A study on cathodic protection against crevice corrosion in dilute NaCl solutions

Potential and current distributions in a cathodically protected crevice between a simulated coating and segmented mild steel electrodes were measured in dilute NaCl solutions. The distributions became more uniform with time due to an increase in solution conductivity and depletion of dissolved oxygen in the crevice. Generally, a negative shift of control potential and an increase in initial solution conductivity and crevice thickness resulted in a higher polarization level on the steel. However, if the control potential is too negative, the polarization level may be lower than that under a suitable control potential because of hydrogen evolution. On the basis of these results, a mechanism of cathodic protection against crevice corrosion in high-resistivity environments was proposed.     

Modeling pipeline crevice corrosion under a disbonded coating with or without cathodic protection under transient and steady state conditions

Underground steel pipelines are protected by coatings and cathodic protection (CP). The pipeline corrosion occurs when the coating is disbonded away from a defect or holiday to form a crevice and the corrosion rate varies temporally and spatially in the crevice. In the presence of dissolved oxygen (O₂) in soil ground water, a differential O₂ concentration cell may develop in the crevice because O₂ diffuses more readily into the crevice through the holiday than through the disbonded coating. CP can decrease or eliminate the O₂ concentration cell depending on the potential applied at the holiday. Since the coatings are usually non-conductive, CP is unable to protect the steel surface deep inside the crevice. The transport of dissolved O₂, and that of dissolved carbon dioxide (CO₂) if present, into the crevice through holiday can be key to determining the crevice corrosion rate. In this work, the transient and steady state behavior of the corrosion process is investigated. The effect of the cathodic portion of iron vs. ferrous ion redox reaction on the crevice corrosion rate, which is often neglected traditionally, is further studied. At steady state, the effect of dissolved O₂ on the crevice corrosion rate and the added effect of dissolved CO₂ are mathematically modeled.     

Measurements of hydrogen activity in iron during cathodic protection using a potentiometric concentration cell

A solid - state potentiometric sensor, based on the well - tried principle of the thermodynamic concentration cell, has been constructed. The sensor uses a conventional bi-electrode design with an entry side and an exit side at which the hydrogen is detected. The sensor has been used for the quantitative determination of the hydrogen activity (equivalent H₂ pressure) generated during cathodic protection. Thus, the background hydrogen pressure in uncharged steel is 10^?17.7 (2 × 10^?18 atm.) with a variation of about 20 times. During cathodic protection this value rises by large amounts. Thus, using a zinc anode, the equivalent hydrogen pressure rose by 10^10.3 (to 4.4 × 10^?8 atm.) in 3.5% NaCl and by 10^16.9 (to 0.16 atm.) in artificial sea water, indicating the much larger amounts of hydrogen present in the latter case. This compares, for example, with literature data of 0.11 μA cm^?2 and 0.60 μA cm^?2 for hydrogen permeation at ?1000 mV (SCE) in 3.5% NaCl and artificial sea water respectively. Hydrogen entry and exit was also considerably slowed in sea water. These differences are caused by local surface pH buffering and deposit formation.     

Effects of alternating current interference on cathodic protection potential and its effectiveness for corrosion protection of pipelines

In this work, the effect of alternating current (AC) interference on cathodic protection (CP) potential on a X65 steel in a near-neutral pH bicarbonate solution was investigated, and the CP performance under AC was evaluated by weight-loss measurements. The CP potential applied on the steel cannot be maintained in the presence of AC interference. The shift of the CP potential depends on the applied CP level and AC current density. No matter if the direct current potential of the steel is shifted negatively or positively upon application of AC, the steel suffers from increased corrosion. The AC decreases the effectiveness of CP for corrosion protection. The CP standard at ?0.850?V (copper sulphate electrode) that does not consider the AC interference is not appropriate for AC corrosion protection.     

Comparison of inhibition efficiency of some azoles on corrosion of type 304 stainless steel in acidic solutions

Abstract

Inhibition efficiencies have been investigated for two types of azole, namely 2-mercaptobenzoazole (CBA) and 2-methylbenzoazole (MBA), containing nitrogen, sulphur, oxygen, or selenium atoms, on the corrosion of type 304 stainless steel in 2M sulphuric acid (H₂SO₄) and 3M hydrochloric acid (HCl). The study was conducted using weight loss, gasometry, and polarisation methods. It was shown that some of the compounds tested provide 90% inhibition efficiency at a concentration of 5 × 10^-4M and that the efficiency depends on the heteroatom and decreases in the order Se > S > N > O. These inhibitors were more efficient in H₂SO₄ than in HCl whereas the opposite is true for inhibitors with nitrogen alone. Adsorption isotherms were fitted to the experimental findings and some thermodynamics functions were obtained.     

