Hydrogen permeation behavior and corrosion monitoring of steel in cyclic wet–dry atmospheric environment

Hydrogen permeation of 16Mn steel under a cyclic wet–dry condition was investigated by Devanathan–Stachurski's electrolytic cell with a membrane covered on the exit side by a nickel layer and the weight loss was measured for each wet–dry cycle. The results show that hydrogen permeation current change with different atmospheric environment: distilled water, seawater, and seawater containing 100 ppm H₂S. The results show that seawater can induce an increase in the hydrogen permeation current due to the hydrolyzation reaction. And after the increase, equilibrium is reached due to the equilibrium of hydrolyzation reaction effect and the block of the rust layer. On the other hand, H₂S contamination also can induce an increase in the maximum hydrogen permeation current due to the hydrolyzation reaction. And H₂S contamination delays the time that hydrogen permeation is detected because of the formation of the FeS_(1?x) film. The FeS_(1?x) film can block the absorption of hydrogen onto the specimen surface. The surface potential change and the pH change of the metal surface control the hydrogen permeation current. And a clear linear correlation exists between the quantities of hydrogen permeated through the 16Mn steel and the weight loss. Based on the linear correlation, we monitored the corrosion rate by monitoring the hydrogen permeation current by a sensor outside. Good coherences were shown between results in laboratory and outside.     

Corrosion behavior of iron thin film in deaerated phosphate solutions by an electrochemical quartz crystal microbalance

The corrosion rates of iron thin film in deaerated phosphate solutions were measured by an electrochemical quartz crystal microbalance as a function of solution pH or phosphorus concentration in solutions. The dependences of corrosion rate and corrosion potential on solution pH and phosphorus concentration have suggested that the corrosion mechanism changes in the vicinity of pH 5 and H₂PO₄⁻ contributes to both anodic dissolution of iron and hydrogen evolution. The corrosion mechanism which contains iron-phosphate-hydroxide complex ion as an adsorbed intermediate was proposed to explain the experimental results.     

The influence of copper upon the atmospheric corrosion of iron

The atmospheric corrosion of pure iron and the binary alloy Fe-0.5Cu has been analyzed by a simultaneous measurement of the anodic current density of the metal dissolution and the cathodic current density of the O₂ reduction reaction during several wet/dry cycles using a magnetic and a gas volumetric technique, respectively. The results show three typical stages of the atmospheric corrosion: stage 1 (wetting of a dry surface): rapid corrosion with rust reduction as cathodic process; stage 2 (wet surface): slow corrosion with O₂ reduction as cathodic process; and stage 3 (drying out of the surface): very rapid corrosion with O₂ reduction as the cathodic process during critical wetting of the surface. The effect of copper is restricted to stage 3, where the corrosion rate is much smaller for the Fe-0.5Cu alloy than for pure iron. Two models are discussed to explain these results.     

Hydrogen permeation and corrosion behavior of high strength steel MCM 430 in cyclic wet-dry SO2 environment

Hydrogen permeation caused by corrosion under a cyclic wet (2 h)-dry (10 h) SO₂ condition was investigated for a high strength steel of MCM 430 by using an electrochemical technique in addition to the corrosion behavior obtained from weight loss measurement and the determination of corrosion products by using X-ray diffraction method. The hydrogen content converted from hydrogen permeation current density was observed in both wet and dry periods. The origin of proton was estimated to be from (1) the hydrolysis of ferrous ions, (2) the oxidation of ferrous ions and ferrous hydroxide, and (3) hydrolysis of SO₂ and formation of FeSO₄, but not from the dissociation of H₂O. With respect to the determination of the corrosion products consisting of inner (adherent) and outer (not adherent) layers, the outer layer is composed of α-FeOOH, amorphous phase and γ-FeOOH, where α-FeOOH increases with the increase in the wet-dry cycle, and amorphous phase shows the reverse trend. The corrosion product in the inner layer is mainly Fe₃O₄ with them. On the basis of the results obtained, the role of the dry or wet period, the effect of SO₂ and the corrosion process during the cyclic wet-dry periods were discussed.     

Comparison of corrosion-resistance and hydrogen permeation properties of Zn-Ni, Zn-Ni-Cd and Cd coatings on low-carbon steel

Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion and hydrogen permeation characteristics of Zn-Ni-Cd alloy coatings electrodeposited from alkaline bath were studied and compared with those of Cd and Zn-Ni coatings obtained using commercial baths. Zn-Ni-Cd alloy was electroplated from an alkaline sulfate bath under potentiostatic conditions. The corrosion potential of this Zn-Ni-Cd coating was −0.62 V vs. SCE, which is still negative potential compared to iron. The corrosion rate of Zn-Ni-Cd coated steel was 0.073 mm y⁻¹, which is estimated in a solution at a pH of 7. This value is much lower than the corrosion rate of Zn-Ni alloy (0.502 mm y⁻¹) and Cd (0.306 mm y⁻¹) coatings deposited from commercial baths. Zn-Ni-Cd alloys are also demonstrated to have superior hydrogen permeation inhibition properties compared to Cd and Zn-Ni coatings. Kinetic parameters of hydrogen permeation such as the transfer coefficient, α, the modified exchange current density, i₀^′, thickness dependent adsorption-absorption rate constant, k^″, recombination rate constant, k₃, surface hydrogen coverage, θ_s, were evaluated by applying a mathematical model to analyze experimental results.     

