© 2006 by Taylor & Francis Group, LLC

Corrosion Testing — Practice 139
TABLE 8.3
Evaluation of the Amount of Flaking
Grade Description
0 Very sharp cuts. No material has flaked.
1 Somewhat uneven cuts. Detachment of small flakes of the coating at the intersections
of the cuts.
2 Clearly uneven cuts. The coating has flaked along the edges and at the intersections
of the cuts.
3 Very uneven cuts. The coating has flaked along the edges of the cuts partly or wholly
in large ribbons and it has flaked partly or wholly on different parts of the squares.
A cross-cut area of no more than 35% may be affected.
4 Severe flaking of material. The coating has flaked along the edges of the cuts in large
ribbons and some squares have been detached partly or wholly. A cross-cut area of
no more than 65% may be affected.
5 A cross-cut area greater than 65% is affected.
tip depends not only on the energy with which the cuts are made (i.e., force and
speed of scribing) but also on the mechanical properties — plastic versus elastic
deformation — of the coating. For example, immediately in front of the knife-edge,
the upper surface of the paint undergoes plastic deformation. This deformation
produces a shear force down at the coating-metal interface, underneath the rim of
indentation in front of the knife-edge [11].
A major drawback to methods using lateral stresses is that they are extremely
dependent on the mechanical properties of the coating, especially how much plastic
and elastic deformation the coating undergoes. Paul has noted that many of these tests
result in cohesive cracking of coatings [11]. Coatings with mostly elastic deformation
commonly develop systems of cracks parallel to the metal-coating interface, leading
to flaking at the scribe and poor test results. Coatings with a high proportion of plastic
deformation, on the other hand, perform well in this test — even though they may
have much poorer adhesion to the metal substrate than do hard coatings.
Elastic deformation means that little or no rounding of the material occurs at
the crack tip during scribing. Almost all the energy goes into crack propagation. As
the knife blade moves, more cracks in the coating are initiated further down the
scribe. These propagate until two or more cracks meet and lead to flaking along the
scribe. The test results can be misleading; epoxies, for example, usually perform
worse than softer alkyds in cross-cut testing, even though, in general, they have
much stronger adhesion to metal.
For very hard coatings, scribing down to the metal may not be possible. Use of
the cross-cut test appears to be limited to comparatively soft coatings. Because the
test is very dependent on deformation properties of the coatings, comparing crosscut
results of different coatings to each other is of questionable value. However, the
test may have some value in comparing the adhesion of a single coating to various
substrates or pretreatments.
© 2006 by Taylor & Francis Group, LLC