Corrosion Control Through Organic Coatings Part 7 docx

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7 Corrosion Testing —

Background and

Theoretical

Considerations

The previous chapter described the aging process of an organic coating, which leads

to coating failure. The major factors that cause aging and degradation of organic

coatings are UV radiation, moisture, heat, and chemical damage. Unfortunately for

coating formulators, buyers, and researchers, aging and breakdown of a good coating

on a well-prepared substrate takes several years to happen in the field. Knowledge

about the suitability of a particular coating is, of course, required on a much shorter

time span (usually “right now”); decisions about reformulating, recommending,

purchasing, or applying a paint can often wait for a number of weeks or even a few

months while test data is collected. Years, however, are out of the question. This

explains the need for accelerated testing methods. The purpose of accelerated testing

is to duplicate in the laboratory, as closely as possible, the aging of a coating in

outdoor environments — but in a much shorter time.

This chapter considers testing the corrosion-protection ability of coatings used

in atmospheric exposure. The term ‘‘atmospheric exposure” is understood to include

both inland and coastal climates, with atmospheres ranging from industrial to rural.

Tests used for underwater or offshore applications are not within the scope of this

chapter. A very brief explanation of some commonly used terms in corrosion testing

of coatings is provided at the end of this chapter.

7.1 THE GOAL OF ACCELERATED TESTING

The goal of testing the corrosion-protection ability of a coating is really to answer

two separate questions:

1. Can the coating provide adequate corrosion protection?

2. Will the coating continue to provide corrosion protection over a long

period?

The first question is simple: Is the coating any good at preventing corrosion?

Does it have the barrier properties, or the inhibitive pigments, or the sacrificial

pigments to ensure that the underlying metal does not corrode? The second question

is how will the coating hold up over time? Will it rapidly degrade and become

useless? Or will it show resistance to the aging processes and provide corrosion

protection for many years?

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