Effect of pulse cathodic protection on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on pipeline steel

Coating disbondments on pipeline steels are regions with high resistivity where conventional cathodic protection (CP) could not fully protect. Therefore, in an attempt to mitigate this challenge, this study investigates the effect of pulse CP on corrosion mitigation and electrochemical conditions under a simulated coating disbondment on X-52 pipeline steel. In this regard, conventional and pulse CP of ?870 mV_SCE were applied to the open mouth of a simulated coating disbondment. For pulse CP, frequencies of 1, 5, and 10 kHz were used. Results showed while the conventional CP was not able to fully protect the 20 cm simulated coating disbondment, for the pulse CP with increase in frequency from 1 to 5 kHz, and from 5 to 10 kHz, improve in CP potential protection under the simulated coating disbondment was achieved. This was accompanied by considerably lower corrosion and a more uniform pH distribution under the simulated coating disbondment.     

Progress in inhibition of metal corrosion and modification of safety nanolayers on metals

The ellipsometric investigations of adsorption combined with corrosion and electrochemical measurements allow one to understand some regularities of iron (steel) passivation in neutral aqueous solutions by various organic inhibitors of metal corrosion (IMCs). The increase in the efficiency of the adsorptive passivation is demonstrated based on the example of using more hydrophobic substituted sodium phenylanthranilate (SPA); more importantly, the compositions on their basis are more effective IMCs than substituted SPAs themselves. This is often connected with ability of one of the components to initiate the adsorption of the other. This is why we investigate the design of the adsorption films during the sequential adsorption of the blend components. It was shown that, for many IMCs, the preliminary chemisorption of some IMC can stimulate an increase in ΔG _A ⁰. The passivating treatment of the metal surface via the layer-by-layer method can provide the best protection of metals from atmospheric corrosion. Progress in the development of the nontoxic compositions to produce conversion coatings and their passivation, as well as the protection of metals by volatile IMC, is considered.     

The inhibition of mild steel corrosion in phosphoric acid solutions by some N-heterocyclic compounds in the salt form

The effect of some quaternary N-heterocyclic compounds on the corrosion of mild steel in solutions of phosphoric acid (H₃PO₄) has been investigated in relation to the concentration of the inhibitor and acid as well as temperature by various monitoring corrosion techniques. Surface morphology was studied by scanning electron microscopy (SEM). Results obtained revealed that these compounds are good mixed-type inhibitors without changing the mechanism of the corrosion process. In general, at constant acid concentration, inhibitor efficiency increased with concentration of the inhibitor. On the other hand, at constant inhibitor concentration, inhibitor efficiency decreased with concentration of the acid up to a critical concentration above which it started to increase. The studied compounds appeared to function through general adsorption following the thermodynamic-kinetic adsorption isotherm. The thermodynamic parameters were determined and discussed. A quantitative correlation between inhibitor structure and inhibition efficiency was discussed.     

The inhibition effect of amides on aluminium corrosion in chloride solutions

The inhibition effect of different concentrations of benzamide (BA), sulfanilamide (SA), and thioacetamide (TA) on the corrosion of aluminium in 0.1 M NaCl solution was investigated by the methods of potentiodynamic polarization, electrochemical impedance spectroscopy, and SEM (scanning electron microscope) in this study. Impedance measurements showed that the charge transfer resistance increased whereas double layer capacitance decreased with the increase in the inhibitor concentrations. Adsorption of these inhibitors followed the Langmuir adsorption isotherm. Thermodynamic parameters of adsorption (K _ads , ΔG _ads ) of studied amides were calculated by using Langmiur adsorption isotherm. The surface films of the aluminium, both in solutions with and without the inhibitors, were then investigated by SEM. The results obtained showed that thioacetamide was much more effective in aluminium inhibition.     

Electrochemical study of the corrosion rate of carbon steel in soil: Evolution with time and determination of residual corrosion rates under cathodic protection

Steel coupons were buried in soil boxes for 2.5 months, with or without cathodic protection. A soil from the field was used and moistened by demineralized water or 5 g L^?1 Na₂SO₄ solution. Instantaneous corrosion rates were deduced from voltammetry experiments while average values were obtained by weight loss measurements. A detailed analysis of the polarization curves led to an estimation of residual corrosion rates (τ_rc) of coupons under cathodic protection. This residual phenomenon could be followed with time and it was observed that τ_rc decreased down to ～10 μm yr^?1. Cathodic protection could be optimised with this methodology.     

Temperature as a pitting and crevice corrosion criterion in the FeCl3 test

The FeCl₃ test is applied to an increasing extent for examining the resistance to pitting and crevice corrosion. Two methods having proved their value are described, the chemical properties of the FeCl₃ solution with regard to hydrolysis, pH and redox potential behaviour at various test temperatures are set forth and finally numerous results of the application of this test to high-alloy stainless steels and nickel alloys are presented. These results have been used to establish, be means of multiple regression, two empirical equations that allow to estimate rather accurately the critical pitting and crevice corrosion temperatures (CPT, CCT) from the contents of the decisive alloying constituents. These temperatures vary by about 2.5°C in the CPT test and by approx. 10°C in the CCT test, which can be reduced, however, by extending the test period beyond 24 hours. This is due to the fact that corrosion potentials in a 10% FeCl₃ · 6H₂O solution take a long time to stabilize. The variation of the critical crevice temperature can be further reduced by pressing the crevice blocks at a higher torque to the specimen. Another section particularly deals with the application of the CPT test for determining the influence of the matrix on the resistance to local corrosion. Consequently, the CPT test lends itself excellently to the examination of welds and as a quality control. Finally, CPT test results are compared with pitting data determined electro-chemically in artificial seawater. This shows that the ranking order with regard to corrosion resistance is identical, although media and processes differ considerably from each other.     

