Behaviour of crevice corrosion in iron

Crevice corrosion was investigated in iron exposed to a strong-buffered acetate solution (0.5 M CH₃COOH + 0.5 M NaC₂H₃O₂), pH = 4.66. The current and the potential gradient within the crevice were measured at crevice depth (L) = 7.35, 8, 10, and 15 mm, for a crevice that was positioned facing the electrolyte in a downward position. A remarkable shift in potential (>1.2 V) in the active direction was measured inside the crevice cavity, when the potential at the outer surface was held at 800 mV(SCE). Experimentation showed that there is a critical depth value, above which little changes occur on the transition boundary between passive and active regions on the crevice wall, x_pass, and below which x_pass location shifts sharply towards the crevice bottom. Steeping of the potential gradient occurred with time indicating enhancement of crevice corrosion, which was seen by the gradual increase in the current. These findings were in close agreement with the IR voltage theory and related mathematical model predictions. Morphological examination showed an intergranular attack around the active/passive boundary (x_pass) on the crevice wall.     

The corrosion behavior of carbon steel in CO2‐saturated NaCl crevice solution containing acetic acid

The corrosion behavior of X52 carbon steel electrodes in CO₂‐saturated NaCl crevice solution containing HAc was investigated by electrochemical measurements. Chemical environment measurements by Cl^? and pH microprobes show an enrichment of Cl^? ions and an increase of pH values inside the crevice. Moreover, both increments could accelerate with the decreasing dimension of the crevice mouth due to the high diffusive resistance. When the electrode in the crevice solution is coupled with the electrode in bulk solution, the alkalization and the enrichment of Cl^? ions in the crevice solution can result in a negative shift of potential of the electrode in crevice solution, while the potential of the electrode in bulk solution shifts positively during the corrosion process. Thus, a galvanic corrosion is established with the electrode in the crevice solution acting as anode while another in the bulk solution as cathode, i.e., the corrosion in the crevice solution was enhanced while the corrosion in the bulk solution was retarded. The anodic dissolution and the cathodic reduction processes dominate in the crevice solution and in the bulk solution, respectively.     

Galvanic reactions during localized corrosion on stainless steel

During localized (crevice and pitting) corrosion, a local cell is established between an anode within a crevice or pit and a cathode on the surrounding passive surface. Data are presented to show that concentrated acidic chloride solutions, simulating corrosion product hydrolysis within a crevice or pit, produce potentials which are active (negative) to the normal surface passive potential. This behaviour explains the previously observed active drift of corrosion potential after initiation of crevice or pitting attack in dilute chloride solutions. The active state in concentrated chloride solutions was quite noble (positive) compared to the active state in more dilute solutions. Thus, there is no need to invoke ohmic resistance effects to account for the active state within a crevice or pit.Experiments were devised in which the local anode within a crevice was physically separated from the nearby passive-surface cathode. When the two were coupled together electrically, the cathode surfaces were polarized nearly to the unpolarized local anode potential, with only a few millivolts anodic polarization at the anode within the crevice. The rate of localized corrosion appears from the data to be limited by the rate of dissolved-oxygen reduction on the cathode surfaces. Thus, localized corrosion in dilute chloride solutions will be increased by (a) raising the temperature, (b) adding an oxidizer such as Fe³⁺ ions, or (c) substituting external anodic polarization for dissolved oxidizers.The overall potential, E_corr acquired by a specimen undergoing pitting or crevice corrosion is demonstrated to be near the protection potential, E_p below which pitting corrosion cannot propagate. Any potential active to E_corr and E_p results in cathodic polarization and suppression of the anode reaction in a crevice or pit. Since both E_corr and E_p vary with the extent of previous localized attack, E_p is not a unique property of the alloy as has been sometimes suggested and is of limited value in classifying alloy resistance to localized corrosion.     

Electrochemical and SEM study on Type 254 SMO stainless steel in chloride solutions

An investigation was conducted to examine the critical crevice potential (E_crev) and the critical protection potential (E_prot) for Type 254 SMO stainless steel in 4% NaCl solution by using potentiodynamic cyclic anodic polarization (PCAP) technique at temperature ranging from 30 to 90 °C. The critical crevice temperature (CCT) and the critical crevice protection temperature (T_prot) were determined by plotting the values of breakdown potential and E_prot versus solution temperature, respectively. The values of CCT and T_prot were recorded at the abrupt transition with increasing the temperature from transpassive corrosion to crevice corrosion and were found to be at 55 and 52 °C, respectively. Above CCT (70 °C) the following points were recorded. The E_crev and E_prot decreased linearly with log [Cl⁻]. The addition of bromide ions to chloride ions at a fixed halide content of 4% increased both E_crev and E_prot. The E_crev value in 4% NaCl increased linearly with increasing pH in the range 1-10. The addition of 0.5 M bicarbonate ions to 4% NaCl completely removed the crevices effect while increasing the addition of sulphate ions to 4% NaCl increased both of E_crev and E_prot. The morphology of the crevice corrosion produced on the steel surface was examined by scanning electron microscope (SEM) after PCAP treatment under different test conditions.     

