Due to their thermal expansion coefficient being close to those of ferritic steels, NiAl atmospheric plasma spraying (APS)-coatings have been chosen to be tested in chloridizing atmosphere. A new type of quasi-stability diagram which couples thermodynamic and kinetic approaches was developed to define the stability domains of the pure metals aluminium and nickel in dynamic chlorine-and oxygen-containing atmospheres. NiAl APS-coatings (300 mm) with low porosity on steels were applied and tested in an atmosphere containing 0.2 v/o chlorine and 3 ppm oxygen. After 280 h of exposure one quarter of the outer coating was transformed into a protective alumina layer. Much nickel depletion, due to the evaporation of nickel chlorides, was observed.

The corrosion of metals at high temperatures in halogen containing environments differs from most other types of high temperature corrosion in that in addition to solid and liquid corrosion products volatile phases are also formed. The latter are not covered by the usual thermodynamic stability diagrams, which are often used for prediction of the corrosion products formed, and indirectly on whether corrosion has to be regarded as critical or not. The present paper aims at including the situation where gas phase corrosion products determine the extent of corrosion and a new type of diagram is developed and discussed for the example of chlorine containing environments. Part I of this paper (Latreche et al. Oxid Met 2009) was dedicated to the establishment of the basis for a new extended approach to a corrosion risk assessment diagram called the ''dynamic'' quasi-stability diagram. The present part deals with the principles of the new type of diagram and the establishment of specific diagrams for the most common alloying elements, i.e. Fe, Ni, Mo, Cr, Si, Al. For the development of the specific diagrams, the calculation of gas viscosities and gas diffusion coefficients of all relevant volatile potential corrosion products was conducted, with methods specified in the paper. The new ''dynamic'' quasi-stability diagrams were then compared to experimental results from kinetics investigations of pure metals, which showed quite good agreement with the experimental observations. Furthermore, the diagrams were used to assess H. Latreche the behaviour of the metallic elements in alloys. Again, in this case good agreement between prediction and experimental results was observed if the specific role of the different elements in the corrosion behaviour of the alloys was taken into account.

a b s t r a c tCommercial 800HT alloy was exposed to 49.1%H 2 -12.8%CO-3.1%CO 2 -1.6CH 4 -33.4%H 2 O gas at 21 bars and 570 • C up to 5000 h. Metal dusting attack by pitting was observed. The kinetics parameters were identified to be the incubation time, pit density and individual pit growth rate. These parameters were introduced in a nucleation-growth model to simulate the pitted surface area kinetics. This model was then extended to the volume considering several geometrical hypotheses. Considering only surface coalescence of the pits without their volume coalescence allowed to correctly reproduce the experimental mass loss kinetics. An even simpler conservative model was proposed for an easy lifetime modelling.

As in many other areas of material science there is an increasing need for computational tools predicting materials behaviour, also in high temperature corrosion. In many cases stability diagrams have been developed for the assessment of high temperature corrosion resistance of metallic materials where the potentially formed corrosion products are plotted as fields of stability as a function of the activities or partial pressures of the species in the reaction environment. One major drawback of these diagrams is that they only contain solid or liquid phases as reaction products, but in halogen induced high temperature corrosion volatile metal halides are also formed. In order to overcome this drawback, the present paper deals with the development of a new type of diagram for high temperature halogen corrosion which will be developed and discussed for the example of oxygen/chlorine environments and which will be named ''dynamic'' quasi-stability diagram. In part I of the paper, the basis for this new approach to the corrosion resistance assessment of metals under chlorine environments is established using the present understanding described in the literature. A comparison is made between the concept of the conventional stability diagram and the recently developed ''static'' quasistability diagram, where in the latter case the evaporation of the gaseous metal H. Latreche chloride phases is taken into account through the use of a critical metal chloride partial pressure of 10 -4 bar, as a criterion distinguishing critical and non-critical corrosion conditions. Concluding from the existing knowledge, the fundamentals for the new improved approach are developed in the form of a ''dynamic'' quasistability diagram. This new type of diagram is based not only on thermodynamic considerations (as for the diagrams existing so far) but also on the products and reactants flow through a gas boundary layer formed on the material surface. As a consequence, in this approach the criterion for corrosion resistance is given in terms of a metal recession rate for dynamic conditions as encountered in most industrial applications. In part II diagrams of the new type will be established for the most common alloying elements, i.e. Fe, Ni, Mo, Cr, Si and Al, and a comparison with experimental data will be performed.

Commercial and model alloys were exposed at 570°C to metal dusting conditions at 1 and 21 bar in CO-H 2-H 2 O and CO-H 2-CO 2-CH 4-H 2 O atmospheres, respectively, with similar a c and P O2. γ" precipitation beneath the surface of 625 superalloy was due to Nb diffusion toward the Cr-depleted zone formed during oxidation. 693 superalloy was more carburised than 690 superalloy. α-Al 2 O 3 formation was believed to induce cracks through the oxide scale, resulting in the alloy carburisation. Cu addition to a FeNiCr model alloy lead to the formation of an almost continuous Cu layer at the oxide/metal interface, greatly improving the metal dusting resistance.