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.     

Theoretical description of the practical possibility of stress corrosion cracking from crevice corrosion sites

Stress corrosion cracking (SCC) from crevice corrosion sites had been found in an experimental work at polarization potential of + 200 mV_SCE. In that work, an occluded U‐bend specimen of Type 316L (UNS S31603) stainless steel was used. The testing was done in sodium chloride (NaCl) solution. Based on that work, the practical possibility of SCC from the occluded U‐bend specimen was described theoretically. It was shown that it would also be possible for SCC to occur in practice (i.e. at practical corrosion potential), but the crevice needs to be tighter. Meanwhile, it would take a longer time for obvious SCC to emerge. For a practical crevice usually formed by placing a crevice former on a large uniform metal surface, the crevice geometry may have little effect on SCC although the crevice can sustain an acidified solution more easily than pitting. The possibility of SCC should mainly depend on the corrosion system itself, i.e. material and environment.     

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.     

Crevice corrosion monitoring of titanium and its alloys using microelectrodes

Crevice corrosion of titanium and its alloys in 10% sodium chloride was investigated at 100°C with the aid of microelectrodes. Potential, pH and chloride ion concentration inside the crevice were monitored using an Ag/AgCl electrode, a tungsten microelectrode and a Ag/AgCl chloride ion selective microelectrode, respectively. The pH and Cl^? concentrations within the crevice were calculated from the standard potential‐pH and potential‐log[Cl^?] calibration curves. The effect of Mo on the crevice corrosion of titanium was also studied. The passivation behavior on the titanium and Ti‐15%Mo alloy was studied using electrochemical impedance studies. There was no apparent change in pH and Cl^? ion activity inside the crevice for the alloy at 100°C, whereas a marginal decrease in pH and increase in Cl^? ion concentration were observed for pure titanium. Thus pure titanium is susceptible to crevice corrosion in hot 10% NaCl solutions at 100°C. The chloride ion activity was found to be reduced for the alloy so that the pH inside the crevice increased. The corrosion reaction resistance (R_t) was found to increase with the addition of Mo as an alloying element. It also increases with externally applied anodic potential. Hence, Mo is an effective alloying element, which enhances the crevice corrosion resistance of titanium.     

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.     

金属缝隙腐蚀的成因及实验验证

在制品和工程结构中,缝隙腐蚀是一种很普遍且隐蔽的局部腐蚀,其结果会导致构件强度降低,既降低服役寿命,也存在安全隐患。指出缝隙腐蚀发生的三个必要条件——制品或工程结构中有金属或合金、有缝隙的存在、缝隙内有腐蚀介质滞留,概述了缝隙腐蚀的特征及三个阶段。分析了缝隙腐蚀的影响因素,包括金属性质、环境因素(溶液中氧、氯离子浓度,温度,p H值以及溶液的流速)和缝隙的几何形状。介绍了在实验室用三氯化铁溶液验证不锈钢及其合金的缝隙腐蚀敏感性的基本步骤,提出了在工程实际中如何预防缝隙腐蚀的基本思路。     

409型不锈钢在冷凝液中的缝隙腐蚀行为

运用冷凝液循环蒸发方法模拟汽车消声器内部腐蚀环境,对比研究了在有/无缝隙条件下409型不锈钢在冷凝液中的腐蚀行为。结果表明:与无缝隙试样相比,有缝隙试样在循环试验过程中表现出相对较低的自腐蚀电位;20次循环试验后,腐蚀阻力较低,腐蚀深度较大;缝隙导致不锈钢局部腐蚀速率约增大2.4倍,这主要是由缝隙内部在蒸发过程中更容易保持电解液腐蚀环境、并不断酸化导致的。     

Susceptibility of 316L stainless steel to crevice corrosion in submersible solenoid valve

The susceptibility of 316L stainless steel to crevice corrosion was investigated by using immersion test and electrochemical test. Three kinds of crevices including 316L‐to‐polytetrafluoroethylene (PTFE) crevice, 316L‐to‐fluoroelastomeric (FKM) crevice and 316L‐to‐316L crevice were tested in artificial seawater at 50°C. The results indicate that 316L stainless steel specimen is the most susceptible to crevice corrosion when it is coupled to 316L stainless steel crevice former, while it is the least susceptible when it is coupled to FKM crevice former. It suggests that during submersible solenoid valve design, the crevice of metal‐to‐metal should be moderately large so that crevice corrosion can not initiate and propagate, and FKM O‐ring rather than PTFE O‐ring should be selected as obturating ring. The corroded surface morphology was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Three regions including passive region, active region and variable region can be observed on crevice corrosion sites.     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