Electrochemical Behavior and Stress Corrosion Sensitivity of X70 Steel Under Disbonded Coatings in Korla Soil Solution

The corrosion of X70 pipeline steel under a model disbonded coating was studied in a simulated solution of Korla soil by combining in situ electrochemical measurements at different locations in the crevice and stress corrosion cracking (SCC) sensitivity analyses in the corresponding simulated environments. The results from electrochemical impedance spectroscopy showed that the corrosion product resistance R _t and charge transfer resistance R _ct of X70 steel first increased and then decreased with increasing distance from the opening of the crevice in the disbonded coating. Scanning electron micrographs showed that pitting in the crevice became more severe at deeper locations in the crevice. Slow strain rate tests showed that the lowest SCC sensitivity of X70 steel was found at 15 cm away from the opening, and the highest SCC sensitivity was at the end of the crevice.     

Repassivation kinetics studies on an austenitic stainless steel in chloride media

A new technique consisting in scratching an electrode kept under a constant load has been developed to study the initial stages of repassivation (10–100 ms) of an austenitic stainless steel in various media (sulphate and chloride) following the rupture of the protective film. S.c.c. criteria have been proposed and particularly a critical potential called transition potential, E_trans. The charge densities passed during repassivation have been measured and the transition between pitting corrosion and stress corrosion cracking in chloride media is also discussed.     

Predicting the chemistry,corrosion potential and corrosion rate in a crevice formed between substrate steel and a disbonded permeable coating with a mouth

A model developed in an earlier work was used in this work to investigate the effect of coating permeability on the evolution of solution chemistry, corrosion potential, and rate in a crevice formed between a steel surface and a coating disbonded from it. The crevice gap varies along distance from the mouth, and the coating is permeable to ions and/or oxygen (O₂). The earlier work focused specifically on modeling the effect of variable gap (on crevice corrosion) with the coating impermeable to either ions or O₂. In both works, the crevice chemistry was an aerated, diluted sodium chloride solution, which at the mouth was set to be different from that initially in the crevice. The results of this work show that a permeable coating behaves like a membrane, which, under a cathodic polarization at the crevice mouth, tends to raise the in-crevice sodium ion concentration and pH more rapidly relative to an impermeable coating. Later, as the sodium ion concentration and pH in the crevice become greater than at the mouth, the permeable coating tends to reverse the transport direction for ions. At a mouth potential of ?0.900 V vs. saturated Cu/CuSO₄, the cathodic current is sufficient to suppress all O₂ penetrating the crevice both from the mouth and through the coating. The practical implication is that in the presence of sufficient cathodic polarization, a permeable coating, when disbonded, can still be capable of protecting the substrate steel from corrosion attack.     

Crevice corrosion of Q235 carbon steels in a solution of NaHCO3 and NaCl

The crevice corrosion of Q235 carbon steels in a solution of NaHCO₃ and NaCl was investigated mainly by electrochemical noise. Three stages of crevice corrosion were found and include an induction stage, a transformation stage and a stable development stage. Principal component analysis and hierarchical agglomerative cluster analysis were used to identify the crevice corrosion stages. The electrode area ratio of the outer to the inner part of the crevice (r) significantly influenced the occurrence and development of crevice corrosion. The induction stage time increased as r increased.     

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

Electrochemical behaviour of Ni-base alloys exposed under oil/gas field environments

The electrochemical behaviour of Ni-base alloys (Inconel 625, Inconel 718, G3 and Incoloy 825) is carried out at 80 °C in CO₂/H₂S corrosion environments using cyclic potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The passivity mechanisms are analysed and discussed. In addition, some significant characterisation parameters such as E_corr, I_pass, E_pit, E_pp, ΔE and I_pass in cyclic polarisation curves are analysed and compared to reveal the corrosion resistance of various Ni-base alloys. The equivalent circuit model and ZsimpWin software are utilised to discuss the Nyquist plots of various Ni-base alloys. The diffusion mechanism in EIS measurement is discussed. The result shows that the corrosion resistance of the Ni-base alloys to CO₂ corrosion or CO₂/H₂S corrosion follows the sequence: Inconel 625 > G3 > Inconel 718 > Incoloy 825. H₂S works as a cathodic depolariser with accelerating initiation of the corrosion process.     

