Korrosionsuntersuchungen an chemischen Sensoren mittels elektrochemischer Rauschanalyse

Investigation of corrosion on chemical sensors by means of electrochemical noise analysis Electrochemical noise analysis has been used increasingly for investigation of electrochemical systems in recent years. In this paper results of noise measurements on different potentiometric electrochemical sensors are presented. The experimental arrangement consists mainly of a high-resolution multimeter and a series connected differential amplifier with a very high input resistance. Miniaturized pH glass electrodes, pH thick film electrodes, ISFET pH sensors as well as miniaturized reference electrodes have been investigated by means of electrochemical noise measurements. The results confirm that measurement of electrochemical noise can be used as an additional method for testing and characterization of electrochemical sensors.     

Thin sensitive organic membranes on selective iron-ion sensors

The organic supramolecular compound synthesized from pyrochatechol units bound in the -position of a linear dicarboxylic acid (AE6) previously used as a stable chelate iron(III) in solution is applied in this work as an ionophoric reagent on ion-sensitive field-effect transistor (ISFET) and electrolyte—insulator—semiconductor (EIS) structures for iron detection. The surface gate oxide (SiO₂) sensitization of sensors was performed, using three deposition methods: chemical grafting to obtain an AE6 monolayer, deposition by vacuum sublimation of an AE6 thin film and incorporation in a polysiloxane gel membrane. Classical electrochemical measurements were made to test the sensitivity towards iron of the AE6 membranes, in EIS and ISFET structures. A Nernstian sensitivity, a good selectivity and a long lifetime were obtained for the evaporated layer and the gel membranes.     

Elaboration of thin films based on p-tert-butyl-calix [8] arene. Application to the chemical sensors type EIS and ISFET

In this paper we report some results about recognition reagents in an EIS (electrolyte—insulator—semiconductor) and ISFET (ion-sensitive field-effect transistor) type sensors, elaborated with a p-tert-butyl-calix [8] arene molecule. This calixarene was deposited by sublimation onto the surface insulator of the samples. Reflexion—adsorption infrared spectroscopy and X-ray diffraction were performed to characterize the chemical properties and the morphology of the layers. Electrochemical measurements were made to study the sensitivity and the selectivity of this sensitive membrane towards earth alkaline cations and the transition metals. A linear sensitivity was obtained only for the Ca²⁺. These devices exhibit a high chemical stability in liquid media and consequently can be used as sensors.     

Über einige Aspekte der atmosphärischen Korrosion von Stahl

To some aspects of the atmospheric corrosion of steel The corrosion of unalloyed steel (St 38 u 2) in pure moist air and by existence of low molecular saturated monocarbonic acids have been studied with microphotometric method. Moreover the electrochemical behaviour of this steel in adequate aqueous solutions has been characterized. It was found that steel - in distinction to pure iron - corroded already in moist air without pollutants in case the relative humidity is (RH) ≥70%. The action of the volatile monocarbonic acids on the corrosion were dependent on the pH value of water film which had been absorbed on the oxide layer of iron material. Oxygen acted as passivator above of pH_crit and the anions of these carbonic acids stabilized the surface hydroxyl groups on the strength of formation of bridging complexes. Final general conclusions about the mechanism of the atmospheric corrosion of iron materials are deduced.     

Korrosionsmonitoring mit elektrochemischen Sensoren unter Einsatz von Evolutionsstrategien

Corrosion monitoring with electrochemical sensors by using evolution-inspired optimization techniques A complex array of sensors for measurements of corrosion influencing parameters was investigated at the example of pitting corrosion of the alloy X 5 CrNi 18 10. Potentiodynamic measurements of pitting potential were performed in a flow circuit by varying chloride ion concentration (10^?3 … 1 mol 1^?1), pH-value (2 … 8), content of dissolved oxygen (0 … 8 mg 1^?1), temperature (10 … 60°C) and flow velocity (0 … 2 m s^?1). The influence of these parameters on the pitting potential was evaluated by using evolution-inspired optimization technique with an equation derived from literature data. This technique is indicated by stable convergence in the present application and results in an empirical equation facilitating the forecast of pitting potential in the investigated parameter field.     

Einfluß von Kupfer auf das Korrosionsverhalten verzinkter Rohre in warmen Brauchwässern

Influence of copper on the corrosion behaviour of galvanized tubing in warm water systems Tests carried out with tap water (65°C) containing 0.1 to 0.3 mg/l Cu²⁺ have revealed that copper ions give rise to accelerated corrosion of tube walls. Corrosion takes place in the form of pitting. The intensity and frequency of pits increase as the copper content is increased and the pH value is decreased. Corrosion products formed on the zinc are predominantly a basic zinc carbonate, while copper is found in a coagulated form at the surface or embedded in the surface layer. The copper content of the corrosion products increases as the copper content of the water is increased and may attain up to 12.5%.     

