CN103510130B - Trivalent hard chromium electro-plating method - Google Patents

Abstract

The invention discloses an electroplating method for electroplating a trivalent chromium hard chromium coating on steel. Using DSA anode and trivalent chromium solution in sulfuric acid system, temperature around 50°C and current density of 35-65 amperes/square decimeter, through the soft-start process of controlling the change of current and tank pressure during the electroplating process and maintaining the acidity within the specified range Inside (pH0.9～2.1), a trivalent chromium coating with a thickness of more than 80 microns is obtained, the hardness of the coating reaches above HV870, the current efficiency exceeds 35%, the highest plating rate can reach 2 microns/minute, and the bonding force of the coating can pass thermal shock Test (heating at 200°C, quenching with brine). The invention provides a process method for trivalent chromium hard chromium electroplating to replace hexavalent chromium hard chromium electroplating that seriously pollutes the environment, and solves the problems that the existing trivalent chromium electroplating technology cannot obtain a thick coating and the bonding force between the coating and the substrate is poor.

Description

Trivalent chromium hard chrome plating method

Technical field The present invention relates to the technology of electroplating hard chromium with trivalent chromium sulfate as main salt

technical background

Hexavalent chromium plating is one of the most widely used plating types in the electroplating industry. It has a large quantity and a wide range. Decorative chromium is plated on steel, aluminum, plastic, copper alloy and zinc-based alloy die castings. Functional electroplating (hard chrome) Plating workpieces include hydraulic cylinders and plungers, crankshafts, printing plates/rollers, internal combustion engine pistons, plastic molds, and fabrication of fiberglass workpieces, cutting tools, etc. Also used to repair worn workpieces such as rollers, molds, cylinders and crankshafts repair. Hexavalent chromium coating has good hardness, wear resistance, corrosion resistance and decorative properties. It is not only used for decorative coatings, but also widely used for functional coatings and repair of workpieces. Micro-cracked chromium coatings can also be used as lubricating coatings. Application (hard chromium) The traditional chromium plating technology has always used hexavalent chromium (chromic acid) as the main electroplating raw material. The hexavalent chromium coating is white and bright, has high hardness, good wear resistance and corrosion resistance, and the process is simple and easy to maintain. But hexavalent chromium plating is one of the most serious and difficult sources of pollution in the electroplating industry. Chromium is one of the necessary elements in the normal metabolic process of organisms. Chromium deficiency will cause metabolic disorders such as sugar and fat, but excessive chromium content will cause serious harm to organisms and human bodies. The U.S. Environmental Protection Agency (EPA) has identified hexavalent chromium as one of 17 highly hazardous toxic substances. Chromium exists in trivalent and hexavalent forms in water, and hexavalent chromium is very toxic, about 100 times that of trivalent chromium. Toxic effects. Hexavalent chromium is highly irritating to the skin and can cause skin ulcers. Long-term intake can cause diseases such as squamous cell carcinoma, sarcoma and adenocarcinoma. The wastewater and waste of hexavalent chromium cannot be degraded naturally in nature like cyanide. It accumulates in organisms and human body and can cause long-term harm. It is a highly toxic carcinogen and a serious corrosive medium and heavy Substances that pollute the environment. There are a large number of hexavalent chromium electroplating baths in various regions of our country, the total amount is estimated to be more than 10 million tons, which seriously pollutes our country's water, soil and atmospheric environment. More than 60% of the annual cost of treating electroplating wastewater is used to treat the electroplating wastewater containing Hexavalent chromium wastewater, waste gas and waste, environmental pollution losses caused by hexavalent chromium electroplating, wastewater treatment costs and occupational diseases of personnel are at least tens of billions of RMB.

In order to replace the heavily polluted hexavalent chromium plating, many studies have been carried out, including low-concentration chromium plating, chromium substitution coating and trivalent chromium plating, among which trivalent chromium plating is the most active and promising.

