The thermogalvanic corrosion of mild steel in alkaline solution—II. Passivated electrodes under conditions of film breakdown

The thermogalvanic corrosion of mild steel in oxygen-saturated alkaline solution (ca. pH 9) with and without the addition of small amounts of Cl^? has been investigated. It is shown that the polarity of the electrodes is time-variable and that spontaneous passivation and subsequent breakdown of the protective film at these electrodes controls the magnitude of both the e.m.f. and the cell current. In the absence of the aggressive ion, the more rapid and complete passivation of the hot electrode leads to the eventual formation of a thermocell with a cold anode. The cell currents in this final stage are small (ca. 0·0016 A/m²) and reflect changes in the polarization of the local action processes rather than any increase in the metal loss attributable to thermogalvanic corrosion. By contrast, in the presence of Cl^?, the hot electrode ultimately becomes the anode of the cell as a result of both the more rapid and the more extensive breakdown of the passive film at higher temperatures. Significant thermogalvanic e.m.f.s (< 100 mV) and cell currents (< 0·16 A/m²) are recorded, the latter realistically representing the increased metal loss due to thermogalvanic action.     

The thermogalvanic corrosion of mild steel in alkaline solution—I. Actively dissolving electrodes

Using a simple model it has been shown that both thermodynamic and kinetic factors contribute to the temperature coefficient of the corrosion potential of a freely corroding electrode when the local action processes are under activation control. In the particular case of a 0·21%C steel immersed in deaerated Na₂B₄O₇/HCl buffer solution (pH 7·6) the kinetic factors predominate resulting in a negative coefficient. A thermogalvanic cell with a hot anode is therefore formed when when two such electrodes, maintained at different temperatures, are connected. In the absence of a significant cell internal resistance, a current will flow which reflects a stimulation of corrosion at the anode and a corresponding protection of the cathode, both attributable to thermogalvanic action. When the thermal e.m.f. is small the cell current overestimates the increased corrosion damage; since the individual electrodes are barely polarized it follows that the kinetics of the local action cathodic process at the anode cannot be ignored.     

The thermogalvanic corrosion of mild steel in alkaline solution—III. The factors controlling etching,passivation and pitting

The thermogalvanic behaviour of mild steel in an environment where active dissolution, passivation and breakdown are all possible has been investigated. The polarity of the electrodes and the magnitude of the cell currents vary with time as these processes occur successively but at different rates at each electrode. In the presence of an aggressive ion the onset of passivity is often undesirable since localized breakdown may ensue, any subsequent thermogalvanic corrosion being predominantly located at the pit sites giving rise to a disproportionate increase in metal penetration relative to the recorded current. Since maximum cell currents of approaching 5 A/m² are possible in these systems, the cells are usually under resistance control; any circumstance which leads to a lowering of this resistance will therefore increase the metal loss. However, thermogalvanic action does not always stimulate the overall corrosion rate of the anode but may, by polarization of this electrode, induce passivity and breakdown at a metal surface which would not undergo these changes spontaneously. Although the metal loss is often lowered, the penetration commonly increases since the corrosion damage which was previously uniform becomes highly localized.     

Study of the pitting corrosion of superduplex stainless steel and X-65 carbon steel during erosion–corrosion by cyclic polarisation technique

Erosion-enhanced corrosion behaviour of X-65 carbon steel and UNS S32750 superduplex stainless steel was investigated by electrochemical cyclic polarisation. The tests were performed using a jet slurry device coupled with a potentio-galvanostat at various jet velocities of 4, 6.5 and 9?m?s^?1 and impingement angles of 30 and 90? in a 3.5?wt-% NaCl water containing 6?wt-% silica sand particles. The results showed that increasing the jet velocity and impingement angle increased the corrosion rate of both alloys. Negative hysteresis and greater E_rp than OCP were observed for superduplex stainless steel in all erosion–corrosion conditions that indicated the pitting resistance of the alloy. However, the low resistance of carbon steel against pitting during erosion–corrosion was demonstrated by positive hysteresis in the cyclic polarisation curves as well as SEM images of the eroded surfaces.     

