Sicherung von Schraubenverbindungen aus nichtrostenden Stählen

Securing screws of stainless steels The author emphasizes the importance of screwed connections in technology and discusses the conclusions which must be drawn from existing standards with respect to the utilization of stainless steel screws. Examples are shown to demonstrate the corrosion resistance of such connections (tests in sea water, 3% NaCl, tap water, salt spray) giving prominence to the problems resulting from the use of spring elements (crevice corrosion danger).     

Oberflächenbehandlung als Korrosionsschutzmaßnahme von nichtrostenden Stählen

Surface treatment as corrosion protection measure of stainless steels The pickling behaviour of several stainless austenitic steels and of one steel with ferritic/austenitic grain structure were investigated in pickling solutions of different compositions based on hydrofluoric acid. Because of uncertainties in practical applications, the influence of temperature, time and acid content on the mass loss is of high interest. In another series of experiments, aqueous solutions of citric acid were tested for their suitability as pickling chemicals for the materials X 6 CrNiTi 18 10 (AISI 321) and X 6 CrNiMoTi 17 12 2 (AISI 316 Ti). Finally, the pickling procedures based on nitric acid/hydrofluoric acid mixtures were compared with mechanical cleansing methods and with pickling procedures based on aqueous citric acid solutions as well, to elucidate their influence on the corrosion resistance of the treated materials. The valuation followed a pitting corrosion test in sodium chloride solutions of different concentrations after Herbsleb and Schwenk. Pickling with hydrofluoric acid solutions is superior to other cleansing procedures, if corrosive environments are present. The ecologically beneficial citric acid solutions are only able to remove the annealing colours from stainless steels.     

Stromdichte-Spannung-Messungen an nichtrostenden Stählen in aktivierenden Medien

Current density-potential measurements on stainless steels in active media In order to elucidate the susceptibility to selective corrosion of weld seams in stainless steels potentio-kinetic current density-potential measurements have been made. When the polarisation curve for the weld seam is more anodic in the vicinity of the rest potential than the curve corresponding to the base material the weld seam is selectively attacked; when, however, the weld seam is moer cathodic than the base metal there is almost no corrosion in the adjacent areas because of the large area ratio (large anode, small cathode). It is only in extreme cases (very high potential difference between cathodic weld seam and anodic plate) that local corrosion is found in the base metal. Tests with Hastelloy B in 20% hydrochloric acid show that this material cannot be welded successfully with identical material; it might, however, be feasible to weld it with cathodically alloyed weld fillers (alloyed with Pt or Pd).     

Korrosionsbeständigkeit von Verbindungen von Rohren aus nichtrostenden Stählen in Wässern

Corrosion of joints for stainless steel tubes in water The most important commonly used joining techniques for stainless steel tubes which are used for the transport of water and gases are welding and brazing. With corrosion attack by dry gases, both connections are resistant against corrosion. However, in water and aqueous condensates limits of application exist with regard to the corrosion resistance. The corrosion resistance of weld connections with stainless steel tubes is diminished by

• – annealing colours (oxide films) and scale layers in the weld area;
• – changes in the microstructure adjacent to the welds (sensitization of the stainless steel material);
• – surface finish of weld seams after welding;
• – welding faults resulting from bad handling and workmanship.

Type and extent of corrosion damage occurring on weld connections with decreased corrosion resistance depend on the composition of the water and condensates, mainly on their chloride content. Typical examples for the causes of degraded corrosion resistance of weld connections, and possible types of corrosion attack, namely pitting, crevice corrosion, and stress corrosion cracking and their mechanisms are described. Furthermore, measures are shown by which the corrosion resistance of weld connections with stainless steel tubes can be increased. Joints of stainless steel tubes by hard soldering with capillary fittings are endangered by knife line attack at the phase boundary between the stainless steel and solder (interfacial corrosion). Knife line attack means in this context the loss of adhesion between steel and hard solder. The severity of the corrosion risk, in particular the incubation time until the occurrence of the corrosion damage, depends on the water quality, mainly on chloride concentration, and pH. The press fitting with non-metallic gasket is a relatively new joining system, and it is used in the cold and warm water domestic installation. This joining technique is described. For domestic water distribution, an installation system with tubes and press fittings made of steel grade AISI 316 SS has been developed. This system is resistant to corrosion attack in potable water of usual composition, and it is already applied in-service in a considerable extent. Other joining systems are stainless steel weld fittings, threaded screw fittings, and compression couplings with cutting or clamping rings. They are used mainly in industrial installations.     

