Abstract
Twin wire arc spray (TWAS) coatings were produced under varying spray conditions (spray angle, traverse rate, and spray distance) to simulate on-site hand spraying operations typically used to coat existing refinery vessels. Two materials, Alloy C276 (commonly used for corrosion protection of refinery vessels) and the newly developed Nicko-Shield 200 chemistry (designed to reduce porosity and oxide content under arc spray conditions) were compared in the testing. Alloy C276 coatings showed good coating performance (>40 MPa adhesion) when sprayed under ideal conditions, but showed a sharp drop off in coating integrity (<20 MPa adhesion) when sprayed at lower traverse rates, sharper angles, and closer spray distances. Deviating from non-ideal conditions resulted in increased porosity and oxide content leading to increased permeability. It was concluded that non-ideal conditions, which intermittently occur in hand spraying operations on large surface areas, can lead to coating patches with unacceptably low adhesion, potential spalling, and high permeability when spraying Alloy C276. Patches of low coating quality require additional maintenance or result in coating failure, creating a lack of confidence in thermal spray technology as a protective solution in the industry. This study shows the results of an effort to develop an alloy solution which is more reliable in spraying large surface areas by hand for corrosion protection. The developed Ni-based material showed improved adhesion (70+ MPa) and greatly reduced permeability (as measured by ferroxyl exposure). This performance was stable across the wide range of spray conditions used in this study. This suggests that alloy design can be used to increase the reliability for twin wire arc spray coatings, and enable confidence for expanded use in this industry.
