Korrosionsschaden durch Elementbildung – Fallbeispiel aus dem Stahlwasserbau

Corrosion damage by element formation – example from hydraulic steel structures Hydraulic steel structures with huge costs of investments generally are protected against corrosive deterioration. Usually the protection is guaranteed by application of a coating system based on polymeres. Additional application of priming coats can raise the efficiency. Research on a weir plant in this work however show, that even protection by a high performed coating system does not necessarely lead to a prevention of corrosion. Immediately there are no hints of a general denial of the applicated coating systems or any causal influence by the immersion medium (river water). The weir plant is constructed by high alloyed steel in combination with carbon steel, partly welded one to another. Thereby the premises of corrosion by galvanic elements are given: an electronic conductivity through the phase boundary of the different types of steel as well as an ionic conductivity by the immersion medium. Delaminations of the coating of the weir gate can be explained by that as well the anodic iron desolution (pitting corrosion) of the carbon steel. Even a renovation of the coating system by an inclusion of the alloyed steel did not stop the corrosion as a whole. An essential improvement of the protection efficiency finally was reached by the installation of a cathodic protection system with sacrificial anodes. With the help of Ohm's and Faraday's law the rate of iron desolution on the one side and the consumption of sacrificial material on the other side can be estimated.     

Untersuchungen an einer zahnärztlichen Gold-Platin-Legierung

Study of a dental alloy based on gold platinum alloy The alloy which contains a certain quantity of silicon is composed of mixed crystals with inclusions of intermetallic compounds. During thermal treatment (for degassing) a surface layer of SiO₂ is formed. This layer acts on the one hand as an adhesive for the dental porcelaine and on the other hand in contact with saliva is transformed into a colloidal condition which is required for appropriate physiological condition.     

Die Inhibition von Kühlkreisläufen

The inhibition of cooling systems The choice of an efficient corrosion inhibitor has to take in account the type of the cooling system, the materials used as well as the specific operating conditions, for instance

• – very low or very high velocities of the cooling water
• – heat transfer
• – the inhibitor may be asked to have some lubricating properties
• – vibrations, that can cause cavitation or fatigue corrosion
• – interaction between the corrosion inhibitor and other electrochemical systems of corrosion protection (cathodic protection).

In numerous applications it is necessary to improve the inhibition by an addition of other effective substances (like dispersants, hardness stabilizers, mixture of inhibitors with selective protection properties).     

Kavitationskorrosion an einer Aluminiumlegierung

Cavitation corrosion of an aluminium alloy The test carried out with an aluminium alloy (ALCuMg 2) in water with synthetically obtained hardness (10° dP) yields the conclusion, that it is not feasible to exactly separate the corrosion losses into those due to corrosion and those to be attributed to the cavitation damage; beyond that the results demonstrate that it is not possible to cathodically protect aluminium in these conditions. These facts are attributed to a mechanism according to which under current flow a surface layer would be formed of oxides and hydroxides being inferior in resistance to the naturally formed oxide layer; consequently, this layer would be removed more easily by the effect of cavitation. It is possible, however, to achieve a certain protective action by addition of an anticorrosion oil, which seems to obstruct the transformation of matter and, consequently, the formation of corrosion products of poor adhesion, Beyond that it is possible that these inhibitors also produce a damping effect on the shocks and, consequently, on cavitation damage.     

Diagnose von Korrosionsfällen an einer Saturationskolonne für Synthesegas

Diagnosis of case histories from a synthesis gas saturation column The saturation column, made of steel type 1Cr18Ni9Ti (wall thickness: 8 mm), in which the gas is cooled down to 240°C by a water jet and, simultaneously saturated with water vapour, failed after only 3000 hours' operation; the failure was attributable to a damaged footing tube. An analysis of the damaged parts suggested, that the main cause of the failure was not the design of the equidistant shells, i. e. the welding connection between the tubes and the two shells, which had originally been assumed to be faulty, but in a first line the material selection which was wrong in view of possible corrosion. The tensile stresses resulting from the rigid connection of the footing pipes are but one of the factors contributing to the development of stress corrosion cracking. This opinion concerning the reason of the first failure was confirmed when another failure occurred after another 1000 hours' operation of the column: a thorough inspection revealed some spots where stress corrosion cracking bad developed.     

Langzeit-Korrosionsuntersuchungen an austenitischen Stählen im Hochdruckteil einer Harnstoffanlage

Longterm corrosion field tests with austenitic steels in the high pressure parts of a urea plant In view of the conditions encountered in urea manufacture, in particular in the high pressure section the corrosion resistance is investigated using various austenitic steels immersed in reaction mixtures typical of urea manufacture. The specimens were placed in an operating plant; different test durations (total 13 000 hours) enable the time dependence of the attack to be assessed. The steels tested were CrNi steels with nitrogen and 17.45?19.8% Cr and 10.3-12% Ni, CrNiMo steels, some of them with Ti and Cu addition and 17% Cr and 20 and 13.8% Ni, respectively. According to the results the corrosion resistance increases with the chromium level while ferrite in particular with longer test durations has a pronounced negative effect. A suitable screening test for corrosion resistance under the conditions of urea manufacture is testing in boiling nitric acid.     

