Electroplating is a process of depositing a layer of one metal on the surface of another metal by taking the help of electric current.
The part to be plated is made the cathode (negative electrode) and the material to be deposited is made the anode (positive electrode). There is an electrolyte which conducts electricity. When electric current passes through the system, positive metal ions flow from the anode and are deposited on the cathode
In zinc electroplating, zinc metal is used as an anode.
Plating is considered a special process, i.e. it is not possible to check for plating properties on the finished article without non destructive testing. Which means that to get a good plated parts, the plating conditions must be controlled very carefully.
Different Types of Electroplating
Zinc Plating
Zinc plating is an electroplating process in which a thin layer of zinc is deposited on the surface of steel components. It is cost-effective, easy to do, and provides corrosion resistance by two methods:
Zinc covers the steel and hence prevents exposure of the base metal to moisture and oxygen
Zinc is more electropositive than iron, which means it loses electrons more easily, and so it more reactive and provides sacrificial protection to iron / steel. This means that even if there is a small hole in the zinc coating, it will not let the exposed steel rust easily but will sacrifice itself first.
Passivation
Since zinc is highly reactive, it tends to get oxidized easily. Hence a passivation coating is done to protect the zinc itself from getting oxidised. Different passivation colours provide varying levels of salt spray life.
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Types of Zinc Plating Based on Passivation
Trivalent Zinc Plating
Available in Blue / Clear, Yellow, Iridescent, and Black colours RoHS and REACH compliant
Hexavalent Zinc Plating
available in Yellow and Olive Green colours
Not acceptable under RoHS and REACH as it is hazardous to the environment and health
Limitations of Zinc Plating
It does not withstand high-temperature
There is a risk of hydrogen embrittlement in high-strength steels
Applications of Zinc Plating in Sensors
Zinc plating is extensively used in sensor components such as housings, brackets, Fasteners and covers
Key Steps in Zinc Electroplating
- Cleaning: Remove all oils, dirt, rust, grease, and scale from the parts using alkaline cleaners, ultrasonic baths, or solvents. This is important as contaminants block adhesion of plating.
- Rinsing: Thoroughly rinse parts cleaning
- Acid Dipping: Dip in an acid bath (such as hydrochloric, sulfuric, or nitric acid) to activate the surface
- Rinsing: Thoroughly rinse parts acid dip
- Electroplating: Carry out the plating itself in a tank
- Rinsing: Rinse thoroughly after plating.
- Passivation / Chromating: Apply a chromate passivation coating
- Sealing: A topcoat sealant can be added for better salt spray life
- Drying: this can be done in a open atmosphere, an oven or even a conveyorized oven
Plating Solution
Composition: Contains zinc ions in the form of zinc oxide or sodium zincate. Additives such as brighteners, wetting agents, and pH buffers help in improve appearance and uniform thickness.
Additional Plating Types Used in Sensors
1. Zinc-Nickel Plating
Zinc-nickel plating usually contains 12–16% nickel and provides much better corrosion resistance compared to pure zinc. It provides uniform thickness even on complicated shapes and is preferred for safety and emission sensors. However, nickel is much more expensive than zinc and so the cost is higher than zinc plating.
2. Nickel Plating
Nickel plating is used where wear resistance, hardness, and conductivity are of importance. It is used in parts such as connector pins, shielding components, and signal contact surfaces. This is indeed very costly.
3. Electroless Nickel (EN) Plating
Electroless nickel is a chemical deposition process that does not involve electrical current at all. It provides uniform coating on complex shapes and has high hardness, wear resistance, and good corrosion protection. This is also quite expensive.
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Frequently Asked Questions in Plating of Sensors
Q1. What are some of the common mistakes which occur in plating lines?
As stated earlier, plating is a special process, and requires good control of plating conditions. Some of the common mistakes are incorrect temperature control, inadequate agitation, improper current density settings, not using flowing water for rinsing, etc.
Q2. What is Hydrogen Embrittlement?
Hydrogen embrittlement is a failure mode in which high-strength steels become brittle and crack. This happens due to the absorption of hydrogen atoms during zinc plating processes.
During electroplating, hydrogen is generated at the cathode and hydrogen atoms can diffuse into the steel part. This can result in a reduction in ductility and toughness, which in turn may result in cracking. Often such cracking is delayed.
Hydrogen embrittlement often affects high-strength steels, especially carbon steel and alloy steel.
Hydrogen embrittlement can be avoided by minimizing acid exposure time, using inhibitors in acid baths, and maintaining the current density during plating.
