Chemical plating on metal surfaces
Chemical plating refers to a surface processing method that uses chemical methods to reduce metal ions in a solution to metals without external current passing through, and deposits them on the substrate surface to form a coating.
During chemical plating, the electrons required for reducing metal ions are directly generated in the solution through chemical reactions. There are three ways to complete the process.
1. Replacement sedimentation
By utilizing the potential of the deposited metal M ₁ (such as Fe) to be more negative than that of the deposited metal M ₂ (such as Cu), the deposited metal ions are displaced from the solution onto the surface of the workpiece. This method is called immersion plating in engineering. When metal M ₁ is completely covered by metal M ₂, deposition stops, so the coating is very thin. Iron immersion copper plating, copper immersion mercury plating, and aluminum galvanizing are such displacement deposits. Immersion plating is difficult to obtain practical coatings and is often used as an auxiliary process for other plating types.
2 Contact deposition
In addition to the plated metal M ₁ and the deposited metal M ₂, there is also a third metal M Å. In a solution containing M ₂ ions, the two metals M Å - M Å are connected, and electrons flow from the high potential M Å to the low potential M Å, causing M ₂ to be reduced and deposited on M Å. When the contact metal M ₁ is completely covered by M ₂, deposition stops. When electroless nickel plating is carried out on functional materials without self catalysis, contact deposition is commonly used to initiate nickel deposition.
3 Reduction sedimentation
The process of reducing metal ions to metal atoms by free electrons released by the oxidation of reducing agents is called reduction deposition.
The chemical plating referred to in engineering mainly refers to this type of reduction deposition chemical plating.
The conditions for chemical plating are as follows:
The reduction potential of the reducing agent in the plating solution should be significantly lower than the potential of the deposited metal, making it possible for the metal to be reduced and deposited on the substrate.
The prepared plating solution does not undergo spontaneous decomposition, and only when in contact with the catalytic surface does the metal deposition process occur.
When adjusting the pH value and temperature of the solution, the reduction rate of the metal can be controlled to adjust the plating rate.
The metal precipitated by reduction also has catalytic activity, so that the redox deposition process can continue and the coating can continuously thicken.
The reaction products do not hinder the normal progress of the plating process, that is, the solution has sufficient service life.
There are many types of metals and alloys for chemical plating, such as Ni-P, Ni-B, Cu, Ag, Pd, Sn, In, Pt, Cr, and various Co based alloys. However, the most widely used are chemical nickel plating and chemical copper plating. Chemical coatings generally have good corrosion resistance, wear resistance, brazing resistance, and other special electrical or magnetic properties, so this surface treatment process can effectively improve the surface properties of materials.