Adhesive bonds corrosion

With the exception of coupling agent technology, primers for structural adhesive bonding have received little theoretical treatment in the literature beyond a discussion of mechanisms of corrosion inhibition by primer additives and limited discussion about statistical techniques for primer formulation. Perhaps because of the much more widespread use and greater economic importance of corrosion-protective coatings, the design and function of primers for these systems have... 

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

Adhesives and sealers can be an important part of a total corrosion protection system. Structural bonding procedures and adhesives for aluminum, polymer composites, and titanium are well established in the aerospace industry. Structural bonding of steel is gaining increasing prominence in the appliance and automotive industries. The durability of adhesive bonds has been discussed by a number of authors (see, e.g., 85). The effects of aggressive environments on adhesive bonds are of particular concern. Minford ( ) has presented a comparative evaluation of aluminum joints in salt water exposure Smith ( ) has discussed steel-epoxy bond endurance under hydrothermal stress Drain et al. (8 ) and Dodiuk et al. (8 ) have presented results on the effects of water on performance of various adhesive/substrate combinations. In this volume, the durability of adhesive bonds in the presence of water and in corrosive environments is discussed by Matienzo et al., Gosselin, and Holubka et al. The effects of aggressive environments on adhesively bonded steel structures have a number of features in common with their effects on coated steel, but the mechanical requirements placed on adhesive bonds add an additional level of complication. 

The extent of adhesive bond failure under corrosive environments is greatly accelerated when cyclic mechanical stresses are imposed on the adhesive bond during exposure. Three to four orders of magnitude reduction in fatigue life of adhesive bonds is observed for bonds exposed to environment prior to fatigue testing. 

Corrosion Testing. Salt spray testing (ASTM-B-117-52,54) was used to determine durability of adhesive bond in corrosive environment. Lap shear samples were exposed to salt spray for 14 days and then immediately tested for lap shear strength. 

The extent of coating adhesion failure was found to be dependent upon the resistance of the polymer in the coating to hydrolysis by corrosion generated hydroxide. In this study, similar trends have been observed for adhesives. Table I shows the results of salt spray corrosion on a series of bonds between cold rolled steel adherends and adhesives of varying chemistry. The results show that there is a direct correlation between the chemistry of the adhesive polymer and the durability of the series of adhesive bonds studied. The locus of adhesion failure also appears to be related to the type of adhesive chemistry. In this study, adhesives based on polymers having a wide range of hydrolysis resistance were examined. 

In a specific example of adhesive bonds between cold rolled steel and SMC adherends (Table II) an adhesive based on hydrolysis resistant epoxy chemistry (i.e., adhesive E) was compared with an adhesive based on hydrolysis prone urethane chemistry (i.e., adhesive C) in composite to cold rolled steel bonds. After corrosion testing, a significant difference in both retention of initial bond strength and locus of failure was observed. For bonds prepared with adhesive E, little if any reduction of the initial bond strength was observed after corrosion testing. The locus of failure for both the tested and untested bonds was largely in the... 

The interfacial adhesive bond surfaces generated as a result of corrosion induced failure (for adhesives C and E) have been examined using x-ray photoelectron spectroscopy. The results of these studies are shown in Table III and Figures 1 and 2. Changes in... 

The Effect of Adhesive Primers. In practice, adhesive bonds involving metal adherends often use primers as pretreatments of the metal surface prior to bonding. Table IV shows the durability of composite-metal bonds prepared with adhesive C over a series of primers (of varying corrosion resistance) in 240 hour salt spray test. The results indicate that the performance of bonds is directly related to the corrosion resistance of the primer used to prepare the adherend surface. In general, the adhesion of the primer to the steel adherend, rather than the adhesive chemistry. 

Surface Morphology. The initial Integrity of an adhesively bonded system depends on the surface oxide porosity and microscopic roughness features resulting from etching or anodization pretreatments. (17) The SAA surface characterized in this study consists of a thick (9 ym), porous columnar layer which provides excellent corrosion resistance in both humid and aggressive (i.e., Cl ) media. I The thinner FPL oxide provides a suitable substrate surface for evaluating the candidate inhibitors. 

Metals such as aluminium, steel, and titanium are the primary adherends used for adhesively bonded structure. They are never bonded directly to a polymeric adhesive, however. A protective oxide, either naturally occurring or created on the metal surface either through a chemical etching or anodization technique is provided for corrosion protection. The resultant oxide has a morphology distinct from the bulk and a surface chemistry dependent on the conditions used to form the oxide 39). Studies on various aluminum alloy compositions show that while the oxide composition is invariant with bulk composition, the oxide surface contains chemical species that are characteristic of the base alloy and the anodization bath40 42). 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

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