ABSTRACT

All ships need periodic maintenance, either onboard a FPSO, in a water ballast tank or a hull side of a marine vessel. When preparing for maintenance of such structures it has been standard practice to sandblast the surface prior to coating application, in recent years it has become more common to use water jetting for this purpose. However, water jetted surfaces are prone to formation of flash rust. The severity of the flash rust increases with temperature, relative humidity and soluble salt contamination on the surface, and the flash rust can create unpredictable surface conditions. Further, after water jetting the surface may be moist. Hence in order to ensure high quality and maintain production speed, coatings for maintenance should be surface tolerant and adhere well to the substrate even if the surface is moist and flash rusted. In order to mitigate the effect of less-than-ideal surface on the protective performance of the coating, a new epoxy coating showing outstanding performance when applied on flash rusted and moist surfaces was developed especially for water jetted surfaces.

INTRODUCTION

All ships need periodic maintenance, either onboard a FPSO, in a water ballast tank or a hull side of a marine vessel. When preparing for maintenance of such structures it has been standard practice to sandblast the surface. Sandblasting creates superb surfaces for coating application with ultimate cleanliness, but due to the dust generated by the process it is facing more local restrictions due to environmental impact and HSE. Hence, in recent years it has become more common to use water jetting for surface preparation. Water jetting can also be a more environmentally friendly method than traditional sandblasting due to lack of abrasives used in the process. However, water jetted surfaces are prone to formation of flash rust after water jetting and in contrast to sandblasting such cleaning does not create any additional surface roughness.

The severity of the flash rust is influenced by various parameters like temperature, relative humidity and soluble salt contamination on the surface. The formation can be quite extensive creating unpredictable surface conditions. Flash rust can be a loose adhering layer on top of the surface and is unwanted prior to coating since it is compromising the adhesion between coating and substrate. Due to the various factors influencing flash rust formation, the flash rust layer will develop differently when exposed in natural environments, in addition to salt content the corrosion will have a different build up with air pockets due to variations in ventilation and humidity. Flash rust is today categorized into light, medium and heavy grade by visual standards 1, 2. The standard is based on visual inspection and a rubbing test using a cloth and is a subjective method that may lack vital information on the properties of the rust layer.

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