Betrachtung zur Verbesserung der Zunderbeständigkeit von Zirkonium und Zirkoniumlegierungen

Some considerations concerning the improvement of the scaling resistance of zirconium and zirconium alloys Improving the scaling resistance of Zr and its alloys requires a reduction of gas solubility in the metal and an improvement of scale adhesion to the base metal. For this reason only such alloy additions are eligible which are either incorporated in the scale layer or form a ductile or molten alloy phase between the base metal and the ZrO₂, layer being formed. In this way one would on the one side prevent the scale layer from becoming porous (thus also eliminating the possibility of hydrogen diffusion into the base metal) while on the other hand the scale layer would be prevented from spalling off under shearing stress. These possibilities are discussed in terms of probable mechanisms for the action of alloying additions such as Sn and Al.     

Allgemeine Beständigkeit von Schweißnähten unter Berücksichtigung der Spannungs- und Schwingungsrißkorrosion

General resistance of weld seams with a view to stress corrosion cracking and corrosion fatigue The corrosion of welds is due to thermal effects during welding which give rise to structural changes and, frequently, compositional changes in the transition zone. The welded material is rapidly cooled and may thus be heterogeneous and may present residual stresses resulting in increased susceptibility to selective and stress corrosion. The manganese content is of high importance in low alloy steels, as well as residual martensite or austenite embedded in a ferrite matrix. Low ferrite contents are generally beneficial because they counteract high temperature cracking; however, ferrite contents should be hept below 10% in order to prevent the formation of a continuous network giving rise to selective corrosion. Corrosion susceptibility may also be produced by carbide or carbonitride precipitation in austenitic and ferritic steels and nickel base alloys. Weld zones in aluminium alloys are attacked in rare cases (e.g. by HNO₃) and the susceptibility of Ta, Zr and Ti depends from the properties of the protective atmospheres.     

Untersuchung zur Korrosionsbeständigkeit von Porzellan und Steinzeug

Investigation into the corrosion resistance of porcelain and stoneware The author communicates the results of corrosion tests made with porcelain and stoneware in contact with acid and alcaline media (sulfuric acid, phosphoric acid, nitric acid, hydrochloric acid, NaHO₂ KHO) under rigid conditions resembling those encountered in practice (oboiling temperature, flow velocities upto 1 m/s). while the acid resistance can as a rule be considered very good there is considerable attack by alcaline media. In this context there is a close interdependence between the attack and the composition of the materials and the nature of the glassy and crystalline phase present. Aluminated materials show good resistance somewhat reduced.     

Mikrobielle Werkstoffzerstörung – Simulation,Schadensfälle und Gegenmaßnahmen: Untersuchung der Beständigkeit von keramischen Werkstoffen

Microbial deterioration of materials – simulation, case histories and countermeasures: Testing of the resistance of ceramic materials Testing of microbiologically influenced corrosion of ceramic materials by biogenic sulphuric and nitric acid corrosion is well described and applied for constantly moist buildings like sewage pipelines and cooling towers. The complex situation on historical buildings of natural sandstones has not yet been investigated in the laboratory. A double-chamber cabinet and a test system for the simulation of chemically (gaseous pollutants), combined chemically and microbiologically (gaseous pollutants plus nitrifying bacteria) and solely micro biologically (nitrifying bacteria) influenced corrosion of natural sandstone is presented. A high stone moisture was essential for the growth of nitrifying bacteria on test stones. Under optimum conditions a nitrifying biofilm developed on the calcareous Ihrlersteiner green sandstone, reducing the evaporation from the stone surface. Biofilm cells adapted well to high concentrations of gaseous pollutants (1,065 μg/m³ sulphur dioxide, 850 μg/m³ nitric oxide, and about 450 μg/m³ nitrogen dioxide): in the simulated smog atmosphere. The mean metabolic activities of ammonia oxidizers were 11 times and those of nitrite oxidizers 30 times higher than mean values of samples from historical buildings. The microbiologically, influenced nitric acid corrosion alone was 8 times stronger than the chemically influenced corrosion by the simulated smog atmosphere. If sulphur dioxide was added, the microbiologically produced nitrite was removed by chemodenitrification. Thus, the combined attack of nitrifying bacteria and gaseous pollutants did not result in an increased corrosion, but the nitrifying biofilm promoted the formation of gypsum.     

