Kontaktkorrosion bei ein- und mehrschichtigen Metallüberzügen

Contact corrosion with one-layer and multilayer metal coatings Contact corrosion on metallic coatings depends on a number of factors which contribute to the difficulty encountered in predicting the progress of corrosion. With metallic coatings the free surface of the base material is normally small by comparison to that of the coating, so that contact corrosion — appearing as locally increased attack, possibly induced by protective measures — comes to bear on the base material only. Excluding some few exceptions the contact corrosion is controlled by the oxygen content of the attacking electrolyte, which — in addition to the type of electrolyte and the metal combination involved — determines the progress of contact corrosion. On the example of sulphur containing and sulphur-free nickel and of electrolytically deposited metals with heavily disturbed lattice it is shown, that the establishment of the contact corrosion potential is controlled by the composition and the condition of the metals involved. With galvanic Ni+Cr coatings the direction of the current in the galvanic element controlling contact corrosion may change with the composition of the attacking electrolyte, even during atmospheric corrosion, so that Cr may either be the anode being dissolved, or the cathode. It may happen that this combination does not yield any element at all, affecting the corrosion process. Model experiments serve to demonstrate the influence of the electrolyte composition and the composition of the metals in contact with each other.     

Automatische Ermittlung von Kontaktkorrosionsdaten und ihre Auswertung mittels Polarisationsdiagrammen

Automatic measuring of contact corrosion parameters and their interpretation by means of polarization diagrams To evaluate the danger of contact corrosion on coupled dissimilar metals, the corrosion potentials of the concerned metals, the mixed corrosion potential and the current between the galvanic couple are of interest. A test stand with 6 sets has been developed to determine these values simultaneously and automatically from 6 variations of metal couple electrolyte/electrolyte's condition/temperature. Every set is equipped with two electrolytic cells. One cell is for measuring the corrosion potentials of the two metal probes in an open circuit; in the other cell an identical metal couple is arranged in a short circuit, so that the mixed potential and the current of the contact couple can be monitored. The values are called in sequence in a pre-selected manner by a scanning unit and are then recorded. From couples obtained from combinations of mild steel, CrNiTi-steel, NiCrFe-alloy, and Molybdenum alloy (0,5 Ti and 0,1% Zr), the mild steel proved to be always the least noble whilst the Molybdenum alloy was the most noble; this was true in both alcaline and acid electrolytes. By plotting the measured values on a polarization diagram, a survey of the possible contact corrosion behaviour in several systems may be obtained together with information on the corrosion mechanisms occurring.     

Beschichtungssysteme für Verbindungselemente in Kontakt mit Leichtmetallen

Coating systems for joining elements in contact with light metals Galvanic corrosion of light metal alloys can only be avoided, if the steel fasteners are given an electrochemically compatible protection or if the corrosion circuit is interrupted by electric insulating layers. Current density vs. potential curves of chromated zinc alloy coatings show, that by means of zinc nickel coatings a largely electrochemical adaption to the equilibrium potential of quenchaged aluminium alloys of the type AlMgSi is possible. On the other hand, the equilibrium potentials of chromated systems based on ZnFe, ZnCo respectively, are too low to avoid galvanic corrosion on aluminium. In practicerelated corrosion tests on joining elements, this result was confirmed. Besides this, zinc- and aluminium-bearing sintersystems, galvanic tin depositions as well as ternary mechanical coatings based on ZnSnAl have been proved to be compatible with AlMgSi-type alloys. For coating systems in contact with magnesium a satisfactory protection against galvanic corrosion can be achieved effectively with insulating top coats. The results with suitable duplex systems are presented and recommendations for practical applications are derived.     

