Corrosion is a relentless and costly threat to the continuous operation of industry. Annual metallic corrosion costs in the United States in 1995 were reported to be 0 billion. At least one third of these costs could have been avoided at this time through use of available technology. Even more savings can be obtained using later technologies. This article describes how to further reduce such costs and obtain quality workmanship by using properly prepared CSI specifications.

Introduction

Corrosion is a relentless and costly threat to the continuous operation of
industry. Annual metallic corrosion costs in the United States in 1995 were
reported (Reference 1) to be $300 billion. At least one third of these costs
could have been avoided at this time through use of available technology. Even
more savings can be obtained using later technologies. This chapter describes
how to further reduce such costs and obtain quality workmanship by using properly
prepared CSI specifications.

What is Corrosion?

Corrosion is defined as "the chemical or electrochemical reaction between
a metal and its environment resulting in loss of the material and its properties."
(Reference 2) With few exceptions (e.g., gold and silver), metallic elements
are found in nature in stable chemical compounds in combination with other elements.
For example, iron is usually found in nature in the form of an ore such as iron
oxide. This chemical compound has a low energy content and is consequently very
stable. Iron is produced from its oxide ore by a high temperature smelting process
that utilizes heat energy to break the strong chemical bonds between the iron
and oxygen. This reaction raises the elemental iron to a higher, less stable
energy state.

Just as water flows to seek its lowest level, all natural processes tend towards
their lowest energy states. Thus, iron and steel have a natural tendency to
recombine with oxygen (rust) to achieve a lower energy, more stable iron oxide
state. The rusting causes metal loss to reduce cross section and strength. The
rust may also cause contamination of stored products or the environment. Localized
corrosion (pitting) may penetrate storage tank walls resulting in loss of stored
products and its contamination of the environment.

The more aggressive the environment, the greater will be the rate of corrosion.
Thus, metals immersed in water or exposed to chemical fumes will corrode much
faster than those in mild atmospheric environments. Higher temperatures will
also increase the rate of corrosion.

It should also be noted that there are different types of steel that require
different types of corrosion control. These include hot and cold rolled steel,
low alloy steel, and different types of stainless steel. For specialized work,
it may be necessary to consult a metallurgist to select the best type of steel
for the job.

Certain structural design features may contribute to early coating failure
and corrosion:

  • Water traps. Configurations with pockets that collect water (e.g., downward
    pointed angle iron) will increase the rate of coating deterioration and metal
    corrosion.
  • Sharp edges. Coatings will retract (draw back) from sharp edges to leave
    only a thin film of protective coating. Sharp edges should be smoothed by grinding
    to permit a greater build-up of coating thickness. Sometimes, edges are "striped",
    brushed with an additional coat to build-up thickness.
  • Crevices. Crevices at bolted seams and back-to-back angle iron accelerate
    corrosion because of reduced oxygen concentrations, and they must be avoided.
    Thus, welding is preferred to other methods of joining metals, because it eliminates
    crevices. However, welds must be ground to remove surface irregularities to
    permit full encapsulation by coatings.
  • Contact of dissimilar metals. Electrical contact of dissimilar metals
    will result in accelerated corrosion of the more chemically reactive metal.
    Thus, aluminum in direct contact with steel will undergo significant accelerated
    corrosion while protecting the steel from corrosion.
  • Areas that are difficult to access. All structural components to receive
    surface preparation and coating must be designed for ready access by workers.
    Access must also be provided for future maintenance painting.

Precisely, What are Protective Coatings?

Protective coatings consist of three basic components:

  1. Pigment. The pigment
    is the heavy, solid portion of the coating that may sink to the bottom of the
    can during prolonged storage. It may provide color, opacity (hiding of substrates),
    corrosion control, or other properties to coatings depending upon their formulations.
  2. Binder. The binder (usually derived from petroleum) combines with the pigment
    to form the dry film. It provides most of the important film properties (ease
    of application, resistance to sunlight, physical forces, water, solvent, and
    chemicals, etc.)
  3. Solvent. An organic solvent, also derived from petroleum, is usually used
    in coatings to reduce their viscosity for easy application. It is lost by evaporation
    after spraying of the coating and enters into the atmospheric environment. Because
    most solvents react in sunlight to produce ozone and other air pollutants, their
    use has been greatly restricted.

Water is used as a dispersant in common latex coatings and in some industrial
coatings. This greatly reduces air pollution.
Normally, three individual coats comprise a total protective coating system
with each coat providing specific properties:

  1. Primer. The primer is designed to bond well to the cleaned substrate and
    provide a good surface for adhesion of the intermediate coat. It may contain
    a corrosion-inhibitive pigment.
  2. Intermediate coat. The intermediate coat provides film thickness to separate
    the metal from the environment. It must also bond well to the primer and provide
    for good adhesion of the finish coat.
  3. Finish coat. If the finish coat is intended for exterior atmospheric service,
    it must provide resistance to sunlight, rain, and damage from wind-blown particles;
    if used to line storage tanks, it must be resistant to the stored materials.

