In recent years, the rate of environmental contamination is being accelerated due to the rapid development of industrial society. Thus, many types of structural steels, which are being widely used in various areas, such as land, sea, etc. are often exposed to severely corrosive environments. In order to protect these structural steels, many protection methods have been developed. These methods can be largely divided with two groups : surface improvement technology, such as nitrizing, carbonizing, ionic injection, metallic diffusion, etc. and surface coating technology such as painting, plating, clad welding, PVD(physical vapor deposition, CVD(chemical vapor deposition), metalizing, etc. Among the metallizing methods, thermal spray is generally explained as follows: metal or nonmetal is changed with fusion or semi fusion conditions by heat sources and then these melted materials are sprayed and striked to the substrate at a high speed, after then it is deposited on the substrate. Metallizing technology is a surface coating method which has been comparatively recently developed compared to the other methods, and its technology has many advantages compared to other surface coating technology.
In recent years, zinc and aluminum are often used with metallizing materials to control the corrosion on structural steel, however, it is generally accepted that these coating films are easy to deteriorate in severely corrosive environments. Furthermore, there was a little experimental result related to the evaluation of corrosion resistance in an atmospheric environment, also there have been barely any examination results associated with corrosion resistance of these coating films in seawater solution.
Therefore, in this study, four types of coating films (pure zinc, pure aluminum, and two types of alloyed films such as Al:Zn=85:15, Al:Zn=95:5 were deposited on the substrate with three types of film thickness such as 200㎛, 300㎛ and 400㎛ by using the thermal spray method, and the corrosion resistance of their coating films was investigated with an electrochemical method in seawater solution.
The pure aluminum sample had the high corrosion resistance with regardless of film thickness when it was exposed to seawater solution and alloy (Al:Zn=95:5, Al:Zn=85:15) samples were followed by the pure aluminum sample. In particular, the alloy film(Al:Zn=85:15), the galvalume which has a higher corrosion resistance in atmospheric environment was exhibited a lower corrosion resistance than that of pure aluminum film in seawater solution. The pattern of general corrosion was observed at the surface of pure zinc film, while, the pure aluminum and alloyed films were exhibited the pattern of localized corrosion. The pure aluminum and zinc films were exhibited more or less a lower porosity ratio compared to alloyed films, and the porosity ratio of pure aluminum and of the other films tended to change with increasing of film thickness. Furthermore, we can see that the characteristics of corrosion resistance is varied with types of film and its thickness.
Consequently, it is suggested that the results of this study may be used as good reference data to develop metallizing technology as well as to apply the metallizing method for corrosion control of structural steel in seawater solution.