The effects of metal binder content and carbide grain size on the aqueous corrosion behaviour of TiC–316L stainless steel cermets

The room temperature electrochemical response of TiC-based cermets, with 10 to 30 vol.% 316L stainless steel binder and either fine- or coarse-grained TiC, has been investigated in an aqueous 3.5 wt.% NaCl solution. The assessment methods included Tafel extrapolation, in combination with potentiodynamic and potentiostatic polarisation. Corroded samples were characterised using SEM, with post-corrosion solutions analysed using ICP-OES. The highest corrosion resistance was achieved at the lowest binder contents, while those with a more coarse-grained structure generally showed superior resistance, due to a reduced TiC–316L interfacial area. Preferential dissolution of the steel binder was observed, leaving the TiC essentially unaffected.     

The a.c. corrosion of stainless steel—II. The polarization of ss304 and ss316 in acid sulfate solutions

An experimental study has been made to examine the effect of alternating current on the corrosion of Types 304 and 316 Stainless Steels in acidic aqueous sulfate solutions. The polarization curves were measured with an alternating voltage (AV) modulation technique and the pitting behavior of the stainless steels was examined with a constant potentiostatic coulometry and SEM photomicrography. The experiments were made with sinusoidal, square and triangular AV over a range of AV frequencies from 60 to 1000 Hz and AV magnitudes from 0 to 1000 mV rms. The results indicate that AV increased the critical current density for passivity and decreased the passive potential regime by shifting the transpassive potential toward the active direction and increasing the current density in the passive regime. The effect was similar to the addition of chloride ions to the corrosion environment. AV enhanced the pitting of the stainless steels at both the passive and transpassive potentials. Passivity was destroyed regardless of the AV waveforms; triangular wave caused the severest destruction, followed by sinusoidal and square waves.     

The effect of nitrogen and carbon on the stress corrosion cracking performance of sensitized AISI 304 stainless steel in chloride and sulfate solutions at 250°C

Slow strain rate tests (SSRT) were conducted on sensitized AISI 304 stainless steels (SS) with varying nitrogen and carbon contents in order to study their susceptibility to stress corrosion cracking (SCC). The tests were performed in de-aerated 0.01 M NaCl at 250°C, at a strain rate of 2 × 10⁻⁶s⁻¹ and at various applied potentials in the range −0.4 to 0.1 V(NHE), after which scanning electron microscopy (SEM) was used to observe the fracture surfaces of the SSRT specimens. SSRT results show that SCC occurs above a certain critical potential (E_SCC) which depends on the carbon and nitrogen contents; E_SCC is in the range −0.3 to 0 V(NHE), with nitrogen additions up to 0.16 wt% increasing E_SCC and carbon additions decreasing E_SCC. This implies that the degree of sensitization (DOS) is the major factor which determines E_SCC. The DOS also determines the fracture mode obtained above E_SCC; at low DOS transgranular stress corrosion cracking (TGSCC) occurs, while at high DOS intergranular stress corrosion cracking (IGSCC) is the predominant mode. The potential ranges in which (1) IGSCC and (2) simultaneous IGSCC, TGSCC and shallow pitting occurred corresponded to (1) the passive range and (2) potentials above the breakdown of passivity on the polarization curves. The results of this investigation are compared with those obtained from similar tests in sulphate solutions [T. A. Mozhiet al., Corrosion42, 197 (1986)], and possible mechanisms discussed.     

Electrodeposition of multi-layer Pd–Ni coatings on 316L stainless steel and their corrosion resistance in hot sulfuric acid solution

Pd–Ni coating shows good corrosion resistance in strong corrosion environments. However, in complex aggressive environments, the performance of the coatings is limited and further improvement is necessary. The effects of the applied plating current density on the composition, structure and properties of Pd–Ni coatings were studied. By changing the current density in the same bath, multi-layer Pd–Ni coatings were prepared on 316L stainless steel. Scanning electronic microscopy, weight loss tests, adhesion strength, porosity and electrochemical methods were used to study the corrosion resistance of the films prepared by different coating methods. Compared with the single layer Pd–Ni coating, the multi-layer coatings showed higher microhardness, lower internal stress, lower porosity and higher adhesive strength. The multi-layer Pd–Ni coating showed obviously better corrosion resistance in hot sulfuric acid solution containing Cl^?.     

