Organic Coatings and Linings,Organic Coatings and Linings,Organic Coatings and Linings
THE TOTAL national yearly cost of metallic corrosion in the United States was estimated at $167 billion in 1985. This
total includes replacement costs and lost production costs as well as the decrease in lifetime and the expected replacement
value of a given component subject to damage by corrosion. The cost of corrosion also includes the means by which the
effects of corrosion are mitigated, such as the use of cathodic protection, inhibitors, alternative materials of construction,
overdesign, and protective coatings.
This latter category, the use of protective coatings, is in itself a very substantial category. In 1975, in a more detailed
survey, the National Association of Corrosion Engineers (NACE) estimated the cost of direct expenditures by NACE
members to combat corrosion at $9.67 billion. Of this amount, $2.9 billion was spent for protective coatings and services.
Coating application accounted for $805 million; purchase of coatings for atmospheric service, $531 million; marine
coatings, $502 million; external pipeline coatings, $310 million; and miscellaneous expenses, $750 million.
Therefore, the use of protective coatings and linings and the costs associated with such use are considerable. More metal
surfaces are protected by organic coatings and linings than by all other methods combined. In addition to protecting
against corrosion, coatings often beautify and provide an aesthetic appeal. Safety colors are used to mark pipes, to
indicate their contents, and to provide warnings of hazardous or dangerous work areas.
The Effect of Legislation on the Coatings Industry
Legislation concerning worker health and environmental protection has had a marked impact on the coatings industry. For
example, although the practice of blast cleaning a steel surface and applying an alkyd, vinyl, or epoxy coating system
remains one of the best methods or corrosion protection for many steel surfaces, the alkyds, epoxies, and vinyls
themselves have changed as a result of legislation restricting the release of volatile organic compounds (solvents) and the
use of toxic pigments.
Surface preparation techniques have also changed drastically; silica sand as a blast-cleaning abrasive has been banned in
virtually all Western countries except the United States and Canada (and there is a strong movement to ban it in these
countries as well). The containment and safe disposal of spent blast-cleaning abrasives are required in many localities to
prevent environmental damage caused by the leaching into water supplies of lead, chromate, and other toxic paint
pigments removed during blast cleaning. Therefore, although a paint layer over a properly cleaned surface still acts as a
barrier against a corrosive environment in most cases, the components that constitute this barrier have changed
considerably within the past 10 years, and the means by which the surface is properly cleaned are rapidly changing at the
present time.
This article will present an overview of the different types of coating and lining materials available, along with
information on the various means of surface preparation and the equipment and techniques of coating application.
However, it must be recognized that all facets of coating technology are strongly influenced by legislative decree; the
alkyds, vinyls, and epoxies currently in use are being reformulated, and within a few years, although the names will be the
same, the compositions of these coatings (and their potential protective capabilities) may be considerably different.
Furthermore, surface preparation and application techniques in current use are also under considerable legislative pressure
and are expected to change considerably in the near future.
Coating Industry Response to Legislative Pressure
The coating industry has been very responsive to enacted legislation and to proposed legislation limiting the use of
potentially harmful or toxic raw materials or surface preparation and/or application techniques. As a result of this positive
industry reaction, the legislators as a whole have been pragmatic in their laws. For example, it is widely recognized that
lead, when ingested by the body, results in anemia, damage to the central nervous system, disruption of the reproductive system, and retardation in child development. Therefore, in 1973, Congress, under the impetus of the Food and Drug
Administration, limited the lead content in all paints used for consumer use to 0.06% solids by weight.
This law effectively banned the use of lead pigments in all paints used around the home. However, because leadcontaining
paint pigments, such as red lead, lead chromate, and lead suboxide, are perhaps the best pigments in use for
corrosion protection, they were not outlawed for paints used for corrosion protection purposes in industry. In fact, even
today, no legislation restricts the use of such pigments for industrial and marine corrosion protection purposes.
However, recently enacted legislation regarding worker and environmental protection is drastically curtailing the use of
lead pigments as well as other toxic pigments, such as those in the chromate, mercury, and tin families. Although such
pigments can be legally used in a paint, the substantial costs due to worker protection and the high costs associated with
containment and disposal of paints containing such pigments are effectively precluding their use in most new coating
formulations.
Similarly, restrictions regarding volatile organic compounds in coatings have been gradually enacted over the last 10
years in order to allow formulators to develop comparable paint materials with complying solvents. Although solvent
restrictions were originally enacted against what were believed to be photochemically reactive solvents (solvents that
reacted under the influence of ultraviolet light and degraded to form smog and to deplete the ozone layer), it was later
found that virtually all volatilizing solvents photochemically degraded and were therefore environmentally detrimental.
This realization, together with the potential for further legislative restriction, has motivated coating formulators to develop
water-base paints or high-solids (low-solvent) coating materials.
Volatile organic content (VOC) legislation has been mandated in California for many architectural coatings. This rule
prohibits the manufacture, sale, or use of designated industrial maintenance primers and topcoats if the VOC content
exceeds 420 g/L (3.5 lb/gal.). Most coating formulators were prepared for this ruling, and the quality of water-base and
high-solids coating formulations is fast approaching, and in some cases exceeding, the performance of conventionally
formulated solvent-containing coating systems.
Conversely, there are many industrial facilities and highway bridges that are coated with old lead-containing paints.