Corrosion behavior of steel under wet and dry cycles containing Cr ion

The corrosion behavior of mild steel has been investigated during the wet and dry cyclic transitions containing Cr³⁺ ion added as sulfate in order to gain a better understanding of the influence of Cr on the atmospheric corrosion of steels. The corrosion rate during drying is greatly suppressed by the existence of Cr³⁺ ion in the electrolyte covered with the surface. Lower corrosion rates are observed during drying even if the surface have been polarized to negative potentials below −200 mV_SHE during the wet corrosion conditions in which the surface-covered electrolyte contains Cr³⁺ ion. This corrosion behavior is identical to the case of Cr-containing steel for the wet and dry cyclic transitions without the addition of Cr³⁺ ion. The composition of rust layer after the wet and dry cyclic transitions is composed of α-FeOOH, γ-FeOOH and Fe_3−δO₄ for both cases of non-Cr³⁺ and Cr³⁺-containing condition, and no significant difference in the mass fraction of the above rust substances between two conditions is observed by means of Mössbauer spectroscopy. The only difference in the rust layer is that the rust formed under the wet and dry cyclic transitions containing Cr³⁺ ion contains a certain amount of Cr near the steel/rust interface. Those results suggest that the role of Cr during the wet and dry cyclic transitions is the inhibition of the rust reduction and the formation of Fe²⁺-state intermediate by the existence of Cr in the rust layer. This can lead to the inhibition of the oxygen reduction during drying.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil II: Korrosion und Deckschichtbildung

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part II: Corrosion and Formation of Surface Layers The effects of C, S, P, Mn, Si, Cr, Ni, Sn and Cu on the formation of surface layer and hydrogen uptake of iron during corrosion in 1 M H₂SO₄/N₂ were investigated using AES, XPS, SEM and electrochemical permeation techniques. Cu, Sn, P and C are enriched on the surface of iron during corrosion in H₂SO₄. Cu is enriched in the metallic form. P forms a phosphate and phosphide containing surface layer. Ni is not enriched. Cr is preferentially dissolved. Cu, Sn and Ni inhibit the dissolution of iron and thus decrease the hydrogen activity. S, P and Mn (MnS) increase the corrosion and hydrogen activity. Cr forms traps in iron which increase the hydrogen uptake.     

Evaluation of hydrogen entry into high strength steel under atmospheric corrosion

Hydrogen entry into high strength steels by atmospheric corrosion has been investigated to evaluate their susceptibility to hydrogen embrittlement. High strength steels were corroded by dry/wet cyclic corrosion after NaCl deposition. The maximum diffusible hydrogen concentration around the surface was successfully obtained by means of thermal desorption analysis after keeping the specimens at high humidity to reproduce enhanced hydrogen entry influenced by the rust layer and to homogenize the hydrogen distribution. Despite decrease in corrosion rate, hydrogen content in specimens did not decrease. Decrease of pH in inner rust layer is responsible for the enhanced hydrogen entry into steel.     

Ex-situ and in-situ X-ray diffractions of corrosion products freshly formed on the surface of an iron-silicon alloy

Ex-situ X-ray diffraction measurements of a small amount of samples extracted from wet corrosion products freshly formed on a pure iron and iron-2 mass% silicon surfaces have been conducted using synchrotron radiation for clarifying the formation process of corrosion products. The results showed that γ-FeOOH was formed on the outer side of wet corrosion products formed on the surface of the pure iron by sodium chloride solution, while γ-FeOOH, α-FeOOH, Fe₃O₄, and green rusts were formed on the inner side. On the other hand, in comparison to the case of the pure iron, a significant formation of β-FeOOH was observed in the iron-silicon alloy. Influences of silicon alloying on corrosion products formed by aqueous solution containing sulfate ions were also observed. Furthermore, in-situ diffraction measurements by a conventional X-ray source were conducted for analyzing corrosion products formed on the pure iron and iron-silicon alloy surfaces by cyclic exposure to wet and dry atmospheres. The results obtained by the in-situ diffraction and ex-situ diffraction measurements on the corrosion products were consistent.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme von Eisen in H2S-haltigen Lösungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in H₂S-solutions Effects of the transition metals Mo, V, Nb, Ti, Zr and their carbides as well as of phosphorous on the corrosion and hydrogen uptake of iron in acid to weakly acid NaCl solutions with and without H₂S are discussed. Investigations were carried out on binary, ternary and quaternary iron based alloys, using electrochemical and surface analytical methods. No specific effect of one of the alloying elements or the carbides on the corrosion or hydrogen uptake is observed. Due to the experimental conditions, sulphur and oxygen enriched surface scales form, by which the kinetics of the corrosion processes are determined. The alloying elements are enriched on the iron surface only as a carbide. Phosphorous is enriched as a phosphide at low pH and as a phosphate at higher pH. H₂S and phosphides increase the corrosion rate and hydrogen uptake. In pure iron or low strength iron alloys, at the very high H₂S affected hydrogen activities new lattice defects are induced permanently resulting in extremely high hydrogen concentrations.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme des Eisens in Schwefelsäure

