Fundamentals of hydrogen evolution reaction and its implications on near-neutral pH stress corrosion cracking of pipelines

In this work, electrochemical techniques were utilized to investigate the hydrogen evolution reaction on X-70 pipe steel and the hydrogen permeation through the steel in near-neutral pH environmental condition. The results demonstrate that the steel has always been in an active-dissolution state in near-neutral pH solution and there is no film formed on the steel surface. Hydrogen evolution is inhibited by anodic polarization of the steel, which is attributed to the alternation of hydrogen evolution mechanism and kinetics on the anodially polarized steel. Combined with slow strain rate tensile tests, it is found that the high susceptibility of steel to stress corrosion cracking (SCC) is always associated with a high hydrogen permeation current. A thermodynamic model was developed, by analyzing the change in free-energy of the steel in the presence and absence of hydrogen and stress, to determine the interactions of hydrogen, stress and anodic dissolution at the crack-tip. The role of hydrogen involvement in pipeline near-neutral pH SCC could be determined quantitatively by characterizing the effect of hydrogen concentration on the dissolution rate of steel and the synergism of hydrogen and stress to promote crack growth.     

Micro-electrochemical characterization of the effect of applied stress on local anodic dissolution behavior of pipeline steel under near-neutral pH condition

Local anodic dissolution behavior of smooth and pre-cracked X70 pipeline steel specimens under applied stress were investigated by micro-electrochemical measurements and numerical simulation. The results demonstrated that the anodic dissolution rate of steel is enhanced by applied tensile stress. Corrosion of the stressed steel is accompanied with the formation of a layer of corrosion product deposit on electrode surface. However, the deposit layer does not provide effective protection to the underlying steel for corrosion attack due to its loose, porous structure. Under small tensile stress, the stress-enhanced dissolution of steel is not significant. With the increase of stress level up to 80% of yielding strength of steel, the activity of the steel increases remarkably, resulting in a significant enhancement of the anodic dissolution of steel. A stress concentration is developed at the crack-tip and enhances significantly the local anodic dissolution of steel. The stress effect factor at the crack tip is as high as 3.6 when a tensile force of 3000 N is applied on the pre-cracked specimen, while that for the low-stress area is 1.102 only. The local dissolution rate at the crack-tip is accelerated with the test time, which would be attributed to the fact that, along with continuous propagation of the crack, the stress concentration at the crack-tip is further increased, which, again, enhances local dissolution. This result reflects exactly the interaction of stress and anodic dissolution at crack-tip during SCC of steel.     

The anodic and cathodic dissolution of Al and Al-Cu-Mg alloy

Atomic emission spectroelectrochemistry (AESEC) was used to monitor the release of Al from 99.99% aluminum (1199 alloy) and Al, Mg, and Cu from 2024 Al alloy in 30 g/l NaCl electrolyte as a function of pH. The cathodic dissolution of Al was demonstrated and attributed to an increase in the pH at the interface due to the water reduction reaction. The dissolution of Mg was also observed but was a more complex function of current probably depending on the interfacial pH and the Al dissolution rate. The detachment of copper-rich particles was observed as very rapid spectroscopic emission transients (peak width < 10 ms).     

The anodic dissolution of zinc and zinc alloys in alkaline solution. I. Oxide formation on electrogalvanized steel

The polarization behaviour of zinc in alkaline solution has been investigated using atomic emission spectroelectrochemistry. By independently measuring the oxidation rate of zinc (electrical current) and the rate of Zn²⁺ dissolution (partial elemental current) it is possible to calculate the amount of insoluble zinc cations produced at any instant. Assuming the insoluble cations are present as a zinc oxide film, the growth of this film as a function of potential and time was determined. On the basis of kinetic evidence, it was found that at least three forms of zinc based oxide/hydroxide films form during polarization experiments. Type I oxide formation occurs when the metal/electrolyte interface becomes locally saturated with Zn²⁺ ions. Type II oxide forms on the metal surface underneath the film of Type I oxide but has little inhibiting effect on zinc dissolution. Type III oxide is produced in much smaller quantity and results in a transition to the passive state. This may be due to a potential induced transition of Type II → Type III oxide.     

Anodic polarization behavior of low-carbon steel in concentrated sodium hydroxide and sodium nitrate solutions

High-level radioactive wastes, primarily consisting of concentrated sodium hydroxide (NaOH) and sodium nitrate (NaNO₃) solutions, are stored in large underground storage tanks made of low-carbon steel. The anodic polarization behavior of low-carbon steel in concentrated solutions of 10 M NaOH and various concentrations of NaNO₃ (0.01-2.0 M) was determined in order to predict the caustic stress corrosion cracking (CSCC) susceptibility of the tanks. The active-passive transition peak exhibited during anodic polarization of low-carbon steel in 10 M NaOH, typically associated with CSCC, at −0.25 and −0.75 V_SCE, is still present at the lower and higher concentrations of nitrate. However, there is a mid-range of nitrate concentrations (0.5-1 M) within which the peak is suppressed by the strongly oxidizing nitrate in the presence of oxygen, a cathodic depolarizer. Temperature also affects the magnitude of this mid-range of nitrate concentrations where CSCC is seen to be electrochemically prevented. The data suggest that the oxygen solubility at the relatively low temperatures tested (<95 °C) controls the preference of the cathodic reaction, i.e. oxygen reduction versus nitrate reduction. When oxygen reduction is the preferred cathodic reaction, E_corr is driven more noble than the active-passive transition peak.     

