Schweitzer P.A. Fundamentals of corrosion. Mechanisms, causes, and preventative methods

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Corrosion of Paint 239

7.7.3.1 Air-Drying

Air-drying coatings cure at room temperatures, forming lms. The lms are

formed by one of three mechanisms:

1. Solvent evaporation. Shellac and lacquers such as nitrocellulose,

acrylic, styrene-butadiene, and cellulose acetate thermoplastic resins

form lms by solvent evaporation.

2. Conversion. As solvents evaporate, lms are formed that are cured

by oxidation, catalysis, or crosslinking. Thermosetting coatings

crosslink to form lms at room temperature by catalysis or oxida-

tion. The addition of catalysts will accelerate the oxidative curing

of drying oils and oil-modied resins. Monomeric materials can

form lms and cure by crosslinking with polymers in the pres-

ence of catalysts, as in the case of styrene monomers and polyester

resins. Epoxy resins will crosslink with polyamine resins to form

lms and cure. Airborne moisture starts a reaction in the vehicle

of polyurethane resin coating systems, resulting in lm formation

and cure.

3. Coalescing. Emulsion or latex coatings, such as styrene-butadiene,

acrylic ester, and vinyl acetate acrylic, form lms by coalescing and

dry by solvent evaporation.

7.7.3.2 Baking

Coatings requiring baking will form lms at room temperature but require

elevated temperatures of 300 to 372°F (150 to 200°C) to cure. Curing is either

by conversion or phase change.

Conversion.• Heating accelerates the cure of many oxidative ther-

mosetting coatings. In resin systems such as thermosetting acryl-

ics and alkyd melamines, the reactions will not take place below a

temperature of 275°F (135°C). Coatings that require heat for curing

are generally tougher than air-drying coatings. In some cases, the

cured lms are so hard that they must be modied with other

resins.

Phase change.• Polyolens, waxes, and polyamides are thermoplastic

coatings that form lms by phase changes — generally from solid to

liquid, then back to solid. Plastisols and organosols undergo phase

changes during lm formation. Fluidized-bed powder coatings, both

thermoplastic and thermosetting, also undergo changes during lm

formation and cure.

240 Fundamentals of Corrosion

7.7.3.3 Force Drying

The cure rate of many thermoplastic and thermosetting coatings can be

accelerated by exposure to elevated temperatures that are below those con-

sidered as baking temperatures.

7.7.3.4 Reﬂowing

Certain thermoplastic coating lms will soften and ow, becoming smooth

and glossy at elevated temperatures. The automotive industry uses this tech-

nique on acrylic lacquers to eliminate bufng.

7.7.3.5 Radiation Curing

Bombardment with ultraviolet and electron-beam radiation with little

increase in temperature will form and cure lms. However, infrared radia-

tion increases the surface temperature and it therefore is considered a bak-

ing process.

7.7.3.6 Vapor Curing

This method is used for two-component coatings. The substrate is coated

with one component of the coating in the conventional manner. It is then

placed in an enclosure lled with the other component — a curing agent in

vapor form — where the catalysis or crosslinking conversion takes place.

7.7.4 inspection

The most important part of a coating operation is to guarantee that the coat-

ing has been properly applied. It is better to have a less effective coating

applied properly than the best coating applied poorly.

There are a wide variety of aids, standards, and inspection instruments

available to check the quality of the coating system. These include devices

for checking the cleanliness of a prepared surface, the depth of a blast-

cleaning anchor pattern or prole, and various magnetic, eddy current, and

nondestructive thickness gauges (capable of measuring the total thickness

or the thickness of each coat in a multicoat system). In addition, there are

instruments available to monitor temperature, humidity, and dew point on

a continuous basis.

After application, adhesion tests and holiday tests (for pinholes and other

discontinuities) can also be specied.

It is also important to realize that even if properly applied, the coating

does not last forever. During the rst 6 to 12 months after application, visual

inspection will be able to detect inadvertent misses, thin spots, or weak areas

in the coating. Repair, if required, should be done at this time.

Corrosion of Paint 241

As time passes, the coating will break down and deteriorate as a result

of the environment. Because of this, scheduled inspections should be con-

ducted. Localized areas of failure should be touched up before deterioration

of the entire surface occurs.

If a scheduled maintenance program of periodic touch-up, followed occa-

sionally by a full coat over the entire area, is followed, the expensive costs of

total surface preparation (such as complete media blasting for removal of all

old coating) can be avoided, sometimes for 30 years or more.

Corrosion protection by coatings can be economically achieved for long

periods of time if the entire coating procedure is followed from beginning

to end, starting with a denition of the environment, selection of the proper

coating system, proper surface preparation and application, inspection, and

periodic maintenance and repair.

