Neue Erkenntnisse auf dem Gebiet der ferritischen rostfreien Stähle

Recent developments in ferritic stainless steels The pitting resistance of ferritic stainless steels in HCl is visibly improved by Mo, in particular in the case of vacuum-melted material. In this context the ratio Cr:Mo = 25:2 is superior ta Cr:Mo = 17:3; addition of Mo prevents, beyond that, crevice corrosion. Ti increases resistance in the Strauß test but not in the Huey test, while Nb turns out to have a positive effect in either test. Steels containing Cr: Mo = 17:l are certainly still susceptible to pitting, but no longer to stress corrosion cracking in boiling MgCl₂, solution; stress corrosion cracking is not observed in 55% boiling Ca(NO₃)₂, and 25% boiling NaOH, but after annealing at 980 °C intercrystalline corrosion takes place. The test duration required for establishing cracking susceptibility is considerably shorter with ferritic than with austenitic steels (100 and 1000 to 2000 hours respectively).     

Localized corrosion properties of low interstitial ferritic,high purity austenitic and high chromium duplex stainless steels

Within the framework of a research aimed at characterizing the behaviour of new materials to pitting and crevice corrosion, an investigation has been made, using electrochemical techniques, of the following materials: ELI ferritic stainless steels (18 Cr-2 Mo-Ti; 21 Cr-3 Mo-Ti; 26 Cr-1 Mo); high chromium duplex stainless steel (Z 5 CNDU 21-08) and high chromium-nickel austenitic stainless steel (Z 2 CNDU 25-20); commercial austenitic stainless steels (AISI 304 L and 316 L) and laboratory heats of austenitic stainless steels with low contents of interstitials (LTM/18 Cr- 12 Ni, LTM/16 Cr- 14 Ni-2 Mo). It was possible to graduate a scale of resistance to pitting and crevice corrosion in neutral chloride solutions at 40 C; in particular the two experimental austenitic stainless steels LTM/18 Cr- 12 Ni and LTM/16 Cr- 14 Ni-2 Mo are at the same level as the AISI 316 L and 18 Cr-2 Mo-Ti, respectively. An occluded cell was developed and used for determining the critical potential for crevice corrosion (E_{localized corrosion}). For the steels under investigation E_{localized corrosion} is less noble than E_pitting especially for ELI ferritic 18 Cr-2 Mo-Ti and 21 Cr–3 Mo-Ti.     

Korrosionsverhalten hochchromhaltiger,ferritischer, chemisch beständiger Stähle

Corrosion behaviour of high chromium ferritic stainless steels Ferritic steels developed for seawater desalination and containing 20 to 28% chromium, up to 5% Mo and additions of nickel and copper have been tested with respect to their corrosion behaviour, in particular in chloride containing media. The materials in the sensibilized state were tested for inter-crystalline corrosion susceptibility in the Strauß-, Streicher-, nitric acid hydrofluoric acid- and Huey-Tests. No intercrystalline corrosion was encountered in the case of the steels with 28% Cr and 2% Mo. The resistance to pitting was assessed on the basis of rupture potentials determined by potentiokinetic tests. The resistance of the steels with 20% Cr and 5% Mo or 28% Cr and 2% Mo is superior to that of the molybdenum containing austenitic types. Addition of nickel yields a significant increase in crevice corrosion resistance; the same applies to resistance in sulfuric acid. In boiling seawater all the materials tested are resistant to stress corrosion cracking. No sign of any type of corrosion was found on nickel containing steels after about 6000 hours exposure to boiling 50% seawater brine even under salt deposits.     

Effect of alloy element contents on caustic stress corrosion cracking of several stainless steels

The stress corrosion cracking behavior in caustic solutions (200 g/l sodium hydroxide, 10 g/l sodium chloride) of three austenitic (18Cr-10Ni-2.5Mo, 20Cr-25Ni-4.5Mo, 27Cr-31Ni-3.5Mo) and three duplex (23Cr-4Ni, 22Cr-5Ni-3Mo, 25Cr-7Ni-4Mo-N) stainless steels was examined. U-bend and Slow Strain Rate (SSR) tests were performed at 200–250°C. The negative influence of nickel in the lower range content for the 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo has been shown; when the nickel content is significantly increased (>30%), as in the case of the steel 27Cr-31Ni-3.5Mo, an increase of SCC resistance has been detected. The negative effect of molybdenum, mainly on the behaviour of duplex stainless steels, has also been evidenced. The duplex stainless steels show better caustic SCC resistance than austenitic stainless steels type 18Cr-10Ni-2.5Mo and 20Cr-25Ni-4.5Mo. The best behaviour has been found for the less-alloyed steel 23Cr-4Ni.     

