Über die Werkstoffzerstörung durch Kavitation und Korrosion in Äthylenglykol-Wasser-Gemischen

The destruction of material through cavitation and corrosion in ethylene glycol-water mixes Destruction of structural parts in contact with liquids through cavitation and corrosion is the consequence of complex mechanical and corrosive stresses. In recent years, extensive research at model test beds has yielded information on the stress conditions encountered in waters. It was now intended to ascertain to what extent the destruction of material through cavitation and corrosion in ethylene glycol-water mixes differs from that in waters. For this purpose, tests have been carried out with magnetostrictive oscillators on grey cast iron and copper in water, in ethylene glycol, and in mixes of both liquids; the tests on grey cast iron were extended to solutions with inhibitor additives. It was found that the mechanical stress through cavitation is about the same in the liquids investigated. Certain differences in the destruction rate are mainly due to the high boiling point of ethylene glycol and the corrosion resistance of the material concerned. In aqueous ethylene glycol solutions as well as in water, the damage can be reduced by the addition of corrosion inhibitors.     

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.     

Korrosion und Korrosionsinhibierung von Aluminiumpigmenten im alkalisch wäßrigen Medium

Corrosion and inhibition of corrosion of aluminium pigments in alkaline aqueous medium Flakelike aluminum pigments (Al-content > 99.5%) were corroded in an aqueous alkaline mixture of water and butyl glycol in the ratio 9: 1. Chelating agents like citric acid or polyacrylic acid inhibit this corrosion reaction. The temporal progress of the corrosion reaction can be determined by volumetric analysis of the evolved hydrogen and furthermore, with addition of the corrosion inhibitors, by measuring the electrical conductivity of the solution. The corrosion reaction consists essentially of two steps:

• 1 In the latency period no or only little corrosion takes place; during this, time the protective layers (normal oxide layer or layers reinforced by reaction products of inhibitor and aluminum) were dissolved.
• 2 After that the actual corrosion reaction takes place.

It was proved that changes of the pH-value or addition of corrosion inhibitors only influence the duration of the latency period; after that period the rate of the corrosion reaction is nearly independent from the examined conditions.     

Untersuchung vielkomponentiger,Nitrilotrimethylenphosphonsäure enthaltender Systeme als Inhibitoren für den Metallschutz in neutralen wäßrigen Medien

Study of multicomponent systems containing nitrilo-trimethylene phosphonic (NTMP) acid as inhibitors for the protection of metals in neutral aqueous media The inhibitor mixtures studied contain nitrilo-trimethylene phosphonic acid, oxalic acid or phthalic acid anhydride and zinc sulfate. On the basis of the measurement of the differential electric double-layer capacity using the bridge method the thermodynamic characteristics of the adsorption of the compounds studied and their mixtures on the surface of armco iron have been determined in aqueous potassium fluoride solutions. The best inhibitor mixture was then used as the starting material for optimizing the composition ratios. Using gravimetric techniques the influence of this mixture on the rate of corrosion of steel ST3S in simulated industrial water of variable chemical composition and in water being chlorinated. In addition the influence of this mixture on the rate of corrosion of copper (Cu 99,9 E), brass (CuZn 37), zinc (97,5) and aluminum alloy (AlCu4Mg1) has been studied as well and considerable decrease of the corrosion rate by the inhibitor mixtures has been established. Nitrilo-trimethylene phosphonic acid, in combination with phthalic acid anhydride and zinc ions can therefore be considered as a perspective inhibitor for the corrosion protection of industrial cooling and heating water installations.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 2: Chemische Korrosionsuntersuchungen mit dem Rotations-Zylinder und zum Schutz gegen Spaltkorrosion

Testing the corrosion protection of heat-transfer fluids Part 2: Chemical corrosion tests using the rotating cylinder and for the protection against crevice corrosion The second part of the report refers to chemical corrosion tests of the protective effect of inhibited heat-transfer fluids provided with anti-freeze mixtures of water and glycol on metals by means of the rotary cylinder testing method. The metals to be tested were plain steel, cast iron, wrought and cast aluminium alloys, copper materials as well as a solder alloy. Moreover the influence of time, dilution ratio and flow on the corrosion protection of some metals was investigated. Based on the test results of part one and two, standard testing methods and limiting maximum values were defined for the evaluation of the protective effects proved by these testing methods. Finally investigations for the protection against crevice corrosion are described. This publication will end with a third part reporting on electrochemical investigations according to the RZ-testing method.     