Influence of activated carbon on crevice corrosion in adsorption tower of advanced liquid processing system

ABSTRACT

The adsorption tower made of type 316L stainless steel (SS) in Multi-nuclide Removal Equipment (Advanced Liquid Processing System) which uses Ag-impregnated activated carbon (Ag AC) as an adsorbent experienced crevice corrosion. The influence of Ag AC on the crevice corrosion susceptibility and E_sp of 316L SS was investigated by performing electrochemical experiments. Crevice corrosion was observed in the specimen in contact with the Ag AC. On the other hand, there was no crevice corrosion without the Ag AC in both pH 7.4 and pH 12 solutions. Clear ennoblement of spontaneous potential (E_sp) by in contact with activated carbon was observed and that was clearly higher than the repassivation potential for crevice corrosion (E_R,CREV). Thus, the presence of the AC notably increased E_sp of 316L SS and this resulted in increased crevice corrosion susceptibility by the galvanic effect.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Rainbow fringes around crevice corrosion formed on stainless steel AISI 316 after ennoblement in seawater

The crevice corrosion occurrence probability of stainless steel (SS) AISI 316 was increased under ennoblement condition due to chemically added H₂O₂ into seawater. The H₂O₂ was used to simulate the important factor causing ennoblement in natural marine biofilm. Morphology of the crevice corrosion was observed using an incident‐light source microscopy. Some interesting “rainbow” fringes were observed around micro‐crevices. The mechanism was discussed from the ions diffusion and potential distribution during the crevice formation. This result shows that under ennoblement condition the colored fringe is a distinct characteristic of the morphology of localized corrosion for stainless steel.     

Initiation and repassivation of crevice corrosion of type 444 stainless steel in chloride solution

Effects of chloride ion concentrations, solution temperature, and crevice-forming materials on the crevice corrosion of type 444 stainless steel were investigated using a potentiostatic method. Critical crevice potential (E_crev) and repassivation potential (E_r) of the creviced alloy decreased with an increase in chloride concentration [Cl⁻], satisfying the logarithmic relationship between E and [Cl⁻]. In addition, E_crev and E_r of the alloy with silicone crevice former were measured to be higher than those of the alloy with an EPDM (Ethylene Propylene Diene Monomer) crevice former, suggesting that silicone was more effective in preventing water from penetrating crevices between a stainless steel sheet and the crevice former. In electrochemical current transient measurements with an applied potential, the intensity of current transients corresponding to the initiation of metastable pits increased abruptly near the E_r of the alloy, indicating that the stability of crevice corrosion is associated with the initiation of metastable pits.     

Active Oxidation of SiC

Silicon carbide (SiC) forms a protective condensed-phase oxide (SiO₂) in passive oxidation and a volatile sub-oxide (SiO(g)) in active oxidation. The transition between these two modes of oxidation and the rates of active oxidation are critical issues. A literature review indicates that impurity effects, the difference between active-to-passive and passive-to-active transitions, and the effect of total pressure on these transitions remain unexplored for SiC. Measurements were made in a thermogravimetric apparatus (TGA) by changing oxygen potentials either by blending O₂/Ar mixtures or changing total pressures in a pure oxygen gas stream to the point where a transition occurs. Specimens were examined with standard optical and electron-optical techniques. Active-to-passive and passive-to-active transitions were measured and found to be similar for SiC, which is in contrast to pure Si. The similarity in SiC is attributed to SiC/SiO₂ interfacial reactions producing the necessary conditions for passive scale formation (active-to-passive) or passive scale breakdown (passive-to-active). Comparable results were obtained in both the O₂/Ar and reduced total O₂ pressure cases for SiC.     

Crevice Corrosion of Titanium in High Temperature-Concentrated Chloride Environments

Crevice corrosion of titanium is activated in concentrated chloride media at 100 °C. This was possible only with the tightest gap (0.005 cm) between Ti-Ti surfaces. No crevice corrosion was observed with greater gap dimensions. The design of the crevice led to the occurrence of two concentric circular rings of corroded areas, with many pits on them. After potentiostating in the passive region for 5 h in 25% NaCl (pH = 4.7)—where hydrogen evolution is thermodynamically prohibited—hydrogen gas bubbles were observed to egress out of the crevice mouth during ongoing crevice corrosion. This indicates that hydrogen evolution occurs within the crevice. The results are compatible with the occurrence of gradually increasing ohmic potential shift and localized acidification in the crevice electrolyte as judged by the measured gradual increase of the crevice corrosion current. The high acidity of the bulk electrolyte does not seem to be sufficient or even a necessary condition for crevice corrosion to occur.     