Miniaturisierte elektrochemische Sensoren zur in-situ-Untersuchung der Spaltkorrosion in Modellsystemen

In-situ-investigation of crevice corrosion in model systems using miniaturized electrochemical sensors Measuring characteristics of miniaturized sensors for the determination of the parameters pH value, oxygen content, concentration of chloride ions and redox potential was tested and optimized with regard to the application in model crevices. The sensors were used for examinations of crevice corrosion in 0.1 molar and 1 molar solution of NaCl at the following alloys: X 20 Cr 13, X 2 CrNi 18 10, X 2 CrNiMoN 17 13 5, X 2 CrNiMoN 17 13 5*, X 2 CrNiMo CuN 20 25 6. Corrosion characteristic of the alloys and the change of crevice electrolyte without external polarization and with galvanostatic polarization (i = 10 μA/cm², i = 200 μA/cm²) were tested and discussed by the hand of the model concepts from Oldfield and Sutton [1]. In experiments with galvanostatic polarization the steels with increasing content of chromium or molybdenum showed a growing stability against crevice corrosion. The results showed that simultaneous measurements of the above mentioned chemical parameters of crevice electrolyte in connection with the corrosion potential can supply additional information about causes and mechanism of crevice corrosion.     

Adsorption und Korrosionsinhibierung von Polyacrylsäuren auf Aluminiumpigment

Adsorption and corrosion inhibition of polyacrylic acids on aluminium pigment The inhibition of the hydrogen corrosion of aluminium pigment at pH 8 and 10 by polyacrylic acids and their adsorption is strongly influenced by the isoelectric point (IEP) of aluminium oxide (pH ≈ 9). Approximately polyacrylic acids are completely dissociated at both pH values. As expected, at pH 8 (below the IEP) polyacrylic acids are adsorbed significantly stronger as at pH 10 (above the IEP). Furthermore, the adsorbed fraction of polyacrylic acid increases with increasing molecular mass; but the protective value of polyacrylic acids decreases with increasing molecular mass. There is the surprising result that with increasing adsorption of polyacrylic acids their corrosion inhibiting effect decreases. A partial explanation of this result is the high content of carboxylat groups (calculated acid number 780 mg KOH/g) in alkaline medium, which may be responsible for specific ionic interactions with the aluminium oxide surface.     

Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41

The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)₂ in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.     

Untersuchungen zum kritischen korrosionsauslösenden Chloridgehalt von Stahlfasern in künstlicher Betonporenlösung

Investigations into the critical corrosion‐inducing chloride content of steel fibres in artificial concrete pore solution It is well known, that reinforcement steel in concrete is normally protected against corrosion due to the high pH‐value of the pore solution of the concrete. This alkalinity leads to a passive layer on the steel surface, which prevents further corrosion. The passive layer can be destroyed by chloride ions diffusing into the concrete. The concentration of chloride in the concrete which leads to a destruction of the passive layer and therefore to corrosion of the steel is defined as the critical chloride content. Investigations in artificial concrete pore solutions show that the critical chloride content of black steel is strongly dependent on the pH‐value of the solution: the higher the concentration of the OH⁻‐ions the higher the critical chloride content. For steel fibres earlier investigations have shown, that steel fibres do not corrode in concrete even at high chloride contents. Therefore it could be assumed, that the critical corrosion‐inducing chloride content of steel fibres in concrete is distinctly higher than of conventional reinforcing steel. To verify this assumption the corrosion‐inducing chloride content of steel fibres is investigated in artificial chloride‐containing concrete pore solutions at different pH‐values. 5 different types of steel fibres, 1 lashing wire and as reference 1 reinforcing steel are investigated at 3 different pH‐value ranges. The concentration of chloride within the pore solution is gradually increased in time steps of 12 h. The beginning of corrosion is determined by current as well as potential measurements. Furthermore additional investigations are carried out with intermediate products of the fibre production (steel wires with different diameters) to investigate if the critical chloride content of the wires is increasing gradually with decreasing diameter. The investigations show, that steel fibres in artificial chloride‐containing pore solutions indicate an distinctly increased resistance against chloride‐inducing corrosion compared with conventional reinforcing steel for high pH‐values. With decreasing diameter of wires the critical chloride content increases gradually.     

Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on Mechanical Properties and Corrosion Resistance of H62 Brass

In this article, the effects of lanthanum oxide (La₂O₃) on the microstructure and mechanical properties of H62 brass were investigated by using the universal testing machine, Brinell hardness tester, optical microscope, and scanning electron microscope (SEM). Immersion corrosion and electrochemical measurements were carried out to identify the influence of La₂O₃ on the corrosion behavior of the H62 brass. The phase constitution, microstructure, and phase composition of the H62 brass were analyzed by x-ray diffraction, SEM, and energy-dispersive spectrometer, respectively. The results show that the microstructure of α phase changes from dendrite grains to equiaxed grains, and the content and distribution of β phase are improved significantly. When the La₂O₃ content reaches 0.8 wt.%, the H62 brass obtains favorable comprehensive mechanical properties and the strength and hardness decrease but elongation increases, which is conducive to plastic processing. In addition, under the optimum amount of 0.8 wt.% La₂O₃ content, the corrosion rate of immersion corrosion attains the minimum values: As 12.6 g m^?2 h^?1, it decreases by 24%; as the corrosion potential changes from ?1.1327 V to ?0.328 V, it increases by 70.9%; and as the corrosion current density decreases from ?2.833 mA mm^?2 to ?3.28 mA mm^?2 corrosion, it decreases by 15.78%, when compared with H62 brass.     

Einflußgrößen der H-induzierten Spannungsrißkorrosion bei niedriglegierten Stählen

Parameters influencing the hydrogen-induced stress corrosion cracking of low alloy steels Wet H₂S corrosion of steel leads to surface blistering, internal cracking at sharp edged inclusions (HIC), and in stressed specimens to transgranular micro cracks transversal to the main stress. The latter grows to SCC. HIC resistant material displays high purity of non-metallic inclusions. The critical stress level for SCC of lien pipe steels with yield strengths up to 600 N/mm² is proportional to the yield strength. In H²S containing media different grades of low alloy and rolled steels show no correlation between life time and yield strength. In general, the susceptibility for H-induced SCC increases with decreasing pH and with cathodic polarisation, whereas the temperature dependence displays a minium of life time at 20°C.     

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.     

Instrumentierung von Sprühnebelprüfverfahren

Instrumentation of salt‐spray test procedures Salt‐spray test procedures are frequently used in industrial practice as a test method for proof of product quality with semi‐finished products and received parts with regard to their corrosion resistance. For this reason, salt‐spray testing is one of the most well‐established corrosion test procedure. However, no statement can be made about the way in which corrosion develops over time (linear, exponential or logarithmic). Furthermore, the test conditions for salt‐spray tests are intensified in order to shorten the measuring times, which can lead to a change in the corrosion mechanism. Nevertheless, also without intensifying test conditions salt‐spray testing can be brought to a qualitatively higher level and the clarity of the results can be improved by means of in‐situ electrochemical noise measurements. In contrast to established electrochemical tests in which the specimens are surrounded by a common bulk electrolyte, electrochemical measurements in the spray chamber are based on the development of an electrolyte film on the surface of the specimen, but also on the inside walls of the testing chamber and on all fittings present (e. g. specimen holder). The measured effect thus has the same basis as in conventional electrochemical measurements. The exploitable signal is limited by the shortest connection route between the specimens, as well as by the thickness and the conducting ability of the electrolyte film. The successful application of noise measurements under salt‐spray conditions is shown on various examples for corrosion testing.     

Effects of polypyrrole modified electrode functionalization on potentiometric pH responses

Two functionalized polypyrrole films, poly(11-N-pyrrolylundecanoic) acid and poly(N-undecylpyrrole), were used as sensitive layers for pH sensors. The functionalized pyrrole monomers were first synthesized before carrying out their electropolymerization onto non-oxidizable electrodes. The deposits were characterized by SEM imaging, XPS and PM-IRRAS to study surface structure and composition. Poly(11-N-pyrrolylundecanoic) acid coating yielded very promising results as sensitive layer in pH sensors. Indeed, the potentiometric responses from pH 4.0 to 9.0 appeared linear, reversible, stable in time over a period of 1 month and highly selective for H⁺ ions. This behavior was attributed to carboxylic acid groups which are sensitive to proton concentration changes. This was confirmed by the fact that poly(N-undecylpyrrole) film led to potentiometric responses being pH dependent only from pH 6.0 to 9.0 with low sensitivity. Poly(11-N-pyrrolylundecanoic) acid is thus promoted to be used as sensitive layer in pH sensors.     

The behavior of chromium and molybdenum in the propagation process of localized corrosion of steels

