cr + 6 ion contents, considered by the WHO, as a carcinogen for operators and personnel who come into contact with the companies where it is implemented, it forms ulcers in the nasal septum. Hexavalent chromium causes a high environmental impact in aquifers. The companies that have chromium plating from hexavalent chromium, coexist with the quality problems associated with this type of processes, such as the low penetration power that leaves yellow in areas such as holes or angles, burning or milking in areas of High current density, stains or streaks. Within these technologies, the processes with the greatest potential for applicability are those of trivalent chromium (Cr + 3) that have been developing and increasingly occupying the attention of industrialists.

Trivalent chromium electrochemical coatings have been stigmatized over time, due to the cost, color and stability of the process; This has caused that its implementation is not very popular in large plants or in small workshops. Trivalent chromium plating baths have been developed from two technologies: sulfate and chloride. Trivalent chromium baths from chloride ions have some disadvantages: the color is darker, they are more sensitive to metal contamination, there is chlorine release in the anodes causing occupational health and safety risks, in addition to corrosion to the equipment . The trivalent chromium baths from sulfate ions, are more noble, have a whiter color, very close to the color of the chromium from hexavalent ions, have greater tolerance to metallic and organic pollutants, do not produce detachments of harmful gases and they do not cause occupational health and safety risks.

Trivalent chromium plating baths from chloride ions work preferably with graphite anode. In the anode, chlorine gas formation occurs, there is no oxygen production, which causes Cr + 6 to not form. In addition, the bath composition may include additives based on Br- and HCOO- ions that prevent the evolution of Cr + 3 to Cr + 6.

Although the trivalent chromium process from chloride ions, mainly uses graphite anodes; The trivalent chromate process based on sulfate ions also uses graphite anodes.

The state of the art is not known the process with graphite anodes in the processes of trivalent chromium plating from sulfate ions. Only the inconvenience of use is specified by the generation of Cr + 6. Cr + 6 is a contaminant of chromium plating baths from trivalent chromium ions and is formed in the anodes by oxidation of Cr + 3 to Cr + 6, causing quality defects in the coating.

In low concentrations it produces, dark color of the coating, streaks, loss of efficiency and collapse of the process in high concentrations. The first trivalent chrome processes from S0 ₄ ^"2 ions, patent 1602404, used a tank with two compartments, in one of the compartments a Pb anode is immersed in an anolyte of H2S04, which is separated by a permeable membrane , and in the other compartment is the solution with Cr + 3 ions that have no contact with the anode and consequently the oxidation of Cr + 3 to Cr + 6 does not occur. The disadvantage of this two-compartment system is the cost and efficiency in a continuous industrial process. US3706639, discloses a process for the preparation of a trivalent chromium plating bath from compounds containing Cr + 6 using inorganic reducers, using graphite anodes and platinized titanium. In addition to the fact that the preparation of the bath from chromic products is very expensive, it does not treat the handling and evolution of Cr + 6 contaminate during the work of the bath.

US 5560815 defines the development of trivalent chromium plating baths from S0 ₄ ^"2 ions, referring to the use of anodes coated with iridium and tantalum oxide with characteristics of having a lower potential than oxygen and consequently inhibiting evolution from Cr + 3 to Cr + 6. This type of anodes has shown that after a time of work in continuous industrial productions, it facilitates the formation of Cr + 6 in the bathroom, due to depletion of the iridium oxide layer, causing also restriction of the passage of the current in the anode and the defects associated with the Cr + 6 pollutant. The trivalent chromium baths based on S0 ₄ ^"2 on the market, work exclusively with titanium anodes coated with platinum, iridium oxide , tantalum, niobium, rhodium, etc., this type of anode prevents the formation of Cr + 6 in the bathroom, but in continuous industrial process, they have a shelf life that depends on the Direct Current used a and of the volume of work. Due to depletion of the layer or cracking due to excess of DC, the anode fails and starts the formation of Cr + 6. On the other hand, the price of this type of anodes drastically affects the final production costs.

