Weatherability of arc Thermal Spray Al-Mg Coating in a Simulated Marine Environment
Authors: Nakano Atsushi , Oshikawa Wataru , Yonezawa Noboru , Morita Chihiro , Shirai Yoshihiro 
Published: December 2020

Plasma thermal spray Al-Mg coating are extensively used in critical infrastructures such as a large-scale bridges, power transmission tower because of their high corrosion protection in corrosive environments. 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 plasma spray coatings, atmospheric field test in a simulated marine environment for 3 years were carried out for plasma spray Al-Mg coatings. By using artificial sea water, the atmospheric field test in a simulated marine environment accelerates corrosion due to chloride in comparison with a general atmosphere field test. Plasma sprayed specimens were prepared using a plasma spraying method with inert gas 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 plasma spraying Al-Mg layer is 95% Al, 5%Mg. The average thickness of plasma spraying layer were 175, 90, 60 and 30µm, respectively, which was obtained by measuring the thickness of plasma spraying layer from confocal laser scanning microscope. After plasma spraying, sealing was not performed to penetrate the inherent pores and void of plasma spraying layer. Furthermore, the average thickness 90µm of hot dip galvanized was prepared as comparison specimens of plasma spraying Al-Mg coating. The atmospheric field test in a simulated marine environment 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. Application quantity of artificial sea water is 2.3×10-3L per one specimen. In additions, a general atmospheric field test was carried out for the same periods. Atmospheric corrosion behavior of plasma 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 plasma spray Al-Mg 30µm coating, red rust occurred on the whole surface of specimen in 3 months, and corrosion protection performance cannot be obtained. In surface observation of Al-Mg 175, 90 and 60, there is no significant changes in a simulated marine environment for 3 years. In XRD spectrum, Mg(OH)2 was identified as the corrosion products formed on Al-Mg coating. The coating thickness of Al-Mg 30µm decreased in a simulated marine environment for 3 years. In EPMA analysis, element Mg always coexisted with Al in the plasma spraying layer. Some voids and pored existed in the inner part of plasma spraying, and oxide which invaded in the inner part of plasm spraying coating occured around these voids and pored.

Consequently, in the coating thickness smaller, plasma spray layer cannot be maintained the high corrosion resistance in a simulated marine environment for 3 years.