Localized Corrosion Mechanism Research Articles

Mechanisms of Localized Corrosion Inhibition of AA2024 by Cerium Molybdate Nanowires

The cerium molybdate nanowires were recently reported as an efficient inhibiting pigment for aluminum alloys. In the present work, the inhibition mechanism of localized corrosion of S-phase intermetallics in AA2024 was studied in detail using a complementary combination of localized and analytical techniques. A significant suppression of dealloying of S-phase was demonstrated in the presence of cerium molybdate nanowires. Microscopic observations clearly show the formation of a conversion layer on the entire alloy surface after immersion in nanowire-containing solutions. A noticeable Volta potential difference (VPD) increase up to around −0.25 V vs Ni reference was measured on alloy after immersion in inhibited solutions. Such VPD changes have been related to the presence of Mo oxides on the alloy surface. Analysis performed by energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) showed that the surface oxide film is mainly composed by Mo(VI) and/or Mo(IV) oxides, and cerium(III...

Failure analysis of one peculiar ‘Yin-Yang’ corrosion morphology on heat exchanger tubes in purified terephthalic acid (PTA) dryer

It is a common knowledge that localized corrosions occur on austenitic stainless steels such as 316L, exposed to halide ions. This paper will discuss such a localized failure that took place not long after beginning of service on the heat exchanger tubes inside a purified terephthalic acid (PTA) dryer in a petrochemical works. Particularly, one peculiar corrosion morphology termed ‘Yin-Yang’ corrosion was observed, i.e. the upside surface of the failed tubes was severely corroded while the downside was intact. Consequently, in order to ascertain the actual causes of this premature failure, samples including the failed tubes and the process media were investigated by a variety of characterization methods. Metallographic structures and chemical compositions of the tube matrix materials were inspected by optical microscope (OM) and photoelectric direct reading spectrometer; both the surfaces and the cross-sections of the corroded areas were microscopically analyzed through scanning electron microscope (SEM) and energy disperse spectroscopy (EDS); and the chemical constituents of the process media were detected via the gas chromatography–mass spectrometric (GC–MS). Finally, the localized corrosion mechanisms were discussed in detail and the pertinent countermeasures were proposed.

SECM and SKPFM Studies of the Local Corrosion Mechanism of Al Alloys – A Pathway to an Integrated SKP‐SECM System

AbstractScanning Kelvin Probe Force Microscopy and Scanning Electrochemical Microscopy were applied for the investigation of localized corrosion on heterogeneous aiming on the investigation of the possible correlation between the local surface potential differences, measured by the Kelvin probe technique in ambient conditions, and corrosion during immersion in a corrosive electrolyte. A model sample mimicking the interaction of Al and Cu in Al alloys was chosen to demonstrate the complementary nature of the information received from SKPFM and SECM. The necessary prerequisites for a future integration of SKP and SECM into a single set‐up are discussed.

Influence of Cerium on the Formation of Micro-Galvanic Corrosion Elements of AZ91

Investigations of AZ91D revealed a strong influence of mainly the following microstructural parameters on corrosion: porosity, the eutectic, the Al-content of the Mg-phase as well as the volume fraction and distribution of gamma-Al12Mg17. Further an improved corrosion resistance was observed by the addition of rare earth elements. The influence of Ce on microstructure and corrosion of sand cast and high pressure die cast AZ91D with 0.5-2.0 wt.% Ce was investigated by SEM before and after potentiodynamic polarization measurements in pH7.5 (bi-distilled water adjusted with KOH). It was observed that Ce significantly improved the corrosion resistance by changing the microstructure. A strong influence of Ce on the average grain size, the fraction of the eutectic, changes in morphology and/or volume fraction of gamma Al12Mg17 and formation of a Ce-rich intermetallic phase was observed. Therefore, not only the chemical composition but also the change of microstructure and the formation of micro-galvanic elements influence the local corrosion mechanism of AZ91D with cerium.