Although the EP0088192 patent defines a trivalent chromium bath from chloride ions, it refers to the use of graphite anodes in trivalent chromium processes from sulfate ions and how the release of oxygen in this type of anodes causes erosion of the surface and release of carbon particles that co-deposit with chromium causing defects in the coating. WO2010051 1 18 refers to the use of manganese ions as an additive to inhibit the formation of Cr + 6 and increase the shelf life of insoluble anodes that are used in the chrome plating process from trivalent ions and sulfate, including lead, lead alloy, platinum titanium anodes, or metal anodes comprising surface coating with iridium oxide, ruthenium oxide or mixed iridium / tantalum oxide. Although the life span of this type of anodes increases, they will eventually collapse. In a continuous industrial production 24/6/360, it is difficult to determine when they run out and the production of Cr + 6 begins. On the other hand, the concentration in the bath of the Mn ion must be strictly controlled, due to the risk of an excess that can cause codeposito with the chromium, changing the color of the coating and affecting the corrosion resistance.

US5413646 "HEAT-TREATABLE CHOMIUN" mentions a bath that starts from a hexavalent chromium and an iron alloy to reduce it to trivalent chromium by means of methanol, ammonium and sodium sulfate format catalysts as a catalyst, where the reaction is carried out starting from chromium Hexvalent pollutant that reacts in the process with platinum and graphite anodes for the purpose of having non-uniform functional chromium. The new invention, based on trivalent chromium, does not use a sodium sultate as a catalyst, but the sodium sultate is a result of the reaction that is removed in a crystallization subprocess, iron or alloys cannot be involved in the new process because iron it is a contaminate in the bathroom that leaves black streaks in the finish and increases the possibility of corrision in the final finish; In addition it does not require changes of temaperatura and the trivalent chrome bath part can be applied to plastics or metals for decorative finishing.

No information is found regarding the use of graphite anodes in trivalent chromium plating processes from S0 ₄ ^"2 ions, the art of the technique is limited to giving information about the dangers of this type of anodes, because it generates Cr +6 pollutant Environmental and occupational health and safety requirements with increasingly stringent regulations in different countries has promoted interest in the development of trivalent chrome plating process with cheaper procedures and controls. According to the problems detailed in the art, a trivalent chrome plating process from S0 ₄ ^"2 ions is required, with anodes at a reasonable cost and process efficiency, both for quality products and for their stability over time in continuous industrial productions.

The present invention solves the problem of contamination when hexavalent chromium is generated in an efficient and low-cost process, proposing a continuous indsutrial process and product obtained resistant to corrision, with uniform and non-contaminating finish from a bath with chromium ions trivalent, sulfate ions and gafite anodes in their own combination for continuous industrial operation. With this invention, difficulties of the prior art are overcome, such as the exhaustion and final collapse of the titanium electrodes coated with noble metals, the control and purification of the contaminants that affect the process has been simplified, generating a constant rejuvenation of the process, reducing risks, costs and allowing the novelty to be replicated without technological difficulties. DESCRIPTION OF THE INVENTION

The invention proposes a chromium plating process from Cr + 3, which contains S0 ₄ ^"2 ions and preferably uses graphite anodes, obtaining a chromium coating with strong adhesion to the substrate, with mechanical, physical and chemical properties, reproducible in productions. industrial continuous The product specifications in terms of color, thickness and resistance to corrosion and wear maintain its stability over time and changes in ambient temperature.

With the process developed, the results of variations in working conditions can be predicted. The composition of the bath has demonstrated stability in continuous productions 24 hours a day with the application of the appropriate controls and maintenance. The bath has been noble to changes in temperature, pH, concentration of components and tolerant of different contaminants. The process of the present invention mainly uses graphite anodes that have qualities such as being a good conductor of electricity, ease of machining to adapt to different conformations, good chemical and mechanical resistance, good resistance to anodic corrosion and a high surface to surface ratio. Volume providing a very good anodic area, they have a low cost compared to titanium electrodes coated with noble metals. The use of graphite anodes is not limited and lead anodes and stainless steel are also used. With the use of graphite anodes in solutions of trivalent chromium plating with SO ² ions, a higher anode: cathode ratio, higher current densities can be used, with no significant effect on the electrodes, compared to titanium electrodes coated with noble metals. which have a tendency to cracking when subjected to high current densities.

The process begins with a preparation of the trivalent chromium solution with graphite anodes so that the bath starts working. In the process of obtaining the coating, hexavalent chromium is produced above the pollutant limit, so hexavalent chromium reducers are added to the bath as explained below. The reducers convert the excess of hexavalent chromium into trivalent chromium, releasing sodium sulfate that must be removed from the bath, and to crystallize it, a crystallization stage is performed by cooling the bath.