Pitting,Crevice and stress corrosion resistance of high chromium und molybdenum alloy stainless steels

This paper presents new data on the resistance of recently developed high-alloy stainless steels to localised corrosion in chloride solutions. Pitting potential was determined in artificial sea water, and critical pitting temperature CPT in very aggressive FeCl₃ solution. Critical crevice corrosion temperature CCT was tested in the same FeCl₃ solution. Stress corrosion measurements, made in a more familiar NaCl solution by the drop evaporation method, demonstrate that alloy stainless steels with high chromium and molybdenum have very long failure times, comparable with those of nickel alloys found to be SCC-resistant under practical conditions. Stainless steels of 20 Cr 25 Ni 6 Mo type showed the best resistance to localised corrosion.     

Korrosion nichtrostender austenitischer Stähle in (kondensierender) chloridhaltiger Salpetersäure

Corrosion of austenitic stainless steels in (condensing) nitric acid containing chlorides The corrosion parameters involved in the behaviour of austenitic stainless steels and nickelbase alloys in chloride containing nitric acid are being stated. Investigation of the corrosion resistance of the austenitic stainless steel 1.4306, ESR grade for application in nitric acid, in nitric acid condensates containing small amounts of chlorides. The condensates were formed from boiling nitric acids of molarities 6 to 10 containing 15 to 45 mg chlorides/1. The calculated corrosion rates of < 1 μm/y can be considered rather small. Especially in cases of insufficient wetting of the heat exchanger surfaces, however, local roughening and pitting corrosion is to be expected, mostly under rust-colored, chloride-containing deposits. The intensity of pitting corrosion increases with increasing chloride contents of the nitric acid solutions that evaporate. The sealing surfaces consisting of steel 1.4306 and PTFE did not exhibit any crevice corrosion. Nitric acid grade ESR-1.4306 definitely does not suffice requirements as described in this paper. It is suggested to use stainless steels which exhibit an improved resistance against chlorides and, at the same time, a high resistance against nitric acid attack. The following steel grades may be considered: X 1 CrNi 25 21 (Mat. No. 1.4335), X 2 CrNiMoN 25 22 2 (Mat. No. 1.4466) and/or X 1 NiCrMoCuN 31 27 4 (Mat. No. 1.4563).     

Localized Corrosion Characteristics of Nickel Alloys:A Review

There are a great variety of commercial nickel alloys mainly because nickel is able to dissolve a large amount of alloying elements while maintaining a single ductile austenitic phase.Nickel alloys are generally designed for and used in highly aggressive environments,for example,those where stainless steels may experience pitting corrosion or environmentally assisted cracking.While nickel alloys are generally resistant to pitting corrosion in chloride-containing environments,they may be prone to crevice corrosion attack.Addition of chromium,molybdenum and tungsten increases the localized corrosion resistance of nickel alloys.This review on the resistance to localized corrosion of nickel alloys includes specific environments such as those present in oil and gas upstream operations,in the chemical process industry and in seawater service.     

Schadensfälle an chirurgischen Implantaten und deren Ursachen

Case histories on surgical implants and their causes Since the first use of chromium nickel molybdenum steels for implants in the thirties rapid progress has been made in the development of stainless steels. Until today these improvements have scarcely been taken into consideration by material standards for implants. In agreement with the characteristic potentials for localized corrosion a statistical evaluation of failures shows that pitting, crevice corrosion and corrosion fatigue as a result from the latter can easily be avoided by higher contents of chromium and molybdenum. Steels in accordance with these requirements are specified in all standards and would solely from the economic point of view justify their use.     

