Steels in steam

A number of authors have reported studies of the oxidation of low-alloy steel in steam. The general observations indicate strong similarities with oxidation in air, the kinetics being typically parabolic and the scales typically comprising FcjO, Fej04, FeO and spinel phases, dependent upon alloy composition, temperature and oxygen partial pressure of the environment... 

For Fe in steam, water vapour or COj below 570°C, a two-layered Fej04 layer is observed, the inner layer growing by Oj diffusion inwards. Similarly, Potter and Mann reported the formation of a duplex Fej04 layer during the oxidation of mild steel in steam between 300°C and 550°C. 

Figure 7.63 Significance of oxygen and chloride content for SCC of an austenitic stainless steel in steam from alkaline-phosphate treated boiler water with intermittent wetting. (From [7.1] after Lee Williams.)...

Martinez-Villafan, A. Almeyara, M. F. Gaona, C. Gonzalez, J. C. Porcayo, J. (1998). High-Temperature Degradation and Protection of Ferritic and Austenitic Steels in Steam Generators. Journal of Materials Engineering and Performance, Vol. 7, No. 1, pp. 108-113, ISSN 15441024... 

N. Otsuka, Y. Shida, H. Fujikawa, Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam, Oxid. Met 32 (1989) 13. 

OtsukaN, Shida Y and Fujikawa H (1989), Intemal-Extemal Transition for the Oxidation of Fe-Cr-Ni Austenitic Stainless Steels in Steam , Oxid Met, 32, 13-45. 

Creep of Thick-walled Cylinders. The design of relatively thick-walled pressure vessels for operation at elevated temperatures where creep caimot be ignored is of interest to the oil, chemical, and power industries. In steam power plants, pressures of 35 MPa (5000 psi) and 650°C are used. Quart2 crystals are grown hydrothermaHy, using a batch process, in vessels operating at a temperature of 340—400°C and a pressure of 170 MPa (25,000 psi). In general, in the chemical industry creep is not a problem provided the wall temperature of vessels made of Ni—Cr—Mo steel is below 350°C. 

Chromium is the most effective addition to improve the resistance of steels to corrosion and oxidation at elevated temperatures, and the chromium—molybdenum steels are an important class of alloys for use in steam (qv) power plants, petroleum (qv) refineries, and chemical-process equipment. The chromium content in these steels varies from 0.5 to 10%. As a group, the low carbon chromium—molybdenum steels have similar creep—mpture strengths, regardless of the chromium content, but corrosion and oxidation resistance increase progressively with chromium content. 

Boiler Water Treatment. Alkanolamines, in general, provide excellent corrosion protection to steel in many appHcations. When used in boiler water treatment, AMP provides excellent protection to steel and copper in steam lines through efficient absorption of CO2, effectual distribution ratio for... 

Laminates are pressed in steam-heated, multiple-opening presses. Each opening may contain a book of as many as ten laminates pressed against pohshed steel plates. Curing conditions are 20—30 min at about 150°C under a pressure of about 6900 kPa (1000 psi). 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

The difference between a mechanical seal in a pump in alcohol service and a pump in steam service is the o-ring. It is not the stainless steel, or the ceramic face of the seal. The difference between a meehanieal seal in... 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

Boiler salts can contain chloride ions. When carried over into the steam (e.g. during priming) this can result in chloride stress corrosion cracking of austenitic stainless steel expansion bellows. In steam systems where freedom from chloride cannot be guaranteed, bellows... 

I2%Cr steel Water droplets in steam. Turbine blades. 

Basically there are two approaches to predicting the occurrence of erosion corrosion. Practical or experience based methods typified by Keller s approach for carbon steels in wet steam. Keller developed an equation that related the erosion corrosion rate as a function of temperature, steam quality, velocity and geometric factor. In recent years this approach has... 

Proc of Specialists Meeting held at Les Renardiers May 1982 on Erosion Corrosion of Steels in High Temperature Water and Wet Steam, Eds P. Berge and E. Khan, Electricite de France Paris (1983)... 

Fine chemicals There are many applications of the austenitic steels in the manufacture and storage of fine chemicals and pharmaceutical products. These include storage tanks, pipelines, valves, stills, steam-jacketed pans, mixing vessels, filters and tableting machinery. Considerable use has been made of these steels in penicillin production. 

Griskin et reported that there is no apparent effect of steam pressure on the rate of oxidation of Cr-Ni steels at temperatures between 600°C and 650°C at 10.1-20.2 MPa. Similar observations for Cr-Mo and Cr-Mo-V steels between 500°C and 600°C have been made by Wiles" . She compared low-alloy steel samples exposed to 101 kPa steam with power plant components that had operated for up to 150000b in steam at 17.25 MPa and found no significant difference in the oxidation rates (Fig. 7.11). 

Fig. 7.11 Comparison of overall scaling kinetics of low-alloy steels in laboratory air and power station steam at 17-25 MPa (after Wiles" )...

Fig. 7.32 Metal wastage of several steels due to oxidation in steam at various temperatures, (a) Mild steel, (b) Fe-2Cr-0-25Mo, (c) Fe-12Cr -I- Mo -I- V, (d) A.l.S.I. 316 and (e) Fe-18Cr-12Ni-lNb. T,oi,i = Ssutface -I- / penetration (after King el at.)...

Alloys of high nickel content also have improved chloride stress-corrosion cracking resistance and Incolloy 825 has replaced type 321 stainless steel for steam bellows on some plants. Occasionally cracking of the latter was experienced due to chloride-contaminated steam condensing in the convolutions on shut-down and being re-evaporated at start-up. 

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