Crevice corrosion initiation and propagation on Alloy-22 under galvanically-coupled and galvanostatic conditions

The initiation and propagation of crevice corrosion on the Ni–Cr–Mo Alloy-22 has been studied in concentrated chloride solutions under galvanically-coupled and galvanostatic conditions. Under galvanically-coupled natural corrosion conditions crevice corrosion initiated but propagation was limited by repassivation. This was attributed to the slow kinetics of oxygen reduction on passive surfaces external to the crevice. Under galvanostatic conditions a potential more positive than 200 mV_Ag/AgCl and an applied current greater than 5 μA were required to stabilize propagation. A minimum critical current density to establish active sites within the crevice was estimated to be around 250 μA cm⁻².     

Elektrochemische Methode zur Prüfung des Korrosionsverhaltens austenitischer CrNiMo-Stähle im Schweißnahtbereich

Electrochemical method for testing of the corrosion behaviour of austenitic CrNiMo-stainless steel weldments Austenitic CrNiMo stainless steel welds can be more susceptible to pitting and crevice corrosion than the base material owing to segregation and precipitation in the heat affected zone, in the high temperature zone and in the welded zone. Suitable test methods are needed to optimize welding technology. Comparison of potential curves (DIN 50919), comparison of critical pitting corrosion temperatures in FeCl₃- or other model solutions, visual estimation after longterm corrosion tests with or without electrochemical load are discussed. A small measurement cell heated from the rear with circulating and temperature controlled electrolyte is shown. With its help separate areas of a weld can be electrochemically investigated. Welds of the materials X 2 CrNiMoN 17 12 2 (1.4406), X 2 CrNiMoN 17 13 5 (1.4439) and X 1 NiCrMoCu 31 27 4 (1.4563) are tested in acidic NaCl solutions in the range of 25 to 75°C. Critical pitting corrosion potentials are obtained for base materials, heat affected zone and weld material. An influence of the welding energy is probable, but cannot be proved in this case without statistical certainty.     

Direct electrosynthesis of polyaniline–montmorrilonite nanocomposite coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline–montmorrilonite (MMT) nanocomposite coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) by using the galvanostatic polarization method. The synthesized coatings were characterized by UV–Vis absorption spectrometry, Fourier transform infrared spectroscopy, X-ray diffraction patterns and scanning electron microscopy. The corrosion protection effect of the coatings was demonstrated by performing a series of electrochemical experiments of potentiodynamic and impedance measurements on Al in 3.5 wt% aqueous NaCl electrolytes. The corrosion current (i_corr) values decreased from 6.55 μA cm⁻² for uncoated Al to 0.102 μA cm⁻² for nanocomposite-coated Al under optimal conditions.     

The localized corrosion of Al 6XXX alloys

This paper presents some recent findings on localized corrosion with a focus on Al 6XXX alloys and on the current times resistance (IR) voltage that exists between the separated anodic and cat hodic reactions. Some newly developed concepts help quantify the relationship between the IR voltage and the polarization curve that exists on the crevice wall, thereby providing for a deeper understanding on how crevice corrosion occurs in many metals and alloys. A signature of the IR voltage form of crevice corrosion in metal/electrolyte systems is the existence of an active/passive transition in their polarization curves. An aluminum/electrolyte system has been found to undergo this form of crevice corrosion. Some new results on intergranular corrosion in Al 6XXX alloys are also presented. For more information, contact B.A. Shaw, The Pennsylvania State University, Department of Engineering Science and Mechanics, University Park, Pennsylvania 16802; (814) 865-7828; fax (814) 865-0122; e-mail bas13.psu.edu.     

Corrosion behavior of Ti‐xNb‐13Zr alloys in Ringer's solution

Ti‐6Al‐4V alloy has been widely used in restorative surgery due to its high corrosion resistance and biocompatibility. Nevertheless, some studies showed that V and Al release in the organism might induce cytotoxic effects and neurological disorders, which led to the development of V‐free alloys and both V‐ and Al‐free alloys containing Nb, Zr, Ta, or Mo. Among these alloys, Ti‐13Nb‐13Zr alloy is promising due to its better biomechanical compatibility than Ti‐6Al‐4V. In this work, the corrosion behavior of Ti, Ti‐6Al‐4V, and Ti‐xNb‐13Zr alloys (x = 5, 13, and 20) was evaluated in Ringer's solution (pH 7.5) at 37 °C through open‐circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Spontaneous passivity was observed for all materials in this medium. Low corrosion current densities (in the order of 10^?7 A/cm²) and high impedance values (in the order of 10⁵ Ωcm² at low frequencies) indicated their high corrosion resistance. EIS results showed that the passivating films were constituted of an outer porous layer (very low resistance) and an inner compact layer (high resistance), the latter providing the corrosion resistance of the materials. There was evidence that the Ti‐xNb‐13Zr alloys were more corrosion resistant than both Ti and Ti‐6Al‐4V in Ringer's solution.     