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in sulphuric acid The effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in 1 M H₂SO₄/N₂/25°C were investigated by electrochemical and surface analytical methods using binary and ternary Fe-Me alloys with about 0.2 at.-%Me. Due to the experimental conditions, no protective surface layers formed. The transition metals were enriched at the iron surface only as a carbide or oxicarbide. Mo or Zr were not markedly enriched. The corrosion current density and the hydrogen activity were decreased only by Mo or Zr. Hydrogen permeation measurements were analysed in terms of the trapping theory. The average binding energy of shallow traps for hydrogen increases with increasing atomic radius of the substituted alloying elements in the order V, Mo, Ti, Nb and Zr. Correspondingly, the hydrogen diffusion coefficient (after saturation of the deep traps) decreases and the hydrogen solubility increases. The steadystate hydrogen permeation rate remains almost unaffected. The total hydrogen content is determined by the density of deep traps and thus mostly independent of the external hydrogen activity. The trapping effect of iron is strongly increased by dissolved Zr or Nb or in the presence of fine dispersed carbides as VC_x. The effect of coarse carbide particles at a lower density is small.     

In situ pH responsive fluorescent probing of localized iron corrosion

A fast and versatile method has been demonstrated to detect the early stage of iron corrosion in situ, with minimum external perturbation, by using a pH sensitive fluorescence dye. The decreasing fluorescence intensity inside corrosion pits induced by hydrogen ions produced during iron corrosion is used as a signal indicator. Identification of corrosion pits (with resolution of <1 μm) and quantification of corrosion rate based on local hydrogen concentration were achieved. The developed technique has been applied to show that surface roughness, under a constant current condition, influences the corrosion of iron. Corrosion occurs preferentially adjacent to local topographic features.     

Effects of corrosion product deposit on the subsequent cathodic and anodic reactions of X-70 steel in near-neutral pH solution

The effects of corrosion product deposit on the subsequent anodic and cathodic reactions of X-70 steel in a near-neutral pH solution were investigated by localized electrochemical impedance spectroscopy (LEIS), scanning vibrating micro-electrode (SVME) and macroscopic EIS measurements as well as surface analysis technique. It is found that the deposit layer formed on the steel surface is porous, non-compact in nature. The presence of a corrosion product layer would enhance adsorption, but significantly inhibit absorption and permeation of hydrogen atoms into steel. It is due to the porous structure of the deposit that generates a spatial separation of cathodic and anodic reaction sites, resulting in an increased effective surface area for hydrogen adsorption and, simultaneously, a “blocking” effect on hydrogen absorption and permeation. The deposit enhances greatly anodic dissolution of the steel, which is attributed to the adsorption of the intermediate species and the resultant “self-catalytic” mechanism for corrosion of the steel in near-neutral pH solution. In the presence of corrosion product deposit on the pipeline steel surface, pipeline corrosion, especially pitting corrosion, is expected to be enhanced. Stress corrosion cracks could initiate from the corrosion pits that form under deposit. However, deposit does not contribute to hydrogen permeation, although the hydrogen evolution is enhanced.     

Untersuchungen zur H-induzierten Rißkorrosion – Teil 2: Vergleichende Untersuchungen zur Wasserstoffpermeation und Spannungsrißkorrosion

Investigation into the hydrogen induced cracking corrosion – Part 2: Comparative investigations into hydrogen permeation Creep specimens of cold rolled pure iron with different strengths have been stressed in H₂SO₄ + As₂O₃ undercathodic polarisation. The critical stress level for stress corrosion cracking is proportional to the yield strength ranging about 60% ys. At higher stresses the life times decrease with increasing strengths. No correlation exists with permeation rate and hydrogen content, but with time lag. Low alloy steel types show similar relationships in the same and in H₂S containing environments. Metallographic examinations show no relation between stress corrosion cracks and surface or inside blisters. SEM examinations show manyfold types of fracture surfaces without systematics.     

