Elektrochemische Untersuchungen zum Korrosionsverhalten von Duplexgusswerkstoffen in Rauchgasentschwefelungsanlagen

Electrochemical investigations on the corrosion behaviour of duplex cast stainless steels in flue‐gas desulphurisation plants This paper deals with results from corrosion examination on duplex cast stainless steels from original components in the flue‐gas desulphurization (FGD) of the VEAG‐power stations Boxberg III and Jaenschwalde under practice and laboratory conditions. In practice the mentioned components have a better corrosion resistance in the FGD medium of the Boxberg power station. The same results were obtained by laboratory examinations, getting a shift in critical potentials of a few hundred millivolts. The tests in a by‐pass of the FGD confirm these results in tendency. However, partially significant differences to laboratory examinations were established on specimens with pre‐damaged original surfaces. This can be traced back to the more severe test conditions in the FGD, like solids and flow. The repassivation behaviour of the materials under investigation was examined by means of electrochemical noise analysis. The values of the free corrosion potentials in the Jaenschwalde power station were higher than potentials, necessary for a safe repassivation of occurring pits. This cause the failures on the components. Remedy steps are discussed in this paper in detail. The free corrosion potential in the Boxberg power station is about 200 mV below the repassivation potential. Because of that, short‐time fluctuations of system parameters leading to enhanced corrosion risk can be tolerated.     

Elektrochemische Untersuchungen zur Korrosion von Nickelüberzügen

Electrochemical investigations into the corrosion of nickel coatings The corrosion behaviour of pure rolled sheet nickel, electrodeposited nickel and electroless Ni-P-alloy-coatings is investigated by electrochemical methods. Some mechanisms of corrosion and passivation in 0,1 N H₂SO₄ and deionated water - both with addition of several anions - are described.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 3: Elektrochemische Untersuchungen und Bewertungskriterien

Testing the corrosion protection of heat transfer fluids Part 3: Electrochemical tests and valuation criteria. The third part of the report demonstrates electrochemical criteria for the assessment of corrosion protection of inhibited antifreezes on glycol base. The measurements were carried out by means of a cylinder of revolution test plant under practice oriented conditions with iron, aluminium and copper materials. The polarisation resistance, the length of passive-range and hysteresis effects at the anodic part of current density-potential curves proved to be the most useful assessment criteria. The most reliable information concerning the inhibitor efficiency is given by a combination of all three criteria. Potentiostatic tests anodic to the rest potential confirm this further. According to this criteria the tested antifreezes offer a good corrosion protection to the metals investigated.     

Elektrochemische Untersuchungen über die Stimulation der kathodischen Teilreaktion und der freien Korrosion von Silber und Chromstählen in Alkalisulfatschmelzen

Electrochemical investigations into the stimulation of the cathodic partial reaction and free corrosion of silver and chromium steels in alkali sulfate melts Information is presented concerning the influence of vanadium pentoxide, sodium metavanadate, sodium vanadate, molybdenum trioxide, tungsten trioxide and potassium chromate on the cathodic accumulated corrosion density potential curve on platinum and on the free corrosion of silver and ferritic chromium steels (Werkstoff- No. 1.4713 and 1.4742) in alkali sulfate melts at temperatures between 625 and 800 °C. All the compounds mentioned, except WO₃, produce a considerable stimulation of the cathodic partial reaction. Contents amounting to about 2 mole-% give rise to an increase of the accumulated current by appr. two orders of magnitude. In all the cases studied limiting diffusion currents are observed. V₂O₅ is reduced to V₂O₄ and potassium chromate to Cr₂O₃. The influence of WO₃ is relatively small because of its insolubility in alkali sulfate melts. Addition of acidic compounds (V₂O₅, NaVO₃, MoO₃) shifts the redox potential of the melt by several hundres millivolts toward more positive potentials, while the addition of salts (Na₃VO₄, K₂CrO₄) has but little influence on the position of the redox potential. The corrosion of silver is accelerated, in particular by acidic compounds (V₂O₅, MoO₃), which shift the free corrosion potential to much more positive values: in the system (K, Li)₂SO₄? V₂O₅ the corrosion rate of silver at 750 °C increases by six orders of magnitude. Below critical potential (rupture potential) the ferritic chromium steels form oxidic surface layers resembling passivation layers. This effect is connected with the formation of porous, sulfide containing layers only in those cases where the free corrosion potential is above the rupture potential. This effect is attained e.g. by addition of V₂O₅, and in the case of material No. 1.4713 also by addition of NaVO₃ to the sulfate melt. Higher contents of Na₃VO₄ in the melt, however, may give rise to a stimulation of the anodic partial reaction of metal dissolution.     

