Shift In Corrosion Potential Research Articles

Improved corrosion resistance through microstructural modifications induced by codepositing SiC-particles with electrolytic nickel

Micron and submicron-sized SiC-particles (5 and 0.3 μm respectively) were codeposited with nickel from a Watts electrolyte. The Ni–SiC composite coatings showed a better corrosion resistance in a 0.6 M NaCl solution than nickel electrodeposited under the same conditions. The corrosion rate of Ni–SiC decreases by two orders of magnitude with respect to pure Ni coatings. This improved corrosion resistance is quite independent of the size and amount of embedded particles, except for the smallest SiC-particles investigated. In that case, the pitting corrosion potential shifts to more noble values indicating a notable reduction of the localized corrosion susceptibility. This improved corrosion resistance of Ni–SiC coatings containing submicrometric SiC-particles is linked to a change in grain morphology and texture of the coatings. That morphology evolves from columnar grains to small and equiaxed grains.

Biofouling and corrosion of stainless steels in natural waters

The noble shift in corrosion potential to values between +300 and +400 mVSCE and the accompanying increase in cathodic current density and polarization slope at mild cathodic potentials that develop during microbial colonization of passive metals, are collectively known as ennoblement. This phenomenon is of concern as the noble shift in the corrosion potential may lead to pitting corrosion. We have demonstrated, by growing pure cultures of manganese oxidizing bacteria (MOB) Leptothrix discophora SP-6 under well defined conditions, that microbial deposition of manganese oxides causes ennoblement of 316L stainless steel (SS). Exposing 316L corrosion coupons in lakes and streams supported this conclusion; the rate and extent of ennoblement were positively correlated with the rates of deposition and the amounts of biomineralized manganese oxides deposited on the surfaces of the SS corrosion coupons. X-ray photoelectron spectroscopy (XPS) analyses of the deposits from the ennobled coupons revealed a mixture of manganese oxides, as expected. Many natural waters can support growth of MOB. When manganese-oxidizing biofilms accumulate on surfaces of passive metals there is a potential for manganese redox cycling on the metal surface. This process is initiated by depositing minute amounts of manganese oxides on the metal surface. These microbially deposited manganese oxides are then reduced by the electrons derived from anodic dissolution of the metal; the metal is corroding and the manganese oxides are reduced to divalent manganese ions. However, since the manganese ions are liberated within the manganese-oxidizing biofilm, the manganese ions are immediately reoxidized, and the cycle continues.

Shift of the Iron Corrosion Potential and Acceleration of the Mass Transport of Dissolved Oxygen by the Micro-MHD Effect

Abstract Change in the corrosion rate of iron under a high magnetic field was examined by determining corrosion potential shifts and polarization curve measurements. With increasing magnetic flux density, the electrode potential shifted to the anodic side. This was induced by a cathodic diffusion current increment via a microscopic stirring effect in the local magnetohydrodynamic flow, which arose from interaction between the electrochemical corrosion current and the applied magnetic field.

Corrosion inhibition of carbon steel in the near neutral media by blends of tannin and calcium gluconate

The efficiency of tannin/calcium gluconate blends, as corrosion inhibitors for carbon steel in near neutral water media, heva been studied.Synrgistic behaviour of two components blend of different concentrations has been demonstrated in solutions prepared with distilled and a tap water. The anodic character of the inhibitor was recognised by the shift of corrosion potential to more positive values and a decrease of the passivation current on potentiodynamic polarisation curves. The cathodic inhibition was also recognised by slowing down the cathodic current of the polarisation run.Soluble iron tannate chelate has been observed as a dark blue flux flowing down from the metal surface into solution of tannin in the tap water, resulting in a corrosion process. Tannin decreased the pH of tap water.Calcium gluconate added in tannin solutions increases the pH maintaining it in the near neutral region enabling the precipitation of a low soluble protctive iron tannate and metal gluconate.

