Die Feuerverzinkung siliziumhaltiger Stähle – Problem und Lösungsmöglichkeiten

Hot dip galvanizing of silicon containing steels – the problem and some attempts to its solution Hot dip galvanizing of silicon containing steels, which are increasingly used today, involves the danger of very thick, poorly adherent, unsightly zinc coatings forming on the steel surface. Quite a number of attempts have been made to prevent this drawback. Chemical surface treatments of the steels to be galvanized did not yield the desired success, whilst mechanical surface treatments by shot blasting gave positive results in certain cases only. Galvanizing in the upper temperature range of conventional working practice (475–485 °C) appears appropriate for steels containing 0.15–0.20% Si, whereas low-temperature galvanizing at 430–440 °C using conventional galvanizing baths should be universally applicable provided that suitable methods are found for pre-heating parts of larger dimensions. Galvanizing with inhibited attack of the zinc caused by small additions of aluminium (less than 0.2%) did not yield coating thicknesses independent of the Si content of the steel. High-temperature galvanizing at more than 530 °C gives satisfactory coatings irrespective of the Si content of the steel; saturation of the galvanizing bath with iron should, however, be avoided. Low-temperature galvanizing at less than 430 °C is possible on principle, provided that the melting point of the zinc is reduced by adding alloying elements such as tin (10–30%), magnesium (3%), or aluminium (4–5%). However, tin is expensive, magnesium deteriorates the wetting properties of the steel, and aluminium requires a very careful pre-treatment, the coatings obtained exhibiting low thicknesses and dependency on the Si content of the steel with iron-saturated baths. Galvanizing with reduced zinc concentrations in zinc-saturated lead baths gave zinc coatings independent of the Si content, while currentless galvanizing in zinc-saturated salt baths was not successful.     

Hochlegierte nichtrostende Stähle und Nickelbasislegierungen im Chemie-Apparatebau

High alloy stainless steels and nickel-base alloys in chemical equipment construction The new ferritic chromium steels and ferritic-austenitic steels, high alloy special austenitic steels and nickel-base alloys presented in recent years have by now yielded some years' positive industrial experience in several applications. The group of austenitic steels containing 6 wt.-% molybdenum are at the threshold for large size application in chemical equipment construction. The results of the first comparative investigations with other high alloy steels are rather promising. The spectrum of nickel-base alloys will certainly be complemented by Hastelloy alloy C-22, so that this alloy will be used in addition to titanium alloys in contact with strongly oxidizing media.     

Gegen die Spannungsrißkorrosion beständige nichtrostende Manganchromstähle

Stress-corrosion resistant stainless manganese chromium steels The following conclusions may be drawn from the results of investigations into the stress corrosion cracking of austenitic and austeno-ferritic MnCr steels (19–22Mn, 13–18Cr, additions of Mo, V, Nb, Ti, N, B): Addition of nitrogen gives rise to a decrease of stress corrosion cracking resistance in magnesium chloride, sodium chloride with potassium dichromate and water at high temperatures. The same applies to the influence if nickel on corrosion in magnesium chloride and water, and for molybdenum in magnesium and sodium chlorides. From among laboratory melts the type 05 Mn 19Cr 13 had the highest resistance, followed by its modifications with additions of boron, vanadium, molybdenum, titanium, niobium and nitrogen. From among the semi-technical melts the nitrogen containing steels turned out to be least resistant, too. During further investigations the chromium level of 13% turned out to be insufficient to prevent pitting in sodium chloride solutions including seawater.     

Untersuchungen zur Lötbrüchigkeit hochlegierter Stähle

A study of the liquid mentla embrittlement of high allowy steels The action of zinc on austenitic stainless CrNi steels at high temperatures may give rise to a type of intercrystalline corrosion called “soldering brittleness” (liquid metal embrittlement) which can be attributed to the formation of a low melting phase rich in nickel. These phenomena have been known for a long time in connection with other metals and alloys and have been described by several authors. As to liquid metal embrittlement of high alloy steel, however, no data have been available so far.     