Chloridinduzierte Korrosion von nichtrostenden Stählen in Schwimmhallen-Atmosphären Teil 2: Einfluß von Hypochloriten

Chloride induced corrosion on stainless steels at indoor swimming pools atmospheres Part 2: influence of hypochlorite The work was started on the occasion of failures of stainless steel components at the indoor swimming pool atmosphere in Uster (Switzerland). Highly-alloyed stainless steels were tested at defined mechanical and environmental conditions. Therefore Ubend specimens with salt spots were examined at 40°C and 35 and 70 % rel. humidity respectively. The results of the experiments with the electrolyte magnesium-chloride (30 %) were presented in part 1. The high nitrogen alloyed steels 1.4529 and 1.4565 showed the best corrosion resistance. The highest corrosion attacks were observed at the steels 1.4401, 1.4462, 1.4439 and 1.4539. Stress corrosion cracking (SCC) was determined at the steels 1.4401, 1.4462 and 1.4439 at 35 % rel. humidity by metallographic methods. A distinctive relationship was observed between the pitting resistance equivalent and the kind of corrosion attack. Furthermore, short time experiments (< 1000 h) can be used to define a qualitative judgement over the occurrence of the kind of long time corrosion attacks at stainless steels.     

Optimaler Nickelgehalt von stickstofflegierten austenitischen Stählen

Optimum nickel content of nitrogen alloyed austenitic steels The authors have studied the effect of a reduction of the nickel content on certain properties which are important with a view to practical applications. The mechanical properties are but little affected by varying nickel contents. As to corrosion resistance, no noticeable influence has been found on pitting, crevice corrosion and acid resistance; the resistance to stress corrosion cracking is a little reduced, but these steels are not considered for such type of service. On the base of the results obtained the nickel content in certain standardized Swedish steels has been reduced: consequently, the steels SIS 2370, 2371, 2374 and 2375 contain – instead of originally 8, 9, 10,5 and 11% – now 7, 8, 9 and 9.5% nickel respectively.     

Untersuchungen über das elektrolytisch-potentiostatische Ätzen von nichtrostenden Stählen in 10 n NaOH unter Anwendung eines Coulometers

Investigations into the electrolytic-potentiostatic etching of stainless steels in 10 n NaOH, using a coulometer Current density/potential characteristics can only provide qualitative indications concerning the phenomena encountered with potentiostatic etching in 10 n NaOH. The process is best carried out in the trans-passive zone. Identification is based, not on the selective dissolution of certain standard components, but on the interference colours which can be observed as a result of the formation of cover layers of different thickness. The charge density/time characteristics also permit quantitative indications of the different phases. Current density/time curves permit a clarification of the mechanism govering the formation of cover layers. A ferritic steel (28 pC Cr) and a NiCr alloy (45 pC Cr) were found to follow a cubic law of growth, whilst CrNi steel 18-8 is initially governed by an approximately logarithmic law, followed by a zone extending over several hours where the growth begins to follow a parabolic law.     

