Lochkorrosionsverhalten von Auftragschweißungen – Schweißlagen‐ und Oberflächeneffekte –

Pitting corrosion behaviour of built‐up welds – Effects of welding layers and tarnish – The pitting corrosion resistance of nickel based deposition welds on a superduplex steel made by active‐gas metal pulsed‐arc welding was studied. Therefore the determination of the CPT (Critical Pitting Temperature) should be carried out corresponding to ASTM G 48 C. However an unexpectedly low resistance of the built‐up welds also at multilayer order was noticed. After visual assessment of the examined specimens a significant effect of the surface condition was assumed. Because the CPT determination according to ASTM does not allow any statement about the corrosion process, this method was not suitable to characterize the corrosion system. For this reason a new method was applied to clarify the causes of the low corrosion resistance. This method determines the CPT with the help of the electrochemical current noise under the same conditions demanded in ASTM G 48 C. The temperature is increased continuously and the characteristic parameters of the system are recorded and evaluated objectively within short time. So it was possible to see the influence of the surface condition on the pitting corrosion behaviour of the examined specimens. The required parameters to the post‐processing of the deposition welds were determined. The comparison of the results show that the surface tarnish formed after the shielded arc welding process influences the pitting corrosion resistance negatively. After its elimination the CPT could be determined in dependence of the welding layers.     

Besondere Aspekte der Temperaturführung und Wärmebehandlung hochlegierter Werkstoffe für den Chemie-Apparatebau

Special aspects of temperature control during heat treatment of high alloyed materials for the Chemical Industry High alloyed materials require a special control of the temperature during any annealing and fabrication procedure to assure the corrosion resistance, which was the main criterion for the selection of the material in the application proposed. Due to this requirement the time-temperature sensitization behaviour with respect to isothermal and polythermal heat treatments has been determined for alloy Cronifer 1925 hMo (1.4529), Nicrofer 3127 hMo (1.4562) as well as for the nickel base alloys Nicrofer 5716 hMoW (2.4819) and Nicrofer 5923 hMo (2.4605). The application of the standard corrosion test according to SEP 1877 II on alloys alloyed with at least 20% chromium shows no correlation between all second phase precipitation which can affect the corrosion resistance under practical conditions. For alloy Cronifer 1925 hMo and Nicrofer 3127 hMo the determination of the critical pitting temperature in the ferric chloride test has been shown to be an effective procedure to evaluate changes in the microstructure due to improper heat treatments during fabrication. The nickel based alloy Nicrofer 5716 hMoW, which is alloyed with only 16% chromium shows already high corrosion rates in the proper solution annealed and quenched condition, when tested according to the strong oxidizing test solution of ASTM G 28 A. Furthermore, the corrosion rate will increase rapidly after sensitization. Due to the fact that the alloy has not been developed for strong oxidizing media, the test media according to ASTM G 28 B should be applied, since it is more appropriate to this alloy. Although alloy Nicrofer 5923 hMo is alloyed with 23% chromium and 16% molybdenum the alloy exhibits a precipitation behaviour which allows easy fabrication of heavy components. Testing according to ASTM G 28 A allows the identification of improper heat treatments.     

Einfluß von Chrom,Molybdän und Stickstoff auf die Korrosionsbeständigkeit des Ni-freien,austenitischen Edelstahles Macrofer 2515MoN (W.-Nr.: 1.4653)

Influence of chromium, molybdenum and nitrogen on the corrosion resistance of the Ni-free, austenitic stainless steel Macrofer 2515MoN (German Alloy No. 1.4653) Nitrogen alloyed, Ni-free, austenitic stainless steels comprising of more than 1 wt.-% nitrogen are a new group of alloys with promising properties. They show a very interesting combination of high strength and toughness with a high corrosion resistance. This combination of properties make the alloys not only suitable for fasteners but also for parts for medical and dental applications. This work shows the influence of chromium, molybdenum and nitrogen on the corrosion resistance of Fe25Mn-alloys in media typical for the above mentioned applications. According to these results Fe25Mn-alloys with appr. 20 wt.-% chromium, about 3 wt.-% molybdenum and appr. 1,3 wt.-% nitrogen have an excellent corrosion resistance in Ringer solution, artificial saliva and artificial sweat. The critical pitting temperature (CPT) as well as the critical crevice temperature (CCT) with 61°C respectively 37°C tested according ASTM G 48A provided significantly higher temperatures when compared to the commercially well established Ni-austenite X6CrNiMoTi17-12-2 (German Alloy No. 1.4571).     

