Experiment and numerical simulation of aluminum silicon alloy corrosive treatment in the water vapor generation autoclaves
Graphical Abstract
Introduction
Aluminum and its alloys are easy to be corroded in the air, especially in the humid air. Their corrosion has been widely studied and applied for protected coatings [1], [2], [3], micro-capsules preparation [4], [5], [6], [7], catalysts and catalyst carriers [8], [9], porous membranes and hydrogen carriers [10] in the past few decades. Hydroxide films composed of boehmite, pseudo-boehmite, or bayerite can be formed on the surface of aluminum alloys by reaction with water. R.U. Din et al. [1] produced boehmite films and oxide layers on aluminum alloy 6060 using a steam-based process. The steam-based process resulted in homogenous growth of oxide layer and superior coverage over inter metallic particles when compared with chromate-based conversion coatings. Nomura and Han [4], [5], [6], [7] fabricated micro-encapsulated phase change materials from Al-Si alloys through two steps. The first step is to form a boehmite precursor shell by corroding Al-Si alloy powders in boiling distilled water or pressured water vapor (WV). All the above studies found the morphologies and structure of hydroxide groups on the metal surface played a key role in the formation of protected coatings and shells.
On the other hand, the temperature, pressure and WV volume fraction of humid air, as well as time and pH have great effect on the corrosion behavior. Aluminum hydroxides with different phase compositions, morphologies and texture characteristics were synthesized by changing the wet oxidation conditions in Kazantsev’s work [11]. Sarah [12] found the hydroxide film with a layered structure was formed on the surface of AA-6061 aluminum alloy after corrosion in the autoclave under temperature 70 °C and 100 °C. The outer layer is very porous and is less protective for the alloy corroded at 70 °C. Din [13] found the thickness of the oxide layer as well as the compactness on the aluminum alloy increased with WV pressure. Our previous studies [4], [14] showed that needle-like and prism-like aluminum hydroxide can be obtained respectively after treatment of Al-Si alloy under different WV pressure and holding times. Chen [15] reported a kind of Al-Si powders that showed popcorn-like shape transformation in the reaction with low-temperature water va por, which demonstrated that circumventing the passivation of aluminum alloy through the self-sustaining shape changes in hydrolysis reaction can become a reality.
Although, wide researches on the reaction of Al-Si alloy with water or WV have been carried out in the past [11], [12], [13], [14], the clear explanation for the coating precursor shell growth and structure on the aluminum alloy surface under WV is deficient. And it was not possible to acquire a new explanation without more information about the WV parameters because it's difficult to measure them directly. Numerical simulation was used to predict the fluid flow, temperature and steam distribution in a steam sterilizer [16], [17], [18], [19], [20]. Feurhuber [16], [17] developed a CFD model to predict the steam quality according to the fluid characteristics and the theoretical inactivation of bacteria in a modern steam sterilizer. Iden et al. [18] estimated the soil hydraulic properties using numerical simulation in the laboratory evaporation experiments. Steam flows during evaporation and condensing have been numerically simulated in other areas [18], [19], [20].
However, few articles have been published to investigate the correlations between parameters of humid air and reactants on the surface of aluminum and its alloys. This study aims to explain the coating precursor shell growth and structure on the aluminum alloy surface under WV through aluminum silicon alloy corrosion experiment and numerical simulation. At first, two different kinds of pressured autoclaves were selected to generate water vapor (WV). The Al-12 wt%Si alloy powders were pretreated in the two autoclaves with different WV parameters. The morphologies and structures were observed. Then, mathematical models of multiphase-flow and heat transfer including water, air and WV, and heat transfer in the two autoclaves were developed. The flow fields and distributions of temperature, pressure, WV and air in the autoclaves were numerically calculated. The shell forming mechanism was analyzed combined with the results of experiments and numerical calculations. The correlations between WV parameters and corrosion behavior of Al-Si alloy under humid air were established. This research work will help people to understand the WV corrosion behavior of aluminum alloys, and provide guidance suggestions on the selection of WV generation establishment and parameters for their coatings and shells preparing.
Section snippets
Experimental equipment
Two kinds of WV generation autoclaves were used in this study. Both vessels were cylindrical shapes and one with heating on the side wall (named as SH), the other one with heating at the bottom wall (named as BH). The liquid water (LW) was put into the vessel and heated, when the temperature (T) was higher than the saturation temperature (Tsat) at the fixed static pressure, WV generated and flew out from top tube. The temperature and static pressure of mixed gas (Tmix and Pmix) were measured by
Simplification and assumptions
In order to simplify the calculation, the following assumptions were proposed.
- (1)
Due to the symmetry of the autoclave, only a half of the geometries were modeled and a symmetry boundary condition is used.
- (2)
The air and WV were treated as ideal gas, while LW was treated as incompressible Newtonian fluids. The density and viscosity of LW were dependent of temperature.
- (3)
Water and air were uniformly distributed in the down zone and up zone of the vessel respectively before creation of vapor.
- (4)
The WV
Conclusions
The present study carried out Al-Si alloy corrosion experiment in two different WV generation autoclaves and developed a mathematical model to calculate the distribution of velocity, temperature, static pressure, volume fraction of WV, LW and air in the autoclaves. The mixture multiphase model was used to describe the multiphase flow in the autoclave, and the evaporation-condensation model was used to describe the phase transition and inter-phase mass transfer between liquid and vapor. The
CRediT authorship contribution statement
Meijie Zhang: Supervision, Funding acquisition, Conceptualization, Methodology, Writing – original draft, Writing – review & editing. Yao Wang: Investigation, Numerical simulation, Writing – original draft. Qiulin Xia: Numerical simulation. Jixiang Zhang: Major revision. Cangjuan Han: Resources. Haifeng Li: Methodology, Writing – original draft. Huazhi Gu: Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 52072276) and the Foundation of Huzhou Municipal Science and Technology Bureau, Zhejiang Province (Grant No. 2020ZD2016). The simulations in this paper have been done on the computing facilities in the High Performance Computing Center of Wuhan University of Science and Technology with the support of Kirill Andreev.
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