Nickel Alloys for applications in steam environments
The attacking characteristics of steam that is used in a plant are mildly considered, specifically if the steam is supplied by a public utility or power unit in which the boiler water treatment is adequate to avoid condensate attack. Although the contaminants in the steam can cause corrosion issues.
In a single plant, high pressure steam was bought from the central power unit and it worked without any issue for fifteen years. Within a few months after the regular work, although cracks and leakage occurred adjacent and across the welded joints in the new pipes. The metallographic tests described that cracking was caused by caustic embrittlement. The deionized water comprising of 10 ppm sodium that produced concentrated sodium hydroxide (NaOH) as water evaporated. The older pipes didn’t crack as they had been stress relieved by the superheated steam over the period of years prior cooling of the steam began.
In many cases, it is not essential to treat system condensate prior reusing and boiler feedwater is prepared by adding makeup water to the condensate. Although, condensate is sometimes contaminated by the corrosion products or by leaking in cooling water or other unwanted materials. In these cases, the attack should be limited or removed prior the recycling of condensate. When steam contains acidic gases like carbon dioxide and oxygen, its condensate will be acidic and it will cause attack of the metal surfaces.
Copper and copper alloys prevent attack by pure steam however if carbon dioxide, oxygen or ammonia is present, the condensate is attacking in nature. However wet steam at large paces can cause intense impingement attack, copper alloys are utilized widely in condensers and heat exchangers. They are also used for feedwater heaters, but their use in the applications is limited due to quick reduction in strength and creep resistance at moderately high temperatures. Copper-nickel alloys are recommended for the elevated temperatures and pressures.
Use of copper in systems handling hot water and steam is controlled by working pressure of tubes and joints. In few conditions, copper-nickel alloys have described a tendency to be damaged by stress corrosion cracking when they are corroded by stress corrosion cracking due to high stress and subjected to steam.
Steam condensate that has been suitably treated so that it is set free of gases in power production units, is comparatively noncorrosive to copper alloys. The corrosion rates in these exposures are smaller than 2.5 micro-m per year. They are not corroded by condensate that includes a large amount of oil for example condensate from a reciprocating steam engine.
Dissolved carbon dioxide, oxygen or both considerably increase the corrosive attack. For instance, condensate with 4.6 ppm oxygen and 14 ppm carbon dioxide and a pH of 5.5 at 68oC or 155oF caused penetration of 175 – 350 micro-m/yr. Steel analyzed in the same conditions was attacked double times the rate shown by copper alloys.
Advanced power utility boiler feedwater treatments normally include the inclusion of organic amines and oxygen scavengers to prevent the attack of iron components in the plants by scavenging oxygen and increasing pH of the feedwater. The chemicals including morphline and hydrazine degrade in service to produce ammonia that is extremely attacking towards copper alloys. In the main part of well-evaluated operating condensers, oxygen and ammonia magnitudes are very small and corrosion is nominal. Irregular service conditions, tube leakage and close- start cycles also accelerate the corrosion in the steam side condition by increasing oxygen level. High oxygen contents are normally more damaging than increased ammonia levels, but copper-nickel alloy 70/30 was nominally affected by the increased oxygen concentration.
The nickel-chromium-iron alloys are famous materials for use in the crucial applications including pressurized and boiling water reactors, due to their outstanding corrosion resistance in steam and water conditions and resistance to chloride stress corrosion cracking. The age hardenable alloy Inconel X-750 is utilized as a spring material for fuel pellet hold down springs, fuel element divide plates and reactor scram springs and for bolting. Age hardenable alloy Inconel 718 due to its high strength and spring properties, is fit for use in fuel assembly divider plates. It also described outstanding wear resistance in sodium fast breeder reactor conditions.
In steam-hot water units for example condensers, considerable corrosion of Nickel 200 and Monel 400 may take place if noncondensables such as carbon dioxide and air in the steam occur in the specific magnitudes. Deaeration of the feedwater or venting of the noncondensable gases will avoid this corrosion.
The single most crucial application of nickel base alloys in the nuclear industry include the use of Inconel 600 in steam generator tubing in the pressurized water reactors. Alloy 600 withstands steam, air and carbon dioxide and hence is specifically significant in contact with steam at the elevated temperatures. Nickel based alloys have been found extremely resistant to oxidation and attack by the superheated steam and hence considerably useful in preventing the critical issues in the nuclear reactor technology. The most promising way for receiving enhanced efficiency in the power production by nuclear reactors is employment of superheated steam system where superheating is influenced by the direct supply of steam through reactor. The construction of this kind of reactor is encouraged by the nickel based alloys that have proved to become the ideal structural materials.
In addition of satisfactory performance in oxidation and corrosion conditions created by super-heated steam, effect on the mechanical characteristics of the material after the prolong exposure to the superheated steam should be taken into consideration. Particularly traditional materials are adversely hardened and embrittleed after many hours of service at temperatures around 340oC to 540oC and long duration.
Nickel based alloys are a technologically significant group of materials that are providing excellent service in the steam conditions. They are a general purpose alloy with excellent high temperature strength and resistance to steam and oxidizing or carburizing gases.
Alloy 800H is significant for several applications including prolong exposure to high temperatures or attacking conditions. In chemical and petrochemical plants, it is utilized in steam or hydrocarbon redevelopment for catalyst tubing, convection tubing, pigtails, outlet manifolds, cooling pipes and distribution pipes in ethylene production for convection and cracking tubes in oxo-alcohol production for tubing in hydrogenation heaters hydrodealkylation systems for heater tubing and in the development of vinyl chloride monomer for cracking tubes, return bends and flanges.
Incoloy 800H is also widely used in the industrial heating applications. In the variety of heat processing furnaces, grade 800H is used in power production for steam superheater tubing and elevated temperature heat exchangers in gas cooled nuclear reactors.
Proven alloys are for use in steam applications are Inconel 600, Inconel 617, Incoloy 800 and Incoloy 800H.