Korrosionsverhalten rostfreier Stähle gegenüber halogenhaltigen Lösungsmitteln

Corrosion behaviour of stainless steels in halogen containing solutions. Halogenated hydrocarbons are widely used for cleaning metallic objects. Since solvents of this type undergo hydrolyzation in presence of water, serious damage is apprehended especially for alloys known to be sensitive against chloride-induced stress corrosion. These tests were intended to check the risk of corrosion when halogenated hydrocarbons are used as a solvent for cleaning metallic surfaces, and if a residual fluid – trapped within small crevices or holes – is stressed by hot water or water vapour. Halogenated hydrocarbons have thus been proved to be dangerous only when metallic parts are exposed to the condensing solvent zone containing just small amounts of water. In excess of water, no corrosion or cracks could be observed. Although, according to results obtained, risks of corrosion can be kept at a low level, it is recommended to avoid the use of halogenated solvents in case the removal of trapped fluid is limited.     

Werkstoff- und Korrosionsprobleme in wäßrigen Lösungen bei hohen Temperaturen und Drücken

Materials and corrosion problems in aqueous solutions at high temperatures and pressures The present report, based on papers and discussions of an international conference on electrochemistry at high temperatures and pressures forms a survey of materials and corrosion problems at high temperatures and pressures. Among the topics of the conference there were, among others, corrosion problems in power stations with particular emphasis on high alloy chromium nickel steels and nickel base alloys which are preferentially used in nuclear power stations. Unalloyed and low alloy steels were lower on the scale of interests. A matter of particular interest, however, were investigations and findings on corrosion cracking failures, which may occur on chromium nickel steels and nickel chromium iron alloys under rather different conditions of heat treatment of the materials (solution annealed or sensitized) and of the corrosive medium. With reference to pertaining literature data a summary is presented of the present state of knowledge concerning the various type of cracking corrosion which may occur on the materials of interest, i. e. chromium nickel steels of the 18 8 type, Incoloy 800 und nickel base alloys of the Inconel 600 type.     

Legierungstechnische Möglichkeiten zur Erhöhung der Korrosionsbeständigkeit Verschleißbeständiger Duplex-Stähle

Alloying variants increasing the corrosion resistance of wear-resistant duplex stainless steels Due to their good corrosion properties, duplex stainless steels are used in many fields of chemical industry as well as power stations, marine anti environmental engineering. Precipitation-hardened variants and alloys with slightly increased carbon content are available for handling media causing simultaneous corrosion and wear. This improvement in wear properties, on the other hand, reduces their corrosion resistance and thus limits their actual fields of applications. Also, the wear resistance achieved by these steels is far inferior to that of white iron, which, however, can only be used in water with close-to-neutral pH-value. So far, handling chemically aggressive media either entailed short service lives or required expensive special materials. Many development activities are therefore aimed at improving the combination of tribological and electrochemical properties of duplex stainless steels. Different chemical compositions and types of hard phases result in varying corrosion and wear properties which are described on the basis of a test melt and a new-developed alloy.     

Korrosion von Chemieemail durch wäßrig-neutrale Medien

Corrosion of chemical service glass-enamel in neutral, aqueous media Chemistry grade enamels are highly resistant glasses fused on to steel, a multi-purpose composite material widely used in chemical plants to protect reactors and accessories against corrosion. This characteristic of glass-enamels is investigated by gravimetric evaluation of laboratory tests with totally glass-lined samples attacked by neutral, aqueous media. The results with pure water verify the known mechanism obtained with solid glass. Chemical production, however, involves also tap water or other aqueous solutions of neutral salts like chlorides, phosphates, or hydrogen carbonates; if these are taken instead of pure water, higher glass corrosion rates are obtained; on the other hand, aluminum(III) or organic compounds dissolved in the water exert a positive effect. Since test parameters were set with reality in mind the quantitative results may be transferred to solve practical problems which promises the user optimum profit of his glass-lined equipment.     