Electrochemical methods study on corrosion of 5% Al–Zn alloy‐coated steel under thin electrolyte layers

The corrosion behavior of a 5% Al–Zn alloy (GF) coated steel was investigated under cyclic wet–dry condition using electrochemical techniques. The wet–dry cycle was conducted by exposure to alternate condition of 1 h immersion in seawater and 7 h drying at ambient temperature. The polarization resistance, R_p of the coating was monitored during the wet–dry cycles by two points AC impedance method and the corrosion potential, E_corr was measured only when the coating was immersed in seawater. Simultaneously, the electrochemical impedance spectroscopy (EIS) of the coating was obtained after it was immersed in different cycles of wet–dry condition. The results obtained by two points AC impedance method had good agreement with those achieved from EIS technique, which proved that the two points AC impedance method was correct and an effective method for atmospheric corrosion study. The monitoring results indicated that the corrosion rate of GF coating firstly increased, then decreased slowly with time, and at last reached a relative steady state with local corrosion under the cyclic wet–dry alternate condition.     

Die Wasserstoffaufnahme von Eisen bei der Korrosion in neutralen bis schwach sauren Elektrolyten

Hydrogen uptake by iron during corrosion in neutral to weakly acid electrolytes During atmospheric corrosion and corrosion by aqueous solutions, hydrogen can enter into steel. The hydrogen activity built up in iron during corrosion by dilute aqueous solutions of hydrochloric acid, sulfuric acid and iron salts has been measured as a function of pH using a permeation technique. Below pH = 5 in oxygen free solutions and pH = 4 in air saturated solutions the hydrogen activity \documentclass{article}\pagestyle{empty}\begin{document}$ a_{\rm H} {\rm = }\sqrt {{\rm p}_{{\rm H}_{\rm 2} } {\rm /(}\mathop {{\rm p}_{{\rm H}_{\rm 2} } }\limits^{\rm o} {\rm = 1}\,{\rm bar)}} $\end{document} reaches values of more than 0.1 sufficient to cause delayed cracking of steels susceptible to hydrogen embrittlement. The anions and Na⁺-ions have no markable influence. The influence of Fe³⁺ and O₂ is discussed.     

A peculiar cathodic process during iron and steel corrosion in sulfate reducing bacteria (SRB) media

Features related to the cathodic reduction of iron sulfides precipitation on iron surface during its exposure to SRB culture were studied. Electrochemical measurements were performed with pure iron and platinum electrodes plated with a thin iron film in de-aerated SRB culture. The study reveals that iron sulfide precipitation is being cathodically reduced just below a potential of −0.1 V_SCE, and if iron corrosion process occurs at potentials below that threshold potential, then the reduction of iron sulfide may provide an alternative cathodic depolarization mechanism in SRB. This cathodic process can maintain iron and corrosion at potentials above RHE potential.     

Corrosion behaviour of oil well casing steel in H2S saturated NACE solution

Abstract

The corrosion behaviour of oil well casing steel in H₂S saturated NACE solution (5 wt-%NaCl+0·5 wt-%CH₃COOH) was investigated by means of electrochemical methods, X-ray photoelectron spectroscopy and scanning electron microscopy. It was found that the investigated oil well casing steel can react with hydrogen sulphide in the H₂S saturated NACE solution at 25°C. During the reaction, corrosion films of mackinawite (FeS) form on the steel surface and the hydrogen atoms diffuse into steel matrix. The further corrosion of iron with H₂S can be retarded, and hydrogen permeation flux decreased, by the presence of the mackinawite film, which acts as a protective barrier provided it remains intact.     

Comparison of influences of NaCl and CaCl2 on the corrosion of 11% and 17% Cr ferritic stainless steels during cyclic corrosion test

During the drying stage of the cyclic corrosion test on ferritic stainless steels in the NaCl environment, the current abruptly increased and then decreased to nearly zero, indicating that pits are initiated as the salt concentration is increased, which are then repassivated when the surface is completely dry. During the wet stage, the current remained high, suggesting that pits mainly propagate during the wet stage. In contrast, in the cyclic corrosion test in the CaCl₂ environment, the current was highest during the drying stage, indicating that the electrolyte is not completely dried and corrosion mainly propagates during the drying stage.     

The acid corrosion of Fe-P alloys

The effect of the P-content in iron (0.002-0.12% P) and the stirring rate on the corrosion rate of five Fe-P alloys in H₂SO₄ and HCl solutions (pH = 0) has been investigated. It was assumed that phosphine generated during corrosion process acts as a stimulator of the cathodic process (at low concentrations) or as an inhibitor of both partial electrode processes (at high concentrations). A mechanism of the corrosion of Fe-P alloys, including the transfer of phosphine from the surface into the bulk of the solution by stirring has been discussed.