Spontaneous passivation observations during scale formation on mild steel in CO 2 brines

Previous study revealed localized corrosion in CO 2 environments was driven by a galvanic cell established between pit surfaces and scaled surrounding area. In order to underpin the understanding of the galvanic mechanism of localized corrosion, the root cause of potential differences between these two surfaces, passivation of mild steel, in CO 2 environments was investigated using transmission electron microscopy technique and electrochemical techniques including potentiodynamic polarization, cyclic polarization and open circuit potential techniques. Potentiodynamic polarization experiments showed that the passivation of the carbon steel surface favorably occurred at pH > 7 and facilitated with the presence of FeCO 3 scale. Cyclic polarization tests showed that polarization rate had an important influence on passivation behavior. At a slower polarization rate, lower passivation potential and current density were observed. Spontaneous passivation was evidenced by a significant increase of corrosion resistance and an open circuit potential without any externally applied current or potential during electrode immersion. This process is affected by pH, temperature, presence of CO 2 and iron carbonate. Nevertheless, iron carbonate film is not the only one responsible for passivation, as demonstrated from depassivation tests where passivity was lost without losing the existing iron carbonate film. Transmission electron microscopy technique was used to determine the structure of the passive layer. An extra phase, most likely magnetite, was observed to be beneath the iron carbonate scale and at the crystal grain boundaries which passivated the mild steel.

Quantitative Interpretation of Half-Cell Potential Measurements in Concrete Structures

The interpretation of half-cell potential measurements in reinforced concrete structures can be a major challenge for civil engineers. The main reason for this is that half-cell potential mapping provides information to predict the probability of corrosion in concrete, but it does not give clear insight on the rate and the nature of corrosion. Although for general uniform corrosion, half-cell data can provide valuable information about the probability of corrosion. In the case of localized corrosion, the predictions of half-cell measurements can be misleading. The main objective of the present work is to provide practicing engineers a tool that they can use to better interpret the results of half-cell potential measurements. This tool quantitatively relates the potential readings on the surface of the concrete to the rate of probable localized reinforcement corrosion through concrete resistivity, cover thickness, and temperature, allowing the engineers to gain supplementary information from half-cell tests. Experimental comparisons presented in this paper demonstrate the potential benefits of the proposed approach when typical half-cell measurements may not be able to predict the localized corrosion mechanism.

Localized Corrosion Mechanisms of 2024 Aluminum Alloy: Atomic Force Microscopy and Kelvin Force Microscopy Contribution

not Available.

Corrosion Propagation Behavior of Magnesium Alloys under Atmospheric Conditions

To investigate corrosion mechanisms of magnesium alloys under atmospheric conditions, field exposure tests were performed. In a marine environment, magnesium alloys experienced localized corrosion, including pitting and filiform corrosion. To reproduce such actual atmospheric corrosion in a laboratory experiment, we employed a wet-dry cyclic test at a constant dew point. From the results of the wet-dry cyclic tests, the propagation mechanism of localized corrosion on magnesium alloys has been discussed. Localized corrosion occurred during the wetting period, and the corroded sites thus formed were repassivated during the drying period. In the next wetting period, new localized corrosion took place, but the repassivated sites were not reactivated. The repassivated sites have higher corrosion resistance than the non-corroded part of the surface because of the barrier effect of the corrosion products. In the case of AZ91D, Al enrichment in the surface region of the metal matrix is responsible for this corrosion resistance.

Electrochemical noise from oxygen reduction on stainless steel surfaces

Oxygen reduction occurring at the passive layer is probably the most important cathodic reaction involved in corrosion processes on stainless steel. Furthermore, the influence of the surface state on the oxygen reduction reaction is a key point for the understanding of the mechanism of localized corrosion on stainless steel. In this study, electrochemical noise measurements under cathodic polarization were carried out to obtain new information about this influence. It has been confirmed that the surface state of stainless steel plays a very important role in the kinetic of this cathodic reaction. Oxygen reduction kinetics was significantly reduced on passivated surfaces and improved on pre-reduced and ground surfaces. In addition, electrochemical current noise measurements allowed to differentiate between the electrochemical activity produced by the oxygen reduction reaction and that due to the reduction of the passive layer, in direct dependence on the characteristics of the different surface states investigated.

Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al–Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al–Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al–Mg matrix and dendrite precipitates of Mg 2Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg 17Al 12. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg 2Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.

Corrosion of steel under the defected coating studied by localized electrochemical impedance spectroscopy

Corrosion of steel under the defected coating in near-neutral pH solution was investigated by localized electrochemical impedance spectroscopy (LEIS) measurements. The LEIS response is dependent on the size of the defect. For small defects, e.g., less than 200 μm in diameter, localized corrosion process and mechanism of steel, as indicated by the measured LEIS plots, change with time. The diffusion process dominates the interfacial corrosion reaction, which is due to the block effect of the deposited corrosion product combined with the geometrical factor of a large coating thickness/defect width ratio. In the presence of a big defect, e.g., up to 1000 μm, the LEIS responses measured at the defect are always featured by a coating impedance in the high-frequency range and an interfacial corrosion reaction in the low-frequency range. The block effect of corrosion product does not apply due to the relatively open geometry. Conventional EIS measurements on a macroscopic-coated electrode reflect the “averaged” impedance results from both coating and defect. The information of the localized electrochemical corrosion processes and mechanisms at the small defect is lost, and the coating impedance information is “averaged” out when a big defect is contained. LEIS measurement provides an essential technique to characterize microscopically the local electrochemical corrosion reaction of steel under the defected coating.