7.8 Maintenance of the Coating

Any paint job, even if properly done, does not last forever. Over a period of

time, the coating will break down and deteriorate as a result of the effect of

the environment. Scheduled inspections should be made to determine the

extent and rate of coating breakdown. Spot touch-up repair should be made

at localized areas of failure before deterioration of the entire coated surface

has taken place. Extensive costs of total surface preparation can be avoided

if a planned maintenance approach of periodic spot touch-up and an occa-

sional recoat of the entire surface is followed.

Many coatings that provide good long-term protection are more dif-

cult to touch up or repair in the event of physical damage or localized fail-

ure. This is particularly true of the thermosetting and zinc-rich coatings.

Application of a subsequently applied paint coat to an older-aged epoxy,

urethane, or other catalyzed coating often results in reduced adhesion that

leads to peeling.

Thermoplastic coatings do not usually present this problem. Solvents of

freshly applied thermoplastic coatings soften and allow for intermolecular

mixing of the new and old coatings with good intercoat adhesion.

Heavily pigmented coatings, such as zinc-rich, require agitated pots to

keep the pigment in suspension during application. Because of this, touch-

up and repair of large areas is not recommended using zinc-rich coatings

unless it is done by spray using an agitated pot.

For the most part, oil-based coatings (alkyds, epoxy esters, and modica-

tions thereof) have a greater tolerance for poor surface preparation and an

ability to wet, penetrate, and adhere to poorly prepared surfaces or old coat-

ings. Consequently, these coatings are often specied for these purposes,

although they do not provide as long a term of corrosion protection.

242 Fundamentals of Corrosion

7.9 Selecting a Paint System

The rst step in selecting a paint system for corrosion protection is to deter-

mine the environment around the structure or item to be painted. Is the

environment predominantly a weathering environment subject to heat,

cold, daily or seasonal temperature changes, precipitation, wind (exing),

exposure to sunlight, or detrimental solar rays? If the structure or item is

located outdoors, are there chemical plants nearby, or pulp and paper mills,

or other industrial facilities that are apt to discharge airborne pollutants? Is

color, gloss, and overall pleasing effect more important than corrosion pro-

tection, or are the normal grays, whites, and pastels of the more corrosion-

resistant paints satisfactory? If located in a chemical facility, what specic

chemicals are used nearby? Is there any chance of a chemical spill on the

painted surface?

Because surface preparation is an important factor in the selection of a

paint system, the suitability or availability of the surface for specic prepa-

ration techniques must be known. In some instances, certain types of sur-

face preparation may not be permitted or practical. For example, many

companies do not permit open blast cleaning where there is a prevalence

of electric motors or hydraulic equipment. Reneries, in general, do not

permit open blast cleaning or any other method of surface preparation that

might result in the possibility of a spark, static electricity buildup, or an

explosion hazard.

If a new facility is being constructed, it is possible that during erec-

tion many areas may become enclosed or covered or so positioned that

access is difcult or impossible. These structures must be painted prior to

installation.

When all this information has been collected, appropriate paint sys-

tems may be selected. In most instances, it will not be practical or pos-

sible to select one single coating system for the entire plant. There will be

areas requiring systems to provide protection from aggressive chemicals,

whereas other areas may require coating systems simply for aesthetics.

If an area is a combination of mild and aggressive conditions, a coating

system should be selected that will be resistant to the most aggressive

condition.

Several typical industrial areas have been illustrated that coating systems

may be exposed to, with recommendations for paint systems to be used in

these areas. The paint systems are shown in Table 7.13 through Table 7.18

with the appropriate surface preparation. These tables have been arranged

based on surface preparation. Each coating system shown in a particular

table requires the same surface preparation.

Corrosion of Paint 243

TabLE 7.13

Multilayer Paint Systems Requiring Commercial Blast (SSPC-SP-6) for Surface

Preparation

System A Inorganic-Zinc/Epoxy Mastic

Paint Layers:

One coat inorganic zinc: 2–3 mil dft (50–75 μm)

One coat epoxy mastic: 4–6 mil dft (100–150 μm)

Properties:

Zinc primer provides outstanding corrosion resistance and undercutting resistance

A barrier protection for the zinc primer is provided by the nish coat of epoxy, which also

provides a color coat for appearance

Suitable for use on carbon steel only

Limitations:

A relatively high level of applicator competence required for the primer

System B Inorganic Zinc/Epoxy Primer/Polyurethane Finish

Paint Layers:

One coat inorganic zinc: 2–3 mil dft (50–75 μm)

One coat epoxy primer: 4–6 mil dft (100–150 μm)

One coat polyurethane nish: 2–4 mil dft (50–100 μm)

