MINIMIZING POLLUTION FROM CHEMICAL PRETREATMENT PROCESSES

Transcription

1 MINIMIZING POLLUTION FROM CHEMICAL PRETREATMENT PROCESSES Copyright Ron Joseph & Associates, Inc. All rights rwerved. January part of this publication may be reproduced or distributed in any form or by MY means, or stored in a data base or a retrieval system, without the prior permission of Ron Joseph & Associates, Inc. Session 4-007

2 11 What is a conversion coating? A conversion coating is a chemical process whereby a very thin coat "plates out" onto the metal surface. During the "plating" process, the coating reacts with the metal surface, thus forming part of that surface. Types of conversion coatings for steel, galvanize, terne-plate and, to a lesser extent, aluminum *: The most common coatings are iron phosphate and zinc phosphate. Manganese phosphate is also used, but not as a pretreatment for painting. Iron phosphate is an amorphous coating. Zinc phosphate is a crystalline coating. Aluminum is sometimes treated in a phosphate system, but in this case, the phosphate only serves as a degreaser. conversion coating is deposited. Types of conversion coating for aluminum: The most common conversion coating for aluminum is a "chromate" conversion coating such as Parker + Amchem's product, "Alodine". Chromate conversion coatings are amorphous. Session

3 Promotes adhesion between paint and the metal substrate. Temporarily prevents corrosion of the unpainted surface. Prevents rust creepage under the film. JJ8 9 f-- ;,$ pl Critical Considerations: /,A --11 /* Metal should have a slightly acidic (low ph) condition prior to paint application. Coatings do not adhere well when metal surface is either neutral or slightly alkaline (high ph). Temperature and contact times must be strictly adhered to. Adequate rinsing must take place to remove all contaminants and dissolved salts. Pretreated surfaces must be dried immediately to prevent flash rusting. gc 'r P.(1 Seal Rinses: Both chromate and non-chromate final seal rinses are available. While chromate rinses often provide improved corrosion resistance, high performance non-chromate rinses are also avai lab1 e. Session

4 Iron Phosphate Zinc Phosphate I Amorphous Crystalline Ql i,p ~ Improves Paint adhesion Provides temporary corrosion protection to substrate until coating system is applied Protects coating system from rust creepage under the film Improves paint adhesion more than iron phosphate Provides better corrosion protection to substrate than iron phosphate Protects coatings system from rust creepage under film more than iron phosphate Coating weights typically Coating weights typically mglff mg/fl? Available in 3-stage or Available in 5-stage process, 5-stage process - not in 3-stage Generally less expensive than zinc phosphate Requires more maintenance of both than iron phosphate More sludge formation than iron phosphate Generally will not affect gloss of primers and topcoats May decrease gloss of primers and topcoats, particularly if heavy, large crystalline zinc phosphate coatings are deposited -k Session

5 1. Alkaline clean 2. Water rinse 3. Water rinse 4. Iron phosphate 5. Water rinse 6. Chrome seal 7. DI Water rinse 8. Dry off 9. Cool down Dwell Time 90 sec. 30 sec. 30 sec. 60 sec. 30 sec. 30 sec. 30 sec. 5 min Temperature 150 F Ambient Ambient 12OoF Ambient Ambient Ambient 250 F Ambient Reference: Focused on Quality. C.Daniels, Product Finishing, v. 1989, 57. J II Session J

6 Final rinse stage (prior to DI water rinse or DI mist spray) often contains a "sealer". "Sealer" may be a low concentrate acidic chromate, or an organic non-chromate. "Seal Rinse" enhances corrosion resistance of the conversion coating. Improves paint adhesion. Removes last traces of contaminants from previous rinse stage. Chromate sealers are often based on hexavalent chrome Cr6+, and although more corrosion resistant, are considered more hazardous as wastes. Session

7 I Rinsing removes all corrosive salts from the previous stages. 9 1 surface. 1 Deionized (DI) rinsing is preferred as the final stage(s) to eliminate all remaining dissolved salts from the treated metal Remaining dissolved salts and other contaminants may shorten corrosion resistance of the conversion coating, and cause other defects under the organic coating (paint) film. 1 Session

8 WHAT IS THE DIFFERENCE BETWEEN CHROME OXIDE AND CHROME P VERSION COATINGS?" Chrome Ox ide Chrome Phew hate Amorphous Based on hexavalent Based on trivalent chrome chrome Cr 6+ (**) Cr 3+ Very corrosion resistant Less mechanically sound Less tolerant to stress Less corrosion resistant More mechanically sound More tolerant to stress Difficult to dispose of * Source: Gruss, Brad, "Pretreatment for Powder Coatings", Metal Finishing, May 1988 ** te that hexavalent chrome is a Hazardous Air Pollutant (HAP) 053 Session

9 1 7 Removes corrosion products from aluminum SVbngly acidic and must be handled with care. '1 contaminants. Etches the metal and removes aluminum oxide layer and other 3 1 Session

