The role of alloying elements in commercial alloys for corrosion resistance in oxidizing‐chloridizing atmospheres. Part II: Experimental investigations

In an extensive study the role of the alloying elements in commercial alloys for corrosion resistance was studied in air without and with 0.1 and 2 vol.% Cl₂, respectively. In the first part of this paper [1] the thermodynamic fundamentals were discussed on the basis of the new concept of the quasi‐stability diagrams. The second part which is presented here reports the results from investigations at 650, 800 and 1000°C and testing times up to 1000 hrs where 14 commercial alloys were tested with regard to their corrosion behavior. The materials were selected so that the role of the alloying elements Mo, C, Si, Al, N, Fe, Ni and Cr would be evident from the results. The exposure tests were followed by extensive microstructural analyses of the corrosion scales and the metal subsurface zones so that type, mechanism and extent of corrosion could be characterized in great detail. At the end a ranking was possible of the different materials and with regard to the detrimental or beneficial role of the different alloying elements. The present results thus provide a much deeper insight into materials resistance in oxidizing‐chloridizing environments at high temperatures.     

Entwicklung von Diffusionsschichten auf Nickelbasiswerkstoffen für den Einsatz in chlorhaltigen Hochtemperaturprozessen

Development of diffusion coatings on nickel base alloys for the use in chlorine‐containing high temperature processes To open up the possibility of using sewage sludge ashes as fertilizers the removal of their heavy metal contents is obligatory. A process newly developed at the BAM Berlin executes this separation in highly chlorine‐containing atmospheres at temperatures of up to 1000 °C [1]. Unfortunately there are no materials available which can withstand such conditions over longer periods of time. This project deals with the development of materials that allow the operation in highly corrosive environments. The corrosion resistance of nickel base alloys against chlorine‐induced high‐temperature corrosion will be optimized by application of aluminum‐ and/or silicon‐containing diffusion coatings. As coating method the pack cementation process was selected. In this process, the metal to be coated is embedded in a powder, consisting of the coating metal, a halogen‐distributor (e.g. ammonium chloride) and aluminum oxide as filler material. During an annealing process of several hours at temperatures of 800 to 1000 °C, gaseous metal halides form. They diffuse through the powder pack and decompose at the substrate surface, thereby depositing the coating metal. Subsequent solid phase diffusion results in the formation of a protective diffusion layer. From the thermodynamic point of view, materials with a high content of aluminum and silicon show best prerequisites to build up slow‐growing, stable oxide layers with a high potential to protect the material against corrosive attacks. The actual performance of the materials will be examined in long‐time tests under simulated field conditions (high temperatures and chlorine‐containing atmospheres).     

Fundamental investigation of the degradation laws of aluminide coatings during high‐temperature oxidation

During high‐temperature oxidation, the aluminide coating degrades by two mechanisms, one is the oxidation of the outer surface, and the other is Al diffusion into the substrate. The relevant laws are investigated for coatings on Co‐base superalloys oxidised in air at high temperatures. The driving force mainly comes from the oxidation reaction for the outer degradation layer, and the chemical potential for the inner degradation layer. By an approximation approach, it can be concluded that the life of the outward‐diffusion coating is roughly directly proportional to the second power of the main‐phase thickness of the coating.     

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solution at high temperature

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solutions as a function of temperature, test time, and sulfur content was investigated by weight loss measurements in this study. The results indicate that the corrosion rate of the steel increases with increasing test temperature. Besides, the corrosion rate increases during the initial test time and then decreases with longer test duration. Moreover, the corrosion rate rises with a higher amount of sulfur in the solution, and consequently a lower amount of sulfur leads to a decreased corrosion rate. In addition, the effect of Cr and Mo elements has been discussed.     

The hot corrosion behaviour of HVOF sprayed MCrAlX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900°C using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na₂SO₄ salt films. In the presence of NaCl in the Na₂SO₄ salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower Al content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat‐treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short‐term corrosion kinetic measurements and SEM analyses.     

High‐Temperature Corrosion of NiTi‐Shape‐Memory Alloys

Semi‐finished products and components made of NiTi‐shape‐memory alloys (NiTi‐SMA) are often subjected to heat treatment after their fabrication. During this heat treatment, oxide layers begin to form which contain a high amount of titanium. In this investigation special attention was drawn to the selective oxidation of Ti because a TiO_X‐layer can represent a Ni‐barrier and may therefore be of special use for medical applications. A comparison of the following three samples was carried out: A sample oxidised at room temperature, another that was heat‐treated in ambient air (600 °C/1min) and a third sample that was subjected to a heat treatment (600 °C/1min) in an atmosphere that oxidises titanium but reduces NiO in order to achieve a selective oxidation of the titanium. The analysis of the oxide layers was carried out by means of x‐ray photoelectron spectroscopy (XPS). It was shown that the ratio of titanium to nickel in the oxide layer can be substantially increased when performing the annealing treatments in a partial reducing atmosphere. Furthermore, a thermo‐gravimetric investigation of the material was carried out at 600 °C in dry air in order to estimate the growth of the oxide layers.