Electroactive conducting polymers for corrosion control

This paper reviews the literature describing the effects of conducting polymer coatings on the corrosion rate of ferrous alloys (iron, steel and stainless steel). The literature is interpreted in terms of the proposed mechanisms of corrosion protection: barrier, inhibitor, anodic protection and the mediation of oxygen reduction. The most intriguing aspect of the reported literature are the studies demonstrating corrosion protection when deliberate defects were introduced into the coating to expose the bare metal. These studies show that protection afforded by conducting polymer coatings is not due to simple barrier protection or inhibition alone. Many studies illustrate that the polymer/metal interface is modified to produce passivating oxide layers and that charge transfer reactions occur between the metal and polymer. These studies support the proposed anodic protection mechanism, as do the reports of significant ennoblism. On the other hand, there is considerable variation in the reported shift in corrosion potential and these highlight the influence of substrate preparation, coating composition and mode of application and the nature of the electrolyte on the corrosion protection provided by the conducting polymer. For example, the evidence suggests that the emeraldine base form of polyaniline is superior to the emeraldine salt in terms of corrosion protection for steel. However, the number of direct comparisons is small and the reasons for the differences are not well understood. Also not well understood are the role of the counterion release and local pH changes on pinhole protection. It is also argued that the conducting polymer reduces the likelihood of large increases in pH at the polymer/metal interface and so stabilizes the coating against cathodic disbondment. Further work is clearly needed to increase the protection period by further studies on the corrosion protection mechanism so that the polymer composition and processing methods may be optimized.

Zinc corrosion in simulated acid rain

Corrosion behavior of zinc in a simulated acid rain has been investigated by electrochemical impedance spectroscopy, atomic emission plasma spectroscopy and X-ray photoelectron spectroscopy. It was found that corrosion rate increases drastically in the solution of lower pH near 3. Corrosion potential shifts to less noble value with increasing pH at the lower pH 2.5 and shifts to more noble value at the higher pH 2.9. An oxide film of about 1 μm thick is revealed on zinc surface at pH 3.6, and no film is detected at pH 2.1. Electrochemical impedance diagram has an inductive loop for the solution of higher pH 3.2, while only depressed capacitive-loops are observed at pH below 2.9. The magnitude of the impedance diagram rises with the increasing period of immersion at higher pH 3.2, and decreases at lower pH 2.3. The phenomena observed are discussed in terms of Zn 2+ diffusion rate and change in the oxide film thickness.

Corrosion behavior of polypyrrole coated mild steel

The effect of surface treatments such as polishing and immersion in acid on the corrosion behavior of polypyrrole coated mild steel was investigated. Polypyrrole films were polymerized electrochemically from the aqueous solution of pyrrole and sodium p-toluenesulphonate at a constant current density of 2 mA/cm 2. Corrosion behavior of mild steel covered by these films was investigated by anodic potentiodynamic polarization technique. Remarkable shift of corrosion potential to the positive direction and strong decrease of reduction oxidation current were observed in all cases.

Deformation and fracture of tungsten heavy alloy: R.Grill et al. (Plansee AG, Reutte, Austria.) Int. J. Refractory Metals Hard Mater., vol 15, no 5/6, 1997, 301–309

The influence of carbon and tungsten additions to cobalt on the electrochemical properties has been studied. Carbon additions up to 0.15 wt% and tungsten additions up to 8 wt%, covering a range of compositions equal to that expected in the binder composition of WC–Co hardmetals, have been investigated using potentiodynamic and potentiostatic polarisation techniques in 1 N sulphuric acid. Pure cobalt does not passivate and remains active with increasing potential. A film forms on Co(W,C) alloys causing the current to become relatively independent of potential but remaining very high. This behaviour is termed here `pseudo-passivation'. The mechanism of film growth is determined. With increasing carbon and tungsten additions, the corrosion current density and critical current density are reduced. The corrosion potential shifts to more positive values with increasing additions. It is shown that carbon and tungsten additions influence the corrosion behaviour by a stabilisation of the fcc phase. Empirical equations are derived which relate quantitatively the volume fraction of the fcc phase to the composition of the alloy, i.e. the carbon and tungsten content in the binary and ternary alloys.