Elektrochemische Untersuchungen an aluminiumreichen intermetallischen Phasen

Electrochemical investigations into aluminium-rich intermetallic phases An electrochemical characterization has been made of Al₃Fe, Al₆Mn, Al₃Mg₂, Al₂Cu and MgZn₂ phases in chloride containing and chloride-free electrolytes of different pH values. The rupture potentials were determined and a study was made on hydrogen overvoltage and passivation behaviour. The results are used to interprete the corrosion behaviour of heterogeneous aluminium alloys. The good corrosion behaviour of low iron AlMn-alloys can be understood in terms of the high hydrogen-overvoltage on Al₆Mn.     

Elektrochemische Untersuchungen zum kathodischen Korrosionsschutz mit Unterbrecherpotentiostaten

Electrochemical investigations into cathodic corrosion protection using an interruption-potentiostat Stationary and instationary electrochemical investigations were carried out on iron in the cathodic and on platinum coated titanium in the anodic region for further developments on the field of the potentiostatic cathodic corrosion protection. Using the results obtained an interruption-potentiostat was developed, which allows to avoid the normally used reference-electrode at the potentiostatic cathodic protection. This becomes possible by using this interruption-potentiostat and an inert anode, whereby the flowing current is interrupted periodically for a short time. In the cut off time the anode, which is normally used as counter electrode, is now after a delay of about 30 μs the reference electrode to measure the existing potential E_ist of the iron which has to be protected. This potential will be compared to the potential E_soll adjusted at the potentiostat and the difference E_ist – E_soll is used for the regulation of the protection current flowing during the switch on time. The investigations carried out for some time with such an interruption-potentiostat show that iron can be protected cathodically in this way without the undesired generation of hydrogen.     

Elektrochemische Untersuchungen an Molybdän

Electrochemical studies on molybdenum The authors show, that the power law found during earlier measurements is valid for at least five decimal powers of the time. The correlation derived from potentiostatic switch-in measurements has been confirmed by galvanostatic tests. The results give rise to the assumption that the kinetic law has its origin in the rates of the reactions resulting in metal dissolution. In order to elucidate these reactions a triangle potential method has been used. The findings enable a first suggestion to be made as to the reaction sequence.     

Zur Korrosion von Eisen und Aluminium in wässerigen Lösungen von Ammoniak und Kohlensäure – 2. Mitteilung: Elektrochemische Untersuchungen

Contributionto the corrosion of iron and aluminum in aqueous solutions of ammonia and carbonic acid. 2nd Communication: Electrochemical investigations Corrosion potential-time curves and current density-potential curves as well as polarization resistances and other electrochemical methods have been used to characterize the corrosion behaviour of aluminum and iron base materials in an aqueous solution of 11% ammonia and 7% carbon dioxide (leaching solution applied in hydrometallurgical nickel raffination) at 50 °C. For comparisonal purposes the behaviour of the same materials has been investigated in ammonia, sodium carbonate and sodium hydroxide solutions at constant pH. Despite of the relatively elevated pH value aluminum is protected against corrosion in the ammonia-carbon dioxide-water-mixture through selfpassivation (fast repassivation of mechanically destroyed passive layer). In contrast, the stability of iron and mild steel in the same solution is seriously impaired by easy activation. Permanent passivation is only achieved through anodic protection by means of an external current source.     

