Corrosion barriers transport properties

Goals of Testing and Classification of Test Methods Nonelectrochemical Methods Electrochemical and Electrical Methods Barrier Characteristics of Coatings Adhesion of Organic Coatings Transport Properties of Coatings Other Film Properties Corrosion Resistance of Painted Metals... 

In order for a polyimide to be useful as an interlevel dielectric or protective overcoat (passivant), additional demanding property requirements must be met In the case of the passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. It has been demonstrated that polyimide filrns can be excellent bulk barriers to contaminant ion motion (such as sodium) [10], but polyimides do absorb moisture [11,12], and if the absorbed moisture affects adhesion to the substrate, then reliability problems can result at sites where adhesion fails. However, in the absence of adhesion failure, the bulk electrical resistance of the polyimide at ordinary device operating temperatures and voltages appears to be high enough to prevent electrochemical corrosion [13]. 

The vast majority of corrosion inhibitors in neutral environment as well as a number of acid corrosion inhibitors form protective 3D films on the metal surface ( interphase inhibition [4]). These films may consist of adsorbate multilayers, ox-ide/hydroxides, salts, or reaction products formed by interaction of the inhibitor with solution species on or near the corroding metal surface (e.g. dissolved metal ions). The type, structure, and thickness of the inhibiting films are strongly influenced by the environmental conditions. The interphase films act as a physical barrier that blocks or retards transport processes and the kinetics of the corrosion reactions at the metal surface. The inhibitive properties could, in some cases, be correlated with the chemical stability of the corresponding insoluble complexes as well as with the solubihty, adsorbabOity, and hydrophobicity of the inhibitor molecules [35]. Often, other ions from the electrolyte, such as... 

Oxygen concentration, temperature and flow velocity have large effects on oxygen transport to the metal surfaee and eonsequently on the corrosion rate in natural waters. These faetors have been dealt with quantitatively in Chapter 6. In addition, variation in eorrosion rate is also eaused by the composition of the water, since this may strongly affeet the barrier properties of surface layers. 

Coatings allow the surface properties of materials (e.g. wear resistance, lower friction, chemical barrier, corrosion resistance, etc.) to be improved while the intrinsic bulk properties (especially mechanical) remain unchanged. Among the most widely used techniques, plasma enhanced chemical vapor deposition (PECVD) and plasma enhanced physical vapor deposition (PEPVD) are widely applied. The difference between PVD and CVD is basically the mechanism used to generate and transport the material as vapor, and the way in which it is later deposited on the substrate. 

Changes in both the adhesion values obtained in before-and-after testing and in the failure loci can reveal quite a bit about aging and failure mechanisms. Changes in barrier properties, measured by electrochemical impedance spectroscopy (EIS), are important because the ability to hinder transport of electrolyte in solution is one of the more important corrosion-protection mechanisms of the coating. 

---