Environmental Protection Agency (EPA) legislation requires the disposal of removed paints (and spent abrasive) in
hazardous waste disposal sites if the leachate after acid digestion (pH 5) contains more then 5 ppm of lead or chromate
and 2 ppm of mercury. The costs of such a disposal, not including collection costs, are estimated by many painting
contractors to be from six to ten times as much as disposal costs in a normal sanitary landfill. The cost of containing the
spent abrasive and paint, as opposed to letting the spent abrasive fall to the ground during blast cleaning, may in itself
double or triple the cost of a paint job.
To date, enactment and enforcement of legislation have been very spotty, and many paint contractors or plant painting
forces have not had to strictly comply with the law. However, this situation is expected to change in the very near future,
and it is felt that the high costs associated with the repainting of many existing bridges and structures will lead to the
development of new surface preparation concepts and enclosure techniques. The surface preparation equipment required
for large-scale coating removal projects will become much more capital intensive, and it is possible that those coating
contractors who tend to disregard specification requirements regarding the extent of surface preparation or coating
thickness, and so on, will find another means of evasion, namely the violation of environmental legislation regarding
containment and disposal of toxic paint residues.
Despite the pending health and environmental legislative influences and the rapidly occurring changes in the coatings
industry, corrosion continues, and painting for corrosion protection must be done. Therefore, the following sections will
discuss the more commonly used coating and lining materials; their characteristics, advantages, and disadvantages;
surface preparation equipment and techniques; application and inspection methods; and equipment.
Coating and Lining Materials
Paints or linings that act as a protective film to isolate the substrate from the environment exist in a number of different
forms. Sheet linings, commonly of the vinyl or vinylidene chloride family, are one such type of coating that can be either
adhered to the surface to be protected or suspended as a bag within a tank, for example, to provide protection. Hot-applied
organisols, or plastisols, again usually of the vinyl family, can also be applied to a surface, typically by dipping or flow
coating, to provide a protective film.

Powder coatings are being increasingly used to protect concrete-reinforcing rod, as pipeline coatings, and as coating
materials in the original equipment manufacturing markets. Fine powders produced from high molecular weight resins of
the thermoplastic vinyl and fluorinated hydrocarbon families or from thermoset resins of the epoxy and polyester families
are applied to the surface to be protected by either electrostatic spray or fluidized-bed deposition. The metal being
protected is usually preheated at the time of application, and after application it is reheated to an elevated temperature
(generally from 150 to 315 °C, or 300 to 600 °F). The specific time/temperature baking schedule depends on the metal
temperature at the time of application and the type of powder being applied.
Alternatively, some coating systems are characterized by the application method used. For example, for coil-coated metal
sheet (commonly steel or galvanized steel), very specialized high-speed roller application equipment is used to coat the
sheet steel as it is unwound from a coil. The paint used in the coil-coating process can be of virtually any generic type,
although alkyds, polyesters, epoxies, and zinc-rich epoxy coatings are the most prevalent.
Certain lining materials, such as hand lay-up fiberglass-reinforced plastics, are also used to protect steel from corrosion.
Such coating systems usually consist of an epoxy primer applied to a blast-cleaned steel surface, followed by one or more
polyester gel coats, with one or more layers of a fiberglass veil or woven roving mat laid within the gel coats as
reinforcement. The system is then sealed with a layer of the polyester gel coat (a semiclear, 100% solids resin coat).
Similarly, rubber linings are used to protect against corrosion. There are various types of rubbers, but they can generally
be categorized as prevulcanized or postvulcanized (vulcanized after application). Similarly, rubbers can be formulated
with different hardnesses and chemical resistances. Commonly, a rubber lining is a composite of two or three different
types of rubbers adhered to each other and to the surface.
The coatings and linings discussed above are mentioned here in summary only; a more thorough description is beyond the
scope of this article. Powder coatings and coil coatings are discussed in more detail in the article "Painting" in Surface
Engineering, Volume 5 of ASM Handbook.
This article will be limited to the liquid-applied (usually by brush, roller, or spray) coating and lining materials that are
commonly used for corrosion protection in atmospheric or immersion service. The rate of base metal corrosion where
such coatings or linings are used should not exceed approximately 1.3 mm/yr (50 mils/yr). For corrosion rates above this,
both in atmospheric and immersion service, or where catastrophic failure is of concern, liquid-applied coatings probably
should not be used, and corrosion-protective measures should include the use of more corrosion-resistant alloys, sheet or
rubber coatings and linings, fiberglass lay-ups, and so on.
The coating systems discussed in this article will be categorized by the generic type of binder or resin and will be grouped
according to the curing or hardening mechanism inherent in that generic type. Although the resin or organic binder of the
coating material is most influential in determining the resistances and properties of the paint, it is also true that the type
and amount of pigments, solvents, and additives such as rheological aids will dramatically influence the application
properties and protective capability of the applied film. Furthermore, hybridized systems can be formulated that are
crosses between the categories. For example, an acrylic monomer or prepolymer can be incorporated with virtually any
other generic type of resin to produce a product with properties that are a compromise between the acrylic and the original
polymer. In many cases, this is advantageous, as in the mixing of vinyls and acrylics or heat-curing alkyds and acrylics. In
other cases, such as with an epoxy, acrylic modification may be a detriment. Table 1 lists the advantages and limitations
of the principal coating resins.

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