In vitro electrochemical investigations of advanced stainless steels for applications as orthopaedic implants

Potentiodynamic anodic polarization experiments on advanced stainless steels (SS), such as nitrogenbearing type 316L and 317L SS, were carried out in Hank’s solution (8 g NaCl, 0.14 g CaCl₂, 0.4 g KC1, 0.35 g NaHCO₃, 1 g glucose, 0.1 g NaH₂PO₄, 0.1 g MgCl₂, 0.06 g Na₂HPO₄ 2H₂O, 0.06 g MgSO₄ 7H₂O/1000 mL) in order to assess the pitting and crevice corrosion resistance. The results showed a significant improvement in the pitting and crevice corrosion resistance than the commonly used type 316L stainless steel implant material. The corrosion resistance was higher in austenitic stainless steels containing higher amounts of nitrogen. The pit-protection potential for nitrogen-bearing stainless steels was more noble than the corrosion potential indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed very little tendency for leaching of metal ions, such as iron, chromium, and nickel, at different impressed potentials. This may be due to the enrichment of nitrogen and molybdenum at the passive film and metal interface, which could have impeded the releasing of metal ions through passive film.     

A study on the modeling of parameters affecting the Ir drop mechanism in the initiation stage of crevice corrosion

The sizable potential difference between the crevice mouth and the crevice tip is difficult to explain from theories mainly based on the chemical changes in crevice solution. In this study, the IR drop theory was adopted to explain the initiation of crevice corrosion in the framework of IR drop in the crevice electrolyte. Furthermore, the parameters linking the two components of potential drop — one due to accumulated charges from chemical changes of ionic species by mass transfer and the physical one caused by the current path through the crevice electrolyte — are examined. The mathematical model is constructed and the IR drop in the crevice is calculated using the FVM method. The chemical composition change in the crevice solution can affect the IR drop behavior through the intermediate parameter form of ion conductivity and viscosity, etc., together with the potential difference generated by the electric charge of locally charged ions. Viewed in this light, it can be concluded that the initiation mechanism only by IR drop is insufficient to explain the initiation of crevice corrosion in stainless steels that have very strong passivity.     

Pitting and crevice corrosion behaviour of high alloy stainless steels in chloride‐fluoride solutions

The localised corrosion resistance (pitting and crevice corrosion) of two high alloy stainless steels, namely superduplex (SD) and superaustenitic (SA), has been studied in chloride‐fluoride solutions at pH values ranging from 2 to 6.5. The pitting potential (E_pit) and crevice potential (E_cre) have been calculated for these test media using electrochemical techniques (continuous current). The Critical Pitting Temperature (CPT) and Critical Crevice Temperature (CCT) are in both materials lower then the room temperature. In spite of this fact and due to the high repassivation rate, the resistance of these materials to localised corrosion is high in the tested media. At the highest tested concentration of aggressive anions and pH 6.5 both materials undergo a generalised attack.     

Stainless steel crevice corrosion testing in natural and synthetic seawater

Susceptibility of stainless steel to crevice corrosion in natural and synthetic seawater was investigated by a new test method, developed by a research group, CrevCorr, of the Marine Working Party of the European Federation of Corrosion. The method was defined on the basis of a Round Robin test performed by twenty laboratories over the world [1, 2]. Among them was our Institute at the Adriatic coast. Geometry of the crevice model, gasket materials, finishing of metal surface and the electrochemical potential increase rate were strictly defined. The monitoring of the free corrosion potential of two austenitic steels and a duplex steel, in the constant flowing conditions, as well as the redox potential, temperature, chloride and oxygen content, have been carried out during six months in the natural seawater from the Adriatic Sea at the location in Dubrovnik, Croatia. Sharp drop of the corrosion potential was used as an indication of the crevice corrosion initiation. After exposure to the seawater eventual crevice damage on a steel specimen was detected by visual and microscopic inspections. In the synthetic seawater the organic component was simulated by glucose and glucose oxidase [3, 4]. The monitoring of the free corrosion potential and the redox potential was performed at 20, 30 and 40°C during five days, and results compared with those obtained in the natural seawater at same temperatures.     