Einfluß von Passivierungs- und Kohlenstoffschichten auf austenitischen CrNiMo-Stählen auf die Beständigkeit gegen Lochfraß und Spannungsrißkorrosion

Effects of passivation and carbon films on austenitic CrNiMo steels on their piting and stress corrosion resistance The influence of passive film and combinations of a passivation and a carbon layer on the resistance to pitting and SCC of austenitic CrNiMo steels has been investigated in physiological sodium chloride solution (Tyrode solution) at pH 6.9 to 7.4 at 37 ± 1°C. The passive film was obtained after electrolytic polishing in H₃PO₄ + H₂SO₄ + C₆H₅NHCOCH₃ + oxalic acid + corrosion inhibitor CS by treatment with 40% nitric acid the carbon film was obtained by CVD. Impurities in the steel (non-metallic inclusions) and the different metallic phases were investigated and the chemical composition of the passive film was determined by quantitative analysis. The resistance to pitting of the steel with and without passive film was determined potentiodynamically in Tyrode's solution at 37 ± 1°C. The resistance to SCC was determined in Tyrode's solution at 37 ± 1°C, in neutral glycerole and in boiling magnesium chloride solution at 154 ± 1°C and evaluated in terms of K_σ and K_τ. The corrosion damage was investigated by optical and scanning electron microscopy. The investigations have revealed that the different surface conditions considerably improve the pitting and SCC resistance of the steels in the media used in this work, so that they make possible the use of these materials as surgical implants.     

Entstehung von Sulfatnestern im Rost bei der atmosphärischen Korrosion von Stahl I

Occurrence of sulphate nests in rust during the atmospheric corrosion of steel. I. A pure FeSO₄, solution (with ³⁵S) was used to generate, on steel surfaces, a continuous layer of FeSO₄, through evaporation. The samples thus treated were exposed to the action of air of different humidity. Within three hours, the sulphate ions tend to concentrate on the surface of the sample and to penetrate into the metal locally, causing nest-like concentrations of corrosion products. The number of the nests increases with the FeSO₄, concentration up to a maximum. Between the frequency and the size of the nests, there is a correlation which depends on air humidity. With increasing air humidity, the size of the nests increases whilst their number decreases.     

Neue Nickel-Chrom-Legierungen mit hoher Beständigkeit gegen Brennstoffaschenkorrosion

New NiCr-alloys with high resistance to oil ash corrosion After giving an account of the origin of oil ash corrosion the authors dwell in particular on the behaviour of the alloys CrNi 35/65, 50/50 and 60/40. In this context they show the range of applications of these materials as derived from laboratory experiments and field tests in refinery furnaces as well as the influence of temperature and firebox conditions. A comparison is discussed of corrosion values obtained under practical conditions with commercial austenitic and ferritic cast materials. Finally the authors discuss the mechanical, physical and processing characteristics of the CrNi alloys, dwelling in particular on their creep properties. CrNi 50/50 and 60/40 can be welded under certain conditions only; welding of load-bearing elements is discouraged.     