In-situ investigation of the interplay between microstructure and anodic copper dissolution under near-ECM conditions – Part 1: The active state

The influence of crystallographic orientation on the anodic dissolution of copper in sodium nitrate under near ECM conditions was investigated in situ by a specially designed electrochemical flow channel cell. The investigation was limited to the active dissolution range where a distinctive influence of the microstructure on the anodic dissolution was expected. It was clearly shown that the topography strongly depends on the crystallographic orientation. The dissolution rate of less-packed {1 0 0} planes is higher than of the close-packed {1 1 1} planes. Additionally, a gas evolution as side reaction was temporarily observed. This process happens preferentially on {1 1 1} planes.     

Influence of pH,time and rate of application on phosphate rock dissolution and availability to pastures

Soil Samples were collected from a field experiment conducted to evaluate the agronomic effectiveness of a reactive phosphate rock (PR), Sechura sand, relative to that of monocalcium phosphate (MCP) at different soil pHs and rates of application. The samples were analysed for P soluble in the soil solution and bicarbonate extractable P. The rate of dissolution of PR was calculated from the data on the fractionation of inorganic P. In MCP plots P in the soil solution decreased sharply with time especially at low pHs and high rates of fertiliser application. In PR plots the concentration remained with time at the same as or a slightly higher level than that was found one month after application. Solution concentration of P was lower at very high rates of PR application than at intermediate rates. In both MCP and PR plots bicarbonate extractable P decreased with increasing pH. Bicarbonate extractable P was linearly related to MCP but not to PR applied. The rate of dissolution and the proportion of PR dissolved decreased with increasing rates of PR application but the amount dissolved increased. Phosphate dissolved at high level of PR application did not seem to enhance proportionately either the concentration of P in soil solution or bicarbonate extractable P.     

Detection of SCC on prestressing steel wire by the simultaneous use of electrochemical noise and acoustic emission measurements

Electrochemical voltage and current noise were measured simultaneously with acoustic emission (AE) measurements during the stress-corrosion cracking (SCC) of prestressing steel wire. Elongation of the specimens was also measured. Constant load tests were performed on specimens made from prestressing steel with a diameter of 3.2 mm: the central wire of a seven-wire strand was used. The specimens were exposed to diluted sodium thiocyanate (a modified version of the test as proposed in EN ISO 15630-3), with and without the addition of an organic corrosion inhibitor. EN was measured between the stressed central cold-drawn wire and the neighbouring wires which acted as reference electrodes for the electrochemical current and voltage measurements. AE was measured by two AE sensors fixed to the specimen.In order to characterize the SCC processes on the prestressing steel wire, the results of all the used techniques were analysed and compared. The effect of the inhibitor on these processes was also studied. A significantly longer time to failure was observed in the experiments with the added inhibitor. The results of the techniques, in combination with SEM and metallographic inspections, confirmed that the inhibitor had a specific influence on SCC. It was concluded that measurements of combined methods are promising for the reliable detection of SCC.     

Initiation and propagation of pitting and crevice corrosion of hydrogen-containing passive films on X70 micro-alloyed steel

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E_p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO₃ solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E_p and open circuit potential E_corr of the steel in 0.5 M NaHCO₃ solution. However, a synergistic effect of hydrogen and Cl⁻ on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO₃ solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed.     

Dissolution of phosphate rocks in soils. 2. Effect of pH on the dissolution and plant availability of phosphate rock in soil with pH dependent charge

The effect of soil pH on the dissolution of phosphate rocks (PRs) and the subsequent availability of the dissolved inorganic phosphorus (Pi) to plants was examined in a volcanic soil adjusted to different pH values. Potassium dihydrogen orthophosphate (KH₂PO₄) and three PRs, Nauru (NPR), Jordan (JPR) and North Carolina (NCPR) were incubated with the pH-amended soils at a rate of 800µg P g^–1 soil for 84 days. The extent of PR dissolution was determined by measuring the increases in the amount of 0.5 M NaOH extractable Pi (NaOH-P) in the PR treated soil over the control soil. The amount of plant available P was measured either by extracting with 0.5 M NaHCO₃ or by growing ryegrass in soil samples incubated with the phosphate sources.At each pH the order of the extent of PR dissolution followed NCPR > JPR > NPR, which was consistent with the decreasing order of their chemical reactivities. As the pH decreased from 6.5 to 3.9 the dissolution of PRs increased from 29.3% to 83.5%, from 18.2% to 78.9%, and from 12.5% to 60.3% for NCPR, JPR and NPR, respectively. In contrast, as the soil pH decreased from 6.5 to 3.9, the proportion of the dissolved P extracted by 0.5 M NAHCO₃ decreased from 38% to 5% and the proportion taken up by ryegrass plants decreased from 46% to 7%. This decrease in plant available P corresponded to an increase in the adsorption of inorganic P with a decrease in pH. However, the uptake of P from PR relative to that from KH₂PO₄ was higher at low pH than at high pH. Further, the amount of P taken up by plants was more closely related to the amount of NaHCO₃ extractable P than to the amount of dissolved P present in the soil.     

低浓度含氯介质中氯离子向局部腐蚀闭塞区内的迁移规律及其对应力腐蚀破裂的影响

The enrichment of chloride anion within the occluded cell （OC） for Type 304 austenitic stainless steel in low chloride concentration solution has been investigated by means of a simulated OC. The influence of the enrichment of chloride anion on stress corrosion crack （SCC） of Type 304 stainless steel has been studied. It was observed that the amount of chloride anion migration was proportional to the charge flowing through the anode. Owning to the effects of enrichment of chloride anion, low chloride concentration solution could induce SCC for Type 304 stainless steel.