Korrosionsuntersuchungen an nichtrostenden,austenitischen Stählen im System Essigsäure/Essigsäureanhydrid

Corrosion tests of austenitic stainless steels in the system acetic acid/acetic anhydride The corrosion behaviour of the austenitic stainless steels X6 CrNiMoTi 17 12 2 (mat.no. 1.4571, AISI 316 Ti) and X2 NiCrMo-Cu 25 20 5 (mat.no. 1.4539, alloy 904 L) was investigated by immersion tests of welded specimens in acetic acid and acetic anhydride and in mixtures thereof, as well. The tests were carried out at 80°C and at the boiling point of the test solutions. The test equipment was either open to the atmosphere or the mixtures were deaerated with nitrogen. The tests show that in the presence of air the mixtures are more corrosive than the neat media. At 80°C in aerated solutions only alloy 904 L (mat.no. 1.4539) performed well, except in a mixture of 70% acetic acid and 30% acetic anhydride. At the boiling point of the test solutions the corrosive attack on both stainless steels increases considerably; the corrosion rates are in the range of 1 mm/a (1.4571) and 0.6 mm/a (1.4539). Deaeration with nitrogen decreases the corrosivity of the solutions to the stainless steels. Electrochemical tests show the detrimental influence of acetic anhydride to the passivation process of stainless steels tested in glacial acetic acid.     

Selektive Korrosion an Verbindungsschweißen von Stählen

Selective corrosion at welded assemblies of steels The paper deals with the possible selective corrosion phenomena at the most important types of weld assemblies of steels, viz. those of unalloyed ferritic ship building steels, austenitic manganesechromium steals, aluminium-killed boiler steels an well as chemically resistant ferritic and austenitic chrom-nickel steels. With unalloyed ship building steels, a strong corrosion element is liable to occur between not fully descaled plates and scaleless welding material, where the latter is particularly liable to be attacked. But even where scalefree plates are welded, a local element may be set up, depending on the killing intensity and on the filler metals, between plate and welding material in such a way that, again, the welding material is liable to be dissolved first. In the case of austenitic manganese-chromium steels, exposed to sea water, intercrystalline corrosion takes place at a certain distance from the weld as a result of chrome-carbide segregation. In the case of welded assemblies of aluminium-killed steels, intercrystalline fissuration is particularly liable to occure in a more or less marked decarbonised zone at the transition. With ferritic chromium steals and austenitic chrom-nickel steels, the welds are decisively influenced by the segregation of chrome-carbides at the grain boundaries which results in a proneness to intercrystalline grain decomposition. Measures are discussed by which the segregation of chrom-carbides during welding can be avoided. Finally, mention is made of the effect of delta-ferrite on the corrosion behaviour of austenitic welding material.     

Untersuchungen zur Schwingungsrißkorrosion von Stählen in Meerwasser

Investigations of corrosion fatigue of steel in seawater Corrosion fatigue of two medium strength steels was studied using CT-specimens at 10, 1 and 0.2 Hz. The threshold values of the amplitude of stress intensity factor, ΔK₁, were lowered in synthetic seawater and in hydrogen gas. The advance of the cracks per cycle in general was maximum at the lowest frequency. The maximum was obtained at low amplitudes ΔK₁ in seawater at corrosion potential, at middle amplitudes in seawater with cathodic polarization and at high amplitudes in gaseous hydrogen.     

Spannungsrißkorrosion von Stählen in flüssigem Ammoniak

Stress corrosion cracking of steels in liquid ammonia The apparatus developped for the investigation of stress corrosion cracking of steel in liquid ammonia under controlled electrochemical conditions is described. The parameters of the experiments were set by a computer which also stored and correlated the experimental data. Cylindrical samples of the welding steel W. Nr. 1.0143 and of the steel STE 355 (W. Nr. 1.0562) in liquid ammonia containing ammonium chloride or lithium perchlorate as the electrolyte developped cracks only at negative electrode potentials in the regions of active dissolution and hydrogen deposition. Other parameters including contamination of the solution by air were unimportant. The results are explained by hydrogen induced stress corrosion cracking. No embrittlement was observed with passive samples. However, ultimate tensile strengths in liquid ammonia were clearly lower than at air also for samples breaking without formation of cracks. Experiments with notched sheets resulted in sharp, essentially transcrystalline cracks. Passivation of these samples was difficult in the region of the notch indicating the danger of anodic stress corrosion cracking.     