The chromium plating layer obtained by trivalent chromium plating is white and bright, with high hardness and excellent mechanical properties. Its dispersion performance and covering ability are much higher than those of hexavalent chromium. Under the same current density, its deposition rate is faster than that of hexavalent chromium plating. Twice that of chromium, and the current efficiency is also higher than that of hexavalent chromium. However, the main advantage of trivalent chromium plating is that its toxicity is only 1/100 of that of hexavalent chromium. Generally, the content of trivalent chromium ions in trivalent chromium plating solution is only 1/100 of that of hexavalent chromium ions in hexavalent chromium plating bath. One-seventh or even less, and the waste water is easy to treat. It is one of the most promising electroplating processes to replace the hexavalent chromium plating that is widely used but seriously pollutes the environment. However, there are also some problems in trivalent chromium plating, mainly due to its low tolerance to impurities. For example, when the cumulative concentration of metal ions such as zinc, copper and nickel in the trivalent chromium plating bath reaches 10-100ppm, the chromium plating layer The quality will drop, resulting in poor stability and difficult to put into actual production and application. The hexavalent chromium ions produced at the anode will also seriously affect the quality of the coating, so that qualified products cannot be produced at all. Generally, the trivalent chromium plating solution sold on the market cannot plate a thick chromium layer, and there are relatively large problems in the stability and maintenance of the bath solution. There is no report on the trivalent chromium electroplating process technology that can replace hexavalent chromium electroplating in all aspects of performance. For this reason, it is of great significance to develop and study trivalent chromium electroplating technology with stable thick coating (that is, hard chromium coating).

Contents of the invention

The invention provides a stable trivalent chromium electroplating technology capable of producing thick coatings (that is, hard chromium), including solution preparation and electroplating process control technology for thick chromium coatings. The cathode current density exceeds 30%, the hardness of the coating reaches 870HV, and after heat treatment is 1058HV, the appearance of the coating is white and bright without defects, and the neutral salt spray does not corrode for 240 hours. The thermal shock test at 300°C shows no cracking.

The invention provides a trivalent chromium sulfate hard chromium electroplating solution and a preparation method thereof, mainly comprising a trivalent chromium main salt, a complexing agent, a buffer and a catalyst component.

The main salt of the trivalent chromium electroplating solution is chromium sulfate or chromium potassium sulfate, and the content of trivalent chromium ions is 5-40g/L.

The complexing agent is nitrilotriacetic acid, oxalic acid, malic acid, glycine, formic acid, acetic acid or its sodium salt and potassium salt, and at least one of the complexing agents is used to prepare the plating solution. The content is 1～60g/L,

The buffering agent is boric acid, aluminum sulfate, phthalic acid, at least one of the buffering agents is used to prepare the plating solution, the content is 50-180g/L, and the catalyst is potassium bromide, potassium chloride, ammonium fluoride, potassium fluoride and sodium fluoride, at least one catalyst 10-30g/L is used to prepare the plating solution, and the conductive salt is potassium sulfate or sodium sulfate at 0-80g/L. The rest is deionized water.

The pH value of the solution is 0.6-2.1, and the acidity can be adjusted with sodium carbonate or potassium carbonate and sulfuric acid, preferably 0.8-1.8.

Preparation method: first put two-thirds of the deionized water into the container and heat it to above 60°C, put the main salt into the deionized water while stirring, then put in the complexing agent, catalyst and buffer in turn, wait for After all dissolved, keep it for later use. Put less than one-third of the measured deionized water into another container and heat it above 60°C. Put the buffer into the deionized water in this container while stirring. After it is completely dissolved, put it in the second container. Pour the solution into the first container, heat it to about 95°C, and keep it warm for more than 2 hours. When the temperature naturally cools down to the process temperature, the preparation of the trivalent chromium plating solution is completed.

Electroplating parameter control method:

The current density range of the solution hard chromium is 25-75A/dm ² , and the current density is preferably controlled at 35-60A/dm ² . If the current density is too small, the precipitation potential of chromium cannot be reached; if it is too large, the stress of the coating will be large and it will easily peel.

In the range of 0.8-2.1, as the pH value increases, the current range expands, and the plating efficiency is higher; the pH value increases, the activation performance of the solution is not enough, and the coating is prone to peeling, which is not suitable for long-term electroplating; control range: pH value It is better to control it between 1.2 and 1.6.

Temperature control of electroplating solution: low temperature, good coverage, fast deposition speed, but too low temperature, slow mass transfer speed, long-term continuous electroplating high-end is easy to burn and peel; when the temperature is high, a higher current density can be used , but the covering ability and deposition rate of the plating solution are reduced; the temperature range of the solution is normal temperature to 60°C, and the preferred temperature range of hard chromium electroplating is 45 to 55°C, and the more preferred temperature range is 40 to 50°C.