Electrochemical behaviour of stainless steels in media containing iron-oxidizing bacteria (IOB) by corrosion process modeling

Localized corrosion mechanism of stainless steel (SS) types UNS S30403 and UNS 31603 in the presence of iron-oxidizing bacteria Sphaerotilus spp. isolated from rust deposits was studied electrochemically. OCP transient, cyclic anodic and cathodic potentiodynamic polarization curves were measured on steel electrodes through their exposure to 3% NaCl solution supplemented with Sphaerotilus culture. The exposure period was composed of three parts: (a) 5 days incubation of steel electrodes in sterile 3% NaCl solution; (b) addition of 3 days-old Sphaerotilus culture to 3% NaCl at 3:2 v/v ratio with subsequent electrodes exposure for 11 days up to complete sedimentation of ferric oxides and (c) subsequent exposure of electrodes for 14 days in upper and bottom (sediments layer) fractions of the experimental medium. The results revealed an instantaneous gradual shift of the transient potential of both steels towards negative potentials from steady-state value of −0.15 V to −0.35 to −0.42 V (SCE) during the whole exposure interval since IOB culture addition into sterile 3% NaCl solution. No evidence of pitting corrosion was found on SS samples subsequent to their exposure to sterile 3% NaCl solution, though in the presence of IOB culture, numerous pits were revealed on 304 L steels specimens exposed to iron hydroxides sediments layer. Electrochemical characteristics (OCP or corrosion potential – E CORR, breakdown potential – E BD, repassivation potential – E RP, passivation current – i PASS) periodically measured by cyclic polarization method, allowed monitoring the electrochemical behavior changes of experimental SS and to establish the initiation of pitting corrosion in the presence of IOB, resulting in crevice effect caused by biogenic ferric oxides deposits precipitated on steel surface. Overall, steel 316L demonstrated higher resistance to pitting corrosion compared to 304L.

Localized corrosion mechanism associated with precipitates containing Mg in Al alloys

To clarify the localized corrosion mechanism associated with precipitates containing Mg in Al alloys, the simulated bulk precipitates of S and β were synthesized through melting and casting. Their electrochemical behaviors and coupling behaviors with α(Al) in NaCl solution were measured. Meanwhile, simulated Al alloys containing S and β particles were prepared and their corrosion morphologies were observed. It's found that there exist two kinds of corrosion mechanisms associated with precipitates containing Mg. The precipitate of β is anodic to the alloy base, resulting in its anodic dissolution and corrosion during the whole corrosion process. While, there exists a corrosion conversion mechanism associated with the S precipitate, which contains active element Mg and noble element Cu simultaneously. At an initial stage, S is anodic to the alloy matrix at its periphery and the corrosion occurs on its surface. However, during its corrosion process, Mg is preferentially dissolved and noble Cu is enriched in the remnants. This makes S become cathodic to α(Al) and leads to anodic dissolution and corrosion on the alloy base at its periphery at a later stage.

Failure of package boilers in a petrochemical plant

AbstractThe paper deals with the failure investigation of furnace‐tubes of a few medium capacity (86 T/H steam generation) package boilers in a petrochemical plant. As a consequence of damage, the boilers required major tube‐repair on urgent basis, and complete tube replacement within a period of two years. Failure of different boilers took place at varying service durations, though all of them had the same design and criteria of operation. The study included metallurgical characterization of the failed components; analysis of operation and process (historical) data; review of inspection and maintenance records; evaluation of design and water‐treatment practices; etc. As a result, water‐side corrosion was attributed to be the cause of tube‐leakage via general and localized corrosion mechanism, while, no metallurgical defects and changes were noticed in the failed components. On the other hand, process and operational abnormalities were diagnosed as the root cause of trouble initiation and propagation. In order to avoid the recurrence of such failures, various preventive measures (inspection, maintenance, operation and process) have been recommended.

Current Density Contour Map of TiNi Shape Memory Alloy in Simulated Physiological Solutions

In order to investigate the corrosion behavior of TiNi shape memory alloy, especially electrochemical behavior of pitting and crevice corrosion in a human body, current density contour(CDC) map of TiNi alloy was constructed by potentiodynamic polarization technique in simulated physiological sodium chloride solutions of pH ranging from 1 to 13 at 37oC. Morphology of pits and corrosion products in sodium chloride solutions of various pH were analyzed by SEM and EDX, and susceptibility and mechanism of localized corrosion were also discussed.

