Nondestructive testing of defects in adhesive joints

other hand, the coatings containing PANI exhibit a self healing effect i.e. although there is a
decrease in the OCP initially, it shifts to more anodic side after some time. This tendency to shift
the OCP to potentials more nobel than the original bare steel increases with the increase in the
TiO2 content in the PANI-nano TiO2 composite based coatings. In fact for PANI-TiO2-4.18%, the
OCP remains on the high anodic side of the other values even after 40 hr of exposure to hot saline
atmosphere. High OCP value compared to that of bare substrate as well as plain PVB coated steel
clearly indicates the high corrosion resistance provided by these coatings. It may also be noted
that the breadth of the Tafel curve in the PANI-TiO2-4.18% is much more than other
composition.
OCP ( V)
0
-0.1
-0.2
-0.3
-0.4
-0.5
0 10 20 30 40 50
Time (Hr)
4.18% TiO2
2.85% TiO2
2.3 % TiO2
1.3%TiO2
Pure PANI
Pure PVB
Figure: 3.OCP Vs Time graph of PVB-PANI containing different percentage of TiO2
The corrosion rate (CR) derived from the Icorr values using the equation
CR = 0.129 Icorr (EW) / (A.d )
Where EW is equivalent weight, A the area and d the density is shown in figure: 4 with the
exposure time to hot saline conditions. It is interesting to observe that the corrosion rate initially
increases but then tends to taper off above 10 hours to 20 hours. However, there is again a second
region above 30 hours where the corrosion rate is seen to increase sharply. These changes can be
attributed to the failure of the resin matrix, self healing due to PANI and then catastrophic failure
above drastic conditions of corrosion testing. It can be noted that these changes are completely
minimized in the coatings containing PANI-TiO2-4.18% which withstand even the drastic
conditions where other coatings are seen to completely give away.
Corrosion rate
(0.1 mm/year)
6
5
4
3
2
1
0
PVB+PANI
+PANI-TiO2 1.3%
+PANI-TiO2 2.8%
+PANI-TiO2 4.18%
0 10 20 30 40 50
Time of exposure (hrs)
Figure: 4.Corrosion rate with respect to prolonged exposure of coatings to hot saline conditions.