Grain growth phenomena in tungsten wire: J.Almanstotter, M.Ruhle. (Max-Planck Inst., Stuttgart, Germany.) Int. J. Refractory Metals Hard Mater., vol 15, no 5/6, 1997, 295–300

The influence of carbon and tungsten additions to cobalt on the electrochemical properties has been studied. Carbon additions up to 0.15 wt% and tungsten additions up to 8 wt%, covering a range of compositions equal to that expected in the binder composition of WC–Co hardmetals, have been investigated using potentiodynamic and potentiostatic polarisation techniques in 1 N sulphuric acid. Pure cobalt does not passivate and remains active with increasing potential. A film forms on Co(W,C) alloys causing the current to become relatively independent of potential but remaining very high. This behaviour is termed here `pseudo-passivation'. The mechanism of film growth is determined. With increasing carbon and tungsten additions, the corrosion current density and critical current density are reduced. The corrosion potential shifts to more positive values with increasing additions. It is shown that carbon and tungsten additions influence the corrosion behaviour by a stabilisation of the fcc phase. Empirical equations are derived which relate quantitatively the volume fraction of the fcc phase to the composition of the alloy, i.e. the carbon and tungsten content in the binary and ternary alloys.

Electrochemical polarisation and corrosion behaviour of cobalt and Co(W,C) alloys in 1 N sulphuric acid

The influence of carbon and tungsten additions to cobalt on the electrochemical properties has been studied. Carbon additions up to 0.15 wt% and tungsten additions up to 8 wt%, covering a range of compositions equal to that expected in the binder composition of WC–Co hardmetals, have been investigated using potentiodynamic and potentiostatic polarisation techniques in 1 N sulphuric acid. Pure cobalt does not passivate and remains active with increasing potential. A film forms on Co(W,C) alloys causing the current to become relatively independent of potential but remaining very high. This behaviour is termed here `pseudo-passivation'. The mechanism of film growth is determined. With increasing carbon and tungsten additions, the corrosion current density and critical current density are reduced. The corrosion potential shifts to more positive values with increasing additions. It is shown that carbon and tungsten additions influence the corrosion behaviour by a stabilisation of the fcc phase. Empirical equations are derived which relate quantitatively the volume fraction of the fcc phase to the composition of the alloy, i.e. the carbon and tungsten content in the binary and ternary alloys.

Polyaniline/epoxy coatings with good anti-corrosion properties

We have studied polyaniline/epoxy blend coatings on mild steel in 0.6 M NaCl and 0.1 M HCl aqueous solutions with electrochemical methods. The corrosion protective performance was characterized by a permanent shift of corrosion potential by about 500 mV to the noble direction and by a decrease of five orders of magnitude in the redox current. The surface of mild steel was covered by a light gray protective layer due to the electrochemical interaction between the coating and steel. The chemical composition of the surface layer has been studied with X-ray photoelectron spectroscopy (XPS).

Influence of alloying elements on electrochemical behaviour of ternary Al-Zn-Ga alloys for sacrificial anodes

Abstract This work underlines the influence of alloying elements zinc and gallium on the electrochemical behaviour of aluminium alloys used as sacrificial anodes. Both the single and joint effects of these two elements on the free corrosion potential, the corrosion current density, and the development of the passive region were studied. Coupled potential and efficiency measurements were also carried out. It has been shown that the influence of gallium on the extent of the negative shift in corrosion potential of aluminium is greater than that of zinc. As regards the joint influence of the two elements, zinc was found to reduce the extent of the gallium induced negative shift in potential. Finally, for any composition the efficiencies were low, possibly because of partial segregation in these alloys.

