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.     

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.     

海水环境中微生物附着对钝性金属开路电位的影响

采用电化学技术和荧光显微技术研究了微生物附着对钝性金属在海水中开路电位的影响．研究中发现，在自然海水中。钝性金属的开路电位逐渐正移达300mV．显微镜原位观察表明，浸入海水后，微生物在金属表面逐渐寄居附着。形成微生物膜．微生物的数量与开路电位按相同方式增加．但在灭菌海水中以及铜电极情况下，开路电位保持不变，微生物也很少附着在表面上．此外，钝性金属界面双层电容在自然海水中逐渐减少。但在灭菌海水中，由于没有微生物膜存在。电容值保持不变．这些结果证实，微生物在表面的附着和寄居生长是导致钝性金属开路电位正移的主要原因．     

Comparative study of the behaviour of AISI 304 SS in a natural seawater hopper, in sterile media and with SRB using electrochemical techniques and SEM

A comparative study of the electrochemical behaviour of stainless steel AISI 304 plates immersed in a natural seawater hopper and, under lab conditions, in sterile natural seawater, were carried out over seven weeks. The behaviour of these plates was, likewise, studied in seawater plus sulphate reducing bacteria (SRB) isolated from the natural seawater medium at the end of the seven week period. Electrochemical studies which measured the voltage over open circuit conditions (E_corr), linear polarisation and electrochemical impedance spectroscopy (EIS) were also carried out. The results have allowed us to profile the varying behaviour of the plates’ rusting in the natural seawater hopper, where a complex bio-film characteristic of natural environments is formed, as opposed to the processes of spreading in zones of high frequency due to the presence of voltage carrier species produced as the result of the transformation of sulphates into sulphides and to the lab conditions where a passive layer is formed, with capacitating behaviour, in the absence of bacteria, and a bio-film with sulphide inclusions is formed in the presence of the sulphate reducing bacteria (SRB) in an inductive semi-circle. The least protective passive layer appears in the presence of the SRB since the iron sulphide increases the susceptibility of steel to corrosion.     

The importance of live biofilms in corrosion protection

As observed before, Al 2024 was passive in artificial seawater (AS) in the presence of a protective biofilm of Bacillus subtilis WB600. When antibiotics were added to the AS to kill the bacteria in the biofilm, pitting occurred within a few hours as indicated by characteristic changes in the impedance spectra. The corrosion potential E_corr decreased at the same time to values observed in sterile AS. Addition of the antibiotics to sterile AS had no effect on corrosion behavior.     

海水环境中微生物附着与钝性金属

对天然海水中微生物膜附着导致钝性金属开路电位正移现象及其作用机理的研究进行了综述.     

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).     

The effect of different exposure conditions on the biofilm/copper interface

The effects of the different exposure conditions on the electrochemical behavior of copper were evaluated in a growth medium containing Shewanella oneidensis MR-1. Impedance spectra were recorded at the corrosion potential (E_corr) in three different cells for one week of exposure followed by cyclic voltammetry. A second time constant was observed in the impedance spectra of copper that was partially immersed in the test cell, where the electrode was in contact with an air/liquid interface (cell B). These spectra resembled those usually observed for metals covered with a polymer coating. Complete immersion of copper in the electrolyte (no air/liquid interface) or deaeration of cell B resulted in one-time-constant spectra that are typical of those found for passive metals. Excellent corrosion protection was provided by MR-1 regardless of exposure condition. E_corr increased with time for the partially immersed Cu electrode exposed to the aerated solution in cell B, while it decreased for the other two exposure conditions. Cathodic polarization curves recorded after exposure for 7 days showed two reduction peaks for copper tested in cell B, while no reduction peaks were observed for the other cases. Similar results were obtained using cyclic voltammetry.     