Corrosion in solar heating systems. I. Copper behaviour in water/glycol solutions

A research programme has been developed in order to investigate the corrosion behaviour of metallic materials commonly used in solar heating systems. This paper presents the results of an experimental study on copper corrosion resistance in ethylene and propylene glycol/water solutions (1:1 by volume) constituting the most common bases of heat transfer fluids. Long time gravimetric tests were carried out on electrolytic copper at 80°C, even in glycol/water solutions previously degraded at their boiling temperature or polluted with 200 ppm chlorides. Chemical compositions, semiconducting properties and morphological characteristics of all surface products were investigated by X-ray diffraction analysis, pulse photopotential technique and SEM observations, respectively. Heat transfer effects on copper corrosion and copper/6351 aluminium alloy couple efficiency were evaluated by electrochemical tests. The following results were obtained:

• – Ethylene and propylene glycol/water solutions are low corrosive media. Nevertheless, chloride pollution and/or high temperature degradation of glycols markedly increase their aggressivity. Under all the experimental conditions, copper corrosion rates are higher in ethylene than propylene glycol solutions.
• – In chloride-free solutions, heat transfer stimulates the cathodic reaction of the copper corrosion process.
• – Galvanic contact between copper and aluminium alloy always causes pitting corrosion on aluminium electrodes. The severity of the pitting attack is enhanced by the presence of heat transfer conditions on copper and/or chloride ions in the solutions, particularly in ethylene glycol.

     

Mößbauer- und ESCA-Untersuchungen zur Bildung oxidischer Eisenphasen in wäßrigen Medien unter der Einwirkung organischer Korrosionsinhibitoren

Mössbauer and ESCA investigations on the formation of oxidic iron phases in aqueous solution under the influence of organic corrosion inhibitors Corrosion layers on steel grown in water of well defined hardness and chloride concentration were studied by Mossbauer and ESCA spectroscopy with particular emphasis on the influence of added organic inhibitors. Relatively thick layers were found with an unexpectedly small iron content (as FeOOH). The layers contain a remarkable amount of constituent ions from the solution and fragments of the inhibitors. The latter seem to be decomposed by the corrosive medium: It is assumed that the whole organic molecule determines the kind of transportation of the inhibitor to the iron metal, but that the inhibition itself is due to functional groups only.     

Simulation atmosphärischer Korrosion durch dünne Elektrolytfilme

Simulation of atmospheric corrosion by thin films of electrolyte A method for investigation on atmospherical corrosion with the aid of thin electrolyte films is described, the special feature of this being the exact adjustability of the corrosion determining parameters. The investigations on the influence of pollutants showed that small additions of sodium chloride and sulphur dioxide into the water film cause two different types of corrosion of electrolytic copper: uniform growth of layer thickness and/or lateral surface growth of the corrosion products. In the initial phase the uniform growth of layer thickness on electrolytic copper depends on t² (t = time), later on proportional on t. The investigation of the corrosion in dependence on the thickness of electrolyte film showed new results regarding the transition of the bulk-electrolytical to thin film electrolyte corrosion (atmospheric corrosion). At electrolytic copper transition from uniform growth of layer thickness to lateral surface growth comes off, when reaching a film thickness of approx. 100 μm. The electrolyte was distilled water with an addition of 0,01%SO₂. The corrosion intensity of grey cast iron GG 25 under a film of pure water passes through a wide maximum of film thickness between 300 and 20 μm, with a weakly marked minimum on 100 μm, the corrosion intensity is decreasing at thinner films. It is herewith demonstrated that also under conditions excluding largely the convective oxygen transfer other corrosion types and -intensities show up when the films are only thin enough. The comparison of the effect of two vapour inhibitors, having been dissolved directly in the electrolyte films (dicyclohexyl-ammoniumnitrite and one usual in the trade of unknown composition) in different concentrations showed on grey cast iron GG 25 the superiority of the latter one.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 1: Chemische Prüfungen,Screening- und Spezial-Tests

Testing the corrosion protection of heattransfer fluids Part 1: Chemical, screening and special tests As supplement to AGK Arbeitsblatt W l “Corrosion testing in heat-transfer fluids of solar heating installations” the different methods applied for testing the corrosion protection in heattransfer fluids provided with anti-freeze mixtures on glycol basis have been checked. The coupon tests corresponding to ASTM D 1384, the EMPA-Test, special tests according to ASTM D 4340 as well as the FVV-Heat-Test were part of these investigations. The investigations were carried out with steel, cast iron, two wrought aluminium alloys, one cast aluminium alloy, with copper and brass as well as with tin solder in non inhibited as well as inhibited heat carriers. The non inhibited heat-transfer fluids were composed of water with ethylene and propylene glycol, the others were made up with antifreeze concentrates prefabricated by well known producers. All investigated products provide excellent protective qualities against corrosion. The coupon test according to ASTM D 1384 has been the most effective of all methods tested up to now. This article will be continued.     