Kinetics of inhibition of aluminum corrosion in hydrogen chloride electrolytes with binary salt mixtures

The kinetics of inhibition of aluminum corrosion with SnCl₂, CdCl₂, PbCl₂, and their equimolar binary mixtures was studied under contact exchange conditions. Prerequisites for synergism in these mixtures were determined. The parameter γ of mutual influence of the components in the mixture was formally separated into the constituents associated with changes in the aluminum surface coverage with the contact deposit, γ_s, and in the free corrosion potential of aluminum, γ_ΔE . In the mixtures studied, γ_s > 1, while γ_ΔE < 1; however, the overall effect is synergism because γ_s > 1/γ_ΔE . Original Russian Text ? O.I. Barteneva, V.V. Bartenev, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 5, pp. 532–538.     

Electrochemical measurements of cathodic protection for reinforced concrete piles in a marine environment using embedded corrosion monitoring sensors

This study developed a sensor to monitor the corrosion of reinforced concrete structures. Concrete pile specimens with embedded sensors were used to obtain data on corrosion and cathodic protection for bridge columns in a real marine environment. Corrosion potential, cathodic protection current density, concrete resistivity, and the degree of depolarization potential were measured with the embedded sensors in concrete pile specimens. The cathodic protection (CP) state was accurately monitored by sensors installed in underwater, tidal, splash, and atmospheric zones. The protection potential measurements confirmed that the CP by Zn-mesh sacrificial anode was fairly effective in the marine pile environment. The protection current densities in the tidal, splash zones were 2–3 times higher than those in underwater and atmospheric zones. The concrete resistivity in the tidal and splash zones was decreased through the installation of both mortar-embedded Zn-mesh (sacrificial anode) and outside an FRP jacket (cover). Considering the CP, the cathodic prevention was more effective than cathodic protection.     

The corrosion inhibition of copper in hydrochloric acid solutions by a tripeptide compound

The efficiency of glutathione as a non-toxic corrosion inhibitor for copper in 0.5 M HCl has been studied by using the weight-loss technique, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. Maximum protection efficiency reaches about 92.7% for glutathione at 10 mM concentration level. Results obtained from potentiodynamic polarization and impedance measurements are in good agreement. The adsorption of glutathione on copper surface follows Langmuir isotherm. The adsorption free energy of glutathione on copper (−32 kJ mol⁻¹) reveals a strong physical adsorption of the inhibition on the metal surface.     

Dielectric properties and corrosion protection of a few red lead pigmented primers exposed to salt solutions. II. The initial wetting period

Using a commercial capacitance bridge the capacitance and dielectric properties of pigmented paint films were measured with a view to the possibility of forecasting the corrosion protection properties of such coatings. According to the results obtained with 8 red lead and 4 iron oxide pigmented primers on steel in sodium chloride and sodium perchlorate solutions the measured values of the capacitance and of the dissipation factor enable really pure and really good primers to be distinguished after only 3 days. However, it is not possible to grade high quality primers of different generic types from dielectric data, irrespective of the duration of exposure (up to 12 months). This finding is ascribed to the differences in the duration of the initial wetting and swelling period.     

A simple approach for assessment of the corrosion inhibition efficiency of triazole,oxadiazole and thiadiazole derivatives as a function of their concentrations without using complex computer codes

Protection of Metals and Physical Chemistry of Surfaces - A new approach is introduced to predict the corrosion inhibition efficiency of triazole, oxadiazole and thiadiazole derivatives. In...     

Steel corrosion in concrete: Determinist modeling of cathodic reaction as a function of water saturation degree

The prediction of the long-term behavior of reinforced concrete structures involved in the nuclear waste storage requires the assessment and the modeling of the corrosion processes of steel reinforcement. This paper deals with the modeling of the cathodic reaction that is one of the main mechanisms of steel rebar corrosion. This model takes into account oxygen reduction and oxygen diffusion through a diffusion barrier (iron oxide and/or carbonated concrete) as a function of water saturation degree. It is demonstrated that corrosion rate of reinforcement embedded in concrete with water saturation degree as low as 0.9 could be under oxygen diffusion control. Thus, transport properties of concrete (aqueous and gaseous phase, dissolved species) are key parameters that must be taken into account to model electrochemical processes on the reinforcement.