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Offshore contractor consults on coating integrity

Connect, no. 128, January/February 2004, p.3

When a leading offshore contractor expressed doubt about the integrity of a crucial protective coating used on one of its installations, it turned to TWI for an independent assessment of the work.

It commissioned two coating contractors to spray key areas of an electrical swivel with an anti-galling tungsten carbide cobalt coating using two different high velocity oxyfuel spraying processes, one using propylene as the fuel, the other using kerosene.

Upon completion it asked TWI for a third party assessment of the work.

TWI carried out extensive testing of coated coupons, supplied by the client, to determine the coating bond strength, porosity, carbide fraction and micro-hardness.

Although bond strengths and carbide fractions exceeded minimum requirements in both cases, coating porosity and micro-hardness measurements highlighted anomalies in the coating contractors' specifications. In particular, the porosity level of 7% in the coating prepared by the propylene-fuelled HVOF process was considered to be excessive, and the micro-hardness of 1200HV of the coating prepared by the kerosene-fuelled HVOF process was more than 100HV below the contractor's specification. Neither of these anomalies would necessarily have reduced their fitness for purpose, but merely highlighted differences between the characteristics of coatings prepared in the laboratory and those prepared on real components.

SEM image of tungsten carbide cobalt coating
SEM image of tungsten carbide cobalt coating
SEM coating image analysis highlighting porosity (black) and tungsten carbide particles (red)
SEM coating image analysis highlighting porosity (black) and tungsten carbide particles (red)

Micrographs of one of the coatings highlighted the presence of blasting grit embedded in the substrate, which is a common feature due to excessive blasting. Although in this case, minimum bond strength requirements were exceeded, this level of embedded grit would generally raise concerns regarding bond strength.

TWI concluded that both coatings were likely to be fit for purpose but recommended that certain refinements should be made to the procedure for the propylene fuelled system. The advice was that hydrogen should be used instead of propylene as the fuel gas on future applications and that the blasting practice should be modified to eliminate embedded grit particles.

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