I. Theoretical Overview
Chemical coating, also known as “autocatalytic coating”, is a process that does not depend on external electrical current.
Instead, it uses the reducing agents in the coating solution to carry out an oxidation-reduction reaction.
This process, in turn, results in the continuous deposition of metal ions on the metal surface, facilitated by the catalytic effect of the surface.
Because chemical coating occurs only on materials with autocatalytic properties, methods that derive metallic coatings from displacement reactions or other chemical reactions, rather than autocatalytic reduction reactions, are not defined as chemical coating.
Chemical coating is a technique that, based on the principle of oxidation-reduction reactions, uses strong reducing agents in a solution containing metal ions to reduce the ions to metal and deposit them on various material surfaces, forming a dense coating layer .
Common solutions used in chemical plating include silver, nickel, copper, cobalt, nickel phosphorus, and nickel boron phosphorus.
Chemical nickel plating involves using a reducing agent to reduce nickel ions in solution and deposit them on a catalytically active surface.
Various reducing agents can be used in chemical nickel plating, but the most commonly used industrial process employs sodium hypophosphite as the reducing agent.
The widely accepted reaction mechanisms are the “atomic hydrogen theory” and the “hydride theory”.
Chemical coating is a new metal surface treatment technology. Its simplicity, energy efficiency and respect for the environment have attracted more and more attention. The application of chemical coating is extensive, providing uniform layers of gold with good decorative attributes.
It increases the corrosion resistance and service life of products in terms of protective performance and elevates the wear resistance, electrical conductivity, lubrication and other specialized functionalities of processed parts, becoming a growing trend in global heat treatment technologies. surface.
Chemical coating technology facilitates metal deposition through controllable oxidation-reduction reactions under the metal catalyzing effect.
Compared to electroplating, electroplating offers advantages such as uniform coating, minimal holes, no need for direct current power supply, depositability on non-conductors and certain special properties.
Furthermore, due to reduced waste discharge, less environmental pollution and good cost-effectiveness, chemical coating is gradually replacing electroplating in many areas, becoming an environmentally friendly surface treatment technique.
It is currently widely used in a variety of industries, including electronics, valve manufacturing, machinery, petrochemicals, automotive, and aerospace.
The application of chemical nickel plating technology in the microelectronics manufacturing industry is growing rapidly.
Reports suggest that Xerox Corporation has adopted selective nickel-phosphorus alloy chemical coating technology in the planarization process of filling interconnects and through holes in multilayer chips of ultra-large-scale integrated circuits. Its products have passed tests for shear strength, tensile strength, high and low temperature cycles and various electrical performances.
These practices indicate that the application of chemical nickel plating technology improves the technical and economic aspects of microelectronics manufacturing and increases product reliability.
II. Project implementation plan
(1) Preparation of electroplating solution
| Galvanizing solution composition and process conditions | Happy |
| Nickel Sulfate (NiSO 4 ·7H 2 O) /g·L -1 | 26 |
| Sodium hypophosphite (NaH 2 PO 2 ·H 2 O) /g·L -1 | 28 |
| Sodium Acetate (CH 3 COONa) /g·L -1 | 10 |
| Propionic Acid (CH 3 CH 2 COOH) /g L -1 | two |
| Lactic Acid (C 3 H 6 O 3 ) /g L -1 | 33 |
| Sulfa /mg.L -1 | 4 |
| PH value | 4.5 |
| Temperature /℃ | 85 |
| Deposition rate /μm·h -1 |
(2) Electroplating solution stability test
The stability of the nickel chemical solution is measured using a boiling method. Two hundred milliliters of the solution are boiled on an electric stove. After boiling for a certain period, the solution is evaluated for decomposition.
If no decomposition occurs after boiling for 30 minutes, this indicates good stability of the solution.
Otherwise, the stability of the solution is poor. The initial solution coating temperature is 60°C. The veneered surface has a level three gloss (semi-gloss). The adhesion of the coating is good, with corrosion resistance of 30~180s and porosity of 16cm².
