Bond failure corrosion

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. 

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... 

Table 111. X-Ray Photoelectron Spectroscopy of Composite to Metal Bond Failure in Corrosion...

Bond failure can also occur if the surface is anodic relative to another joint component. An example would be clad aluminum adherends where a thin layer of pure aluminum overlays the base alloy. Such a surface layer is designed to be more corrosion resistant than the alloy, but to act as a sacrificial anode should corrosion occur. Although this approach works well for corrosion protection of the substrate material, it can be a disaster for bonded material if the adherend surface/interface corrodes. As a result, American companies tend to use unclad aluminum for bonding and provide other means of corrosion protection, such as painting [1,70]. On the other hand, European companies commonly use clad adherends, but with a thicker oxide (CAA) [6,18,71-73] that provides bondline corrosion protection. 

Bond failure and blistering, resulting from the accumulation of fluids at the bond when the substrate is less permeable than the liner or from corrosion/reaction products if the substrate is attacked by the permeant. 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

When bonding metals with adhesives, water may influence corrosion behaviour. The goal is to prevent the spread of corrosion beneath the adhesive layer which leads to failure of the joint (so-called bond-line corrosion). Elastic adhesives are particularly well suited to this type of application, since they are compatible with a wide range of corrosion protection systems and often aid in their function. 

The relationship between delamination and failure for the lifetested specimens was more complex, and it was only for manufacturer F (the one that used a nitride passivation) that (i) delamination was a necessary precondition for failure, and (ii) the corrosion was restricted to the exposed aluminium at the bond-pads. For the other manufacturers, less than 50% of the lifetest failures delaminated prior to failure and, although the delaminated specimens showed signs of bond-pad corrosion, there were two other types of corrosion which were more prevalent. On the early failures there were small areas of severe corrosion scattered over the die and the frequency of such sites was correlated with that of passivation crack and pin-hole density for each manufacturer (up to 36 per die in the worst case), whereas for the late failures (and survivors) there was uniform corrosion of all the cathodi-cally biased tracks. This uniform corrosion was similar to that usually ascribed to high levels of phosphorus in CVD silicon dioxide passivations, so it would appear that this type of corrosion can even occur when, as in the cases reported here, the phosphorus contents are within the range 3-1-5-5%. 

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

The Alclad alloys have been developed to overcome this shortcoming. Alclad consists of a pure aluminum layer metallurgically bonded to a core alloy. The corrosion resistance of aluminum and its alloys tends to be very sensitive to trace contamination. Very small amounts of metallic mercury, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fully acceptable. When alloy steels do not give adequate corrosion protection—particularly from sulfidic attack—steel with an aluminized surface coating can be used. 

CRS which had been phosphated prior to bonding exhibited a significant enhancement of durability and corrosion resistance under the same accelerated conditions (Figure 4). The crystalline barrier layer restricted the exposure of the metal oxide to moisture by reducing the rate of water penetration at the interface. Even samples exposed to the cycle test were able to maintain failure within the adhesive for up to 10 days, after which varying amounts of interfacial failure were noted. Again, room temperature control samples maintained initial joint strength and failure remained cohesive within the adhesive. 

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