CAUSES OF TUBING FAILURES
The purpose of the following information is to focus attention on these facts:
- The environmental conditions within the boiler can be highly aggressive and alter the microstructure of tubing.
- Stresses caused by external loads, or induced by cold forming operations, uneven cooling or welding, may substantially lower the resistance of tubing to attack by certain corrosive media.
Immediately following are contributing factors that may cause failures.
Pitting is a type of extremely localized attack which can be difficult to detect. Pitting is a destructive form of corrosion that affects the water side of boiler tubes. Surface imperfections and deposits can serve as initiation sites for pitting, and a consequent breakdown of the protective scale. The corrosive penetration depends on factors such as temperature, oxygen concentration, and lack of flow of fresh solution to the pitted area.
mproper chemical cleaning procedures may also lead to pitting. Pitting can be prevented by:
- treating the water to eliminate the halide ions.
- providing a continuous flow of water to eliminate stagnant conditions
- minimizing the oxygen content in the water.
- following proper chemical cleaning procedures.
Sulfidation or sulfide corrosion is a problem often encountered if there are reducing conditions in coal and oil fired boilers. Sulfidation can become a problem when temperatures exceed 500F. Sulfides may form scale on the tubing surface and cause damage, because the scale is friable and subject to exfoliation; or the sulfides with their comparatively low melting point may fuse with the tubing surface and cause rapid integranular penetration.
SA213-T22 and T5 offer some resistance to sulfidation in the absence of hydrogen.
Departure from nucleate boiling (DNB) occurs when the bubbles of steam forming on the hot tube surface begin to interfere with the flow of water to the inside tube surface, and eventually coalesce to form a film of superheated steam over part or all of the heating surface. Consequently, the tube wall may be subjected to local overheating and eventual rupture.
Ribbed tubing was developed to inhibit the onset of DNB.
CAUSES OF TUBING FAILURES
Fuel ash corrosion
Fuel ash corrosion is a high-temperature phenomenon in coal and oil-fired boilers. In oil-fired boilers, sodium sulfate combines with vanadium pentoxide to form a very corrosive coating of molten “slag” on the fireside tube surface of superheater and reheater tubes. The molten slag tends to dissolve any steel it contacts, thereby weakening the structure of the tubing. No steel is immune. However, the higher the chromium content of the alloy, generally the more resistant it is to attack.
Although the melting point of vanadium pentoxide is much higher than boiler tube metal temperatures, the addition of sodium sulfate lowers the melt point of the ash to below the temperature of the boiler tube.
There are a number of ways to minimize fuel ash corrosion.
- Fuel additives such as calcium oxide and magnesium oxide can be used to raise the melting temperature of the ash.
- Frequent cleaning of the tubes by soot blowers.
During exposure at elevated temperatures between 750 F and 1,000 F, high-chromium ferritic and martensitic steels, as well as the ferrite phase in duplex austenitic-ferritic stainless steels are subject to a form of embrittlement. This condition is known as 885 F embrittlement because maximum embrittlement occurs at this temperature. The condition is characterized by an increase in hardness and a loss in toughness. At chromium levels greater than 15% the embrittlement may be observed in long-time exposures at temperatures at least as low as 500F, depending upon the chromium and alloy content. At chromium levels of 13% or 14%, a modest increase in hardness may be seen in long-time exposure at 900 F.
The carbide phase of carbon and carbon-moly steels may be converted to graphite after long-time exposure to high temperature. If this occurs locally as sometimes associated with weldment, especially in headers and steam piping, structural integrity may be adversely affected. If it occurs, generally slight losses of strength and ductility can be expected. In carbon steels, such conversion to graphite may occur on prolonged exposure to temperatures exceeding about 800 F. In carbon-moly, the conversion may occur on prolonged service above about 875 F.
Chrome-moly steels containing 1/2% or more chromium are normally considered to be resistant to graphitization.
Dew Point corrosion
During periodic shutdowns of the boiler unit, tube failures can occur. Quite often they are related to dew point corrosion, an insidious condition that can penetrate up to 0.50” of steel per year. Thin walled tubes that normally last 10 years can become badly pitted during shutdown, and replacement tubing becomes an unanticipated expense.