Inhibition der Lochkorrosion von Eisenwerkstoffen in neutralen Lösungen durch organische Phosphonsäurederivate

Inhibition of pitting corrosion of Iron and steel in neutral solutions by organic phosphonic acid derivatives Pitting corrosion and inhibition by organic phosphonic acid derivatives were investigated for pure iron, mild steel, 99,5% nickel and annealed austenitic steel (AISI 304) electrodes in aerated, not stirred and chloride ions containing borate buffer solution of pH = 8.0. Inhibition efficiency is different for the electrode materials investigated. Best results were obtained for nickel, were the inhibition efficiency decreases following the row: benzole-1.4-diphosphonic acid, hexane-l.6-diphosphonic acid-monohydrate, propane- 1.3-diphosphonic acid-monohydrate, amino-tris(methylene-phosphonic acid) and 1-hydroxyethane-1.1-diphosphonic acidmonohydrate. For iron and mild steel the last two substances do not show any inhibition efficiency. For stainless steel none of the compounds investigated can be used as inhibitor for pitting corrosion. For all materials inhibition depends significantly on the ratio of the inhibitor to the chloride concentration. Potentiodynamic measurements as well as cathodic discharging experiments of passive electrodes show that the inhibiting molecules are not incorporated into the passive layers but work by the adsorption mechanism. Results of Photopotential- and AES-investigations are in agreement with this statement.     

Untersuchungen der Spannungsrißkorrosion austenitischer Stähle durch saure Chloridlösungen bei Niedrigen Temperaturen

Investigation of stress corrosion cracking of austenitic steels in acid chloride solutions at low temperatures Tests were carried out on materials 1.4301, 1.4571, 1.4439 and 1.4558 in cold hydrocloric acid (c(Cl^?) = 1.5 mol/L and c(H⁺) = 1 mol/L, a few tests also at 0.1 and 0.01 mol/L). Chronopotentiostatic tests yielded data on active corrosion, passivity and pitting corrosion. CERT tests (10^?6s^?1, a few tests also at 2 · 10^?7s^?1) showed superposition of general corrosion on stress corrosion under free corrosion condition, while the rest potential was relatively negative in the active range. Oxygen purging has only a minor effect. The extent of cracking decreases with decreasing c(H⁺). In the case of cathodic polarisation straining induced surface notches occur which can be attributed to hydrogen induced effects. In the case of anodic polarisation pittings are generated without any crack initiation. Characteristic features of stress corrosion increase with decreasing strain rate. It follows from the results that high acid concentrations are necessary for stress corrosion cracking in the active state to occur. CERT tests cannot be used as an accelerated test for this kind of stress corrosion cracking. Few CERT tests carried out in warm NaCl solution did not show any stress corrosion cracking though these solutions are known to cause stress corrosion cracking after long periods. Furthermore, predamage in the form of pitting does not alter the situation.     

Vermeidung der Wasserstoffversprödung vergüteter Stähle bei galvanotechnischen Prozessen durch thermische Legierungsbildung

Avoidance of hydrogen embrittlement of high strength steels during electroplating processes by thermal alloying Low alloyed high strength steels are often electroplated by metal layers protecting against corrosion. For ultra high strength, quenched and tempered steels with yield strengths > 1000 Nmm^?2 embrittlement by hydrogen being envolved during the electrochemical pretreatment as well as metal deposition has to be avoided. More over the corrosion protecting layers should form a diffusion barrier for hydrogen which can be formed during corrosion processes under special circumstances. In this paper two problem solutions including thermal alloying processes will be discussed. Plating the steel substrate with a nickel layer subsequently annealed at a temperature above 800°C in an inert gas atmosphere an austenitic iron-nickel-alloy at the boundary is formed, being a high efficient diffusion barrier for hydrogen. Further zinc plating is improving the corrosion resistance avoiding at the same time pitting corrosion problems. Plating the steel substrate with a copper and a following nickel layer on top and annealing it at the temperature of 800°C a highly corrosion resistant copper-nickel-alloy is formed showing excellent barrier behaviour for hydrogen diffusion. In both cases hydrogen being formed during the plating process itself and penetrating into the base metal does not lead to embrittlement as it is effusing during the annealing procedure.     

Untersuchungen zur Schädigung höherfester niedriglegierter Stähle durch Druckwasserstoff bei statischer und dynamischer Beanspruchung

Investigations on hydrogen embrittlement of high strength low alloy steels in high pressure hydrogen under static and dynamic loading CERT experiments with high strength low alloy steels with ultimate tensile strengths in the range 700-1200 N/mm₂ were performed with electropolished and ground surfaces with elongation rates of 1 to 277 · 10^?7s^?1 in nitrogen and hydrogen (100 bar) at 20°C under conditions of unidirectional straining as well as cyclic loading with saw tooth characteristics. Under unidirectional straining the highest hydrogen effect was observed at ε = 2,2 · 10^?5s^?1. While no influence was detectable of the alloy elements Mn, Cr, Ni and Mo, the concentration of nonmetallic inclusions proved to be very important. Cyclic loading with load steering gave no sufficient discrimination of H effects. However, cyclic loading with a maximum load defined by σ_max = σ_(F=0), yielded good results. The effect of the loading amplitude is poor. A significant decrease of the time to failure in hydrogen is observed only at ultimate tensile strengths above 1000 N/mm². Comparable results were obtained from cyclic loading experiments steered by elongation.     