Preferably, the graphite anodes should not present pores, since these cause the bath solution to penetrate and in the electrochemical reaction when oxygen is released, they erode prematurely leaving too much carbon residue. The graphite anode should be placed in acid-resistant cloth bags, such as those used in the nickel plating process to prevent the passage to the solution of eroded particles of the anode. In a continuous industrial process the use of continuous filtration at a speed of 4 times to 8 times and preferably 4 times to 6 times the volume of the bath per hour is used. This ensures that the carbon particles in the bath are controlled and do not harm the quality of the coating. With the use of graphite anodes in the bath of the present invention, the ratio in the anode / cathode bath should be 3: 1 and preferably a ratio of 2: 1, a higher ratio and a higher direct current improve the efficiency of the process, achieving a greater cathodic area of coating. The applied current density should be between 4dm2 at 12 A / dm2 and preferably between 5 A / dm2 at 8 A / dm2.

The bath prepared for use with graphite anodes, contains Cr + 3 ions, the concentration of Cr + 3 ions in the bath is between 10g / I at 30 g / l, preferably between 15-g / la 25 g / l . A salt of Cr ₂ {SO) z is used as the source of Cr + 3 ions.

The prepared bath contains chromium complexes of organic and inorganic type that are stable, forming bonds that allow the release of the Cr + 3 ion and its reduction to CrO on the cathode surface to form a metal film with mechanical, physical and chemical properties of strong adhesion to the substrate and reproducible in continuous industrial productions.

The chromium complexes of this preparation also have properties that allow them to be easily degraded when the sewage treatment is carried out, which guarantees a reduction of environmental impacts.

The organic acid salts HCOONa I HCOONH ₄ I NaCH ₃ COO I NH3CH3COO have been used as a source of products that form complexes with trivalent chromium; lactate ions C ₃ H ₅ 0 ^~ ₃ , Oxalate ions (COO) ₂ ^{~ 2} , malate C ₄ H ₆ <¾, glycine NH ₂ CH ₂ COOH. These types of compounds are used individually or a mixture of these, with C + 3 to form complexes. The concentration should be between 50/1 to 150 g / l, preferably between 70g / l to 120 g / l.

It is well known in the art that the presence of H3B03 in an electrolytic solution, between 40-g / 60 g / l, preferably between 45g / 50 g / l increases conductivity and acts as a buffering agent and also as an anode inhibitor reducing the attack and the collapse of the graphite anode by the presence of B- ions that reduce the overproduction of 02.

To improve the conductivity of the bath, Na ₂ S0 ₄ is added as conductive salt, between 30 g / l to 60 g / l, preferably between 40 g / l to 50 g / l. In the present invention this salt is added only for the formation of the bath since during the process it is being produced and increasing its concentration, due to the effect of the different oxide-reduction reactions. The bath of the present invention also includes salts containing Na +, K + and (NH) ₄ ⁺ ions.

The chromium complex decomposes by action of the electric current and deposits CrO on the cathode and releases the anion S0 ₄ ^'2 that is combined with the sodium ions that are present in solution forming Na ₂ S0 ₄ . Also during the mechanism of reduction of Cr + 6 to Cr + 3, there is an increase in acidity by formation of H ₂ S0 ₄ , acidity control is carried out with NaOH, Na ₂ C0 ₃ , neutralizing the solution with formation of Na ₂ S0 ₄ . In this type of bath using graphite anodes, there is always a tendency to become acidic.

The pH should be controlled between 3.4 pH to 4.0 pH. High pH values favor the formation of oxygen and promote the attack of the graphite anode.

The surface tension of the bath should be handled between 30 dynes to 70 dynes. The adjustment is made with the addition of wetting agents such as octyl alcohol. The process temperature should be between 40 ° C to 60 ° C, preferably between 45 ° C to 50 ° C. Due to the constant reduction of Cr + 6 to Cr + 3, the bath is concentrated from Na ₂ S0 ₄ , and a temperature reduction can cause crystallization of this salt.