Vorwort

Critical potentials in corrosion chemisry and their measurement It is shown with the aid of a potential-pH-diagram that the corrosion behaviour of a given system can be evaluated on the basis of the border lines between the different zones of the metal behaviour. Such regions are: thermo-dynamic stability of the metal, active and transpassive corrosion, surface layer passivity (= thermodynamic stability on the surface layer), chemical passivity etc. These border lines are thermodynamic equilibrium lines or correspond to such lines. With the exception of the equilibrium line between stability and corrosion of the metal, the other transition must be interpreted in cinetic rather than in thermodynamic term. The influences of Cr-content, pH, temperature and specimen pretreatment are discussed. Under certain conditions local corrosion may appear within certain potential regions; such corrosion phenomena are e.g. pitting and stress corrosion cracking. The border potentials of these corrosion regions are discussed with particular attention to measurement problems involved. With corrosion protection by cathodic or anodic polarisation the border potentials have the meaning of protection potentials. The latter are, consequently, no equilibrium potentials but border potentials having an analogous significance; they can be measured best with the aid of stationary methods.     

Cathodic polarization of commercially pure aluminium

Cathodic corrosion, particularly alkaline pitting and repassivation, of 1S aluminium has been investigated by potential-controlled methods in unbuffered chloride media. The results are mainly for de-aerated solutions; the influence of dissolved oxygen and stirring, which are likely to be present in a practical situation, are also discussed. The metal undergoes stable pitting and then uniform etching as the applied potential is decreased below — 1.35 V(SCE). At potentials more positive than this threshold value, metal dissolution still occurs possibly near the cathodic impurities on the surface due to local alkalinization resulting from hydrogen evolution; but, the surface repassivates slowly as the cathodic sites are covered up by the oxide film. The polarization behaviour of the partially passive surface is studied by fast-scan potential measurements and is associated with hydrogen evolution kinetics. The measured current decays exponentially with time during repassivation at constant potential. This is explained in terms of a model based on film growth controlled by metal dissolution.     

Das potentialabhängige Repassivierungsverhalten der Titanlegierung Ti6A14V in neutralen Elektrolyten

The potential dependent repassivation behaviour of the titanium alloy Ti6A14 V in neutral electrolytes The potential dependent repassivation behaviour of Ti6A14V is very similar in aqueous sodium halogenide solutions (except NaF). Characteristic differences are found only at potentials in the vicinity of the particular pitting potentials. Comparative current-potential measurements have shown that it is possible from repassivation experiments to determine the pit passivation potential which is at 700 mV in NaCl and at 1200 mV in Nal. At potentials above–400 mV oxygen dissolved in the electrolyte has no effect on the repassivation processes; at lower potentials it has a bearing on the current; this effect is attributed to the oxygen reduction reaction. The method of investigation chosen for this particular case enables the measurement of current-potential curves on active Ti6A14V. The corrosion potential resulting therefrom is ?1100 mV (in the active state), the corrosion current density is about 10 mA/cm² (the corrosion current density for the passive material is about 10^?9 to 10^?10 A/cm²).     

Neuere Untersuchungen über das Korrosionsverhalten und Korrosionsschutzmaßnahmen von Leichtmetall-Legierungen im Schiffbau und ähnlichen Anwendungsgebieten

Recent investigations into the corrosion behaviour and corrosion protection of light metal alloys in shipbuilding and similar applications The corrosion of Al and its alloys in seawater is due to the high anion defect concentration in the (natural or artificial) oxide skin; this structure facilitates anion adsorption (in particular adsorption of the small chloride ions), so that Al atoms are dissolved from the crystal and pitting corrosion may appear. This danger can be eliminated by Zn coatings on Al or by coupling Al with galvanized steel. This measure results in a certain anodic protection and an enforced general uniform corrosion. Flame-sprayed Zn coatings turn out to be most efficient while Cd coatings are clearly inferior. On the basis of experience gained with the stell/Al couple important variables affecting the corrosion behaviour are potential difference and current density (area ratio) and, in addition, the formation of electrolyte bridges. It is therefore important to prevent the formation of such bridges by careful sealing; it is necessary, however, to take account of the ageing behaviour of the particular sealant.     

An in situ optical second harmonic generation study of carbon steel in contact with an alkaline aqueous solution containing glutamic acid

In this paper we report a spectroelectrochemical investigation—relevant to the elucidation of rebar corrosion processes—based on in situ optical second‐harmonic generation spectroscopy. We studied the corrosion behaviour of a carbon steel electrode in contact with an alkaline aqueous solution—simulating the electrolyte of freshly prepared cement paste—containing glutamic acid, a candidate corrosion inhibitor 1 . The non‐linear time‐dependent optical response to exciting radiation at 1064 nm has been recorded as a function of applied potential. This approach allows to obtain detailed information regarding the dynamic electronic structure of the corrosion interface, which is not available from other electrochemical or spectroelectrochemical approaches.     