Die Lochkorrosion austenitischer Chrom-Nickel- und Chrom-Nickel-Molybdän-Stähle in schwefelsaurer Bromidlösung und ihre Inhibition durch Nitrationen

Pitting corrosion of austenitic chromium nickel and chromium nickel molybdenum steels in sulfuric acid containing bromides, and its inhibition nitrate ions In acidified bromide solution CrNi steels are attacked under pitting when a certain critical potential has been exceeded; this potential is higher than in the case of chloride containing solutions. Bromides are, consequently, less active than chlorides, but the pit density is considerably higher under idential corrosion conditions. While the pitting corrosion in chloride solutions can be considerably reduced by molybdenum addition to the steel, this effect is but little pronounced in the case of bromide solutions (with Mo additions up to 4% the potential is displaced by 0.2 V toward positive values). Mo additions around 2% are even dangerous since the pitting density is considerably increased in that range. Similar to the conditions in chloride solutions corrosion in bromide solutions is inhibited by nitrate additions; the potential limit is considerably higher in the bromide solution; this phenomenon points to stronger adsorption of bromide ions at the metal surface.     

Die Inhibition der Aktivkorrosion nichtrostender Stähle in Säuren durch Kohlenmonoxid

The inhibition of the active corrosion of stainless steels in acids due to carbon monoxide The active corrosion of stainless austenitic chrome-nickel steels in H₂SO₄ is effectively inhibited by CO below the boiling temperature of the acid. The potential range of active corrosion is narrowed down, and the passivation current density as well as the maximum dissolution rate are reduced. In the boiling solution, pitting corrosion occurs in certain potential ranges, which can be reduced or completely prevented by increasing the Mo-content of the steels.     

Möglichkeiten zur Früherkennung von Wasserstoffschädigungen in metallischen Werkstoffen durch Anwendung der Schallemissionsanalyse

Feasibility of early detection of hydrogen in metals by acoustic emission analysis Acoustic emission measurements are performed with the steels Ck 35 and 35 NCD 16. In function of the tensile strength tensile specimens are charged electrochemically with hydrogen. It is shown, that even in the charging phase acoustic emission signals can be registered. Especially during the repeated tensile test with hydrogen-charged specimens material damages can be detected with acoustic emission. In this case the well-known Kaiser effect in acoustic emission is omitted. The effect of the absorbed hydrogen is documented by changes of the mechanical properties of the material and by microfractographic investigations.     

Korrosionsprüfung durch Stromdichte-Potential-Messungen in flüssigen Medien

Corrosion testing by current density potential measurement in liquid media Metal dissolution processes in liquid media are among heterogeneous chemical reactions and follow an electrolytic mechanism. It is thus possible to add to the measurable quantities used in chemical cinetics also the current and the potential. The current density which, under certain conditions, is a direct measure of corrosion rates offers a particular interest to cinetic investigations. For the determination of the corrosion rate the following methods are used:

• 1 extrapolation of the anodic and cathodic partial curves of the current density/potential curves, as far as they follow the Tafel equation;
• 2 polarisation resistance measurement;
• 3 interpretation of the current density potential curve in view of rate determining boundary currents;
• 4 determination of the corrosion current density from the current potential curve of passivable metals and the potential of a given redox system;
• 5 corrosion current measurement in the local cell model;
• 6 current distribution determination in the electrolytes with local corrosion.

Different examples show the general applicability and the lilmits of the individual methods. In addition, particular reference is made to problems encountered in practice.     

