Electrolytic Phosphating (Electrolytic Phosphating) is the process of forming a phosphate film on a metal surface by electrochemical methods, which is more complicated than chemical phosphating, but the film formation speed is faster and the film layer is more uniform. The following is a typical electrolytic phosphating process and key control parameters:
Author: Robby
Electrolytic Phosphating (Electrolytic Phosphating) is the process of forming a phosphate film on a metal surface by electrochemical methods, which is more complicated than chemical phosphating, but the film formation speed is faster and the film layer is more uniform. The following is a typical electrolytic phosphating process and key control parameters:
1. Pre-treatment (pretreatment)
Similar to chemical phosphating, the metal surface needs to be thoroughly cleaned to ensure uniform adhesion of the phosphate film:
Degreasing: alkaline or neutral degreasing agent to remove oil (electrolytic degreasing is more effective).
Water rinse: Rinse off residual degreaser with water (deionised water is preferred).
Acid washing (optional): For workpieces with serious corrosion, use dilute hydrochloric acid or sulphuric acid to remove the oxidised skin.
Surface adjustment (optional): titanium salt or colloidal titanium phosphate solution to activate the surface, improve the denseness of the phosphate film.
2. Electrolytic phosphating treatment
(1) electrolyte composition
Main ingredient:
Phosphate (such as Zn²⁺, Mn²⁺, Ca²⁺, etc.).
Oxidising agents (e.g. nitrates, chlorates) to promote film formation.
Additives (e.g. nickel, fluoride) improve film properties.
pH value: usually controlled at 2.0~3.5 (acidic environment).
Temperature: 20~50℃ (some processes need to be heated).
(2) Electrolysis parameters
Parameter Typical range Function
Current density: 0.5~5 A/dm² Influence on film forming speed and densification
Voltage: 5~20 V (DC) Adjusted according to the shape of the workpiece.
Time: 1~10 minutes Film thickness increases with time.
Electrode configuration: Workpiece as anode or cathode Anodic phosphating (good corrosion resistance) / Cathodic phosphating (high hardness)
(3) Process type
Anodic phosphating:
The workpiece is used as the anode to produce a film with good corrosion resistance, but may be accompanied by metal dissolution.
Applicable to steel, aluminium alloy, etc.
Cathodic phosphating:
Workpiece as the cathode, film hardness, low porosity, but need to prevent hydrogen embrittlement.
Commonly used for high strength steel or precision parts.
3. Post-treatment
Water washing: Remove the residual electrolyte (multi-stage counter-current water washing is required).
Passivation (optional): chromate or chromium-free passivate to seal pores and improve corrosion resistance.
Drying: Drying (80~120℃) or air drying.
4. Key control points
Electrolyte maintenance:
Regularly test metal ion concentration, pH, temperature.
Filter to remove impurities and prevent film roughness.
Current uniformity:
Complex workpieces need to design auxiliary electrodes or adjust the pole distance.
Hydrogen embrittlement risk:
Dehydrogenation treatment is required after cathodic phosphating (e.g. baking at 200°C for 2 hours).
5. Application examples
Automotive industry: gears, pistons and other wear-resistant parts (cathodic phosphating).
Electronic components: precision connectors (anodic phosphating + passivation).
Aerospace: high strength steel parts (low hydrogen embrittlement process).
Comparison of chemical phosphating process differences
Steps Electrolytic phosphating Chemical phosphating
Reaction drive Imposed current Chemical displacement reaction
Film formation time 1~10 minutes 5~30 minutes
Film properties Thinner, more uniform Thicker, porous structure
Equipment Requirements Power supply and electrode system are required. Tanks needed only
6. Precautions
Electrolytic phosphating is sensitive to the shape of the workpiece, deep holes or complex structures need to optimise the electrode layout.
Waste water contains phosphates and heavy metals and must be treated and discharged.
Through precise control of electrolysis parameters, high-performance phosphating film can be obtained, suitable for areas with strict requirements on corrosion resistance and abrasion resistance.