High temperature alloy chloridation at 850°C. Part I: Comparison of Ni‐based and Fe‐based alloy behaviour

Eight alloys were tested under Ar/Cl₂ atmosphere at 850°C for 15 min and 1 h. Their gross and net weights were evaluated together with the base metal consumption. Macroscopic and microscopic micrographs, associated with chemical analyses and X‐ray diffraction gave the composition and microstructure of the corrosion products. Huge differences were observed if one compared the nickel based alloy behaviour to that of the iron based alloy. Molybdenum and tungsten could also play a role, but it was not clearly defined until now. A tentative evaluation of the best candidates will be given, according the experimental conditions of this work and the chosen criteria. A corrosion index was established after each observation and helped to classify the different material behaviour under chloridation. The Ni‐based alloys possessed the best chloridation behaviour in the present conditions, followed by the Ni‐based alloys containing more Cr and finally by the Fe‐based materials. The chloridation behaviours were discussed according to the volatility of the chlorides species formed on the different alloys.     

Pitting and crevice corrosion of superaustenitic stainless steels

Superaustenites are mainly used in offshore applications, oil production and chemical industry. Most important types of localised corrosion of these steels are pitting and crevice corrosion. Investigated materials were N08028, S31254 and three modified alloys. Chromium content of investigated alloys varied between 20 and 27%, molybdenum between 3.2 and 6.0%, nitrogen between 0.1 and 0.36% and copper between 0 and 1.1%. For means of comparison stainless steel AISI 316L has been included in the study. Pitting and crevice corrosion of these highly corrosion resistant steels has been investigated by use of standardized tests. Critical pitting temperature and critical crevice temperatures were determined according to ASTM G 48, Methods C and D, respectively. Electrochemical measurements for determination of pitting potentials were done according to ASTM G 61 as well as for determination of critical pitting temperatures according to ASTM G 150. Results are presented as function of MARC (Measure of alloying for resistance to corrosion) defined by Speidel since linear correlation coefficients were higher when compared to conventional PREN. Results obtained by different testing methods must not be compared directly. Every test however is sensitive to microstructural defects like precipitations and segregations that decrease corrosion resistance. The higher alloyed a material is, the higher is its tendency to form microstructural defects, and the more difficult is it to reach its theoretical corrosion resistance at given chemical composition.     

Corrosion characteristics of the wrought Ni‐Cr‐Mo alloys

This paper concerns the wrought, nickel‐chromium‐molybdenum (Ni‐Cr‐Mo) alloys, a family of materials with a long history of use in the chemical process industries. Their attributes include resistance to the halogen acids and resistance to pitting, crevice attack, and stress corrosion cracking in hot, halide salt solutions. The purpose of this paper is to characterize the performance of the Ni‐Cr‐Mo alloys in several key chemicals, using iso‐corrosion diagrams. These indicate the expected corrosion rates over wide ranges of concentration and temperature. Furthermore, the differences between individual Ni‐Cr‐Mo alloys, and their behavior relative to the stainless steels are defined. The data indicate benefits of both a high chromium content and a copper addition, as used in Hastelloy® C‐2000® alloy.     

Long‐term exposure of austenitic steels and nickel‐based alloys in lignite‐biomass cofiring

The aim of this study was to assess the long‐term impact that the addition of biomass provokes on superheater materials exposed to fireside corrosion environments. Alloys covering a broad range of commercially available materials were investigated. Their corrosion kinetics under different corrosive deposits and atmospheres was evaluated, and their corrosion products analyzed to deepen understanding of the underlying corrosion mechanisms. Therefore, three nickel‐based alloys and three austenitic steels containing 20–24 wt.% Cr were tested at 650°C for 7,000 hr. The long‐term exposure shows new mechanistic aspects of Type II hot corrosion that were revealed by accelerated material depletion. The formation of Ni–NiS eutectic and the formation of a Cr depleted zone close to the substrate corrosion product interface are indicative of the breakaway occurrence. Differences in the corrosion behavior are related to the balance of Ni, Mo, Co, and Cr and can serve as the material selection argument. The evaluation concluded with the finding that alloys presenting Mo and Ni might be preferentially used in fireside corrosion in the presence of biomass, whereas the use of austenitic steels suffer less corrosion if no biomass is present in the corrosive atmosphere.     

