Calcareous scales formed by cathodic protection—an assessment of characteristics and kinetics

This paper reports the effect of boiling synthetic seawater on the performance of damaged Thermally Sprayed Aluminum (TSA) on carbon steel. Small defects (4% of the sample's geometric surface area) were drilled, exposing the steel, and the performance of the coating was analyzed for corrosion potential for different exposure times (2 h, 335 h, and 5000 h). The samples were monitored using linear polarization resistance (LPR) in order to obtain their corrosion rate. Scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used for post-test characterization. The results showed that a protective layer of Mg(OH) 2 formed in the damaged area, which protected the underlying steel. Additionally, no coating detachment from the steel near the defect region was observed. The corrosion rate was found to be 0.010-0.015 mm/year after 5000 h in boiling synthetic seawater.

Section: Discussionmentioning

confidence: 78%

Section: Discussionmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

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Thermally Sprayed Aluminum Coatings for the Protection of Subsea Risers and Pipelines Carrying Hot Fluids

Cé

Paul

2016

“…The following increase of Rct gradually may be attributed to the repair of the passive film because of the alkaline environment formed due to cathodic polarization [42,43]. At the first stage before the 10-h polarization, Figure 4a is considered to be suitable for representing the electrode processes, which are dominated by the corrosion reaction of metal and the effect of passive film [30,39,40]. At the second stage after that time, a new semi-circle appeared at the high frequency region in the Nyquist plot as shown in Figure 3a, which is related to the produced calcareous deposits.…”

Section: Eis Evolution Of 304 Ss Withmentioning

confidence: 99%

Evolution of Calcareous Deposits and Passive Film on 304 Stainless Steel with Cathodic Polarization in Sea Water

Sun

Huang

et al. 2018

Abstract:The change of protective current density, the formation and growth of calcareous deposits, and the evolution of passive film on 304 stainless steel (SS) were investigated at different potentials of cathodic polarization in sea water. Potentiostatic polarization, electrochemical impedance spectroscopy (EIS), and surface analysis techniques of scanning electron microscopy (SEM), energy dispersive X-ray (EDX) microanalysis and X-ray diffraction (XRD) were used to characterize the surface conditions. It was found that the protective current density was smaller for keeping polarization at −0.80 V (vs. saturated calomel electrode (SCE), same as below) than that at −0.65 V. The calcareous deposits could not be formed on 304 SS with polarization at −0.50 V while it was well protected. The formation rate, the morphology, and the constituent of the calcareous deposits depended on the applied potential. The resistance of passive film on 304 SS decreased at the first stage and then increased when polarized at −0.80 V and −0.65 V, which was related to the reduction and the repair of passive film. For the stainless steel polarized at −0.50 V, the film resistance increased with polarization time, indicating that the growth of oxide film was promoted.

“…In previous studies it has been established that the deposit has a two layer structure made up of a fine layer of Mg-rich brucite followed by a Ca-rich polymorph [7,14,[16][17][18][19][20][21]. Leeds and Cottis [22] found that a CaCO3 only deposit occurred at a potential of −0.7 V (SCE) and Barchiche et al [19] suggested an aragonite deposit would be formed at a potential −0.9 ≤ E ≤ −1.1 V (SCE), a combination of aragonite and brucite at −1.2 V (SCE) with brucite the only phase forming at a potential more negative than −1.3 V (SCE) in the 10-30 °C range with rotating electrodes.…”

Section: Introductionmentioning

confidence: 99%

Natural Deposit Coatings on Steel during Cathodic Protection and Hydrogen Ingress

Smith

Paul

2015

Abstract:The calcareous coating formed during cathodic protection (CP) in seawater is known to reduce the current demand by hindering the transport of species required to support the cathodic reactions and, thereby, improve the economic performance of CP systems. There is, however, uncertainty as to whether the coating reduces hydrogen uptake or indeed enhances it. To ascertain this, two sets of samples were polarized at −1.1 V (standard calomel electrode, SCE) in 3.5% w/v NaCl and synthetic seawater (ASTM D1141) at 20 °C and the diffusible hydrogen content measured over a period of 530 h. Under such conditions reports suggest a deposit with two distinct layers, comprising an initial brucite layer followed by an aragonite layer. Contrary to other findings, a fine initial layer containing Ca and Mg followed by a brucite layer was deposited with a few specks of Ca-containing zones in synthetic seawater. The hydrogen uptake was found to occur within the initial 100 h of exposure in synthetic seawater whilst it continued without the benefit of a deposit coating, i.e., in 3.5 wt % NaCl solution.