Lochkorrosion nichtrostender Stähle und hochlegierte Fe/Ni/Cr/Mo‐Werkstoffe ‐ Entwicklungsstufen und Untersuchungsverfahren

Pitting corrosion of stainless steels and high‐alloyed Fe/Ni/Cr/Mo materials – Stages of development and test methods In relation to the stages of development of pitting the characteristics and factors of influence are explained. After the explanation of distinct pitting potentials different ways of the detection of the stages of pitting are highlighted and their influencing through characteristic featues of the metal, of the chemical composition of the medium, and of the physical conditiones are pointed out. Based upon these facts, loading and methods of analyses and evaluation for the test of pitting resistance are discussed and chemical and electrochemical tests are described which are important to solve technical problems. Furthermore a test for the determination of the pitting resistance of welds directly performed at apparatus and equipments is explained which is still in the development stage.     

Optimierung des Schweißprozesses hochlegierter Stähle und Verbesserung der Korrosionsbeständigkeit der Schweißverbindungen

Optimization of the welding process of high alloyed steels and improvement of corrosion behaviour of welded joints The optimization of welding processes is necessary to obtain a good durability of the welded joints connected with a minimization of the corrosion performance. Welding processes represent a considerable influence of the material. The formation of precipitations, strong structure changes, increasing of the residual stress and not at all undefined surface layers are possible. All these changes have a great influence on the corrosion behaviour. Particularly tempering tarnish changes the passive layer which is decisive for the corrosion resistance. But also surface treatment methods can influence the corrosion behaviour. Therefore both the welding process and an “after‐care” coordinated with the respective welding process had to be optimized. The optimization of the welding process was carried out by variation of the energy per unit length and the use of different protective gases. For a selection of a surface treatment method it has to be taken into account that an obvious remove of the tempering tarnish doesn't lead to an improvement in the corrosion behaviour. Traces of the working tool which can have a negative effect on the corrosion behaviour often remain on the surface. The influence of these different parameters on the corrosion property could be proved by electrochemical and surface analytical examinations. The investigations were carried out at specimens of two typical representatives of high alloyed austenitic steels and at welded joints, which had different surface treatments.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels Austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.5 to 3.6%) had been gas‐nitrided. By stepwise removal, samples could be prepared with various surface content of nitrogen up to 0.45%. The susceptibility against pitting corrosion of these samples had been tested by two methods: – determination of the stable pitting potential in 0.5 M NaCl at 25°C – determination of the critical pitting temperature in artificial sea water (DIN 81249‐4) The influence of nitrogen to both determined parameter can be described well by PRE = Cr + 3,3 · Mo + 25 · N That means for the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen.     

Untersuchungen zum Einfluss des Stickstoffs auf das Lochkorrosionsverhalten hochlegierter austenitischer Cr‐Ni‐Mo‐Stähle (Teil II)

Investigation of the influence of nitrogen on the pitting corrosion of high alloyed austenitic Cr‐Ni‐Mo‐steels (Part II) Austenitic stainless steel (18% Cr, 12% Ni, Mo gradation between 0,06 to 3,6%) had been solution nitrided. By step‐by‐step removing, the samples could be prepared with various surface contents of nitrogen from 0.04 to 0.42%. In two test series the influence of nitrogen had been determined. The susceptibility against pitting corrosion of these samples had been tested by the chronopotentiostatical method. For the investigated steel composition and the used corrosion system there is no infuence of molybdenum on the effectiveness of nitrogen. The effectiveness of nitrogen can be described by the factor 25 in the PRE. By the investigation of the surfaces with the XPS analysis, it could be shown that the passivation and the pit nucleation is influenced by nitrogen. In these ranges NO_x, NH_x, and NH_z‐spectra have been detected. Bound Mo was found in steels containing molybdenum. It is assumed that the repassivation mechanisms of N and Mo work independently of each other. With the results efforts are supported to improve the pitting corrosion resistance also at molybdenum poor steels by surface nitriding or nitrogen alloying. The achieved results justify the assumption that the observed positive effect of the nitrogen may be extented to even higher nitrogen contents. A prerequisite for this is avoiding secondary phases in the matrix. The adverse influence of small particles is known well.     

Ausscheidungshärtbare Nickelbasis‐Superlegierungen für den Verschleißschutz im Hochtemperaturbereich

Precipitation‐hardenable nickel‐based super alloys for wear resistance in the high‐temperature range Precipitation‐hardenable nickel based filler metals are already available for the original manufacture as well as regeneration of high‐temperature‐resistant components. Their special strength properties at operating temperatures of about 800°C are based on their capability of being hardened by the so‐called γ′‐phase Ni₃(Ti, AI), which precipitates after a multistage heat treatment, among other factors. However, the weld‐surfaced components cannot be heat‐treated in this manner without impairing the mechanical qualities of the base metal. For this reason, a search has been conducted for a weld surfacing alloy which does not require multistage annealing, or which can be applied with a heat treatment which does not damage the substrate. The objective was the development of a precipitation‐hardenable test alloy for MIG weld surfacing with flux‐cored wire. For this purpose, the chemical composition of the alloy had to be selected for achieving high‐temperature properties similar to those of nickel‐based super alloys in the hardened condition without the need of post‐weld heat treatment which damages the substrate. The alloys were applied by multi‐pass weld surfacing and subjected to different heat treatments. The characteristic strength values were determined by hardness tests. Effects of the welding parameters as well as the resulting solidification shapes were characterised by metallographic analyses. Scanning electron micrographs of selected test alloys have confirmed the precipitation of the γ′‐phase and the formation of further hard phases. From the results, recommendations can be derived for weld surfacing of high‐temperature‐resistant claddings with respect to process control, alloy composition, and post‐weld heat treatment.     