Why use the CSI format?

Many specifiers for coating work use their own format or vary the format with
the job. This makes it very difficult for owners or bidders to find and understand
all the specification requirements. The CSI format is so effectively divided
into the three-part format (1. General, 2. Products, and 3. Execution) that
are well defined that preparation of a quality document is much more readily
achieved using it.

How do Coatings Control Corrosion?

By far, the most common mechanism of coating control of metal corrosion is
by forming a barrier isolating the metal from its aggressive environment. To
be able to reduce penetration of water, salts, and other aggressive materials,
the coating barrier must have inherent moisture resistance, adequate thickness,
and absence of film imperfections commonly called holidays. These concerns must
be addressed in the pertinent section of the job specification (e.g., dry film
thickness in the section describing materials requirements and absence of holidays
in the section describing inspection requirements.

Micaceous iron oxide pigment in coatings has a unique way of providing metal
protection. Flakes of the pigment are treated so that they tend to form overlapping
platelets in the wet film much like the shingles on a roof. This effectively
increases the path that water and salt on the coating surface must travel to
reach the metal.

Zinc coating products (e.g., galvanizing, metallizing, and zinc-rich coatings)
have been very effectively used to provide protection to steel. The initial
protection is a galvanic (cathodic) protection. If these barrier films become
scratched, zinc metal will preferentially corrode to protect the steel in the
manner described for dissimilar metal corrosion. The voluminous zinc corrosion
products gradually fill the scratch until it is healed to restore barrier protection.

For years, red lead and zinc chromate-pigmented coatings were very effectively
used to protect steel by inhibiting the corrosion reaction at the metal-coating
interface. Their use has subsequently been greatly reduced because of their
toxicities. Alternative, acceptable corrosion-inhibitive pigments haven’t performed
nearly so well.

How does the Specification Address Unique Coating Terms?
As occurs in many trades, the coatings industry is fraught with many specialized
terms that mean different things to different people. The inclusion of these
vague terms in specifications has lead to many expensive disputes. Different
interpretations over such apparently simple terms such as "trim coat",
"heavy sandblasting", and "paint coverage" have lead to
extensive litigation. To avoid such problems, all terms that are not universally
understood should be defined using industry-accepted definitions such as those
in Reference 3. Definitions belong in Part 1 (General) of the CSI specification
format.

How do I Address the Multitude of Government Restrictions?

Coating specifications must address a multitude of governmental restrictions
concerning cleaning and coating materials; surface preparation materials, pressures,
and contamination; and coating application (e.g., tip guards and trigger locks
for airless spray guns). These requirements are usually available on the web
sites of various agencies (e.g., EPA and OSHA). On large coating jobs, the services
of an industrial hygienist are often used to ensure protection of the workers
and the environment. Such requirements belong in Part 1 (General) of the CSI
format.

How do I Specify Coatings Materials?

It is important to specify a total protective coating system that will not
only meet the requirements for corrosion control and governmental regulations
but also offer other desired features. These may include appearance; resistance
to mildew, marine fouling, or slipping; or reflectivity. All component coatings
of the system should come from the same manufacturer and be recommended by the
manufacturer for the intended purposes. The coatings manufacturer may greatly
assist the specifier in preparing specifications for coatings work.

Because coating manufacturers have had to virtually continuously change their
formulations to meet increasingly restrictive governmental regulations or to
contend with the unavailability of specific raw materials, they have not had
time to sufficiently test their coatings for performance before offering them
for sale. Neither have plant owners had sufficient time to field test new products
to determine their long time performance. Thus, owners and coating suppliers
must cooperate with each other to utilize the best available information concerning
the work.

Part 2 (Products) of the CSI specification format may limit the coating products
to one manufacturer, if this is a very unique product. More commonly, the job
specification will list three or more commercial systems from different manufacturers
that provide generally similar performances. Coating contractors like to work
with products that they have successfully used in the past.

Governmental agencies frequently are not permitted to specify commercial products.
In this case, government specification products, field-tested and approved products,
or a performance type specification must be used.
Of course, labeling must conform to ANSI Z129.1 "Hazardous Industrial Chemicals
Precautionary Labeling and materials safety data sheets must be provided in
accordance with OSHA requirements. The specification should also require that
coatings have not exceeded their shelf lives, normally one year.

Methods of delivery and storage must be specified, along with requirements
for collection, storage, and disposal of waste materials.

How do I Specify Surface Preparation Requirements?

Inadequate surface preparation of metals is one of the chief sources of early
coating failure. If grease, oil, or dirt contaminates otherwise clean surfaces,
the available bonding sites for the primer coating may be insufficient to permit
good coating adhesion. If residual soluble salts remain on cleaned metal surfaces,
osmotic blistering of the coating is likely to result.