Elektrochemische Untersuchungen von handelsüblichen Kupferrohren mit verschiedenen Oberflächenzuständen

Electrochemical potential measurements of commercial copper tubes with different surfaces Besides the quality of the water, the design and the operating conditions of an installation the surface condition of the tubes plays an important role in influencing the corrosion behaviour of copper tubes in drinking water. For this reason commercial copper tubes have for some time already been supplied with a very clean bore, a condition which can he achieved by different processes, such as a very clean manufacturing process, oxidising treatment or abrasive cleaning as a final manufacturing step. The development of the free corrosion potential, the pitting potential and the repassivation potential were investigated in commercial copper tubes with different surface conditions in a sulphate/chloride/carbonate electrolyte similar to drinking water.     

Elektrochemische Rauschmessungen zum Nachweis der Wirksamkeit von neuen Korrosionsschutzschichten auf Aluminium und Magnesium

Electrochemical noise measurements for the determination of the effectiveness of new corrosion protection layers on aluminium and magnesium The corrosion behaviour of the aluminium alloy AlMg1 and of the magnesium alloy AM60 have been investigated by electrochemical noise measurements in chloride and sulfate solutions. It should be proved, whether protecting layers based on self assembling molecules, which have been adsorbed on the samples before the electrochemical noise measurements, could inhibit pitting corrosion. The measurements do show, that several systems have an inhibitive effect, others not. Potentiostatic tests in combination with the measuements of the current noise are useful for a quick determination of the break through potential.     

Elektrochemische Untersuchungen zur Hochtemperaturkorrosion hochlegierter Werkstoffe in Alkalisulfatschmelzen

Electrochemical investigation into the high temperature corrosion of high alloy materials in alkali sulafte metals The investigations have been carried out in alkali sulfate melts under air at 625 and 800°C with iron, chromium, four ferritic chromium steels, two austenitic chromium nickel steels, one each nickel and cobalt base alloy and three chromium alloys. Clear statements concerning the potential dependence of the corrosion behaviour are feasible only on the basis of mass loss/potential curves. It results from these curves that the behaviour of all the materials studied is characterized by two potential regions with different corrosion behaviour. At relatively negative potentials a protective layer is formed on the material surface and the corrosion rate is low and almost independent from potential. Above a rupture potential (which depends from the system material/melt) porous surface layers are formed. In this potential range the corrosion rate shows a steep increase as potential increases. With increasing chromium contents of the materials the rupture potential is displaced towards more positive potentials, but chromium contents exceeding 20% have no markable influence on the rupture potential. Temperature, too, does not noticeably affect the position of the rupture potential. The interpretation of potentiodynamic accumulated current/potential curves requires great care; the shape of such curves depends from the variation of potential with time. The approximate position of the rupture potential can frequently be recognized at very low potential variations only. At larger potential variations in time the values of the rupture potential appear as a rule to be too negative; this phenomenon may probably attributed to potential dependence growth processes in the passive layer.     

Elektrochemische und chemische Korrosionsprüfung von 13%igem Chrom-StahlguM in chloridhaltigen Medien

Electrochemical and chemical corrosion testing of 13% Cr cast steel in chloride containing media The corrosion rate of 13-1 Cr-Ni, 13-4 Cr-Ni, 12-5-1 Cr-Ni-Mo and 14-5-1 Cr-Ni-Mo cast steel is studied in 3% NaCl solution, sea water and 2 N H₂SO₄ containing up to 0.35 N NaCl; test methods include electrochemical ones, i.e. by measuring the slope of the current density/potential curve at the corrosion potential, and chemical methods, i.e. by analysis. In 3% NaCl solution both methods yield a series of decreasing rates (13-1, 12-5-1, 13-4 and 14-5-1 Cr-Ni-Mo cast steel), weight loses ranging from 0,7 to 0,2 g/m²d at 40° C, and from 0,4 to 0,1 g/m²d at 20° C. These results confirm the quantitative agreement between electrochemical and chemical evaluation as well as the agreement between short term and long term testing. The influence of flow is investigated and overall current density/potential curves are determined in various media and are interpreted. In addition, the authors describe an apparatus for the determination of corrosion rates between 1000 and 0,003 g/m²d.     

