Furnace waterwall tube failures represent a leading cause of forced outages in power boilers. These failures occur as a result of a variety of causes, including under deposit corrosion, overheating, erosion, wastage, graphitization, and fatigue.
Under deposit corrosion mechanisms such as hydrogen damage and caustic gauging are often the most difficult damage mechanisms to detect. This is due to the fact that damage initiates on the inside diameter and often does not show any appreciable wall loss prior to failure. Therefore, these potential failures will not be identified through conventional ultrasonic thickness surveys.
Hydrogen damage initiates on the inside surface of waterwall tubing as a result of low pH corrosion combined with high metal temperatures. The pH corrosion may result from inadvertent release of acidic regeneration chemicals form make-up demineralizers or in the leakage of acid-producing salts into the condenser. The high metal temperature or “hot spots” may be caused by over-firing, misadjusted burners, changes in fuel, gas channeling, or excessive blowdown. The hydrogen embrittlement and damage occur from the inside surface of the tubing.
The following mechanism normally occurs:
1) Corrosion along the inside surface and oxidation
2) Scale layer formation
4) Formation of steam underneath the scale
5) Diffusion of atomic hydrogen into the steel
6) Carbide decomposition and carbon diffusion within the steel (hydrogen embrittlement)
7) Methane Formation
8) Microcracking, fissuring along the grain boundaries (hydrogen damage)
9) Major cracking
10) Failure of the tube by rupture
Stages 5 to 7 are referred to a hydrogen embrittlement. Stages 8 to 10 represent hydrogen damage, which is irreversible and involves permanent tube deterioration. Final tube failures generally occur as brittle ruptures.
Deterioration associated with hydrogen embrittlement and damage is quite common in boilers particularly with improper water treatment or inadequate acid washing. In some boilers, it has occurred after only 2,000 hours of service. However, it generally takes significantly longer periods of time. In most instances, hydrogen damage represents a localized condition.
In waterwall tubing, hydrogen damage and caustic gouging occur intermittently in specific waterwall sections such as the nose arch slope and around the burner openings. The affected areas can extend from 1 ft. to 10 ft. or more in length and width. The damage can also be extremely isolated with pit-like indications as small as 1/2″ diameter, which can result in failure.
The initial corrosion is generally associated with oxidation. “Bonded” oxygen (H2O) and dissolved oxygen have been considered responsible. Probably under specific operating conditions either stage, singly or in combination, may result in the localized oxidation and pitting of steel tube materials.
The initial oxidation produces a thin, essentially uniform, iron oxide layer along the entire inside surface of the carbon steel tube. As this layer is tightly adherent, it is considered to inhibit further oxidation. With increasing thickness of scale deposit, that rate of oxidation thus would be expected to become negligible and, in time, should stop. However, periodic pickling or specific concentration of acid, salt of caustic loosens the scale in some of the localized pit areas, exposing fresh metal surface areas to further oxidation. As the pit sharpness and depth increase, the cavity in the metal surface might also not be able to accommodate the iron oxide which forms subsequently, since the lower density of the iron oxide requires more space than is available. This may result in the cracking of the scale, and reveal more or a fresh steel surface. Thermal cycling (fatigue), mechanical vibrations, or acid pickling may also loosen the scale in larger pit areas. Thus, after a specific size of pit cavity has developed along the inside wall, the rate of further oxidation and growth may tend to become substantial.
In the majority of failures, a relatively heavy, hard and tightly adherent scale forms in the final oxidation stage, which preceded or accompanied the hydrogen embrittlement.
Within the steel tube above the pit surface decarburization along the grain boundaries adjacent to microfissures is associated with the diffusing of hydrogen into steel. Hydrogen diffusion is usually based on permeable characteristics. Although diffusion of atomic hydrogen through metal tends to increase with increasing temperatures, permeability may vary with the method of introducing the hydrogen. In boiler tube failures, the hydrogen embrittlement generally is associated with decarburization of the iron carbide (Fe3C) particles in the steel by hydrogen and the formation of methane, particularly along the grain boundaries.
The methane is formed by the following reaction:
Fe3C + 2H2 = CH4 + 3Fe
This reaction is reversible in the laboratory, depending upon the temperature and pressure involved. At each particular temperature and pressure, a specific quantity of methane is in equilibrium with a specific quantity of hydrogen. At approximately 875°F, the reaction is normally considered to be in equilibrium. At lower temperatures, such as in waterwall tubing, the reaction tends to proceed to the right and form methane, while at higher temperatures, it tends toward hydrogen formation.
The hydrogen in the grain boundary voids of the steel combines with the carbon atoms to form methane, which is unable to diffuse through the steel. With increasing methane pressure, fissuring along the grain boundaries then takes place.
At this stage, the hydrogen embrittlement condition is also referred to as “hydrogen damage”, since the steel cannot be restored to its original properties by heat treatment.
Other factors complicating analysis are the varying temperatures which occur from boiler start-up to normal operation causing variations in the different reactions and diffusion processes. Moreover, heavy scale deposits on the inside of waterwall tubing within the corroded cavity areas tend to result in localized overheating, which further complicates the analysis.
Thielsch Engineering, Inc. utilizes an ultrasonic attenuation examination technique, which allows for detection of under deposit corrosion. Access to the examination areas by staging and/or sky climbers and surface preparation by grit blasting or hydro blasting is initially required.
The ultrasonic examination technique, which monitors the attenuation of longitudinal waves can pinpoint grain boundary microfissuring developing and progressing from the inside surface of waterwall tubing. By monitoring the sound transmission properties associated with the microfissuring, areas of extensive damage can be clearly defined.
The areas are initially scanned to determine the general ultrasonic response obtained from the tube areas in question. This is then further refined by utilizing high frequency transducers to evaluate the severity and level of damage occurring through the cross- sectional thickness of the tube wall. This technique has been proven reliable by subsequent removal of tube samples and confirmation of damage by metallurgical evaluations.
Thielsch Engineering Inc. has successfully conducted this scanning technique in over 200 utility boiler furnaces throughout the United States over the last 20 years.
For additional information regarding methods for identifying hydrogen damage in furnace waterwall tubing please contact Mr. Peter Kennefick at firstname.lastname@example.org.