Corrosion and scaling problems present in some desalination plants in Abu Dhabi

All the Arabian Gulf States suffer from a shortage of fresh water resources. This deficiency is remedied through the desalination of seawater, mainly by the multi- stage flash (MSF) evaporation technique. The number of MSF distillers in Abu Dhabi is 382, producing a total quantity of 5,465,784 m3/d of potable water. It has been reported that these units have a tendency to suffer, at varying degrees, from a variety of corrosion and scaling problems. This paper consists of two sections. The first section describes some of the corrosion problems facing the desalination plants, concentrating mainly on the vapour-side corrosion (VSC) of condenser tubes made of cupronickel alloys, 70/30 and 90/10. The research in this section is necessary because condenser tubes are the part most prone to failure in desalination plants. Almost all of the previous cases of corrosion of condenser tubes reported were focused on water-side corrosion of the tubes. The present paper will discuss two contrasting cases of attack related to both acidic and alkaline corrosion in the vapour side. This is the first such research to be cited anywhere in this field. The second section of the paper will discuss the mechanism of the scale formation reported in the literature, and will present the actual scale formed in desalination plants, which is different from scale formed in the laboratory.

Fig. 7. Cross-section of the corrosion product of the tube. The Umm Al Narr power station showed very severe VSC of the 70/30 Cu-Ni condenser tubes of cell no. 2. Another distiller operating in the same location, built by a different manufacturer, also suffered severe VSC of the condenser tubes, but this attack was mainly confined to cell no. 1. The general appearance of the failed tubes was completely different from that reported at Jebal Dhana. Fig. 6 shows the nature of the failed tubes from the UAN station. It was clear that there was no pitting corrosion or fatigue-corrosion cracking, but the attack appeared to be general in nature, involving grooving, or localized. Fig. 7 illustrates the cross-section of part of the tube, showing the formation of a thick scale and some areas with deep penetration of corrosion products inside the matrix. It is believed that the grooving attacks which took place in the condenser tubes used at UAN were along the bottom of horizontal piping where ammonia-rich condensate collects. Oldfield and Todd have reported the possibility of the presence of both ammonia and hydrogen sulphide on the vapour side of the condenser tubes [19]. The shape of the failure of the present tubes is very similar to that reported in literature relating to copper alkaline attack [22,23]. Further details about this type of attack are to be published in the near future.