Corrosion problems in waste incinerators are usually very severe, also due to the presence of low-melting point heavy and alkali metal chlorides. In this paper, the corrosion behavior of several Fe-based alloys with different Cr content, as well as three pure metals Fe, Cr, and Ni, was studied at 450°C beneath ZnC12-KC1 deposits in flowing pure oxygen. All the materials suffered from accelerated corrosion, characterized by an important separation and spaUation of the oxide scales. The Cr-containing alloys did not exhibit better corrosion resistance to this environment due to the poor adherence of the scales to the matrix, while higher Ni content could improve the corrosion resistance. Chlorine was always enriched on the substrate surface beneath the scale, and in some cases chlorine-rich corrosion products were also present within the alloy matrix. For the HP stainless steel, Cr-rich CrTC 3 carbides were preferentially attacked in comparison to the matrix. The effect of Cr is discussed with respect to the nature of the scales.
1. INTRODUCTION
2.
Incineration has been adopted as an effective and hygienic method for disposal of the ever-increasing voluminous industrial and municipal solid waste world-wide, especially during the last twenty years, and especially in developed countries. For example, in Western Europe, nearly 600 plants are in operation and the number is still increasing. In contrast, incineration is still a new business in China, where the first waste incineration plant, equipped with two small waste incinerators with capacity of 13.1 t/h for each, was built in 1987. However, many big cities such as Shanghai, Beijing, Shenyang and so on, are planning now to build more large-scale waste incineration factories in the near future[i], encouraged by the fact that the average heat of combustion of municipal waste in China has increased greatly and may reach 4180KJ/kg or above, which is very suitable for incineration disposal.
The corrosion problems during incineration are usually very severe due to the complex reactions between the combustion gases containing HCI/CI2 or SO2 and salt deposits (ZnCl2, KC1, NaC1, Na2SO4, etc,). For example, the superheater tubes used in the waste incinerator for Shenzhen degraded seriously in less than three months of service, and had to be replaced completely. Similar examples have been reported in the literature [2,3]. Even though the corrosion mechanisms caused by sulfate salts or gaseous chlorides have been examined thoroughly in the past years, the individual corrosion effects of the heavy metal (Sn, Pb, Zn) or alkali metal (K, Na) chlorides, which are actually the major components in deposits from waste incinerators, are not well understood at present [2,3]. This paper investigated the corrosion behavior of several Fe-based alloys as well as pure Fe, Cr, Ni in the presence of ZnC12-KC1.
EXPERIMENTALS
The materials employed in this study were commercially pure Fe, Cr, Ni, and the following five Fe-based alloys: carbon steel, 5Cr-l/2Mo, P91, SS310 and HP, in order of increasing Cr content. Their chemical compositions are shown in Table. 1. The materials were machined into specimens of size 10 x 15 x 1.5 mm, ground to 600 SiC paper and subsequently cleaned with acetone. After being preheated, the surface of the samples was coated uniformly with ZnC12-KCI (55: 45 molar ratio). The amount of the salt coating was about 3-4 mg/cm 2.
All experiments were carried out at 450°C in pure 02 with a flow velocity of 80ml/s controlled by capillary flowmeters, using a horizontal furnace with a quartz working tube. Each salt-coated sample was hung in an alumina crucible with a platinum wire, after which the crucibles were place