Electrochemical studies of packed iron powder electrodes: Effects of common constituents of natural waters on corrosion potential

Using a powder disk electrode (PDE) made with micron-sized, high purity iron metal we investigated how the corrosion of this material is affected by solution conditions that are relevant to the degradation of contaminants in environmental remediation applications. Changes in corrosion potential (E_corr) with time showed that low pH, high concentrations of chloride, and natural organic matter led to breakdown of the passive film. Bicarbonate caused E_corr to decline rapidly into the active potential region, but then E_corr rose back into the passive region over 10s of hours. The rate of decline in E_corr was greatest at higher pHs, suggesting a specific effect of rather than a general effect of pH.     

海洋环境中微生物膜吸附动力学过程对钝态金属开路电位变化特征的影响

采用荧光显微技术研究了自然海水中微生物在两种钝态金属表面的吸附动力学过程与其开路电位正移之间的关系。高钼钢和1Cr18Ni9Ti电极在海水中浸泡了10d左右,1d后现金种钝态金属腐蚀电位正移约200mV。研究发现微生物向钝 态金属表面附着的初期对金属的开路电位影响最大,随后腐蚀电位增加缓慢,在吸附过程中微生物在钝态金属表面附着数量随时间增加呈负指数增长。给出了微生物附着的动力学方程,并对其进行了讨论。两种钝态金属开路电位的变化和其表面微生物数量增加有着相似的趋势。开路电位在微生物附着过程中迅速增加,微生物附着达到平衡后钝态金属开路电位增加缓慢。这说明在微生物附着初期对钝态金属电位正移影响最大,随后阶段影响逐渐减弱。     

Microstructure and corrosion resistance of molybdenum and aluminum coatings thermally sprayed on 7075-T6 aluminum alloy

Pitting corrosion upon 7075-T6 high strength aluminum alloy, often associated with cathodic intermetallic particles decreases its fatigue life by a factor of about 6 to 8. In order to improve the corrosion resistance of this alloy, arc spray coatings of molybdenum and aluminum are applied. The open circuit potential and potentiodynamic polarization measurements made in 3.5% NaCl naturally aerated solution reveal that the molybdenum coating, which has an excellent hardness, shifts the 7075-T6 corrosion potential (E _corr) to noble values and increases slightly the corrosion current density (i _corr). On the contrary, when the aluminum coating alloy is applied, both E _corr and i _corr are shifted to better values. The increase of i _corr of the alloy when molybdenum coating is applied can be attributed to the high porosity present into the coating. On the other hand, microstructure observations of the aluminum coating reveal a small porosity, which helps the formation of passive oxide film that protects the coating against a further corrosion.     

海水环境中弧菌对45钢腐蚀行为及力学性能的影响

通过热带海洋气候条件下在海水中培养弧菌,并对比45钢在自然海水、无菌海水和弧菌海水中的腐蚀行为,研究了弧菌对45钢腐蚀行为及力学性能的影响。结果表明,弧菌可以接种于海水中大量培养至高浓度(非培养基中培养),避免了培养基成分的缓蚀作用,从而更接近碳钢的自然腐蚀状态。海水中弧菌对45钢的平均腐蚀速率及力学性能有显著的影响。弧菌能加速45钢的平均腐蚀速率,但微生物的协同作用比单种弧菌更能加速材料的平均腐蚀速率。弧菌能显著降低局部pH值,引起材料表面严重的局部腐蚀,材料表面局部腐蚀促使其在拉伸过程中应力集中而发生断裂。     

The use of electrochemical techniques for the investigation and monitoring of microbiologically influenced corrosion and its inhibition – a review

The background of several electrochemical techniques – recording of the corrosion potential E_corr, electrochemical impedance spectroscopy (EIS), electrochemical noise analysis (ENA) and recording of potentiodynamic polarization curves – has been discussed and examples of the use of these techniques in studies of microbiologically influenced corrosion (MIC) and MIC inhibition have been presented. These examples range from the use of the polarization resistance R_p and the noise resistance R_n in corrosion monitoring in a sewer environment and a gas production field, respectively, to the application of EIS and ENA in the evaluation of the interaction of bacteria with protective coatings on steel exposed to natural seawater (NS). A more detailed analysis based on EIS and ENA of Corrosion protection of Al 2024, mild steel and brass in artificial seawater (AS) by bacteria such as B. subtilis, B. licheniformis, E. coli and Shewanella has been presented. Determination of the changes of R_p and R_n as well as E_corr allows to propose appropriate mechanisms of MIC inhibition. These mechanisms have been confirmed in the case of S. algae by recording of anodic and cathodic potentiodynamic polarization curves after exposure to AS containing the bacteria for extended time periods.     

