Adhesives corrosion protection system

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. 

A corrosion protection system should provide protection of the oxides and, in addition, should provide a good adhesive base for subsequent coating. The conventional corrosion protection system consists of alkaline cleaning and deoxidization of the surface followed by the application of a chromate conversion coating. 

In a damaged (scribed) corrosion protection system, the wet adhesion of a coating becomes the most important factor because both liquid water and corrosive species attack the interface. A water-delaminated coating layer does not provide any corrosion protection. Thus, water-insensitive adhesion of a coating to a substrate is a mandatory requirement for the prevention of corrosion-induced delamination. 

When water molecules permeate through the coating and reach the interface, the water sensitivity of a coating adhesion becomes a crucial factor in an undamaged corrosion protection system. Water permeates through a flawless polymer layer... 

The most significant marine use of resins is actually in the form of paint corrosion protection systems for hulls. These include polyurethane and epoxide systems, the latter giving good alkali and solvent resistance in addition to providing superior adhesion to most substrates. Such systems take the form of zinc-rich epoxy and epoxy coal-tar combination hull paints. Epoxy powder coatings are also commonly used for the protection of steel pipelines, both on land and offshore. 

Finish systems can be one coat or multicoat that use primers, intermediate coats, and topcoats. Primers provide adhesion, corrosion protection, passivation, and solvent resistance to substrates. Topcoats provide weather, chemical, and physical resistance, and generally... 

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. 

Generally, coatings for corrosion protection are applied as layered systerrrs, where each layer has been optimized to maxinuze a different functionality (adhesion, corrosion protection, hydrophobicity, gloss, UV resistance). For example, an anticorrosive paint system in the aerospace indnstiy has three main layers (Garcia et al, 2011a) ... 

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... 

Primers. Primers tend to hinder adhesive strength degradation in moist environments by providing corrosion protection to the adherend surface. A fluid primer that easily wets the interface presumably tends to fill in minor discontinuities on the surface. Qrganosilane, organotitanate, and phenolic primers have been found to improve the bond strength of many adhesive systems. 

The usual approach to good bonding practice is to prepare the aluminum surface as thoroughly as possible, then wet it with the adhesive as soon afterward as practical. In any event, aluminum parts should ordinarily be bonded within 48 h after surface preparation. However, in certain applications this may not be practical, and primers are used to protect the surface between the time of treatment and the time of bonding. Primers are also applied as a low-viscosity solution which wets a metal surface more effectively than more viscous, higher-solids-content adhesives. Corrosion-resistant epoxy primers are often used to protect the etched surface during assembly operations. Primers for epoxy adhesive systems are described in Chap. 10. 

The same plasma polymer deposited in a closed-system reactor has a graded elemental composition with a carbon-rich top surface, and the oligomer content is much lower [10], both of which increase the level of adhesion. The adhesion of the same water-borne primer is excellent and survives 8 h immersion in boiling water. When this surface is treated with O2 plasma, the adhesion does not survive 1 h of boiling, while the dry tape test still remains at the level of 5. The water-sensitivity of adhesion depends on the chemical nature of the top surface as depicted by XPS data shown in Figure 28.12. Water-insensitive tenacious adhesion, coupled with good transport barrier characteristics, provides excellent corrosion protection, as supported by experimental data [1-4], and constitutes the basic principle for the barrier-adhesion approach. 

An ideal model system was selected to study the interfacial factors with EIS [19]. The model system was Parylene C-coated Alclad (aluminum-clad aluminum alloy). In this system, the surface state of the top surface (salt solution/coating interface) and the adhesion of the coating (coating/metal interface) were modified to study the influence of these factors on the corrosion protection performance of the system. 

A nanofilm of plasma polymer of TMS (typically 50 nm) is applied on an appropriately prepared surface of an aluminum alloy. Then a corrosion-protective primer coating (typically 30 pm) is applied onto the surface of the plasma nanolilm. Adhesion of a multilayer coating system is the prerequisite for success of the SAIE approach. Therefore, adhesion of the first layer of the nanofilm prepared by plasma polymerization is the most crucial factor in this approach because if this layer delaminated from the substrate surface the rest of coatings could not function at all. It should be emphasized, however, that the issue is not the adhesion of a plasma polymer per se, but rather the adhesion that leads to better corrosion protection by the principle of SAIE, which requires the water-insensitive adhesion relevant to the corrosion protection. 

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. 

The corrosion protection of plasma interface-engineered coating systems relies on the tenacious water-insensitive adhesion and good barrier characteristics of the coatings [3]. DC cathodic polymerization and plasma treatment have been demonstrated as efficient in improving the primer adhesion to metallic substrates. 

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. 

It should be emphasized that the primers in the plasma coating systems applied to the IVD Al-coated A1 alloys could not be removed by the commercial Turco paint stripper solution. This tenacious and water-insensitive adhesion at the primer/IVD interface achieved by TMS cathodic polymerization in a closed reactor system must be responsible for the excellent corrosion protection performance of these plasma coating systems. In other words, excellent corrosion protection of IVD Al-coated A1 alloys can be accomplished with chromate-free primer coatings with the aid of tenacious and water-insensitive interface adhesion. 

They are typically applied as adhesive sealants in bodywork manufacturing (fold bonding and flange bonding, vibration insulation, corrosion protection) and as sealants in bottle and glass caps. For environmental reasons (hydrochloric acid separation in the case of thermal disposal), PVC plastisols are increasingly being replaced by acrylate-plastisols and epoxy systems. 

This paper wiU build on previous reviews which have sought to explore the marmer in which surface analysis methods can be purposefully employed to understand adhesion phenomena [4—6], with an emphasis on the elucidation of interphase chemistry. The rationale behind such an approach is that it is this critical region of a polymer/metal or polymer/polymer couple that will influence the performance of the overall system, be it the durability of an adhesive joint or the corrosion protection performance of an organic coating. 

A complete coating system, recommended for highly demanding situations, consists of several layers - the primer, the intermediate coats and the topcoat. The primer is an essential component for corrosion protection, because it is responsible for the adhesion of the coating to the substrate and also it provides most of the anticorrosive properties, through the use of inhibitors. Its requirements are a good adhesion to the substrate and to the intermediate coat, flexibility and cohesion and good chemical resistance. 

Choice of an appropiate surface treatment and a suitable primer are important because adhesion to the substrates presents difficulties [11.37]. Primers based on modified alkyd resins or two-pack epoxy-resins for derusted ferrous metals mainly contain zinc phosphate and zinc OKide as corrosion protection pigments. Nonferrous metals are first washed with an ammoniacal wetting agent before applying the primer that contains a binder based on synthetic resins (e.g., PVC copolymers, chlorinated rubber) which ensure good adhesion to the substrate. The same primer must be used on zinc or galvanized surfaces because the use of alkyd resins causes embrittlement [11.38] The primed surfaces are largely topcoated with alkyd resin systems. 

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