During shutdown, surface temperatures of fireside boiler tubing drop below the dew point and form condensate. The condensate reacts with sulfurous parts of the ash and acid deposits, then hydrolizes, forming a highly acidic solution. The end result is rapid, localized attack of the metal surface.
Dew point corrosion can be prevented by the following:
- Clean ash, soot and other products of combustion from fireside tube and tube sheet surfaces immediately after shutdown.
- Flush these surfaces completely with water to dissolve, dilute and remove any remaining sulfur compounds. Dry by blowing air through the boiler.
- All cleaned surfaces should be coated with a good grade of light oil to prevent rust.
- A tray of unslaked lime should be placed in the ash pit during the idle period. This will lower the relative humidity inside the boiler and lessen the chances of condensate forming. The lime must be renewed whenever it becomes mushy.
- Finally, the present fuel should be evaluated for sulfur content. If it is high, switch to one containing less sulphur.
CAUSES OF TUBING FAILURES
Steel tubing exposed to elevated temperatures for extended periods of time will oxidize; i.e., accumulate a protective coating of scale. At some minimum temperature, the scale will lose adhesion, gradually flake and cause a progressive thinning of the tube walls.
Resistance to oxidation is one of the most important characteristics of the alloy and stainless grades. The chromium in these grades reacts with oxygen to form a tight, adherent scale that retards oxidation at elevated temperatures. As the chromium content increases, the degree of protection and the maximum operating temperature increases for the tubing.
Stress corrosion cracking
Austenitic chromium-nickel steels that are highly stressed in tension may develop transcrystalline or intercrystalline cracks when simultaneously exposed to a specific aqueous corrosive medium. The austenitic stainless steels are very susceptible to chloride stress corrosion cracking. Dissolved oxygen is essential to the cracking of the austenitic stainless steels in solutions containing chlorides or other halides
The chances of stress corrosion cracking occurring can be reduced by the following:
- Substituting an alloy which is not susceptible to cracking in the particular environment.
- Removing the particular ion associated with the cracking.
- Removing all dissolved oxygen.
- Annealing the material to relax any residual stresses.
Formation of sigma phase
Sigma phase may form in certain austenitic stainless steels and may reduce its room temperature ductility and toughness. Sigma phase is formed in certain stainless steel alloys, such as 310, 316, 321 and 347, in the temperature range between 1050 F (566C) and 1700 F (927C). However, 304 and 304H tubing are not subject to this embrittlement mechanism in normal boiler use. Factors contributing to the formation of sigma phase include the progressive solidification, and the presence of ferritizers — particularly molybdenum, columbium and titanium.
Sigma phase may also form in the ferritic stainless steels.
Formation of sigma phase may be minimized or prevented by proper selection of alloy.
CAUSES OF TUBING FAILURES
Intergranular corrosion (IGA) is a form of localized corrosive attack of areas adjacent to the grain boundaries. IGA is only of concern with austenitic stainless steels when the material is subject to caustic or halide solutions as from improper chemical cleaning or water treatment.
Intergranular corrosion can be prevented by the following:
- Avoiding contact with certain aggressive media, particularly inorganic acids, and by keeping the oxygen contents of the aqueous solutions low. Superheaters and reheaters are rarely exposed to aqueous solutions, and these are the main boiler components in which austenitic seamless tubing is used.
- Substituting a low-carbon grade or a grade which is alloyed with strong carbide stabilizers. These alloys have lower allowable stresses and require a heavier wall design.
Creep is a time-dependent deformation which occurs when a material is stressed at high temperature. Over a period of time with a continued load, the material will eventually rupture.
The temperature at which creep becomes important depends on the particular metal. For carbon steel, creep rupture becomes a design consideration at 800 F, for alloy steels at about 900 F and for austenitic stainless steels at about 1040 F.
Creep rupture can be avoided in tubing by selecting a grade of steel whose creep rupture strength is sufficient to withstand the specific operating conditions. For boilers creep and creep rupture data have been used as part of the criteria for establishing maximum allowable working pressures. The values are listed in ASME Boiler and Pressure Vessel Code, “Section I, Power Boilers.” Designing to, or below, these maximum values is required by law in most states and Canadian provinces. Operating at the corresponding design conditions will prevent almost all failures of this type.