化学工場における冷却水環境でのNi基合金の微生物腐食事例

There are cases of corrosion which are supposed to have been influenced by microbiological activities. It is not easy work to make the involvement of bacteria clear because of difficulties in reproducing the environmental conditions of the corrosion in question in laboratory tests. A case study introduced in this paper is a localized corrosion from cooling water environment on Ni-base alloy used in plate-type heat exchanger in chemical plants. It was made clear from observation of corrosion samples and through reproduction corrosion test in laboratory that some bacteria have contributed in causing the corrosion. With a series of laboratory tests, sterilization treatment of water was found quite effective to prevent bacteria-influenced corrosion and continuous application of biocide demonstrated a remarkable efficiency in preventing corrosion. The study results have already been adopted in actual plants and the corrosion in plate-type heat exchanger with Ni-base alloy have been largely prevented. No measurable shift of corrosion potential to noble side was observed by corrosion test. It is thought from the observation and examination above that the local environment in the plate had been severed by the influence of microbaiological activities.

On electrochemical techniques for interface inhibitor research

The electrochemical behavior related to different modes of inhibition effect is discussed for interface inhibitors. The action coefficients of an inhibitor on the anodic and cathodic reactions ( f a and f c , respectively) of a corrosion process are defined. An equation is deduced which denotes that if the inhibition effect is caused by geometrically blocking the surface of metal electrode by adsorbed inhibitive species, the corrosion potential in the inhibitor-containing solution will be nearly equal to that in the solution without the inhibitor. In this case the inhibition efficiency (η) equals the coverage (Θ) of the adsorbed species and can be estimated by polarization resistance ( R p ) measurements. In addition, the plot of η vs the “relative coverage” (μ) estimated from interfacial capacitance measurements will be a straight line through the original point of the coordinate. If the inhibition is not due to the geometric blocking effect, which can be judged by the noticeable shift of corrosion potential as the inhibitor is added into the solution, then η cannot be estimated by R p and does not equal Θ. Instead, transfer resistance ( R t ) can always be employed to estimate inhibition efficiency whatever the mode of the inhibition effect. It is argued that theoretically no Tafelian straight lines can be measured in solutions with interface inhibitors unless Θ is independent of the electrode potential E or f a and f c of the inhibitor change linearly with E. Equations of faradaic admittance at the corrosion potential in inhibitor-containing solution for different modes of inhibition effect are deduced. In addition to the estimation of R t , R p and interfacial capacitance, information on the adsorption of the inhibitive species can also be gathered by EIS measurement. In the case of the geometric blocking effect, the EIS display in the inhibitor-containing solution will be similar to that in the black solution when η is low, while a single capacitive loop will display when η is high. If the inhibition is not due to the geometric blocking effect, two time constants will be involved in the EIS display measured in the inhibitive-containing solution no matter what the displays of EIS in the blank solution are, and in this case the effect of E on Θ can be predicted based on the EIS and polarization curve measurements.

The inhibition action of 1(benzyl) 1-H-4,5-dibenzoyl-1, 2,3-triazole on mild steel in hydrochloric acid media

The recently synthesized compound 1(benzyl)1-H-4,5- dibenzoyl-1,2,3-triazole (BDBT) possesses a considerable number of aromatic systems. A significant number of electrons are available on the three nitrogen atoms of the triazole ring. The inhibition effect of BDBT on the corrosion activity of mild steel in acid media has been investigated. Tafel polarization technique, AC impedance measurements and continuous linear polarization resistance method were employed in conjunction with a rotating cylinder electrode apparatus. The corrosion rate of mild steel in 1 % HCl was reduced by more than 95% in the presence of 50 ppm of BDBT. The BDBT has a mixed inhibition effect with a significant shift in the free corrosion potential to the cathodic direction. Film persistency tests showed that BDBT forms a stable film on the surface of the electrode.

Study of effect of pollutant additions on corrosion behaviour of Al-brass in sea water

AbstractThe corrosion behaviour of Al-brass was studied in stagnant and flowing sea water in the absence and presence of 5 ppm Mn and 4 ppm Cl. The addition of Mn or Cl to stagnant sea water had little effect on the free corrosion potential. In flowing sea water, Mn caused a shift in the corrosion potential to more noble values at a flowrate of 0·1 m s−1 and to more active values at 2·2 m S−1. Addition of Cl caused an electropositive shift at both flow velocities. Under all testing conditions, the linear polarisation resistance decreased in the presence of Mn and increased in the presence of Cl. Potentiodynamic polarisation tests indicated that the presence of either pollutant in stagnant sea water causes cathodic depolarisation, electropositive shift in corrosion potential, and elimination of active-passive transition. Microscopic examination revealed that the film formed in the presence of Mn was thicker and more porous than that formed in clean sea water. In the presence of Cl, the film was smooth and ...

