Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 52 of 331

Guidelines for quality manufacturing practices are covered by specifications from agencies such as American Society for Testing and Materials (ASTM), Amer- ican Architectural Manufacturers Association (AAMA), military specifications (MIL), and AeroSpace Material Specifications (AMS). See Table II for typical ex- amples. Other Test Requirements Salt spray resistance tests are also required as standard quality-control practices. Comprehensive testing procedures listed in the following standards and speci- fications evaluate the performances of the composite film consisting of the con- version coating and paint topcoat. ASTM D 1730: Standard practices for preparation of aluminum and aluminum-alloy surfaces for painting. ASTM B 449: Standard practice for chromate treatments on aluminum. MIL-C-81706: Chemical conversion materials for coating aluminum and aluminum alloys. MIL-C-5541: Chemical conversion coatings on aluminum and aluminum alloys. AAMA 603.8: Voluntary performance requirements and test proce dures for pigmented organic coatings on extruded aluminum. AAMA 605.2: Voluntary specification for high-performance organic coatings on architectural extrusions and panels. AMS 2473: Chemical treatment for aluminum-base alloys, general purpose coating. What then are the critical factors in a coating system? Under film corrosion protection is an independently quantifiable property of prepaint treatments. You can measure the unpainted salt spray or humidity resistance and assess per- formance and quality. On the other hand, top coat paint adhesion, as it relates top pretreatment quality can sometimes be difficult to measure. This property is jointly dependent on the two coating systems considered. A third factor is substrate metal. Alloy qual- ity significantly affects the overall performance of the coating system. Other factors being equal, paint quality should be reviewed for inclusion in quality planning, especially if an adhesion problem occurs. Significant quali- ty defects may be contributed by high levels of low-molecular-weight resin and additive fractions in the formulation. These fractions usually have poor wet- ting properties. This results in weak boundary layers along the underlying substrate. Closely related to the adhesion properties of the paint system is moisture permeability. Corrosion and loss of adhesion is accelerated by poor permeability properties of the paint topcoat. 51

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