Evaluation of Painted Steels for Transmission Towers Under Accelerated Deterioration Tests
Authors: Maeda Mari , Yasumoto Kenji , Tani Junichi 
Published: December 2024

Painted transmission towers are repainted after removing the former paint film and corrosion products, because the deterioration of paint film and under-film corrosion take place with time. To judge the appropriate timing to repaint for transmission towers, it is of great importance to estimate the deterioration of painted steels. While atmospheric exposure tests can provide the deterioration behavior close to the actual environment, considering the lifetime of paintings, it takes a long time to obtain data. Thus, it is necessary to establish a test method that can evaluate a variety of painting specifications in a short period of time. The accelerated deterioration tests have been performed using salt spray and ultraviolet (UV) irradiation for various painted structures. However, these test conditions are diverse, and the relationship between them and the degree of deterioration remains to be elucidated.

In this study, the deterioration behavior of painted steels for transmission towers was investigated by electrochemical impedance spectroscopy (EIS) and paint film analysis during the accelerated deterioration test.

The painted specimens were prepared with the paint film thickness of 100 μm or less using a type of paint that has been widely used for transmission towers in recent years. The painting specifications were the double-layer paint with polyurethane resin for primer and topcoat (PP), single-layer paint with acrylic modified alkyd resin (A), double-layer paint with epoxy resin for primer and polyurethane resin for topcoat (EP), and single-layer paint with polyurethane resin for primer (P). The accelerated deterioration test of each painted specimen was performed using combined corrosion test (CCT) consisting of salt spray, dry, and wet processes, and artificial UV weathering test with fluorescent UVA lamps (UVT). This test was repeated alternatively 50 cycles of CCT and 50 cycles of UVT as one set until 10 sets. The upper area of specimens was exposed to CCT and UVT environments, and the other area was exposed to only CCT environment by shielding UV radiation. The deterioration behavior of painted steels was monitored by measuring EIS each one set. In addition, the surface condition of paint film was observed, and evaluated by measuring of gloss and thickness.

The paint film resistance R
f was obtained by analyzing impedance spectra, assuming an equivalent circuit for painted metals. In the area exposed to CCT and UVT environments, the R
f of A-painted steel decreased significantly at the earliest timing, followed by P-painted steel. On the other hand, the R
f of PP-painted steel after 10 sets was almost same as that before tests. The gloss retention of all painted steels in the area exposed to CCT and UVT environments decreases to about 50 %, and that in the area exposed to only CCT environment decreases to 70-80 %. The R
f of PP-painted steel kept a high value even when the gloss was decreased. Accordingly, it was showed that the slight initial change in surface roughness did not significantly affect the environmental barrier properties. For all painted steel except P-painted steel, there was almost no change in the paint film thickness after the test with or without UV irradiation. The R
f, gloss retention, and paint film thickness of P-painted steel in the area with UV irradiation were lower than in the area without UV irradiation. Moreover, many blisters and corrosion products were observed on the surface of P-painted steel only in the area with UV irradiation. These results revealed that the paint film was accelerated to deteriorate by UV radiation, and under-film corrosion progressed.

In conclusion, the initial paint deterioration for painted steels in some specifications was clarified through the accelerated deterioration tests combining CCT and UVT. However, the deterioration of painted steels was not accelerated enough to decrease the paint film thickness and R
f of the double-layer painted steel in this test condition.