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.     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

Korrosionsreaktiorren von elementarem Schwefel mit unlegiertem Stahl in wäßrigen Medien

Corrosion reactions between elemental sulphur and plain carbon steel in aqueous media Plain carbon steels are rather severely attacked by elemental sulphur at room temperature in the presence of aqueous media. The corrosion occurs preferentially at the places where the two solid substances iron and sulphur are in contact with each other and results in shallow pit formation. At the same time the pH is also decreased slightly and small amounts of H₂S and sulphate ions are formed. Neutral salts stimulate the corrosion process whereas phosphates inhibit it and the alkaline media such as Na₂CO₃ and ethylamine with pH > 12 prevent it completely. The latter can be made use of for corrosion protection. At high salt concentrations (c > 1 mol/l) the corrosion rate, however, decreases with increasing salt concentration. The corrosion rate may increase with increasing flow velocity of the medium, but the corrosion takes place uniformly. The results of electrochemical investigations show that the reduction of sulphur occurs at the corrosion product FeS and is the rate controlling step. No sulphur reduction is observed on platinum electrodes when no FeS is present. It is assumed that the starting reaction to initiate corrosion in the system Fe/S/H₂O is a slight disproportionation of S to H₂SO₄ resulting in the formation of FeS.     

Corrosion in solar heating systems. II. Corrosion behaviour of AA 6351 in water/glycol solutions

A research was carried out in order to investigate the corrosion behaviour of the metals most commonly used as construction materials for solar absorber plates. With this view, an attempt was made to test the corrosion resistance of the aluminium alloy AA 6351 (nominal composition: 1% Si, 0.6% Mg, 0.3% Mn, the balance Al) towards common uninhibited heat transfer fluids, such as ethylene and propylene glycol/water mixtures. Long time gravimetric tests consisted in up to 60 day exposures of the aluminium specimens to pure, chloride-polluted, or degraded glycol/water solutions, at the temperature of 80°C. The degradation into acidic products, experienced by heat transfer liquids in service, was simulated by keeping the ethylene and propylene glycol/water solutions at their boiling temperature for 30 days, in contact with copper. In glycol/water solutions the presence of chlorides at low concentration (200 ppm) caused the aluminium corrosion rates to increase by more than one order of magnitude, while in degraded solutions, containing 143 or 86 ppm cupric ions, corrosion rates higher than two order of magnitude with respect to pure solutions were obtained. During the gravimetric tests, pitting corrosion was observed in some cases and its extent was rated by evaluating the deepest and the average metal penetration, the pit density and the average pit size. The influence of heat transfer on the alloy AA 6351 corrosion and on the couple copper/AA 6351 efficiency was evaluated by gravimetric and electrochemical tests. Heat transfer through aluminium was found to significantly increase the aluminium alloy pitting potential. On the contrary, it stimulated the aluminium galvanic corrosion, when applied on either aluminium or copper. Under galvanic coupling conditions, the aluminium corrosion rates calculated from the average galvanic currents were a very little contribution to the gravimetric corrosion rates. This demonstrates that in low conductive solutions the risk of matching such dissimilar metals as copper and aluminium does not reside in the galvanic contact itself, but mainly in the mere presence of the noblest metal in the same solution where aluminium is immersed.     

Korrosionsinhibitoren für Kupfer – Eine vergleichende Untersuchung unter besonderer Berücksichtigung oberflächenanalytischer Methoden

Corrosion inhibitors for copper – A comparative investigation with emphasis on surface analysis methods Surface and micro analysis methods like ESCA, AES, SIMS, ISS and LAMMA have been used to determine the composition and the thickness of the protective layers formed by adsorption of benzotriazole, tolyltriazole, mercaptobenzothiazole and 2-(5-aminopentyl)-benzimidazole on copper in drinking water and in artificial sea-water. Gravimetric and electrochemical corrosion tests have been carried out to study the effectiveness and the mechanism of these inhibitors. It is found that all the inhibitors form effective protective layers on copper after very short immersion time. The thickness of the layers is 1 to 2 nm and depends on the immersion time. The effect and the mechanism of these inhibitors are discussed.     

Klärung der Einzelmechanismen bei der Schädigung von Kupfer unter Kavitationsbeanspruchung in wäßrigen Medien

Study of the individual mechanisms giving rise to the destruction of copper under cavitation in aqueous media An experimental method developed by the authors enables the overall destruction mechanism during cacitation corrosion to be subdivided into the two main components “mechanical destruction” and “corrosion”. With the aid of impressed current it is then possible in a defined way largely to control the ratio of the two components of attack. Mechanical sollicitation due to the successive formation and collapse of bubbles in the liquid corresponds to the pattern found in corrosion fatigue; the authors have indeed found the phenomenological pattern of corrosion fatigue in some systems (grain boundary attack by electrochemical action, glide line attack by mechanical sollicitation).     

