Stability Of Passive Layer Research Articles

The Effect of Water Vapor on Passive-Layer Stability and Corrosion Behavior of Fe–Al–Cr Base Alloys

Although it has been reported that additions of water vapor to high-temperature corrosion environments accelerate corrosion, the role of water vapor on increasing the corrosion rate of Fe–Al–Cr alloys and coatings is not very well understood. In order to better understand these effects, multiple Fe–Al–Cr base alloys were tested at 500°C for 100 hr in three different multi-component corrosive gases with and without water vapor. The three gases were a sulfidizing gas, a mixed oxidizing/sulfidizing gas, and an oxidizing gas. Thermogravimetric testing showed that the corrosion kinetics increased when water vapor up to 6% was added to the atmospheres. The surfaces of the exposed samples were considered carefully to determine if the addition of water vapor changed the morphology of the corrosion products and, more importantly, affected the passive layer. It has previously been shown that the formation of nodules can be caused by the inability of the passive layer to re-heal itself after the presence of a defect allowing fast growing non-protective corrosion products to externally grow from the surface. In this study, the amount of external nodules that formed on the surface of the alloys was shown to increase with the addition of water vapor. An increase in the amount of external nodules present due to additions of water vapor gives an indication that water vapor accelerates the passive-layer breakdown. It was observed that the scale morphology correlated well with the corrosion kinetics for the exposed alloys, which showed that water vapor increased the corrosion rates of the alloys.

In-vitro corrosion resistance study of hot worked Ti-6Al-7Nb alloy in a isotonic medium

This investigation reports the results of linear polarization of hot upset Ti-6Al-7Nb bar samples. Current-potential curves recorded in Hank’s solution were analyzed by correlating characteristics of passivation and microstructures obtained after processing. Results have shown that it is important to select temperature process and deformation rate as parameters when more noble potential values are required. Low deformation rate facilitates the formation of beta phase that is retained in the structure at room temperature shifting the corrosion potential to more positive values. However, samples hot compressed from 750 °C to 1030 °C showed passive layer stability over a wide range of potentials extending from -0.15 V to 1.75 V vs SCE . Furthermore, passive films grown onto the Ti-6Al-7Nb samples surfaces have shown no sign of rupture for the processing conditions selected for this study.

Material compatibility tests with flowing Pb17Li eutectic

The material compatibility studies are related to the self-cooled liquid metal blanket concept. The studies are mainly concerned with the corrosion of the martensitic steel X18CrMoVNb 12 l (MANET) in flowing liquid Pb17Li eutectic at 500 and 550°C (773 and 823 K) in the PICOLO loop. Additional, some specimens of the austenitic steel Type 316 L(N) and of the loop material, the ferritic steel X10CrAl 7, were exposed to the eutectic alloy, the flow velocity of which was maintained at 0.3 m/s. The temperature gradient in the two tests series was 240 and 190 K, respectively. The specimens were corroded by the following liquid alloy by means of the removal of material from their surfaces. The material loss generally reaches a region of linear rates after an initial period of slower corrosion rates. It is shown that the linear rates for iron rich materials are in good agreement with rates calculated on the basis of an hydraulic model for turbulent flow conditions. The faster corrosion of the austenitic steel is due to the selective leaching of Ni and Mn out of surface layers which change their structure into the α - Fe type and get a large number of cavities. The initial period of slower corrosion seems to be related to the composition of the material. At lower Cr contents, the initial period becomes very short or disappears completely. This indicates an influence of the stability of passivation layers on the inhibition of the Pb17Li corrosion of steels.

NH4OH-based etchants for silicon micromachining: Influence of additives and stability of passivation layers

Abstract Solutions based on ammonium hydroxide and water were investigated for anisotropic etching of monocrystalline silicon. In 2.65 M NH3 solutions at 80 °C, the addition of H2O2 in a concentration between 0.65 × 10−2 M and 1.84 × 10−2 M increases the silicon etch rate in the (100) direction to 75 μm/h, which is a factor of about 2.5 higher compared to pure etchants. In addition, when the pH value was adjusted above 11.3 silicon etching proceeds in H2O2-doped solutions without formation of pyramidal-shaped hillocks. The influence of the pH value and H2O2 concentration on the etching behaviour is discussed in detail. Passivation layers, like silicon oxides and silicon nitrides formed under various preparation conditions, and metals show excellent stability in AHW solutions. The temperature dependence of etch rates as well as the influence of post-treatments on the etching behaviour is discussed.