Praxisnaher Nachweis zur Wirksamkeit von VCI‐Korrosionsschutzmitteln an flächigen Proben –Prüfverfahren nach BFSV –

Practical demonstration of the efficacy of volatile corrosion inhibitors using plane specimens – BFSV test method – In the sphere of temporary corrosion control for semifinished metal products the VCI (Volatile Corrosion Inhibitor) method has established itself successfully on the market. In the past the efficacy of the VCIs has been demonstrated in laboratory tests (delivery standards) which take little account of exposure under practical working conditions. The reproducibility of the results is also limited. In the context of AIF project 12115 (supported financially by the Federal Ministry of Economics) the BFSV Institute at Hamburg University of Applied Sciences has developed and verified a test method that correlates strongly with metal surfaces under practical conditions and produces results with good reproducibility. Procedures for determining actual climatic load profiles and an objective method of assessing the degree of corrosion have also been elaborated.     

Pitting corrosion of sheets of a nickel‐base superalloy

Cold rolled sheets of a nickel‐base superalloy are used in building gas turbine engines. After rolling to the final gauge, the sheets were annealed at 1180 °C. Pickling of the sheets was carried out to remove the scale and the oxidized layer. It was observed that there was heavy incidence of pitting with perforations at several locations due to pits penetrating through the thickness. The sheets got rejected, resulting in heavy loss. The superalloy contains 25 wt% chromium, 15 wt% tungsten. Tungsten provides high resistance to pitting corrosion. With such a high level of W, the grade has a high Pitting Resistance Equivalent. Further it is a single‐phase material. Pitting attack is thus totally unexpected. The failure was analyzed. Pickling process for this material normally involves the usage of salt bath, sulfuric acid, nitric acid, and/or hydrofluoric‐nitric acid baths. In the case under study, satisfactory pickling was not achieved this way; discoloration and deposits were persistently present on the sheet surface. In order to improve the surface quality, the sheets were dipped in hydrochloric acid bath for a long time. Hydrochloric acid bath was in aged condition; by that point of time, the bath went through extensive usage for pickling of different grades of steels. Consequently there was significant accumulation of iron salts in the bath. It is known that ferric and chloride ions cause significant acceleration in pitting. The bath was stagnant and the time of immersion of the sheets in the bath was long. The material failed under the combination of these adverse conditions. The paper brings out the details of the failure analysis carried out.     

Concrete cover cracking owing to reinforcement corrosion – theoretical considerations and practical experience

Reinforcement corrosion might lead to cracking and spalling of the concrete cover owing to the volume expansion associated with the deposition of some of the possible corrosion products. This is not only aesthetically unpleasing, it might also accelerate deterioration processes or become a safety issue for passing traffic. The present paper discusses first the mechanisms of carbonation‐ and chloride‐induced reinforcement corrosion and considers the chemistry of aqueous iron in order to identify the type of corrosion products as well as their location of formation. Furthermore, practical examples are summarised in order to compare the documented behaviour of a number of real structures with the theoretical considerations made. It is shown that for the case of purely chloride‐induced (pitting) corrosion, precipitation of corrosion products is strongly delayed or may even not occur. Implications are discussed with respect to time‐to‐corrosion prediction models and visual inspection of reinforced concrete structures. Both the theoretical considerations and the practical experience illustrate that relying on outwardly visible signs to detect internally on‐going corrosion must be done with caution if localised reinforcement corrosion cannot be excluded.     

Field test of superheater corrosion in a CFB waste boiler: Part I – Metal loss characteristics

The corrosion was investigated on a superheater test coil in a CFB waste boiler. The alloys ranged from ferritic steel T22 to nickel‐based Alloy 65 and the metal temperatures were between 460 and 540°C. The thickness of the deposit was alloy and temperature dependent. The low‐alloyed steels developed thick deposits at all temperatures while the deposit thickness increased with the temperature on the high‐alloyed steels and the nickel‐based alloy. The corrosion attack was alloy dependent and related to the deposit crest. The nickel‐based Alloy 65 was preferentially attacked directly under the crest of the deposit while the other alloys were preferentially attacked at the edge. The corrosion rate increased with temperature for X20, Alloy 304L, Alloy 310 and Alloy 825; decreased on Alloy 65; and was bell shaped on T22 and Alloy 28. Alloy 310 suffered from severe pitting corrosion in a line following the edge of the deposit crest. The best overall corrosion resistant alloy was Alloy 28.     

Field test of superheater corrosion in a CFB waste boiler: Part II – Scale formation characteristics

This study concerns the scales formed on the steels T22 , Alloy 310, Alloy 28 and the nickel‐based Alloy 65 in a superheater test coil at 460–540°C in a CFB waste boiler. The methods used for the characterisation of the scales included SEM, EDX, Auger spectroscopy and XRD. The deposits on the tubes consisted mainly of alkali chlorides and calcium sulphate. The scales formed consisted of Fe₂O₃ and Fe₃O₄ on the T22 steel, NiFe₂O₃ and Cr₂O₃ on Alloy 310 and Alloy 28, and Cr₂O₃ and NiO on Alloy 65. Rapid corrosion on the steel T22 was associated with the growth of an open columnar iron oxide below a thick porous chlorine‐containing scale. Pitting corrosion on Alloy 310 occurred and it may be associated with selective corrosion, first following the grain boundaries then uniformly attacking the metal. The only protective oxide was observed on Alloy 28 that formed an inner chromium oxide separating the chlorides from the metal. Dense thin chromium oxides were observed in the scale on Alloy 28, but no major cracks were found perpendicular to the tube. Alloy 65 suffered from grain boundary attack and was locally attacked under thick porous chromium oxide with nickel chlorides in the advancing front. Molybdenum was enriched at the interface to the metal on both Alloy 28 and Alloy 65.