Hochfeste Stangen aus NiCr23Mo16Al – Einfluss der Kaltverfestigung auf die mechanischen Eigenschaften und das Korrosionsverhalten

High‐strength NiCr23Mo16Al bars – Influence of cold reduction on mechanical properties and corrosion behaviour For application in highly corrosive environments with high demands on the mechanical properties of the alloys, cold reduced NiCrMo alloys are often the alloys of choice. This work examines the influence of cold reduction up to 42%, introduced by cold drawing on the mechanical properties and the corrosion behaviour of the alloy NiCr23Mo16Al. Tensile tests performed at 20°C, 204°C and 371°C show a linear increase in 0.2% yield strength up the maximum cold reduction of 42%. For all cold reductions and all test temperatures at least 16% elongation to fracture were achieved. Also the impact strength was as high as approx. 200 J even for the lowest test temperature of ? 40°C. Corrosion tests performed according to ASTM G 28 A/B and G 48 C/D showed no deterioration of the 42% cold reduced alloy in comparison to the solution annealed condition. Only for material, which was 15–30% cold reduced a minor, slightly unsatisfactory behaviour was observed, which was attributed to the strong inhomogenity of the microstructure, thus resulting in the formation of a local element between the severe deformed rim and the undeformed centre of the material. Current density‐potential curves measured in 1 M NaCl according to ASTM G 61/98 revealed no influence of cold deformation on the electrochemical behaviour of the alloy.     

Electrochemical evaluation of Ti‐13Nb‐13Zr,Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys for biomedical application by long‐term immersion tests

In this investigation the electrochemical behaviour of the Ti‐13Nb‐13Zr, Ti‐6Al‐4V and Ti‐6Al‐7Nb alloys, for application as implant materials was evaluated in Hanks' solution by electrochemical techniques. The alloys were immersed in this solution for 410 days and periodically they were tested by electrochemical impedance spectroscopy. At the end of this period, polarization curves of the three titanium alloys were obtained. The electrochemical impedance experimental results were interpreted using an equivalent electrical circuit that simulates a duplex structure oxide composed of an inner compact layer, here called barrier layer, and an outer and porous layer. The results indicated that all the alloys present a very high corrosion resistance in the electrolyte used, typical of passive alloys, and that the corrosion resistance is mainly due to the barrier layer. The passive like behaviour was maintained during the whole period of test.     

Zur Entwicklung der elektrochemischen potentiokinetischen Reaktivierungsmethode – des EPR‐Tests

On the development of the electrochemical potentiokinetic reactivation method – the EPR‐test Electrochemical potentiokinetic reactivation methods, but also other polarisation methods, are suitable as quasi non destructive testing methods to describe the corrosion resistance of steels and alloys without needing a very pretentious surface preparation. They may be used also to check finished components or even on site such in service, especially welding areas or weld overlays. Compared to standard tests they are quicker, more sensitive, more objective and quasi quantitative. They may be applied not only with austenitic standard steels but also with high alloyed ferritic, martensitic or duplex steels as well as with nickel based alloys. After any case of heat treatment the sensitivity to intergranular corrosion but also to pitting or selective attack can be determined by these testing methods. They may inform on the general corrosion resistance or the microstructural changes which may occur with materials, too. Based on the experience of many years with such methods which, starting from the early developments, have now been applied to very different materials, one may attend new fields of application in research and also as a nationally and internationally testing standard.     

