STATE-OF-THE-ART OF THERMAL SPRAY COATINGS FOR CORROSION PROTECTION

摘要

Thermal-spray coatings are widely used in marine structures including offshore pipelines without external cathodic protection (CP). Al, Zn and Zn-Al thermal-spray coatings protect steel by acting both as barrier coatings and as sacrificial anodes at local defects where corrosion would otherwise occur. Zn provides better galvanic protection whereas Al is better as a less-reactive barrier layer. Zn-Al alloys appear to combine the protective properties of both Zn and Al. Although further research is required in order to specify the optimal alloy compositions for specific applications, 85 percent Zn-15 percent Al alloy is widely used. The best long-term protection is provided by suitably primed, sealed, and painted thermal-spray coatings.Thermal-spray coatings of acceptable structures and properties can be produced by flame spraying (wire or powder), arc spraying or plasma processing. However, due to economical reasons low melting point metals and their alloys are sprayed either by arc or flame.Surface preparation is considered to be a key factor in the production of uniform high quality coatings with maximum bond strength. Also of equal importance are the control of process facilities, equipment selection, and quality of consumable material for applying thermal-spray coatings.Well-bonded, relatively dense, sealed coatings have the ability to provide effective long term corrosion protection (10-20 years), with minimum periodic maintenance.Standards for evaluating thermal spray coatings have recently been developed.