Simulation study on function mechanism of some precipitates in localized corrosion of Al alloys

The function mechanism of different types of aging precipitates in localized corrosion of Al alloys was studied. The function mechanism of the precipitates of θ (Al 2Cu) and η (MgZn 2) is validated. The precipitate of θ containing noble element Cu is cathodic to the alloy base, resulting in the anodic dissolution and corrosion of the alloy base at its adjacent periphery. The precipitate of η containing active element Mg is anodic to the alloy base, anodic dissolution and corrosion occur on its surface. Meanwhile, a localized corrosion mechanism conversion associated with the precipitate of T1 (Al 2CuLi) is advanced, which contains noble element Cu and active element Li simultaneously. The precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface at the beginning. However, during its corrosion process, the preferential dissolution of Li and the enrichment of noble element Cu make its potential move to a positive direction. As a result, the corroded T1 precipitate becomes cathodic to the alloy base at a later stage, leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery.

Study on localized corrosion mechanism of 2195 Al‐Li alloy in 4.0% NaCl solution (pH 6.5) using a three‐electrode coupling system

AbstractLocalized corrosion morphologies of 2195 Al‐Li alloy with various heat treatment in 4.0% NaCl solution (pH 6.5) were investigated, and its corrosion mechanism was studied using a three‐electrode coupling system of α (Al) substituting for the precipitate‐free zone (PFZ), simulated bulk θ′ (Al2Cu) and T1 (Al2CuLi). θ′ acts as cathodic zone in the alloy. At the initial stage, T1 phase is active with respect to θ′ and α (Al), and endures the main anodic current, indicating that anodic dissolution occurs on T1. However, its potential moves to positive direction with immersion time, due to dealloying of Li from T1. As a result, the main anodic dissolution occurs on α (Al) at a later stage. At this stage, as only T1 and α (Al) are coupled, T1 is cathodic to α (Al). In real 2195 alloy, T1 phase is very tiny, and anodic dissolution of T1 and PFZ occurs alternately. These results show that its intergranular corrosion or intersubgranular corrosion is caused by alternate anodic dissolution of T1 phase and PFZ along grain and subgrain boundaries.

Localized corrosion mechanism of 2×××-series Al alloy containing S(Al 2CuMg) and θ′(Al 2Cu) precipitates in 4.0% NaCl solution at pH 6.1

To study localized corrosion mechanism of 2×××-series Al alloys containing θ′ and S precipitates, the electrochemical behaviors of a three-electrode coupling system of α(Al) substituting for alloy base, simulated bulk θ′ and S in 4.0% NaCl solution at pH 6.1 have been investigated. The potential of θ′ is positive to that of α(Al) and S, indicating that θ′ acts as cathodic zone in the alloy. S is main anodic phase, and the anodic dissolution of S will cause its pitting corrosion. However, during the corrosion of S, Mg is preferentially dissolved, resulting in noble Cu-rich remnants, which makes its potential move to positive direction with immersion time. As a result, corroded S becomes cathodic to α(Al), leading to anodic dissolution of α(Al) and pitting corrosion of the alloy base at adjacent periphery of S precipitate.

Pitting Corrosion of Bulk Glass‐Forming Zirconium‐Based Alloys.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pitting corrosion of bulk glass-forming zirconium-based alloys

The polarization behavior of glassy Zr 59Ti 3Cu 20Al 10Ni 8 alloy samples was investigated in 1N H 2SO 4 solution to understand their passivation characteristics in strongly acidic environment. Pitting corrosion tests were performed in acidic chloride medium of 1N H 2SO 4 containing 0.01–0.2N NaCl. Electrochemical investigations were carried out by potentiodynamic and potentiostatic anodic polarization methods at room temperature. The corroded sample surfaces were examined using scanning electron microscopy, and the alloying element distribution across the pits was analyzed using energy dispersive X-ray analysis. The passive films developed at potentiostatic anodic polarization were examined using Auger electron spectroscopy (AES). The results of the present investigation revealed (i) corrosion resistance of Zr-based amorphous alloys is very high in acidic medium and moderate in acidic chloride medium (ii) structural defects and clustered zones of alloying elements deteriorate the corrosion resistance, and (ii) copper plays a significant role in enriching below the passive film and affecting the corrosion behavior. In conclusion, a localized corrosion mechanism is proposed.