Conversion coatings are chemical surface treatments that transform the outermost layer of a metal substrate to improve corrosion resistance, paint adhesion, electrical insulation, and abrasion resistance. The processes of anodizing (aluminum), phosphating (steel), and passivating (stainless steel) are common in the aerospace, automotive, defense, and consumer electronics sectors.

Process Cycle

1. Pre-cleaning (alkaline, acid, mechanical)
2. Rinsing and deoxidation
3. Conversion coating treatment (immersion/spray)
4. Neutralization (if needed)
5. Post-treatment: sealing, passivation, drying
6. Inspection: thickness of coating, color, and adhesion

Equipment

• Immersion tanks or spray chambers
• pH, temperature, and conductivity control units
• Filtration and recirculation systems
• Anodizing rectifiers (for aluminum)
• Exhaust and fume extraction systems
• Automated transport/hoist systems for batch lines

Tooling

Primary:

• Workpiece racks, baskets, or jigs
• Electrical contact clips (for anodizing)

Secondary:

• Fixtures for masking or selective coating
• Sealing and post-treatment tanks
• Thickness measurement probes (eddy current)

Materials

Possible Defects

• Irregular coating or streaking
• Pitting caused by poor cleaning
• Irregular finish or discoloration
• Delamination of coating in thermal cycling
• Hydrogen embrittlement (if not properly controlled)

Design Guidelines

• Avoid hard internal corners or blind holes
• Provide good drainage and access for immersion
• Select compatible alloys for uniform coating
• Specify standard thickness in tolerance stack-ups
• Address masking and fixturing early in design

Cost Drivers

• Size and surface area of the substrate
• Coating type and thickness
• Chemical replenishment and treatment time
• Energy consumption (particularly in anodizing)
• Environmental and waste control regulations

Conversion coatings are utilized for the improvement of surface properties without a change in part geometry. Substrate choice, chemical control, and design compatibility guarantee durability and performance in extreme applications.