## 4 Discussion  
### Corrosion Performance  
The equilibrium potential assumed by the metal (or the coating system) in the absence of any current flowing through the circuit is called the open circuit potential (OCP) or the rest potential. When the potential of a material in solution is forced away from OCP by using a potentiostat, it is referred to as polarising the material. The response of the material, current, is measured as it is polarised. This gives the potential versus current curve, which approximates to a straight line near the OCP. The slope of this line is called the polarisation resistance (Rp). Polarisation resistance is inversely proportional to the corrosion current and hence the corrosion rate.  
It is generally recommended to allow sufficient time for the OCP to stabilise before beginning the electrochemical test. A stable OCP is an indication that the system being studied has reached a'steady state'. However, in many cases the corrosion reaction rates are such that it may take many hours/days before the so called'steady state' is reached. This might explain the fluctuations in the potentials of some of the samples tested. The potentials attained by the samples (with different damage areas) after a few months are similar. In the absence of a thermally sprayed layer, the open circuit potential would assume the value of the carbon steel substrate. Thus, the presence of TSA polarises the entire specimen to potentials well below the so called protective potential of approximately \(-800\) mV, and while doing so the TSA gets preferentially consumed.  
Although polarisation resistance gives an indication of possible corrosion performance, the corrosion rate is related to other parameters (like the Tafel slopes) which are dependent on the material and the environment. As the nature of the surface governs the polarisation resistance, these values can change depending on the surface properties. For TSA coatings with damage, the corrosion rate values of TSA are also governed by the area of steel exposed. However, in the corrosion rate data this difference was only noticed in the first week of testing, indicating a reduction in the area of steel exposed with time. This'reduction' is related to the deposition of calcareous matter on the steel surface. Further, the surface of TSA had some corrosion products which also increased the polarisation resistance or decreased corrosion rate.  
The reduction in the corrosion rate with time of exposure, even when damaged, is an added benefit for the use of TSA in offshore service. The initial corrosion rate might be a bit higher when holidays are present, where the exposed steel is acting as a cathode and causing accelerated corrosion of the anodic TSA. However, the deposition of calcareous matter reduces the effective area of exposed steel. The nature of the calcareous matter, when present, governs the intensity of galvanic interaction between the TSA and the steel. The ionic path between the steel and the TSA is at least partially blocked by the calcareous matter. The conductivity of the calcareous matter-electrolyte combination somewhat determines the galvanic interaction. Thus, the calcareous matter plays a vital role is protecting the TSA-coated steel when damaged prior to service.