Effect of the reinforced boron carbide particulate content of AA6061 alloy on formation of the passive film in seawater

The effect of boron carbide (B₄C) reinforcement on the corrosion of AA6061 alloy was studied by investigating passive films formed in seawater. The higher passive current and its potential-dependence for these composites indicated formation of porous passive film. Electrochemical impedance spectroscopy (EIS) graph suggests that the alloy surface is partly or totally active. The formed passive film is n-type semiconductor junction in nature. The difference between corrosion potential (E_corr) and potential at zero charge (PZC) suggests that the chloride ions responsible for film breakdown exist within the passive film. A suitable mechanism is proposed for the passive film breakdown.     

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%.     

Electroplating of nickel from acetate based bath – Hull Cell studies

Abstract

Electroplating of nickel onto steel substrates from an electroplating bath containing nickel acetate, nickel chloride, boric acid and acetic acid has been investigated under different levels of operating conditions such as bath composition, current density and temperature. A detailed study has been made on the influence of these variables by measuring cathodic current efficiency and throwing power for various baths. Deposition parameters have been optimised using Hull Cell experiments. From potentiodynamic polarisation studies on Ni deposits it was revealed that E _corr moved towards the more negative direction. It is inferred from this fact that there is cathodic control of the corrosion reaction. The R _t value increases with increasing deposit thickness and the corresponding C _dl value is decreased.     

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.     

The corrosion behavior of stainless steels and copper alloys exposed to natural seawater

The corrosion behavior of stainless steels, titanium and copper alloys exposed to flowing Pacific Ocean water was characterized using surface analytical and electrochemical techniques. Biofilm formation on stainless steels and titanium resulted in thin films of bacteria and diatoms that did not cause significant changes of the corrosion potential (E_corr) or surface properties. Rotating cylinder experiments indicated that both E_corr and corrosion rates for stainless steels and titanium were independent of mass transport. Four surface layers were identified on copper-containing materials: substratum metal; an inorganic chloride corrosion layer that contained alloying elements; a biofilm; and crystalline, spherical phosphate-rich deposits. All copper surfaces were colonized by bacteria independent of alloy composition. The complexity of the impedance spectra for copper-containing materials was attributed to formation of surface layers and contributions of charge transfer and mass transport controlled reactions mediated by the layers. Both anodic and cathodic reactions for copper-based materials were affected by mass transport.     

Electrochemical behavior of some copper alloys in sulfuric acid solutions

In this article, the corrosion behavior of Cu? Fe and Cu? Al? Fe was studied in H₂SO₄ solutions in the absence and presence of different concentrations of some inorganic additives, Na₂MoO₄, K₂Cr₂O₇, KIO₃, and Na₂B₄O₇. The techniques of measurements used were open‐circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy. The polarization measurements indicated that the corrosion current, I_corr, of Cu? Fe is less than that of Cu? Al? Fe. The investigated additives in H₂SO₄ shifted E_corr to more positive values and increases I_corr. Impedance measurements indicated that Nyquist plots for Cu? Fe may be regarded as a semi‐circle in early stages of immersion and tends to be a part of semi‐circle with elapse of time and a well‐defined inductive loop appears. In case of Cu? Al? Fe, the Nyquist plots tends to be semi‐circle or depressed semi‐circle. The values of R_p for Cu? Fe are higher than those for Cu? Al? Fe. On the other hand, the values of C_dl for Cu? Fe are lower than those obtained for Cu? Al? Fe. The presence of these additives in H₂SO₄ solutions decreases the values of polarization resistance (R_p) and increases the values of corrosion current density (I_corr) and capacitance of double layer (C_dl).     

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.