Investigation of adsorption and corrosion inhibition effect of 1,1’-thiocarbonyldiimidazole on mild steel in hydrochloric acid solution

The adsorption and inhibition effect of 1,1′-thiocarbonyldiimidazole (TCDI) on the corrosion of mild steel (MS) in 0.5 M HCl solution was studied in both short and long immersion time (120 h) with the help of electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques. For long-time tests, the hydrogen gas evolution (VH₂-t) and the change of open circuit potential with immersion time (E _ocp-t) were also utilized in addition to the former two techniques. The surface morphology of MS after its exposure to 0.5 M HCl solution with and without 1.0 × 10⁻² M TCDI was examined by scanning electron microscopy (SEM). It was demonstrated that the inhibition efficiency of studied inhibitor is concentration depended and increased with TCDI concentration. The higher value of inhibition efficiency was obtained after longer immersion time merely on the basis of strong increase of corrosion rate of mild steel in the blank solution. The high inhibition efficiency was discussed in terms of adsorption of inhibitor molecules and protective film formation on the mild steel surface which was substantiated by SEM micrographs. The adsorption of TCDI on MS was found to obey Langmuir adsorption isotherm.     

Stress corrosion cracking at constant load of 304L stainless steel in molten NaCl-CaCl2 at 570°C

The incubation and propagation times of cracks in 304L in molten NaCl-CaCl₂ at 570°C were related to the applied stress value, from creep and creep rate curves. Rest potential versus time curves were recorded simultaneously. The results showed intergranular stress corrosion cracking. When the temperature was kept at 570°C, precipitation of chromium carbide M₂₃C₆ which promoted cracking propagation, was induced. Determination of the crack rate shows that anodic dissolution at the bottom of the cracks is the main process during the stress corrosion crack propagation of 304L stainless steel in the stress range used.     

Atmospheric corrosion kinetics and dynamics of electrogalvanized mild steel in southeastern coastal area of China–Pakistan Economic Corridor

This study aims to examine atmospheric corrosivity, corrodants, and corrosion products of southeastern coastal area of China–Pakistan Economic Corridor as per ISO protocols 9223 and 9225, and ASTM standards G1, G50, G140-02, D4458-94, and D2010. Test sites are located at National Institute of Oceanography (NIO) and Karachi Port Trust (KPT) at the banks of the Arabian Sea. Electrogalvanized mild steel test coupons were exposed, and levels of corrodants (sulfur dioxide, chloride, and time of wetness) were measured for a period of 24 months, from May 2014 to May 2016. Corrosivity category C5+ is established in terms of the corrosion rate for both NIO and KPT test stations, which does not coincide with the corrosivity category C5 ascertained by employing environmental characteristics and atmospheric corrodants. Corrosion kinetic parameter “n” and correlation coefficient (R²) are 0.71 and 0.97 for NIO and 0.96 and 0.97 for KPT, respectively. Scanning electron microscopy/energy-dispersive spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction spectroscopy have corroborated the presence of simonkolleite and hydrozincite, zinc oxide, zinc hydroxychloride, and zinc in corrosion products at both test sites.     

Preconditioning of AISI 304 stainless steel surfaces in the presence of flavins—Part I: Effect on surface chemistry and corrosion behavior

Stainless steel AISI 304 surfaces were studied after a mild anodic polarization for oxide growth in the presence and absence of two derivatives of vitamin B2 (riboflavin and flavin mononucleotide) that can be secreted by metal-reducing bacteria and act as a chelating agent for iron species. The alterations in oxide chemistry were studied by means of surface-sensitive techniques such as X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analysis. The complementary electrochemical characterization revealed a preferential growth of an oxide/hydroxide iron-rich film that is responsible for an altered pit initiation and nucleation behavior. These findings suggest that as the corrosion behavior is determined by the interplay of the chemical and electronic properties, only a mild anodic polarization in the presence of redox-active molecules is able to alter the chemical and electronic structure of the passive film formed on stainless steel AISI 304. This helps to achieve a profound understanding of the mechanisms of microbially influenced corrosion (MIC) and especially the possible effects of the redox-active biomolecules, as they may play an important role in the corrosion susceptibility of stainless steel surfaces.     