When the solution is used for electroplating, the pre-treatment of degreasing, derusting and activation must be carried out.

Electroplating start method: In order to ensure a good bonding force between the coating and the substrate, a soft start method must be used for electroplating. The meaning of soft start in the present invention is that the applied current density gradually increases from the initial 0 ampere to the specified current density within 1 minute to 5 minutes. Or from the initial plating voltage of 0 volts, gradually adjust the voltage to the specified voltage within 1 minute to 5 minutes for plating.

Cathode movement and solution forced flow are not allowed:

In order to ensure a good bonding force between the coating and the base metal, during the electroplating process, the electroplating cathode (that is, the workpiece to be plated) cannot move, and the solution cannot use other methods such as circulating filtration and mechanical stirring that cause the solution to flow.

The anode is titanium alloy anode coated with iridium and tantalum.

Performance test data, the cathode current density exceeds 30%, the hardness of the coating is 870HV, and after heat treatment is 1058HV, the appearance of the coating is white and bright without defects, the neutral salt spray has not corroded for 240 hours, and the 300-degree thermal shock test has no cracks.

The power supply can be DC power supply or pulse power supply. The deposition speed of DC power supply is faster, and the deposition layer surface of pulse power supply is finer.

Embodiment 1 solution preparation

Using potassium chromium sulfate 230g/L, buffering agent 120g/L, complexing agent 30g/L, catalyst 20g/L, conductive salt 50g/L, prepare the solution according to the following method.

First, put two-thirds of the deionized water into the container and heat it to above 60°C, put the main salt into the deionized water while stirring, then put in the complexing agent, catalyst and buffer in turn, and wait until all are dissolved , reserved for later use; . Put less than one-third of the measured deionized water into another container and heat it above 60°C. Put the buffer into the deionized water in this container while stirring. After it is completely dissolved, put it in the second container. Pour the solution into the first container, heat it to about 95°C, and keep it warm for more than 2 hours. When the temperature naturally cools down to the process temperature, use deionized water to make up the volume, and the preparation of the trivalent chromium plating solution is completed.

Embodiment 2 Base material No. 45 steel shaft-shaped part (diameter 30*long 200mm)

Mechanical polishing——pre-chemical degreasing for 10 minutes—hanger, installation of shields—anode electrolytic degreasing for 5 minutes—secondary water washing—activation for 1 minute—secondary water washing—pure water washing—hard chrome plating (directly start to Specified current density) - secondary washing - pure water washing - unloading

The trivalent chromium plating is peeling off.

Embodiment 3 Substrate 65Mn steel shaft-shaped part (diameter 30*length 200mm, wherein the middle part of electroplating is 50mm long)

Mechanical polishing——pre-chemical degreasing for 10 minutes—hanger, installation of shields—anode electrolytic degreasing for 5 minutes—secondary water washing—activation for 1 minute—secondary water washing—pure water washing—hard chrome plating (mechanical stirring) ——Secondary water washing——Pure water washing——Unloading hanger

Plating cracked.

Embodiment 4: Base material 65Mn steel standard test piece (100*50*2mm)

Mechanical polishing——pre-chemical degreasing for 10 minutes—hanger and shield installation——anodic electrolytic degreasing for 5 minutes——secondary water washing——activation for 1 minute——secondary water washing—pure water washing—hard chrome plating (air stirring) ——Secondary water washing——Pure water washing——Unloading hanger

The coating is cracked and peeled.

Embodiment 5 Base material No. 45 steel shaft-shaped parts (diameter 30*long 200mm)

Process flow:

Mechanical polishing——pre-chemical degreasing for 10 minutes—hanger, installation of shields—anode electrolytic degreasing for 5 minutes—secondary water washing—activation for 1 minute—secondary water washing—pure water washing—hard chrome plating (soft start, Cathode kept still, solution without stirring)——secondary water washing—pure water washing—unloading tool—hydrogen removal treatment

Hard chromium electroplating conditions: 50°C, 90A, PH=1.4, electroplating for 20min, the coating is bright and uniform, the thickness reaches 44um, the hardness reaches 880HV (1050HV after heat treatment), and the bonding force passes the thermal shock test (heating at 200°C, quenching with salt water).

