Study of the electrochemical behaviour of 304 stainless steel in postgate medium in the presence and absence of desulfovibrio desulfuricans

A comparative study has been undertaken on the behaviour of AISI 304 steel in Postgate medium with and without the presence of Desulfovibrio desulfuricans subspecies desulfuricans bacteria (strain DSM 642). Electrochemical studies were carried out with open circuit potential measurements, linear polarization, electrochemical impedance spectroscopy (EIS) and scanning electronic microscopy (SEM). In Postgate medium without bacteria, and as expected in stainless steels, the formation is observed of a protective surface passive film on the AISI 304 steel. On measuring the corrosion potentials (E_corr), it is noted that the potential tends to rise with the immersion time, and in the impedance graphs an increase is observed in the charge transfer resistance and a higher phase angle with respect to the other systems tested. However, in Postgate medium with Desulfovibrio desulfuricans (strain DSM 642), the tendency of the corrosion potentials (E_corr) is not uniform, though the recorded values are less noble than those for the medium without bacteria. A variation is observed in the properties of the layer of corrosion products in the presence of bacteria as the immersion time advances (due to the effects caused by the formation of corrosion products which gives rise to changes in the corrosion speed).     

Ennoblement of stainless steel in natural seawater – A new explanation

The ennoblement of corrosion potential (E_corr) of passive metal immersed in seawater was investigated with electrochemical technology and epifluorescence microscopy. The in situ observation showed that the bacteria number increased on the metal surface according to an exponential law which was in the same way with the ennoblement of E_corr. At the same time, the anodic polarization current of high‐Mo stainless steel decreased in the initial days. According to the mix‐potential theory and the characteristics of polarization curves of high‐Mo steel in natural seawater, the ennoblement of corrosion potential may be induced by the decrease of the passive current density.     

Electrochemical study of stainless steels used in an oil company by electrochemical impedance

Localized corrosion is a serious problem for stainless steel exposed in a chloride solution. In this context, the present work concerns the study of electrochemical behavior of 316 (A and B) and 430 (C and D) stainless steels, where A and C are laboratory steels, while steels B and D were taken from heat exchangers tubes (after 10 years of operation). This study has addressed three different aqueous environments: (1) monoethanolamine (MEA) 15%, (2) natural seawater (NSW), and (3) NaCl 3%, using the technique of potentiodynamic polarization curve complemented by electrochemical impedance spectroscopy (EIS). The obtained experimental electrochemical parameters (E_corr, J_corr, E_r, C_d, R_tc, and constant phase element) were used to compare the corrosion resistance of the tested steels. As a result, the nobility of these steels in NaCl 3% and in MEA 15% solutions is established in the following order: 316 (A) > 316 (B) > 430 (C) > 430 (D), where E_r moves increasingly toward the positive direction, indicating a good protection against corrosion. In addition, the results show that the effect of the electrolyte (aggression) is characterized by increased corrosion potential and a decrease in the passivity domain for all samples. The hierarchy of the nobility of steels A and C in these electrolytes can be determined as follows: MEA 15% > NSW > NaCl 3%.     

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells was investigated using electrochemical tests and XPS surface analyses. The addition of copper to the alloy deteriorated the corrosion resistance at a passive region of 0.6 V_SCE. With an increase of the copper content, the passivity of the alloy degraded due to an increase of the formation of Cr-containing inclusions, and a decrease of the Cr oxide and hydroxide in the passive film.     

Relationship between the ennoblement of passive metals and microbe adsorption kinetics in seawater

The relationship between microbial colonization of two kinds of passive metals and ennobling of their corrosion potentials (E_corr) were studied. Two types of passive metal coupons were exposed to natural seawater for about ten days. Under laboratory conditions, all corrosion potentials of the samples ennobled for about 200 mV. Epifluorescence microscopy showed that bacteria adsorption was the main process during about the first day immersion and bacteria reproduced in the following days. The bacteria number increased on the metal surface according to an exponential law and the kinetics of bacteria adsorption at the metal surface during this period was proposed. The ennoblement of E_corr was similar to the increasing bacteria number: E_corr increased quickly during the bacteria adsorption process and increased slowly after biofilms had formed.     

