Definition of Pinholes in Electrocoating: Pinholes refer to small, needle-like pits or holes on the surface of the dry coating film after baking, often exposing the substrate. Below are the specific causes and solutions for pinhole formation in cathodic electrocoating films.
Comparison Chart: With vs. Without Pinholes
1. Low Bath Liquid pH
Issue: Low pH causes difficulty in film deposition and redissolution pinholes.
Causes:
-
Acidic pretreatment liquid carryover into the bath.
-
Excessive addition of acidic additives.
-
Leaking anode liquid from anode tubes/frames.
-
Blocked anode membranes, hindering acid discharge.
-
Use of bare anodes, preventing acid discharge.
Solutions:
-
Add appropriate process holes to workpieces and strengthen pretreatment controls to minimize liquid carryover, ensuring thorough pre-coating rinsing.
-
Reduce or stop the addition of acidic additives (e.g., acetic acid, lactic acid).
-
Inspect the anode system for leaks (check if the anolyte shows the bath liquid’s color).
-
Replace blocked anode membranes with new ones.
-
Use anode tubes or frames instead of bare anodes.
2. Low Bath Liquid Temperature
Causes:
-
Insufficient heating equipment power or aging equipment failing to meet production temperature requirements.
-
Low ambient temperature causing the bath liquid temperature to drop when workpieces are introduced.
Solutions:
-
Ensure heating equipment operates normally.
-
Slightly increase the bath liquid temperature to compensate for temperature loss when workpieces enter.
3. Abnormal Solvent Content in Bath Liquid
Causes:
-
Low solvent content leads to poor workpiece wetting, causing pinholes.
-
High solvent content exacerbates redissolution, resulting in loose wet films with high small-molecule content, leading to gas pores during baking and pinhole formation.
Solutions:
-
Add solvent to correct low content.
-
Perform appropriate ultrafiltration drainage to reduce excessive solvent content.
4. Abnormal Pigment-to-Binder Ratio
Cause:
-
High pigment-to-binder ratio.
Solution:
-
Add emulsion to balance the ratio.
5. Abnormal Bath Liquid Conductivity
Causes:
-
Low conductivity leads to thin film deposition and poor coverage, causing pinholes.
-
High conductivity causes intense reactions, generating excessive bubbles that cannot be expelled in time.
Solutions:
-
Reduce ultrafiltration drainage and add fresh material; if bacterial growth causes low conductivity, sterilize promptly.
-
Drain ultrafiltration to lower conductivity.
6. Bath Liquid Aging Due to Low Update Volume
Cause:
-
Minimal bath updates lead to severe aging, reducing breakdown voltage and causing intense reactions, resulting in pinholes.
Solution:
-
Drain ultrafiltration, add additives, and introduce fresh paint to accelerate bath liquid updates.
7. Workpiece Material Issues
Causes:
-
Special materials or coatings on the workpiece surface result in poor wettability, uneven film formation, blooming, or pinholes.
Solutions:
-
Improve bath liquid wettability by adding solvent, reducing the pigment-to-binder ratio, or increasing bath temperature.
-
Shift the electric field backward (remove anode plates at the bath entry) to extend workpiece wetting time.
-
Add specialized additives to enhance bath liquid wettability.