To achieve stability in the coating color, electrolytic performance and compliance with specifications of corrosion resistance and penetration into the bath, the presence of contaminants must be controlled. To control the presence of contaminants in this bath, purification with false cathode has been effective in controlling of parasitic metals such as Ni, Fe, Cu and Zn, because these metals do not form strong complexes with bath components. For high-capacity industrial productions that do not allow suspension of activities, it is applied externally to the process, continuous false cathode, with existing equipment in the market for this type of operations, with the progressive accumulation of contaminants in the bathroom is avoided. When industrial productions allow it, false cathode is applied inside the tank and periodically, with process stop.

It has been found that with the process of the present invention the use of purification with selective resins is not required allowing a reduction of pollutants produced in regeneration and a reduction in economic costs. The use of sequestering agents such as EDTA for metallic purification is not recommended in this process, because it produces collateral effects such as the progressive accumulation by masking of strong organometallic complexes that affect the quality of the coating and cause problems in the treatment of effluents.

The addition of the reducers is carried out in a controlled way by consumption of amp hours, trying to keep Cr + 6 between Oppm at 40 ppm, preferably between Oppm 20 ppm, a range in which there is no adverse effect on the quality of the coating.

Reduction compounds for Cr + 6 are well known in the art. In our trivalent chroming process from S0 ₄ ^"2 ions, preferably using graphite anodes, where Cr + 6 formation is mandatory, we use components with S0 ₃ ^'2 anions, mainly NaHS0 ₃ , Na ₂ S ₂ 0 ₅ , A / a ₂ S (¾, {NH) ₄ HS0 ₃ , Na ₂ S ₂ 0 ₄ , and optionally sulfur compounds derived from S ₂ Q¡ ^'2 , or a mixture of the above. One of the characteristics of These products do not cause adverse side effects to the bathroom by decomposition.

Results

The results obtained at industrial level in continuous productions, demonstrate the degree of development of the present process, several hundreds of thousands of dm2 of products for export and local use, show the stability and quality of the process. Results have been obtained with the bath of the present invention from Cr + 3 ions and S0 ₄ ^'2 using graphite anodes, Hull Cell penetration between 80% to 92% even after five trials with the same solution and The photometric analysis in the presence of diphenylcarbazide gave Cr + 6 results between 20ppm at 30ppm, a concentration that did not affect the quality of the coating, the color was very close white to the color of the hexavalent chromium and without dark streaks.

The thickness of the coating obtained in decorative coatings of Cr + 3 is between 0.3 μιη to 2 μιη, very similar to those obtained with the traditional Cr + 6 process.

It has been found in tests conducted at the industrial level that when the concentration of Cr + 6 in the bath has not been corrected, levels <50 ppm are not detrimental to the quality of the coating. Cr + 6 concentration levels between 50-ppm to 150 ppm affect the color and produce dark streaks. Cr + 6 concentration levels> 300 ppm dramatically reduce bath efficiency and coating quality.

It has been found that in the present invention of a trivalent chromium plating bath with SO ₄ ² ions preferably working with graphite anodes and with continuous false cathode purification, the concentrations of contaminants were kept within permissible ranges without affecting the quality of the Coating and smooth operation of the process, Cr + 6 Oppm at 20 ppm, Ni 30ppm at 150 ppm, Fe 10 ppm at 100 ppm. The Cu was the pollutant with the lowest possibility of reaching the trivalent chromium plating bath and with a false cathode it was controlled at ranges <1 ppm

The inventors have managed to replicate the results in a continuous commercial production with a bath of 1 .500 liters of trivalent chromium plating provided by the present invention with S0 ₄ ^"2 ions working preferably with graphite anodes, controlling the production of Cr + 6 with reducers mentioned above, applying crystallization to maintain the content of Na ₂ S0 ₄ within the working ranges without discarding the bath in whole or part of the bath since it can be infinitely reused when controlling the production of hexavalent chromium in the process. The invention is a novel preparation of bath trivalent chromium from S0 ₄ ^'2 ions which offers the advantage of preferably working with graphite anodes and have provided solutions to control the generation of Cr + 6 and the control and elimination of different pollutants, obtaining a chromium coating of strong adhesion to the substrate with excellent mechanical, physical and chemical properties, commercially reproducible in continuous industrial productions. The specifications of the product in terms of color, thickness and resistance to corrosion and wear have been stable over time.