Elektrochemische Untersuchungen zum Einfluß von Nitrat und Nitrit auf die Ausgangsruhepotentiale und den Kurzschlußstrom des Elementes Eisen/Zinn in Fruchtsäuren und -säften

Electrochemical investigations into the influence of nitrate and nitrite on the starting rest potentials and the short circuit current of the element iron/tin in fruit acids and juices With the aid of electrochemical measurements the influence is studied of nitrate and nitrite additions on the corrosion behaviour of tin and the iron/tin element in various fruit acids (citric, acetic, tartaric, malic and lactic acids) and vegetable and fruit juices; the results are compared with storage tests conducted in parallel. The electrochemical short time test is generally little suitable for showing the influence of nitrates, in particular because of the slow conversion of nitrate into nitrite which is a corrosion accelerator; the nitrite influence, on the other hand, can be shown quite easily since it produces on the one hand a potential shift toward more positive values and, on the other hand, gives rise to an increase in the short circuit current. These results can be applied to coated cans, too, where the nitrite gives rise to strong dissolution of iron with pitting attack.     

Elektrochemische Untersuchungen zur Hochtemperaturkorrosion von Chrom-Nickel-Stählen in Alkalisulfatschmelzen bei 700°C

Electrochemical investigations into the high temperature corrosion of chromium nickel steels in alkali sulfate melts at 700°C Specimens from austenitic stainless steels, with and without susceptibility against intergranular corrosion attack, decarburized and carburized, and stabilized with Ti or Nb, have been corroded in eutectic mixture of alkali sulphates at 700°C at constant potentials. Furthermore additions of 10% NaCl and 1% SO₃ have been investigated. Any selective corrosion of both carbides and Cr depleted zones did not take place. With decreasing potentials there was a preferred attack at grain boundaries – especially in the case of NaCl additions. At very negative potentials an “active” corrosion state exists without formation of protective oxide layers with the consequence of fissured surfaces and internal oxidation. The effect is supported by NaCl addition and by Ni as alloying element.     

Untersuchungen zum Zusammenhang von Stofftransportgeschwindigkeit und Abtragsgeschwindigkeit unter Erosionskorrosionsbedingungen

Investigations on the correlation between mass transfer rate and mass loss rate depending on conditions of erosion corrosion Interactions of flow conditions, mass transfer rates and mass loss rates are investigated in two phase flows depending on conditions of simulation of erosion - corrosion. The velocity and the particle dose of the flow are varied. The limiting current density technique is applied to measure the mass transfer rate. By means of a potassium ferric/ferrocyanide redox system the limiting current density is measured at a platinum electrode. At mild steel and austenitic CrNi-steel the mass loss rate is gravimetrically determined. The mass transfer rate is a characteristic number of erosive treatment in two-phase flows. In certain conditions bistabilities are observed in both mass transfer rate and mass loss rate.     

Elektrochemische Charakterisierung von Phosphatschichten auf verzinktem Stahl

Electrochemical characterization of phosphate layers on zinc coated steel The electrochemical test for determination of the coverage of phosphated steel [1] is transferred to zinc coated steel. Potentiodynamic current and capacity measurements as well as impedance spectroscopy were used to evaluate the coverage. For electrogalvanized steel the free metal area decreases to 20% within 60 s phosphating time. The layer growth kinetics are determined for different systems by gravimetric measurements. Time dependent measurement of the potential shows a nickel cementation during trication phosphating. The combination of electrochemical methods and XPS-/EDX-measurements shows the incorporation of manganese and nickel from the trication bath into the phosphate layer. Inhibiting passive layers of aluminium and lead were also detected. The influence of iron originating from the alloy layer on galvannealed steel on the phosphating process is examined.     