The Inhibition of the Iron Corrosion in aqueous solutions containing oxygen

The corrosion behaviour of iron and steel in aerated aqueous 0.5 M Na₂SO₄ solutions in the range 7 ≤ pH ≤ 9 was studied on rotating disc electrodes. The polarization curves measured cyclic voltammetrically or under steady-state potentiostatic conditions show a membrane inhibition effect caused by the time-dependent formation of three-dimensional porous oxide layers on the electrode surface. In presence of the inhibitors hexane (1, 6) biphosphonic acid, sodium-dihydrogenphosphate, Preventol VP OC 2003 and Aktiphos the inhibition effect is markedly enhanced leading to more homogeneous and compact protecting layers.     

铸铁及其表面碳化钨涂层的电化学行为

考察了甲醇汽油发动机缸套材料铸铁及其表面碳化钨涂层在甲酸水溶液中的腐蚀性能,并研究了其电化学腐蚀行为;采用电化学腐蚀中的极化曲线和电化学阻抗来表征其电化学腐蚀性能。结果表明：碳化钨涂层可有效地改善甲醇汽油发动机缸套材料铸铁的耐腐蚀性能;与铸铁相比,碳化钨涂层的自腐蚀电位高、自腐蚀电流小;同时碳化钨涂层使铸铁表面电荷的传输电阻变大,抑制了铸铁在甲酸溶液中的自放电功能,阻碍了电化学腐蚀的进行,从而有效地保护发动机缸套材料铸铁。     

4种不同金相组织的仿古铸铁在土壤中局部腐蚀行为的差异

为了加强铸铁文物的保护，采用动电位扫描极化、模拟闭塞电池、交流阻抗等电化学方法，以及光镜、扫描电镜等显微组织分析手段，研究了4种仿古铸铁在模拟土壤介质（0．06mol／L NaCl＋0．03mol／L Na2SO4＋0．01mol／L NaHCO3溶液）中的局部腐蚀行为，详细探讨了组织差异对仿古铸铁局部腐蚀的影响。结果表明：珠光体灰口铸铁的自腐蚀电位负移最为明显，在模拟土壤条件中裂纹和腐蚀孔洞相对其它几种仿古铸铁较多。在闭塞电池中，仿古铸铁的pH值下降，Cl^-和SO4^2-增加，仿古铸铁耐蚀性依次递减的顺序为白口铸铁、麻口铸铁、珠光体＋铁素体灰口铸铁、珠光体灰口铸铁。     

Zur Inhibition der Eisenkorrosion in wäßrigen Lösungen

Inhibition of the corrosion of iron in aqueous solutions The inhibition effect of propan(1,3)-biphosphonic acid and hexan (1,6)-biphosphonic acid on the corrosion of highly pure iron (polycrystalline, recrystallized) in aqueous sulfate solutions in the range 4 ≤pH ≤6 in presence and absence of oxygen has been studied at T = 298 K. Steady state and cyclic voltammetric measurements on rotating disc electrodes have been carried out. Both substances act as interface inhibitors. Their measured inhibition effects at the corrosion potentials are relatively low. The inhibition mechanism is discussed on the basis of kinetic data and the well known anodic iron dissolution mechanism.     

Aminomethylenphosphonsäuren als Korrosionsinhibitoren für Aluminiumpigmente in wäßrigen Medien

Aminomethylenephosphonic acids as corrosion inhibitors for aluminum pigments in aqueous media Flakelike aluminum pigments (aluminum content >99.5%) are corroded in aqueous alkaline mixtures of water and butyl glycol. Aminomethylenephosphonic acids inhibit this corrosion reaction. The temporal progress of the corrosion is determined by volumetric analysis of the evolved hydrogen. Direct comparison of aminomethylenephosphonic acids with analogous aminomethylenecarboxylic acids shows that the phosphonic acid group inhibits the corrosion significantly better than the carboxylic group. The actual corrosion inhibitors are in both cases (soluble) aluminum(III)-chelates, which are formed by a chemical reaction of the aminomethylenphosphonic resp.-carboxylic acids with the aluminum surface. This is proved by measuring the concentration of dissolved aluminum(III) in the corrosion medium with the help of atomic absorption spectroscopy and by using aluminum(III)chelates as corrosion inhibitors.