Impacts of Microorganisms (Bacteria,etc.) in Electrodeposition Coating and Control Measures
Impacts of Microorganisms (Bacteria,etc.) in Electrodeposition Coating and Control Measures
Electrodeposition paint is an environmentally friendly coating material with reduced organic solvent content. Using water as the dispersion medium, its aqueous system inherently serves as a favorable culture medium for microorganisms. Modern electrodeposition coating formulations contain little or no heavy metal components such as lead and tin, which weakens their inherent inhibitory effect on bacteria and other microbes. The operating temperature of typical electrodeposition lines ranges from 20 to 35 °C, coupled with high humidity and poor ventilation—conditions perfectly suitable for the survival and reproduction of various bacterial strains.
Microbial contamination in electrodeposition bath liquid rarely involves a single strain; instead, it forms complex mixed microbial communities. Contamination sources include pure water supply systems, tap water pipelines, ultrafiltration systems, post-rinse tanks, anode circulation systems, airborne microbial deposition, and contaminants carried by coated workpieces. The specific microbial species present depend on on-site environmental conditions. Once microbes proliferate in the bath liquid, bath stability deteriorates, triggering a wide range of coating defects. In severe cases, the entire bath liquid must be discarded.
The primary microorganisms proliferating in electrodeposition baths include bacteria, molds, and yeasts:
1. Bacteria: Microscopically tiny and ubiquitous with an extremely fast reproduction rate. Mass bacterial growth first causes abnormal bath parameters, followed by visible white flocculent bacterial sediment.
2. Molds: A category of fungi widely found in soil, water, air, and moldy organic matter. They favor acidic environments and reproduce slower than bacteria. Molds adhere firmly to tank and pipe inner walls, appearing as fluffy white floating flocs or slippery filamentous slime.
3. Yeasts: Another group of fungi widespread in soil, water, air, and animal organisms. They survive under both aerobic and anaerobic conditions, thriving within a pH range of 3.0–7.5. Yeast contamination manifests as excessive persistent foam and foul odors in the bath liquid.
Harms Caused by Bacterial Contamination in Bath Liquid
1. Deviations in Bath Liquid Parameters
• Cathodic electrodeposition: Reduced MEQ value, decreased conductivity, elevated pH value
• Anodic electrodeposition or contamination by specific strains: Increased conductivity, lowered pH value
2. Deteriorated Bath Application Performance
• Reduced breakdown voltage; film deposition becomes highly sensitive to voltage fluctuations
• Diminished bath stability, increased sediment volume, degraded filtration efficiency
• Accumulated surface foam and elevated viscosity
3. Degraded Cured Film Properties
Severe orange peel, rough texture, blisters, excessive particulate defects, and weakened coating adhesion.
4. Equipment Damage
• Heavy sediment clogs filter bags and ultrafiltration membranes, lowering ultrafiltration permeate flux
• Higher operational load on circulation motors
• Increased replacement frequency for filter bags and ultrafiltration membranes
Bacteria Quantification Test Method: Microbial Test Slide Assay
Immerse the test slide in the target liquid for several seconds, or flush the slide surface directly with the liquid sample. Place the slide back into its sealed original protective tube and incubate in a constant temperature & humidity chamber for 2–5 days (3 days as standard), then observe microbial colony growth on the slide. The detection limit of this method is approximately 1 CFU/cm² or 100 CFU/mL.
• Nutrient agar: For bacterial detection. Optimal incubation temperature is 30 °C; visible bacterial colonies form after 1–2 days.
• Rose Bengal agar: For fungal detection (yeasts & molds). Optimal incubation temperature is 25 °C. Yeast colonies develop in 2–3 days, while mold colonies require 5–7 days of incubation.
Note: Compare microbial growth on mikrocount® slides with the reference chart to determine total microbial count. Minor differences in agar color exist across different mikrocount® slide models.
Remediation Measures for Established Microbial Contamination
Biocides are applied to eliminate proliferated microbes by disrupting their cellular metabolism. Biocide selection must account for compatibility with both microbial strains and electrodeposition coating formulas to avoid damaging finished film performance. Common biocides include isothiazolinone (Kathon), hydrogen peroxide, and silver nitrate. These substances destroy cell membrane structures and denature proteins of bacteria and algae to achieve sterilization.
Long-term continuous use of a single biocide induces microbial resistance. Alternate biocides with different sterilization mechanisms weekly or per production cycle for optimal control. The standard dosing strategy combines continuous low-concentration feeding with periodic shock dosing to sustain effective disinfection.
The core workflow for microbial elimination in electrodeposition systems follows the sequence: mechanical cleaning first → chemical biocidal treatment → filtration removal of microbial biomass. Relying solely on biocide dosing without physical cleaning is strictly prohibited.
Long-Term Microbial Inhibition Strategies
1. Select electrodeposition coatings with inherent superior antibacterial properties to optimize microbial resistance at the raw material stage.
2. Standardize production operation and equipment maintenance procedures in strict accordance with process specifications.
3. Source water control: Install UV sterilizers, special precision filters, chlorination units, or ozone generators on pure water supply lines to eliminate bacteria in source water.
4. Regular sampling and testing of electrodeposition bath liquid, rinse tank liquid, and pure water supply; implement targeted biocidal treatment based on test results.
5. Periodically add appropriate biocides to suppress microbial reproduction and maintain stable bath performance.
6. Tank draining & deep cleaning:
￮ Pretreatment and rinse tanks are mostly static with severe dead zones prone to biofilm buildup. Increase cleaning frequency, drain and refill with fresh solution according to production schedules, and thoroughly scrub biofilm adhered to tank surfaces.
￮ For recirculating, filtered tanks: Maintain continuous circulation or adjust flow intensity to reduce stagnant zones. Regularly clean filters, pipelines, and tank walls to remove attached biofilm.