Verformungsvorgänge bei erosiver Beanspruchung und ihre Beeinflussung durch korrosive Medien

Deformation processes by erosive treatment and influences of corrosive media As a result of our solid impact experiments on steel increased wear was established, if aqueous media are added to the particle stream. Because of the absence of any corrosion mark on the worn surfaces in SEM topography changed deformation behaviour was supposed as the cause of the enhanced material loss. This paper deals with a survey of the deformation mechanisms of the erosive surface damage. Environmental influences of deformation processes are discussed by experimental results and by a literature review.     

Untersuchungen über den interkristallinen Kongrenzenangriff von austenitischen Mangan-Chrom-Stählen durch Wasser und wäßrige Lösungen

Research on intercrystalline grain boundary corrosion of austenitic manganesechrome steels caused by water and aqueous salt solutions Stressed bar specimens of welded and tempered austenitic manganesechrome steel such as as X 40 MnCr 18 and X 22 MnCrNi 885, exposed to sea water, tap water or destilled water, are liable to suffer intercrystalline stress corrosion cracking. The occurrence of intercrystalline corrosion is not necessarily dependent on mechanical tensil stress. In particular, even unstressed specimens exposed to tap or distilled water are liable to show marked intercrystalline corrosion. Additional tensile stress merely has the effect of speeding up the inter-crystalline corrosion. The cause for the intercrystalline corrosion must be seen in the segregation of chrome-rich carbids at the grain boundaries and the consequent reduction in the chrome content within the grain boundary zone. It is possible to prove the segregation of a chrome-rich carbide of the structure M₇C₃ at the grain boundaries of steel X 40 MnCr 18, and the aggregation of a chrome-rich carbide of the structure M₂₃C₆ at the grain boundaries of steel X 22 MnCrNi 885. Through contact with zinc, it is possible to obtain a complete corrosion protection with the manganese-chrome steels investigated. A short circuit with passive austenitic chrome-nickel steels increases the general or selective corrosion rate through anodic polarisation. By adding chromate to the sea water, the corrosion rate is retarded. In tap water, an addition of chromate result in complete corrosion protection.     

Die Korrosion austenitischer Überhitzerstähle durch Alkalisulfat/Alkalichlorid-Gemische in Luft und in Verbrennungsgasen

The corrosion of austenitic superheater steels by alkali sulphate/alkali chloride mixtures in air and combustion gases In view of the fact that, in superheaters, with metal temperatures in excess of 590° C, deposits with sulphate content give rise to heavy corrosion, investigations have been carried out into the behaviour of three austenitic steels exposed to alkali sulphate/alkalie chloride mixtures at temperatures ranging from 540 to 760° C. The atmosphere above the crucibles during the tests consisted of air and a synthetic combustion gas to which SO₂ and SO₃ had been added. The tests showed that the pure sulphates are harmless, but that even a small quantity of NaCl (0.5 to 2 percent.) may result in catastrophic oxidation (at temperatures between 630 and 760° C). In the synthetic combustion. gas, potassium sulphate was found to be more corrosive that sodium sulphate. A chloride additon has no major effect except at temperatures above 630° C, presumably due to complex alkali-iron-sulphates. With all tests, corrosion was intergranular.     

Untersuchung der Spannungsrißkorrosion eines niedriglegierten Röhrenstahls durch CO?CO2 in wäßriger Lösung

Investigation on stress corrosion cracking of low alloy pipeline steel by CO? CO₂ in aqueous solution Round bar tensile specimens of 5.2 mm gauge diameter taken from grade X 52 pipeline steel have been exposed to a 10^?3 molar NaCl solution purged with CO/CO₂ up to 5000 hrs at ambient temperature and pressure. The CO partial pressure was 0.1 and 0.3 bar; the remainder being from CO₂. All specimens had a weld in the gauge length. The applied load was 85% YS of the base material. Test parameters were: electropolished and anodically pre-corroded surface; 60%-V-notch in the weld. In the latter case the un-notched part of the gauge length was stressed to 60% YS only. External and internal cracks as well as different types of localised attack had been observed. Some of them can be described as typical for strain-induced corrosion cracks. In no case did the fracture occur in the notch region, which can be attributed to the increased YS of the weld metal. The lifetime markedly depends on CO pressure, and is about 5000 hrs for 0.1 bar. CO is a critical component for stress corrosion and pitting corrosion to occur. For high pressure service, condensation of water must be prevented or limited to a small period depending on the CO partial pressure.