Alloys and metals get corroded when exposed to high temperature in air or in actual boiler environment in thermal plant

Alloys and metals get corroded when exposed to high temperature in air or in actual boiler environment in thermal plant. In the present study, a Ni-20Cr coating was deposited by Detonation Gun process on ASTM–AS-213, T-11 boiler steel. Coating characterization was done using SEM/EDS analysis, XRD analysis. Cyclic corrosion studies were carried out in molten salt environment at 900oC for 50 cycles. Each cycle consist of 1 hours heating in Silicon carbide tube Furnace followed by 20 Minutes cooling in air. The kinetics of weight gain or loss was measured after each cycle and visually examined, the surface studies of the exposed samples were characterized by SEM/EDS analysis, XRD analysis. The results obtained showed the better performance of Ni-20Cr coated T-11 boiler steels than the uncoated T-11 boiler steel.
Keywords:- D-Gun coatings, Ni-20Cr, Corroison.
1. INTRODUCTION
To attain high temperature is very important for the development of civilization in almost every country. Materials in high technology areas need to operate in harsh conditions of corrosive environment, temperature and pressure etc. Invention and discovery of high temperature alloys or materials are looked with an expectation of increasing the life time of boilers, gas turbines etc by giving improved strength efficiency the corrosion resistant properties etc. Gas turbines in aircraft, fossil fueled power plants refineries and heating elements for high temperature furnaces are some examples where corrosion restricts their use and reduce their life, severely reducing the efficiency. The corrosion that occurs at high temperature is called hot corrosion or also sometimes dry corrosion. Hot corrosion is the accelerated oxidation of a material at elevated temperature induced by a thin film of fused salt deposit 1. The annual direct loss of natural resources i.e. metals due to environmental degradation is also substantial. In USA, the loss due to corrosion is around 4% of GDP. In India, the corrosion losses will be around Rs.1 lakh crore per annum. Around 80% of the unscheduled shutdowns and breakdowns in industries are due to corrosion and process fouling.
Metallic components in coal fired boilers are exposed to severe corrosive atmospheres and high temperatures Selection of material and its preparation are very important for the efficient functioning of the system components. Alloys used at high temperature should possess good mechanical properties along with erosion-corrosion resistance. Therefore composite system of a base material providing necessary mechanical properties with a protective surface layer, different in structure or chemical composition can be an optimum choice in combining mechanical properties
This particular chapter is concerned with high temperature oxidation, hot corrosion and its mechanism and related salt chemistry. The main focus is on hot corrosion of boiler steels, its preventive measures, coatings techniques.
Hot corrosion became a topic of important and popular interest in the late 60s as the gas turbines engines of military suffered severe corrosion during the Viet Nam conflict during operation over seawater. Metallographic inspection of failed parts often showed sulfides of nickel and chromium, so the mechanism was initially called “sulfidation” 2.
An increasing demand for more electricity, reduced plant emissions and greater efficiency is forcing power plants to increase the steam temperature and pressure of boilers. Ultra supercritical steam conditions greater than 31MPa and 600 ?C have been adopted, and the thermal efficiency of a pulverised coal-fired boiler of up to 45% has been obtained. Super heater and re-heater materials will therefore be required which have high creep rupture strength and high corrosion resistance at temperatures of about 900 oC and above 3,4. Super alloy can be used to meet these stringent material targets 5, 6 but they are unable to meet both the high-temperature strength and the high temperature corrosion resistance requirements simultaneously 7, 8. Protective coatings can be used on super alloys to meet the latter requirement. Coatings can add value to products up to 10 times the cost of the coating 9. Even if the material withstands high temperature without a coating, the coating enhances the lifetime of the material.
1.1 Oxidation
Oxidation is a type of corrosion involving the reaction between a metal and air or oxygen at high temperature in the absence of water or an aqueous phase. It is also called dry corrosion. The rate of oxidation of a metal at high temperature depends on the nature of the oxide layer that forms on the surface of metal 10.
Most metals are thermodynamically unstable in air and react with oxygen to form an oxide. As this oxide usually develops as a layer or scale on the surface of the metal or alloy, it can give protection by acting as a barrier that separates the metal from the gas. The establishment of an oxide scale on an alloy occurs by a nucleation and growth process. 11.
