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17 NACE INTERNATIONAL: VOL. 56, NO. 8 MATERIALS PERFORMANCE AUGUST 2017 Information on corrosion control and prevention Using Thermal-Sprayed Aluminum to Protect Deep Sea Steel Structures F or many years, thermal-sprayed aluminum (TSA) has effectively protected offshore structures from the corrosive effects of seawater and the marine environment. Thermal spray, also known as metalizing, is a process that uses an energ y source to melt metal in powdered or wire form, and then propel or spray small molten or softened metal particles onto the surface of another metal or structure. 1 When they impact the substrate, the hot metal particles f latten and form pancake-shaped "splats." Many thermal-spray processes are avail- able, including electric arc spray, f lame spray, plasma spray, high-velocity oxy-fuel spray, detonation gun deposi- tion, and cold spray. The process selected depends on the desired coating proper- ties, component size, and cost. 2 According to NACE International member Shiladitya Paul with TW I (Cam- bridge, United K ingdom), TSA coatings are applied on subsea assets to protect carbon steel (CS) from corrosion because they act as a barrier as well as provide sacrif icial protection. In seawater, alumi- num has a low self-corrosion rate and is also anodic to CS. Because the galvanic potential of aluminum is more active than CS, TSA acts as an anode and pro- vides a substantial level of cathodic pro- tection (CP) at ambient and elevated tem- peratures. "TSA is essentially an evenly distributed anode," Paul says. "Instead of having anodes attached to the structure at specif ic points, you have a distributed anode that is a 250- to 300-micron thick layer. When you minimize the steel sur- face that is exposed, you drastically mini- mize the cathode area; and the dissolu- tion rate of your anode, in this case the TSA coating, is reduced quite a lot." Even when the TSA coating is dam- aged, corrosion of the CS substrate is not accelerated. The aluminum in the coating polarizes the CS in seawater to potentials in the protective range: –800 mV or below vs. a silver/silver chloride (Ag/AgCl) Continued on page 18 On or Near the Water… Canadian Office: 780-447-1114 Email: sales@irtrectifier.com U.S. Office: 918-805-7587 Website: www.irtrectifier.com We have the solution to help you Protect those Assets and our Environment !