Korrosionsverhalten von Plasma- Heißdraht-Auftrag-Schweißungen. Teil I: Beständigkeit von auftraggeschweißten Nickelbasislegierungen gegen Loch- und interkristalline Korrosion

The corrosion properties of Plasma-Hot-Wire-Surfacing Part one: resistance of Nickel-base-alloy claddings against pitting and intergranular attack This paper describes measurements at claddings of nickel-based-alloys C 276, C 4 and C 22 on the St 52-3 steel substrat produced by Plasma-Hot-Wire-Surfacing (PHS). The correct heat control during the welding process is a crucial factor for the preparation of nearly precipitate-free claddings. For this reason it was necessary to optimize all the process data. The claddings itself as well as the rolled reference material (both of nearly the same chemical composition) were examined by means of metallografical and microanalytical techniques. Corrosion measurements show, that the composite materials exhibit high resistance both against pitting and intergranular corrosion and comparable to the properties of the rolled reference material. This was shown by determination of the critical pitting temperature in quinary salt solution, by electrochemical techniques in acidic chloride solution and by investigation in iron-(III)-sulfate-sulphuric acid solution.     

Untersuchung des SwRK-Verhaltens des martensitaushärtenden Stahls X 5 CrNiCuNb 17 4 PH im Vergleich zum weichmartensitischen Stahl X 4 CrNiMo 16 5 1 ESU in chloridhaltigen wäßrigen Medien. Teil 1: Korrosionsuntersuchungen und SpRK-Untersuchungen zur Optimierung der Wärmebehandlung bezüglich der SpRK-Beständigkeit

Investigations of the corrosion fatigue behaviour at a super pure martensitic stainless steel (X 5 CrNiCuNb 17 4 PH) in comparison to the soft martensitic stainless steel X 4 CrNiMo 16 5 1 ESR in chloride containing aqueous media — Part 1: Corrosion investigations and stress corrosion tests to optimize the heat treatment according to the stress corrosion resistance The aim of this investigation was to improve the reliability of operation and the economy of corrosion fatigue stresses of structural elements, especially at elevated temperatures, by optimizing the material. This investigation is of great interest both to the industry and to related fields. It concerns the influence of a higher degree of purity achieved by the secondary metallurgical aftertreatment of soft martensitic steel X4CrNiMo 16 5 1 ESR (ESR-electroslag remelting) in comparison to the corrosion fatigue behaviour of the precipitation hardening steel X5CrNiCuNb 17 4 PH, which was specially heat-treated with respect to stress corrosion cracking resistance. The stress corrosion was investigated for all heat-treatments of the 17-4 PH in 22% NaCl(pH3) solution. The precipitation hardening steel was most resistant to stress corrosion in concentrated NaCl-solution after a three-stage heat-treatment. There was no improvement of corrosion fatigue resistance after metallurgical aftertreatment of soft martensitic steel compared to the untreated material. This is due to the instable passive behaviour of the material which led to crack initiation, especially during the 150°C experiments, at chloride-induced places of pitting. The investigation of the electrochemical corrosion behaviour of both materials showed that the pH-value hardly influences corrosion restistance. An increase of the salt content leads to higher pitting induction. At temperatures of 80°C in a saturated NaCl-solution the material showed no corrosion resistance. In potentiokinetic investigations, a direct transition from the active area to the pitting potential was observed. In accordance with both the corrosion fatigue and the stress corrosion cracking investigations, it was found that pitting at the martensite precipitator starts primarily around Cu-containing or oxidic inclusions.     

Die Entstehungs- und Ausbreitungskinetik von Sulfatnestern bei der atmosphärischen Korrosion von Stahl

Kinetics of formation and propagation of sulfate nests on steel The atmospheric corrosion of steel starts at individual sites, the number of which increases until the steel surface is covered by a rust layer. The initial distribution of corrosion sites seems to be connected with active sulfide inclusions (in particular MnS). The manganese sulfates formed are hydrolised, so that sulfuric acid is formed which initiates the formation of sulfate nests. The ferrous sulfate formed in such nests is again hydrolised and this reaction controls the rate of sulfate nests propagation. Good rust protection requires the removal of such sulfate nests which can be achieved only by removal of the rust layer (either mechanically or by pickling); mere rinsing or the use of rust stabilizers are not sufficient.     

