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2 Corrosion Control Through Organic Coatings All of these forms of structural steel have at least two things in common: 1. Given a chance, the iron in them will turn to iron oxide. 2. When the steel begins rusting, it cannot be pulled out of service and sent back to a factory for treatment. During the service life of one of these structures, maintenance painting will have to be done on-site. This imposes certain limitations on the choices the maintenance engineer can make. Coatings that must be applied in a factory cannot be reapplied once the steel is in service. This eliminates organic paints, such as powder coatings or electrodeposition coatings, and several inorganic pretreatments, such as phosphating, hot-dip galvanizing, and chromating. New construction can commonly be protected with these coatings, but they are almost always a one-time-only treatment. When the steel has been in service for a number of years and maintenance coating is being considered, the number of practical techniques is narrowed. This is not to say that the maintenance engineer must face corrosion empty-handed; more good paints are available now than ever before, and the number of feasible pretreatments for cleaning steel in-situ is growing. In addition, coatings users now face such pressures as environmental responsibility in choosing new coatings and disposing of spent abrasives as well as increased awareness of health hazards associated with certain pretreatment methods. 1.1.2 SPECIALTIES OUTSIDE THE SCOPE Certain anticorrosion coating subspecialties fall outside the scope of this work, including those dealing with automotive, airplane, and marine coatings; powder coatings; and coatings for cathodic protection. These methods are all economically important and scientifically interesting but lie outside of our target group for one or more reasons: • The way in which the paint is applied can be done only in a factory, so maintenance painting in the field is not possible. (Automotive and powder coatings) • Aluminium — not steel —is used as the substrate, and the coatings experience temperature extremes and ultraviolent loads that earth-bound structures and their coatings never encounter. (Airplane coatings) • The circumstances under which marine coatings and coatings with cathodic protection must operate are so different from those experienced by the infrastructure in the target group that different coating and testing technologies are needed. These exist and are already well covered in the technical literature. 1.2 PROTECTION MECHANISMS OF ORGANIC COATINGS This section presents a brief overview of the various mechanisms by which organic coatings provide corrosion protection to the metal substrate. Corrosion of a painted metal requires all of the following elements [1]: • Water • Oxygen or another reducible species © 2006 by Taylor & Francis Group, LLC
Introduction 3 • A dissolution process at the anode • A cathode site • An electrolytic path between the anode and cathode Any of these items could potentially be rate controlling. A coating that can suppress one or more of the items listed above can therefore limit the amount of corrosion. The main protection mechanisms used by organic coatings are: • Creating an effective barrier against the corrosion reactants water and oxygen • Creating a path of extremely high electrical resistance, thus inhibiting anode-cathode reactions • Passivating the metal surface with soluble pigments • Providing an alternative anode for the dissolution process The last two protection mechanisms listed above are discussed extensively in Chapter 2. This section will therefore concentrate on the first two protection mechanisms in the list above. It must be noted that it is impossible to use all these mechanisms in one coating. For example, pigments whose dissolved ions passivate the metal surface require the presence of water. This rules out their use in a true barrier coating, where water penetration is kept as low as possible. In addition, the usefulness of each mechanism depends on the service environment. Guruviah studied corrosion of coated panels under various accelerated test methods with and without sodium chloride (salt). Where salt was present, electrolytic resistance of the coatings was the dominant factor in predicting performance. However, in a generally similar method with no sodium chloride, oxygen permeation was the rate-controlling factor for the same coatings [2]. 1.2.1 DIFFUSION OF WATER AND OXYGEN Most coatings, except specialized barrier coatings such as chlorinated rubber, do not protect metal substrates by preventing the diffusion of water. The attractive force for water within most coatings is simply too strong. There seems to be general agreement that the amount of water that can diffuse through organic coatings of reasonable thickness is greater than that needed for the corrosion process [2–8]. Table 1.1 shows the permeation rates of water vapor through several coatings as measured by Thomas [9,10]. The amount of water necessary for corrosion to occur at a rate of 0.07 g Fe/cm 2 /year is estimated to be 0.93 g/m 2 /day [9,10]. Thus, coatings with the lowest permeability rates might possibly be applied in sufficient thickness such that water does not reach the metal in the amounts needed for corrosion. Other coatings must provide protection through other mechanisms. Similar results have been obtained by other studies [2,11]. However, the role of water permeation through the coating cannot be completely ignored. Haagan and Funke have pointed out that, although water permeability is not normally the rate-controlling step in corrosion, it may be the rate-determining factor in adhesion loss [11]. © 2006 by Taylor & Francis Group, LLC
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4 Blast Cleaning and Other Heavy Su
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