Oxidation behaviour of zirconium in molten (Na,K)NO3eutectic containing halide ions

AbstractThe oxidation behaviour of zirconium in molten (Na,K)N03 eutectic containing various halide ions at 300°C was studied under open circuit and polarisation conditions. Under open circuit conditions, the metal is passivated in the presence of Cl-, Br-, and I- ions and the corrosion potential shifts to more positive values with time. In the presence of F- ions active corrosion occurs. In all cases the steady corrosion potential was found to decrease linearly with the logarithm of the halide ion concentration. The results in the presence of Cl-, Br- , and I- ions were explained in terms of the dependence of the Zr/ZrO2 electrode on the oxide ion concentration which depends on the halide ion concentration. In the presence of F- ions, a metal/metal ion electrode, is proposed. Under anodic polarisation, Cl- ions have no significant influence on the passivation behaviour of zirconium while I- and F- ions inhibit oxide formation. Br- ions showed an intermediate effect between Cl- and I- ions. The main anodi...

2-aminobenzimidazole as corrosion inhibitor for 70-30 brass in ammonia

AbstractCorrosion of 70-30 brass in 1·0M ammonia solution and its inhibition by 2-aminobenzimidazole (2-ABZ) has been studied from mass loss and galvanostatic polarisation measurements. The effects of inhibitor concentration (10−6 to 10−3 M) and of temperature (293 to 313 K) on inhibition efficiency were investigated. At a given temperature with increasing inhibitor concentration the polarisation curves shift to higher potentials, corrosion potentials shift slightly to the less noble direction, and the Tafel slopes remain virtually constant. The energy of activation is lowered in the presence of the inhibitor. The results indicate that 2-ABZ acts as a mixed inhibitor, does not affect the mechanism of the electrode processes, and inhibits corrosion by blocking the reaction sites. In the presence of 2-ABZ a protective film is formed on the surface of brass. From ir spectral studies the film has been shown to consist of complexes of Cu(II) and Zn(II) with 2-ABZ acting as a bidentate ligand coordinated to the...

The passivation of electrodeposited chromium due to electrochemical pretreatment

Electrodeposited chromium was pretreated by subjecting it to direct current (DC) and alternating voltage (AV). The corrosion behavior of the pretreated chromium samples was examined in 0.5 M H 2SO 4 using standard potentiodynamic polarization techniques. The data indicate that the application of DC modulation results in a dramatic decrease (up to 93%) in the corrosion current, while the corrosion potential shifts in a more noble direction for the samples from which the oxide layer was removed prior to the electrochemical treatment. Although alternating voltage (AV) modulation of chromium deposits also results in a significant decrease in the corrosion current (up to 89%) for the samples from which the oxide layer was removed prior to the electrochemical treatment, the DC pretreatment in general appears to be more suitable than the AV modulation. DC pretreatment of the as-deposited chromium from which the oxide layer was not removed does not result in a decrease in corrosion current.

圧縮負荷を受けるＴｉＮ被覆材の皮膜破壊評価法

Evaluation method of film fracture of TiN coatings doposited by CVD and PVD was investigated under compressive load to estimate adhesive strength of the film which is important to apply ceramic coatings to instruments in engineering use. The proposed method consisted of corrosion potential monitoring and acoustic emission (AE) monitoring. The results obtained in this study are summarized as follows:(1) Corrosion potential shift due to film fracture was recognized at higher substrate strain in CVD coatings having higher adhesive strength than in PVD coatings having lower adhesive strength.(2) AE signal due to film fracture was also recognized at higher substrate strain in CVD coatings than in PVD coatings.(3) Critical substrate strain at film fracture initiation estimated by corrosion potential measurement and AE monitoring methods correlated quantitatively with adhesive strength of CVD and PVD coatings.