Beitrag zur Hitzebeständigkeit der Stahltypen Cr 24 und Cr 24 Ni 20

Contribution to the heat resistance of Cr24 and Cr24Ni20 steels The maximum permissible temperature for the use of the above steels depends from the character of the temperature profile. The Cr24 type is superior under cyclic temperature, while the other steel shows better behaviour with uniform temperature conditions (test temperatures 1000 and 1100°C). The shape of the specimens (cylindric rods and cubes) has no essential influence, but it is indispensable to take into account the surface area because the number of material defects increases as the surface increases. The differences in behaviour are attributed, in the case of the steel Cr24, to the catalytic influence of the iron-rich oxides which are formed in the chromium-depleted zone and which are intermittently removed during temperature cycling. In the case of steel Cr24Ni20, however, the chromium carbonitride formed in the chromium-rich zone is the essential factor; it is destroyed when exposed to constant temperature, while it becomes stabilized during temperature cycling and enhances oxidation during any subsequent heating.     

Die Prüfung von Halbzeug aus Kupferwerkstoffen auf Beständigkeit gegenüber Spannungsrißkorrosion

Testing of copper and copper-alloy semis for resistance to stress corrosion The mercurous nitrate test currently used to test the resistance to stress corrosion of copper and copper alloys is problematic. The testing agent is toxic and, moreover, it is not encountered as a corrosive agent. For these materials, the main source of corrosion is ammonia and its compounds. Therefore, a test on this basis is proposed, capable of replacing the mercurous nitrate test for the purposes of quality assurance, as experience has shown for about a year.     

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 Ermittlung der Beständigkeit gegen Spannungsrißkorrosion (SpRK) von hochlegierten Sonderstählen in chloridhaltigen wäßrigen Medien

Determination of resistance to stress corrosion cracking (SCC) of high-alloy special steels in chloride-containing aqueous media The 18 Cr 10 Ni(Mo) based stainless steels have been continually improved by raising the Cr, Ni and Mo contents. The behavior of these high-alloy steels towards SCC was determined in test media generally used in practice since the question of the resistance to stress corrosion cracking (SCC) had still remained unanswered to a large extent. SCC tests on U-bend samples in boiling 62% CaCl₂ solution showed a good differentiation depending on the Ni and Mo contents. With increasing Ni content, the susceptibility of special high-alloy steels to SCC is shifted towards longer service lives, alloys containing ≧ 42% by weight of Ni being resistant. High-Mo special alloy steels are more resistant to SCC than low-Mo special alloy steels. These results could be confirmed by tests carried out on circular cross section samples in boiling 62% CaCl₂ solution under constant load and potentiostatic control. The free corrosion potentials recorded for 25% Ni special alloy steel and Ni-based alloys are within the potentiostatically determined range of insusceptibility to SCC. The high-Mo special alloy steel X 2 NiCrMoCu 25 20 6 (1.4529) shows the same critical SCC potential on the anodic side as the Ni alloy NiCr21 Mo (2.4858). Superferrit X 1 CrNiMoNb 28 4 2 (1.4575) and austenitic ferritic steel X 2 CrNiMoN 22 5 (1.4462) showed that the SCC behavior was unsatisfactory in both tests as in the case of steel X 10 CrNiMoTi 18 10 (1.4571). Tests in boiling 4 m NaCl showed no SCC, not even under the aggrevated test conditions in the test set-up. The great influence of the oxygen content was demonstrated in tests carried out in the autoclave with defined oxygen and chloride concentrations. The resistance of the steels to SCC decreases under air-saturated conditions (8 … 10 ppm O₂) whereas the chloride concentration (200 and 2000 ppm Cl^?) does not exercise an important influence. U-bend samples should be given preference to Erichsen samples for SCC tests. SCC break characteristics could be determined metallographically and by scanning electron microscope.     