Betrachtungen über den Zusammenhang zwischen bakteriellem Wachstum und der mikrobiell induzierten Potentialverschiebung bei nichtrostenden Stählen

An examination of the interrelationship between bacterial growth and microbially induced potential shift of stainless steels Corrosion damage was found in an earlier examination of stainless steel piping used for cooling circuits at temperatures below 50°C. This attack was caused by a microbial slime layer which was responsible for the shifting of the open circuit potential to a region of instable pitting resistance. Both the bacterial cell densities of the biofilm and the final values of the open circuit potential in laboratory experiments showed temperature dependence. Furthermore, a comparison of the open circuit potential with the redox situation of the surrounding aqueous phase leads us to postulate a direct coupling between the potential shift and the growth of the bacterial population. According to these observations the cell density can only increase until a minimal potential difference is reached. This value is the limit necessary for maintaining the biomass.     

Korrosionsbeständigkeit von Installationsteilen aus nichtrostenden austenitischen Chrom-Nickel-Stählen bei erhöhter Temperatur

Corrosion resistance of installation elements of stainless austenitic chromium-nickel steels at high temperatures The test results presented prove the statements made in DIN 50 929 Part 2. In accordance with DIN 50 930 Part 4, the corrosion probability for stainless steels increases when chloride containing water evaporates on warm material surfaces, whereby chloride ions concentrate. Under these conditions, ferritic chromium steels and austenitic chromium-nickel steels can suffer pitting corrosion, austenitic chromium-nickel steels also stress corrosion cracking. In the latter case, the corrosion cracks start from pits. The molybdenum-containing material no. 1.4571 withstands pitting and stress corrosion cracking in wet, chloride-containing environments at 90°C over some weeks. With increasing exposure time and at temperatures above 45°C, however, corrosion damage cannot be excluded. Then, according to DIN 50 929 Part 2, coating of the external surfaces of installation components, e.g., tubes, is required. The coating must be thick, free from pores and holidays, and resistant to heat and aging.     

Korrosionsverhalten von unlegierten Stählen bei Berührung mit Baustoffen

Corrosion of unalloyed steels in contact with building materials Embedded in concrete no corrosion of steel occurs even in the presence of water as long as the water in the concrete pores is alkaline enough to maintain the passivity of iron. The passivity can be destroyed by chloride-ions, which may be components of concrete additives. Another possible reason for loss of passivity is the decrease of alkalinity due to the reaction with carbon dioxide. In contact with non alkaline building materials as e.g. insulating material corrosion is guided mainly by the access of water. A very high risk of corrosion damage occurs due to a galvanic element when steel in contact with non alkaline material is connected by metallic and electrolytic conductor with steel in concrete.     

Die Verzahnung von Zunder und Grundmetall in niedriglegierten Stählen

The indentation of scale and base metal in low-alloyed steels Surface defects due to role-in scale may be caused by the indentation between metal and scale, and possibly also by the concentration of fayalite at the boundary face. Both factors are favoured by high temperatures, i.e. temperatures of 1200° C and more. In the case of low-alloyed steels, the furnace atmosphere should be set for strong oxidation; in the case of steels with no more than “residual” nickel content, however, the atmosphere should be set for weak oxidation. During the early stages of oxidation, the oxide remains in contact with the metal itself, by “creeping after it”. Owing to the inward creeping of the scale, nickel is concentrated in a thin layer. As soon as the creeping is replaced by the inward diffusion of oxygen, the metal particles remain fixed in space whilst the boundary face continues to migrate further into the metal. The thickness of the expanded indentation zone is a measure for the quantity of scale produced by the inward diffusion of oxygen. The metal filaments lose iron to the surrounding oxide. The nickel concentration, on the other band, is associated with a reduction in volume. Molten fayalite-eutecticum faciliates the creeping of the oxide layer past the projecting filaments so that, in this case, the contact between oxide and metal is preserved.     

