© 2006 by Taylor & Francis Group, LLC

138 Corrosion Control Through Organic Coatings
substrate, depending on which is the weakest link in the system. Therefore, adhesion
of the primer to the metal is not necessarily what this method measures, unless it
is at this interface that the adhesion is the weakest.
DPO methods suffer from some additional disadvantages:
• Tensile tests usually involve a complex mixture of tensile and shear forces
just before the break, making interpretation difficult.
• Stresses produced in the paint layer during setting of the adhesive may
affect the values measured (a glue/paint interactions problem).
• Nonuniform tensile load distributions over the contact area during the
pulling process may occur. Stress concentrated in a portion of the contact
area leads to failure at these points at lower values than would be seen
under even distribution of the load. This problem usually arises from the
design of the pulling head.
Unlike lateral stress methods, DPO methods can be used on hard or soft coatings.
As previously mentioned, however, for a well-adhering paint, these methods tend to
measure the cohesive strength of the coating, rather than its adhesion to the substrate.
With DPO methods, examination of the ruptured surface is possible, not only
for the substrate but also for the test dolly. A point-by-point comparison of substrate
and dolly surfaces makes it possible to fairly accurately determine interfacial and
cohesive failure modes.
8.2.2.3 Lateral Stress Methods
Methods employing lateral stresses to detach a coating include bend or impact tests
and scribing the coating with a knife, as in the cross-cut test.
In the cross-cut test, which is the most commonly used of the lateral stress
methods, knife blades scribe the coating down to the metal in a grid pattern. The
spacing of the cuts is usually determined by the coating thickness. Standard guidelines
are given in Table 8.2. The amount of paint removed from the areas adjacent
to, but not touched by, the blades is taken as a measurement of adhesion. A standard
scale for evaluation of the amount of flaking is shown in Table 8.3.
Analysis of the forces involved is complex because both shear and peel can
occur in the coating. The amount of shearing and peeling forces created at the knife
© 2006 by Taylor & Francis Group, LLC
TABLE 8.2
Spacing of Cuts in Cross-Cut Adhesion
Coating thickness Spacing of the cuts
Less than 60 µm 1 mm
60 µm–120 µm 2 mm
Greater than 120 µm 3 mm