1.2 Hot Corrosion
Hot corrosion may be defined as accelerated corrosion, resulting from the presence of salt contaminants such as Na2SO4, NaCl that combine to form molten deposits which damage the protective surface oxides 12. Corrosion is the deterioration or destruction of metals and alloys in the presence of an environment by chemical or electrochemical action. Corrosion is an irreversible interfacial reaction of a material (metal, ceramic and polymer) with its environment which results in its consumption or dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered.
Hot corrosion is a high-temperature analogue of aqueous atmospheric corrosion. A thin film deposit of fused salt on an alloy surface in a hot oxidizing gas causes accelerated corrosion kinetics. Recognition of the problem and a search toward a mechanistic understanding and engineering abatement were initiated in response to the severe corrosion attack of military gas turbines during the Viet Nam conflict. Initially, the researchers were misled by the observation of corrosion product sulphides beneath a fused film of sodium sulphate to denote the problem and mechanism as “sulfidation”. Later, studies by Bornstein and Decresente 13, 14 and Goebel and Pettit15, 16 demonstrated that the principal corrosive environmental component was not a vapor species, but rather the contact of the fused salt with the surface. This fused salt (sodium sulphate) exhibited an acid-base character which at the time was quite uncertain and undefined. The electrolytic nature of the fused salt film and its similarity to atmospheric corrosion led to the more proper naming of the problem as “hot corrosion”.
High-temperature corrosion is chemical deterioration of a material (typically a metal) as a result of heating. This non-galvanic form of corrosion can occur when a metal is subjected to a hot atmosphere containing oxygen, sulphur or other compounds capable of oxidizing (or assisting the oxidation of) the material concerned. For example, materials used in aerospace, power generation and even in car engines have to resist sustained periods at high temperature in which they may be exposed to an atmosphere containing potentially highly corrosive products of combustion. Another popular name of this type of corrosion is hot-corrosion, which is also extensively used in the scientific community. Hot-corrosion is a serious problem in power generation equipment in gas turbines for ships and aircraft, and in other energy conversion and chemical process systems. The severity of hot-corrosion in combustion processes can vary substantially and is significantly affected by the type of fuel used, its purity and the quality of the air required to support the combustion. The hot-corrosion attack involves more severe degradation of the alloy. The hot-corrosion process depends on parameters such as alloy composition, deposit composition, and temperature.
2.0 COATINGS
2.1 Coating Techniques
If a material is added or deposited onto the surface of another material (or the same Material), it is known as a coating. Coatings are frequently applied to the surface of Materials to serve one or more of the following purposes:
To protect the surface from the environment that may produce corrosion or other
? Deteriorative reactions.
? To improve the surfaces appearance.
There are many coating deposition techniques available, and choosing the best process depends on the functional requirements, (size, shape and metallurgy of the substrate), adaptability of the coating material to the technique intended, level of adhesion required, and availability and cost of the equipment.
2.2 Thermal Spraying Technology
Thermal spraying is a process of depositing a superior material layer over a base metal or substrate either to improve the surface characteristics like corrosion resistance, wear resistance, surface fatigue or to get the desired dimension, size, surface appearance etc. In thermal spraying the feed stock material, in the form of a wire or powder of metallic or non metallic materials is melted or softened by flame or electricity and propelled on to prepared work piece to form a coating. Thermal spray coating processes are not only capable of applying coatings with excellent wear resistant properties, but also the range of materials capable of being sprayed so wide that applications for thermally sprayed wear resistant coatings are unlimited 9.
2.3 Corrosion Protection
Plating, painting, coating and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material. Platings usually fail only in small sections, and if the plating is more noble than the substrate (for example, chromium on steel), a galvanic couple will cause any exposed area to corrode much more rapidly than an unplated surface would. Painted coatings are relatively easy to apply and have fast drying times although temperature and humidity may cause dry times to vary. Corrosion can be prevented through using multiple products and techniques including painting, coating, sacrificial anodes, cathodic protection (electroplating), and natural products of corrosion itself.
2.4 Hot-Corrosion Protection
In general, thermal spray coating are used to define a group of processes that deposit finely divided metallic or non metallic materials onto a prepared substrate to form a coating. “Thermal spraying is a technique that is capable to solve the problems like wear, corrosion and thermal stability by depositing thin layer on surface of substrate.” The coating we apply on a prepared substrate may be in powder, rod or wire form. The thermal spray gun uses plasma arc, combustible gases or an electric arc to generate the heat necessary to melt