In order to clarify the effect of Cr and Mo on the propagation of localized corrosion of steels, the electrochemical behavior of pure chromium and molybdenum and of some stainless steels was studied under conditions similar to those existing inside occluded corrosion cells of steels. Oxygen-free FeCl₂ and/or CrCl₃ solutions were used for experiments. The surface films formed in these conditions were analyzed by means of AES and XPS. Cr is passive if the pH value >ca. 1.8 and Cr may retard the propagation of localized corrosion of steels in its early stage. The hydrolysis of Cr³⁺ may change the pH to even lower values. At such low pH and high Cl^? concentrations, Cr is active in the potential range of interest for localized corrosion of steels. Mo is passive under these more acidic conditions and the passivity of Mo in this potential range is attributed to the formation of a film of MoO₂ (or hydrated oxide), which is extremely thin but very protective. Mo is thus effective to retard the propagation of localized corrosion of steels in its more advanced stage.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme von Eisen in H2S-haltigen Lösungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in H₂S-solutions Effects of the transition metals Mo, V, Nb, Ti, Zr and their carbides as well as of phosphorous on the corrosion and hydrogen uptake of iron in acid to weakly acid NaCl solutions with and without H₂S are discussed. Investigations were carried out on binary, ternary and quaternary iron based alloys, using electrochemical and surface analytical methods. No specific effect of one of the alloying elements or the carbides on the corrosion or hydrogen uptake is observed. Due to the experimental conditions, sulphur and oxygen enriched surface scales form, by which the kinetics of the corrosion processes are determined. The alloying elements are enriched on the iron surface only as a carbide. Phosphorous is enriched as a phosphide at low pH and as a phosphate at higher pH. H₂S and phosphides increase the corrosion rate and hydrogen uptake. In pure iron or low strength iron alloys, at the very high H₂S affected hydrogen activities new lattice defects are induced permanently resulting in extremely high hydrogen concentrations.     

Der Einfluß von pH-Wert und Sauerstoffgehalt von Pufferlösungen auf das Korrosionsverhalten metallischer Werkstoffe

Influence of pH and oxygen content of buffer solutions on the corrosion behaviour of metallic materials The application of solutions to the decontamination of materials in nuclear installations is based on the condition that their corrosion behaviour is clearly understood. Since electrochemical corrosion is due to cathodic and anodic partial reactions which are influenced in different ways by the pH of hte solution and the oxygen content it is suggested that the results of electrochemical experiments with buffer solutions be used as a model for predicting the corrosion behaviour of materials in other solutions. In the tests described here potentiocinetic current-potential-curves have been traced and galvanic corrosion tests have been made. The results obtained in ascorbic acid, postassium hydrogen phthalate, ammonium citrate and acetate, sodium and potassium tartrate, ammonium hydrogen phosphate, sodium carbonate, hexamethylene tetramin, ethylene diamine enable – on the basis of summarized current-potential-curves – the metals studied to be classified in four groups characterized by clear differences concerning the influence of pH on the corrosino behaviour.     

Einfluß des pH-Wertes,des Sauerstoffgehaltes und der Strömungsgeschwindigkeit von kaltem Trinkwasser auf das Korrosionsverhalten und die Schutzschichtbildung bei feuerverzinkten Stahlrohren

Influence of pH-value, oxygen content and flow velocity of cold drinking water on corrosion behaviour and surface layer forming of galvanized steel tubes During a test program of 28 months determinations of weight loss, metallographical examinations and chemical analysis data of surface layers showed clearly that in the chosen variation range mainly the pH is important for the longterm behaviour and the expected working life of galvanized steel tubes. Lower pH-values of 6.9 lead to increased rates of Zinc attack and to inhomogene, disturbed surface layers, whereas higher pH-values of 7.9 show decreased Zinc loss and favour the forming of well protecting layers. Oxygen content and flow velocity become important only in the early stage of Zinc corrosion. Additional electrochemical measurements gave a good impression of Zinc corrosion rate development, corrosion attack morphology and properties of surface layers.     

Einfluß der Festigkeit unlegierter und niedriglegierter Stähle auf H-induzierte Korrosionserscheinungen bei zügiger plastischer Belastung

Influence of mechanical strength on hydrogen-induced corrosion effects on unalloyed and low-alloysteels subjected to slow strain rate tests Three steels were subjected to various heat treatments or cold working to produce 7 variants of strength. Specimens from these materials were tested in 9 aqueous solutions containing various acidic components by the constant strain rate technique under cathodic polarisation or free corrosion conditions. Hydrogen induced cracks were only observed after passing the maximum load. Hydrogen induced cracking and the decrease in the reduction of area (hydrogen embrittlement) show a good correlation with the mechanical properties of the materials. For less ductile materials the number of cracks decreased with increased embrittlement. The magnitude of hydrogen embrittlement depends on the concentration of undissociated acid in the test solution and is independent of pH value. O₂ can reduce the embrittlement. The effect of test solution composition decreases as the cathodic polarisation potential becomes more negative, because hydrogen will then be generated from H₂O. There is no correlation between the type of corrosion effects and the strength of the material, except in the case of highly sensitive high strength variants with R_m in excess of 1000 N/mm². Quenched and tempered low alloy steels, even at high strength levels, have significantly higher resistance to hydrogen embrittlement than significantly higher resistance to hydrogen embrittlement than unalloyed steels. The same is also ture for workhardened variants. For unalloyed steels, metallurgical cleanness seems to have a favourable influence. No hydrogen induced corrosion effects were observed in specimens tested at 907°C under free corrosion conditions.