In many paint testing applications, it is more convenient to use aluminum test panels. Aluminum is much lighter than steel and is impervious to corrosion in most environments. Aluminum sheets can be made from many different alloys and can be prepared in a variety of ways. Since it is not possible to describe all the different types of aluminum panels, an effort will be made to describe standardized aluminum panels for common testing.
Aluminum sheet for general applications should be produced from readily available common grades of polished aluminum sheet. Aluminum Institute alloys 3003, 3105 and 2024 have all been found suitable for test panel production. Standardized panels need to be cleaned in a strictly controlled process to keep them free of oil and particulate dirt. Both solvent and water cleaning are acceptable methods. Regardless of the cleaning method employed, key process parameters need to be identified, narrow ranges established for each parameter, and parameters closely monitored to ensure they are always on track. The cleaning process should be evaluated by performing water break and white cloth wipe tests on the treated panels at predetermined intervals.
surface finish
The Aluminum Institute defines ground sheet as “sheet with an uneven surface finish that may vary from sheet to sheet and from within the sheet, and may not be completely free of stain or oil” (from Aluminum Association literature). In terms of appearance, the standard finish produced by one factory can be very different from the standard finish produced by another factory. Some mills tend to roll a very glossy, reflective mill finish, while others produce a darker finish, less reflective. There can also be batch-to-batch variation in mill production by a given mill finish. This variation in surface finish can affect the results of some color and appearance evaluations. To somewhat reduce this variability, the aluminum used in the production of the test panels can only be purchased from pre-qualified factories that have been evaluated for surface quality and consistency.
preprocessing
Untreated aluminum surfaces oxidize immediately upon exposure to air or water, forming a protective surface oxide layer. This oxide layer protects aluminum from corrosion in most environments. However, when the aluminum surface needs to be painted, some problems can also arise. First, the oxide layer tends to be non-uniform, which inhibits adhesion between the coating and the aluminum surface. In addition, surface oxides are soluble in environments with pH values below 4 or above 10. For these reasons, untreated aluminum surfaces are not suitable for critical performance evaluations such as coating adhesion and corrosion resistance. There are many different types of pretreatments designed to improve the properties of painted aluminum.
The basic design purpose of most aluminum pretreatment systems is to modify the surface oxide layer to improve its stability in weakly acidic and alkaline environments and to significantly improve the paint adhesion characteristics of the surface. Several different types of pretreatment are described in ASTM D 1730, Standard Practice for Surface Preparation of Aluminum and Aluminum Alloys for Paint. It is not necessary to discuss each preprocessing method in detail. It can be amply stated that when designing specifications for test panel preconditioning, it is important to identify critical process parameters, establish narrow tolerances for each parameter, and develop controls to ensure that the process remains within specified tolerances.
Test panels may be provided with pretreatment following any of the methods described in ASTM D 1730. However, many of these methods are application or alloy specific. In this case, it is not practical to prepare standardized panels for general testing in this way. On the other hand, chromate conversion coating is the most widespread method of preparing aluminum surfaces for painting. A chromate conversion coating that greatly improves paint bonding and corrosion resistance on aluminum surfaces. Standardized test panels pretreated with chromate conversion coatings can be used to evaluate various performance characteristics of the coatings, including adhesion and corrosion resistance.
However, the sheer importance of chromate panels for performance testing being produced in a tightly controlled process with close monitoring of all important process parameters cannot be overemphasized. Variations in chromate process conditions can result in different chromate coating characteristics, which in turn can lead to inconsistent performance under paint. Some of the process parameters that need to be closely controlled include the temperature and concentration of the process bath, and the contact time and conductivity of the water used in the rinse and process solutions. Controlling these factors within normal commercial tolerances is not sufficient as this will likely not produce the consistency of results expected from standardized test substrates. For example, ASTM B 449, Standard Specification for Chromates on Aluminum, classifies paint-based chromate coatings as having a coat weight of 10 to 35 mg/ft2. This is a fairly wide range, and in some cases a panel with a coat weight at the lower end of the range cannot be expected to perform as well under paint as a panel with a coat weight at the higher end of the range. test type. Therefore, a narrower coat weight range, such as 15 to 20 mg/ft2, is required when preparing the specification for chromized standardized test panels.
Application field
Untreated aluminum panels are suitable for taking appearance-related measurements such as color, gloss and clarity of image. They can also be used to evaluate pretreatment and coating systems for aluminum, as well as in many general applications. When testing for adhesion, flexibility, impact resistance, corrosion resistance, and similar properties, chromate-pretreated aluminum panels should be used.
