Ⅰ Principle of chemical plating
Chemical plating is in the absence of current through (no external power) with the help of reducing agent in the same solution of redox, so that the metal ion reduction deposited on the surface of the parts of a plating method. Therefore, chemical plating can be described as a kind of metal deposition, controllable, self-catalyzed chemical reduction process.
Ⅱ the characteristics of chemical plating
With the development of industry, chemical plating has become a process technology with great prospects for development, compared with other plating methods, chemical plating has the following characteristics:
1 ）Can be made of metals, semiconductors and non-conductors and other materials such as metal plating on the parts.
2 ）Regardless of the complexity of the geometry of the part, wherever the solution can be contacted, a uniform thickness of the coating can be obtained.
3 ）For self-catalyzed chemical plating, it is possible to obtain a large thickness of plating and even electroforming.
4 ）No current is required.
5 ）The plating layer is dense with few pores.
6 ）The plated layer often has special chemical and mechanical properties.
Because of the many advantages of chemical plating, chemical plating is becoming more and more rapid in the industrial and electronic industries. The following is an example of nickel plating:
(1) Chemical nickel plating due to the use of sodium hypophosphite as a reducing agent, so the coating generally obtained for nickel-phosphorus alloy.
According to the different phosphorus content can be divided into low phosphorus (1%-4%) in phosphorus (4%-10%) and high phosphorus (10%-12%). From different PH value of the plating solution can be obtained from different phosphorus content of the plating layer, in the PH = 4-5, can be obtained in the phosphorus and high phosphorus alloy. Low-phosphorus and medium-phosphorus alloys can be obtained from weakly alkaline solutions at pH=8-10.
NI-P alloys containing more than 8% phosphorus are amorphous coatings with particularly good corrosion resistance due to the absence of grain boundaries.
Through heat treatment 300-400 degrees into a mixture of amorphous and crystalline hardness of up to HV = 1150, chemical plating heat treatment hardness close to the hardness of hard chrome, is the ideal alternative to hard chrome layer plating, but also in the application of aluminum on the good plating (can be suitable for all aluminum and aluminum alloy products, including forging, die casting, aluminum profiles, etc.).
Disadvantage: The plating is brittle, but the plasticity of the product is greatly improved after heat treatment. ( Generally, the plasticity will be improved by heat treatment of more than 400 degrees for four hours. )
(2) Aluminum chemical nickel plating process:
Ultrasonic degreasing → De-oxidation film → First zinc replacement → Harmonization → Second zinc replacement → Chemical nickel plating → Passivation → Ultrasonic Cleaning → Baking → Full inspection → Packaging
(3) Removal of unqualified plating layer:
Steel, aluminum, copper, plastic and other chemical nickel degradation can be concentrated nitric acid degradation. But the parts should be dried into the tank, if the moisture will lead to excessive corrosion of the base metal. The working temperature is below 35 degrees.
(4) Quality requirements:
a, film thickness in the range of 5-30um
b, salt spray test to 24 hours or more, up to CNS standard 9 or more.
c, through the 60-degree bending test, no peeling phenomenon
d, the appearance of no bruises, scratches, water stains and other bad phenomena.
e, the film layer is uniform, there can be no fog, white spots, dew plating phenomenon.
Ⅲ Common Exception Handling
| Abnormal phenomenon | Possible causes and methods of correction |
| Slow reaction and low deposition rate | 1) low pH, adjust with NH4OH; 2) low temperature; 3) insufficient sodium hypophosphite, analyze and supplement. |
| The plating solution reacts vigorously, boils, and dark gray nickel powder appears (a sign of natural decomposition of the plating solution). | 1) Too much loading; 2) Too high temperature or local overheating plating solution dropped into the metal chips or solid particles; 3) Too high hypophosphite content; 4) Too high PH value. |
| Nickel metal deposited on walls and bottoms of tanks | 1) localized high temperature; 2) groove lining broken, revealing the base metal, replace the groove. |
| Nickel phosphite precipitation occurs when phosphite in the plating solution does not reach the rated level. | 1) PH is too high; 2) Insufficient complexing agent, should be given supplemental. |
| Dullness of plating | The plating solution is contaminated by zinc, copper and other metal impurities, can be used to deal with low current density, or appropriate supplementation of sodium hypophosphite with the waste parts for processing, until the plating layer is normal as well. |
| Plating with pitting | Hydrogen stays on the part, which can be avoided by stirring the tank or shaking the workpiece. |
| The plating is easy to peel off | 1) Incomplete treatment before plating; 2) Temperature fluctuation is too large. |