Aqueous corrosion behaviour of sintered stainless steels manufactured from mixes of gas atomized and water atomized powders

The electrochemical corrosion improvement of a powder metallurgical (PM) stainless steel is studied in this work. Water atomized (WA) ferritic AISI 434L powders have been mixed with gas atomized (GA) austenitic (AISI 316L type) and ferritic (AISI 430L type) powders and processed through the traditional PM route. The addition of GA powder to the usual WA powder decreases the mean size of the pores of the sintered stainless steels. As the bigger pores are the ones that are able to act as crevices, unlike the smaller ones - that act as closed porosity, reduction in the number of big pores tends to improve the corrosion behaviour of PM stainless steels. Reductions of the corrosion rate (i_corr) and increases of the corrosion potential (E_corr) have been measured in neutral media, with and without chlorides. Moreover, the additional beneficial effect of achieving a duplex microstructure through the addition of GA austenitic powders to the WA ferritic powders has also been verified.     

Corrosion resistance and corrosion behavior of high-copper-bearing steel in marine environments

Thus far, research on the corrosion resistance of copper-containing steel has been limited to Cu content of less than 1%, and the corrosion resistance of antibacterial Cu-containing steel with Cu content above 3% has not been reported. In this study, 0Cu3 carbon steel (CS), 0Cr15Cu3 stainless steel (SS), and Q345 CS were investigated. The corrosion resistance and corrosive behavior of high-copper (high-Cu)-bearing steel in a marine environment were examined by scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, electrochemical impedance spectroscopy, and potentiodynamic polarization. Coupon test results showed that the Cu in the 0Cr15Cu3 SS and 0Cu3 CS can promote the formation of stable α-FeOOH from γ-FeOOH in the outer rust layers, and make the rust layers more thick and dense. In the electrochemical experiment, the impedance loop diameters and R_ct values of the 0Cr15Cu3 SS and 0Cu3 CS were higher than those of Q345, while the I_corr was less than that of Q345, which indicates that the anticorrosion property of these two types of high-Cu-bearing steel was higher than that of Q345. The aim of this study was to define the properties of corrosion resistance and corrosive behavior in high-Cu-bearing steels to promote their application in marine engineering.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Influence of biofilms growth on corrosion potential of metals immersed in seawater

The changes of corrosion potential (E_corr) of metals immersed in seawater were investigated with electrochemical technology and epifluoresence microscopy. In natural seawater, changes of E_corr were determined by the surface corrosion state of the metal. E_corr of passive metals exposed to natural seawater shifted to noble direction for about 150 mV in one day and it didn't change in sterile seawater. The in‐situ observation showed that biofilms settled on the surfaces of passive metals when E_corr moved in noble direction. The bacteria number increased on the metal surface according to exponential law and it was in the same way with the ennoblement of E_corr. The attachment of bacteria during the initial period played an important role in the ennoblement of E_corr and it is believed that the carbohydrate and protein in the biofilm are reasons for this phenomenon. The double layer capacitance (C_dl) of passive metals decreased with time when immersed in natural seawater, while remained almost unchanged in sterile seawater. The increased thickness and reduced dielectric constant of C_dl may be reasons.     

Characterization of Corrosion Product Layers from CO2 Corrosion of 13Cr Stainless Steel in Simulated Oilfield Solution

The influence of temperature and flow rate on the characterization and mechanisms of corrosion product layers from CO₂ corrosion of 13Cr stainless steel was carried out in simulated oilfield solution. Cyclic potentiodynamic polarization method as well as weight loss tests in autoclave were utilized to investigate pitting corrosion behavior at various temperatures. Weight loss tests were performed at 100 and 160 °C under dynamic and static flow conditions. At the same time, the significant pitting parameters such as E _corr, E _pit, E _pp, ∆E, and I _pass in cyclic polarization curves at various temperatures were analyzed and compared for revealing the pitting behavior of 13Cr stainless steel. The surface measurement techniques such as SEM, XRD, and XPS were used to detect the corrosion product layers. The results showed that both temperature and flow rate had significant effects on characterization of corrosion product layers or passive films formed on 13Cr stainless steel in CO₂ corrosion system. At high temperature, lots of pits were formed at the localized corrosion areas of metal surfaces. Corrosion rates under the condition of 5 m/s were higher than those under the static condition regardless of the test temperatures.     

Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K₂HPO₄ have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, i_corr, increased, corrosion potential, E_corr, shifted toward more positive values and charge transfer resistance, R_ct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses.     

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 pair in aqueous LiBr solutions

The influence of Reynolds number on the galvanic corrosion of the copper/AISI 304 stainless steel pair in a concentrated lithium bromide solution was investigated according to the mixed potential theory. A hydraulic circuit was designed to study dynamic corrosion processes in situ. A potential relation between corrosion current density (i_corr) and Reynolds number (Re) was found for copper, showing a mixed control of a chemical step and mass transport through the corrosion products film with the predominance of the former. No dependence of i_corr on Re could be established for AISI 304, showing a chemical step control. Moreover, under stagnant conditions, partial passivation may occur in AISI 304; however, under flowing conditions passivation is not possible. Copper is the anodic element of the pair under all flowing conditions analysed. The galvanic phenomenon is more important as Re increases, but the results show compatibility of both materials at all Re values analysed. Similarly, a potential relation between galvanic current density (i_G) and Re was found, showing a mixed control of a chemical step and mass transport with the predominance of the latter. Copper corrosion resistance decreases more rapidly as Re increases due to the AISI 304 galvanic effect: there is a synergy between the galvanic effect and the hydrodynamic conditions. Under stagnant conditions, the galvanic behaviour of the materials is close to the compatibility limit and an inversion of the anodic element of the galvanic pair takes place.     

含铜抗菌不锈钢的抗菌特性研究

研究了铁素体和奥氏体含铜抗菌不锈钢的抗菌特性及相关机制.实验结果表明,含铜抗菌不锈钢具有优良的广谱杀菌作用,对本文选择的大部分革兰氏阴性菌(G-)和革兰氏阳性菌(G )的杀灭率均在99.0%以上.通过铁素体抗菌不锈钢对大肠杆菌作用不同时间的研究证明,抗菌不锈钢的杀菌率和其与细菌作用时间有关,铁素体抗菌不锈钢与大肠杆菌作用150 min左右时间后的杀菌率才会达到99.9%的最大值.原子力显微镜的观察表明,抗菌不锈钢对大肠杆菌的杀灭表现为菌体内大量物质流失,细菌出现干瘪现象.电化学实验结果表明,抗菌不锈钢与大肠杆菌作用后,会由于其表面抗菌相中铜离子溶出的加快,而表现为其耐点蚀电位的下降.     

Effect of Nitrogen Partial Pressure on the Electrochemical Evaluation of (Ti-A1)N Coatings Deposited by Reactive Magnetron Sputtering

Titanium aluminium nitride films were deposited on stainless steel substrates by reactive magnetron sputtering under various nitrogen partial pressures, using a composite target consisting of alternate arc segments of titanium and aluminium. Electrochemical evaluation of these coatings, carried out by the potentiodynamic measurement technique in deaerated 1N H₂S0₄ solution at room temperature, has shown that initially there is a rapid increase in corrosion resistance of the coatings with increase in partial pressure of nitrogen; a further increase in nitrogen partial pressure leads to a much lower increase in the corrosion rate. The corrosion potential (E_corr) increased from -339.8 to -268.0 mV with the increase in nitrogen partial pressure from 0.18 to 0.63 mtorr. With further increase in partial pressure of nitrogen to 1.08 mtorr, E_corr decreased to -303.6 mV. The corrosion current density (I_corr) was found to be least 4.6 μA cm^?2 at nitrogen partial pressure of 1.08 mtorr. Coatings were characterized by X-ray diffraction phase analysis, which showed the presence of microcrystalline cubic TiN structure for nitrogen partial pressures of up to 0.88 mtorr. A cubic TiN plus hexagonal AIN structure was present at 0.98 mtorr, while only hexagonal AIN structure was observed at 1.08 mtorr nitrogen partial pressure. Surface hardness measured by microhardness tester using a Knoop indenter showed an increase in surface hardness values with increase in partial pressure of nitrogen. The maximum hardness of 2790 HK₂₅ was observed at a nitrogen partial pressure of 0.98 mtorr. At nitrogen partial pressure of 1.08 mtorr the hardness value decreased drastically to 1746 HK₂₅     

Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels

Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pitting potential, E_b, also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed.     

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.     