After treatment by the chemical precipitation method, the filtered water becomes colorless. Gradually increase the temperature of the coating solution from a lower point.
When heated to a certain temperature, a prepared iron sample is immersed into the plating solution. If a reaction (bubble overflow) occurs, this indicates that the chemical plating reaction has started at that temperature, which is the initial plating temperature.
In this project, the heating temperature is divided into five levels: 50°C, 60°C, 70°C, 80°C and 90°C.
(3) Sample surface pretreatment
Steel and copper parts are polished with sandpaper to remove surface oxides and other impurities.
(4) Chemical Nickel Plating
Sample preparation – Mechanical polishing – Organic solvent degreasing – Chemical degreasing – Hot water washing – Cold water washing – Activation – Cold water washing – DI water washing – Chemical nickel plating – Water washing – Air drying
(5) Galvanizing performance test
Plating Performance:
Visual inspection of the galvanized layer on metal parts is the most basic and commonly used method. Galvanized parts that fail visual inspection do not need to undergo additional testing. Inspections are carried out visually and, based on appearance, galvanized parts can be classified as acceptable, defective or waste.
Surface defects include holes, blemishes, pimples, blisters, peeling, peeling, shadows, stains, burns, dark areas, dendritic and spongy deposits, and areas that should be coated but are not.
Testing for Coating Surface Defects
Types and characteristics of defects: The coating surface must not present defects such as holes, stains, peeling, burrs, bubbles, stains, pimples, shadows, cloudy areas, burns, dendritic and spongy coatings. During testing, these must be strictly differentiated. Below is a brief introduction to its features for visual assessment.
Coating porosity test
Coating Corrosion Resistance Test
(6) Liquid Waste Treatment (Chemical Precipitation Method)
Collect the residual liquid → Heat → Add 15% sodium hydroxide until the pH of the residual liquid is between 10 and 12 → Stir and maintain the temperature for 1 hour → Add precipitant → Filter → Cool to 50 degrees Celsius and then , use dilute sulfuric acid to adjust the pH of the solution to 8.0 → Add Ca(ClO)2 powder (the ratio of Ca(ClO)2 to total P is 3.5:1.0) → Stir for 2 hours → Add appropriate amount of precipitant → Precipitate and filter.
Chemical precipitation is a common method for treating heavy metal wastewater. When the pH of the aging liquid is adjusted above 8 using caustic soda, lime, or sodium carbonate, Ni(OH) 2 may be formed. After decantation, the residue can be separated, achieving the purpose of removing nickel from the aging liquid.
In addition, iron sulfide, insoluble yellow starch xanthate (ISX) and others can also be used as precipitants for nickel wastewater treatment. The above research is generally for the treatment of nickel wastewater with a nickel concentration of less than 500/mg L -1 .
The phosphorus in the electrolyte-free nickel plating aging liquid can be treated by chemical oxidation precipitation method, that is, oxidants such as potassium permanganate and hydrogen peroxide are used to destroy the chromium complex in the plating solution and oxidize hypophosphite and others to phosphate.
Then, the phosphate salt is precipitated with a precipitant to reduce the total discharge of phosphorus into the wastewater. Nickel and phosphorus wastewater treatment by chemical precipitation will produce a large amount of waste.
If not handled properly, it will cause secondary pollution. Currently, there is no better method to treat waste than burying it.
III. Advantages of electroless nickel plating:
1. Gloss of the coating
Compared with light yellow electroplated nickel, most electroless nickel platings are silvery white, with excellent resistance to discoloration, and the brightness can be maintained for a longer time.
For nickel-phosphorus electrolytic coating, the brightness of the coating increases with increasing phosphorus content.
After adding a certain amount of brightener to the electroplating solution, the reflectivity of the coating can reach more than 80%. Recent studies show that the brightness of the nickel-copper-phosphorus-free alloy is good and has greater resistance to discoloration.
2. Coating Hardness
The hardness of electroplated hard chrome is 960HV. Its hardness drops drastically when heated. The hardness of a chemically plated nickel layer, after being heat treated at 400°C for 1 hour, can reach 1100HV.