Exfoliation and intergranular corrosion resistance of the 2198 Al–Cu–Li alloy with different thermomechanical treatments

In this study, the resistance to exfoliation and intergranular corrosion (IGC) of the 2198 Al–Cu–Li alloy submitted to different thermomechanical treatments (T3, T8, and T851) was investigated. The tests were carried out following the standard practices, ASTM G34-18 and ASTM G110-15, respectively. All the tested alloys showed susceptibility to exfoliation and some alloys showed susceptibility to IGC, but the artificially aged alloys presented a higher tendency to exfoliation. The extensive hydrogen evolution reaction (HER) was observed on the surfaces of artificially aged alloys when immersed in the EXCO solution. The HER resulted in an increase in solution pH with the time of immersion. Also, the weight losses related to the artificially aged alloys were higher than that of the naturally aged ones. The T8 treatment was the only condition that resulted in susceptibility to both, intergranular and transgranular corrosion, whereas the T851 treatment did not show IGC susceptibility, only transgranular corrosion. Finally, the 2198-T3 condition showed the highest corrosion resistance among the thermomechanical treatments tested. The results of the 2198 alloy subjected to various treatments were compared with that of the 2024-T3 alloy. This last alloy showed higher resistance to exfoliation and IGC as compared with the 2198 alloy.     

Corrosion behavior of Al2024‐T3, Al5052‐H32, and Al6061‐T6 aluminum alloys coated with DLC films in aviation fuel medium,Jet A‐1 and AVGAS 100LL

In aircraft and storage fuel tanks, many environmental corrosion factors can be present, which can cause breakdown to the passive film of aluminum causing severe localized corrosion. Aiming to a solution to this problem, diamond‐like carbon (DLC) films were deposited on substrates Al 2024‐T3, Al 5052‐H32, and Al 6061‐T6. The substrates with and without DLC films were submitted to a corrosive immersion test with aviation fuel Jet‐A1 and AVGAS 100LL. The surface morphology from aluminum substrates with and without DLC films was analyzed before and after 7 and 30 days of corrosive immersion tests by SEM. The adhesion of the DLC films was evaluated according to the ASTM‐C1624 scratching test standard, and the optical profilometry was used to measure the thickness and roughness. Silver nanoparticles were also added in DLC films samples due to its antibacterial properties to check the stability and resistance against corrosive fuel medium. The results indicated an improvement of corrosion resistance just with DLC films. The silver nanoparticles did not present any advantages in this corrosion protection, and the aluminum alloys suffered more corrosion than when protected by DLC films.     

New engine coolant for corrosion protection of magnesium alloys

The global trend toward decreasing of atmospheric pollution, by saving fuel consumption in vehicles, has led to extensive interest of using lightweight metals such as magnesium alloys, in engine and cooling system components. The modern coolant is not intended to prevent corrosion of magnesium alloy in the engine cooling systems. We have developed a new coolant that aims to protect Mg alloy parts together with all other commonly used metals. Several inhibitor formulations were tested, according to glassware test (ASTM D1384) and heat transfer conditions (ASTM D4340). Mg alloys EZ33 and WE43 were added to the standard sets of metals and the corrosivity of different types of formulations was determined by weight loss measurements. The new anticorrosive coolant showed high performance in all tested metals including magnesium alloys and it satisfied the requirements (ASTM D3306). Cyclic potentiodynamic polarization curves have been used to study electrochemical corrosion behavior of the magnesium alloys EZ33 and WE43 in aqueous solution containing the inhibitors and ethylene glycol (33 vol%‐EG prepared with corrosive water according to ASTM D1384) and compared to a reference coolant with no inhibitor. It was found that a passive film was created upon the Mg alloys, which exhibited high corrosion resistance against pitting.     

Corrosion behavior of Ti‐xNb‐13Zr alloys in Ringer's solution

Ti‐6Al‐4V alloy has been widely used in restorative surgery due to its high corrosion resistance and biocompatibility. Nevertheless, some studies showed that V and Al release in the organism might induce cytotoxic effects and neurological disorders, which led to the development of V‐free alloys and both V‐ and Al‐free alloys containing Nb, Zr, Ta, or Mo. Among these alloys, Ti‐13Nb‐13Zr alloy is promising due to its better biomechanical compatibility than Ti‐6Al‐4V. In this work, the corrosion behavior of Ti, Ti‐6Al‐4V, and Ti‐xNb‐13Zr alloys (x = 5, 13, and 20) was evaluated in Ringer's solution (pH 7.5) at 37 °C through open‐circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Spontaneous passivity was observed for all materials in this medium. Low corrosion current densities (in the order of 10^?7 A/cm²) and high impedance values (in the order of 10⁵ Ωcm² at low frequencies) indicated their high corrosion resistance. EIS results showed that the passivating films were constituted of an outer porous layer (very low resistance) and an inner compact layer (high resistance), the latter providing the corrosion resistance of the materials. There was evidence that the Ti‐xNb‐13Zr alloys were more corrosion resistant than both Ti and Ti‐6Al‐4V in Ringer's solution.     

