Steps forward in the implementation of cathodic protection based on sediment energy

PARADISO, AGUSTÍN; SIMISON, SILVIA; BUSALMEN, JUAN PABLO

Nice

Simposio; 61st Annual Meeting of the International Society of Electrochemistry; 2010

ISE

Stainless steel and nickel alloys are susceptible to fail by localized corrosion when exposed to seawater. Cathodic protection can be employed in order to avoid damage, but specialists have not arrived yet to a consensus in the definition of a protection criterion for this kind of materials. The criterion most widely used in practice is that coming from the implementation of cathodic protection of carbon steels (-0.805 V vs. Ag/AgCl), which lead to extremely high current density requirements to maintain polarization in passive alloys and to severe problems of hydrogen embrittlement in susceptible ones. It has been recently demonstrated that using energy stored at the sediment/water interface it is possible to sustainably feed low amperage devices. In the same way, experiments carried out in our laboratory showed that sediment energy can be used to cathodically protect stainless steel UNS30403 against localized corrosion. In that work a protection criterion of ­-0,2 V (Ag/AgCl) has been stablished. The same protection criterion has been used here to protect UNS30316 stainlees steel and a nikel-based alloy (UNS N06200). Based on previous evidences, we implemented a laboratory scale seawater/sediment flow system. Graphite electrodes buried in marine sediment, developing a potential of -0.45 V vs. SCE, were used as anodes to cathodically polarize stainless steel and nickel based alloys coupons exposed to seawater. Graphite anodes were connected to the samples to protect through a variable resistor of 100 kΩ. The potential of the samples connected to graphite anodes was maintained at −0.2 V (SCE) by the iterative adjustment of the external load throughout the experiments (120 days). Electrochemical reactions involved in current generation were analyzed by chronopotenciometric and voltammetric measurements. Chronopotenciometric experiments performed on burried graphite electrodes showed that sediment reactions can sustain an equilibrium potential of about −200 mV at a load as high as 0,15 μA.cm2. On the other hand, the cathodic current density needed to protect UNS304 stainless steel was determined to range from 0,01 to 0,015 μA.cm2, while that needed for UNS30316 ranged from 0,005 to 0,01 μA.cm2. Presented results demonstrate that sediments can be an inexpensive and environmentally friendly source of energy for extending the service life of stainless steels in seawater by cathodic polarization.