Austenitische gegen Spannungsrißkorrosion in konzentrierten Chloridlösungen beständige Manganchromstähle

Austenitic manganese chromium steels resistant to stress corrosion cracking in concentrated chloride solutions The testing of stress corrosion susceptibility in MgCl₂ and CaCl₂ shows that the classical Austenitic CrNi and CrMnNiN steels are not sufficiently resistant to this type of corrosion. On the other hand MnCr steels exhibit good resistance even in hot solutions. The resistance, however, is considerably deteriorated by addition of Ni even in small quantities; with Ni contents up to 0.5%, however, the time to failure is still considerable. This fact is important since such nickel contents must be counted with because of production conditions in metallurgy. On the basis of experiments including investigation into mechanical properties, microstructure, metallic phases present and stress corrosion resistance (under constant tensile load) in MgCl₂ solution (35%, 115–120 °C) the authors selected out of 62 types of steels tested four experimental austenitic MnCr steels meeting the following complex of requirements: high resistance in MgCl₂ solution, sufficiently high resistance to intercrystalline corrosion, certain resistance to pitting and practically monophasic (austenitic) structure.     

In vitro electrochemical investigations of advanced stainless steels for applications as orthopaedic implants

Potentiodynamic anodic polarization experiments on advanced stainless steels (SS), such as nitrogenbearing type 316L and 317L SS, were carried out in Hank’s solution (8 g NaCl, 0.14 g CaCl₂, 0.4 g KC1, 0.35 g NaHCO₃, 1 g glucose, 0.1 g NaH₂PO₄, 0.1 g MgCl₂, 0.06 g Na₂HPO₄ 2H₂O, 0.06 g MgSO₄ 7H₂O/1000 mL) in order to assess the pitting and crevice corrosion resistance. The results showed a significant improvement in the pitting and crevice corrosion resistance than the commonly used type 316L stainless steel implant material. The corrosion resistance was higher in austenitic stainless steels containing higher amounts of nitrogen. The pit-protection potential for nitrogen-bearing stainless steels was more noble than the corrosion potential indicating the higher repassivation tendency of actively growing pits in these alloys. The accelerated leaching study conducted for the above alloys showed very little tendency for leaching of metal ions, such as iron, chromium, and nickel, at different impressed potentials. This may be due to the enrichment of nitrogen and molybdenum at the passive film and metal interface, which could have impeded the releasing of metal ions through passive film.     

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.     

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).     

New stainless steels for sea-water applications. Part II: Comparison of corrosion and mechanical properties of laboratory and commercial ELI ferritic and superaustenitic stainless steels

This paper represents a follow-up to the first part of the work on new stainless steels for sea-water service. Four laboratory ELI (Extra Low Interstitial) ferritic stainless steels (types 25 Cr-4 Ni-4 Mo), two commercial ELI ferritic stainless steels (types 25 Cr-4 Ni-4 Mo? Ti and 26 Cr-2.5 Ni-3 Mo? Ti) and two highly alloyed austenitic stainless steels (types 20 Cr-25 Ni-4.5 Mo? Cu and 20 Cr-18 Ni-6 Mo? N) have been investigated. With a view to establish the performance of these new alloys in chloride containing environments, systematic electrochemical and laboratory exposure tests have been carried out to define how various factors affect its susceptibility to intergranular, pitting, crevice and stress corrosion. Tension tests were also performed. From the comparison of the localized corrosion resistance and mechanical properties it has been concluded that the laboratory Ti, Ti + Nb or Nb stabilized ELI ferritic stainless steels and the commercial type 25 Cr-4 Ni-4 Mo? Ti of analogous composition could be a valuable alternative to the more expensive highly alloyed stainless steel type 20 Cr-25 Ni-4.5 Mo? Cu which has been especially developed and already used for industrial sea-water applications.     

钛的缝隙腐蚀行为研究

采用电化学测试技术，研究了工业纯钛和Ｔｉ－０．３％Ｍｏ－０．８％Ｎｉ合金在２５％ＮａＣｌ和ＨＣｌ沸腾溶液中的缝隙腐蚀行为。结果表明，由于合金元素Ｎｉ具有低的氢超电势，促进了阴极极化过程，从而提高了钝化能力，且缝内表面Ｎｉ的富集增强了膜的钝化稳定性，因此，Ｔｉ－０．３％Ｍｏ－０．８％Ｎｉ合金抗缝隙腐蚀性能优于工业纯钛。     

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.     

