ABSTRACT
Thermal Spray Aluminum (TSA) can be used to reduce anode demand or to extend anode life on projects with long design lives (i.e., 40 to 50 years). However, for subsea structures, TSA has not been used to replace the functionality of sacrificial anodes. In conventional CP design, TSA should not degrade while it remains connected to the CP system, draining current from sacrificial anodes, which ensure adequate cathodic protection.
During the CORROSION 2017 conference, a new concept named CP by distributed sacrificial anodes (DSA) was presented.1 The main principle was to convert the cathode area to anode area by distributing anode mass over the surface of the equipment to be protected. CP by DSA is achieved by the deposition of a single-layer metallic coating. In this work, DSA was applied by thermal spray (TS). DSA reduces the total exposed cathode area to small defects and imparts active cathodic protection.
In previous work, the outcome of exposure testing in flowing natural seawater at 10°C was discussed. In this paper, exposure in seawater at 50 °C and 80°C and in mud, are discussed. Freely exposed samples thermally sprayed with DSA and conventional TSA as well as galvanic couplings between DSA and both TSA and carbon steel were investigated.
INTRODUCTION
Over the last 50 years, only minor changes have been made to cathodic protection (CP) design. In this regard, optimization of the CP system has not been considered as a major cost saving opportunity to date. For subsea applications, sacrificial anodes, combined with organic coatings, is the main corrosion protection strategy per DNVl RP B4012 (structures) and ISOll 1558923 (pipelines). The total anode mass can be substantial, depending on the design life of the subsea system, the size and complexity of the structure to be protected, and the environmental conditions. Weight increase for subsea structures due to sacrificial anodes has not, up to now, been considered an aspect that can be substantially improved without designing for costly retrofit solutions. The total anode weight can be considerable, i.e., exceed 500-600 metric tons, for structures on field developments with a long design life, especially, uninsulated gas subsea systems.4 The weight of the CP system adds to the total structure weight, and can put special constraints on lifting vessels and cranes. It follows, therefore, that a lower anode mass can significantly reduce both fabrication and installation costs of the offshore structure. Additionally, less complex lifting operations can result in safer installation campaigns.