10 Does it provide a Does it remove: ~ Conversion Coating? Oils? Rust? Oxides? Aqueous Cleaners Wash Primer Marginal Vapor/Liq u id Degreasers Solvent Washing Solvent Wiping Steam Cleaning Steam + Phosphate Cleaning Marginal 2-stage Iron Phosphate Yes 3-stage Iron Phosphate Yes 5-stage Iron Phosphate Yes 5-stage Zinc Phosphate Yes 7-stage Zinc Phosphate Yes Chromate Conversion Coating of Aluminum Yes Yes Yes Some Most Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes* * A multistage chromate conversion coating for aluminum does remove aluminum corrosion products. Session

11 Cleaning and Treating of Steel Surfaces Steam Cleaning Fl Stage (Iron) Phosphate 3-Stage (Iron) Phosphate 4-Stage (Iron) Phosphate 5- Stage Iron or Zinc Phosphate 7- Stage Iron or Zinc Phosphate 0 w m

12 Chromate Types: &b?j 1.b 4' True conversion coatings. Gold in color. Used extensively for military and non-military purposes. Provide excellent corrosion resistance (>I 68 hours salt spray) Provide excellent paint adhesion. Tend to degrade if heated above approximately 1 4OoF, unless it has already been overcoated. Chromates are a concern regarding hazardous waste and water pollution. Session

13 m m rsion Coama of Aluminum Typical Seven Stage Process: (t Always Necessary) c

14 n-chromaie Types: t true conversion coatings. Clear in color. Have been shown to provide good salt spray resistance. Have not yet been accepted by the military as a substitute for the chromate type. Provide good paint adhesion. Do not lead to serious water pollution and hazardous waste problems as do the chromate types. Applied in many industries, such as the beverage can industry. Session

15 Dried-implace Types: t true conversion coatings. Clear coatings. Do contain chromates. final chrome rinse. Coatings are dried in place so there is little or no water pollution or hazardous waste. Good corrosion resistance, but not as good as chromate type. Good paint adhesion. t accepted by military as substitute for chromate type. Typical uses include coils for aluminum siding and other fabrications. l SessIon 4-00? 13 J

16 Steel Surfaces; Specification TT-C-490C Cleaning Method II. I II. IV. V. Alkaline derusting VI. 1. Mechanical or Abrasive Cleaning Solvent (immersion, spray or vapor) Hot alkaline (immersion, spray or electrolytic) Emulsion (with or without water added) Phosphoric acid (alcoholic, detergent or solventtype with detergent) Chemical Conversion and Pretreatment Type I Zinc phosphate Type II Iron phosphate Type I I Organic pretreatment coating Type IV n-aqueous iron phosphate Typical specification call-out for steel would be: Clean per Methods II through VI, treat per Type II of Specification TT-C-490 Aluminum Surfaces; Specification MIL-C-5541, Class 1 A Typical specification call-out for aluminum would be: Clean and treat per MIL-C-5541, Class 1A Session

17 Mechanical Cleaning Crude methods, such as wire brushing, do not adequately remove surface contaminants. Degreasing should precede mechanical cleaning, otherwise oil and grease are simply spread. Abrasives are reused too often and become contaminated. Surface profile from abrasive blast cleaning is too coarse. (Coatings can't cover the peaks). Incorrect blend of grit sizes are selected. Session

18 Chemical Cleaning and Treatment Rinse water is over-contaminated. Rinse water is not continuously being overflowed to drain (or to counter rinse tank). Rinse water not evaporated fast enough in oven, resulting in flash rusting. Tap water containing high concentration of dissolved solids, used as final rinse stage. Surfaces are inadequately degreased. Chemicals and solvents are reused until their levels of contamination are unacceptably high. Temperature of chemicals too low so that inadequate cleaning or treating takes place. Parts stay in chemicals and rinse stages too long due to poor operator procedures or conveyor stoppages. Inadequate draining of chemicals causes excessive carryover (dragout) into next stage. Chemical concentration not monitored often enough. Chemicals purchased based on price rather than quality, Pretreatment coating weights too heavy, causing splitting or delamination. Parts remain suspended between stages resulting in premature corrosion. Sesslon

19 1 7 ' _1 1 7 Pretreatment primer (wash primer) applied over chemical pretreatments. Cleaned and treated parts stored in fabrication shop for too long before receiving primer. Pretreated (and often primed) parts stored outdoors in inclement weather, before receiving top coats. Poor maintenance of equipment such as clogged nozzles in spray washers. Inadequate motion of parts in immersion tanks inhibits adequate cleaning and treating. Inadequate impingement from nozzles in spray washers inhibit adequate cleaning and treating. Spray nozzles either missing or misdirected so that they do not direct solutions to the parts. zzles are aimed so that the solutions deposit in the neighboring stage. Session

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