Patent Publication Number: US-4929287-A

Title: Method for surface treatment of metal alloys

Description:
TECHNICAL FIELD 
     The invention relates to the field of methods for improving the corrosion/erosion resistance of a high-temperature metal alloy in systems included in power generating plants for combustion in a fluidized bed by surface treatment of the metal alloy. 
     BACKGROUND ART 
     It is known to improve the corrosion resistance of high-temperature metal alloys, intended for internal components in combustion chambers and the like, by adding small amounts of some rare earth metal to the alloy. A disadvantage with this method is that also the strength could be influenced by the alloy additive. For land-based gas turbines and the like, it is common to provide high-strength alloys with protective surface layers. Swedish patent No. SE 7711357 describes how the corrosion resistance of a high-temperature alloy is increased by applying a surface layer by means of electrochemical deposition. The surface layer consists of a coherent matrix of Co or Ni with discrete particles containing some added substances from the group Cr, Si, Al, Ti, Ta, Be or a rear earth metal. Protective layers containing Al, Pt, Cr, Y, Sc, B and spraying of layers of Cr, Al with small additives of Y, Zr, Hf, or a rare earth metal, are also known in the art. 
     During combustion in a fluidized bed of coal of a varying quality and also other fuels, such as waste from the pulp industry or other waste, wood, peat, oil, brown coal, and the like, combustion gases and ash particles are formed which may have a corrosive and also erosive influence on the construction materials, primarily in the combustion chamber, the ash separator and the gas turbine. A corrosive effect from formed SO 3  may lead to the protective surface layer of the metal being broken through by the formation of porous sulphides which are not capable of protecting the metal from further attack, which involves a risk of accelerated erosion attack. 
     In systems operating under potentially corrosive conditions, it is important to ensure that the construction materials have a good resistance to degradation caused by the attack of corrosion and erosion. 
     DISCLOSURE OF THE INVENTION 
     The invention relates to a method for improving the resistance to corrosion and erosion attacks on construction materials included in combustion chambers, cyclones and gas turbines in power plants with combustion in a fluidized bed. The constuction materials are primarily low-alloy or high-alloy high-temperature steels or alloys based on Co or Ni. During combustion of coal in a fluidized bed, lime (CaO) or dolomite (CaMg(CO 3 ) 2 ) is added to bind sulphur (S) in the flue gas. This leads to the formation of anhydrite (CaSO 4 ) which may become deposited on internal surfaces. A greatly contributory cause of the attack of corrosion on the construction materials is the formation of SO 3  which may lead to sulphide attack on the materials. To improve the resistance of the construction materials to corrosive and erosive attacks, it is possible, in accordance with the invention, to improve the surface layers of the materials by treating the surface in a multi-step combination thus obtaining a surface layer which is thin and adherent and protects the metal without influencing the strength of the material. The method is applicable to alloys which are based on Fe, Co or Ni and which contain Cr and/or Al. The method comprises (a) causing the material first to oxidize selectively, for example by heating the material at 500°-1200° C. in an atmosphere of air or hydrogen, air, argon, nitrogen or helium mixed with water vapour. Thereafter (b) a substance is added which contributes to increase the ductility and adhesitivity of the oxide layer, such as a rare earth metal, for example Ce or Y, or Si, B, Al, Ta, Pt, Zr, Hf, Cr, Nb, Ti or Mg. Mixtures or compounds containing at least one of these substances can also be used. The addition can be performed by immersing the material in a slurry or the like, which contains any of the above-mentioned substances, or by some form of electrolytic, electrochemical, or chemical deposition of any of the substances on the surface of the material. Spray depositing on the material surface of a solution or mixture, containing any of the above-mentioned substances, can also be performed. It is an advantage if the material is hot; preferably a temperature within the range 500°-1200° C. is maintained, as during the preceding heat treatment, or a somewhat lower temperature considering the fact that the material cools during the transport and in view of the waiting times between the steps in the surface treatment. To achieve an improved incorporation of an added substance and to further strengthen the oxide layer, a further heat treatment of the surface is preferably carried out at 500°-1200° C. in an atmosphere of air, moist air, oxygen gas, nitrogen gas or mixtures thereof, after the application of the additive. One or more of the steps in the surface treatment can be carried out repeatedly. By surface treatment in three steps, a surface layer is obtained which is relatively thin (≦20 μm), has good adhesion to the metal and has good ductility. Contrary to other methods in which additives are applied on metal surfaces, a three-step surface treatment provides very thin oxide layers in which the added substance is well incorporated. The third step (c) entails an improved method of ensuring that the added substance is incorporated into the surface layer without the thickness of the surface layer exceeding 20 μm. The treatment according to the invention provides a considerable improvement of the ability of the metal to withstand the attack of corrosion and erosion in an environment characteristic of coal combustion in a fluidized bed. In addition, the surface layer can be rapidly applied and does not influence the strength of the metal. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     EXAMPLE 1 
     To improve the corrosion/erosion resistance of a binary austenitic Fe-based alloy, for example 2338, which is subjected to flue gases containing, inter alia, sulphur compounds, a threestep surface treatment is carried out. Prior to the surface treatment, the metal surface is carefully cleaned, possibly blasted if this should be necessary. In the first step (a) of the surface treatment, the object is heated in a furnace at a temperature of 500°-700° C. in an atmosphere of hydrogen gas and water vapour for about 5-30 minutes to obtain a selective oxidation of chromium on the surface of the alloy. Thereafter, the object is immersed, while still hot, into a slurry of cesium nitrate and alcohol. After drying the surface in air, the third step (c) of the surface treatment is carried out by heating the object to 500°-700° C. in air to convert the nitrate into oxide and to further strengthen the oxide layer. After completed surface treatment, the thickness of the oxide layer is about 10-20 μm. 
     EXAMPLE 2 
     Surface treatment of Ni-based or Co-based superalloys containing Al can be carried out as described in Example 1. During heat treatments (a) and (c), however, it is advantageous to raise the temperature to about 1000°-1200° C. In the second step (b), one or more of the substances specified in claim 5, e.g. yttrium, can be added and thereafter step (a) can be repeated before step (c) is carried out. For superalloys containing Cr, steps (a) and (c) can be carried out at 800°-950° C. 
     The method as described above can be varied in many ways within the scope of the following claims.