Einfluß von Schutzfremdströmen auf die Wirkung von Kühlwasserinhibitoren

The influence of imposed protection currents on the efficiency of inhibitors for cooling waters In case where the cathodic protection of a cooling system is not feasible a combination of cathodic protection and inhibition may be of advantage. The authors have studied the potential influence of a direct current on the efficiency of an inhibitor; during the experimental work they have used a current intensity corresponding to the maximum intensity of the protection current used in practice. Using potentiokinetic methods they have found that the number of inhibitors which may be used in such conditions is rather small. In the case of inhibitors based on nitrites or amines the protecting efficiency is rather reduced after some hundred hours' operation under direct current, and in some case even a corrosion stimulation has been experienced. Inhibitors based on chromates and anticorrosion oil emulsions exhibit improved electrochemical stability, though, in the case of oils emulsion breaking must be taken into account after relatively short durations of operation.     

Laboruntersuchungen zur Korrosion von unlegiertem Stahl in Kühlwässern

Laboratory studies on corrosion of unalloyed steel in cooling waters Corrosion tests were carried out in recycle apparatures to prepare cooling-water treatment. The corrosion behaviour of unalloyed steel St 38b-2 in dependence on m-value of water was tested. The corrosion-inhibiting effect of sodium polyphosphate, zinc sulfate, 1-hydroxy-1,1-diphosphonic acid (HEDP) and the trisodium salt of HEDP and polyacrylate and their combinations in water with m-value 1.0–1.5 mmole per litre was tested in comparison with unalloyed steel. The corrosion rate increases strongly below a m-value of 2.2 mmol per litre. Sodium polyphosphate, sodium polyphosphate/zinc sulfate and zinc sulfate-polyacrylate show good protection features in water with m-value 1.0–1.5 mmole per litre. The trisodium salt of HEDP has an insufficient effect.     

Korrosions- und Kavitationsverhalten von Leichtmetallen in Kühlkreisläufen von Verbrennungskraftmaschinen

Corrosion and cavitation behaviour of light metals in cooling systems of combustion engines Combustion engines are liquid-cooled, in general, in order to transport away dissipated heat and to avoid overheating. Initially, pure water without any further additives was employed for this purpose. With the introduction of high-speed marine diesel engines, however, sometimes after only 20 000 operating hours there occurred damage at the cylinder liners that had been exposed to the cooling water. More than forty years ago, the Research Association Internal Combustion Engines, Registered Association (FVV), Frankfurt am Main, which was founded in the year 1956, commissioned the Institute of Materials Science of the Darmstadt University of Technology to investigate and, if possible, clarify damage of this type and to elaborate corrective action. Since that time, the problem of corrosion in cooling systems has been worked on.     

Untersuchungen an Korrosionsprodukten und Ablagerungen in Kühlrohren aus Kupfer und dessen Legierungen mittels Röntgenbeugung und Spektrophotometrie im Infrarot

Investigations on corrosion products and deposits in cooling pipes of copper and its alloys by means of X-ray diffraction and spectro-photometry in infrared The results are communicated of 41 investigations on pipes carrying fresh water, mainly in Swiss hydro-electric power stations. The deposits most frequently encountered were mica, quarts and calcite, the most frequent corrosion products cuprite and malachite. On SoMs 71 (?Admiralty Alloy”?), zinc hexahydroxostannate ZnSu(OH)₆ was repeatedly observed, and compared 'with the synthetic product. The corrosion product found on SoMs 71 and CuNi 30 Fe was the Berthollide compound 3 Cu(OH)₂·CuSO₄·1.5 (?) H₂O which was termed parangalte. This compound was also produced synthetically and in corrosion tests. The paper also reports on langite as a synthetic and corrosion product. On SoMs 76 (aluminium brass) used for a cooler working with sea water, Zn₆Al₂CO₃(OH)₁₆ 4 J₂O (?) was found in analogy to hydrotalcite. X-ray infraction data and infrared spectra of these products are quoted.     

Erosionskorrosions-Untersuchungen in einer Naßdampfversuchsstrecke

Erosion corrosion in a wet steam loop In a technical loop of a power plant erosion corrosion behaviour of the steels C 22.8, 15 Mo 3 and 10 Cr Mo 9 10 was investigated in test periods up to 78 days under definite conditions in wet steam of a relative steam content of about 75% at a temperature of 185 to 200°C and flow velocities up to 180 m/s. For comparison one test was carried out also in hot steam. By gravimetric, metallographic, SEM- and Xray-diffraction analysis essential information was obtained on the kinetics of erosion corrosion and the structure of oxide coatings. The rate of erosion corrosion shows a high dependence on time and steel composition and decrease in the above mentioned order. Structure and quality of the oxide layer also show a remarkable dependence on steel composition.     