Lochkorrosion an Nichtrostenden Stählen

Pitting corrosion of stainless steels Stainless steels can get pitting corrosion in halide containing solution, which make them a big risk in industrial production. Many investigations were made in the past in order to understand processes involved in pitting corrosion, pit initiation and pit growth. Results about the influence of alloying elements, their contents, the state of the structure, the condition of the surface, the content of chloride, the temperatures, the pH-value, the velocity of flow and of the oxidizer on the chloride induced pitting corrosion of passive stainless steels are presented. Electrochemical measurements and the application of surface analytical methods (SEM, SAM, XPS) with high lateral resolution are carried out. A part of the samples received a diffusion annealing in order to obtain reproducible results. Pitting Resistance Equivalents (PRE) – Pitting Index – with different multipliers are given and discussed critical. An electrochemical method for selecting materials without susceptibility to pitting corrosion are also presented.     

Messerlinienkorrosion von automatisch hergestellten Schweißverbindungen aus korrosionsbeständigen Stählen

Knife-line corrosion of weld connections of corrosion resistant steels, produced with automatic welding equipment The investigation of submerged arc weldings (obtained with automated equipment) and electro-slag weldings (obtained by manual methods) of a steel of CSSR production and of AISI 316 L steel have revealed, that the behaviour of both is practically identical, i.e. that both of them are not susceptible to knife-line attack (testing in standard solutions, i.e. sulfuric acid/copper sulfate and 65% nitric acid). In the case of the submerged arc weldings the damaged zone is some what narrower, but in either case the cause of attack in the adjacent region is due to overheating to a temperature close to the solidus line. An important influence must be attributed to heat treatment: after sensitizing, the weld metal, too, may become susceptible to intercrystalline corrosion, while after certain types of heat treatments even sensitizing has no deleterious effects.     

Wasserstoffinduzierte Korrosion von niedriglegierten Stählen in wäßrigen Medien

Hydrogen induced corrosion of low alloy steels in hydrous media The absorption of hydrogen from media can lead to cracking in steels with a ferritic structure. The interaction of absorbed hydrogen and steel is governed by the hydrogen activity and by stress- and structure-dependent threshold values. Different types of hydrogen-induced corrosion are illustrated by means of tests performed on lowalloy structural steels. Surface blistering and internal cracking can occur on the one hand without external stresses. In the case of hydrogen-induced stress corrosion cracking on the other hand a differentiation is made between corrosion systems in which constant loading displays already cracking and systems in which the cracking is tied to slow strain rates within a critical range.     

Korrosionselemente durch Gefügeheterogenität in nichtrostenden Chrom-Nickel-Stählen

Corrosion elements set up by structure heterogeneity in stainless chromium-nickel steels Transformation processes and precipitation of carbides could be the reason for corrosion elements in austenitic stainless steels. Examples are the intergranular corrosion and the selective martensite corrosion. The intergranular corrosion is based on a change of the electrochemical properties of the grain boundary regions by impoverishment of chromium. This “classical intergranular corrosion” takes place at all potentials. Resulting from the impoverishment of chromium after colling to very low temperatures, or by cold deformation, an α′-phase can be produced. This phase is only attacked in the active region of the current density-potential-curve. Both the intergranular corrosion and the corrosion of martensite are caused by local anodes. A selective attack of grain boundaries is also observed in the transpassive region. The intensity of this attack increase with precipitation of carbides. During the sensitivisation cathodes are produced, which cause a polarisation in the intergranular corrosive regions. Therefore, this attack of grain boundaries is not only connected with the formation of local anodes but also influenced by the formation of local cathodes.     

Dehnungsinduzierte Spannungsrißkorrosion an Stählen

Strain-induced stress corrosion cracking of steels Characterization of mechanical influencing factors and their effects on the corrosion of low alloy and high alloy steels. Stress- and strain-induced corrosion of steels. Influence of strain on the occurrence of stress corrosion cracking of low alloy steels in nitrate-, carbonate- and sodium hydroxide solutions and of high alloy steel in magnesium chloride solution. Significance of critical strain-rates.