In metalworking laboratories, the gradual growth of a red metal layer on the surface of the cathode as
current passes through a blue solution is always fascinating. This phenomenon, known as electrodeposition,
is not only the basis of industrial electroplating, it is an excellent example for understanding
electrochemical reactions. In this paper, we will take the electrolytic deposition of copper as
the core, systematically analyze the whole process of metal growth on the surface of the
electrode, and reveal the wonderful transformation from copper ions in solution to solid metal.
Microscopic mechanism of copper deposition
When direct current passes through a copper sulfate solution, the Cu²⁺ ions in solution migrate toward the
cathode driven by the electric field. After reaching the electrode surface, each copper ion captures two
electrons (Cu²⁺ + 2e- → Cu), completing the transition from the ionic state to the metallic state. This
process is expressed crystallographically:
Nucleation stage: random electron collisions cause copper ions to form an initial nucleus on the
electrode surface
Lattice growth: subsequent copper atoms are arranged in an orderly manner along a specific
lattice direction.
Three-dimensional stacking: Layers grow to form a dense metallic structure.
This deposition process is characterized by significant directionality, and typical (111) faceted optimum
growth can be observed under the microscopic electron microscope. The deposition rate is directly
controlled by the current density, which is usually kept at 0.5-2A/dm² to obtain a uniform coating.
The five major factors affecting deposition quality
Electrolyte concentration
When the concentration of CuSO₄ is lower than 0.1 mol / L, the deposited layer is prone to porous
structure; concentration of more than 1.5 mol / L may produce dendrites. The optimum working
concentration is usually controlled in the range of 0.5-1.2 mol/L.
Solution pH
Acidic environment (pH=1-3) is conducive to improve the conductivity of the solution, but too
low a pH will trigger the precipitation of hydrogen side reaction, resulting in pinholes in the
coating. The pH can be stabilized by adding a boric acid buffer.
Temperature control
The temperature range of 20-50°C ensures that the ion migration rate is balanced with the
deposition rate. Every 10°C increase in temperature increases the deposition rate by about
15%, but over 60°C accelerates the decomposition of the solution.
Role of additives
Trace amount of gelatin (0.1-0.5g/L) can refine the grain, and thiourea type brightener can
make the plating layer show a mirror effect. These organic molecules change the crystallization
process by adsorbing on the electrode surface.
Current waveform regulation
Compared with direct current, pulse current can improve the density of plating layer, and reverse
current can effectively eliminate deposition stress. Modern electroplating equipment mostly
adopts periodical commutation current technology (PRC).
Phenomenon Observation Guide for Deposition Process
Initial stage (0-5 minutes)
Star-shaped red spots appear on the surface of the cathode, and the solution interface produces slight
convection. At this time, the thickness of the deposited layer is about 2-5μm, and the metallic
luster is visible to the naked eye.
Stable deposition period (10-30 minutes)
The plating layer grows uniformly at a speed of 0.1-0.3μm/s, and oxygen bubbles begin to precipitate
at the anode. The blue color of the solution gradually becomes lighter, and color layering appears
around the electrode.
Identification of abnormal phenomena
Black spots: solution impurity contamination
Plating flaking: Incomplete pre-treatment or sudden change of current
Haze-like surface: decomposition of organic additives
Edge darkening: uneven current distribution
Key Technologies for Industrial Applications
PCB Manufacturing
20-30μm copper layer is deposited on the substrate through graphic plating, and the line
accuracy can reach ±5μm. Acidic copper sulfate system with photoresist technology is
used to realize micron-level circuit molding.
Decorative Plating
Depositing a 0.2-0.5μm copper layer as an intermediate layer on top of the underlying nickel
layer not only improves the surface gloss, but also enhances the adhesion of the subsequent
plating layer. This process is used for automobile wheel plating.
Metal purification
Crude copper anode is dissolved in electrolytic refining, and electrolytic copper of 99.99%
purity is deposited at the cathode. The current efficiency of a modern copper electrolyzer
can reach 98%, with a power consumption of about 220kWh per ton of copper.
3D printing assistance
Electrolytic deposition is combined with 3D printing to directly mold metal structures on
complex curved surfaces. The latest research shows that the conductivity of copper components
prepared by this method can reach 95% of pure copper.
Suggestions for operational optimization of teaching experiments
Electrode pretreatment
Use sandpaper to polish the carbon rod to the surface roughness Ra ≤ 0.8μm, hydrochloric acid
immersion to remove oxides. Copper anode needs to be alkaline washing → acid washing →
activation of the three-step treatment.
Solution preparation technique
Dissolve copper sulfate in deionized water at 60℃ first, and then add sulfuric acid after cooling.
Suggested ratio is: CuSO₄-5H₂O 200g/L + H₂SO₄ 50g/L.
Parameter setting reference
Student experiment: voltage 3V, time 10 minutes, pole distance 4cm
Quantitative study: current density 1A/dm², temperature 25±2°C
Plating characterization: use digital calipers to measure the thickness, magnifying glass to
observe the surface morphology
Safety Precautions
Wear acid-proof gloves when configuring the solution, and keep the electrolysis process ventilated.
Waste solution should be neutralized with lime to pH=6-9 before discharge.
Frequently Asked Questions
Q：Why are there streaks in the sedimentary layer?
A: This is usually caused by uneven convection of the solution, which can be improved by
magnetic stirring or increasing the temperature to 40℃.
Q：How to get mirror effect plating layer?
A: Add 0.02g/L sodium polydisulfide dipropane sulfonate to the solution and reduce the current
density to 0.8A/dm².
Q: What should I do if the deposition rate suddenly drops?
A：Check whether the anode is passivated, remove the electrode and clean it with 10% nitric acid if
necessary. If the solution copper ion concentration is too low, need to supplement copper sulfate.
Electrolytic deposition of copper is like a microcosmic architectural feast, each copper atom is a carefully
stacked masonry. From the glass beaker in the laboratory to the giant electrolyzer in the factory, this
technology has always sought a balance between precision and efficiency. Understanding the nature
of the deposition process not only enhances the effectiveness of laboratory teaching, but also provides
basic theoretical support for the development of new materials. When we see the familiar red color at
the cathode, what we witness is not only the transformation of the metal form, but also the wisdom of
human beings to master the microscopic particles.