Kinetics and mechanism of corrosion of iron in phosphoric acid

A study of the anodic and cathodic reactions involved in the corrosion of iron in phosphoric acid shows that the rate determining steps for the two reactions are similar to those reported for corrosion of iron in sulphuric acid. However, the dependance of i_corr and E_corr on time and solution parameters shows that an additional factor in the corrosion of iron in phosphoric acid is the slow coverage of the iron surface with phosphate resulting in a decrease of corrosion rate with time.     

The cavitation erosion and erosion‐corrosion behavior of carbon steel in simulating solutions of three rivers of China

The corrosion behaviour of the Q235 carbon steel in simulation solution of Yangtze, Yellow and Talimu river of China was investigated by mass loss, electrochemical measurements (polarization curves, linear polarization resistance and electrochemical impedance spectroscopy) and the damaged surfaces were observed by a scanning electron microscope (SEM) under static, cavitation erosion and erosion‐corrosion conditions. It was shown that under static condition, the E_corr was almost the same in the three solutions but the anodic current density in Talimu river was about one order of magnitude larger than that in Yangtze and Yellow river. Under dynamic conditions including cavitation erosion and erosion‐corrosion, the E_corr in the three solutions all shifted in positive direction from that under static condition, but the extent of ΔE_corr increased in the order of Yangtze, Yellow and Talimu river. The corrosion rate under dynamic conditions greatly enhanced, especially in Talimu river. It was confirmed that the corrosion played an important role in the synergistic effect under cavitation erosion and erosion‐corrosion conditions. The test results suggested that when the structure materials used in the different rivers were chosen, the difference of corrosiveness of the rivers must be taken into account, especially under dynamic conditions. The materials used in the aggressive rivers such as Talimu river under dynamic conditions must resist the synergistic effects between erosion and corrosion.     

An efficient protection of stainless steel against corrosion: Combination of a conversion layer and titanium dioxide deposit

In the present work, a novel process has been developed to improve the corrosion properties of ferritic stainless steels. Titanium oxide coatings have been deposited onto stainless steel by sol-gel process after a pre-functionalization of the substrate in a conversion bath. Gel titania was prepared by hydrolysis of a titanium butoxide through a sol-gel process. Duplex systems “conversion layer/uniform TiO₂ coating” have been prepared on stainless steels using a dipping technique and thermal post-treatments at 450 °C. The preparation of sol-gel coatings with specific chemical functions offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The morphology and structure of the coatings were analysed using scanning electron microscopy with field effect gun (SEM-FEG), Mass spectroscopy of secondary ions (SIMS) and X-ray diffraction (XRD). The anticorrosion performances and the ageing effects of the coatings have been evaluated in neutral and aggressive media by using several normalized tests.The results show that the conversion layer was not sufficient to protect steel but sol-gel TiO₂ coatings, anchored on the metal substrate via the conversion layer, show good adhesion with the substrate and act as a very efficient protective barrier against corrosion. So, duplex layers with TiO₂ nanoparticle coatings on steels exhibit an excellent corrosion resistance due to a ceramic protective barrier on metal surface. Analysis of the data indicates that the films act as geometric blocking layers against exposure to the corrosive media and increase drastically the lifetime of the substrate.     

Effects of chloride and crevice on corrosion resistance of stainless steels buried in soil within Seoul Metropolitan

Field and laboratory tests were conducted to find the factors affecting corrosion of stainless steels in soil. During one-year exposure, corrosion occurred within a joint and on the surface of type 304 pipe with the joint, which was buried at the site with a high chloride concentration of about 3680 ppm; however, corrosion was not observed at any of the other sites independent of the stainless steel grade and the presence of joints. At some sites, a seasonal fluctuation of corrosion potential was observed in the soil though corrosion did not occur. This observation may be due to the activity of sulfate reducing bacteria because a decrease of corrosion potential with the inoculated bacteria did not cause corrosion of stainless steels. These results indicate that both the level of chloride and the presence of crevices are the main factors affecting corrosion of stainless steels in soil but that the activity of bacteria is not. From measurements of pitting potential, a guideline for stainless steel use in soil is drawn as follows: Corrosion of stainless steels in soil occurs when the pitting potential of stainless steel under crevices in synthetic ground water that contains the same chloride concentration as the soil is less than the saddle potential. Finally, the guideline for stainless steels applications was provided in this paper according to this criterion.