A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater

Abstract

Calcareous deposits were formed on steel under conditions of cathodic protection in artificial seawater at applied constant current densities ranging from 50 to 400 mA·m −2. The calcareous layers were characterized using a Field Emission Gun Scanning Electron Microscope (FEG SEM) in conjunction with Energy Dispersive X-Ray Analysis (EDX), and Electrochemical Impedance Spectroscopy (EIS). At cathodic current densities of 50-100 mA·m −2 where corrosion was still occurring, a clear correlation existed between the iron containing corrosion product and the overlying magnesium hydroxide layer. This revealed that the mapping of magnesium rich areas on a steel surface can be used in the identification of local corrosion sites. At current densities of 150-200 mA·m −2 , a layered deposit was shown to occur consisting of an inner magnesium-containing layer and an outer calcium-containing layer. At current densities of 300-400 mA·m −2 , intense hydrogen bubbling through macroscopic pores in the deposits gave rise to cracking of the deposited film. Under such conditions deposits do not have a well-defined double layer structure. There is also preferential formation of magnesium-rich compounds near the steel surface at the early stages of polarisation and within the developing pores and cracks of calcareous deposits later on. Based on SEM/EDX investigation of calcareous depositions the impedance model was proposed and used to monitor in situ variations in steel corrosion resistance, and to calculate the thickness of formed deposits using the length of oxygen diffusion paths. OPEN ACCESSMetals 2015, 5 440

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

“…The potential is an important factor to consider for cathodic protection. By shifting the potential negatively from open circuit potential (OCP), the thermodynamic trends of corrosion reaction can be mitigated, the adsorption of aggressive chloride ions can be held back, and the pH value close to the surface can be elevated [5]. All the above results can exert a positive effect on the anti-corrosive performance of 304 SS.…”

“…The weak Fe signal may come from the metal substrate. Although the presence of codeposit of Mg(OH)2 together with iron hydroxide as the thin base layer well before precipitation of CaCO3 on steel has been reported in the literature [5], the calcareous deposits formed at −0.65 V without any Mg element indicate that there is no such co-deposit containing Mg and Fe precipitated as a base layer on 304 SS. Hence, the Fe element is not from the deposit itself.…”

“…It should be noted from Figure 10 that only the deposits formed by polarization at −0.80 V contained the Mg element. Usually, magnesium hydroxide precipitates in sea water at a critical pH value as high as 9.3 or more [5,44]. This implies that applied polarization at −0.80 V can result in local environment of high pH at the interface of metal and electrolyte, which promotes the deposit of Mg(OH) 2 .…”

“…EIS spectra were recorded at intervals of cathodic polarization as well as at the beginning without polarization to characterize the surface conditions. The EIS test was taken at the steady potential that the specimen had immediately prior to conducting the measurement, without decaying back to a natural potential [5]. The impedance spectra were measured in the frequency range from 100 kHz to 10 mHz with 49 points recorded, the disturbing signal is a sinusoidal wave with an amplitude of ±10 mV.…”

“…The composition and structure of calcareous deposits can be influenced in a complex manner by many factors, such as environment, applied polarization, and metals of substrate [5,7,20,25,26]. The morphologies of calcareous deposits formed under different conditions of cathodic protection for mild steel in sea water were analyzed in detail by Yang et al [5], who demonstrated that the deposits consisted of inner layer of co-deposited Mg(OH) 2 and iron oxide due to the presence of corrosion at the early stage of calcareous deposit formation, and the outer layer of CaCO 3 with some Mg(OH) 2 precipitated in the pores or cracks.…”