Untersuchung der Korrosionsbeständigkeit von Auftragschweißungen der Hartlegierungsfülldrahtelektroden Stelly Fe12 und Stelloy 6

Investigation of the corrosion resistance of the hard-facing alloys stelloy Fe12 and stelloy 6 Cobalt hard-facing alloys are often used under complex conditions with combined wear and corrosion attack at higher temperatures. For using in equipments of atomic power plants cobalt free claddings are required to guarantee a low radioactivity level. Beside this there is an economic interest of the industry to use cobalt free welding filler metals. The problem consists in developing of cobalt free hard-facing alloys with the same wear and corrosion resistance as cobalt base alloys. The resistance of claddings of the electrode stelloy Fe12 with about 19% Cr and 10% Ni has been investigated in 0.5 M sulfuric acid at 66°C during a full immersion test in comparison with the cobalt base alloy stelloy 6. Both investigated materials have no satisfactory corrosion resistance. But according to the EPR-test they were resistant against intergranular attack (IK). The high matrix dissolution of grinded samples was disadvantageous to evaluate IK resistance by EPR-test.     

Der Einfluß des Salzgehaltes auf die Korrosion von Eisen und niedriglegiertem Stahl in belüfteten Lösungen

Influence of the salt content on the corrosion of iron and low-alloy steel in aerated solutions The influence of borate and carbonate on the corrosion of iron and steel was investigated in aerated, neutral electrolytes. Both salts were inhibiting the corrosion. But only borate in higher concentrations was able to prevent the corrosion by chloride. The convection of the electrolyte was of only small influence.     

Zur Prüfung von Schweißplattierungen mit austenitischen Bandelektroden der Art X2 CrNiNb 2413 auf Beständigkeit gegen interkristalline Korrosion

Testing of weld-claddings with austenitic band electrodes of the type X2CrNiNb24 13 for resistance against intercrystalline corrosion Two-phase, austenite- and ferrite-containing weld claddings of the 24 Cr-13 Ni-type may become susceptible to intercrystalline corrosion after heat-treatment in the 550°C temperature range due to a sensitization of the austenite. Thus, both austenite/austenite- and austenite/ferrite grain boundaries are attacked with the attack starting from the austenite phase in the latter case. Testing the resistance to intercrystalline corrosion in a sulphuric acid/copper sulphate media in the presence of copper turnings according to DIN 50914, the attack of grain boundaries ferrite/austenite becomes evident only if the specimens are bent after testing. Non-bent specimens do not show any indication of grain boundary attack by light-optical or even electron-microscopical metallographic examination. At non-bent specimens, the attack may become recognizable with considerably prolonged (several hundred hours) testing time. Referring to the investigations performed, the mechanism of intercrystalline corrosion of two-phase, ferritic-austenitic stainless steels and the influence of heat-treatment on susceptibility to intergranular attack are discussed.     

Einfluß von Verformung und spannungsinduzierten Ausscheidungen auf die Korrosionsbeständigkeit von siliciumlegiertem,nichtrostendem Stahl X2CrNiSi 18 15 in Salpetersäure

Effect of deformation and stress-induced precipitations on the corrosion resistance of silicon alloyed stainless steel X2CrNiSi 18 15 in nitric acid The corrosion resistance of silicon alloyed X2 CrNiSi 18 15 stainless steel with about 4 wt.-% Si in nitric acid depends on both deformation degree of the material and number and distribution of precipitations in the microstructure. After cold deformation and subsequent heat-treatment at 700°C (1292°F), the corrosion rate in 5 M HNO₃ is higher than that of non-deformed material. This result was established with specimens isolated from each other but was particularly pronounced with specimens being in galvanic contact. By deformation and subsequent annealing at 700°C, precipitations are formed at the grain boundaries the number of which increases with increasing degree of deformation. With higher degree of deformation, the precipitations are preferably arranged at slip-lines. They were identified as chromium carbide, M₂₃C₆, and silicide of the Cr₅Ni₃FeSi₂ type. It is shown by electrochemical measurements that these precipitations stimulate the cathodic partial reaction, i.e., the reduction of nitric acid. Hence, the corrosion resistance of the silicon alloyed stainless steel is impaired by deformation and stress-induced precipitations formed during annealing at 700°C.