Properties:

Zinc primer provides outstanding corrosion resistance and undercutting resistance

The zinc primer is protected by a barrier coating of epoxy primer, while the nish coat of

polyurethane provides color and gloss retention

This is a premium industrial nish for steel surfaces

Can only be used on carbon steel

Limitations:

A relatively high level of applicator competence required for the primer

System C Inorganic Zinc/Acrylic Finish

Paint Layers:

One coat inorganic zinc: 2–3 mil dft (50–75 μm)

One coat acrylic nish: 2–3 mil dft (50–15 μm)

Properties:

Zinc primer provides outstanding corrosion resistance and undercutting resistance

Water-based single-package nish has excellent weathering and semigloss appearance

Limitations:

A relatively high level of applicator competence required for the primer

The nish coat has low temperature curing restrictions

—continued

244 Fundamentals of Corrosion

TabLE 7.13 (continued)

Multilayer Paint Systems Requiring Commercial Blast (SSPC-SP-6) for Surface

Preparation

System D Aluminum Epoxy Mastic/Epoxy Finish

Paint Layers:

One coat aluminum epoxy mastic: 4–6 mil dft (100–150 μm)

One coat epoxy nish: 4–6 mil dft (100–150 μm)

Properties:

Can be used on tight rust and marginally prepared surface

The epoxy nish coat is available in a variety of colors and has good overall chemical

resistance

May be used on carbon steel or concrete

Concrete must be clean, rough, and cured at least 28 days

Hand or power tool cleaning, including water blasting, may be used for surface preparation

System E Aluminum epoxy mastic/acrylic nish

Paint Layers:

One coat aluminum epoxy mastic: 4–6 mil dft (100–150 μm)

One coat acrylic nish: 2–3 mil dft (50–75 μm)

Properties:

May be used on tight rust and marginally prepared surfaces

The acrylic nish coat is available in a variety of colors and has good overall chemical

resistance

This is an excellent maintenance system

Normally used on carbon steel and concrete

System F Epoxy Mastic/Epoxy Mastic

Paint Layers:

One coat epoxy mastic: 4–6 mil dft (100–150 μm)

Properties:

May be used on tight rust and marginally prepared surfaces

The substrate is protected by the formation of a tight barrier stopping moisture from reaching

the surface

Normally used on steel or concrete

Concrete must be clean, rough, and cured at least 28 days

If necessary, hand or power tools may be used for cleaning

System G Epoxy Primer/Epoxy Finish

Paint Layers:

One coat epoxy primer: 4–6 mil dft (100–150 μm)

One coat epoxy nish: 4–6 mil dft (100–150 μm)

—continued

Corrosion of Paint 245

TabLE 7.13 (continued)

Multilayer Paint Systems Requiring Commercial Blast (SSPC-SP-6) for Surface

Preparation

Properties:

An easily applied two coat high build barrier protection is provided with ease of application

Used on carbon steel or concrete

Limitations

Because these are two-component materials, they must be mixed just prior to application

They require additional equipment and more expertise to apply than a single-packed product

Many epoxy nish coats will chalk, fade, and yellow when exposed to sunlight

System Η Epoxy Primer

Paint layers:

One coat epoxy primer: 4–6 mil dft (100–150 μm)

Properties:

Normally applied to carbon steel or concrete in protected areas such as the interiors of

structures, behind walls and ceilings, or for temporary protection during construction

Limitations:

This is a two-component material requiring mixing just prior to application

System I Epoxy Novalac/Epoxy Novalac

Paint Layers:

One coat epoxy novalac: 6–8 mil dft (150–200 μm)

Properties:

An exceptional industrial coating for a wide range of chemical resistance and physical

abuse resistance

Has a higher temperature resistance than standard epoxy

May be used to protect insulated piping or for secondary containment

Normally used on carbon steel and concrete surfaces

246 Fundamentals of Corrosion

TabLE 7.14

Multilayer Paint Systems Requiring That Surface Be Abrasive Blasted in

Accordance with SSPC-SP-10 Near-White Blast

System A Aluminum-Epoxy Mastic/Aluminum-Epoxy Mastic

Paint Layers:

One coat aluminum-epoxy mastic: 4–6 mil dft (100–150 μm)

Properties:

Tolerates poorly prepared surfaces and provides excellent barrier protection

A third coat may be added for additional protection

Can be used on carbon steel and concrete

Concrete must be clean, rough, and cured at least 28 days

If necessary, this system can be applied to surfaces that are pitted or cannot be blasted, but

the service life will be reduced

System B Epoxy Phenolic Primer/Epoxy Phenolic Finish/Epoxy Phenolic Finish

Paint Layers:

