Metal Finishing Guide Book


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Page 144 of 903

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 performance 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 quality 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 quality defects may be contributed by high levels of low-molecular-weight resin and additive fractions in the formulation. These fractions usually have poor wetting 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. 141

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