Corrosion performance and tribological behavior of diamond-like carbon based coating applied on Ni−Al−bronze alloy

The effect of diamond-like carbon (DLC) coating (fabricated by cathodic arc deposition) on mechanical properties, tribological behavior and corrosion performance of the Ni?Al?bronze (NAB) alloy was investigated. Nano-hardness and pin-on-plate test showed that DLC coating had a greater hardness compared with NAB alloy. Besides, the decrease in friction coefficient from 0.2 for NAB substrate to 0.13 for the DLC-coated sample was observed. Potentiodynamic polarization and EIS results showed that the corrosion current density decreased from 2.5 μA/cm² for bare NAB alloy to 0.14 μA/cm² for DLC-coated sample in 3.5 wt.% NaCl solution. Moreover, the charge transfer resistance at the substrate–electrolyte interface increased from 3.3 kΩ·cm² for NAB alloy to 120.8 kΩ·cm² for DLC-coated alloy, which indicated an increase in corrosion resistance due to the DLC coating.     

提高KF-AlF3-Al2O3熔体中Cu-Al基阳极的耐腐蚀性

采用恒电流和动电位极化技术研究经预氧化的和未经预氧化的Cu-Al基阳极(Cu-10Al和Cu-9.8Al-2Mn)在KF-AlF₃-Al₂O₃熔体中的阳极行为。将合金在700℃下进行短时间(8 h)氧化,然后在800℃下施以0.4 A/cm²的电流密度进行1 h恒电流极化。测定扫描速率为0.01 V/s的动电位曲线。对在阳极表面冻结的熔体样品进行XRD分析,并在实验后对阳极进行SEM分析,以研究合金表面形成的氧化皮的物相。所有阳极材料的稳态电位都在2.30~2.50 V(vs Al/AlF₃)范围内。根据动电位极化获得的数据计算阳极的腐蚀速率。结果表明,经预氧化的阳极比未经预氧化的阳极具有更低的腐蚀速率。     

Transformation between localized and general corrosion behavior of die‐cast AZ91D magnesium alloy in 3.5 wt% NaCl aqueous solutions

Transformation behavior between localized and general corrosion of die‐cast AZ91D magnesium alloy in anodizing processes was investigated by galvanostatic polarization. The results revealed that, at electric current density 5, 10, and 20 mA/cm², a localized corrosion state appears on the specimen surface in the initial stage (10 min), and is changed into general corrosion with the galvanostatic measurement time. On the other hand, the increase in electric current density has caused the increase of the corroded area on the specimen surfaces at the same galvanostatic measurement time. In comparison, the appearance of localized corrosion based on general corrosion can be attributed to the localized absorption along with enhanced activity and reactivity of Cl^? at the higher electric current density (10 and 20 mA/cm²). Meanwhile, the penetration of Cl^? throughout the corrosion products has caused the destruction of the integrity of these corrosion products, which promotes the continuous growth of the localized corrosion pits under the galvanic effect.     

Corrosion behaviour of carbon steel coated with Zr‐based metallic glass

Zr₅₅Cu₃₀Al₁₀Ni₅ glassy alloy samples were prepared by the single roller melt spinning technique in an argon atmosphere. Amorphous ribbons were coated on carbon steel by the electron beam technique. Subsequently, the coated region was remelted to study its effect on the corrosion performance. The microstructure of the ribbons and of coated steel samples was characterized by X‐ray diffraction (XRD) and optical microscopy (OM). Potentiodynamic polarization studies were carried out in 0.1 M Na₂SO₄ electrolytes at pH = 8 and pH = 2. The coated samples showed a significant increase in corrosion resistance compared to the base carbon steel in both sulphate electrolytes. The pitting corrosion behaviour of the coated samples has been examined in 0.01 M NaCl electrolyte and was found to be poor. Furthermore, the galvanic and crevice corrosion behaviour of these alloys has been evaluated in the chloride electrolyte.     