One coat epoxy phenolic primer: 8 mil dft (200 μm)

One coat epoxy phenolic nish: 8 mil dft (200 μm)

Properties:

Because of this system’s outstanding chemical resistance, it is often used in areas subject to

frequent chemical spills

The nish coats are available in a limited number of colors

Normally used on carbon steel and concrete. Concrete must be clean, rough, and cured at

least 28 days

System C Epoxy Phenolic Primer/Epoxy Phenolic Lining/ Epoxy Phenolic Lining

Paint Layers:

One coat epoxy phenolic primer: 8 mil dft (200 μm)

One coat epoxy phenolic lining: 8 mil dft (200 μm)

Properties:

Because of this system’s overall chemical resistance, it is suitable for lining areas subject to

owing or constant immersion in a variety of chemicals

Normally used on carbon steel and concrete

When used on concrete, the surface must be clean, rough, and cured at least 28 days

System D Epoxy/Epoxy

Paint Layers:

One coat epoxy: 4–6 mil dft (100–150 μm)

—continued

Corrosion of Paint 247

7.9.1 area 1: Mild Exposure

This is an area where structural steel is embedded in concrete, encased

in masonry, or protected by noncorrosive type re-proong. In many

instances, no coating will be applied to the steel. However, it is a good

idea to coat the steel substrate with a protective coating to protect it dur-

ing construction and in case it ends up being intentionally or unintention-

ally exposed.

A good practice would be to apply a general-use epoxy primer, 3 to 5 mil

dry lm thickness (dft) (75 to 125 μm). If the surface cannot be abrasive-

blasted, a surface-tolerant epoxy mastic may be used.

Recommended systems are found in Table 7.16, systems A and C.

TabLE 7.14 (continued)

Multilayer Paint Systems Requiring That Surface Be Abrasive Blasted in

Accordance with SSPC-SP-10 Near-White Blast

Properties:

Two coats of the same product are applied, providing a high build protection

Can be used in immersion service without the addition of corrosion inhibitors

When used in potable water systems, the product must meet Federal Standard 61

A third coat may be added for additional protection

Normally used on carbon steel and concrete

System E Coal Tar Epoxy/Coal Tar Epoxy

Paint Layers:

One coat coal tar epoxy: 8 mil dft (200 μm)

Properties:

Provides excellent barrier protection and is the most economical of the water lining systems

for water immersion

Normally used on carbon steel and concrete

System F Solventless Elastomeric Polyurethane

Paint Layers:

One coat solventless elastomeric polyurethane: 20–250 mil dft (500–6250 μm)

Properties:

Excellent barrier protection

Normally used on carbon steel and concrete

Limitations:

Must be applied by a knowledgeable contractor

248 Fundamentals of Corrosion

7.9.2 area 2: Temporary Protection, Normally Dry interiors

This area consists of ofce space or dry storage areas (warehouses) or

other locations exposed to generally mild conditions or areas where oil-

based paints presently last for 10 or more years. If located in an industrial

environment, there is the possibility of exposure to occasional fumes,

splashing, or spillage of corrosive materials. Because of this, it is sug-

gested that an industrial-grade acrylic coating system or a single coat of

epoxy be applied.

This recommendation is not suitable for interior surfaces that are frequently

cleaned or exposed to steam cleaning. Refer to Area 4. Recommended for

this area are systems A and C in Table 7.16.

7.9.3 area 3: Normally Dry Exteriors

This includes such areas as parking lots, water storage tanks, exterior stor-

age sheds, and lighting or power line poles that are exposed to sunlight in a

relatively dry location. Under these conditions, oil-based paints should last 6

or more years. Those materials that are resistant to UV rays and are normally

rated for exterior use include acrylics, alkyds, silicones, and polyurethanes.

Epoxies will lose gloss, normally chalk, and fade rapidly when exposed to

UV rays. Recommended systems include A in Table 7.15, C in Table 7.16, A in

Table 7.17, and A in Table 7.18.

TabLE 7.15

Multilayer Paint Systems Requiring Surface to Be Clean, Dry, and Free of Loose

Dirt, Oils, and Chemical Contamination

System A Aluminum Epoxy Mastic/Polyurethane Finish

Paint Layers:

One coat aluminum epoxy mastic: 4–6 mil dft (100–150 μm)

One coat polyurethane nish: 2–4 mil dft (50–100 μm)

Properties:

Excellent over tight rust

Tolerant of minimally prepared steel

May be applied to a wide range of surfaces, but normally used on carbon steel and concrete

This is a premium system to use when cleaning must be minimal

Limitations:

To cure properly, temperature must be above 50°F (10°C)

For lower temperature requirements, other aluminum epoxy/urethane mastics may be

substituted