A comparative study of corrosion inhibition of steel and stainless steel in hydrochloric acid by N,N,N′,N′-tetramethyl-p-phenylenediamine

Protection of Metals and Physical Chemistry of Surfaces - The corrosion of mild steel (MS) and AISI type 321 stainless steel (AISI 321) in 1 M HCl solution and the inhibitive mechanism of...     

Dithizone and thiosemicarbazide as inhibitors of corrosion of type 304 stainless steel in 1·0M sulphuric acid solution

Abstract

Dithizone and thiosemicarbazide were investigated as inhibitors of the corrosion of type 304 stainless steel in 1·0M sulphuric acid solution by studying weight loss, polarisation curves, and polarisation resistance and using scanning electron microscopy and Auger electron spectroscopy. Various corrosion parameters such as Tafel slopes, corrosion rate, heat of adsorption, and activation energy were evaluated to understand the inhibition mechanism. Inhibition efficiency increased with an increase in concentration of both inhibitors and decreased with an increase in temperature. Both inhibitors seemed to obey the Langmuir adsorption isotherm equation. Dithizone was chemisorbed, whereas thiosemicarbazide was physisorbed on the surface of type 304 stainless steel. Dithizone acted as a mixed inhibitor, while thiosemicarbazide was cathodic in nature.     

Preconditioning of AISI 304 stainless steel surfaces in the presence of flavins—Part II: Effect on biofilm formation and microbially influenced corrosion processes

Biofilm formation and microbially influenced corrosion of the iron-reducing microorganism Shewanella putrefaciens were investigated on stainless steel surfaces preconditioned in the absence and presence of flavin molecules by means of XANES (X-ray absorption near-edge structure) analysis and electrochemical methods. The results indicate that biofilm formation was promoted on samples preconditioned in electrolytes containing minute amounts of flavins. On the basis of the XANES results, the corrosion processes are controlled by the iron-rich outer layer of the passive film. Biofilm formation resulted in a cathodic shift of the open circuit potential and a protective effect in terms of pitting corrosion. The samples preconditioned in the absence of flavins have shown delayed pitting and the samples preconditioned in the presence of flavins did not show any pitting in a window of −0.3- to +0.0-V overpotential in the bacterial medium. The results indicate that changes in the passive film chemistry induced by the presence of minute amounts of flavins during a mild anodic polarization can change the susceptibility of stainless steel surfaces to microbially influenced corrosion.     

Electrical mapping of AISI 304 stainless steel subjected to intergranular corrosion performed by means of AFM–LIS in the contact mode

The paper presents results of the AFM-based approach to local impedance spectroscopy (LIS) measurements performed in a 20 × 20 μm grid within an austenite grain–grain boundary region for sensitized AISI 304 stainless steel (SS). Maps of electrical parameters obtained on the basis of localized impedance spectra were demonstrated, presenting their changes and correlation with the sample topography. Performed research revealed significant differences in the electrical distribution of the contact resistance considered as the passive layer resistance and contact capacitance considered as the passive layer capacitance determined for austenite grain interiors and austenite grain boundaries affected by intergranular corrosion.     

The corrosion fatigue and stress corrosion cracking of α-brass in ammoniacal solutions

A study of the corrosion fatigue and stress corrosion cracking under a constant strain-rate in 70 Cu?30 Zn brass as a function of pH has been made. Correlation between the two has been discussed. Corrosion fatigue was very dependent on pH both with respect to the lifetime and to the nature of cracks as well as those of stress corrosion cracking. A minimum in fatigue life and a large fatigue crack growth rate were observed in neutral tarnishing solutions of pH 6.5–7.0. The fatigue crack propagation rate in neutral tarnishing solution was about twice as great as that in an acidic non-tarnishing solution. Fractographic examination on the fracture surfaces of corrosion fatigue and stress corrosion cracking showed that intergranular cracking predominated both in neutral tarnishing and acidic non-tarnishing solutions, and transgranular cracking predominated in alkaline solution. It is concluded that the tarnish film is not a primary factor controlling the cracking mode in the acidic to neutral pH range and that intergranular cracking may result from the localized dissolution of alloy at grain boundaries.