With this invention, difficulties of the prior art are overcome, such as the exhaustion and final collapse of the titanium electrodes coated with noble metals, the control and purification of the contaminants that affect the process has been simplified, generating a constant rejuvenation of the process, reducing risks, costs and allowing the novelty to be replicated without technological difficulties.

Claims

one . Trivalent chrome plating process for decorative coating on a metallic substrate or a plastic substrate of a coating of thickness of 0.3 μηι to 2 μιη, CHARACTERIZED because it is a process with a continuous bath containing chromium complexes of organic and inorganic type from of Cr + 3 ions taken from salts preferably Cr ₂ (S0 ₄ ) 3 with S0 ₄ ^"2 ions and graphite anodes with anode ratio: 2: 1 cathode resistant to cracking by current density, with production control of Cr + 6 with reducers that generate sodium sulfate and removal by crystallization of said sodium sulfate generated in the process; and with purification with false cathode for the control of parasitic metals that have weak complexes in the bath such as Ni, Fe, Cu and Zn for coating color stability, electrolytic performance and compliance with corrosion and penetration resistance specifications.

2. Trivalent chromium plating process according to claim 1 characterized in that it begins with a preparation of the trivalent chromium solution with graphite anodes and in the process of obtaining the coating hexavalent chromium is produced which is controlled with reducers that convert the excess of hexavalent chromium in trivalent chromium releasing sodium sulfate that is removed in a crystallization stage by cooling the bath.

3. Trivalent chromium plating process according to claim 1, characterized in that the graphite anode must be free of pores and placed in acid-resistant cloth bags that prevent the passage to the solution of eroded particles of the anode in a continuous filtration process. a speed of 4 times to 6 times the volume of the bath per hour so that the carbon particles in the bath are controlled and do not impair the quality of the coating.

4. Trivalent chrome plating process according to claim 1 characterized in that the applied current density must be between 4dm2 at 12 A / dm2 and preferably between 5 A / dm2 at 8 A / dm2.

5. Trivalent chromium plating process according to claim 1 characterized in that the concentration of Cr + 3 ions in the bath is between 10g / I to 30 g / l, preferably between 15-g / l to 25 g / l.

6. Trivalent chromium plating process according to claim 1 characterized in that the source of products that complex with trivalent chromium comes from the salts of organic acids HCOONa I HCOONH ₄ 1 NaCH ₃ COO I NH ₃ CH ₃ COO; lactate ions C ₃ H ₅ 0 ^~ ₃ Oxalate ions (COO) ₂ ^{~ 2} , malate C ₄ H ₆ 0 ₅ , glycine NH ₂ CH ₂ COOH individually or a mixture of these with C + 3 at a concentration between 50 / I at 150 g / l, preferably between 70g / l and 120 g / l.

7. Trivalent chromium plating process according to claim 1, characterized in that Na ₂ S0 ₄ is added to the bath as a conductive salt, between 30 g / l and 60 g / l, preferably between 40 g / l and 50 g / l only for formation bath and includes salts containing Na +, K + and {NH) ₄ ions.

8. Process trivalent chromium according to claim 1 characterized in that the chromium complex is decomposed by the electric current and deposited CrO on the cathode and releases S0 ₄ ^'2 anion is combined with the sodium ions that are present in solution forming A / a ₂ S0 ₄ and also during the mechanism of reduction of Cr + 6 to Cr + 3 there is an increase in acidity by formation of H ₂ S0 ₄ where the acidity control is carried out with NaOH, Na ₂ C0 ₃ neutralizing the solution with Na ₂ S0 ₄ formation.

9. Trivalent chromium plating process according to claim 1 characterized in that the pH must be controlled between 3.4 pH to 4.0 pH, the surface tension of the bath must be handled between 30 dynes at 70 dynes and the temperature of the The process must be between 40Ϊ at 60 ° C, preferably between 45 ° C and 50 ° C because the constant reduction of Cr + 6 to Cr + 3 in the bath concentrates Na ₂ S0 ₄ and a temperature reduction can cause crystallization of the concentrated salt of Na ₂ S0 ₄ .

10. Trivalent chromium plating process according to claim 1, characterized in that the addition of the reducers is carried out in a controlled manner by consumption of amp hours, trying to maintain the Cr + 6 between Oppm at 40 ppm, preferably between Oppm 20 ppm, range in which there is no adverse effect on the quality of the coating.