Die wirtschaftliche Verwendung von hitzebeständigen Werkstoffen in Prozeßöfen der Petrochemie

Rational utilisation high temperature materials in petrochemical furnaces While tubular furnaces in petrochemical industries were originally manufactured of cast materials, advantage is being taken now increasingly from wrought materials processed by hot deformation. These materials are characterized by a high degree of homogeneity and enable long tubular material to be obtained without transverse weld seams; in addition, they allow the use of additional specific alloying elements. In the EDC furnaces (ethylene-dichloroethylene) for the manufacture of vinyl chloride use is made of the high corrosion resistance of nickel alloys; these materials possess a remarkable resistance in chlorinated media, in particular at temperatures exceeding 600°C. Another property of importance in such conditions is the resistance to stress corrosion cracking produced by the formation of hydrogen chloride and chlorides. In the enthylene production furnaces operating at temperatures above 800°C steels are used with 32.5% Ni and 21% Cr; they resist carburazation and are creep resistant. These materials are also used for reforming furnaces and in particular for the catalyst tubes where high resistance to creep cracking is indispensable.     

Eigenschaftsprofil des neuentwickelten Stahls X2 CrNiMnMoN 23 1764

Properties of the newly developed steel X 2 CrNiMnMoN 23 17 6 4 A well balanced alloying composition allows to produce the high alloyed stainless steel X 2 CrNiMnMoN 23 17 6 4 containing more than 0.4% nitrogen by conventional processes. Features of this material are a stable fully austenitic microstructure, 0.2% yield strength higher than 420 N/mm², excellent corrosion resistance and good working and welding properties. The critical pitting temperature evaluated in ferric chloride solution according to ASTM G 48–76 standard is higher than 85 deg C and it is only lightly lowered when tested in the as welded condition. By these properties Remanit 4565 S may bridge the gap between high-alloyed stainless steels and non-ferrous materials for applications in the chemical process industries.     

Hot rolled clad plates for application in flue gas desulfurization (FGD)-systems

A newly developed roll cladding technique is described in this article. This so called on-line-technique allows roll claddings with materials that were not yet considered to be suitable for roll cladding. Herewith it was made possible to successfully produce hot rolled clad plates with base metal RSt 37-2 and clad metals 1.4529, alloy 625, alloy C-22 and alloy C-276. Several investigations were made on these clad plates: The mechanical and technological properties of the base metal and the clad metals all met the specified limits. The plates showed excellent bonding behaviour. The microstructures of the claddings were free of precipitates. The corrosion resistance of the claddings was checked using standard tests, and compared with solution or soft annealed solid plates: The resistance against intergranular corrosion was tested in the Streicher test according to ASTM or SEP, in the ASTM G28B-test and in the Huey test. The susceptibility to pitting or crevice corrosion was checked by determination of CPT and CCT according to ASTM or Cabot. In all these tests the corrosion resistance was equivalent to the corresponding solid materials. Finally the welding process of such clad plates is outlined, and some experiences in FGD-plants are reported.     

High temperature corrosion of structural materials under gas‐cooled reactor helium