Herstellung und Eigenschaften stickstofflegierter,korrosionsbeständiger Stähle und Sonderstähle mit niedrigen Kohlenstoffgehalten

Production and properties of nitrogen alloyed, corrosion resistant steels and special steels with low carbon contents Alloying with nitrogen has favourable influence in particular on the mechanical properties of CrNiMo steels (X 2 CrNiMoN 17 12, materials No. 1.4406, X 2 CrNiMoN 17 13 5, materials No. 1.4439 und X 2 CrNiMoN 22 5, materials No. W.-Nr. 1.4462). This comes to bear when ambient temperature and low temperature strength and toughness are concerned. With respect to the corrosion behaviour the data concerning the effect of nitrogen are contradictory. It has become clear that nitrogen improves pitting corrosion resistance; this applies, however, only to pit initiation but not to pit growth. Stress corrosion cracking is not delayed by nitrogen but different results have been obtained with different media: while the duplex steel X 2 CrNiMoN 22 5 is attacked considerably faster than the corresponding nitrogen-free steel in 42% boiling magnesium chloride solution the time-to-failure of both steels are comparable in 30% boiling MgCl₂-solution. The nitrogen alloyed steels can be welded by all known welding procedures, provided fully austenitic welding rods are used.     

Influence of histidine as an electrolyte additive on the electrochemical performance of Al electrodes in 4.0 M KOH electrolyte

In this study, the influence of histidine as an electrolyte additive on the electrochemical performance of Al electrodes in 4.0 M KOH electrolyte is studied by measuring open-circuit potential–time curves, potentiodynamic polarization, potentiostatic oxidation, and electrochemical impedance spectroscopy. The surface morphology of Al electrodes is characterized by scanning electron microscopy. The results show that the battery performance is significantly improved by adding histidine. Samples with 50 mM histidine exhibit the highest corrosion resistance and discharging current density among all the specimens, with the corrosion current density (I_corr) being 7.471 mA/cm² and corrosion inhibition efficiency (η) being 43.33%.     

Corrosion resistant coatings based on synergistic effects of alkaline etching and molybdate treatment for AZ31D magnesium alloy

Eco-friendly, non-toxic protective coatings deposited from molybdate–based solutions have been developed as undercoat thin-films (for subsequent organic top coats) for AZ31D magnesium alloy. Direct treatment of Mg AZ31D substrates with molybdate–based solutions has no significant effect on the overall surface resistance (charge transfer resistance). Alkaline etching of Mg AZ31D surfaces using KOH solution prior to molybdate conversion coating showed significant enhancement in the corrosion resistances The optimum conditions of alkaline etching and molybdate treatment steps have been determined. The total surface resistance was improved from 2.1?×?10³ Ω.cm² (for as-polished AZ31D) to be 3.2?×?10³ Ω.cm² for the alkaline etched samples followed by 10?g?L^?1 molybdate treatment. The resistance to localised corrosion (pitting and crevice) improved significantly after applying the alkaline etching step. Molybdate–based coatings formed on Mg AZ31D exhibited a network of flower-like and needle-like protective molybdenum oxide structures which are belived to be responsible for the improvement in the pitting and crevice corrosion resistance. They impede the corrosive media from reaching the bare metal and hence improve the pitting and crevice corrosion resistances. This simple eco-friendly, low-toxicity pretreatment approach seems very promising and effective for improving the corrosion resistance of magnesium alloys in chloride containing solutions.     

Influence of potassium pyrophosphate in electrolyte on coated layer of AZ91 Mg alloy formed by plasma electrolytic oxidation

The effect of potassium pyrophosphate in the electrolyte on plasma electrolytic oxidation (PEO) process for AZ91 Mg alloy was investigated. The morphologies and chemical compositions of the coating layer on the AZ91 Mg alloy were evaluated and corrosion resistance was also estimated by potentiodynamic polarization analysis. The coating layer on AZ91 Mg alloy coated from the Bath 2 containing 0.03 mol/L of potassium pyrophosphate for 360 s exhibited considerably dense structure and contained 11%–18% (mass fraction) of phosphorous. The higher content of phosphorous of coating layer coated from Bath 2 could be detected at the bottom of oxide layer, which strongly implied that the phosphorous ion might be concentrated at the barrier layer. Corrosion potential of coating layer of AZ91 Mg alloy increased and corrosion current density decreased with increasing the concentration of potassium pyrophosphate. The polarization resistance (R_p) of coating layer of AZ91 Mg alloy coated from Bath 2 was 4.65×10⁷ Ω/cm², which was higher than that (R_p=3.56×10⁴ Ω/cm²) of the sample coated from electrolyte without potassium pyrophosphate. The coating layer coated from Bath 2 containing 0.03 mol/L potassium pyrophosphate exhibited the best corrosion resistance.     

