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Structural Corrosion Introduction
Over-specifying a corrosion protective system will result in unnecessary high cost implications. On the other hand, under estimating the required degree of corrosion protection can cause a problem especially when proper care of the steel while it is stored at the jobsite and during erection, is not taken. A poor choice of the corrosion protection system, improper site storage conditions, rough handling, and improper application of finish coats at the site (when required) is a sure way of shortening the service life of the steel structure. Over-specifying a corrosion protective system will result in unnecessary high cost implications. On the other hand, under estimating the required degree of corrosion protection can cause a problem especially when proper care of the steel while it is stored at the jobsite and during erection, is not taken. A poor choice of the corrosion protection system, improper site storage conditions, rough handling, and improper application of finish coats at the site (when required) is a sure way of shortening the service life of the steel structure Elements of Corrosion Protection
Although Zamil Steel is in a position to recommend a complete corrosion protection system, we generally limit ourselves to the steel surface preparation and the application of a shop primer. The Zamil Steel standard 25 microns Sigmazam red-oxide primer (conforming to SSPC-PS-7.00 1982 (or equivalent), applied over solvent cleaned steel, is intended to protect the steel against excessive rusting during transport (from factory to jobsite) and during the relatively short period of erecting the steel. The performance of this red oxide primer on its own and without the application of any further coatings has proven to be adequate in the majority of applications, particularly when the erected building is enclosed, ventilated and is not exposed to a corrosive environment. It is strongly recommended that the owner or specifier of a preengineered building evaluate the necessity for additional site painting and ensure that any paint, applied at the site, is compatible with this primer. In a Zamil Steel pre-engineered building the structural components that require corrosion protection comprise of primary built-up plate members and hot rolled members such as columns, rafters, mezzanine beams, crane runway beams, etc. and secondary cold-formed members such as roof purlins, wall girts, etc. As part of its standard product line, Zamil Steel offers galvanized and Zincalume coated secondary members, which provide maximum service life, under almost all service conditions, at a very reasonable cost. Zamil Steel is in a position to help customers to specify the most cost-effective corrosion protection system for almost all building usage applications. To enable us to do that the customer must follow these steps:
Consequently Zamil Steel can then recommend a detailed specification of surface preparation, primer and additional paint coats (if required) in addition to recommending a paint manufacturer and the manufacturer’s product number. If a specific paint manufacturer and product is desired by the customer, Zamil Steel will use that paint system as long as that product satisfies the quality and application requirements of Zamil Steel. Main Generic Types Of Paint and their Properties
Surface Preparation Surface preparation addresses the suitability of the steel substrate to accept a primer and additional paint coats. It involves two factors:
No coating system will perform as intended if surface preparation is ignored or is inappropriate. On the other hand, over specifying the surface preparation will be costly and may not be of significant benefit to the desired result. When a specification for surface preparation is not available Zamil Steel will use the recommended surface preparation given by the paint manufacturer as listed in its product data sheet. The cleanliness of the steel substrate is important for proper wetting and adhesion during and after the application of the coating and for the successful long-term performance of the coating system. The presence of water, oil, grease and other contaminants prevents paint from adhering properly to the substrate. Excessive rust in the substrate results in eventual flaking and peeling of the paint. Loose mill scale results in immediate paint flaking whereas tightly adhering mill scale may allow the coating to last for several years. Different exposure conditions and paints require different level of cleanliness. Table 4 gives the recommended surface preparation technique for various exposure conditions (this table should be used together with Table 2). The vast majority of applications will normally fall in the first and second categories of exposure conditions. Bear in mind that the exposure condition is for the completed building, which in the majority of cases is enclosed and well ventilated. Table 5 is a summary of the different surface preparation specifications given by the Steel Structures Painting Council. Zamil Steel is able to perform all of these except SP4 and SP5, which are not part of our standard offering. The Zamil Steel standard of solvent cleaning (augmented when necessary by hand tool cleaning or brush-off blasting) for Zamil Steel’s standard red-oxide primer is adequate for the majority of applications and has proven to be suitable even with other types of paint which do not require deep anchor profiles. Our steel is generally clean and of a superior rust resistant grade due to our policy of storing the plate inventory of raw materials indoors and our shop flow which keeps materials in an enclosed building up to the time it is painted in a covered area. The surface profile of the steel substrate is a measure of the roughness required for optimum adhesion of paint. It is important because it increases the surface area and provides a mechanical anchor. Paints with excellent wetting characteristics (and the majority of oil-based and alkyd primers) are tolerant of relatively smooth surfaces like those resulting from tightly adhering mill scale. High build epoxies and inorganic paints normally require deeper surface profiles achieved only by blasting the substrate. As a rule, thick coatings require a deeper profile than thin coatings to bond properly to the substrate. The profile anchor pattern or roughness is the average depth (or height) from peak to valley caused by the impact of the abrasive onto the substrate. Figure 1 is an illustration, which shows “R” as the roughness. Excessive roughness should be avoided since corrosion can easily occur on peaks where the paint coating is thinner. Furthermore, the deeper the anchor profile, the more paint is required to achieve a desired Dry Film Thickness (DFT). Zamil Steel uses a combination of shot and grit blasting in an enclosed cabinet when a blasted surface is necessary. This allows us to achieve the level of cleanliness and roughness required for almost all types of paint applications. Many specifiers often ask for a specific type of abrasive blasting either sand, shot or grit. For cleaning purposes, there is no significant difference between the three except for the media and method of application. Comments on other surface preparation methods:
Paints Paints are composed primarily of pigments dispersed in a film form, or binder, which is either dissolved in solvent or emulsified in water to makethe paint fluid enough to apply by brush, roller or spray. The basic composition of paint dictates its suitability to a particular combination of application conditions and protection requirements. Paints are normally used in combination with other paints to create a paint system. A paint system generally consists of a primer, an intermediate coat and a finish coat applied in different thicknesses to achieve the total desired film thickness. For most painting systems, increasing the film thickness decreases permeability and improves performance and durability. Zamil Steel generally applies only a shop primer to the steel structures of its buildings in order to provide adequate protection during transport, storage at site for a limited period, and erection. Finish coats and intermediate coats are preferably applied at the job site just before the steel is erected in order to minimize handling damage. Shop applied primers reduce the risk of corrosion of steel by preventing direct contact between moisture and air and the surface of the steel. Zincrich primers prevent the rusting of steel even if water gets through missed spots, breaks or pinholes in the coating. With the exception of zinc-rich paints, primers are not formulated to be exposed to the environment and require a finish coat for protection. However, if there is a risk of exposure to very acidic or alkaline chemicals, even zinc-rich primers, with all their durability attributes, must be finish-coated. Zamil Steel has the capability to apply many types of primers from different paint manufacturers. Over the years, certain paints have proven to be reliable under different combinations of building applications and environmental exposures. Specifying these systems assures the owner of having proven paint products from reputed paint manufacturers with expert primer application. When intermediate and finish coats are specified for a certain project, Zamil Steel consults with the paint manufacturer on compatibility and ease of application of the coat before a recommendation is made. The following recommended paint systems, for the primary and secondary steel, are a result of Zamil Steel’s extensive experience in this field. The actual paint system may be changed depending on the buyer’s definition of the specific application. Zamil Steel Area Offices will guide the buyer in specifying the exact painting system required. When a customer specifies a different painting system or paint manufacturer, Zamil Steel reserves the right to review the specifications against the quality and application requirements of Zamil Steel. Secondary Steel |
