© 2006 by Taylor & Francis Group, LLC

Corrosion Testing — Practice 135
8.2.1 GENERAL CORROSION
Direct evidence of corrosion can be obtained by macroscopic measurement of creep
from scribe, rust intensity, blistering, cracking, and flaking.
8.2.1.1 Creep from Scribe
If a coating is properly applied to a well-prepared surface and allowed to cure, then
general corrosion across the intact paint surface is not usually a major concern.
However, once the coating is scratched and metal is exposed, the situation is dramatically
different. The metal in the center of the scratch has the best access to
oxygen and becomes cathodic. Anodes arise at the sides of the scratch, where paint,
metal, and electrolyte meet [9]. Corrosion begins here and can spread outward from
the scratch under the coating. The coating’s ability to resist this spread of corrosion
is a major concern.
Corrosion that begins in a scratch and spreads under the paint is called creep or
undercutting. Creep is surprisingly difficult to quantify, because it is seldom uniform.
Several methods are acceptable for measuring it, for example:
• Maximum one-way creep (probably the most common method), which is
used in several standards, such as ASTM S1654
• Summation of creep at ten evenly spaced sites along the scribe
• Average two-way creep
None of these methods is satisfactory for describing filiform corrosion. The maximum
one-way creep and the average two-way creep methods allow measurement
of two values: general creep and filiform creep.
8.2.1.2 Other General Corrosion
Blistering, rust intensity, cracking, and flaking are judged in accordance with the
standard ISO 4628 or the comparable standard ASTM D610. In these methods, the
samples to be evaluated are compared to a set of standard photographs showing
various degrees of each type of failure.
For face blistering, the pictures in the ISO standard represent blister densities
from 2 to 5, with 5 being the highest density. Blister size is also numbered from
2 to 5, with 5 indicating the largest blister. Results are reported as blister density
followed in parentheses by blister size (e.g., 4(S2) means blister density = 4 and
blister size = 2); this is a way to quantify the result, “many small blisters.”
For degree of rusting, the response of interest is rust under the paint, or rust
bleed-through. Areas of the paint that are merely discolored on the surface by rusty
runoff are not counted if the paint underneath is intact. The scale used by ISO 4628
in assigning degrees of rusting is shown in Table 8.1 [10].
Although the ASTM and ISO standards are comparable in methodology, their
grading scales run in opposite directions. In measuring rust intensity or blistering,
© 2006 by Taylor & Francis Group, LLC