Comparative corrosion studies of zirconia particle reinforced high‐alloyed 14‐6‐6 CrMnNi‐steel by potentiodynamic polarisation and one year outdoor exposure

The corrosion behaviour of a spark plasma sintered CrMnNi high‐alloyed steel, without and with 10 vol.% zirconia particle reinforcement was investigated by potentiodynamic polarisation in 5 wt.% sodium chloride solution and by an one year outdoor exposure test. After the polarisation test both materials revealed a similar damage behaviour as in the outdoor exposure test. The unreinforced material was attacked by pitting corrosion, whereas the particle reinforced material showed a more homogeneous corrosion attack involving an intensified particle detachment. Furthermore, investigations of the unreinforced steel by white light interferometry revealed pit depths of approximately 65 µm after potentiodynamic polarisation and 36 µm after one year outdoor exposure. In contrast, for the composite a damage evolution due to particle detachment was found in both experiments.     

Theoretical and practical aspects of seeding technology for fusion welding

In numerous applications, contradictory properties are demanded from welded joints; for example, high seam strength and good formability or high hardness value and wear resistance with the highest possible toughness. Controlling the solidification behaviour by means of seeding the molten weld metal represents a very effective method of fulfilling such contrary demands. For this purpose, seeding methods were modified for welding technology in such a way that microstructures can be specified. The choice of seeding materials is based on a theoretical model which describes the functioning mode of complex inoculants in a melt. The effectiveness of the procedure was investigated on ferritic chromium‐steels by using various seeding methods and complex inoculants. Moreover, pronounced seeding effects were verified on nickel based superalloys and titanium.     

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.     

Initiation and propagation of stainless steel pitting corrosion under heat flux

An effect of heat flux on initiation and propagation of pitting corrosion of austenitic stainless steel in chloride environment has been studied using electrochemical and exposure methods. The experiments were performed at constant surface temperature of 60°C and heat flux from ? 15 to + 74 kW m^?2. The presence of heat flux from metal to solution shifted the breakdown potential towards noble values and lowered the pit density, whereas the pit propagation rate increased. Presence of maximum heat flux caused, in comparison with isothermal conditions, increase of the breakdown potential by approx. 100 mV, reduction of pit density by 30% and increase of the average pit depth by 40%. The positive effect of heat flux from metal to solution was given mainly by improving the protective ability of the passive film, as the amount of oxygen available at the metal surface increased. With a smaller significance, the effect of intensified mass transfer, which made accumulation of the chloride ions on the surface more difficult, occurred.     

Impedance of Cr18Ni10 stainless steel in sulphate solutions after a low‐temperature plasma nitriding

The 00Cr18Ni10 stainless steel was examined before and after plasma nitriding in a 80%N₂/20%H₂ gas mixture at 425°C for 30 h, resulting in the formation of a supersaturated solid solution of nitrogen in austenite (S phase) with 12.9 wt.% N at the surface. Electrochemical measurements were made on the as‐nitrided surface and after abrasion to various depths. Polarisation curves and impedance were measured in 0.1 M Na₂SO₄ acidified to pH 3.0; some measurements were also made at pH 6 and 9. It was found that nitrogen decreased the corrosion resistance of the steel in the acidified solution (pH 3.0), but increased it in the neutral and near‐neutral solutions (pH 6.0 and 9.0). The deleterious effect of nitrogen at pH 3.0 was at potentials of the active state much stronger and more dependent on the nitrogen concentration than in the passive state. It was suggested that the increased anodic reactivity of the nitrided steel in the active state is inherent for the lower thermodynamic stability of the supersaturated solid solution of nitrogen, whereas the increased corrosion resistance in the neutral and near‐neutral sulphate solutions is associated with the effect of nitrogen on anodic films.     

Interactions between polysulphides and stainless steel materials

Ability of polysulphide to enhance the potential of digester materials (S30400, S31603, S32205, S32304) was studied in the impregnation stage of the pulping process. Polysulphides were generated by adding elemental sulphur to alkaline sulphide solutions. Polysulphide levels 0.3–0.48, 1.25–2, 5–8 and 12.5–20 g/l enhanced the potentials of the steels by 70, 90, 170 and 190 mV, respectively. The polysulphide concentrations were calculated values, because concentrations of polysulphides are very difficult to measure in kraft liquors. No weight changes were observed, but slight accumulation of sulphur onto surfaces, particularly onto S31603 was seen in X‐ray fluorescence analysis. Otherwise, differences in behaviour among the steel materials were minimal.     

Simulation of the distortion of 20MnCr5 parts after asymmetrical carburization

The carburization of steel parts changes the chemical composition in the surface. According to the change of chemical composition a change in density occurs. If the change in density is distributed in an asymmetrical way, distortion can not be avoided. In order to find out the effects of specifying factors on distortion, several finite element simulations of asymmetrically carburized 20MnCr5 discs were carried out. All simulated activities during the carburizing process occur at austenitization temperature. First, an analysis with several geometries – with changed radii and changed thicknesses – shows, that the influence of the partial carburization on distortion decreases exponentially with increasing thickness of the disc. Second, an analysis with four different carbon profiles shows, that a change in carbon profile leads to different distortion behaviour. In order to check the quality of the calculated results, the distortion of a partially carburized and afterwards oil‐quenched disc was measured by coordinate measurements. This result was compared with the simulated distortion and shows a good agreement.