Loch- und Spaltkorrosion von Chrom- und Chromnickelstählen in chloridhaltigen Lösungen

Pitting and crevice corrosion of stainless steels in chloride solutions In practice stainless steels in chloride containing waters are found to be susceptible to crevice corrosion and pitting. Corrosion tests were carried out on AISI 304 L stainless using a simulated crevice and the compositions of the electrolyte in the crevice determined. Long term potentiostatic tests were used to determine the critical potentials for crevice corrosion (U_S), for various steels in sodium chloride solutions at different concentrations and temperatures. The steels studied were 22 CrMo V 121, X 22 CrNi 17 and AISI 304 L. Like the critical pitting potential (U_L), U_S was found to have a strong dependence on the chloride content of the external solution. At higher concentrations the two potentials were similar. At lower concentrations the U_S was lower than U_L. The knowledge of these critical potentials together with well known rest potentials for a steel in an electrolyte of known concentration, allows conclusions to be drawn about its susceptibility to pitting and crevice corrosion. The method is suitable also for other passive metals.     

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.     

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).     

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field

The effect of a magnetic field on chloride induced pitting of stainless steel was studied by potentiodynamic measurements in aqueous NaCl solution in a cylindrical cell with the field perpendicular to the surface under test. Compared to identical tests without field, a significant shift of the repassivation potential E_r in the cathodic direction was observed, together with the formation of small pits at high density in the periphery of the electrode. These pits develop under the influence of a vortical flow induced by the magnetohydrodynamic effect. The shift of E_r is explained as the consequence of the occluded morphology of these pits.     

An alternate explanation for the abrupt current increase at the pitting potential

A critical transition potential is shown to be associated with the abrupt increase in the number of metastable pits and the onset of spatial clustering of pits amongst nearest neighbor defects in both commercial and model stainless steels. This critical transition potential is correlated with the parameter, E_pit commonly observed in near-neutral NaCl solutions when stainless steels form pits. The explosive growth of pit sites, the onset of clustering of pits, and E_pit all depend on the diffusion length associated with aggressive corrosion products formed at pits and thus on the stirring rate. E_pit is observed to increase significantly and pit sites remain random to high potentials in both experiments and modeling when the diffusion length is decreased below the nearest neighbor distance (NND) associated with surface defects such as non-metallic inclusions that form a random array of susceptible sites across the otherwise passive surface. This phenomenon is confirmed by experiments and modeling using model stainless steel alloys with decreased inclusions densities and, thus, increased defect NNDs. If the NND is greater than the lateral diffusion length, then E_pit will increase, clustering of pit sites will decrease and explosive growth in the number of pits will be suppressed at a given applied point.     

Influence of chromizing treatment on the corrosion behavior of AISI 316 stainless steel in supercritical water oxidation

SCWO, sometimes referred to as hydrothermal waste processing, uses the solvating traits of water in its supercritical condition to effectively destroy liquid organic wastes. One major problem in the supercritical water oxidation process is corrosion, because all metallic tubes in the process are exposed to high temperature and high pressure as well as severe corrosive species such as Cl⁻, F⁻, S²⁻, and O²⁻. The presence of Cl⁻ when the pH of a solution is very low and the solution has excess oxygen causes active corrosion and metal loss by metal-chloride and/or oxychloride formation. This study performed a chromizing treatment on 316 stainless steel and immersion tests in supercritical water. Weight change of chromized steels and untreated steels was measured, and the chemical state and composition of oxide films on 316 stainless steel were investigated. On the basis of SCWO tests using distilled water, the oxide layer was found to be very thin and homogeneous and weight gain was observed regardless of testing temperature, while the chromizing treatment slightly reduced weight gain. In the case of SCWO tests using salt water, weight loss was observed regardless of testing temperature and its corrosion mode was pitting by chloride ion, while chromizing treatment greatly decreased the corrosion rate.     

A crevice corrosion study by Electrochemical Noise Analysis

The use of the Electrochemical Noise Analysis (ENA) for the evaluation of crevice corrosion is illustrated in the case of AISI 430 stainless steel in 3% sodium chloride. A crevice former was used in order to induce a crevice corrosion attack. Current and potential noise signals were simultaneously recorded allowing the determination of the noise resistance (R_n). ENA was able to detect the four stages mechanism of crevice corrosion. The comparison of R_n with the Polarization Resistance (R_p) determined by Electrochemical Impedance Spectroscopy (EIS) gave good agreement in particular during the initiation and propagation of the attack. The evaluation of the noise data in the frequency domain gave interesting results in particular in the evaluation of the roll‐off slopes in the Power Spectrum Density (PSD) plot that are correlated to the corrosive status. Finally a new analysis for noise data is shown. The application of spectral ratio discriminant function to noise data in the frequency domain, permits to deduce the best sampling frequency and sampling duration for ENA acquisition, able to discriminate between two different situations.