Rational Control of Redissolution (Re-solubilization) of Cathodic Electrophoretic Coating Films
During electrophoretic coating, rational control is mainly achieved by maintaining a proper pH value to regulate hydrogen ion (H⁺) concentration, an appropriate organic solvent content, as well as suitable rinsing duration and rinsing pressure.
pH Value
Electrophoretic bath: The pH of normal electrophoretic bath is generally maintained at 5.6–6.0 (some product systems range from 5.2–5.6). The lower the bath pH, the higher the hydrogen ion concentration and the more severe the redissolution. At the optimal pH, the film redissolution rate (10 min immersion test) shall be within 5%. If pH drops by 0.4 (e.g., from 5.8 down to 5.4), the film redissolution rate will exceed 30%.
UF bath: Under normal operation, the ultrafiltration system runs continuously and UF filtrate recirculates back to the electrophoretic bath, forming a balanced system between the UF bath and electrophoretic bath. When the electrophoretic bath pH is kept within the process window, the UF filtrate will also meet process requirements.
At initial tank filling, neutralizer shall be added into the UF tank to lower pH and guarantee rinsing performance. Bath pH may decrease due to excessive addition of acidic neutralizer, prolonged operation with bare anodes, anode liquor leakage and other causes, which will trigger severe redissolution.
Solvent Content
Electrophoretic bath: Solvent content of electrophoretic bath is normally controlled at 0.8–2.0%.
UF bath: Under normal conditions, the UF bath and electrophoretic bath form a balanced system with solvent content around 0.5–1%. When the electrophoretic bath solvent content is controlled within specification, the UF filtrate will also comply with process requirements.
Rinsing Pressure
In post-rinsing, adequate UF spray pressure and UF immersion circulation volume are beneficial for good rinsing efficiency. However, excessively high spray pressure will aggravate redissolution.
Rinsing Time
Insufficient rinsing time leads to poor cleaning, while excessive rinsing time causes film redissolution and coating defects. On production lines, over-immersion in the electrophoretic bath or prolonged UF spray / immersion may occur due to equipment failure or unexpected line stoppage. Normally spray time is controlled at 20–30 s and immersion time around 60 s.
Abnormal Case
Abnormal Phenomenon
During blow-off of wet electrophoretic film in production, sheet-like peeling, film pinholes and low film thickness were observed.
Root Cause Analysis
Severe redissolution greatly reduces the hydrophobicity of wet electrophoretic film. Water penetrates into or underneath the coating film, weakening adhesion and resulting in film peeling.
Troubleshooting
Phosphating and bath parameters were normal. The pH of fresh UF filtrate was approximately 0.7 lower than usual; the normal fluctuation range of fresh UF filtrate pH is within 0.2, so abnormal UF filtrate parameters were confirmed.
Root Cause Confirmation
On-duty operators cleaned the ultrafiltration membrane during production. Acid and solvent were dosed for thorough membrane cleaning. Incorrect valve operation during membrane cleaning allowed chemicals to enter the UF filtrate storage tank without complete drainage, resulting in low pH of fresh UF filtrate. The acidic and solvent-containing UF filtrate contacted workpieces via the spray system. Both acid and solvent strongly promote redissolution of wet electrophoretic film. Coupled with the relatively thin film at that time, peeling of wet film occurred during air blow-off.
Summary
Redissolution control in electrophoretic coating requires systematic monitoring of key parameters including pH, solvent content, rinsing pressure and rinsing time. Abnormal pH fluctuation significantly aggravates film redissolution. Excessive solvent content, over-high rinsing pressure or prolonged rinsing time will also induce coating defects. In actual production, the status of bath and UF filtrate must be strictly monitored, and operating procedures standardized. Parameter abnormalities caused by equipment failure or human error should be avoided to stabilize coating quality and prevent peeling, pinholes and other failures originating from redissolution.
