Corrosion Control Through Organic Coatings Part 6 ppsx

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6 Weathering and Aging

of Paint

This chapter presents a brief overview of the major mechanisms that cause aging,

and subsequent failure, of organic coatings. Even the best organic coatings, properly

applied to compatible substrates, eventually age when exposed to weather, losing

their ability to protect the metal.

In real-life environments, the aging process that leads to coating failure can

generally be described as follows:

1. Weakening of the coating by significant amounts of bond breakage within

the polymer matrix. Such bond breakage may be caused chemically (e.g.,

through hydrolysis reactions, oxidation, or free-radical reactions) or

mechanically (e.g., through freeze–thaw cycling, which leads to alternat￾ing tensile and compressive stresses in the coating).

2. Overall barrier properties may be decreased as bonds are broken in the

polymeric backbone — in other words, as transportation of water, oxygen,

and ions through the coating increases. The polymeric network may be

plasticized by absorbed water, which softens it and makes it more vul￾nerable to mechanical damages. The coating may begin to lose small,

water-soluble components, causing further damage. Flaws such as micro￾cracks develop or, if preexisting, are enlarged in the coating.

3. Even more transportation of water, oxygen, and ions through the coating.

4. Deterioration of coating-metal adhesion at this interface.

5. Development of an aqueous phase at the coating/metal interface.

6. Activation of the metal surface for the anodic and cathodic reactions.

7. Corrosion and delamination of the coating.

Many factors can contribute in various degrees to coating degradation, such as:

• Ultraviolet (UV) radiation

• Water and moisture uptake

• Elevated temperatures

• Chemical damage (e.g., from pollutants)

• Thermal changes

• Molecular and singlet oxygen

• Ozone

• Abrasion or other mechanical stresses

The major weathering stresses that cause degradation of organic coatings are the

first four in the list above: UV radiation, moisture, heat, and chemical damage. And,

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