Atmospheric Corrosion Behavior of Arc Thermal Spray Zn-Al Coatings Using Accelerated atmospheric Field Test for Ten-Years
Authors: Nakano Atsushi , Oshikawa Wataru , Yonezawa Noboru , Morita Chihiro , Shirai Yoshihiro 
Published: December 2020

Thermal spray such as Al, Zn-Al and Al-Mg is one of the surface treatments technologies that long-term high corrosion resistance can expect. Therefore, in recent years, these coatings technologies is widely used for infrastructures such as a large-scale bridges, wind-power generating tower and a intricately shaped pole transformer. However, there are few reports about the corrosion life investigations while a sprayed coating is applied in various environment.

In this study, as a basic study on corrosion life of thermal spray coatings, accelerated field test with artificial sea water for 10 years were carried out for thermal spray Zn-Al coatings. By using artificial sea water, accelerated field test accelerates corrosion due to chloride in comparison with a general atmosphere field test. Thermal sprayed specimens were prepared using an arc thermal spraying method with compressed air on carbon steel subjected surface treatment. The dimensions of specimens for corrosion field tests are 70×150×2.3mm. The edges of each specimens were protected by a coating of epoxy resin. The chemical composition (mass%) of arc spraying Zn-Al layer is 70% Zn, 30% Al. The average thickness of arc spraying layer were 150, 120 and 90µm, respectively, which was obtained by measuring the thickness of arc spraying layer from confocal laser scanning microscope. After arc spraying, sealing was not performed to penetrate the inherent pores and void of arc spraying layer. Furthermore, the average thickness 90µm of hot dip galvanized was prepared as comparison specimens of arc spraying Al-Zn coating. The scratched were made straight on a lower part center of each specimen surface. The accelerated field test were performed with artificial sea water in conformity with ASTM D1141. The artificial sea water was applied to the surface of each specimens uniformly by a brush as 24 hour is set one cycle. In additions, a general atmospheric field test was carried out for the same periods. Atmospheric corrosion behavior of arc thermal spray coating were evaluated by surface observation, X-ray diffraction (XRD), coating thickness measurement, element analyses using electro probe micro-analyser (EPMA) and electrochemical measurement.

In arc spray Zn-Al coating for 10 years under accelerated field test, the granular white corrosion product were observed uniformly on the whole surface of each coating thicknesses. In hot dip galvanized coating, the powdery white corrosion product of Zn were observed uniformly on the whole surface. Furthermore, red rust was slightly observed at the edge of specimen. As a results of XRD, the corrosion product of the arc sprayed coating formed by an accelerated field test using the artificial sea water formed the product which is the same as a general atmosphere field test. In the coating thickness of Zn-Al 150µm, there is no significant difference in the coating thickness in atmospheric field tests for 10 years. On the other hand, the coating thickness of Zn-Al coating 90µm decreased. In addition, the effects of artificial sea water cannot be recognized in each thickness of arc spray Zn-Al. In EPMA analysis, corrosion products of Zn6Al2(OH)16CO3・4H2O were preferentially formed in the scratched regions of the arc spray Zn-Al surface.

Consequently, arc spray Zn-Al coating maintained the high corrosion resistance in an accelerated field test for 10 years.