Corrosion low temperature

Additives for high-temperature oil and coal-ash fouling and corrosion can be applied as powders or slurries. The form of the additive determines the 

Magnesium oxide particles in the 2- to 7-p.m median particle size range deposit in the convection pass preferentially, while finer sized particles are not deposited and tend to get carried into the back end of the boiler. When an MgO of 2-p.m median particle size is fed into the fuel at treatment rates of 0.5-1.0 parts by weight Mg to 1.0 part V in the oil, the quantity of MgO deposited on the superheater tubes is 2-3 times that of the V and Na. This treatment rate results in a high Mg V ratio, which is needed to raise the ash-melting point and reduce tube deposits. 

Application of aqueous MgO-based slurry via the soot blowers should theoretically result in lower treatment rates than other modes of application. However, data suggests that dosage rates are in the same range as oil-based MgO dispersions. However, with this mode of application, inadequate dosage or poor distribution can result in an increased ash burden, thus exacerbating the problem. 

The feed rate for powders injected into the economizer outlet for boilers firing high-sulfur oil (2% or greater S) and operating with low excess air is 165-621 lb MgO equivalent per 1000 barrels of oil. The feed rate for oil-based magnesia dispersions is 127 lb MgO equivalent per 1000 bbl of oil. Coal-fired boilers, using a high-sulfur bituminous coal, typically require 

27 lb Mg(OH)2 per ton of coal when dosed into the upper furnace as an aqueous slurry (Radway and Rohrbach, 1976, and Radway and Boyce, 1977). Oil-based dispersions fed into the upper furnace require approximately 0.4-0.6 lb MgO equivalent per ton of coal (Dainoff and Schenck, 1984). 

---

The formation of a layer of metal oxide on the surface of this steel provides better corrosion resistance in oxidizing environments than under reducing conditions. Common steels 304, 304L, 347, 316 and 316L are used for equipment exposed to aqueous solutions of acids and other low-temperature corrosive conditions. For high-temperature regimes involving... 

The alloys containing less than 11% silicon have resistances to low-temperature corrosion not substantially different from those of low-silicon irons containing similar amounts of other alloying elements, and will not be further discussed in this section. 

Increased thermal efficiency is possible if boiler back-end temperature is reduced. Theoretically, without modifying boiler output, a reduction of 8 °C (14.4 °F) in back-end temperature results in a saving in fuel oil consumption of 0.3%. Low temperature corrosion can occur, however, when boilers are operated with back-end temperatures close to the S03 dew point. 

There are many types of corrosion, as would be expected from its general definition. It has been traditional (4) to divide the study of corrosion into two areas the study of low temperature corrosion by aqueous or other solutions, controlled by electrochemical processes (wet) and the study of gaseous corrosion at high temperatures, controlled by thermodynamics and diffusion processes (dry). In addition to the obvious differences, the two areas have many phenomena in common. 

Manganese compounds have been used in coal as well as oil firing equipment to alleviate slagging and fouling problems. Low temperature corrosion has so been reduced by the use of these chemicals. The relatively high cost of manganese compounds may reduce their universal applicability. 

There are means to combat low-temperature corrosion using corrosion resistant materials. All these means do not contribute to an improvement in the availability of the entire installation. Often a lower efficiency than would be economically justified is accepted, just because of availability considerations. 

As dealt with in previous ehapters, the eorrosion resistanee of stainless steel is due to passivation by a surface film of chromium oxide. The chromium content is higher than about 11%, and the low-temperature corrosion resistance as well as the resistance to oxidation and mill scale formation at high temperature increase with increasing content of chromium. Pure chromium steels are either ferritic (low C-content, non-hardenable by heat treatment) or martensitic (traditionally higher C-content for most grades, hardenable by heat treatment). Wifli sufficient content of Ni the structure becomes austenitic, which gives increased formability, weldability. 

Thermal spraying. The prineiples of thermal spray methods are shown in Figure 10.25. Flame and are spraying are most common for prevention of low-temperature corrosion. These elassieal methods have been thoroughly described by Ballard [10.42]. Coated test samples have been studied after long-term exposure [10.43]. 

The critical regions in oil-fired furnaces are the evaporator tubes, steam superheater tubes, air heater, and channel. Evaporator tubes are affected by hot gas corrosion from hydrogen sulfide as the temperature exceeds 280°C. Superheater tubes suffer from sulfate/sulfite corrosion induced by molten sulfates above 620"C. Superheater tube holders undergo vanadic corrosion caused by molten vanadate species in the range of 550 C-600 C. Air heaters are subject to low-temperature corrosion from liquid sulfuric acid at 100°C-140°C. Flue gas channels suffer from acid deposits at the dew point (Balajka 1980). 

Elevated tenperature damage to refinery furnace tubes may consist of corrosion-dependent failures and temperature-related defects connected with degradation of the steel microstructure and creep damage. At high temperatures, a steel tube may fail due to deformation and creep fracture even at a stress level well below the yield stress, whereas at low temperatures corrosion and microstmcture degradation processes prevail. These two ranges can be determined by yield strength and rupture stress vs temperature curves [8]. 

The first task in any experimental study of low temperature corrosion is to prepare a metal surface free of oxide and adsorbed species. For this the sample must be placed in an ultra- high vacuum (UHV) chamber. One then eliminates the natural oxide layer and adsorbed impurities by ion sputtering. Because ion bombardment disturbs the uppermost atomic layers of the metal (Chapter 3), a thermal treatment is sometimes applied in order to reestablish the original surface structure. Once a clean metal surface is available one introduces a known amount of oxygen into the UHV chamber by setting its partial pressure, and one follows the evolution of the reaction using a suitable method. Surface sensitive methods for the study of adsorption and thin film growth include surface analysis by AES, XPS, SIMS, optical methods, in particular ellipsometry, or mass sensitive methods such as the quartz microbalance. 

Low Temperature Corrosion Problems in Fossil Power Plants— State of Knowledge. EPRI, Palo Alto, CA, December 2003, 1004924. 

Already from Clause 3.2.5 it is known how major accidents/incidents are defined. It is now wise to consider some commonly used guided words in HAZID Unignited hydrocarbon (HC) released Hydrocarbon (HC) released—fire HC released—explosion Toxic exposure High pressure High/low temperature Corrosion Object dropping Improper access/escape Radiation Maintenance Construction/startup Explosives Electrical Mechanical Stmctural Effluent disposal Biological and others... 

The Ni-Cr-Fe alloys are also extensively used in refining and petrochemical plant equipment for both liquid and gaseous low-temperature corrosion resistance and for heat-resistant applications. Table 8.24 describes the practical behavior of the main high-performance alloys and highly alloyed stainless steels in some of the very demanding operational situations in which these alloys are expected to perform satisfactorily. The chemical composition of these alloys can be foimd in App. E. 

For practical purposes, corrosion in oil, gas, petrochemical, and chemical plants can be classified into low-temperature corrosion and high temperature corrosion. Low temperature corrosion occurs below 260 °C in the presence of water. Fligh temperature corrosion takes place above 260 °C. The presence of water is not necessary in this case because corrosion occurs by direct reaction between the metal and the environment. 

No increased risk for low temperature corrosion at the back end of the boiler was detected when the ChlorOut additive was in use. The air preheaters in Nykoping are made of the austenitic stainless steel AISI316 (SS2343, WNr 1.4436) which contains 16-18% Cr, 11-14% Ni and 2.5-3% Mo. Corrosion probe tests near the air pre-heaters, with a temperature gradient of 60-130°C, showed that corrosion with and without ChlorOut was negligible in both cases [7]. 

---