Kelvin探头参比电极技术在大气腐蚀研究中的应用

Kelvin探头参比电极技术是近年来大气腐蚀领域中最为活跃的研究方向之一．它能够不接触、无破坏的检测薄液膜甚至涂层下金属表面的腐蚀电位分布，文中介绍了该技术的发展历史、技术要点，着重讨论了该技术在研究金属表面有机涂层的剥离和薄液层下金属的腐蚀两方面的应用。     

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.     

Untersuchungen über die Lochfraßkorrosion des passiven Eisens in chlorionenhaltiger Schwefelsäure

Investigation into pitting corrosion of passive iron in sulphuric acid containing chloride ions Pitting corrosion of metallic materials is generally connected with presence of a surface layer giving rise to a local differentiation of the electrochemical behaviour of the metal surface. The pitting corrosion by halogen ions on passive metals is investigated using passive iron in chloride ion-containing sulphuric acid as the model system. Quantitative data are presented concerning the mechanism and kinetics of the individual processes giving rise to pitting corrosion in a chloride ion concentration range covering three powers of ten, and in the whole potential range of iron passivity, from the Flade potential to the transpassive breakthrough potential. Pit formation normally follows a linear kinetic law, the rate depending in particular from the chloride ion concentration and from the thickness of the passive layer. The growth of pit diameters follows a linear kinetic law, too; the dissolution current density in the pits depends from the chloride ion concentration. Comparative investigations carried out on active iron, and potential distribution as measured in the pits show that the metal is active in the pits, too. The heterogeneous mixed electrode condition — active pit/passive metal surface — is stabilised by resistance polarisation. The investigations so far do not permit any statement concerning the specific effect of the chloride ions.     

Potential- und Polarisationsmessungen an Korrosionselementen

Potential and polarization measurements on corrosion elements The polarization behaviour of heterogeneous mixed electrodes is of interest in connection with the assessment of the corrosion state of large objects involving macroelements and localized anodes, and in connection with the evaluation of the corrosion behaviour of weld seams. Measurements on a tube section have shown, that the influence of polarization and stray currents on local tube potentials and potential gradients in the electrolyte is increased by increased anodic polarization levels, provided, however, the current density: potential curves of the two partners diverge. Local anodes on welded connection can be found by measuring the surface potential; anodic polarization makes this method, too, more sensitive.     

Korrosionsschutzmaßnahmen bei erdverlegten Rohrleitungen und ihre Überwachung

Measures for corrosion protection of buried pipelines and their control Corrosion protection of buried pipelines involves a combination of proper coating and electrochemical protective measures. The cathodic protection is based on the potential dependence of corrosion rates of steel in water. The theory is easy to understand. But problems in service arise with respect to questions on current distribution as well as the control by pipe-to-soil potential measurements. Proper coating is an essential requirement for an adequate current distribution. In the neighbourhood of large holidays disturbances of the current distribution can occur which can go undetected by normal potential control in steps of 1 to 2 km depending on the soil conditions. For a better control the intensive measuring technique was employed. By this method pipe-to-soil potentials and potential gradients for both “on” and “off” conditions of the polarisation current are measured in steps of 5 m. The values are evaluated through a computer assisted programme. Pipe sections with large holidays and with insufficient polarisation can thus be detected. In special cases that are characterized by high cell currents caused by contact with foreign structures or by stray currents from foreign dc sources it is difficult to measure IR-free potentials. In this case external measuring probes are installed which simulate holidays of the object to be protected. The potential of these probes can be measured without IR-drops with the aid of capillaries. The probes inform on proper cathodic protection, provided the holidays of the object to be protected are smaller than the testing area of the probe. With the aid of the intensive measuring technique as well as the application of the measuring probes it is possible to control the total object to be protected. But in the case of very high ac densities difficulties cannot be excluded. Diversion of these current is a protective measure. Questions occur as to possible corrosion danger beneath disbonded coatings and the occurrence of stress corrosion cracking. Experiments have shown that corrosion resistance is not restricted provided the construction and the service conditions of the pipeline comply with the standards.     