The profile height of the prepared surface is also important in coating performance.
Abrasive blast cleaning is the best method of providing the recommended levels
of cleaning and profile to steel surfaces, but costly containment of blasted
particulates may be required to protect the environment. If the profile is too
high, the primer may not adequately cover the peaks; if too low, it may not
provide adequate surface area for good adhesion of the coating to the metal.

The most commonly used abrasives generate steel surface profiles from 1 to
3 mils (thousandths of an inch). A 4 or 5 mil profile may be specified for thick
coatings over 20 mils after curing. Deeper profiles are usually obtained by
increasing the abrasive size or velocity. Particle shape and hardness have lesser
effects. ASTM D 4417, "Test Methods for Field Measurement of Surface Profile
of Blast Cleaned Steel" can be specified for measuring profile.

The best way to ensure proper metal surface preparation is for Part 3 (Execution)of
the CSI specification format to utilize industry-accepted standards (Reference
4) that fully define the cleaned surface rather than describing how to achieve
it. Both the type of coating and the level of surface preparation will be dictated
by the service environment. Thus, an oil-based coating over a commercial blast
cleaned surface (SSPC-SP 6) may be adequate for steel in a mild environment,
but an epoxy coating over a near-white blast cleaned surface (SSPC-SP 10) is
usually required for steel in an aggressive (e.g., sea water immersion) environment.

There are robotic systems available for both abrasive blast cleaning and waterjetting
systems for preparation of steel surfaces for coating. Reference 5 and the Surface
Preparation Commentary in Reference 4 describe both the advantages and limitations
of these and other surface preparation systems.

How do I Specify Coatings Application?

New high-solids (some are solvent-free) and waterborne coatings have been
developed to meet new low solvent requirements legislated to reduce the amount
of paint solvent that pollutes the air. High-solids products are much more difficult
to apply to metal surfaces than were previous coatings of higher solvent content
and may require special application techniques.

The high-solids coatings may be so viscous that they require a heating system
to reduce their viscosities to a suitable level for spray application. Also,
they are more chemically reactive than products containing more solvent and
may cure within minutes after mixing of the two components of epoxy and polyurethane
coatings. In such cases, plural-component spraying equipment that mix components
immediately before or after leaving the spray tip must be used.

With multi-component coating products that are not mixed at the nozzle, the
applicator must be concerned about:

  • complete mixing of components but not overmixing that will allow air
    to get entrapped in the wet coating;
  • induction times for those coatings that require time, especially at cold
    temperatures, for the chemical reaction between components to get started before
    application;
  • pot life time after which the mixed coating can no longer be successfully
    applied before the viscosity becomes too great for application;
  • recoat time range for successful overcoating. When using systems with
    multicomponent coatings (e.g., epoxies and polyurethanes) or single component
    moisture-cured polyurethanes, the applicator must be concerned about the time
    interval during which these newly applied coatings can be successfully overcoated.
    If the overcoat is applied too soon after application of an undercoat, it will
    interfere with the curing of the undercoat; if applied too late, the overcoat
    will not bond well to the undercoat.

Specifications should require that all coatings be applied in accordance with
the manufacturers’ written instructions. This is especially important with multiple
component coatings. Guidance documents from SSPC, such as SSPC-PA 1, "Shop,
Field, and Maintenance Painting of Steel", found in Reference 4, provides
more detailed information on application techniques.
Reference 4 also includes qualification procedures for contractors who conduct
field painting, removal of hazardous paint, and shop painting, respectively.
Selecting a quality contractor is necessary for procuring quality work.

How do I Specify Coatings Inspection?

Details of the inspection requirements for coating work also belong in Part
3 (Execution) of the CSI specification format. Again, industry-accepted inspection
standards (those of ASTM, SSPC, and other technical organizations) must be specified
without variation in order to obtain unbiased, reproducible test results.

Deviations from specification requirements should be resolved as soon as possible
and at the lowest level possible. All pertinent observations and test data must
be fully described and recorded by the inspector on standard inspection forms
(Reference 5).
Inspection is best accomplished by using independent inspectors certified by
NACE International and inspection companies certified by the SSPC-QP 5 procedure
(Reference 4). The inspectors should work with all contract parties to prevent
costly delays.

Commonly Encountered Specification Mistakes

There are several errors commonly encountered in specifications prepared for
coatings work. These are usually found in documents prepared by field personnel
who are not familiar with proper specifying techniques.

Perhaps the most common mistake is repeating a requirement in several places
in the specification. Some writers feel that this adds emphasis, but it only
adds confusion, because each time the requirement is expressed somewhat differently.

Another common specification error is site variation. The specifier sometimes
does not examine the job site, even though it may be close at hand, but he relies
on old drawings. Modifications may have been made over the years that will require
subsequent change orders and significantly increase the cost of the work.