Kühlwasserseitige Korrosionsbeständigkeit von metallischen Werkstoffen zur Handhabung von Schwefelsäure

Cooling water side corrosion resistance of high alloyed materials for handling of process side sulfuric acid The approved materials for use in sulfuric acid alloy 825 (German material No. 2.4858) and alloy 20 (German mater. No. 2.4660) have only a low resistance against localized corrosion in chloride containing water and are unsuitable for handling of sulfuric acid. The newly developed austenitic Cr-base alloy, alloy 33, (X1CrNiMoCuN 33-32-1, German mater. No. 1.4591) with 33 % Cr, 31 % Ni, 0,6 % Mo and 0.4 % N should have an excellent resistance against pitting and crevice corrosion additional to its high sulfuric acid resistance, too, because its Pitting Resistance Equivalent No. calculated according to PREN = %Cr + 3,3 · %Mo + 30%N runs to 50. Pitting and crevice corrosion properties of the alloy 33 are tested in comparison to those of reference materials in high chloride containing solutions (1M NaCl, artificial and modified sea water, 10% FeCl₃ · 6H₂O; 500 g/l CaCl₂ ). Pitting potentials and potentials of repassivation of pitting, critical temperatures of localized corrosion (FeCl₃-test, CaCl₂-test, artificial sea water), potentials of repassivation of crevice corrosion as well as depassivation pH values of crevice corrosion following Crolet have been determined. The results confirm that the localized corrosion behaviour of the alloy 33 corresponds to its PREN. With regard to pitting corrosion alloy 33 is comparable with the special stainless steel alloy 31 (mater. No. 1.4562), with regard to crevice corrosion it is comparable with alloy 926 (German mater. No. 1.4529).     

Die Abhängigkeit der Korrosion von Kohlenstoffstahl von der Zusammensetzung des Kühlwassers

Relation between the corrosion of carbon steel and the composition of cooling water The rate of uniform corrosion of carbon steel depends on the calcium, magnesium and bicarbonate ion content, the pH and the temperature of the cooling water. This dependence has been expressed as a mathematical relation which yields an accuracy of ±30% for longterm measurements. On principal it appears feasible to formulate the relation between corrosion and pH_s and I_s but the error would be significantly increased. So far it has not been possible to find a relation between the cooling water composition on the one hand and the formation of shallow grooves and corrosion pits on the other.     

Metallkundliche und elektronische Untersuchungen über die interkristalline Korrosion an einer AlMg-9,56 Knetlegierung

Metallographic and electrochemical investigations into intercrystalline corrosion of AlMg 9,56 wrought alloy The intercrystalline corrosion has been studied with a 50% cold worked AlMg9,56 wrought alloy in 1 N sodium chloride solution; the metal has been used in various heat treatment states. Electron transmission photographs were made of the grain boundary precipitates formed by heat treatment. The electrochemical behaviour was studied by potentiostatic and galvanostatic methods. The corrosion behaviour of a heterogeneous alloy having a high proportion of intermetallic AlMg ß-equilibrium phase was compared with the behaviour of heat treated specimens of the alloy. The current potential curve could be traced for the ß-equilibrium phase. Indications were found of two different rate controlling and potential dependent reactions for the dissolution of the ß-equilibrium phase and of the grain boundary precipitates of the heat treated wrought alloy. In the course of heat treatment two different typs of precipitates are found which show different corrosion behaviour. After a short time heat treatment the grain boundaries are covered with continuous precipitates of unknown composition. After extended precipitation coagulation takes place which results in isolated globulites of the intermetallic ß-equilibrium phase. Intercrystalline corrosion of the AlMg9,56 wrought alloy is thus consistently due to the selective corrosion of grain boundary precipitates.     

Einfluß der Kationenzusammensetzung von Chloridlösungen beim Modellieren von Korrosionseinflüssen der Küstenatmosphären

Influence of the cation composition of chloride solutions in modelling corrosion effects due to caoastal atmospheres Pure sodium chloride solutions cannto be used for accelerated weathering tests in artificial sea atmosphere. A spray liquid suitable for such tests is artificial seawater containing (grams/litre) 27 NaCl, 6 Mg chloride, 1 Ca chloride and 1 KCl. The corrosion behaviour of the two metals studied – iron and zinc – is characterized by typical differences, in particular variations depending on the salt quantity applied. While the corrosion of iron at first increases with the salt concentration applied and achieves a maximum at about 1000 mg/m²d, the losses of zinc increase just to a salt quantity of 200 mg/m² d and are then practically constant up to about 1000 mg/m²d; at higher concentrations there is again a slight diminuation of corrosion. The corrosion in artificial seawater is more pronounced than in sodium chloride solutions; this fact can be attributed to the hydrolysis of the chlorides in seawater, which gives rise to the formation of anodic and cathodic regions on the metal surface; as a consequence, pH differences are created and corrosion is enhanced. In order to simulate natural conditions as closely as possible the application of the salt spray should be interrupted by extended dry periods.