Plug-In Specimens for Measurement of the Corrosion Rate of Mg Alloys

Magnesium alloy corrosion is often nonlinear. The corrosion rate accelerates to steady state after an initial period of low corrosion. Plug-in specimens permit simultaneous measurement of the corrosion rate using hydrogen evolution, P _H, and Tafel extrapolation of cathodic polarization curves, P _i. Moreover, weight loss allows independent verification. P _H is consistently greater than P _i. The data, for short exposure periods up to 10?days, are consistent with the unipositive Mg⁺ ion being a short-lived intermediate. Tafel extrapolation needs to be used with caution for estimation of Mg corrosion, as the measured corrosion rate can have a significant contribution from crevice corrosion. Furthermore, measurements made at short immersion times may not reflect the steady-state corrosion rate, and the corrosion reaction at the Mg surface may be decoupled from the electrochemical measurement.     

A Study on the Passivation Behavior and Semiconducting Properties of Gamma Titanium Aluminide in 0.1 N H2SO4, HNO3, and HClO4 Acidic Solutions

The study focuses on the passivation behavior of single-gamma-phase titanium aluminide in acidic solutions with a particular emphasis on the role of oxidizing strength in characteristics of passive layer. The report includes potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) studies along with Mott-Schottky analysis in order to determine the corrosion behavior of the alloy and the semiconducting properties of the scale formed during exposure to acidic environment. Passive current density measured from potentiodynamic polarization curve, polarization resistance (R _p) estimated by EIS, defect density, and flatband potential drawn from Mott-Schottky analysis are mainly used in estimating the ability of passive film formed on alloy in protecting it against high corrosion rates in Sulfuric acid (a non-oxidizing acid), perchloric acid, and nitric acid (oxidizing acid with different oxidizing strength). The results show that passive current density (i _pass) in Sulfuric acid is 2.67 × 10^?5 A cm^?2, which is 2.5 and 3 times greater than the values obtained in perchloric acid (i _pass = 9.91 × 10^?6) and nitric acid (i _pass = 7.98 × 10^?6), respectively. EIS data reveal that the value of R _p in sulfuric acid (20 kΩ cm²) is about three and five times smaller than that its value in perchloric acid and Nitric acid, respectively. Mott-Schottky analysis shows that the passive layer exhibits an n-type semiconducting characteristics irrespective of acidic environment. The greatest and the smallest values of donor density (N _D) are obtained for the passive scale formed in sulfuric acid (N _{D, H2SO4} = 18.36 × 10¹⁹) and nitric acid (N _{D, HNO3} = 13.13 × 10¹⁹), respectively. The report concludes that characteristics of the passive scale are directly affected by reduction potential of the acid, which is the criterion of its oxidizing strength. An increase in the oxidizing strength of the acidic solution results in formation of more protective and less conductive layer on γ phase titanium aluminide.     

Crevice corrosion damage function for grade-2 titanium of iron content 0.078 wt% at 95 °C

A crevice corrosion damage function has been developed for grade-2 titanium in 0.27 mol/dm³ NaCl at 95 °C. Crevice corrosion experiments were conducted for various durations using a galvanic coupling technique, and corrosion depth profiles subsequently measured using a combination of metallographic and image analysis techniques. The damage function was then determined by plotting the maximum penetration depth (d_max) in μm as a function of time (days). This function possessed two clear stages; an initial stage in which d_max = 89.4t^0.87, and a second stage in which corrosion spread laterally without an increase in d_max, before the crevice finally passivated. The initial high penetration rate was associated with an intergranular attack down Fe-containing grain boundaries. In the second stage, further increases in d_max were limited by the IR drop between the propagation sites deep within the grain boundaries and the external cathode.     

The role of electrolyte concentration on crevice corrosion of pure nickel

In this work an artificial crevice electrode was used in conjunction with a fine microprobe assembly to measure the potential inside the crevice. Using this setup crevice corrosion of commercially pure nickel was investigated in sulfuric acid with concentrations: 0.5, 1 and 2N. The outer surface of the Ni was held at a passive potential of 530 mV(SCE) while the experiment was running. The results showed a steep potential decay observed in all concentrations. For 0.5, 1 and 2N H₂SO₄, the total potential drop inside the crevice was: 681, 619 and 593 mV, respectively. This indicates the higher the acid concentration is the lower the potential drop will be. On the other hand, the measured current was highest (4.09 mA) for 2N and lowest (1 mA) for 0.5N. On the crevice wall a boundary was found to exist between the passive and the active regions. These findings point toward the IR voltage drop mechanism operating for this system.     

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.