The nature of crevice corrosion of aluminum in chloride solutions

To study crevice corrosion of pure aluminum, polished specimens partly covered with a glass foil were polarized potentiostatically in 1 N NaCl-solution at potentials negative to the critical potential for stable pitting (pitting potential). For comparison, non-crevice experiments were performed on polycrystalline and singlecrystalline material in neutral as well as acidified 1 N NaCl-solution and in AlCl₃-solutions. Corrosion morphology was examined by scanning electron microscopy. In current-time plots recorded during experiments on crevice corrosion, both an incubation and a propagation stage are discernible. If experiments were interrupted during the induction period, micropits were found inside the crevice. This unstable micropitting is detectable down to 0.30 V below the pitting potential. In contrast, during crevice corrosion propagation, the aluminum surface undergoes general attack. In a range of 0.2 V below the pitting potential, dimpled surfaces are produced. At more negative potentials, metal dissolution occurs crystallographically oriented. An identical behaviour was detected on unshielded samples polarized in the same potential range in both 1 N AlCl₃- and acidified 1 N NaCl-solution. Hence, the build-up of an acidic electrolyte is considered the sufficient requirement for crevice corrosion initiation.     

Corrosion resistance in different environments and structural features of dental casting alloys for resin bonding

The influence of different electrolytes on the corrosion behaviour of three dental casting alloys for resin bonding with different nobility was investigated. The alloys were studied in the metallurgical state representing that in the oral cavity. Structural and morphological characterisation was performed by means of optical and electronic microscopy and EDS microanalysis. The corrosion resistance was evaluated by means of potentiodynamic anodic polarization curves recorded in three different aggressive environments: artificial saliva (pH = 6.7), Ringer's solution and 0.1 N NaCl solution added with lactic acid (pH = 2). The latter solution is recommended by UNI EN ISO 8891 for the static immersion corrosion test. For comparison purposes the electrochemical behaviour of the pure alloying metals in this solution was also investigated. Results showed a satisfactory corrosion resistance of the alloys examined. The synthetic saliva was the least aggressive electrolyte for all of them but it did not allow to distinguish the corrosion behaviour of these alloys although their differences in nobility and structure. The chloride‐containing solutions showed a better distinction in the corrosion behaviour, as matter of facts, as the chloride content increased, the higher (Cu + Ag) amount was in the alloy, the higher was its current density in the immunity region of the anodic polarisation curves. The environment recommended by the normative UNI EN ISO 8891 resulted the most sensitive and effective for the discrimination of the corrosion resistance of precious metal alloys for resin bonding with different nobility by means of the potentiodynamic test.     

Galvanic corrosion of carbon steel coupled to antimony

Galvanic corrosion of carbon steel coupled to antimony was studied in aerated and N₂-purged electrolytes at ambient and 60 °C temperatures. Free corrosion potential of antimony and carbon steel shifts to more active values with increasing temperature and N₂ purging of the electrolyte. Under all experimental conditions, antimony remains less electronegative than carbon steels. Aeration and temperature affect potentiodynamic behaviour of both materials. As a consequence, the corrosion current for the antimony–carbon steel couple increases with increasing temperature and with aeration. There was a good agreement between the corrosion currents obtained through the Evans’ experiment and super-imposed potentiodynamic scans.     

Contact corrosion of steel 3 caused by its different aeration or chloride concentration conditions

Corrosion processes caused by a contact of metal samples with initially different potentials were studied with specially designed electrochemical simulator. Probable corrosion cells were determined for carbon and low-alloyed steels. It is shown that irrespective of factors causing the initial potential difference between the samples in contact, the corrosion type and rate are determined by their potentials settled upon the contact and referred to the corresponding anodic polarization curve. Any current measured in the external circuit corresponds to a difference in the dissolution rates of surface areas in contact rather than to the metal dissolution rate. A local corrosion of phase-heterogeneous carbon, as well as low-alloyed, steels at their active dissolution potentials is caused by the different dissolution rates of their phase components at the corresponding potential. Original Russian Text ? N.N. Glazov, S.M. Ukhlovtsev, I.I. Reformatskaya, A.N. Podobaev, I.I. Ashcheulova, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. I, pp. 43–51.     