不锈钢海水潮汐区16年腐蚀行为

在青岛、厦门和榆林3个试验站的潮汐区对5种不锈钢暴露16年，总结其腐蚀行为和规律。在潮汐区暴露的不锈钢受点蚀和缝隙腐蚀破坏。不锈钢在潮汐区暴露1至4年的点蚀速度较大，以后点蚀速度减慢。耐点蚀性能较好的不锈钢，耐缝隙腐蚀性能也较好。不锈钢在潮汐区的腐蚀随暴露地点的海水温度升高而加重。增加Cr含量、添加Mo能明显提高不锈钢在潮汐区的耐蚀性。Ni对提高的耐蚀性有效，但影响效果较小。海生物污损能引起不锈钢的局部腐蚀，它对不锈钢在潮汐区的腐蚀有显著影响。     

New stainiess steels for sea water applications. Part. 1: Corrosion and mechanical properties of ferritic stainless steels

Two experimental ELI ferritic stainless steels (22 Cr – 2.5 Ni – 3 Mo and 22 Cr – 2.5 Ni – 3 Mo – Ti) prepared in laboratory and a commercial one (21 Cr – 3 Mo – Ti) were investigated. Electrochemical and laboratory exposure tests were carried out to define the localized corrosion resistance (pitting and crevice) of such steels in chloride solution. Intergranular and stress corrosion resistance was also evaluated. Room temperature tension tests and impact tests were performed. 22 Cr – 2.5 Ni – 3 Mo – Ti and 21 Cr – 3 Mo – Ti steels are immune to intergranular corrosion whatever temperature they are heat treated at and have the same pitting corrosion resistance as a function of temperature; crevice corrosion of 22 Cr – 2.5 Ni – 3 Mo is decidely better than in the commercial 21 Cr – 3 Mo – Ti. The experimental steels were immune to stress corrosion in hot chloride environment.     

Temperature as a pitting and crevice corrosion criterion in the FeCl3 test

The FeCl₃ test is applied to an increasing extent for examining the resistance to pitting and crevice corrosion. Two methods having proved their value are described, the chemical properties of the FeCl₃ solution with regard to hydrolysis, pH and redox potential behaviour at various test temperatures are set forth and finally numerous results of the application of this test to high-alloy stainless steels and nickel alloys are presented. These results have been used to establish, be means of multiple regression, two empirical equations that allow to estimate rather accurately the critical pitting and crevice corrosion temperatures (CPT, CCT) from the contents of the decisive alloying constituents. These temperatures vary by about 2.5°C in the CPT test and by approx. 10°C in the CCT test, which can be reduced, however, by extending the test period beyond 24 hours. This is due to the fact that corrosion potentials in a 10% FeCl₃ · 6H₂O solution take a long time to stabilize. The variation of the critical crevice temperature can be further reduced by pressing the crevice blocks at a higher torque to the specimen. Another section particularly deals with the application of the CPT test for determining the influence of the matrix on the resistance to local corrosion. Consequently, the CPT test lends itself excellently to the examination of welds and as a quality control. Finally, CPT test results are compared with pitting data determined electro-chemically in artificial seawater. This shows that the ranking order with regard to corrosion resistance is identical, although media and processes differ considerably from each other.     

Zur Lochkorrosionsneigung von austenitischen CrNiMo-Stählen in konzentrierten Ammoniumrhodanidlösungen

Pitting corrosion of austenitic CrNiMo-steels in concentrated ammoniumrhodanide solutions Quasipotentiostatic and potentiokinetic polarisation measurements at various 18 Cr-10 Ni steels with molybdenum contents up to 4,3% were performed in 25 and 45% ammoniumrhodanide solutions. It was found that pitting corrosion is caused by incomplete passivation in the potential range of –300 to +250 m V _H. At these potentials the formation of stable passive layers is hindered by the formation and local oxidative dissolution of sulfidic layers. Above +250 m V _H rhodanide ions act in these weak acidic ammoniumrhodanide solutions as agents which destroy passive layers, comparable with chloride ions. The limiting potentials for stable pitting corrosion, obtained from potentiostatic experiments, are shifted from –300 to –150 m V _H with increasing molybdenum content of the steel. The least tendency of pitting corrosion was found for that steel with the highest molybdenum content.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.