Plating solution composition: acid copper plating, nickel plating, zinc plating, etc. (e.g., providing electrical conductivity and pH adjustment in sulfate copper plating solution). Pre-treatment: for descaling and activating metal surfaces (e.g. pickling of steel parts). Characteristics: Strong acid, highly corrosive, extremely absorbent. When diluting, the acid should be added to the water s
What are the common acids used in electroplating production?
Author: Robby
In electroplating production, a variety of acids are used in different process steps, such as pre-treatment, plating solution preparation, activation, and de-plating. The following are the commonly used acids in electroplating and their main uses, characteristics and precautions:
1. Sulphuric acid (H₂SO₄)
Uses:
Plating solution composition: acid copper plating, nickel plating, zinc plating, etc. (e.g., providing electrical conductivity and pH adjustment in sulfate copper plating solution).
Pre-treatment: for descaling and activating metal surfaces (e.g. pickling of steel parts).
Characteristics:
Strong acid, highly corrosive, extremely absorbent.
When diluting, the acid should be added to the water slowly to avoid violent exothermic splashing.
Precautions:
Acid-resistant equipment is required (e.g. PP/PVC tank).
Waste liquid needs to be neutralised (separate recovery is required when heavy metal ions are contained).
2. Hydrochloric acid (HCl)
Uses:
Pre-treatment: Removal of oxides (e.g. pickling of steel parts, activation of stainless steel).
Plating solution composition: chloride zinc plating, tin plating, etc. (provide Cl- to promote anode dissolution).
Characteristics:
Highly volatile, produces irritating HCl gas, requires ventilation and protection.
Passivation effect on stainless steel, titanium and other metals.
Precautions:
Avoid mixing with oxidising agents (e.g. nitric acid), toxic chlorine gas may be produced.
3. Nitric acid (HNO₃)
Uses:
De-plating: Stripping off undesirable plating (e.g. de-chroming, de-nickeling).
Passivation: passivation of stainless steel or galvanised layers (to improve corrosion resistance).
Characteristics:
Strong oxidising acid, may cause explosions in contact with organic materials.
Produces toxic NOx gas (exhaust gas treatment system required).
Precautions:
Use of oxidising acid resistant materials (e.g. HDPE or PTFE lined equipment) is required.
4. Phosphoric acid (H₃PO₄)
Uses:
Phosphate treatment: to generate phosphate conversion film on the metal surface (such as iron system, zinc phosphating).
Plating solution conditioning: pH buffer for chemical nickel plating.
Characteristics:
Medium strong acid, low corrosivity, corrosion inhibition.
Precautions:
Phosphate slag should be filtered and cleaned regularly.
5. Hydrofluoric acid (HF)
Uses:
Pre-treatment: remove silicates, aluminium oxide (such as activation of aluminium parts before zinc dipping).
Titanium/zirconium alloy treatment: dissolve surface oxide film.
Characteristics:
Highly toxic, corrodes glass and silicate materials, penetrates skin (strict protection required).
Precautions:
PP/PTFE equipment must be used, operators need to wear anti-HF gloves and face masks.
6. Chromic acid (H₂CrO₄, in the form of CrO₃)
Uses:
Chromium plating: main component of hexavalent chromium chromium plating solution (to be used in conjunction with sulphuric acid).
Passivation: coloured/blue-white passivation of zinc plating.
Characteristics:
Strongly oxidising, carcinogenic (hexavalent chromium is a high risk pollutant).
Precautions:
Need to comply with the strict emission standards of GB 21900-2008, the waste liquid needs to be reduced (hexavalent chromium → trivalent chromium).
7. Boric acid (H₃BO₃)
Uses:
Buffer: Stabilise pH in nickel plating solution (maintain 3.5~4.5).
Characteristics:
Weak acid, solubility increases with temperature.
Cautions:
Excessive amount will lead to increased brittleness of plating layer.
8. organic acid (such as citric acid, acetic acid)
Uses:
Environmental alternative: complexing agent for cyanide-free gold and silver plating.
Deprocessing: Gentle removal of plated layers (e.g. citric acid for nickel).
Characteristics:
Low corrosiveness, easy to treat waste water.
Safety and Environmental Points
Ventilation system: Hydrochloric acid, nitric acid, hydrofluoric acid and other volatile acids need to be equipped with induced draft fan + exhaust gas treatment (such as spray tower).
Anti-corrosion materials: acid tanks need to be made of PP/PVC/PTFE to avoid metal corrosion.
Waste liquid classification: heavy metal-containing (e.g. chromic acid, nickel-plating waste liquid) needs to be recycled separately, and mixed discharge is prohibited.
Personal protection: acid-resistant gloves, goggles, gas mask (especially HF, NOx occasions).
Summary
The choice of acid for plating depends on the process requirements (e.g. degreasing, activation, bath formulation) and the type of substrate (steel, aluminium, copper, etc.). Strong acids (sulphuric acid, hydrochloric acid) are mostly used for pre-treatment, while chromic acid and phosphoric acid are used for functional treatment. With the increase of environmental protection requirements, alternative technologies such as chromium-free passivation and organic acid plating solutions are gradually being promoted.