ABSTRACT
The method of aluminum surface preparation ultimately selected for a particular commercial bonding application can vary significantly. Without question, surface preparation can be considered as one of the most vital factors in assuring adequate joint strength and long-service potential. In Table 18, Baun reviewed the various sources of contamination that could adversely affect the adhesive bond strength and the joint durability in manufacturing. He has further identified in Table 19 the presence of a variety of elements which he found present on a 7075-T6 aluminum surface even after pretreating in various fashions. The desired goal of any aluminum surface-cleaning and oxide-regeneration procedure is to produce a dry, firm, stable, and chemically wettable aluminum adherend surface with known and reproducible surface characteristics. The degree of the chemical stability of the treated aluminum surface necessary to achieve the desired goal of long-service life may vary depending on the strength of bond desired to maintain the structural integrity of any particular manufactured product and the severity of the service environment. Listed in order of increasing effectiveness for producing this chemical stability, the aluminum surface pretreatments can be separated into four general categories as follows:
1. Operations designed only to remove surface soil contaminants 2. Operations necessary to remove the above soiling plus some mechanical abrasion with subse-
quent operations to remove the residual surface contamination produced by the abrading step 3. Operations to remove surface soils plus a special chemical processing designed to create a new
stable surface oxide followed by final rinsing to remove any chemical contamination or residues left by the chemical processing, followed by final air drying of the surface
4. Operations combining the steps of (3) as a pretreating before an overcoating with a special primer designed to create maximum chemical compatibility with the selected adhesive and temporary protection from environmental contamination until final bonding operations
A 1984 discussion of the need to prepare adherend surfaces before adhesive application has been offered by Adams and Wake (901). More recently, Clearfield et al. (902) developed a chapter on "Surface Treatment of Metals" with the major emphasis on treating aluminum adherends (902), including observations on morphology and relative joint durability. It should be clear that it is absolutely mandatory to have the best possible conditions for adhesion on any aluminum surfaces for best service in long-term service applications under severe weathering conditions. It may often be satisfactory for less critical applications to simply confirm whether reasonable wetting can be achieved by the adhesive with removal of gross surface contamination. The simplest test is to observe whether the flow of water over the surface to be bonded is uniformly free flowing. Those areas where the water tends to bead up must be considered suspicious at best. This latter condition is certainly not acceptable for lOOo/o solids structural adhesive pastes, films, or tapes. For a more quantitative measure of wettability, the procedure of making accurate measurements on the spread of a drop of water of constant volume needs to be viewed through a transparent gauge placed over the drop area. As the angle of contact approaches zero, the degree of anticipated wetting is correspondingly more confirmed. Bijlmer and Schliekelmann (779), as mentioned in Chapter 4, have particularly studied the relationship of the surface condition of aluminum adherends after pretreatment for bonding Fokker commercial aircraft which have achieved good service results for many years. Bijlmer (903) has described the use of the Fokker Contamination Tester for evaluation of adherend surface cleanliness (including aluminum) for bonding. An oscillating probe is used to measure the electron emission energy of the surface in this device. (More references about the theoretical basis for operation and field success of this instrumentation will be given in Chapter 9 on testing.) The instrument reading can vary greatly depending on the degree of surface contamination and offers a testing device for some relative prediction of bonding success. It has been shown to be sensitive even to residues from an alkaline cleaning operation on aluminum where that method of surface preparation has been employed in manufacturing.
