Research On Corrosion Behaviors Of Aluminium Tubes In Hirizontal Tube Falling Film Evaporator

The low-temperature multi-effect distillation technology has been widely used as it has many advantages over other methods, such as high product quality, high reliability and small temperature difference. However, the initial investment of this method is high, which severely restricts its further development. Using aluminum tubes instead of copper tubes in horizontal tube falling film evaporator can effectively reduce the cost, however, the corrosion problem of aluminum tubes in seawater environment has always been a trouble in practical engineering. To solve this problem, the study of the corrosion mechanism of aluminium tubes in seawater environment must be completed in the first place.An experimental equipment of horizontal tube falling film evaporator was designed and built. Firstly, the corrosion experiment was carried out in the horizontal tube falling film evaporator using natural seawater as the corrosive medium, the corrosion morphology changes of aluminum tubes and the influence factors on aluminum corrosion were studied. Then a comparative experiment using seawater concentrated by evaporation as the corrosive medium was carried out to study the effects of salinity onthe corrosion behavior of aluminum tubes in horizontal tube falling film evaporator. The results of these two experiments were compared and analyzed.Experimental results showed that aluminum tubes were vulnerable to cavitation erosion due to the special temperature and pressure environment in the horizontal tube falling film evaporator. Tubes were corroded after a period of time, the main form of corrosion was pitting corrosion. Under the experimental conditions, the starting time of the occurrence of corrosion in horizontal tube falling film evaporator was about38hours when using natural seawater (salinity2.9%) as the corrosive medium. At the initial stage of the corrosion, the corrosion rate was relatively high, then as the accumulation of corrosion products and the development of corrosion, the corrosion rate began to decrease. At the beginning of the corrosion, corrosion products played a catalytic role to accelerate the corrosion rate; latterly, corrosion products played a role to reduce the corrosion rate. The starting time of the occurrence of corrosion was about6hours using seawater concentrated by evaporation (salinity6%) as the corrosive medium. Compared with the condition using natural seawater as the corrosive medium, the diameter of the etched hole was larger, the pitting corrosion products were difficult to cover the hole formed by corrosion completely. The protective effect of corrosion products on the aluminum tubes was then weakened, making seawater concentrated by evaporation destructive to aluminum tubes.When using aluminum tubes in horizontal tube falling film evaporator, it is important to try to avoid corrosion of aluminum by reducing the generation of bubbles in the evaporator and controlling the salinity of seawater. The preparing work must be done according to the starting time of the occurrence of pitting corrosion correspondingly.