Corrosion and corrosion testing of magnesium alloys

The corrosion behaviour of magnesium alloys is not substantially comparable to other metals, such as iron, nickel and copper. It is always accompanied by hydrogen evolution. More hydrogen is evolved at a more positive potential or a higher anodic current density. The ‘strange’ hydrogen evolution behaviour is a common phenomenon for magnesium alloys and it is called negative difference effect (NDE). The NDE continues to receive considerable discussion. Furthermore, the corrosion behaviour of magnesium alloys depends mainly on the pH value of the surrounding electrolyte. Voluminous reaction products, formed in neutral electrolytes, lead to a diffusion‐controlled dissolution on the surface of the underlying magnesium alloy. Therefore, influences from structure and alloying are suppressed very strongly. In alkaline environments, passivation occurs as a result of the formation of a hydroxide layer on the magnesium surface. Therefore, differences in the corrosion behaviour between the alloys are hardly detectable. Measurable effects can only be detected using very ‘aggressive’ corrosion conditions. Present methods do not adequately take into account the specific character of the corrosion of magnesium alloys. It can be better characterized using a rotating disc electrode for electrochemical measurements, which enables model defined flow conditions on the surface. Furthermore, the application of electrochemical noise offers the possibility of a simple and sensitive assessment of the corrosion susceptibility of magnesium alloys. Due to the high sensitivity of this measurement procedure, it is also possible to carry out examinations under more practical conditions.     

Polarisationsmessungen an Kurzschlußelementen und Schweißverbindungen aus Aluminiumlegierungen

Polarization measurements on short-circuit couples and weldments of aluminium alloys In order to study the corrosion behaviour of welded AlZnMg alloys potential/path profiles were registered under anodic polarization in sodium chloride solutions. Using theoretical considerations it is shown that this method is particularly suitable for aluminium alloys because current break-through occurs in such solutions. The performance of the method was checked with the aid of model couples of two alloys having different break-through potentials.     

Potential-Sonden-Messungen und Potential-Profile über Schweißverbindungen korrosionsbeständiger Nickel-Basis-Werkstoffe und Sonderstähle

Potential probe measurements and potential profiles across weldments of corrosion resistant nickel-base alloys and special stainless steels A great number of potential profiles was measured across weldments (SMAW, GTAM, GMAW, Plasma) of the highly corrosion-resistant nickel-chromium-molybdenum alloys 22, C-276, C-4, of the nickel-chromium-iron-molybdenum-copper alloy G-3 as well as of the highly corrosion-resistant austenitic steel Cronifer 1925 hMo. After several tests had been made in a high-ohmic electrolyte solution of 90 vol.-% of methanol + 10 vol.-% of 10^?3 M HCl ( χ = 18 to 20 μs/cm) the tests were continued, for reasons of easier handling, in deionized water whose conductivity was adjusted by using 0.1 M HCl to χ = 10 to 20 μs/cm. It clearly proved that the potential profiles of various weld joints were dependent on the electrolyte. The potential profiles measured across weldments of the NiCrMo alloys which were made with matching filler metal revealed only insignificant or no differences between the potentials of the base material and the weldment. Consequently, in chloride-containing electrolytes it is very unlikely that matching NiCrMo weldments are subject to a preferred corrosion attack. This corresponds to practical experience. The potential profiles in the aqueous HCl electrolyte across weldments of the 6% Mo stainless steel Cronifer 1925 hMo and of the NiCrFeMoCu alloy G-3 welded with a NiCrMo-filler metal showed a potential drop of up to 420 mV over the weldment when compared with the base material. These potential differences can be explained by means of current density-potential curves, showing the electrochemical reactions of base material and weldment with the electrolyte to be different. Nevertheless, the corrosion resistance of the weldment is not inferior to that of the base material.