Electrochemically corrosion protection/adhesion

The corrosion widths of Prohesion salt spray-tested alloys are calculated and summarized in Figure 32.14. E-coated IVD controls (CC/E), i.e., the combination coating systems of chromate conversion coating with nonchromated E-coat, showed very large corrosion widths for all the IVD Al-coated aluminum alloys. This combination did not provide good corrosion protection, which could be taken as proof that the two completely different approaches (electrochemical corrosion protection and corrosion protection by barrier adhesion principle) should not be mixed. 

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]. 

While a copper-enriched surface has the implication of always causing accelerated electrochemical corrosion, replacing the native, hydroxylated, mixed Al-Mg oxide layer with a thin stable oxide layer seems to allow the plasma films to tightly adhere to the alloy surface. This adhesion, coupled with the barrier properties of the films, appears to provide additional protection of the oxide layer from contact with corrosive agents. 

The main aim of SAIE is the complete elimination of heavy metals from the coating systems an approach that primarily relies on tenacious water-insensitive adhesion and good barrier characteristics of a primer has been taken. It should be pointed out that this approach is theoretically incompatible with the approach that utilizes the primers with corrosion inhibitors, e.g., chromated primers. This is because a primer with super barrier characteristics would not allow the migration of inhibitors and would not provide enough water for their electrochemical reaction to form corrosion protection products, as described in Chapter 28. In order to further elucidate the SAIE concept, both chromated and nonchromated spray primers were employed to generate two types of plasma coating-modified systems, and their corrosion protection behaviors were investigated in this study. 

Corrosion Protection by Nonintact Films. In many end uses, there will be breaks in the films. Then it is desirable to design coatings to suppress electrochemical reactions. If there are gouges through the film down to bare metal, and wet adhesion is not adequate, water creeps under the coating, and the coating comes loose from the metal over a wider area. Poor hydrolytic stability exacerbates the situation. 

Since wet adhesion is critical for corrosion protection, techniques for studying wet adhesion can be useful. Electrochemical impedance spectroscopy (eis) is used to study coatings on steel. Many papers (99,100) are available, covering various applications of eis. When a coating film begins to delaminate, there is an increase in apparent capacitance. The rate of increase of capacitance is proportional to the amount of area delaminated by wet adhesion loss. Onset of delamination can be determined by eis studies (101). Results of eis tests are subject to considerable variation (102). Other problems with eis are discussed in Reference 103. 

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. 

The electrochemical deposition approach was initially used for depositing orga-nofunctional silane sol-gel films for promoting adhesion between aluminum alloy and epoxy resin [Ij. Later, it was used for preparing silane films for corrosion protection of metals, electrochemical stripping analysis of metal ions, and microextraction of organic compounds [18,20,22,42,45]. The recent development of the electrochemical deposition of sol-gel-based composite films opens various applications such as encapsulating proteins, enzymes, and bacteria. In this section, we will show three main applications of eiectrodeposited sol-gel films, that is, corrosion protection and adhesion promotion, electrochemical sensors, and finally biocomposite films. Other applications such as solid-phase microextraction (SPME), nonlinear optics, antireflection, electrocatalysis, and superhydrophobic films are also discussed. 

A variety of techniques has been developed to measure the condition of a coating so that some evaluation of its protective ability can be made. Many of these are based on electrochemical measurements [2]. The four techniques used in this study are (1) corrosion potential, (2) AC conductance, (3) tensile adhesion, and (4) weight gain. 

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