Augerspektroskopische Untersuchungen zur Lochkorrosion von Fe-Cr-, Fe-Mo- und Fe-Cr-Mo-Legierungen

Auger-spectroscopic investigations into pitting corrosion of FeCr, FeMo and FeCrMo alloys A single pit was produced on binary iron-chromium (Fe17Cr), iron-molybdenum (Fe1Mo, Fe3Mo, and Fe5Mo) and ternary iron-chromium-molybdenum alloys (Fe17CrxMo, x = 1, 3, 7 wt.%) in 0.5 M H₂SO₄ with Cl^? additions (0.02 to 0.08 molar) at room temperature. Compositional changes at the bottom of the pit and in the surrounding passive film were determined using high resolution Auger-Electron Spectroscopy (AES). The chromium and molybdenum content of the passive layers are not changed in the presence of Cl^? and there is no incorporation of chlorine in the passive film as long as the films were formed in a chlorine-free electrolyte and Cl^? was added later. At the bottom of the pits salt films could be determined in all cases after removing the electrolyte with filter paper. These salt films are strongly enriched in chromium, molybdenum and chlorine. The salt film was not detectable for Fe-Cr after rinsing the samples with distilled water, whereas in the case of molybedenum additions the salt film could not be washed away with water.     

Elektrochemische Charakterisierung von TiC- und Ti(C,N)-Schichten auf WC/Co-Hartmetallen

Electrochemical characterization of TiC and Ti(C,N) layers on WC/Co hard metals TiC and Ti(C,N)layers deposited on WC/Co hard-metal substrates by Chemical Vapour Deposition (CVD) can be characterized using a combination of electrochemically controlled dissolution and voltammetric signals with respect to stoichiometry (C/N ratio) and depth profile. A regular layer-by-layer dissolution with proportionality between charge and depth as well as with a good etching quality (edges, lateral homogenity) could be realized in sulphuric acid. The influence of the C/N ratio in Ti(C,N) on the electrochemical signal has been investigated with defined powders using the Carbon-Paste-Electrode. These results were used for calibration of the signals at CVD layers.     

Elektrochemische Untersuchungen der Korrosion hitzebeständiger Fe- und Ni-Basislegierungen in eutektischer Alkalisulfatschmelze bei 700 °C

Electrochemical Investigation of the Corrosion of Heat Resistant Fe- and Ni-Base-Alloys in Eutectic Alkali Sulphate Melt at 700 °C Eight Fe-Cr-Ni-alloys with (mass-%) I5-29 Cr, 0-76 Ni, Fe balance have been corroded in eutectic alkali sulphate melt at 700 °C, I bar O₂, potentiostatically for 23 h. The corrosion behaviour at different potential in the active, passive and transpassive range was investigated by mass loss measurements and metallographically. Cr was found to have a markedly effect on the passive/transpassive behaviour and for the corrosion resistance at all. But there is no further improvement at > 20% Cr. The effect of Ni was found to be negative or indifferent in the active range. The greater effect was found with the “minor” elements, e. g. Al and Si, which can support protective layers formation in the active range, whereas in the transpassive range they can enhance the corrosion process. Ti was found to have a negative effect due to local sulfide formation. Heat resistant alloys with high contents of Cr and Ni have been found to have necessarily no increased corrosion resistance to sulphate attack. In the range of 20-30% Cr the resistance can alter markedly showing the greater effect of minor elements.     

Untersuchungen zur Morphologie von Aluminiumhydroxiden und anodischen Aluminiumoxiden mit dem Rasterelektronenmikroskop

Investigations on the morphology of aluminium hydroxides and anodic aluminium oxide films using scanning electron microscope Hydroxide films of spongy structure are originated on aluminium surfaces if aluminium is trated with hot water for short periods. By increasing the reaction time the hydroxide layers start to become more compact until films are growing coherent showing a porous surface. If the reaction time exceeds 4 hours at a reaction temperature of 98°C crystalline scales appear on the top of the films. During the anodic oxydation of hydroxide coated aluminium in citric acid solutions, thin hydroxide films transform into barrier layer type oxides. If the anodization occurs in aggressive electrolytes the existing hydroxide dissolves and oxide layers with porous structure are obtained. Oxide layers produced by anodizing in hot citric acid solutions in the absence of a hydroxide film show a current density dependent morphology. At higher current densities barrier layer type oxides are growing. Using low current densities oxide layers with a character are predominating.