Biocide Application Guide for Electrophoretic Coatings: Controlling Microorganisms and Biofilm

Microbial contamination is one of the most underestimated challenges in electrophoretic coating (e-coat) lines. When bacteria and fungi proliferate in the bath, they form flocculent or viscous substances, block pipelines and control valves, impair electrical conductivity and ultimately degrade coating quality. This application guide summarizes the sources of contamination, testing methods, routine control measures and the correct use of biocides to keep e-coat lines clean and productive.

01 Sources and Impacts of Microbial Contamination

Identifying where microorganisms enter the system is the first step toward effective control. The main contamination sources are:

• Pretreatment processes (washing, phosphating, passivation, etc.)
• External water sources (reverse osmosis water, deionized water, ion exchangers)
• Electrophoresis systems (UF filtrate, circulating water, recovery systems)
• Equipment surfaces (tank walls, pipelines, filters, etc.)

Once established, contamination causes serious operational problems:

• Bacteria form flocculent or viscous substances in the bath
• Block pipelines and control valves, impair electrical conductivity and coating quality
• Form biofilms that shield bacteria from biocidal action

02 Microbial Testing and Monitoring

Regular monitoring allows operators to detect contamination before it affects film quality. Common testing methods include:

Control criteria:

• Bacterial count < 500 CFU/mL (or DipSlide result < 10⁴)
• HY-LITE RLU < 150 (UF filtrate < 100)

03 Routine Microbial Control Measures

The following methods are commonly adopted to inhibit microbial growth during normal operation of electrophoretic coating lines:

• Select electrophoretic coatings with good bacteriostatic performance (note that lead-free and organotin-free eco-friendly coatings are more susceptible to bacterial growth)
• Install auxiliary UV sterilization units on pure water, rinsing water and anode compartments
• Deploy copper-ion devices in anolyte for bacteriostasis
• Periodically add hydrogen peroxide, biological biocides and other substances for microbial inhibition

When routine bacteriostasis fails and microorganisms proliferate massively in the bath, biocides must be applied for sterilization.

04 Precautions for Biocide Application

Selection and dosing principles

• Use a single biocide at a time to avoid bacterial adaptation
• Shock dosing: active ingredients (e.g. CIT/MIT) ≥ 10 ppm
• Avoid continuous low-dose feeding to prevent microbial resistance

Common biocide types: isothiazolinones (CIT/MIT, OIT, BIT); aldehydes (glutaraldehyde, formaldehyde); peroxides (hydrogen peroxide, peracetic acid); quaternary ammonium salts; bromides / iodides.

Dosing procedure

• Dilute before addition: dilute to 20% with deionized water and feed via the UF rinsing tank
• Add during non-production periods and circulate for at least 24 hours
• Inspect filter bags and replace them in a timely manner
• Tank draining and cleaning is normally required

Safety protection: wear acid-alkali resistant gloves and goggles, and avoid direct skin contact to prevent allergic reactions.

05 Biofilm Removal and System Cleaning

Cleaning approaches include alkaline cleaning (e.g. NaOH, pH 11-12) plus surfactants, acidic cleaning (e.g. peracetic acid, hydrogen peroxide) and special cleaning additives (e.g. NHF-05 additive). After cleaning, apply biocides frequently to prevent biofilm regeneration and perform deep system cleaning periodically (e.g. every 10 weeks).

06 Recommended Biocide Products

All of the above biocides share identical application procedures. Dilute to 20% with deionized water before use, then add to the ultrafiltration rinsing tank for overflow into the electrophoresis bath. Direct addition into the electrophoresis bath tends to cause coagulation and precipitation, and overdosing may hinder film deposition.

07 Summary and Recommendations

• Prevention over remediation: regular cleaning, water quality monitoring and system sanitation maintenance
• Combine multiple control strategies: biocides + physical treatment + system maintenance
• Record-keeping and analysis: document bacterial test results and biocide dosing records to optimize control protocols
• Safety and environmental compliance: follow local regulations, use approved biocides and implement proper personnel protection