The Generation IV International Forum has selected six promising nuclear power systems for further collaborative investigations and development. Among these six concepts, two candidates are Gas Cooled Reactors (GCR), namely the Very High Temperature Reactor (VHTR) and the Gas‐cooled Fast Reactor (GFR). The CEA has launched a R&D program on the metallic materials for application in an innovative GCR. Structural GCR alloys have been extensively studied in the past three decades. Some critical aspects for the steels and nickel base alloys resistance under the service conditions are microstructural stability, creep strength and compatibility with the cooling gas. The coolant, namely helium, proved to contain impurities mainly H₂, CO, CH₄, N₂ and steam in the microbar range that interact with metals at high temperature. Surface scale formation, bulk carburisation and/or decarburisation can occur, depending on the atmosphere characteristics, primarily the effective oxygen partial pressure and carbon activity, on the temperature and on the alloys chemical composition. These structural transformations can notably influence the mechanical properties: carburisation may induce a loss in toughness and ductility whereas decarburisation impedes the creep strength. There is a valuable theoretical as well as practical knowledge on the corrosion of high temperature alloys in the primary circuit of a GCR but this past experience is not sufficient to qualify every component in a future reactor. On the one hand, the material environment could be significantly different from the former GCR's, especially regarding the higher temperature. On the other hand, the materials of interest are partly different. Ni‐Cr‐W alloys, for instance, may offer significant improvement in the maximum operating temperature as far as the mechanical properties are concerned. However, their corrosion resistance toward the GCR atmosphere is still unknown. We describe here our first corrosion tests of Haynes 230, a high strength Ni base alloy containing tungsten, under low oxidising helium at 750 °C or 950 °C. The experiments were carried out in a purposely‐designed device that allows controlling low impurity partial pressure. The test duration was up to 1000 h. The corrosion behaviour was assessed through gravimetry and microscopy. Some specimens were investigated using EDS and DRX. The results under low oxidizing potential are compared to the oxidation kinetics in air. Tentative interpretations are proposed based on published models.     

Corrosion of magnesium alloys in commercial engine coolants

A number of magnesium alloys show promise as engine block materials. However, a critical issue for the automotive industry is corrosion of the engine block by the coolant and this could limit the use of magnesium engine blocks. This work assesses the corrosion performance of conventional magnesium alloy AZ91D and a recently developed engine block magnesium alloy AM‐SC1 in several commercial coolants. Immersion testing, hydrogen evolution measurement, galvanic current monitoring and the standard ASTM D1384 test were employed to reveal the corrosion performance of the magnesium alloys subjected to the coolants. The results show that the tested commercial coolants are corrosive to the magnesium alloys in terms of general and galvanic corrosion. The two magnesium alloys exhibited slightly different corrosion resistance to the coolants with AZ91D being more corrosion resistant than AM‐SC1. The corrosivity varied from coolant to coolant. Generally speaking, an organic‐acid based long life coolant was less corrosive to the magnesium alloys than a traditional coolant. Among the studied commercial coolants, Toyota long life coolant appeared to be the most promising one. In addition, it was found that potassium fluoride effectively inhibited corrosion of the magnesium alloys in the studied commercial coolants. Both general and galvanic corrosion rates were significantly decreased by addition of KF, and there were no evident side effects on the other engine block materials, such as copper, solder, brass, steel and aluminium alloys, in terms of their corrosion performance. The ASTM D 1384 test further confirmed these results and suggested that Toyota long life coolant with 1%wt KF addition is a promising coolant for magnesium engine blocks.     

Pitting and Intergranular Corrosion Resistance of AISI Type 301LN Stainless Steels

The pitting and intergranular corrosion (IGC) resistance of AISI type 301LN stainless steels were evaluated using ASTM methods, anodic polarization, and electrochemical impedance techniques. The IGC results indicated that the microstructure of the samples after sensitization heat treatment at 675 °C for 1 h shows step or dual structure for both imported and indigenous materials indicating insignificant Cr₂₃C₆ precipitation. The results of immersion tests in boiling 6% copper sulfate + 16% sulfuric acid + copper solution for 24 h followed by the bend test (ASTM A262 Practice-E method) indicated no crack formation in any of the tested specimens. Pitting corrosion resistance carried out in 6% FeCl₃ solution at different temperatures of 22 ± 2 and 50 ± 2 °C (ASTM G 48) up to the period of 72 h revealed pitting corrosion attack in all the investigated alloys. The potentiodynamic anodic polarization results in 0.5 M NaCl revealed variation in passive current density and pitting potential depending on the alloy chemistry and metallurgical condition. The passive film properties studied by electrochemical impedance spectroscopy (EIS) correlated well with the polarization results. The x-ray diffraction (XRD) results revealed the presence of austenite (γ) and martensite (α′) phases depending on the material condition. The suitability of three indigenously developed AISI type 301LN stainless steels were compared with imported type 301LN stainless steel and the results are highlighted in this article.