Implementation 6: Substrate 65Mn steel shaft-shaped parts (diameter 30*length 200mm, of which 50mm in the middle of partial electroplating) process flow:

Mechanical polishing—pre-chemical degreasing for 10 minutes—hanger, installation of shields, insulation treatment for non-plating positions—anodic electrolytic degreasing for 5 minutes—secondary water washing—activation for 1 minute—secondary water washing—pure water washing—electroplating Hard chromium (soft start, the cathode remains still, the solution is not stirred) - secondary water washing - pure water washing - hanger removal - hydrogen removal treatment

Hard chromium electroplating conditions: 50°C, 22A, PH=1.4, electroplating for 20min, the coating is bright and uniform, the thickness reaches 45um, the hardness reaches 870V (1048HV after heat treatment), and the bonding force passes the thermal shock test (heating at 200°C, quenching in salt water).

Embodiment 7: Base material 65Mn steel standard test piece (100*50*2mm)

Process flow:

Mechanical polishing——pre-chemical degreasing for 10 minutes—hanger, installation of shields—anode electrolytic degreasing for 5 minutes—secondary water washing—activation for 1 minute—secondary water washing—pure water washing—hard chrome plating (soft start, Cathode kept still, solution without stirring)——secondary water washing—pure water washing—unloading tool—hydrogen removal treatment

Hard chromium electroplating conditions: 50°C, 45A, PH=1.4, electroplating for 20 minutes, the coating is bright and uniform, the average thickness reaches 40um, the hardness reaches 890V (1030HV after heat treatment), and the bonding force passes the thermal shock test (heating at 200°C, quenching with salt water).

Claims (5)

1. a method for electroplating of trivalent chromium hard chromium, comprising the steps: a) Pre-treatment of the workpiece, the workpiece must be degreased, derusted and cleaned, and activated with dilute sulfuric acid; b) Put the trivalent chromium hard chromium electroplating solution after the workpiece is mounted; c) The start of electroplating must adopt soft start, that is, the time is 1 to 5 minutes, the potential is gradually changed from 0V to the specified potential; the current density is gradually changed from 0A to the specified current density; d) The electroplating parameters control the current density range to 25-75A/dm ² ; e) The temperature of the electroplating solution is controlled at normal temperature to 60°C; f) During the electroplating process, the electroplating cathode cannot move, and the solution cannot adopt the method of circulating filtration and mechanical stirring to force the solution to flow; the trivalent chromium hard chromium electroplating solution mainly includes trivalent chromium sulfate, complexing agent, catalyst and buffering agent , wherein the trivalent chromium sulfate can be chromium sulfate or chromium potassium sulfate, and the concentration of trivalent chromium ions in the solution is 23.9-40g/L; the complexing agent can be nitrilotriacetic acid, oxalic acid, malic acid, glycine, formic acid, acetic acid Or its sodium salt, potassium salt, containing at least one of the complexing agents, the content is 1-60g/L; the catalyst can be potassium bromide, potassium fluoride, ammonium fluoride or sodium fluoride, containing at least one of the catalysts , the content is 5-50g/L; the buffering agent can be aluminum sulfate, boric acid, phthalic acid, at least one of them is contained, the content is 120-150g/L; the pH value of the solution is 1.2-2.1. 2. the electroplating method of trivalent chromium hard chrome as claimed in claim 1, is characterized in that, the preparation method of described trivalent chromium hard chromium electroplating solution is: first put into container the deionized water of measuring 2/3rds and heated to above 60°C, add the main salt into the deionized water while stirring, then put in the complexing agent, catalyst and buffering agent in turn, and keep it for later use after all are dissolved; Put the deionized water into another container and heat it to above 60°C. Put the buffer into the deionized water in this container while stirring. After it is completely dissolved, pour the solution in the second container into the first container and heat it. Keep warm at about 95°C for more than 2 hours. When the temperature naturally cools down to the process temperature, use deionized water to make up the volume, and the electroplating solution is prepared. 3. The method according to claim 1, characterized in that the current density in step (4) is 35-60A/dm ² . 4. The method according to claim 1, characterized in that the temperature control range in step (5) is 40-55°C. 5. The method according to claim 4, characterized in that the temperature control range is 45-50°C.