Korrosion nichtrostender austenitischer Stähle in (kondensierender) chloridhaltiger Salpetersäure

Corrosion of austenitic stainless steels in (condensing) nitric acid containing chlorides The corrosion parameters involved in the behaviour of austenitic stainless steels and nickelbase alloys in chloride containing nitric acid are being stated. Investigation of the corrosion resistance of the austenitic stainless steel 1.4306, ESR grade for application in nitric acid, in nitric acid condensates containing small amounts of chlorides. The condensates were formed from boiling nitric acids of molarities 6 to 10 containing 15 to 45 mg chlorides/1. The calculated corrosion rates of < 1 μm/y can be considered rather small. Especially in cases of insufficient wetting of the heat exchanger surfaces, however, local roughening and pitting corrosion is to be expected, mostly under rust-colored, chloride-containing deposits. The intensity of pitting corrosion increases with increasing chloride contents of the nitric acid solutions that evaporate. The sealing surfaces consisting of steel 1.4306 and PTFE did not exhibit any crevice corrosion. Nitric acid grade ESR-1.4306 definitely does not suffice requirements as described in this paper. It is suggested to use stainless steels which exhibit an improved resistance against chlorides and, at the same time, a high resistance against nitric acid attack. The following steel grades may be considered: X 1 CrNi 25 21 (Mat. No. 1.4335), X 2 CrNiMoN 25 22 2 (Mat. No. 1.4466) and/or X 1 NiCrMoCuN 31 27 4 (Mat. No. 1.4563).     

An electrochemical study on the effect of pitting inhibition in weakly alkaline solution by copper addition in pure iron

The effects of copper on the corrosion rafe of steel have been investigated in order to propose the application of scraps as material of reinforcing steels in concrete structures. The corrosion behaviors of pure iron and several Fe−Cu alloys were examined by conducting potentio-dynamic and galvanostatic polarization in pH 10, 12.5 and 13.5 Ca(OH)₂ solutions added to several chloride concentrations. Values of the pitting potential (E _p) of pure iron were plotted at negative potential. In contrast to pure iron, theE _p values of Fe−Cu alloys showed much more positive values. The polarization curves in weakly alkaline solutions containing chloride ions indicated that the addition of copper up to 3 mass% significantly improved the corrosion resistance of steel.     

Die Adsorption von Chlorid an einem austenitischen Cr-Ni-Stahl bei hohen Temperaturen

The adsorption of chloride on austenitic Cr-Ni-steel at high temperatures Using the radioactive isotope chlorine-36 and nuclear physical measuring methods (gas-flow radiation detector, autoradiography) chloride adsorbed on metal surfaces can be detected with a detection limit of 10^?10 g cm^?2 and a local resolution of at least 5 μm. This method was developed for investigating the chloride adsorption on the protective layers on an austenitic Cr-Ni-Steel (X 8 CrNiTi 18.10) under high temperature conditions at 250°C. Three different kinds of chloride adsorption could be distinguished: homogeneous chloride distribution across the whole surface, high localized chloride concentrations in places of local corrosion attack and local deposition of chloride by heat and mass transfer. The results obtained enabled attack concepts for the mechanism of action of the chloride were derived. Potential applications of this method to the rapid detection of chlorideinduced corrosion processes (pitting corrosion, stress corrosion cracking or crevice corrosion) or to the quantitative estimation of thermohydraulic concentration processes are mentioned.     

Entwicklung,Verarbeitung und Einsatz des stickstofflegierten,hochmolybdänhaltigen Stahles X 3 CrNiMoN 17 13 5

Development, processing and use of nitrogen alloyed high molybdenum steel X3CrNiMoN 17 13 5 Addition of 0,15% Ni to the steel X5-CrNiMo 17 13 delays the precipitation of intermetallic phases and chromium carbides. The nitrogen containing steel can be welded up to a plate thickness of 30 mm without precipitation; hot cracking has not been observed either. The weld metal corresponds to the base material with respect to ductility and corrosion resistance. It is resistant to pitting, crevice corrosion and stress corrosion cracking, in particular in mixed acids. Successful applications include textile industry (bleaching equipment, heating equipment), shipbuilding (driving shafts, exhaust gas coolers, waste gas condenser tubing), water desalination equipment, nuclear industry (evaporator for radioactive waste water, processing of radioactive waste in fluoride containing nitric acid), chemical industry (pressure container material, heat exchangers for brackish water).     

