Investigation of Spray Parameters on Coating Performance of Corrosion Resistant Nickel-Based Twin Wire Arc Spray Coatings

Twin wire arc spray (TWAS) coatings were produced under a varying spray conditions (spray angle, traverse rate, and spray distance) to simulate the on-site  hand spraying operations of existing refinery vessels.  Two materials, Alloy C276 (commonly used for corrosion protection of refinery vessels) and a newly developed Ni-based (designed to reduce porosity and oxide content under arc spray conditions) were compared in the testing.  Alloy C276 coatings showed good coating performance (as measured by adhesion and permeability testing) when sprayed under ideal conditions, but showed a sharp drop off in coating integrity when sprayed at lower traverse rates, sharper angles, and closed spray distances.  The drop in performance resulted in immediate coating spalling in the worst case, but generally resulted in increased porosity and oxide concentration coupled with increased permeability.  Non-ideal conditions, which intermittently occur in hand spraying operations on large surface areas lead to coating patches with unacceptably low adhesion and high permeability.  Such patches lead to additional maintenance or coating failure and a lack of confidence in the thermal 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 and greatly reduced permeability (as measured by ferroxyl exposure).  This performance was stable across the wide spray conditions range 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.