Pretreatment Processes for the Aluminum Extrusion Industry Harry Adams - BCI Surface Technologies Bruce Chambeau Republic Chemical Co., Inc.
Purpose of Pretreatment Process Clean/etch, condition and treat aluminum surfaces for subsequent coating: Extends the corrosion resistance of the under-film substrate Enhances bonding of powders, paints and adhesives Improve surface appearance
Process Parameters Each item is important to the pretreatment process integrity: Time Temperature Chemical Management Water Quality Line Oversight/Management
Process Types Horizontal or Vertical Spray Cascade Immersion Spray Wand
Traditional Chrome Phosphate Process Heat in Water in Heat in Water in Cleaner Water Rinse Chrome Coating Water Rinse Acid. Water Rinse Water out Water out Work Flow Observations: Hexavalent chrome bath High contamination/chemical consumption High rinse water usage and of waste water treated Two heated tanks Two component pretreatment
Hexavalent Chrome Exposure Limits 2006 - OSHA reduces hex chromium PEL from 0.052 milligrams per cubic meter of air to 0.005 mg/m 3 in an 8 hours period (90% reduction) Monitoring of hex chrome begins at 0.0025 mg/m 3 NIOSH (Center for Disease Control) recommended exposure limit (REL) at 0.0002 mg/m 3 Trivalent chrome PEL/REL =.5 mg/m 3
Dry In Place Process Heat in Water in Cleaner Water Rinse Water Rinse or Conditioner Water Rinse Pretreat Coating Work Flow Observations: No hexavalent chrome One waste stream, low waste treat expense Low contamination due multiple rinses between cleaner and pretreatment coating RO/DI water required in stages 4/5 Low rinse water usage due to counter-flows One heated tank
Cleaner Types Alkaline Etching provides a controlled chemical attack on surface Active Alkalinity vs. Concentration Acid Removes surface oxides Lower temperature, less sludge Most cleaners include a surfactant to remove surface shop dirts, oils, etc.
Comparison of Alkaline and Acid Cleaners CLEANER TYPE EQUIP TYPE EQUIP COSTS BATH LIFE ETCH RATE TEMP SLUDGE/ SCALE WASHER MAINT ACID Acid Resistant ALKALINE Mild Steel
Surface Cleaning Morphology (pictures magnified 500X) Fig. 1 Mill Finish Fig. 2-5 min. Acid Cleaning Fig. 3-1 min. Alkaline Etching Fig. 4-5 min. Alkaline Etching
Clean Surface Identification Water Break = Not Clean Water-Break Free = Clean
Water Rinses Reduces contamination between chemical stages Removes, neutralizes and/or dilutes residual chemical on surface Prevents parts from drying Removes non-adherent debris from surface
Water Quality Tap water is acceptable for all stages except the final rinse stage DI or RO water should be used in the final rinse stage and/or dried in place chemical stage Contamination measured by conductivity or ph
Pretreatment Should have a measurable coating weight, within a recommended range Coating weight ranges may vary based on AAMA specification Dried in Place or rinsed pretreatments Dried in Place technologies may include trivalent chrome or chrome-free chemistries
Bad Actors Items to avoid trapping under powder/paint coatings: Anions: chlorides, nitrates, sulfates Cations: potassium, sodium, calcium, iron, copper
Controlling the Process Ensure temperature, time and chemical/ rinse stages are within specifications. Titrations determine chemical level or activity and provides immediate information. Automated equipment (conductivity and ph controls) is used to maintain consistency. Quality Tests: Boiling Water Test Other Physical Tests
Sources of Failure Lack of Adhesion Poor cleaning/etching Errors in rinsing (blistering after boiling test) No/low/high pretreat coating weight Bad parameters of baths Lack of Corrosion Resistance No or low coating weights Drag in of contaminants No or low etch rate
Troubleshooting Aluminum Condition Time/Temperature/Chemical Activity Rinse/Water Quality Bath Contamination (drag out) Equipment Line Operations
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