Evaluation of nanometer-sized zirconium oxide incorporated Al—Mg—Ga—Sn alloy as anode for alkaline aluminum batteries

Zirconium oxide nanoparticles with 0.4 wt.% and 0.8 wt.% are incorporated into the Al—0.65Mg—0.05Ga— 0.15Sn (wt.%) alloy anode (base alloy) in order to improve the performance of the resulting anodes. Electrochemical characterization of the reinforced alloys was done by potentiodynamic polarization, electrochemical impedance spectroscopy and galvanostatic discharge and corrosion behavior was evaluated using self-corrosion rate and hydrogen evolution in 4 mol/L KOH solution. The surface morphology of the alloys was also studied using field emission scanning electron microscope (FESEM). The obtained results indicate that the base alloy shows high corrosion rate in 4 mol/L KOH solution by releasing 0.47 mL/(min·cm²) hydrogen gas, whereas the alloy containing 0.8 wt.% ZrO₂ provides the lowest hydrogen evolution rate by releasing 0.32 mL/(min·cm²) hydrogen gas. Furthermore, by increasing zirconium oxide nanoparticles, the corrosion current density of the aluminum anodes is decreased and their corrosion resistance increases significantly compared to the base alloy in alkaline solution. In addition, nanometer-sized zirconium oxide incorporated anodes exhibit the improved galvanic discharge efficiencies, so that 0.8 wt.% nano-zirconium oxide incorporated base alloy displays the highest power density and anodic utilization compared with the others in 4 mol/L KOH solution.     

Analysis of the galvanostatic polarization method for determining reliable pitting potentials on stainless steels in crevice-free conditions

In this paper the potential of the galvanostatic polarization technique as accelerated method for determining the characteristic pit potentials on stainless steels in crevice-free conditions is examined. Measurement of the potential change as a function of time shows a maximum that agrees with the nucleation pit potential. Thereafter, a stationary potential is reached corresponding to the protection potential against pit. Possible limitations of this kind of measurements have been remedied by refinements in the test procedure and conditions. The state of the surface oxide film and the applied anodic current are two basic parameters that must be well defined because they govern the pitting susceptibility. It has been found that with applied anodic currents in the range 40–200 μA/cm² and with prior electrode exposures to solution between 30 and 60 min it is possible to obtain results in excellent agreement with the conventional potentiodynamic tests with the advantage of smaller data scattering and absence of crevice at electrode/holder interfaces. These effects are the result of the rapid pitting stimulated in the galvanostatic method. This implies a short duration of the experiment thus also favouring the elimination of the time-dependent crevice, which notoriously contributes to the poor reproducibility of pit potentiodynamic potentials. A detailed series of experiments have been conducted on several stainless steels and in different test conditions to validate the accuracy of the galvanostatic polarization method.     

Auswirkungen des elektrochemischen Probenpotentials auf das SwRK-Verhalten des Duplexstahles X 2 CrNiMoN 22 5 3 in NaCl-Lösung

Effect of electrochemical potential on the corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 in NaCl-solution The influence of the electrochemical potential on the corrosion fatigue behaviour of the nitrogen-alloyed ferritic-austenitic steel X 2 CrNiMoN 22 5 3 (german material-number 1.4462) is investigated in 3% NaCl-solution with pH = 7 at room temperature. Smooth and notched (α_k = 2,0) specimens are tested under cyclic tension load (σ_μ/σ₀ = 0) with a frequency of 25 Hz up to 10⁷ cycles. The fatigue limits for both specimen geometries in corrosive environment at free corrosion potential reach only about 78% of the according values in air. In potentiostatic controlled corrosion fatigue tests the fatigue life respectively the fatigue limit is higher than under free corrosion. The chosen cathodic (?300 mV_SHE) and anodic (+ 700 mV_SHE) protection potentials cause important improvement to 500 N/mm² or 510 N/mm² for smooth specimens (fatigue limit in air: 490 N/mm²). For notched specimens there is an improvement to values of 98% respectively 91% of the air fatigue limit. The recordings of the polarisation current show a sudden increase of the current at the fatigue crack initiation and a further increase during crack propagation. ESCA-analytical results of the passive films for different potentials verify the better corrosion fatigue behaviour for the tested potentials.