Elsevier

Corrosion Science

Volume 44, Issue 11, November 2002, Pages 2561-2581
Corrosion Science

Corrosion behaviour of HVOF sprayed SUS316L stainless steel in seawater

https://doi.org/10.1016/S0010-938X(02)00030-6Get rights and content

Abstract

SUS316L stainless steel was coated onto the SUS316L plate by the high velocity oxy-fuel (HVOF) spray technique. Its corrosion behaviour in seawater was investigated by the electrochemical method and the microscopy. The coating had corrosion resistance inferior to the bulk plate. The corrosion of the HVOF sprayed SUS316L coating was related to both its porosity and its oxygen content. Depending on them, the corrosion took place at the small pore and the boundary between the spray particles on the surface.

Introduction

An offshore or a marine structure is located in the corrosive environment containing abundant sea salt, sea wave, and sunlight. To prevent the corrosion of the structural steel under such a severe condition, various methods are applied corresponding to each site of the structure. Under the sea, the electrical protection using the sacrificial anode is effective. In the mild tidal zone, the steel is covered with the insulating varnish or paint. In the severe tidal and the splash zones, the coating requires the sufficient physical strength in addition to corrosion resistance. Therefore, cladding of the anti-corrosion metal and alloy is adopted in these zones.

In the Ultra Steels Research Project (STX-21 Project), the thermal spraying was suggested as a coating method alternative to cladding and as a field-mending method of the damaged clad steel. This research has been studied since 1997 [1]. The high velocity oxy-fuel (HVOF) spraying is able to make a denser and less-oxidized coating, compared with other method such as the plasma spraying [2]. Furthermore, this spraying system enables the metal and the alloy with the high melting point up to about 2000 °C to be deposited on the target substrate. These features are suitable for an application to the corrosion resistant coating.

So far, several HVOF spray coatings have been subjected to the corrosion test in seawater. In addition to the cermets [3], [4], [5], the anti-corrosion alloys [6], [7], [8] were adopted as the coating materials. These studies revealed that the HVOF method was superior to other spraying techniques to make the coating having higher corrosion resistance.

In general, the coating is formed on the steel, based on the assumption that the sprayed material has the same corrosion resistance as the original bulk material. Although this assumption may be acceptable, it is essential to reveal the corrosion behaviour of the spray deposit itself. In this paper, SUS316L stainless steel was coated onto the SUS316L bulk plate by the HVOF spray technique to avoid formation of the galvanic couple by combination of the noble coating and the less-noble substrate. SUS316L was one of the widely used stainless steel and was selected as a spray material. The corrosion behaviour of the sprayed coating in seawater was investigated using the electrochemical measurements and the microscopic observations.

Section snippets

Specimen preparation

The HVOF spraying of SUS316L was carried out with a TAFA apparatus (JP-5000). The schematic illustration of this spraying system is shown in Fig. 1. The spray process was described as follows; the mixture of an oxygen gas and a fuel (kerosene) was ignited by the spark plug, resulting in making a flame with the sonic speed. Then, the powder was fed into the flame and was accelerated to become a flight particle. Immediately, the flight particle was impinged and deposited onto the substrate,

Physical properties of HVOF sprayed SUS316L coating

Fig. 5 shows the relation between the combustion pressure and the porosity of the HVOF sprayed SUS316L coating, superimposing the relation between the combustion pressure and the oxygen content of the coating. The content of other elements of the coating was almost the same as that of the supply powder, regardless of the spray condition. As the combustion pressure increased, the porosity decreased while the oxygen content increased. The comparatively high oxygen content indicated the presence

Conclusion

The coating of SUS316L stainless steel deposited on the SUS316L substrate with the HVOF technique possessed a similar characteristic to the ordinary stainless steel in artificial seawater. In addition to the comparatively high current value in the passive region and the low starting potential of pitting in the anodic polarization curve, however, the low polarization resistance by the AC impedance measurement indicated poor corrosion resistance of the coating, compared with the SUS316L bulk

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