Furthermore, there is little change in the hardness of the coating from room temperature to 400°C.
Therefore, chemically plated nickel is a heat-resistant coating, suitable for use in conditions where friction generates heat, while electroplated hard chrome can only be used at room temperature.
3. Wear resistance
For medium-high phosphorus coatings, after suitable heat treatment, nickel-phosphorus alloy coatings have good self-lubricating properties. Coatings with low phosphorus content have greater hardness.
However, wear resistance tests show that the wear resistance of high-phosphorus coatings is higher than that of low-phosphorus alloy coatings.
In order to improve the wear resistance of nickel-phosphorus coatings, highly hard tungsten (W) is added to the nickel-phosphorus coating to form a ternary alloy coating, which significantly improves its wear resistance.
4. Corrosion resistance
Chemically plated nickel is a uniform amorphous structure. It does not present defects such as grain boundaries, dislocations and stacking faults. Each substrate is densely bonded, making it difficult for corrosive media to pass through the bonding interface to corrode the substrate metal, resulting in better corrosion resistance than chrome plating.
Furthermore, chemically coated nickel is almost unaffected by corrosion from seawater, saltwater and freshwater. Its corrosion resistance in HCL and sulfuric acid is better than that of stainless steel, and it can withstand corrosion from various media such as high-concentration caustic soda, hydrogen sulfide, lactic acid and more.
5. Weldability
The main application of chemically plated nickel in the electronics industry is in discrete devices. This not only requires the coating to have good wear resistance, corrosion resistance and electrical contact performance, but also good weldability.
For example, the weldability of aluminum heat sinks in generators is poor. However, by coating a 7-8μm layer of chemically plated nickel on the surface of the aluminum substrate, its solderability can be improved, solving the connection problem between the aluminum heat sink and the transistor.
Additionally, chemically coated nickel can be used in high-energy microwave devices, connectors, and underwater communications components. Generally, the weldability of chemically coated nickel is measured by the expanded area method. A φ1.5 mm welding wire is placed on the coating surface.
After heating at 400°C in a hydrogen-nitrogen gas mixture for 30 minutes, the expanded area is measured to determine the relationship between weldability and phosphorus content in the coating. The larger the diffusion area, the better the weldability of the coating.
4. Equipment and Materials
A constant temperature water bath with two holes, four 100 mL beakers, three 200 mL beakers, one 500 mL beaker, one each of 10 mL and 50 mL graduated cylinders, a 0.001 g electronic balance (or 0 .0001g), a 0.2 gram balance, a micrometer, a pH meter, a 1000 watt electric oven, two thermometers, a wash bottle, a set of tongs, filter paper, iron rings for support and retort stand, glass funnel, glass rod, medicine spoon, semi-automatic burette, pipette, ear bulb, test tube brush, acid-resistant gloves, a saw blade, half a sheet of sandpaper (No. 100 ~ 800) , four No. 1 dry batteries.
Chemicals: ammonium purpurate, sodium chloride, sodium hydroxide, EDTA, sodium carbonate, sodium phosphate, OP-10, hydrochloric acid, sulfuric acid, nitric acid, nickel sulfate, monosodium phosphate, propionic acid, acetate sodium, lactic acid, sodium dodecylbenzenesulfonate, thiourea, potassium ferricyanide, calcium hydroxide, hydrogen peroxide, sodium tungstate, coagulants such as polyaluminum chloride, various samples of steel and copper.
V. conclusion
The basic principles and process of electroless nickel plating have been summarized. The stability of the electrolyte-free nickel plating solution and the initial plating temperature were tested.
Electrolytic nickel plating tests were performed on the surfaces of steel and copper samples; The appearance, porosity, corrosion resistance, thickness, adhesion, brittleness, hardness and other properties of the electrolyte-free nickel plating layer were tested.
The residual nickel plating solution without electrolyte was treated using the chemical precipitation method. The process and experiment results indicate that the electrolytic nickel plating solution used in this experiment has excellent stability and does not decompose even in a boiling state for 30 minutes.
