Hot corrosion is typically defined as the accelerated high temperature oxidation caused by a thin fused salt deposit in contact with the surface of metallic material. When alloys used in air and land-base gas turbines experience hot corrosion, usually the dominant salt found in the deposit due to the thermodynamic stability of this compound in the presence of sodium and sulfur in an oxidizing gas. In an effort to study the hot corrosion of several Fe- and Ni-base commercial wrought alloys in a gas turbine environment, a burner rig test program was executed. The study was conducted at900°C(1650°F) and incorporated various test durations and two fuel subtle contents (0.4 and 1.Owl%). Also, a synthetic sea salt was injected into the combustion zone of the burner rig, thereby maintaining a specific concentration of the salt in the combustion gas. Tests with the injection of Parts per million (ppm), 5ppm, or 50 ppm salt were conducted. The principal constituents of the solution were NaCl, MgCl2,Na2S04, CaCl2,and KCl.
In general, the Ni-base alloys tested exhibited superior hot corrosion resistance over the Fe-base alloys. When exposed to a 1000hour test with the injection of 50 ppm salt, the Fe content of aNi-22Cr- 18Fe-9Mo alloy appeared to be detrimental to the hot corrosion resistance of this alloy when comparedtoaNi-21 Cr-9Mo alloy. The W content in a Ni-22Cr- 14Walloy did not show a detrimental affect on the hot corrosion resistance of this material. Also, Fe-base alloys containing less than 25 wt% Cr did not perform well in a 500 hour test burning fuel containing 1.0 wt% sulfur and 50 ppm salt injected into the combustion zone. Reasons for the performance of the alloys are suggested to be the quality or protectiveness of the oxide scale grown on a given alloy, the composition of the oxide scale grown on a given alloy -- making the scale more or less susceptible to a synergistic hot corrosion attack, and/or the alloy composition in relation to its ability to resist attack by chloride salts.
Hot corrosion can be defined as the accelerated oxidation of materials caused by the presence of a thin film of fused salt deposit in a high temperature oxidizing gas environment. The compound Na2S04 is the most common and dominant salt deposit encountered because Na is found in the fuel and/or air (usually in the form of NaCl), S is found in the fuel, and Na2S04has a high thermodynamic stability. The hot corrosion reaction can be considered to consist of three stages: an incubation period, an initiation step, and a propagation stage. The incubation period is characterized by typical parabolic oxidation kinetics, as would be expected for the performance of an alloy in a high temperature oxidizing environment. An accelerated corrosion rate caused by the failure of a protective oxide scale marks the beginning of the initiation step, and rapid corrosion typifies the propagation stage. Hancock listed the following possible causes for the failure of a protective oxide scale, thereby causing the initiation step: (1) dissolution of the scale by the molten salt (2) local reduction of the scale due to local reducing conditions, (3) failure by mechanical erosion, (4) mechanical faults present in the as-grown oxide, (5) thermal stresses, (6) superimposed operating mechanical stresses, and/or (7) contaminant reaction at the scale/metal interface (e.g. Cl effects at the scale/metro interface).
Dissolution of the scale by the molten salt (i.e. fluxing) is generally accepted as being the dominant scale failure and propagation mechanism in type I hot corrosion. The dissolute reprecipitation reaction in the fused salt fin was frost suggested by Bomstein and DeCrescente3, developed b
