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Distributed Sacrificial Cathodic Protection – A New Cost Effective Solution to Prevent Corrosion on Subsea Structures

In this paper, a new concept named CP by distributed sacrificial anodes (DSA) is presented. The main principle of CP by DSA is to convert cathode area to anode area by distributing anode mass over the surface of the equipment to be protected.

Product Number: 51317--8941-SG
ISBN: 8941 2017 CP
Author: Roy Johnsen
Publication Date: 2017
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Sacrificial anodes combined with organic coatings are the main corrosion protection strategy used to prevent corrosion on equipment submerged in seawater. Depending on the lifetime of the subsea system the complexity of the structure to be protected and the environmental conditions the total anode mass can be substantial. For subsea structures the anode mass not only increases fabrication costs but also affects the total structure weight in a way that puts special requirements on lifting vessels and cranes.Thermal Spray Aluminum (TSA) has occasionally been used to replace organic coating on subsea structures especially to reduce current demand at elevated temperatures or to extend anode life on projects with long design lives (i.e. 40 to 50 years). However TSA has not been used subsea as an anode replacement to protect subsea structures. In conventional CP design TSA remains connected to the CP system draining current from sacrificial anodes to ensure adequate cathodic protection.In this paper a new concept named CP by distributed sacrificial anodes (DSA) will be presented. The main principle of CP by DSA is to convert cathode area to anode area by distributing anode mass on the surface of the equipment to be protected. CP by DSA is achieved by the deposition of either a dual- or a single-layer metallic coating which in this work was applied by thermal spray (TS). In a dual layer approach a conventional TSA coating is first applied directly on the carbon steel surface with a second sacrificial outer Al-Zn-In layer on top of the TSA. In a single layer design only the sacrificial Al-Zn-In coat is applied. The outer sacrificial coating reduces the total exposed cathode area to small defects and imparts active cathodic protection (CP) further reducing CP current demand. Results from exposure testing in flowing natural seawater at 10C and 40C are presented and discussed. Freely exposed samples thermally sprayed with DSA and conventional TSA as well as galvanic couplings between DSA TSA conventional anodes and carbon steel with area ratios of 100:1 and 10:1 are included in the test program. The performance of DSA and conventional TSA is compared with that of traditional sacrificial anodes. Results have shown that DSA can be used interchangeably with conventional anodes providing an equal amount of cathodic current.

Keywords; thermal sprayed aluminum, distributed sacrificial anode, seawater, corrosion, cathodic polarization, crevice corrosion

Sacrificial anodes combined with organic coatings are the main corrosion protection strategy used to prevent corrosion on equipment submerged in seawater. Depending on the lifetime of the subsea system the complexity of the structure to be protected and the environmental conditions the total anode mass can be substantial. For subsea structures the anode mass not only increases fabrication costs but also affects the total structure weight in a way that puts special requirements on lifting vessels and cranes.Thermal Spray Aluminum (TSA) has occasionally been used to replace organic coating on subsea structures especially to reduce current demand at elevated temperatures or to extend anode life on projects with long design lives (i.e. 40 to 50 years). However TSA has not been used subsea as an anode replacement to protect subsea structures. In conventional CP design TSA remains connected to the CP system draining current from sacrificial anodes to ensure adequate cathodic protection.In this paper a new concept named CP by distributed sacrificial anodes (DSA) will be presented. The main principle of CP by DSA is to convert cathode area to anode area by distributing anode mass on the surface of the equipment to be protected. CP by DSA is achieved by the deposition of either a dual- or a single-layer metallic coating which in this work was applied by thermal spray (TS). In a dual layer approach a conventional TSA coating is first applied directly on the carbon steel surface with a second sacrificial outer Al-Zn-In layer on top of the TSA. In a single layer design only the sacrificial Al-Zn-In coat is applied. The outer sacrificial coating reduces the total exposed cathode area to small defects and imparts active cathodic protection (CP) further reducing CP current demand. Results from exposure testing in flowing natural seawater at 10C and 40C are presented and discussed. Freely exposed samples thermally sprayed with DSA and conventional TSA as well as galvanic couplings between DSA TSA conventional anodes and carbon steel with area ratios of 100:1 and 10:1 are included in the test program. The performance of DSA and conventional TSA is compared with that of traditional sacrificial anodes. Results have shown that DSA can be used interchangeably with conventional anodes providing an equal amount of cathodic current.

Keywords; thermal sprayed aluminum, distributed sacrificial anode, seawater, corrosion, cathodic polarization, crevice corrosion

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